Peter Michael | The University of Tulsa (original) (raw)
Conference Presentations by Peter Michael
The Northern Tofua Arc and adjacent NE Lau Basin host a wide variety of volcano types reflecting ... more The Northern Tofua Arc and adjacent NE Lau Basin host a wide variety of volcano types reflecting dynamic and diverse tectonic settings (e.g., rapidly opening basin, migrating microplate boundaries, within-plate discrete rear arc volcanoes, and weak on-arc magmatism, all in close spatial proximity). Frequent contemporaneous eruptions of high-MgO magmas and their differentiates from closely spaced volcanoes provide an uncommonly detailed view of the diversity of parent magma types formed in the broader suprasubduction zone environment. They show how mantle lithological variations couple with local tectonic setting to control magmatism, and influence magma accumulation, storage, and subsequent eruption. The results of 6 recent research expeditions to this relatively small region show how these attributes collectively result in systematic relationships between eruption style, inferred duration, size, intensity, location, and compositional variability. In particular, we can constrain magmatic conditions and timescales from single eruption deposits that we sampled with high-spatial-resolution in the context of detailed deposit mapping, high resolution 210 Po-210 Pb eruption chronologies, and U-Th-Ra disequilibrium. At the same time, variations between eruption deposits reveal how quickly parent melt compositions drawn from the mantle wedge can change in space and time. We will discuss and interpret diverse phenomena from the region, including high rates of temporal variability seen in magma compositions within single eruptions, ultra-high variability in source compositions sampled at sub-km scale on neighboring small Mata volcanoes, highly productive and frequent volcanism on the NE Lau Spreading center, and the petrogenetc links between boninite and high MgO basalt magmas with extensive contemporaneous dacite volcanism in the region.
Papers by Peter Michael
Nature Communications, Aug 6, 2021
Goldschmidt2021 abstracts, 2021
Journal of Geophysical Research, Aug 10, 1998
Chlorine and major elements in >400 mid-ocean ridge basalt (MORB) glasses from 20 suites are used... more Chlorine and major elements in >400 mid-ocean ridge basalt (MORB) glasses from 20 suites are used to examine how spreading rate, magma flux, tectonics, and hydrothermal activity influences assimilation and crystallization beneath MOR. Crystallization depths were determined for fractionated glasses using published models that describe liquids saturated with olivine+ clinopyroxene+ plagioclase. Calculated depths are minima for the onset of crystallization and maxima for the completion of crystallization for each liquid. Glasses from fast spreading ridges and from medium and slow spreading ridges with low Na8.0 define low-pressure liquid lines of descent (LLDs). Higher crystallization pressures and greater variability are obtained from slow and medium spreading ridges with high Na8.0. Crystallization pressures do not vary regularly along individual segments. The correlation between average crystallization pressure and Na8.0 suggests that magma supply (and perhaps mantle temperature) plays an important role in determining magma ascent and crystallization depths. C1/K in glasses is an indicator of assimilation of hydrothermally influenced material. Suites of MORB with high crystallization pressures have low C1/K: from below detection limits (=0.01) in normal MORB (NMORB) to about 0.05-0.08 in enriched MORB (EMORB). We propose that this trend defines the mantle limit of C1/K and that higher values are related to assimilation. C1/K is highest (up to 1.1) and is negatively correlated with MgO along the superfast spreading southern East Pacific Rise (EPR) and the propagating, low-Na8.0 Galapagos Spreading Center (GSC) at 85øW. C1/K is also above mantle values, but is not well correlated with MgO, in MORB from fast and medium spreading ridges and from slow spreading ridges that have low Na8.0 and low crystallization pressures, e.g., Reykjanes Ridge. C1/K is not correlated with crystallization pressure for individual samples within any suite. We propose that the spreading rate and the extent of melting act together to determine the total magma flux to a ridge, which influences crustal temperatures and determines how magmas ascend. At the highest magma fluxes, C1/K is correlated with MgO, consistent with continuous assimilation of material that has a uniform C1 content: crystallization and assimilation are steady state processes that occur in crustal magma bodies that are larger than the scale of crustal heterogeneity in C1. The lowest magma fluxes occur on slow spreading ridges that have formed by small extents of melting. In this cooler environment, magma crystallizes at the base of the strong lithosphere, below the level of alteration, and then ascends rapidly with little crystallization at shallow levels, so C1 contamination is avoided. On slow and medium spreading ridges with high extents of melting, magma flux is intermediate and forms small or transient crustal magma bodies: whether a magma batch becomes enriched in C1 depends upon the particular crust that it encounters.
Earth and Planetary Science Letters, Apr 1, 1995
... Geochim. Cosmochim. Acta, 57 (1993), pp. 2907–2912. [7] TC Hoering and PL Parker, The geochem... more ... Geochim. Cosmochim. Acta, 57 (1993), pp. 2907–2912. [7] TC Hoering and PL Parker, The geochemistry of the stable isotopes of chlorine. Geochim. Cosmochim. Acta, 23 (1961), pp. 186–199. [8] CJ Eastoe and JM Guilbert, Stable chlorine isotopes in hydrothermal processes. ...
