Thallium isotopes in Iceland and Azores lavas — Implications for the role of altered crust and mantle geochemistry (original) (raw)
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Thallium elemental and isotopic systematics in ocean island lavas
Geochimica et Cosmochimica Acta, 2021
The Earth's mantle exhibits marked chemical heterogeneity, sampled via oceanic basalts. We provide a comprehensive examination of thallium systematics in ocean island basalts (OIB): new high-precision trace element analyses, including Tl, and Tl isotopic compositions for 48 OIB spanning the entire range of observed Sr-Nd-Hf-Pb isotope ratios. All investigated OIB are characterised by ubiquitous Tl depletion requiring OIB mantle sources to have Tl concentrations as low as 0.2 ng/g, which is an order of magnitude lower than estimates for the primitive mantle and similar to Tl concentrations inferred for the depleted mantle. The low Tl concentrations inferred for OIB mantle sources are interpreted to reflect near quantitative removal of Tl during subduction and inefficient Tl recycling into the deeper mantle. If true, the Tl isotopic composition of surface materials may not be readily translated to the mantle sources of OIB. The new OIB dataset shows a >10 ε-unit range in primary isotopic variation, from ε 205 Tl =-6.4 to +6.6. However, the majority of samples (32 of 48) are within uncertainty of mantle values (ε 205 Tl =-2 ± 1), and show no co-variation with radiogenic isotopic composition. Notably, OIB with only minor Tl depletion (11 samples) have Tl isotopic compositions outside the mantle range. The Tl concentration contrast between the mantle and inputs such as sediments and altered basalt is so great that minor additions (<1% by mass) of high-Tl material will dominate the isotopic budget of a lava, with decoupling of Tl and radiogenic isotopic compositions as an expected result. Thallium isotopic compositions of OIB are therefore difficult to link directly to radiogenic isotope variations and the mantle components they may reflect. Indeed, if isotopically distinct Tl from altered oceanic crust and/or sediments were efficiently recycled into the mantle and sampled via OIB, more variation in the Tl isotopic composition of OIB would be expected than is observed. The markedly unsystematic primary Tl isotopic variations in OIB therefore likely reflect the residual Tl isotopic composition of subducted material, and/or Tl acquired en route to the surface via shallow-level crustal assimilation.
Geochimica et Cosmochimica Acta, 2019
Thallium (Tl) isotope compositions of ocean island basalts (OIBs) have been proposed as a novel tracer of subducted ocea-nic crust and sediments in ocean island basalt sources, which could act as direct confirmation that deep mantle recycling eventually resurfaces through mantle upwelling to form ocean island basalt magmatism. However, it is unknown if oceanic crust that went through an active subduction zone would retain the Tl isotope compositions recorded in hydrothermally altered oceanic crust and authigenic marine sediments. In this study we present Tl isotope and concentration data for samples of sub-ducted oceanic crust from five different locations: Zambezi Belt, Zambia; Cabo Ortegal complex, Iberian Massif, Spain; Ras-pas Complex, southwest Ecuador; Syros island, Cyclades, Greece; Tian Shan, northwest China. Thallium concentrations in most samples follow strong linear relationships with K, Rb, Cs and Ba, which strongly suggest that the mineral phengite is the primary control of Tl abundances in subducted oceanic crust. This conclusion is consistent with recent Tl data sets for arc lavas that imply residual phengite in the arc lava source regions as a strong control of Tl recycling. We find that Tl isotope compositions vary widely and systematically in each location depending on the protolith, metamorphic and metasomatic history of the samples. Samples from Cabo Ortegal and Raspas Complex reveal Tl isotope compositions similar to their pro-toliths, which were comprised of low-temperature altered oceanic crust. Tian Shan metamorphic rocks and Zambian eclogites reveal invariant Tl isotope values indistinguishable from average mantle, which is best explained by overprinting by metamorphic fluids that contained high concentrations of Tl and other alkali metals. Samples from Syros reveal a range of Tl isotope compositions from normal mantle towards values for pelagic clay sediments. The sediment-like values in Syros likely arose from fluids released from the surrounding mélange matrix that consists of serpentinite, metagabbros and metased-iments. Each of the three Tl isotope ranges observed for subducted oceanic crust samples here are mirrored by individual OIB locations. Cabo Ortegal and Raspas Complex display Tl isotope compositions identical to St. Helena, suggesting that the HIMU component likely comprises subduction modified low-temperature altered oceanic crust. Thallium isotope ratios in Zambia and Tian Shan eclogites and blueschists are identical to lavas from Iceland, whereas Syros metamorphic rocks overlap almost exactly with lavas from Hawaii. Our data, therefore, show that Tl isotope compositions of oceanic crust and sediments can be traced through the subduction process and eventually is expressed largely unmodified in ocean island basalts.
