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Papers by Alan Brandon
Geochemical characterization of mid-Cretaceous granitoids of the Kootenay Arc in the southern Canadian Cordillera
Canadian Journal of Earth Sciences, 1993
Contributions To Mineralogy and Petrology, 1995
Many objections have been raised as to the ability of subcontinental lithospheric mantle to prod... more Many objections have been raised as to the ability of subcontinental lithospheric mantle to produce voluminous amounts of basalt, because this upper part of the mantle is thought to be refractory, and the geotherm is rarely above the peridotite solidus at these depths under continents. However, in the Pacific Northwest of the USA during the Neogene, the subcontinental lithospehric mantle has been proposed as a key source for basalts erupted within the northern Basin and Range, and for the Columbia River flood basalts erupted on the Columbia Plateau. An alternative explanation to melting in the subcontinental lithospheric mantle, which equally well explains the chemical compositions thought to originate there, is that these magmas were contaminated by crust of varying ages. Calc-alkaline lavas, which occupy the Blue Mountains in the center of this region, hold clues to the latter process. Their elevated trace element ratios (e.g., Ba/Zr, K2O/P2O5), coupled with differentiation indicators such as Mg? [molar Mg/(Mg + Fe)], and Sr, Nd, and Pb isotopic compositions, can most reasonably be explained by crustal contamination. Appraisal of continental peridotite xenolith data indicates that high trace element ratios such as Ba/Zr in continental basalts cannot result from melting in the subcontinental lithospheric mantle. Instead, as with the calc-alkaline lavas, these high ratios in the tholeiites most likely indicate crustal contamination. Furthermore, the peridotite xenoliths do not have a relative depletion in Nb and Ta that is observed in most of the lavas within the region. Relatively minor volumes of tholeiites erupted in late Neogene times in the northern Basin and Range (Hi-Mg olivine tholeiites) and Columbia Plateau (Saddle Mountains basalts), are the only lavas which have trace element and isotopic compositions consistent with being derived from, or largely interacting with a subcontinental lithospheric mantle in the Pacific Northwest. In contrast to the prior studies, we suggest that the mantle sources for most of the basalts in this region were ultimately beneath the lithospheric mantle.
Earth and Planetary Science Letters, 1988
Most of the recent discussions on the number and kinds of components which can be distinguished i... more Most of the recent discussions on the number and kinds of components which can be distinguished in the Columbia River Basalt Group (CRBG) magmas have used arguments developed from isotopic evidence. In this paper, we consider relative contents of excluded trace elements, interpreted by means of spidergrams and abundance ratios, and compare data from the CRBG and related lavas with those of selected oceanic basalts. Although many recent models for CRBG petrogeneses do not include a plume component, spidergrams for the American Bar flows of the Inmaha Basalt of the CRBG have broad humps at Ba, Th, and Nb (Ta). This feature suggests that an enriched mantle plume component is present in the American Bar magmas, an inference which is weakly confirmed by 87Sr//86 Sr data.
Contributions To Mineralogy and Petrology, 1993
Crustal contamination of basalts located in the western United States has been generally under-em... more Crustal contamination of basalts located in the western United States has been generally under-emphasized, and much of their isotopic variation has been ascribed to multiple and heterogeneous mantle sources. Basalts of the Miocene Columbia River Basalt Group in the Pacific Northwest have passed through crust ranging from Precambrian to Tertiary in age. These flows are voluminous, homogenous, and underwent rapid effusion, all of which are disadvantages for crustal contamination while en route to the surface. The Picture Gorge Basalt of the Columbia River Basalt Group erupted through Paleozoic and Mesozoic oceanic accreted terranes in central Oregon, and earlier studies on these basalts provided no isotopic evidence for crustal contamination. New Sr, Nd, Pb, and O isotopic data presented here indicate that the isotopic variation of the Picture Gorge Basalt is very small, 87Sr/86Sr=0.70307–0.70371, ɛNd=+7.7-+4.8, δ18O=+5.6±6.1, and 206Pb/204Pb=18.80–18.91. Evaluation of the Picture Gorge compositional variation supports a model where two isotopic components contributed to Picture Gorge Basalt genesis. The first component (C1) is reflected by low 87Sr/86Sr, high ɛNd, and nonradiogenic Pb isotopic compositions. Basalts with C1 isotopic compositions have large MgO, Ni, and Cr contents and mantle-like δ18O=+5.6. C1 basalts have enrichments in Ba coupled with depletions in Nb and Ta. These characteristics are best explained by derivation from a depleted mantle source which has undergone a recent enrichment by fluids coming from a subducted slab. This C1 mantle component is prevalent throughout the Pacific Northwest. The second isotopic component has higher 87Sr/ 86Sr and δ18O, lower ɛNd, and more radiogenic Pb isotopic compositions than C1. There is a correlation in the Picture Gorge data of Sr, Nd, and Pb isotopes with differentiation indicators such as decreasing Mg#, and increasing K2O/TiO2, Ba, Ba/Zr, Rb/Sr, La/Sm, and La/Yb. Phase equilibrium and mineralogical constraints indicate that these compositional characteristics were inherited in the Picture Gorge magmas at crustal pressures, and thus the second isotopic component is most likely crustal in origin. Mixing and open-system calculations can produce the isotopic composition of the most evolved Picture Gorge flows from the most primitive compositions by 8 to 21% contamination of isotopic compositions similar to accreted terrane crust found in the Pacific Northwest. Therefore, in spite of the disadvantages for crustal contamination and their narrow range in isotopic compositions, the process controlling isotopic variation within the Picture Gorge Basalt is primarily crustal contamination. We suggest that comprehensive analyses for basaltic suites and careful consideration of these data must be made to test for crustal contamination, before variation resulting from mantle heterogeneity can be assessed.
Contributions To Mineralogy and Petrology, 1993
Crustal contamination of basalts located in the western United States has been generally under-em... more Crustal contamination of basalts located in the western United States has been generally under-emphasized, and much of their isotopic variation has been ascribed to multiple and heterogeneous mantle sources. Basalts of the Miocene Columbia River Basalt Group in the Pacific Northwest have passed through crust ranging from Precambrian to Tertiary in age. These flows are voluminous, homogenous, and underwent rapid effusion, all of which are disadvantages for crustal contamination while en route to the surface. The Picture Gorge Basalt of the Columbia River Basalt Group erupted through Paleozoic and Mesozoic oceanic accreted terranes in central Oregon, and earlier studies on these basalts provided no isotopic evidence for crustal contamination. New Sr, Nd, Pb, and O isotopic data presented here indicate that the isotopic variation of the Picture Gorge Basalt is very small, 87Sr/86Sr=0.70307 0.70371, ɛNd=+7.7-+4.8, δ18O=+5.6±6.1, and 206Pb/204Pb=18.80 18.91. Evaluation of the Picture Gorge compositional variation supports a model where two isotopic components contributed to Picture Gorge Basalt genesis. The first component (C1) is reflected by low 87Sr/86Sr, high ɛNd, and nonradiogenic Pb isotopic compositions. Basalts with C1 isotopic compositions have large MgO, Ni, and Cr contents and mantle-like δ18O=+5.6. C1 basalts have enrichments in Ba coupled with depletions in Nb and Ta. These characteristics are best explained by derivation from a depleted mantle source which has undergone a recent enrichment by fluids coming from a subducted slab. This C1 mantle component is prevalent throughout the Pacific Northwest. The second isotopic component has higher 87Sr/ 86Sr and δ18O, lower ɛNd, and more radiogenic Pb isotopic compositions than C1. There is a correlation in the Picture Gorge data of Sr, Nd, and Pb isotopes with differentiation indicators such as decreasing Mg#, and increasing K2O/TiO2, Ba, Ba/Zr, Rb/Sr, La/Sm, and La/Yb. Phase equilibrium and mineralogical constraints indicate that these compositional characteristics were inherited in the Picture Gorge magmas at crustal pressures, and thus the second isotopic component is most likely crustal in origin. Mixing and open-system calculations can produce the isotopic composition of the most evolved Picture Gorge flows from the most primitive compositions by 8 to 21% contamination of isotopic compositions similar to accreted terrane crust found in the Pacific Northwest. Therefore, in spite of the disadvantages for crustal contamination and their narrow range in isotopic compositions, the process controlling isotopic variation within the Picture Gorge Basalt is primarily crustal contamination. We suggest that comprehensive analyses for basaltic suites and careful consideration of these data must be made to test for crustal contamination, before variation resulting from mantle heterogeneity can be assessed.