Journal of Petrology, Aug 23, 2018
Journal of Petrology, May 1, 2016
Major 6 trace element and Sr-Nd-Pb-Hf-He isotope data are presented for more than 300 geochemical... more Major 6 trace element and Sr-Nd-Pb-Hf-He isotope data are presented for more than 300 geochemically diverse basalt samples collected by submersible from the Inflated Central Endeavour Segment of the Juan de Fuca Ridge. Seven chemically distinct basalt types are present, from depleted (D-) to enriched mid-ocean ridge basalt (E-MORB). By combining the geochemical data with high-resolution bathymetry and age determinations, the detailed spatial and temporal scale of on-axis mantle-derived basalt heterogeneity is determined. The basalts define binary mixing arrays in all isotope plots that are usual in their correlations, but unusual in the limited range of Sr-Nd-Hf isotope compositions for D-to E-MORB, and greater range in Pb isotopes. The basalts also define two different styles of enrichment of moderately incompatible elements. Geochemical enrichment began when the currently inflated axial ridge formed <10 5 years ago. One enrichment style (the Inflated Ridge Trend) characterizes basalts erupted across the 5kmwideridgefrom>10000to5 km wide ridge from >10 000 to 5kmwideridgefrom>10000to4000 years ago, whereas the other enrichment style (the Graben Trend) characterizes most basalt types erupted within the axial graben after it formed 2300yearsago.WeattributetheInflatedRidgeTrendtoarelativelyhighproportionofpyroxenite(ormeltderivedtherefrom)toenrichedperidotiteinthemantleduringaphaseofridgeinflationthatlastedatleast6000years.TheGrabenTrendreflectsthereducedeffectofpyroxeniteaftertheaxialgrabenformed.Becauseatleast14differentsamplingsofmantlecomponentsoccurredwithin<1kmofridgelengthandwidthduringatimewhen<1kmofupwellingoccurred,weinferthatthescaleofmantleheterogeneityfarfromaplumeis<1km.TheenrichedmantlecomponentatEndeavourisyoungwith206Pb/204Pb2300 years ago. We attribute the Inflated Ridge Trend to a relatively high proportion of pyroxenite (or melt derived therefrom) to enriched peridotite in the mantle during a phase of ridge inflation that lasted at least 6000 years. The Graben Trend reflects the reduced effect of pyroxenite after the axial graben formed. Because at least 14 different samplings of mantle components occurred within <1 km of ridge length and width during a time when <1 km of upwelling occurred, we infer that the scale of mantle heterogeneity far from a plume is < 1 km. The enriched mantle component at Endeavour is young with 206 Pb/ 204 Pb 2300yearsago.WeattributetheInflatedRidgeTrendtoarelativelyhighproportionofpyroxenite(ormeltderivedtherefrom)toenrichedperidotiteinthemantleduringaphaseofridgeinflationthatlastedatleast6000years.TheGrabenTrendreflectsthereducedeffectofpyroxeniteaftertheaxialgrabenformed.Becauseatleast14differentsamplingsofmantlecomponentsoccurredwithin<1kmofridgelengthandwidthduringatimewhen<1kmofupwellingoccurred,weinferthatthescaleofmantleheterogeneityfarfromaplumeis<1km.TheenrichedmantlecomponentatEndeavourisyoungwith206Pb/204Pb19Á0; Hf and He isotope ratios trend toward HIMU characteristics. These traits are regionally widespread and are shared with the next two ridge segments to the north (West Valley and Explorer).
Journal of Geophysical Research, Jun 30, 2009
We report major, trace, and volatile element data on basaltic glasses from the northernmost segme... more We report major, trace, and volatile element data on basaltic glasses from the northernmost segment of the Eastern Lau Spreading Center (ELSC1) in the Lau back-arc basin to further test and constrain models of back-arc volcanism. The zero-age samples come from 47 precisely collected stations from an 85 km length spreading center. The chemical data covary similarly to other back-arc systems but with tighter correlations and welldeveloped spatial systematics. We confirm a correlation between volatile content and apparent extent of melting of the mantle source but also show that the data cannot be reproduced by the model of isobaric addition of water that has been broadly applied to back-arc basins. The new data also confirm that there is no relationship between mantle temperature and the wet melting productivity. Two distinct magmatic provinces can be identified along the ELSC1 axis, a southern province influenced by a ''wet component'' with strong affinities to arc volcanism and a northern province influenced by a ''damp component'' intermediate between enriched mid-ocean ridge basalts (E-MORB) and arc basalts. High-field strength elements and rare earth elements are all mobilized to some extent by the wet component, and the detailed composition of this component is determined. It differs in significant ways from the Mariana component reported by E. Stolper and S. Newman (1994), particularly by having lower abundances of most elements relative to H 2 O. The differences can be explained if the slab temperature is higher for the Mariana and the source from which the fluid is derived is more enriched. The ELSC1 damp component is best explained by mixing between the wet component and an E-MORB-like component. We propose that mixing between water-rich fluids and low-degree silicate melts occurs at depth in the subduction zone to generate the chemical diversity of the ELSC1 subduction components. These modified sources then rise independently to the surface and melt, and these melts mix with melts of the background mantle from the ridge melting regime to generate the linear data arrays characteristic of back-arc basalts. The major and trace element framework for ELSC1, combined with different slab temperatures and compositions for difference convergent margins, may be able to be applied to other back-arc basins around the globe.