Closing the loop: subducted eclogites match thallium isotope compositions of ocean island basalts
Geochimica et Cosmochimica Acta, 2019
metagabbros and metasediments. Each of the three Tl isotope ranges observed for subducted oceanic crust samples here are mirrored by individual OIB locations. Cabo Ortegal and Raspas Complex display Tl isotope compositions identical to St. Helena, suggesting that the HIMU component likely comprises subduction modified low-temperature altered oceanic crust. Thallium isotope ratios in Zambia and Tian Shan eclogites and blueschists are identical to lavas from Iceland, whereas Syros metamorphic rocks overlap almost exactly with lavas from Hawaii. Our data, therefore, show that Tl isotope compositions of oceanic crust and sediments can be traced through the subduction process and eventually is expressed largely unmodified in ocean island basalts.
The metasomatic alternative for ocean island basalt chemical heterogeneity
Earth and Planetary Science Letters, 2005
Subduction of oceanic lithosphere is thought to be responsible for producing heterogeneity in Earth's mantle; the heterogeneity is most clearly preserved in the compositions of ocean island basalts (OIB). The variation of trace element and isotopic ratios in OIB is commonly explained by recycling of ancient oceanic crust associated with terrigenous or pelagic sediment. However a variety of chemical and physical arguments seem to indicate that subducted oceanic crust is not the source of OIB. In particular, experimental petrologic studies indicate that the most plausible source for OIB is the partial melting of peridotite in the presence of CO 2 or silica-deficient pyroxenites. Alternative hypotheses for the source of OIB are subducted oceanic basal lithosphere enriched by metasomatic liquids and delaminated metasomatised continental lithosphere. Lithospheric metasomatism is common to both hypotheses and could produce silica-deficient pyroxenite; however, the exact chemical and physical nature of the process and how it leads to chemical variations in recycled lherzolite that produce the various OIB isotopic end-member bsignaturesQ is unclear. Chemical variations observed in the Cantal basalt (France) are interpreted as the result of a lithospheric metasomatic mechanism and provide important new constraints on the nature of this metasomatic process. The Cantal basalts, similar to OIB in composition, show unusual variations in Nb / Th, Nb / U, La / Nb and Ce / Pb ratios from the first (13-9 Ma) to the last (9-3 Ma) emitted basalt. The basalts are homogeneous with respect to their Sr, Nd, and Pb isotopic composition, ruling out variable sediment contamination of their mantle sources. We postulate that these trace element variations result from an evolution of metasomatic vein compositions present in a vein plus enclosing lithospheric mantle source.
Earth and Planetary Science Letters, 2006
We report U-Th disequilibria data for a suite of 13 young basaltic samples from the Samoan Islands, which represent the endmember mantle component EM2, and 4 historic lavas from Mt. Erebus, typifying young HIMU. The Samoan samples have low 230 Th/ 232 Th and 238 U/ 232 Th, consistent with the enriched nature (EM2) of the Samoan mantle source, whereas the Mt. Erebus samples have high 206 Pb/ 204 Pb and intermediate (230 Th/ 232 Th) and (238 U/ 232 Th). When considered in the context of the global oceanic basalt database, the Samoan samples' low 230 Th/ 232 Th and 238 U/ 232 Th greatly extend the global correlations between 230 Th/ 232 Th and 238 U/ 232 Th with 87 Sr/ 86 Sr and 143 Nd/ 144 Nd and change the functional form of these correlations from linear to hyperbolic. Using a maximum likelihood non-linear inversion method, we show that these correlations of 230 Th/ 232 Th and 238 U/ 232 Th with 87 Sr/ 86 Sr and 143 Nd/ 144 Nd can be approximated by two-component mixing. However, the global oceanic basalt data also show considerable scatter about the best-fit mixing curves. This scatter is attributed to additional source components and melting processes influencing the lavas (230 Th/ 232 Th) and (238 U/ 232 Th). Additional source components are supported by the Pb isotope data, which clearly require more than two endmember mantle components. We also show that the extent and variability in (230 Th/ 238 U) decreases as a function of source enrichment, with "depleted" MORB showing the largest extents and greatest variability in (230 Th/ 238 U) and "enriched" OIB, like Samoa, showing smaller extents and less variability of (230 Th/ 238 U). We interpret this observation in terms of differences in the melting regimes beneath mid-ocean ridges and ocean islands.