Journal of Geophysical Research, 1994
The trace element and isotopic compositions of Mesozoic granitoids in southeast British Columbia ... more The trace element and isotopic compositions of Mesozoic granitoids in southeast British Columbia constrain the timing and mechanisms for granitoid production in the western North American Cordillera. The granitoids were eraplaced in Middle Jurassic and mid-Cretaceous pulses into Paleozoic-Mesozoic accreted terranes, Paleozoic metasediments, and Proterozoic metasediments of the Purcell-Windermere Group. Middle Jurassic compositions range from quartz diorite to tonalite to granite, with high Ba and low Rb and Nb abundances similar to those in granitoids found within Mesozoic arc complexes of western North America. Isotopic compositions range for eSrT from -27 to +33 and eNdT from 0 to -7 with initial 2ø6pb/2ø4pb, 2ø8pb/2ø4Pb, and 2ø7pb/2ø4Pb lying in an array between upper and lower crust reservoirs. These compositions can be produced by addition of 5-25% of Precambrian crust to basalt. The Mid-Jurassic granitoids formed as part of an arc magmatic complex during subduction of ocean crust and as accretion of outboard terranes occurred west of the North American continent. In contrast, mid-Cretaceous batholiths are composed of weakly peraluminous hornblende-biotite granitoids and strongly peraluminous two-mica granites. These granitoids have lower eNdT, and eSrr, 2ø6pb/2ø4pb, 2ø8pb/2ø•Pb, and 2ø7pb/2ø4pb ratios extending to more radiogenic compositions than the Middle Jurassic granitoids and have complete overlap with isotopic compositions of southeast British Columbia Precambrian crust. Their trace element and isotopic compositions are consistent with a model in which crustal anatexis of Precambrian basement gneisses produced the hornblende-biotite granitoids and anatexis of Proterozoic metapelites produced the two-mica granites as a melt zone migrated up through the crust. Collision and accretion of exotic terranes resulted in a maximum in crustal thickening in the Late Jurassic to Early Cretaceous (~150-130 Ma) in the Canadian Cordillera, 2040 m.y. prior to mid-Cretaceous (~110 Ma) granitoid magmatism. The timing of these events are consistent with anatexis being a response to crustal thickening. in California, and the Coast Ranges batholith in British Columbia, Yukon, and Alaska [Pitcher, 1984; Grotnet and Silver, 1987; Batetnan, 1983; Barker and Arth, 1990]. These granitoids were emplaced within larger Mesozoic magmatic arc complexes. Belts dominated by lesser volumes of high-K20 granodiorites and granites (with little or no tonalite) are mostly found within the Cordilleran interior inboard of the main magmatic arc complexes [Miller and Barton, 1990]. The Paper number 94JB00336. 0148-0227/94/94JB-00336505.00 mechanisms which produced granitoids in the Cordilleran interior remain controversial but are likely the consequence of several interrelated factors including heat, magma, and fluid transport from the mantle to the crust [Hyndtnan and Foster, 1988; Hoisch and Hatnilton, 1990; Barton, 1990], and crustal thickening [Pati•o Douce et al., 1990], all of which may induce crustal anatexis.
Geochimica Et Cosmochimica Acta, 2009
The Moon likely accreted from melt and vapor ejected during a cataclysmic collision between Proto... more The Moon likely accreted from melt and vapor ejected during a cataclysmic collision between Proto-Earth and a Marssized impactor very early in solar system history. The identical W, O, K, and Cr isotope compositions between materials from the Earth and Moon require that the material from the two bodies were well-homogenized during the collision process. As such, the ancient isotopic signatures preserved in lunar samples provide constraints on the bulk composition of the Earth. Two recent studies to obtain high-precision 142 Nd/ 144 Nd ratios of lunar mare basalts yielded contrasting results. In one study, after correction of neutron fluence effects imparted to the Nd isotope compositions of the samples, the coupled 142 Nd-143 Nd systematics were interpreted to be consistent with a bulk Moon having a chondritic Sm/Nd ratio Neodymium isotope evidence for a chondritic composition of the Moon. Science 312, 1369-1372]. The other study found that their data on the same and similar lunar mare basalts were consistent with a bulk Moon having a superchondritic Sm/Nd ratio [Boyet M. and Carlson R. W. (2007) A highly depleted Moon or a non-magma origin for the lunar crust? Earth Planet. Sci. Lett. 262,[505][506][507][508][509][510][511][512][513][514][515][516]. Delineating between these two potential scenarios has key ramifications for a comprehensive understanding of the formation and early evolution of the Moon and for constraining the types of materials available for accretion into large terrestrial planets such as Earth.
Contributions To Mineralogy and Petrology, 1992
The peraluminous Drammen batholith (650 km2) is the largest granite complex within the mainly alk... more The peraluminous Drammen batholith (650 km2) is the largest granite complex within the mainly alkaline province of the Permo-Carboniferous Oslo Rift, and peraluminous to metaluminous granites are also present in the southern part of the otherwise alkaline Finnemarka complex (125 km2). The emplacement of the Drammen granite, and probably most of the other biotite granite complexes, predate the alkaline syenites and granites. The eight separate petrographic types of the Drammen batholith range in SiO2 from 70 to 79 wt.% and have experienced variable amounts of fractionation of feldspars, biotite, zircon, apatite, titanite and Fe−Ti-oxides. The initial Sr, Nd and Pb isotopic ratios and a decoupling between the variations in the SiO2 content and the aluminum saturation index [ASI=Al2O3/(CaO+Na2O +K2O)] show that the various intrusive phases are not strictly comagmatic. The εNd values of the southern part of Finnemarka (+3.5 to +4) and the northern part of the Drammen granite (+1 to +1.5) are high and indicate insignificant (for Finnemarka) to minor Precambrian crustal or enriched mantle contributions. The very low εSr values of all of these samples (−1 to −12, outside the main Oslo Rift magmatic array), point to a time integrated Rb-depleted crustal contaminant or an EM1 mantle component. The earliest extruded alkali basalts along the southwestern margin of the Oslo Rift are the only other samples within this low εSr area, but their isotopic signature may also be linked to a mantle enrichment event (involving an EM1 component), e.g. associated with the Fen carbonatite magmatism 540 Ma ago. For a given 206Pb/204Pb, the 208Pb/204Pb ratios of the Drammen and Finnemarka batholiths are distinctly lower than those of the Skien alkaline volcanics and all other magmatic Oslo Rift rocks. This may indicate that the lithosphere of the central part of the rift had a time integrated Th-depletion. The samples from the southern part of the Drammen batholith, characterized by the presence of abundant miarolitic cavities, have εNd near 0 (−0.7 to +0.4) but strongly elevated εSr of +35 to +67. The combined Pb isotopic ratios of all the samples analyzed indicate that the Precambrian crustal anatectic contribution is in the form of time integrated Th-and U-depleted lower crust, and the high +Sr of the sourthern part of the Drammen granite results from shallow level wallrock assimilation or magma-fluid interactions. The remarkably low contribution of old crustal components to the Finnemarka and the northernmost Drammen batholiths may result from extensive late Precambrian intracustal differentiation in southwestern Scandinavia, leading to widespread upper crustal granites (≈ 900 Ma) and a correspondingly dense and refractory lower crust, in particular in a zone intersecting the central part of the rift. Liquidus phase relations and mass-balance constrainst permit derivation of the granites from mildly alkaline to tholeiitic melts by extensive crystal fractionation of clinopyroxene-and amphibole-rich assemblages. It is equally possible to form the granitic magmas by partial melting of Permian gabbros of similar composition. Either scenario is consistent with the isotopic constrainst and with the presence of dense cumulates and/or residues in the lower crust. The lack of igneous rocks of intermediate composition associated with the Drammen and Finnemarka batholiths point to an efficient upper crustal density filtering. Considerable amounts of heat would be accumulated in this region if differentiated, intermediate melts could not escape to shallower levels. Successive magma injections would therefore easily result in partial melting of already solidified mafic to intermediate melts and cumulates, and it is suggested that the peraluminous granites formed mainly by water-undersaturated anatexis of mafic material.