Geochemistry Geophysics Geosystems, Oct 1, 2003
We report measurements of U‐series disequilibria, Sr, Nd, Hf, and Pb isotopic compositions and ma... more We report measurements of U‐series disequilibria, Sr, Nd, Hf, and Pb isotopic compositions and major and trace element abundances in a suite of well‐located, off‐axis MORBs that span the East Pacific Rise (EPR) ridge crest from 9°48′–52′N and across it for ∼4 km on either side. The geological context of the samples are well constrained as they were collected by submersible in an area that has been extensively imaged by remote sensing techniques. Sr, Nd, Hf and 208Pb/206Pb isotopic compositions of the off‐axis N‐MORB are identical to the axial lavas from this same region, suggesting that their sources are similar and that melting processes are the dominant influence in establishing the U‐Th‐Ra disequilibria and trace element fractionations. A majority of off‐axis samples have U‐Th and Th‐Ra disequilibria that are larger, and model ages that are younger, than would be predicted from their off‐axis distance and the time‐integrated spreading rate. There are, however, a few off‐axis samples with U‐Th ages that are consistent with their spreading rate ages. It is likely that these samples erupted within or close to the axial summit trough (AST) and aged at a rate proportional to the spreading rate. The anomalously young ages determined for most of the off‐axis lavas suggest that volcanic construction along this region is occurring over a zone that is wider (at least 4 km) than the AST (10s to 100s of meters). The combined observational, chemical and isotopic data support a model for the 9°0′N area that includes a significant component of crustal accumulation resulting from lavas that breach the AST and flow down the flanks of the EPR ridge crest. However, these data also require a minor component of off‐axis eruptions that occur on distinct pillow mounds and ridges. This suggests that MOR construction involves several volcanic and tectonic processes acting in concert to form a complex patchwork of lava ages and compositions along, and across, this fast spreading ridge crest.
Earth and Planetary Science Letters, Feb 1, 1994
A segment of the slow-spreading Mid-Atlantic Ridge (MAR) at 33°S changes dramatically as its cent... more A segment of the slow-spreading Mid-Atlantic Ridge (MAR) at 33°S changes dramatically as its center is approached. Towards the center of the segment, the axis shoals from 3900 to 2400 m and a deep median valley nearly disappears. There is a prominent bullseye gravity low ...
Subduction of oceanic crust has long been considered a major cause of mantle heterogeneity. By fa... more Subduction of oceanic crust has long been considered a major cause of mantle heterogeneity. By far the largest volume of recycled plates, however, is the lithospheric mantle, which is often inferred but rarely observed. Here we report evidence that the Gakkel ridge sampled a section of recycled refractory ocean lithosphere: (1) a unique group of high-Ti basalts occurs adjacent to a long sparsely magmatic zone (SMZ) that is consistent with melting of oxide gabbro hosted in ancient ocean lithosphere; (2) the SMZ itself which is 300km of ancient mantle emplaced at the surface; (3) sparse enriched basalts from the SMZ with no residual garnet signature suggesting shallow melting of ancient refractory but metasomatized lithosphere with low mantle potential temperature; (4) a gradient in composition approaching the SMZ suggesting lower extents of melting of more depleted sources. The rarity of these occurrences elsewhere shows that ordinarily recycled ocean crust and refractory lithosphere...
The AMORE expedition recovered igneous rocks from over two hundred stations from the Gakkel Ridge... more The AMORE expedition recovered igneous rocks from over two hundred stations from the Gakkel Ridge, permitting the first regional scale investigation of its composition. These samples permit for the first time the evaluation of petrological systematics from an ultra-slow spreading ridge (0.7029 west of the SMZ and &lt; 0.7028 to the east, even for the same La/Sm). Thus the amagmatic
ABSTRACT The distance of the Eastern Lau Spreading Center (ELSC) from the volcanic front of the T... more ABSTRACT The distance of the Eastern Lau Spreading Center (ELSC) from the volcanic front of the Tonga Arc varies progressively from 40 km at the Valu Fa Ridge in the south to 100km in the north. Previous studies showed a marked contrast between "arc-like" lavas in the Valu Fa region and "MORB-like" lavas from the north of the Eastern Lau spreading Center. We report more than 60 new isotopic analyses from samples with an average spacing of 5 km that complement our major (over 200 analyses) and trace element (130 analyses) dataset. The isotopic data provide new constraints on the "MORB-like" mantle, the composition of the subduction-related component and its variation in the basin, and the relationship between the geochemical variations and the distance behind the arc. Pb isotope ratios decrease overall toward the north away from Valu Fa, providing an along-axis gradient in 206Pb/204Pb from 18.13 to 18.66. Values for the southern Valu Fa lavas intersect the Pb isotope array of the Tonga arc. The northern end of the array is offset to high 207Pb/204Pb and ^{208}Pb/204Pb compared to the N. Atlantic-Pacific data. These results are consistent with mixing between a MORB-like end-member having a DUPAL signature and a Valu Fa-like end-member with strong arc signature. The presence of a Dupal component explains the radiogenic 87Sr/86Sr (0.70323±8) that is observed in this region. In detail, both the Sr and Pb isotopic data requires that the DUPAL signature of the "MORB-like" mantle decreases in the south of the basin. Indeed, in the Pb-Sr space, the southernmost segment defines a trend, distinct from the other segments, toward lower 87Sr/86Sr and 206Pb/204Pb. This trend becomes further extended by some samples of the northernmost segments that have especially unradiogenic 87Sr/86Sr. Overall, the Pb isotope ratios correlate well with trace element ratios such as Nb/U or Ba/Nb in a manner consistent with an important Pb contribution from the subduction-related component(s). Coupled with mobile incompatible element ratios such as Ba/La, the Pb isotope ratios confirm that the subduction-related components vary in composition from one segment to another. Our major and trace element data showed that the transition from basaltic to andesitic magmatism along the ELSC occurs almost entirely along a single 15 km long segment located 70 km behind the Tonga arc. Over 30% of the Pb isotopic variation occurs within this area, confirming the importance of this short segment as a boundary between two different types of mantle process. South of the basalt-to-andesite transition, Pb isotopes define a regular variation with latitude, sometimes showing smooth oscillations within a single segment. The two northernmost segments, located north of the transition, have a less regular latitudinal Pb isotopic variation than other segments, but the variability still correlates with higher mobile/immobile element ratios. These results show that, in the arc vicinity, a large part of the mantle regime below the back-arc is affected by subduction-related fluids or melts, while far from the arc, only local spikes of subduction-related signature occur.
Earth and Planetary Science Letters, 2021
Basalts erupted within intra-transform spreading centers can be valuable probes of geochemical co... more Basalts erupted within intra-transform spreading centers can be valuable probes of geochemical components in Earth's upper mantle, and provide constraints on the proportions of mantle carbon that are juvenile (primordial) vs. tectonically recycled. Here we present new results for submarine basalt glasses, erupted within the Garrett Transform Fault (GTF) and to its north and south along the East Pacific Rise (EPR). Analyses of 3 He/ 4 He, and He and CO 2 concentrations in vesicles and glass were performed, through a series of crushing and melting experiments plus FTIR spectroscopy. Trace elements were analyzed by laser ablation ICP-MS. The GTF basalts provide further tests for the origin of volatile-undersaturated basalts using CO 2 /Ba and CO 2 /Nb systematics. CO 2 is highly correlated with Ba and Nb in basalts erupted in the transform domain (n = 13, including 5 undersaturated basalts) and does not show the variability expected from mixing between undegassed and variably degassed melts. Rather, the melts appear to originate from a heterogeneous mantle source that was variably depleted through partial melting, and limited mixing of melts is involved in their generation. The CO 2 /Ba and CO 2 /Nb weight ratios of 106 ± 8 and 308 ± 27, respectively, are similar to values determined previously for a global suite of undersaturated mid-ocean ridge basalts (MORBs). The ridge and transform domains have distinct 3 He/ 4 He ratios. Along the nearby EPR, 3 He/ 4 He = 8.5-9.1 R A , while within the Garrett Transform Fault 3 He/ 4 He = 9.2-10.1 R A. These two basalt populations are also distinct in their Pb-Sr-Nd isotope compositions based on earlier regional studies. The distinct populations result from partial melting of two different mantle source compositions. Melting of depleted mantle containing a small amount (∼1 to 5%) of enriched, ancient heterogeneities occurs beneath the EPR. Melting of ultradepleted mantle (in which the heterogeneities have been removed by earlier melting beneath the EPR) occurs beneath the GTF. This explains the distinction between intra-transform and spreading ridge domains for 3 He/ 4 He, if the heterogeneities were enriched in U, Th and He and had low 3 He/ 4 He as would be found in tectonically recycled material. The enriched mantle component sampled by the EPR basalts has molar CO 2 / 3 He = 2 × 10 9 , and it dominates the CO 2 / 3 He ratio generally ascribed to the upper mantle source for mid-ocean ridge basalts. In contrast, the ultradepleted MORB mantle component sampled by the GTF basalts has CO 2 / 3 He = 3 × 10 8 or less. This indicates that most of the carbon in Earth's upper mantle originates from tectonic recycling.