Geochimica Et Cosmochimica Acta, 2009
New measurements of Os, He, Sr and Nd isotopes, along with major and trace elements, are presente... more New measurements of Os, He, Sr and Nd isotopes, along with major and trace elements, are presented for basalts from the three volcanic flank zones in Iceland and from Jan Mayen Island. The 187 Os/ 188 Os ratios in lavas with <30 ppt Os (n = 4) are elevated compared to ratios in coexisting olivine and appear to be contaminated at a shallow level. The 187 Os/ 188 Os ratios in the remaining lavas with >30 ppt Os (n = 17) range between 0.12117 and 0.13324. These values are surprisingly low for oceanic island basalts and include some samples that are less than putative present-day primitive upper mantle (PUM with 187 Os/ 188 Os of 0.1296). These low 187 Os/ 188 Os preclude significant shallow-level contamination from oceanic crust. The 187 Os/ 188 Os ratios for Jan Mayen lavas are less than PUM, severely limiting the presence of any continental crust in their mantle source. A positive correlation between 143 Nd/ 144 Nd and 187 Os/ 188 Os ratios in Iceland and Jan Mayen lavas likely reflects the presence in their source of ancient subcontinental lithosphere that has undergone incompatible trace element enrichment that did not affect the Re-Os system. In addition, the Jan Mayen lava isotopic signature cannot be explained solely by the presence of subcontinental lithospheric mantle, and the influence of another geochemical component, such as a mantle plume appears required. Combined 87 Sr/ 86 Sr, 143 Nd/ 144 Nd, 3 He/ 4 He and 187 Os/ 188 Os data indicate a genetic relationship between Jan Mayen Island and the Iceland mantle plume. Material from the Iceland mantle plume likely migrates at depth until it reaches the tensional setting of the Jan Mayen Fracture Zone, where it undergoes low-degree partial melting. At a first-order, isotopic co-variations can be interpreted as broadly binary mixing curves between two primary end-members. One end-member, characterized in particular by its unradiogenic 187 Os/ 188 Os and 143 Nd/ 144 Nd, low 3 He/ 4 He and high 87 Sr/ 86 Sr, is represented by subcontinental lithospheric mantle stranded and disseminated in the upper mantle during the opening of the Atlantic Ocean. The second end-member corresponds to a hybrid mixture between the depleted-MORB mantle and the enriched Iceland mantle plume, itself resulting from mixing between recycled oceanic crust and depleted lower mantle. This hybrid accounts for the high 3 He/ 4 He ($28 Ra), high 143 Nd/ 144 Nd ($0.5132), high 187 Os/ 188 Os ($0.14) and low 87 Sr/ 86 Sr ($0.7026) composition observed in Iceland. Two different models may account for these observed mixing relationships between the end-members. In this first model, the Iceland mantle entrains pristine depleted material when rising in the upper 0016-7037/$ -see front matter Ó
Geochimica Et Cosmochimica Acta, 2007
Picrites from the neovolcanic zones in Iceland display a range in 187 Os/ 188 Os from 0.1297 to 0... more Picrites from the neovolcanic zones in Iceland display a range in 187 Os/ 188 Os from 0.1297 to 0.1381 (c Os = + 2.1 to +8.7) and uniform 186 Os/ 188 Os of 0.1198375 ± 32 (2r). The value for 186 Os/ 188 Os is within uncertainty of the present-day value for the primitive upper mantle of 0.1198398 ± 16. These Os isotope systematics are best explained by ancient recycled crust or melt enrichment in the mantle source region. If so, then the coupled enrichments displayed in 186 Os/ 188 Os and 187 Os/ 188 Os from lavas of other plume systems must result from an independent process, the most viable candidate at present remains core-mantle interaction. While some plumes with high 3 He/ 4 He, such as Hawaii, appear to have been subjected to detectable addition of Os (and possibly He) from the outer core, others such as Iceland do not.
Geochimica Et Cosmochimica Acta, 2010
Here we search for evidence of the existence of a sub-chondritic 142 Nd/ 144 Nd reservoir that ba... more Here we search for evidence of the existence of a sub-chondritic 142 Nd/ 144 Nd reservoir that balances the Nd isotope chemistry of the Earth relative to chondrites. If present, it may reside in the source region of deeply sourced mantle plume material. We suggest that lavas from Hawai'i with coupled elevations in 186 Os/ 188 Os and 187 Os/ 188
Geochimica Et Cosmochimica Acta, 2009
New measurements of Os, He, Sr and Nd isotopes, along with major and trace elements, are presente... more New measurements of Os, He, Sr and Nd isotopes, along with major and trace elements, are presented for basalts from the three volcanic flank zones in Iceland and from Jan Mayen Island. The 187Os/ 188Os ratios in lavas with <30 ppt Os ( n = 4) are elevated compared to ratios in coexisting olivine and appear to be contaminated at a shallow level. The 187Os/ 188Os ratios in the remaining lavas with >30 ppt Os ( n = 17) range between 0.12117 and 0.13324. These values are surprisingly low for oceanic island basalts and include some samples that are less than putative present-day primitive upper mantle (PUM with 187Os/ 188Os of 0.1296). These low 187Os/ 188Os preclude significant shallow-level contamination from oceanic crust. The 187Os/ 188Os ratios for Jan Mayen lavas are less than PUM, severely limiting the presence of any continental crust in their mantle source. A positive correlation between 143Nd/ 144Nd and 187Os/ 188Os ratios in Iceland and Jan Mayen lavas likely reflects the presence in their source of ancient subcontinental lithosphere that has undergone incompatible trace element enrichment that did not affect the Re-Os system. In addition, the Jan Mayen lava isotopic signature cannot be explained solely by the presence of subcontinental lithospheric mantle, and the influence of another geochemical component, such as a mantle plume appears required. Combined 87Sr/ 86Sr, 143Nd/ 144Nd, 3He/ 4He and 187Os/ 188Os data indicate a genetic relationship between Jan Mayen Island and the Iceland mantle plume. Material from the Iceland mantle plume likely migrates at depth until it reaches the tensional setting of the Jan Mayen Fracture Zone, where it undergoes low-degree partial melting. At a first-order, isotopic co-variations can be interpreted as broadly binary mixing curves between two primary end-members. One end-member, characterized in particular by its unradiogenic 187Os/ 188Os and 143Nd/ 144Nd, low 3He/ 4He and high 87Sr/ 86Sr, is represented by subcontinental lithospheric mantle stranded and disseminated in the upper mantle during the opening of the Atlantic Ocean. The second end-member corresponds to a hybrid mixture between the depleted-MORB mantle and the enriched Iceland mantle plume, itself resulting from mixing between recycled oceanic crust and depleted lower mantle. This hybrid accounts for the high 3He/ 4He (˜28 Ra), high 143Nd/ 144Nd (˜0.5132), high 187Os/ 188Os (˜0.14) and low 87Sr/ 86Sr (˜0.7026) composition observed in Iceland. Two different models may account for these observed mixing relationships between the end-members. In this first model, the Iceland mantle entrains pristine depleted material when rising in the upper mantle and allows refractory sub-lithospheric fragments to melt because of excess heat derived from the deep plume material. A second model that may better account for the Pb isotopic variations observed, uses the same components but where the depleted-MORB mantle is already polluted by subcontinental lithospheric mantle material before mixing with the Iceland mantle plume. Both cases likely occur. Though only three principal components are required to explain the isotopic variations of the Iceland-Jan Mayen system, the different possible mixing relationships may be accounted for by potentially a greater number of end-members.
Earth and Planetary Science Letters, 1999
Primitive Hawaiian picrites have 187 Os/ 188 Os as high as V0.145 and are more radiogenic than th... more Primitive Hawaiian picrites have 187 Os/ 188 Os as high as V0.145 and are more radiogenic than the depleted upper mantle, reflecting a time-integrated suprachondritic Re/Os ratio. The high Re/Os may be explained either by an ancient recycled crustal component or an evolved outer core component in the Hawaiian plume. New high precision 186 Os/ 188 Os measurements for these picrites, combined with previous analyses, show that the Hawaiian plume source has 186 Os/ 188 Os that range from chondritic mantle values of V0.119834 to more radiogenic values as high as V0.119848. The higher 186 Os/ 188 Os reflects long-term suprachondritic Pt/Os and is coupled with higher 187 Os/ 188 Os in all but the Koolau picrites. The latter have near-chondritic 186 Os/ 188 Os but with radiogenic 187 Os/ 188 Os. The Pt/Re of crustal materials that may make up ancient recycled slabs ranges from V0.1 to 33. Recycled slab material with such Pt/Re ratios evolved for 1^3 Ga and added to the plume source may explain the Koolau Os isotopic compositions. Pt/Re ratios of 88^100, however, are required for the ancient recycled crust to generate the coupled enrichments of 186 Os/ 188 Os and 187 Os/ 188 Os in picrites from Loihi and Hualalai. These high Pt/Re ratios do not occur in any known crustal materials, but they are consistent with the observed partition coefficients for Os s Re s Pt, during metal crystallization from an initially chondritic molten core (Pt/Re V21^24). Such partitioning may have produced an evolved outer core with suprachondritic Pt/Os, Re/Os and Pt/Re, resulting in the production of suprachondritic 186 Os/ 188 Os and 187 Os/ 188 Os over time. Small amounts of outer core metal (9 1.2%) mixed into the Hawaiian plume source can explain the coupled 186 Os/ 188 Os and 187 Os/ 188 Os enrichment in some of the Hawaiian picrites. In addition, the most radiogenic 186 Os/ 188 Os in the Hawaiian picrites is correlated with higher 3 He/ 4 He, consistent with an undegassed, and likely, lower mantle source. These data provide compelling geochemical evidence that the Hawaiian plume was generated at the core-mantle boundary. ß 1999 Elsevier Science B.V. All rights reserved.