Goldschmidt2021 abstracts, 2021
The Northern Tofua Arc and adjacent NE Lau Basin host a wide variety of volcano types reflecting ... more The Northern Tofua Arc and adjacent NE Lau Basin host a wide variety of volcano types reflecting dynamic and diverse tectonic settings (e.g., rapidly opening basin, migrating microplate boundaries, within-plate discrete rear arc volcanoes, and weak on-arc magmatism, all in close spatial proximity). Frequent contemporaneous eruptions of high-MgO magmas and their differentiates from closely spaced volcanoes provide an uncommonly detailed view of the diversity of parent magma types formed in the broader suprasubduction zone environment. They show how mantle lithological variations couple with local tectonic setting to control magmatism, and influence magma accumulation, storage, and subsequent eruption. The results of 6 recent research expeditions to this relatively small region show how these attributes collectively result in systematic relationships between eruption style, inferred duration, size, intensity, location, and compositional variability. In particular, we can constrain magmatic conditions and timescales from single eruption deposits that we sampled with high-spatial-resolution in the context of detailed deposit mapping, high resolution 210 Po-210 Pb eruption chronologies, and U-Th-Ra disequilibrium. At the same time, variations between eruption deposits reveal how quickly parent melt compositions drawn from the mantle wedge can change in space and time. We will discuss and interpret diverse phenomena from the region, including high rates of temporal variability seen in magma compositions within single eruptions, ultra-high variability in source compositions sampled at sub-km scale on neighboring small Mata volcanoes, highly productive and frequent volcanism on the NE Lau Spreading center, and the petrogenetc links between boninite and high MgO basalt magmas with extensive contemporaneous dacite volcanism in the region.
Nature Communications, Aug 6, 2021
Goldschmidt2021 abstracts, 2021
Journal of Geophysical Research, Aug 10, 1998
Chlorine and major elements in >400 mid-ocean ridge basalt (MORB) glasses from 20 suites are used... more Chlorine and major elements in >400 mid-ocean ridge basalt (MORB) glasses from 20 suites are used to examine how spreading rate, magma flux, tectonics, and hydrothermal activity influences assimilation and crystallization beneath MOR. Crystallization depths were determined for fractionated glasses using published models that describe liquids saturated with olivine+ clinopyroxene+ plagioclase. Calculated depths are minima for the onset of crystallization and maxima for the completion of crystallization for each liquid. Glasses from fast spreading ridges and from medium and slow spreading ridges with low Na8.0 define low-pressure liquid lines of descent (LLDs). Higher crystallization pressures and greater variability are obtained from slow and medium spreading ridges with high Na8.0. Crystallization pressures do not vary regularly along individual segments. The correlation between average crystallization pressure and Na8.0 suggests that magma supply (and perhaps mantle temperature) plays an important role in determining magma ascent and crystallization depths. C1/K in glasses is an indicator of assimilation of hydrothermally influenced material. Suites of MORB with high crystallization pressures have low C1/K: from below detection limits (=0.01) in normal MORB (NMORB) to about 0.05-0.08 in enriched MORB (EMORB). We propose that this trend defines the mantle limit of C1/K and that higher values are related to assimilation. C1/K is highest (up to 1.1) and is negatively correlated with MgO along the superfast spreading southern East Pacific Rise (EPR) and the propagating, low-Na8.0 Galapagos Spreading Center (GSC) at 85øW. C1/K is also above mantle values, but is not well correlated with MgO, in MORB from fast and medium spreading ridges and from slow spreading ridges that have low Na8.0 and low crystallization pressures, e.g., Reykjanes Ridge. C1/K is not correlated with crystallization pressure for individual samples within any suite. We propose that the spreading rate and the extent of melting act together to determine the total magma flux to a ridge, which influences crustal temperatures and determines how magmas ascend. At the highest magma fluxes, C1/K is correlated with MgO, consistent with continuous assimilation of material that has a uniform C1 content: crystallization and assimilation are steady state processes that occur in crustal magma bodies that are larger than the scale of crustal heterogeneity in C1. The lowest magma fluxes occur on slow spreading ridges that have formed by small extents of melting. In this cooler environment, magma crystallizes at the base of the strong lithosphere, below the level of alteration, and then ascends rapidly with little crystallization at shallow levels, so C1 contamination is avoided. On slow and medium spreading ridges with high extents of melting, magma flux is intermediate and forms small or transient crustal magma bodies: whether a magma batch becomes enriched in C1 depends upon the particular crust that it encounters.
Earth and Planetary Science Letters, Apr 1, 1995
... Geochim. Cosmochim. Acta, 57 (1993), pp. 2907–2912. [7] TC Hoering and PL Parker, The geochem... more ... Geochim. Cosmochim. Acta, 57 (1993), pp. 2907–2912. [7] TC Hoering and PL Parker, The geochemistry of the stable isotopes of chlorine. Geochim. Cosmochim. Acta, 23 (1961), pp. 186–199. [8] CJ Eastoe and JM Guilbert, Stable chlorine isotopes in hydrothermal processes. ...