Earth and Planetary Science Letters, 2000
Rhenium^osmium isotopic systematics of primitive calc-alkaline lavas from the Lassen region appea... more Rhenium^osmium isotopic systematics of primitive calc-alkaline lavas from the Lassen region appear to be controlled by mantle wedge processes. Lavas with a large proportion of slab component have relatively low Re and Os abundances, and have radiogenic Os and mid ocean ridge basalt-like Sr and Pb isotopic compositions. Lavas with a small proportion of slab component have higher Re and Os elemental abundances and display mantle-like Os, Sr, Nd, and Pb isotopic compositions. Assimilation with fractional crystallization can only generate the Re^Os systematics of the Lassen lavas from a common parent if the distribution coefficient for Re in sulfide is V40^1100 times higher than most published estimates and if most incompatible element abundances decrease during differentiation. High Re/Os ratios in mid ocean ridge basalts makes subducted oceanic crust a potential source of radiogenic Os in volcanic arcs. The slab beneath the southernmost Cascades is estimated to have 187 Os/ 188 Os ratios as high as 1.4. Mixing between a slab component and mantle wedge peridotite can generate the Os isotopic systematics of the Lassen lavas provided the slab component has a Sr/Os ratio of V7.5U10 5 and Os abundances that are 100^600 times higher than mid ocean ridge basalts. For this model to be correct, Os must be readily mobilized and concentrated in the slab component, perhaps as a result of high water and HCl fugacities in this subduction environment. Another possible mechanism to account for the correlation between the magnitude of the subduction geochemical signature and Os isotopic composition involves increasing the stability of an Os-bearing phase in mantle wedge peridotites as a result of fluxing with the slab component. Melting of such a source could yield low Os magmas that are more susceptible to crustal contamination, and hence have more radiogenic Os isotopic compositions, than magmas derived from sources with a smaller contribution from the slab. Thus, the addition of the slab component to the mantle wedge appears to result in either the direct or indirect addition of radiogenic Os to arc magmas. ß
Isotopic constraints on time scales and mechanisms of slab material transport in the mantle wedge: evidence from the Simcoe mantle xenoliths, Washington, USA
Chemical Geology, 1999
... 1996). These xenoliths have largely granular to porphyroclastic textures, with little evidenc... more ... 1996). These xenoliths have largely granular to porphyroclastic textures, with little evidence of pervasive brittle deformation. ... 1994). Strontium, Sm, Nd, U, Th, and Pb abundances were obtained by isotope dilution. 87 ...
Geochimica Et Cosmochimica Acta, 2005
The Tagish Lake meteorite is a primitive C2 chondrite that has undergone aqueous alteration short... more The Tagish Lake meteorite is a primitive C2 chondrite that has undergone aqueous alteration shortly after formation of its parent body. Previous work indicates that if this type of material was part of a late veneer during terrestrial planetary accretion, it could provide a link between atmophile elements such as H, C, N and noble gases, and highly siderophile element replenishment in the bulk silicate portions of terrestrial planets following core formation. The systematic Re-Os isotope and highly siderophile element measurements performed here on five separate fractions indicate that while Tagish Lake has amongst the highest Ru/Ir (1.63 Ϯ 0.08), Pd/Ir (1.19 Ϯ 0.06) and 187 Os/ 188 Os (0.12564 -0.12802) of all carbonaceous chondrites, these characteristics still fall short of those necessary to explain the observed siderophile element systematics of the primitive upper mantles of Earth and Mars. Hence, a direct link between atmophile and highly siderophile elements remains elusive, and other sources for replenishment are required, unless an as yet poorly constrained process fractionated Re/Os, Ru/Ir, and Pd/Ir following late accretion on both the Earth and Mars mantles.
Earth and Planetary Science Letters, 2004
Precise Pt -Re -Os isotopic and highly siderophile element (HSE) data are reported for 10 drill c... more Precise Pt -Re -Os isotopic and highly siderophile element (HSE) data are reported for 10 drill core samples collected across a 2.7 m thick, differentiated komatiite lava flow at Pyke Hill in Ontario, Canada. The Pt -Re -Os isotopic data for five cumulate samples yield Re -Os and Pt -Os isochrons with ages of 2725 F 33 and 2956 F 760 Ma and initial 187 Os/ 188 Os = 0.10902 F 16 and 186 Os/ 188 Os = 0.1198318 F 18, respectively. These ages are consistent with the time of lava emplacement determined by other geochronometers. The initial 187 Os/ 188 Os isotopic composition of the Pyke Hill source is similar to the 187 Os/ 188 Os isotopic compositions of enstatite and ordinary chondrites and to that of PUM at 2725 Ma. Using the Pt -Os isotopic data for the Pyke Hill source, the present-day 186 Os/ 188 Os isotopic composition of PUM was calculated to be 0.1198387 F 18. The regression of global 187 Os/ 188 Os isotopic data for terrestrial materials shown to represent Os isotopic composition of the contemporary mantle indicates that the LILE-depleted mantle evolved from the solar system starting composition and, over Earth's history, was on average f 8% depleted in Re vs. Os relative to PUM. From the HSE composition of the emplaced Pyke Hill komatiite lava, the mantle source was inferred to contain Re (0.30 ppb), Os (3.9 ppb), Ir (3.6 ppb), Ru (5.4 ppb), Pt (6.4 ppb), and Pd (6.3 ppb) in relative proportions similar to those in average enstatite chondrites, except for Pd, which was enriched over Pt by f 30% in the Pyke Hill source compared to enstatite chondrites. From these data, the 190 Pt/ 188 Os of PUM was calculated to be 0.00157. If the HSE abundances in the terrestrial mantle were inherited from chondritic material of a late veneer, the latter should have had the HSE composition of ordinary or enstatite chondrites. Alternatively, if the reenrichment of the mantle with HSE was the result of an open-system transport of material across the core -mantle boundary, it was likely a global event that should have occurred shortly after core formation. D
Astrophysical Journal, 2007
Correlated isotopic anomalies in osmium (186Os, 188Os, 190Os, measured with respect to 189Os) ext... more Correlated isotopic anomalies in osmium (186Os, 188Os, 190Os, measured with respect to 189Os) extracted from primitive carbonaceous chondrites tightly constrain the σn(190Os)/σn(188Os) ratio to be 0.859+/-0.042 (+/-5%). A recent measurement of the Maxwellian-averaged neutron-capture cross sections (MACSs) for 186,187,188Os lowered the σn(188Os) by 27% but did not measure σn(190Os). From the σn(190Os)/σn(188Os) ratio, we infer σn(190Os)=249+/-18 mbarns for internal consistency with the new MACSs for the other Os isotopes. This approach is applicable to other isotopic anomalies in r-process/s-process ratios derived from meteorites for nuclei that do not have branching points between them. Branching at 185W and 186Re makes the 186Os/188Os ratio a neutron dosimeter for the s-process that, with the new cross sections, yields an average neutron density, nn~3×108 cm-3. This low neutron density is consistent with previous results from Sr, Zr, Mo, and Ba isotopes that indicated a minor contribution from the 22Ne(α, n)25Mg neutron source relative to the 13C(α, n)16O neutron source.
190Pt– 186Os and 187Re– 187Os systematics of abyssal peridotites
Earth and Planetary Science Letters, 2000
Abstract Abyssal peridotites are normally thought to be residues of melting of the mid-ocean ridg... more Abstract Abyssal peridotites are normally thought to be residues of melting of the mid-ocean ridge basalt (MORB) source and are presumably a record of processes affecting the upper mantle. Samples from a single section of abyssal peridotite from the Kane Transform area in the Atlantic Ocean were examined for 190 Pt-186 Os and 187 Re-187 Os systematics. They have uniform 186 Os/188 Os ratios with a mean of 0.1198353+/-7, identical to the mean of 0.1198340+/-12 for Os-Ir alloys and chromitites believed to be representative of the upper ...
Earth and Planetary Science Letters, 2005
Some hotspot volcanism is likely the surface manifestation of hot upwelling mantle plumes that ri... more Some hotspot volcanism is likely the surface manifestation of hot upwelling mantle plumes that rise from the core-mantle boundary (CMB) or from compositional boundaries within the mantle. Plumes that rise from the CMB may generate magmas that carry compositional information about the diversity of materials present in the lowermost mantle. Some plume-derived materials have coupled enrichments in 186 Os/ 188 Os and 187 Os/ 188 Os relative to upper mantle materials. The coupled enrichments are consistent with Os isotopic compositions predicted for the liquid outer core that might result from elevated Pt/Os and Re/Os generated via progressive crystallization of a solid inner core over Earth history. If these enrichments are a reflection of coremantle interaction, then the mechanisms of material transport between the core and the mantle, rates of whole mantle convection at present and over time, and cooling histories for the core and the mantle can potentially be constrained. However, fractionation may also occur between Pt/Re/Os via mantle and crustal processes and in particular, Pt and Pt/Re may be elevated in some portions of ancient recycled slabs thought to be present in certain plume sources. This possibility brings into question whether the coupled Os isotope enrichments are a product of core-mantle exchange. Hence, a lively debate over the causes of Os isotopic heterogeneities in plume-derived materials has ensued. Resolution of this debate has been hampered by limited constraints on the behavior of these elements in the Earth's core, mantle and crust. A variety of analytical and experimental tasks will have to be pursued to test different aspects of both the core-mantle interaction and crustal recycling models, and to further constrain the behaviors of these elements. These future endeavors may help to unravel the compositional complexities of plumederived materials and their mantle sources, and ultimately provide important insights to processes operating at the CMB. D
Geochemical characterization of mid-Cretaceous granitoids of the Kootenay Arc in the southern Canadian Cordillera
Canadian Journal of Earth Sciences, 1993
Contributions To Mineralogy and Petrology, 1995
Many objections have been raised as to the ability of subcontinental lithospheric mantle to prod... more Many objections have been raised as to the ability of subcontinental lithospheric mantle to produce voluminous amounts of basalt, because this upper part of the mantle is thought to be refractory, and the geotherm is rarely above the peridotite solidus at these depths under continents. However, in the Pacific Northwest of the USA during the Neogene, the subcontinental lithospehric mantle has been proposed as a key source for basalts erupted within the northern Basin and Range, and for the Columbia River flood basalts erupted on the Columbia Plateau. An alternative explanation to melting in the subcontinental lithospheric mantle, which equally well explains the chemical compositions thought to originate there, is that these magmas were contaminated by crust of varying ages. Calc-alkaline lavas, which occupy the Blue Mountains in the center of this region, hold clues to the latter process. Their elevated trace element ratios (e.g., Ba/Zr, K2O/P2O5), coupled with differentiation indicators such as Mg? [molar Mg/(Mg + Fe)], and Sr, Nd, and Pb isotopic compositions, can most reasonably be explained by crustal contamination. Appraisal of continental peridotite xenolith data indicates that high trace element ratios such as Ba/Zr in continental basalts cannot result from melting in the subcontinental lithospheric mantle. Instead, as with the calc-alkaline lavas, these high ratios in the tholeiites most likely indicate crustal contamination. Furthermore, the peridotite xenoliths do not have a relative depletion in Nb and Ta that is observed in most of the lavas within the region. Relatively minor volumes of tholeiites erupted in late Neogene times in the northern Basin and Range (Hi-Mg olivine tholeiites) and Columbia Plateau (Saddle Mountains basalts), are the only lavas which have trace element and isotopic compositions consistent with being derived from, or largely interacting with a subcontinental lithospheric mantle in the Pacific Northwest. In contrast to the prior studies, we suggest that the mantle sources for most of the basalts in this region were ultimately beneath the lithospheric mantle.
Earth and Planetary Science Letters, 1988
Most of the recent discussions on the number and kinds of components which can be distinguished i... more Most of the recent discussions on the number and kinds of components which can be distinguished in the Columbia River Basalt Group (CRBG) magmas have used arguments developed from isotopic evidence. In this paper, we consider relative contents of excluded trace elements, interpreted by means of spidergrams and abundance ratios, and compare data from the CRBG and related lavas with those of selected oceanic basalts. Although many recent models for CRBG petrogeneses do not include a plume component, spidergrams for the American Bar flows of the Inmaha Basalt of the CRBG have broad humps at Ba, Th, and Nb (Ta). This feature suggests that an enriched mantle plume component is present in the American Bar magmas, an inference which is weakly confirmed by 87Sr//86 Sr data.
Contributions To Mineralogy and Petrology, 1993
Crustal contamination of basalts located in the western United States has been generally under-em... more Crustal contamination of basalts located in the western United States has been generally under-emphasized, and much of their isotopic variation has been ascribed to multiple and heterogeneous mantle sources. Basalts of the Miocene Columbia River Basalt Group in the Pacific Northwest have passed through crust ranging from Precambrian to Tertiary in age. These flows are voluminous, homogenous, and underwent rapid effusion, all of which are disadvantages for crustal contamination while en route to the surface. The Picture Gorge Basalt of the Columbia River Basalt Group erupted through Paleozoic and Mesozoic oceanic accreted terranes in central Oregon, and earlier studies on these basalts provided no isotopic evidence for crustal contamination. New Sr, Nd, Pb, and O isotopic data presented here indicate that the isotopic variation of the Picture Gorge Basalt is very small, 87Sr/86Sr=0.70307–0.70371, ɛNd=+7.7-+4.8, δ18O=+5.6±6.1, and 206Pb/204Pb=18.80–18.91. Evaluation of the Picture Gorge compositional variation supports a model where two isotopic components contributed to Picture Gorge Basalt genesis. The first component (C1) is reflected by low 87Sr/86Sr, high ɛNd, and nonradiogenic Pb isotopic compositions. Basalts with C1 isotopic compositions have large MgO, Ni, and Cr contents and mantle-like δ18O=+5.6. C1 basalts have enrichments in Ba coupled with depletions in Nb and Ta. These characteristics are best explained by derivation from a depleted mantle source which has undergone a recent enrichment by fluids coming from a subducted slab. This C1 mantle component is prevalent throughout the Pacific Northwest. The second isotopic component has higher 87Sr/ 86Sr and δ18O, lower ɛNd, and more radiogenic Pb isotopic compositions than C1. There is a correlation in the Picture Gorge data of Sr, Nd, and Pb isotopes with differentiation indicators such as decreasing Mg#, and increasing K2O/TiO2, Ba, Ba/Zr, Rb/Sr, La/Sm, and La/Yb. Phase equilibrium and mineralogical constraints indicate that these compositional characteristics were inherited in the Picture Gorge magmas at crustal pressures, and thus the second isotopic component is most likely crustal in origin. Mixing and open-system calculations can produce the isotopic composition of the most evolved Picture Gorge flows from the most primitive compositions by 8 to 21% contamination of isotopic compositions similar to accreted terrane crust found in the Pacific Northwest. Therefore, in spite of the disadvantages for crustal contamination and their narrow range in isotopic compositions, the process controlling isotopic variation within the Picture Gorge Basalt is primarily crustal contamination. We suggest that comprehensive analyses for basaltic suites and careful consideration of these data must be made to test for crustal contamination, before variation resulting from mantle heterogeneity can be assessed.
Contributions To Mineralogy and Petrology, 1993
Crustal contamination of basalts located in the western United States has been generally under-em... more Crustal contamination of basalts located in the western United States has been generally under-emphasized, and much of their isotopic variation has been ascribed to multiple and heterogeneous mantle sources. Basalts of the Miocene Columbia River Basalt Group in the Pacific Northwest have passed through crust ranging from Precambrian to Tertiary in age. These flows are voluminous, homogenous, and underwent rapid effusion, all of which are disadvantages for crustal contamination while en route to the surface. The Picture Gorge Basalt of the Columbia River Basalt Group erupted through Paleozoic and Mesozoic oceanic accreted terranes in central Oregon, and earlier studies on these basalts provided no isotopic evidence for crustal contamination. New Sr, Nd, Pb, and O isotopic data presented here indicate that the isotopic variation of the Picture Gorge Basalt is very small, 87Sr/86Sr=0.70307 0.70371, ɛNd=+7.7-+4.8, δ18O=+5.6±6.1, and 206Pb/204Pb=18.80 18.91. Evaluation of the Picture Gorge compositional variation supports a model where two isotopic components contributed to Picture Gorge Basalt genesis. The first component (C1) is reflected by low 87Sr/86Sr, high ɛNd, and nonradiogenic Pb isotopic compositions. Basalts with C1 isotopic compositions have large MgO, Ni, and Cr contents and mantle-like δ18O=+5.6. C1 basalts have enrichments in Ba coupled with depletions in Nb and Ta. These characteristics are best explained by derivation from a depleted mantle source which has undergone a recent enrichment by fluids coming from a subducted slab. This C1 mantle component is prevalent throughout the Pacific Northwest. The second isotopic component has higher 87Sr/ 86Sr and δ18O, lower ɛNd, and more radiogenic Pb isotopic compositions than C1. There is a correlation in the Picture Gorge data of Sr, Nd, and Pb isotopes with differentiation indicators such as decreasing Mg#, and increasing K2O/TiO2, Ba, Ba/Zr, Rb/Sr, La/Sm, and La/Yb. Phase equilibrium and mineralogical constraints indicate that these compositional characteristics were inherited in the Picture Gorge magmas at crustal pressures, and thus the second isotopic component is most likely crustal in origin. Mixing and open-system calculations can produce the isotopic composition of the most evolved Picture Gorge flows from the most primitive compositions by 8 to 21% contamination of isotopic compositions similar to accreted terrane crust found in the Pacific Northwest. Therefore, in spite of the disadvantages for crustal contamination and their narrow range in isotopic compositions, the process controlling isotopic variation within the Picture Gorge Basalt is primarily crustal contamination. We suggest that comprehensive analyses for basaltic suites and careful consideration of these data must be made to test for crustal contamination, before variation resulting from mantle heterogeneity can be assessed.