Journal of Petrology, Aug 23, 2018
Journal of Petrology, May 1, 2016
Major 6 trace element and Sr-Nd-Pb-Hf-He isotope data are presented for more than 300 geochemical... more Major 6 trace element and Sr-Nd-Pb-Hf-He isotope data are presented for more than 300 geochemically diverse basalt samples collected by submersible from the Inflated Central Endeavour Segment of the Juan de Fuca Ridge. Seven chemically distinct basalt types are present, from depleted (D-) to enriched mid-ocean ridge basalt (E-MORB). By combining the geochemical data with high-resolution bathymetry and age determinations, the detailed spatial and temporal scale of on-axis mantle-derived basalt heterogeneity is determined. The basalts define binary mixing arrays in all isotope plots that are usual in their correlations, but unusual in the limited range of Sr-Nd-Hf isotope compositions for D-to E-MORB, and greater range in Pb isotopes. The basalts also define two different styles of enrichment of moderately incompatible elements. Geochemical enrichment began when the currently inflated axial ridge formed <10 5 years ago. One enrichment style (the Inflated Ridge Trend) characterizes basalts erupted across the 5kmwideridgefrom>10000to5 km wide ridge from >10 000 to 5kmwideridgefrom>10000to4000 years ago, whereas the other enrichment style (the Graben Trend) characterizes most basalt types erupted within the axial graben after it formed 2300yearsago.WeattributetheInflatedRidgeTrendtoarelativelyhighproportionofpyroxenite(ormeltderivedtherefrom)toenrichedperidotiteinthemantleduringaphaseofridgeinflationthatlastedatleast6000years.TheGrabenTrendreflectsthereducedeffectofpyroxeniteaftertheaxialgrabenformed.Becauseatleast14differentsamplingsofmantlecomponentsoccurredwithin<1kmofridgelengthandwidthduringatimewhen<1kmofupwellingoccurred,weinferthatthescaleofmantleheterogeneityfarfromaplumeis<1km.TheenrichedmantlecomponentatEndeavourisyoungwith206Pb/204Pb2300 years ago. We attribute the Inflated Ridge Trend to a relatively high proportion of pyroxenite (or melt derived therefrom) to enriched peridotite in the mantle during a phase of ridge inflation that lasted at least 6000 years. The Graben Trend reflects the reduced effect of pyroxenite after the axial graben formed. Because at least 14 different samplings of mantle components occurred within <1 km of ridge length and width during a time when <1 km of upwelling occurred, we infer that the scale of mantle heterogeneity far from a plume is < 1 km. The enriched mantle component at Endeavour is young with 206 Pb/ 204 Pb 2300yearsago.WeattributetheInflatedRidgeTrendtoarelativelyhighproportionofpyroxenite(ormeltderivedtherefrom)toenrichedperidotiteinthemantleduringaphaseofridgeinflationthatlastedatleast6000years.TheGrabenTrendreflectsthereducedeffectofpyroxeniteaftertheaxialgrabenformed.Becauseatleast14differentsamplingsofmantlecomponentsoccurredwithin<1kmofridgelengthandwidthduringatimewhen<1kmofupwellingoccurred,weinferthatthescaleofmantleheterogeneityfarfromaplumeis<1km.TheenrichedmantlecomponentatEndeavourisyoungwith206Pb/204Pb19Á0; Hf and He isotope ratios trend toward HIMU characteristics. These traits are regionally widespread and are shared with the next two ridge segments to the north (West Valley and Explorer).
Journal of Geophysical Research, Jun 30, 2009
We report major, trace, and volatile element data on basaltic glasses from the northernmost segme... more We report major, trace, and volatile element data on basaltic glasses from the northernmost segment of the Eastern Lau Spreading Center (ELSC1) in the Lau back-arc basin to further test and constrain models of back-arc volcanism. The zero-age samples come from 47 precisely collected stations from an 85 km length spreading center. The chemical data covary similarly to other back-arc systems but with tighter correlations and welldeveloped spatial systematics. We confirm a correlation between volatile content and apparent extent of melting of the mantle source but also show that the data cannot be reproduced by the model of isobaric addition of water that has been broadly applied to back-arc basins. The new data also confirm that there is no relationship between mantle temperature and the wet melting productivity. Two distinct magmatic provinces can be identified along the ELSC1 axis, a southern province influenced by a ''wet component'' with strong affinities to arc volcanism and a northern province influenced by a ''damp component'' intermediate between enriched mid-ocean ridge basalts (E-MORB) and arc basalts. High-field strength elements and rare earth elements are all mobilized to some extent by the wet component, and the detailed composition of this component is determined. It differs in significant ways from the Mariana component reported by E. Stolper and S. Newman (1994), particularly by having lower abundances of most elements relative to H 2 O. The differences can be explained if the slab temperature is higher for the Mariana and the source from which the fluid is derived is more enriched. The ELSC1 damp component is best explained by mixing between the wet component and an E-MORB-like component. We propose that mixing between water-rich fluids and low-degree silicate melts occurs at depth in the subduction zone to generate the chemical diversity of the ELSC1 subduction components. These modified sources then rise independently to the surface and melt, and these melts mix with melts of the background mantle from the ridge melting regime to generate the linear data arrays characteristic of back-arc basalts. The major and trace element framework for ELSC1, combined with different slab temperatures and compositions for difference convergent margins, may be able to be applied to other back-arc basins around the globe.