Journal of Geophysical Research, 1994
The trace element and isotopic compositions of Mesozoic granitoids in southeast British Columbia ... more The trace element and isotopic compositions of Mesozoic granitoids in southeast British Columbia constrain the timing and mechanisms for granitoid production in the western North American Cordillera. The granitoids were eraplaced in Middle Jurassic and mid-Cretaceous pulses into Paleozoic-Mesozoic accreted terranes, Paleozoic metasediments, and Proterozoic metasediments of the Purcell-Windermere Group. Middle Jurassic compositions range from quartz diorite to tonalite to granite, with high Ba and low Rb and Nb abundances similar to those in granitoids found within Mesozoic arc complexes of western North America. Isotopic compositions range for eSrT from -27 to +33 and eNdT from 0 to -7 with initial 2ø6pb/2ø4pb, 2ø8pb/2ø4Pb, and 2ø7pb/2ø4Pb lying in an array between upper and lower crust reservoirs. These compositions can be produced by addition of 5-25% of Precambrian crust to basalt. The Mid-Jurassic granitoids formed as part of an arc magmatic complex during subduction of ocean crust and as accretion of outboard terranes occurred west of the North American continent. In contrast, mid-Cretaceous batholiths are composed of weakly peraluminous hornblende-biotite granitoids and strongly peraluminous two-mica granites. These granitoids have lower eNdT, and eSrr, 2ø6pb/2ø4pb, 2ø8pb/2ø•Pb, and 2ø7pb/2ø4pb ratios extending to more radiogenic compositions than the Middle Jurassic granitoids and have complete overlap with isotopic compositions of southeast British Columbia Precambrian crust. Their trace element and isotopic compositions are consistent with a model in which crustal anatexis of Precambrian basement gneisses produced the hornblende-biotite granitoids and anatexis of Proterozoic metapelites produced the two-mica granites as a melt zone migrated up through the crust. Collision and accretion of exotic terranes resulted in a maximum in crustal thickening in the Late Jurassic to Early Cretaceous (~150-130 Ma) in the Canadian Cordillera, 2040 m.y. prior to mid-Cretaceous (~110 Ma) granitoid magmatism. The timing of these events are consistent with anatexis being a response to crustal thickening. in California, and the Coast Ranges batholith in British Columbia, Yukon, and Alaska [Pitcher, 1984; Grotnet and Silver, 1987; Batetnan, 1983; Barker and Arth, 1990]. These granitoids were emplaced within larger Mesozoic magmatic arc complexes. Belts dominated by lesser volumes of high-K20 granodiorites and granites (with little or no tonalite) are mostly found within the Cordilleran interior inboard of the main magmatic arc complexes [Miller and Barton, 1990]. The Paper number 94JB00336. 0148-0227/94/94JB-00336505.00 mechanisms which produced granitoids in the Cordilleran interior remain controversial but are likely the consequence of several interrelated factors including heat, magma, and fluid transport from the mantle to the crust [Hyndtnan and Foster, 1988; Hoisch and Hatnilton, 1990; Barton, 1990], and crustal thickening [Pati•o Douce et al., 1990], all of which may induce crustal anatexis.
Geochimica Et Cosmochimica Acta, 2009
The Moon likely accreted from melt and vapor ejected during a cataclysmic collision between Proto... more The Moon likely accreted from melt and vapor ejected during a cataclysmic collision between Proto-Earth and a Marssized impactor very early in solar system history. The identical W, O, K, and Cr isotope compositions between materials from the Earth and Moon require that the material from the two bodies were well-homogenized during the collision process. As such, the ancient isotopic signatures preserved in lunar samples provide constraints on the bulk composition of the Earth. Two recent studies to obtain high-precision 142 Nd/ 144 Nd ratios of lunar mare basalts yielded contrasting results. In one study, after correction of neutron fluence effects imparted to the Nd isotope compositions of the samples, the coupled 142 Nd-143 Nd systematics were interpreted to be consistent with a bulk Moon having a chondritic Sm/Nd ratio Neodymium isotope evidence for a chondritic composition of the Moon. Science 312, 1369-1372]. The other study found that their data on the same and similar lunar mare basalts were consistent with a bulk Moon having a superchondritic Sm/Nd ratio [Boyet M. and Carlson R. W. (2007) A highly depleted Moon or a non-magma origin for the lunar crust? Earth Planet. Sci. Lett. 262,[505][506][507][508][509][510][511][512][513][514][515][516]. Delineating between these two potential scenarios has key ramifications for a comprehensive understanding of the formation and early evolution of the Moon and for constraining the types of materials available for accretion into large terrestrial planets such as Earth.
Contributions To Mineralogy and Petrology, 1992
The peraluminous Drammen batholith (650 km2) is the largest granite complex within the mainly alk... more The peraluminous Drammen batholith (650 km2) is the largest granite complex within the mainly alkaline province of the Permo-Carboniferous Oslo Rift, and peraluminous to metaluminous granites are also present in the southern part of the otherwise alkaline Finnemarka complex (125 km2). The emplacement of the Drammen granite, and probably most of the other biotite granite complexes, predate the alkaline syenites and granites. The eight separate petrographic types of the Drammen batholith range in SiO2 from 70 to 79 wt.% and have experienced variable amounts of fractionation of feldspars, biotite, zircon, apatite, titanite and Fe−Ti-oxides. The initial Sr, Nd and Pb isotopic ratios and a decoupling between the variations in the SiO2 content and the aluminum saturation index [ASI=Al2O3/(CaO+Na2O +K2O)] show that the various intrusive phases are not strictly comagmatic. The εNd values of the southern part of Finnemarka (+3.5 to +4) and the northern part of the Drammen granite (+1 to +1.5) are high and indicate insignificant (for Finnemarka) to minor Precambrian crustal or enriched mantle contributions. The very low εSr values of all of these samples (−1 to −12, outside the main Oslo Rift magmatic array), point to a time integrated Rb-depleted crustal contaminant or an EM1 mantle component. The earliest extruded alkali basalts along the southwestern margin of the Oslo Rift are the only other samples within this low εSr area, but their isotopic signature may also be linked to a mantle enrichment event (involving an EM1 component), e.g. associated with the Fen carbonatite magmatism 540 Ma ago. For a given 206Pb/204Pb, the 208Pb/204Pb ratios of the Drammen and Finnemarka batholiths are distinctly lower than those of the Skien alkaline volcanics and all other magmatic Oslo Rift rocks. This may indicate that the lithosphere of the central part of the rift had a time integrated Th-depletion. The samples from the southern part of the Drammen batholith, characterized by the presence of abundant miarolitic cavities, have εNd near 0 (−0.7 to +0.4) but strongly elevated εSr of +35 to +67. The combined Pb isotopic ratios of all the samples analyzed indicate that the Precambrian crustal anatectic contribution is in the form of time integrated Th-and U-depleted lower crust, and the high +Sr of the sourthern part of the Drammen granite results from shallow level wallrock assimilation or magma-fluid interactions. The remarkably low contribution of old crustal components to the Finnemarka and the northernmost Drammen batholiths may result from extensive late Precambrian intracustal differentiation in southwestern Scandinavia, leading to widespread upper crustal granites (≈ 900 Ma) and a correspondingly dense and refractory lower crust, in particular in a zone intersecting the central part of the rift. Liquidus phase relations and mass-balance constrainst permit derivation of the granites from mildly alkaline to tholeiitic melts by extensive crystal fractionation of clinopyroxene-and amphibole-rich assemblages. It is equally possible to form the granitic magmas by partial melting of Permian gabbros of similar composition. Either scenario is consistent with the isotopic constrainst and with the presence of dense cumulates and/or residues in the lower crust. The lack of igneous rocks of intermediate composition associated with the Drammen and Finnemarka batholiths point to an efficient upper crustal density filtering. Considerable amounts of heat would be accumulated in this region if differentiated, intermediate melts could not escape to shallower levels. Successive magma injections would therefore easily result in partial melting of already solidified mafic to intermediate melts and cumulates, and it is suggested that the peraluminous granites formed mainly by water-undersaturated anatexis of mafic material.