Geochemistry Geophysics Geosystems, Oct 1, 2003
We report measurements of U‐series disequilibria, Sr, Nd, Hf, and Pb isotopic compositions and ma... more We report measurements of U‐series disequilibria, Sr, Nd, Hf, and Pb isotopic compositions and major and trace element abundances in a suite of well‐located, off‐axis MORBs that span the East Pacific Rise (EPR) ridge crest from 9°48′–52′N and across it for ∼4 km on either side. The geological context of the samples are well constrained as they were collected by submersible in an area that has been extensively imaged by remote sensing techniques. Sr, Nd, Hf and 208Pb/206Pb isotopic compositions of the off‐axis N‐MORB are identical to the axial lavas from this same region, suggesting that their sources are similar and that melting processes are the dominant influence in establishing the U‐Th‐Ra disequilibria and trace element fractionations. A majority of off‐axis samples have U‐Th and Th‐Ra disequilibria that are larger, and model ages that are younger, than would be predicted from their off‐axis distance and the time‐integrated spreading rate. There are, however, a few off‐axis samples with U‐Th ages that are consistent with their spreading rate ages. It is likely that these samples erupted within or close to the axial summit trough (AST) and aged at a rate proportional to the spreading rate. The anomalously young ages determined for most of the off‐axis lavas suggest that volcanic construction along this region is occurring over a zone that is wider (at least 4 km) than the AST (10s to 100s of meters). The combined observational, chemical and isotopic data support a model for the 9°0′N area that includes a significant component of crustal accumulation resulting from lavas that breach the AST and flow down the flanks of the EPR ridge crest. However, these data also require a minor component of off‐axis eruptions that occur on distinct pillow mounds and ridges. This suggests that MOR construction involves several volcanic and tectonic processes acting in concert to form a complex patchwork of lava ages and compositions along, and across, this fast spreading ridge crest.
Earth and Planetary Science Letters, Feb 1, 1994
A segment of the slow-spreading Mid-Atlantic Ridge (MAR) at 33°S changes dramatically as its cent... more A segment of the slow-spreading Mid-Atlantic Ridge (MAR) at 33°S changes dramatically as its center is approached. Towards the center of the segment, the axis shoals from 3900 to 2400 m and a deep median valley nearly disappears. There is a prominent bullseye gravity low ...
Subduction of oceanic crust has long been considered a major cause of mantle heterogeneity. By fa... more Subduction of oceanic crust has long been considered a major cause of mantle heterogeneity. By far the largest volume of recycled plates, however, is the lithospheric mantle, which is often inferred but rarely observed. Here we report evidence that the Gakkel ridge sampled a section of recycled refractory ocean lithosphere: (1) a unique group of high-Ti basalts occurs adjacent to a long sparsely magmatic zone (SMZ) that is consistent with melting of oxide gabbro hosted in ancient ocean lithosphere; (2) the SMZ itself which is 300km of ancient mantle emplaced at the surface; (3) sparse enriched basalts from the SMZ with no residual garnet signature suggesting shallow melting of ancient refractory but metasomatized lithosphere with low mantle potential temperature; (4) a gradient in composition approaching the SMZ suggesting lower extents of melting of more depleted sources. The rarity of these occurrences elsewhere shows that ordinarily recycled ocean crust and refractory lithosphere...
The AMORE expedition recovered igneous rocks from over two hundred stations from the Gakkel Ridge... more The AMORE expedition recovered igneous rocks from over two hundred stations from the Gakkel Ridge, permitting the first regional scale investigation of its composition. These samples permit for the first time the evaluation of petrological systematics from an ultra-slow spreading ridge (0.7029 west of the SMZ and &lt; 0.7028 to the east, even for the same La/Sm). Thus the amagmatic
ABSTRACT The distance of the Eastern Lau Spreading Center (ELSC) from the volcanic front of the T... more ABSTRACT The distance of the Eastern Lau Spreading Center (ELSC) from the volcanic front of the Tonga Arc varies progressively from 40 km at the Valu Fa Ridge in the south to 100km in the north. Previous studies showed a marked contrast between "arc-like" lavas in the Valu Fa region and "MORB-like" lavas from the north of the Eastern Lau spreading Center. We report more than 60 new isotopic analyses from samples with an average spacing of 5 km that complement our major (over 200 analyses) and trace element (130 analyses) dataset. The isotopic data provide new constraints on the "MORB-like" mantle, the composition of the subduction-related component and its variation in the basin, and the relationship between the geochemical variations and the distance behind the arc. Pb isotope ratios decrease overall toward the north away from Valu Fa, providing an along-axis gradient in 206Pb/204Pb from 18.13 to 18.66. Values for the southern Valu Fa lavas intersect the Pb isotope array of the Tonga arc. The northern end of the array is offset to high 207Pb/204Pb and ^{208}Pb/204Pb compared to the N. Atlantic-Pacific data. These results are consistent with mixing between a MORB-like end-member having a DUPAL signature and a Valu Fa-like end-member with strong arc signature. The presence of a Dupal component explains the radiogenic 87Sr/86Sr (0.70323±8) that is observed in this region. In detail, both the Sr and Pb isotopic data requires that the DUPAL signature of the "MORB-like" mantle decreases in the south of the basin. Indeed, in the Pb-Sr space, the southernmost segment defines a trend, distinct from the other segments, toward lower 87Sr/86Sr and 206Pb/204Pb. This trend becomes further extended by some samples of the northernmost segments that have especially unradiogenic 87Sr/86Sr. Overall, the Pb isotope ratios correlate well with trace element ratios such as Nb/U or Ba/Nb in a manner consistent with an important Pb contribution from the subduction-related component(s). Coupled with mobile incompatible element ratios such as Ba/La, the Pb isotope ratios confirm that the subduction-related components vary in composition from one segment to another. Our major and trace element data showed that the transition from basaltic to andesitic magmatism along the ELSC occurs almost entirely along a single 15 km long segment located 70 km behind the Tonga arc. Over 30% of the Pb isotopic variation occurs within this area, confirming the importance of this short segment as a boundary between two different types of mantle process. South of the basalt-to-andesite transition, Pb isotopes define a regular variation with latitude, sometimes showing smooth oscillations within a single segment. The two northernmost segments, located north of the transition, have a less regular latitudinal Pb isotopic variation than other segments, but the variability still correlates with higher mobile/immobile element ratios. These results show that, in the arc vicinity, a large part of the mantle regime below the back-arc is affected by subduction-related fluids or melts, while far from the arc, only local spikes of subduction-related signature occur.
Earth and Planetary Science Letters, 2021
Basalts erupted within intra-transform spreading centers can be valuable probes of geochemical co... more Basalts erupted within intra-transform spreading centers can be valuable probes of geochemical components in Earth's upper mantle, and provide constraints on the proportions of mantle carbon that are juvenile (primordial) vs. tectonically recycled. Here we present new results for submarine basalt glasses, erupted within the Garrett Transform Fault (GTF) and to its north and south along the East Pacific Rise (EPR). Analyses of 3 He/ 4 He, and He and CO 2 concentrations in vesicles and glass were performed, through a series of crushing and melting experiments plus FTIR spectroscopy. Trace elements were analyzed by laser ablation ICP-MS. The GTF basalts provide further tests for the origin of volatile-undersaturated basalts using CO 2 /Ba and CO 2 /Nb systematics. CO 2 is highly correlated with Ba and Nb in basalts erupted in the transform domain (n = 13, including 5 undersaturated basalts) and does not show the variability expected from mixing between undegassed and variably degassed melts. Rather, the melts appear to originate from a heterogeneous mantle source that was variably depleted through partial melting, and limited mixing of melts is involved in their generation. The CO 2 /Ba and CO 2 /Nb weight ratios of 106 ± 8 and 308 ± 27, respectively, are similar to values determined previously for a global suite of undersaturated mid-ocean ridge basalts (MORBs). The ridge and transform domains have distinct 3 He/ 4 He ratios. Along the nearby EPR, 3 He/ 4 He = 8.5-9.1 R A , while within the Garrett Transform Fault 3 He/ 4 He = 9.2-10.1 R A. These two basalt populations are also distinct in their Pb-Sr-Nd isotope compositions based on earlier regional studies. The distinct populations result from partial melting of two different mantle source compositions. Melting of depleted mantle containing a small amount (∼1 to 5%) of enriched, ancient heterogeneities occurs beneath the EPR. Melting of ultradepleted mantle (in which the heterogeneities have been removed by earlier melting beneath the EPR) occurs beneath the GTF. This explains the distinction between intra-transform and spreading ridge domains for 3 He/ 4 He, if the heterogeneities were enriched in U, Th and He and had low 3 He/ 4 He as would be found in tectonically recycled material. The enriched mantle component sampled by the EPR basalts has molar CO 2 / 3 He = 2 × 10 9 , and it dominates the CO 2 / 3 He ratio generally ascribed to the upper mantle source for mid-ocean ridge basalts. In contrast, the ultradepleted MORB mantle component sampled by the GTF basalts has CO 2 / 3 He = 3 × 10 8 or less. This indicates that most of the carbon in Earth's upper mantle originates from tectonic recycling.
Goldschmidt2021 abstracts, 2021
Nature Communications, 2021