Geochimica Et Cosmochimica Acta, 2009
New measurements of Os, He, Sr and Nd isotopes, along with major and trace elements, are presente... more New measurements of Os, He, Sr and Nd isotopes, along with major and trace elements, are presented for basalts from the three volcanic flank zones in Iceland and from Jan Mayen Island. The 187 Os/ 188 Os ratios in lavas with <30 ppt Os (n = 4) are elevated compared to ratios in coexisting olivine and appear to be contaminated at a shallow level. The 187 Os/ 188 Os ratios in the remaining lavas with >30 ppt Os (n = 17) range between 0.12117 and 0.13324. These values are surprisingly low for oceanic island basalts and include some samples that are less than putative present-day primitive upper mantle (PUM with 187 Os/ 188 Os of 0.1296). These low 187 Os/ 188 Os preclude significant shallow-level contamination from oceanic crust. The 187 Os/ 188 Os ratios for Jan Mayen lavas are less than PUM, severely limiting the presence of any continental crust in their mantle source. A positive correlation between 143 Nd/ 144 Nd and 187 Os/ 188 Os ratios in Iceland and Jan Mayen lavas likely reflects the presence in their source of ancient subcontinental lithosphere that has undergone incompatible trace element enrichment that did not affect the Re-Os system. In addition, the Jan Mayen lava isotopic signature cannot be explained solely by the presence of subcontinental lithospheric mantle, and the influence of another geochemical component, such as a mantle plume appears required. Combined 87 Sr/ 86 Sr, 143 Nd/ 144 Nd, 3 He/ 4 He and 187 Os/ 188 Os data indicate a genetic relationship between Jan Mayen Island and the Iceland mantle plume. Material from the Iceland mantle plume likely migrates at depth until it reaches the tensional setting of the Jan Mayen Fracture Zone, where it undergoes low-degree partial melting. At a first-order, isotopic co-variations can be interpreted as broadly binary mixing curves between two primary end-members. One end-member, characterized in particular by its unradiogenic 187 Os/ 188 Os and 143 Nd/ 144 Nd, low 3 He/ 4 He and high 87 Sr/ 86 Sr, is represented by subcontinental lithospheric mantle stranded and disseminated in the upper mantle during the opening of the Atlantic Ocean. The second end-member corresponds to a hybrid mixture between the depleted-MORB mantle and the enriched Iceland mantle plume, itself resulting from mixing between recycled oceanic crust and depleted lower mantle. This hybrid accounts for the high 3 He/ 4 He ($28 Ra), high 143 Nd/ 144 Nd ($0.5132), high 187 Os/ 188 Os ($0.14) and low 87 Sr/ 86 Sr ($0.7026) composition observed in Iceland. Two different models may account for these observed mixing relationships between the end-members. In this first model, the Iceland mantle entrains pristine depleted material when rising in the upper 0016-7037/$ -see front matter Ó
Geochimica Et Cosmochimica Acta, 2007
Picrites from the neovolcanic zones in Iceland display a range in 187 Os/ 188 Os from 0.1297 to 0... more Picrites from the neovolcanic zones in Iceland display a range in 187 Os/ 188 Os from 0.1297 to 0.1381 (c Os = + 2.1 to +8.7) and uniform 186 Os/ 188 Os of 0.1198375 ± 32 (2r). The value for 186 Os/ 188 Os is within uncertainty of the present-day value for the primitive upper mantle of 0.1198398 ± 16. These Os isotope systematics are best explained by ancient recycled crust or melt enrichment in the mantle source region. If so, then the coupled enrichments displayed in 186 Os/ 188 Os and 187 Os/ 188 Os from lavas of other plume systems must result from an independent process, the most viable candidate at present remains core-mantle interaction. While some plumes with high 3 He/ 4 He, such as Hawaii, appear to have been subjected to detectable addition of Os (and possibly He) from the outer core, others such as Iceland do not.
Geochimica Et Cosmochimica Acta, 2010
Here we search for evidence of the existence of a sub-chondritic 142 Nd/ 144 Nd reservoir that ba... more Here we search for evidence of the existence of a sub-chondritic 142 Nd/ 144 Nd reservoir that balances the Nd isotope chemistry of the Earth relative to chondrites. If present, it may reside in the source region of deeply sourced mantle plume material. We suggest that lavas from Hawai'i with coupled elevations in 186 Os/ 188 Os and 187 Os/ 188
Geochimica Et Cosmochimica Acta, 2009
New measurements of Os, He, Sr and Nd isotopes, along with major and trace elements, are presente... more New measurements of Os, He, Sr and Nd isotopes, along with major and trace elements, are presented for basalts from the three volcanic flank zones in Iceland and from Jan Mayen Island. The 187Os/ 188Os ratios in lavas with <30 ppt Os ( n = 4) are elevated compared to ratios in coexisting olivine and appear to be contaminated at a shallow level. The 187Os/ 188Os ratios in the remaining lavas with >30 ppt Os ( n = 17) range between 0.12117 and 0.13324. These values are surprisingly low for oceanic island basalts and include some samples that are less than putative present-day primitive upper mantle (PUM with 187Os/ 188Os of 0.1296). These low 187Os/ 188Os preclude significant shallow-level contamination from oceanic crust. The 187Os/ 188Os ratios for Jan Mayen lavas are less than PUM, severely limiting the presence of any continental crust in their mantle source. A positive correlation between 143Nd/ 144Nd and 187Os/ 188Os ratios in Iceland and Jan Mayen lavas likely reflects the presence in their source of ancient subcontinental lithosphere that has undergone incompatible trace element enrichment that did not affect the Re-Os system. In addition, the Jan Mayen lava isotopic signature cannot be explained solely by the presence of subcontinental lithospheric mantle, and the influence of another geochemical component, such as a mantle plume appears required. Combined 87Sr/ 86Sr, 143Nd/ 144Nd, 3He/ 4He and 187Os/ 188Os data indicate a genetic relationship between Jan Mayen Island and the Iceland mantle plume. Material from the Iceland mantle plume likely migrates at depth until it reaches the tensional setting of the Jan Mayen Fracture Zone, where it undergoes low-degree partial melting. At a first-order, isotopic co-variations can be interpreted as broadly binary mixing curves between two primary end-members. One end-member, characterized in particular by its unradiogenic 187Os/ 188Os and 143Nd/ 144Nd, low 3He/ 4He and high 87Sr/ 86Sr, is represented by subcontinental lithospheric mantle stranded and disseminated in the upper mantle during the opening of the Atlantic Ocean. The second end-member corresponds to a hybrid mixture between the depleted-MORB mantle and the enriched Iceland mantle plume, itself resulting from mixing between recycled oceanic crust and depleted lower mantle. This hybrid accounts for the high 3He/ 4He (˜28 Ra), high 143Nd/ 144Nd (˜0.5132), high 187Os/ 188Os (˜0.14) and low 87Sr/ 86Sr (˜0.7026) composition observed in Iceland. Two different models may account for these observed mixing relationships between the end-members. In this first model, the Iceland mantle entrains pristine depleted material when rising in the upper mantle and allows refractory sub-lithospheric fragments to melt because of excess heat derived from the deep plume material. A second model that may better account for the Pb isotopic variations observed, uses the same components but where the depleted-MORB mantle is already polluted by subcontinental lithospheric mantle material before mixing with the Iceland mantle plume. Both cases likely occur. Though only three principal components are required to explain the isotopic variations of the Iceland-Jan Mayen system, the different possible mixing relationships may be accounted for by potentially a greater number of end-members.
Earth and Planetary Science Letters, 1999
Primitive Hawaiian picrites have 187 Os/ 188 Os as high as V0.145 and are more radiogenic than th... more Primitive Hawaiian picrites have 187 Os/ 188 Os as high as V0.145 and are more radiogenic than the depleted upper mantle, reflecting a time-integrated suprachondritic Re/Os ratio. The high Re/Os may be explained either by an ancient recycled crustal component or an evolved outer core component in the Hawaiian plume. New high precision 186 Os/ 188 Os measurements for these picrites, combined with previous analyses, show that the Hawaiian plume source has 186 Os/ 188 Os that range from chondritic mantle values of V0.119834 to more radiogenic values as high as V0.119848. The higher 186 Os/ 188 Os reflects long-term suprachondritic Pt/Os and is coupled with higher 187 Os/ 188 Os in all but the Koolau picrites. The latter have near-chondritic 186 Os/ 188 Os but with radiogenic 187 Os/ 188 Os. The Pt/Re of crustal materials that may make up ancient recycled slabs ranges from V0.1 to 33. Recycled slab material with such Pt/Re ratios evolved for 1^3 Ga and added to the plume source may explain the Koolau Os isotopic compositions. Pt/Re ratios of 88^100, however, are required for the ancient recycled crust to generate the coupled enrichments of 186 Os/ 188 Os and 187 Os/ 188 Os in picrites from Loihi and Hualalai. These high Pt/Re ratios do not occur in any known crustal materials, but they are consistent with the observed partition coefficients for Os s Re s Pt, during metal crystallization from an initially chondritic molten core (Pt/Re V21^24). Such partitioning may have produced an evolved outer core with suprachondritic Pt/Os, Re/Os and Pt/Re, resulting in the production of suprachondritic 186 Os/ 188 Os and 187 Os/ 188 Os over time. Small amounts of outer core metal (9 1.2%) mixed into the Hawaiian plume source can explain the coupled 186 Os/ 188 Os and 187 Os/ 188 Os enrichment in some of the Hawaiian picrites. In addition, the most radiogenic 186 Os/ 188 Os in the Hawaiian picrites is correlated with higher 3 He/ 4 He, consistent with an undegassed, and likely, lower mantle source. These data provide compelling geochemical evidence that the Hawaiian plume was generated at the core-mantle boundary. ß 1999 Elsevier Science B.V. All rights reserved.
Earth and Planetary Science Letters, 2000
Rhenium^osmium isotopic systematics of primitive calc-alkaline lavas from the Lassen region appea... more Rhenium^osmium isotopic systematics of primitive calc-alkaline lavas from the Lassen region appear to be controlled by mantle wedge processes. Lavas with a large proportion of slab component have relatively low Re and Os abundances, and have radiogenic Os and mid ocean ridge basalt-like Sr and Pb isotopic compositions. Lavas with a small proportion of slab component have higher Re and Os elemental abundances and display mantle-like Os, Sr, Nd, and Pb isotopic compositions. Assimilation with fractional crystallization can only generate the Re^Os systematics of the Lassen lavas from a common parent if the distribution coefficient for Re in sulfide is V40^1100 times higher than most published estimates and if most incompatible element abundances decrease during differentiation. High Re/Os ratios in mid ocean ridge basalts makes subducted oceanic crust a potential source of radiogenic Os in volcanic arcs. The slab beneath the southernmost Cascades is estimated to have 187 Os/ 188 Os ratios as high as 1.4. Mixing between a slab component and mantle wedge peridotite can generate the Os isotopic systematics of the Lassen lavas provided the slab component has a Sr/Os ratio of V7.5U10 5 and Os abundances that are 100^600 times higher than mid ocean ridge basalts. For this model to be correct, Os must be readily mobilized and concentrated in the slab component, perhaps as a result of high water and HCl fugacities in this subduction environment. Another possible mechanism to account for the correlation between the magnitude of the subduction geochemical signature and Os isotopic composition involves increasing the stability of an Os-bearing phase in mantle wedge peridotites as a result of fluxing with the slab component. Melting of such a source could yield low Os magmas that are more susceptible to crustal contamination, and hence have more radiogenic Os isotopic compositions, than magmas derived from sources with a smaller contribution from the slab. Thus, the addition of the slab component to the mantle wedge appears to result in either the direct or indirect addition of radiogenic Os to arc magmas. ß
Isotopic constraints on time scales and mechanisms of slab material transport in the mantle wedge: evidence from the Simcoe mantle xenoliths, Washington, USA
Chemical Geology, 1999
... 1996). These xenoliths have largely granular to porphyroclastic textures, with little evidenc... more ... 1996). These xenoliths have largely granular to porphyroclastic textures, with little evidence of pervasive brittle deformation. ... 1994). Strontium, Sm, Nd, U, Th, and Pb abundances were obtained by isotope dilution. 87 ...
Geochimica Et Cosmochimica Acta, 2005
The Tagish Lake meteorite is a primitive C2 chondrite that has undergone aqueous alteration short... more The Tagish Lake meteorite is a primitive C2 chondrite that has undergone aqueous alteration shortly after formation of its parent body. Previous work indicates that if this type of material was part of a late veneer during terrestrial planetary accretion, it could provide a link between atmophile elements such as H, C, N and noble gases, and highly siderophile element replenishment in the bulk silicate portions of terrestrial planets following core formation. The systematic Re-Os isotope and highly siderophile element measurements performed here on five separate fractions indicate that while Tagish Lake has amongst the highest Ru/Ir (1.63 Ϯ 0.08), Pd/Ir (1.19 Ϯ 0.06) and 187 Os/ 188 Os (0.12564 -0.12802) of all carbonaceous chondrites, these characteristics still fall short of those necessary to explain the observed siderophile element systematics of the primitive upper mantles of Earth and Mars. Hence, a direct link between atmophile and highly siderophile elements remains elusive, and other sources for replenishment are required, unless an as yet poorly constrained process fractionated Re/Os, Ru/Ir, and Pd/Ir following late accretion on both the Earth and Mars mantles.
Earth and Planetary Science Letters, 2004
Precise Pt -Re -Os isotopic and highly siderophile element (HSE) data are reported for 10 drill c... more Precise Pt -Re -Os isotopic and highly siderophile element (HSE) data are reported for 10 drill core samples collected across a 2.7 m thick, differentiated komatiite lava flow at Pyke Hill in Ontario, Canada. The Pt -Re -Os isotopic data for five cumulate samples yield Re -Os and Pt -Os isochrons with ages of 2725 F 33 and 2956 F 760 Ma and initial 187 Os/ 188 Os = 0.10902 F 16 and 186 Os/ 188 Os = 0.1198318 F 18, respectively. These ages are consistent with the time of lava emplacement determined by other geochronometers. The initial 187 Os/ 188 Os isotopic composition of the Pyke Hill source is similar to the 187 Os/ 188 Os isotopic compositions of enstatite and ordinary chondrites and to that of PUM at 2725 Ma. Using the Pt -Os isotopic data for the Pyke Hill source, the present-day 186 Os/ 188 Os isotopic composition of PUM was calculated to be 0.1198387 F 18. The regression of global 187 Os/ 188 Os isotopic data for terrestrial materials shown to represent Os isotopic composition of the contemporary mantle indicates that the LILE-depleted mantle evolved from the solar system starting composition and, over Earth's history, was on average f 8% depleted in Re vs. Os relative to PUM. From the HSE composition of the emplaced Pyke Hill komatiite lava, the mantle source was inferred to contain Re (0.30 ppb), Os (3.9 ppb), Ir (3.6 ppb), Ru (5.4 ppb), Pt (6.4 ppb), and Pd (6.3 ppb) in relative proportions similar to those in average enstatite chondrites, except for Pd, which was enriched over Pt by f 30% in the Pyke Hill source compared to enstatite chondrites. From these data, the 190 Pt/ 188 Os of PUM was calculated to be 0.00157. If the HSE abundances in the terrestrial mantle were inherited from chondritic material of a late veneer, the latter should have had the HSE composition of ordinary or enstatite chondrites. Alternatively, if the reenrichment of the mantle with HSE was the result of an open-system transport of material across the core -mantle boundary, it was likely a global event that should have occurred shortly after core formation. D
Astrophysical Journal, 2007
Correlated isotopic anomalies in osmium (186Os, 188Os, 190Os, measured with respect to 189Os) ext... more Correlated isotopic anomalies in osmium (186Os, 188Os, 190Os, measured with respect to 189Os) extracted from primitive carbonaceous chondrites tightly constrain the σn(190Os)/σn(188Os) ratio to be 0.859+/-0.042 (+/-5%). A recent measurement of the Maxwellian-averaged neutron-capture cross sections (MACSs) for 186,187,188Os lowered the σn(188Os) by 27% but did not measure σn(190Os). From the σn(190Os)/σn(188Os) ratio, we infer σn(190Os)=249+/-18 mbarns for internal consistency with the new MACSs for the other Os isotopes. This approach is applicable to other isotopic anomalies in r-process/s-process ratios derived from meteorites for nuclei that do not have branching points between them. Branching at 185W and 186Re makes the 186Os/188Os ratio a neutron dosimeter for the s-process that, with the new cross sections, yields an average neutron density, nn~3×108 cm-3. This low neutron density is consistent with previous results from Sr, Zr, Mo, and Ba isotopes that indicated a minor contribution from the 22Ne(α, n)25Mg neutron source relative to the 13C(α, n)16O neutron source.
190Pt– 186Os and 187Re– 187Os systematics of abyssal peridotites
Earth and Planetary Science Letters, 2000
Abstract Abyssal peridotites are normally thought to be residues of melting of the mid-ocean ridg... more Abstract Abyssal peridotites are normally thought to be residues of melting of the mid-ocean ridge basalt (MORB) source and are presumably a record of processes affecting the upper mantle. Samples from a single section of abyssal peridotite from the Kane Transform area in the Atlantic Ocean were examined for 190 Pt-186 Os and 187 Re-187 Os systematics. They have uniform 186 Os/188 Os ratios with a mean of 0.1198353+/-7, identical to the mean of 0.1198340+/-12 for Os-Ir alloys and chromitites believed to be representative of the upper ...
Earth and Planetary Science Letters, 2005
Some hotspot volcanism is likely the surface manifestation of hot upwelling mantle plumes that ri... more Some hotspot volcanism is likely the surface manifestation of hot upwelling mantle plumes that rise from the core-mantle boundary (CMB) or from compositional boundaries within the mantle. Plumes that rise from the CMB may generate magmas that carry compositional information about the diversity of materials present in the lowermost mantle. Some plume-derived materials have coupled enrichments in 186 Os/ 188 Os and 187 Os/ 188 Os relative to upper mantle materials. The coupled enrichments are consistent with Os isotopic compositions predicted for the liquid outer core that might result from elevated Pt/Os and Re/Os generated via progressive crystallization of a solid inner core over Earth history. If these enrichments are a reflection of coremantle interaction, then the mechanisms of material transport between the core and the mantle, rates of whole mantle convection at present and over time, and cooling histories for the core and the mantle can potentially be constrained. However, fractionation may also occur between Pt/Re/Os via mantle and crustal processes and in particular, Pt and Pt/Re may be elevated in some portions of ancient recycled slabs thought to be present in certain plume sources. This possibility brings into question whether the coupled Os isotope enrichments are a product of core-mantle exchange. Hence, a lively debate over the causes of Os isotopic heterogeneities in plume-derived materials has ensued. Resolution of this debate has been hampered by limited constraints on the behavior of these elements in the Earth's core, mantle and crust. A variety of analytical and experimental tasks will have to be pursued to test different aspects of both the core-mantle interaction and crustal recycling models, and to further constrain the behaviors of these elements. These future endeavors may help to unravel the compositional complexities of plumederived materials and their mantle sources, and ultimately provide important insights to processes operating at the CMB. D