Thallium Isotopes and Their Application to Problems in Earth and Environmental Science
2012
This paper presents an account of the advances that have been made to date on the terrestrial stable isotope geochemistry of thallium (Tl). High precision measurements of Tl isotope ratios were only developed in the late 1990s with the advent of MC-ICP-MS and therefore we currently only have limited knowledge of the isotopic behavior of this element. Studies have revealed that Tl isotopes, despite their heavy masses of 203 and 205 atomic mass units, can fractionate substantially, especially in the marine environment. The most fractionated reservoirs identified are ferromanganese sediments and low temperature altered of oceanic crust. These display a total isotope variation of about 35 e 205 Tl-units, which is over 50 times the analytical reproducibility of the measurement technique. The isotopic variation can be explained by invoking a combination of conventional mass dependent equilibrium isotope effects and the nuclear field shift isotope fractionation, but the specific mechanisms are still largely unaccounted for.
Initial Reports of the Deep Sea Drilling Project, 82, 1985
Sr and Nd isotopic composition of 23 basalts from Sites 556-559 and 561-564. are reported. The 87 Sr/ 86 Sr ratios in fresh glasses and leached whole rocks range from 0.7025 to 0.7034 and are negatively correlated with the initial 143 Nd/ 144 Nd compositions, which range from 0.51315 to 0.51289. The Sr and Nd isotopic compositions (in glasses or leached samples) lie within the fields of mid-ocean ridge basalts (MORB) and ocean island basalts (OIB) from the Azores on the Nd-Sr mantle array/fan plot. In general, there is a correlation between the trace element characteristics and the 143 Nd/ 144 Nd composition (i.e., samples with Hf/Ta > 7 and (Ce/Sm) N < 1 [normal-MORB] have initial 143 Nd/ 144 Nd > 0.51307, whereas samples with Hf/Ta < 7 and (Ce/Sm) N > 1 (enriched-MORB) have initial 143 Nd/ 144 Nd compositions < 0.51300). A significant deviation from this general rule is found in Hole 558, where the N-MORB can have, within experimental limits, identical isotopic compositions to those found in associated E-MORB. The plume-depleted asthenosphere mixing hypothesis of Schilling (1975), White and Schilling (1978) and Schilling et al. (1977) provides a framework within which the present data can be evaluated. Given the distribution and possible origins of the chemical and isotopic heterogeneity observed in Leg 82 basalts, and some other basalts in the area, it would appear that the Schilling et al. model is not entirely satisfactory. In particular, it can be shown that trace element data may incorrectly estimate the plume component and more localized mantle heterogeneity (both chemical and isotopic) may be important.
Geochimica et Cosmochimica Acta, 2011
Lithium (Li) isotopes are thought to provide a powerful proxy for the recycling of crustal material, affected by low temperature alteration, through the mantle. We present Li isotope compositions for basaltic volcanic rocks from Hengill, Iceland, and Jan Mayen in order to examine possible links between ocean island volcanism and recycled oceanic crust and to address recent suggestions that mantle 3 He/ 4 He is also related to recycling of ancient slabs. Basaltic glasses spanning a range of chemical enrichment from the Hengill fissure system define an inverse correlation between d 7 Li (3.8-6.9&) and 3 He/ 4 He (12-20 R A). The high-3 He/ 4 He basalts have low d 18 O as well as excess Eu and high Nb/U, but carry no Li isotope evidence of being the product of recycling of altered slab or wedge material. In fact, there is no clear correlation between Li or He isotopes on the one hand and any of the other fingerprints of recycled slab components. The low-3 He/ 4 He samples do have elevated Nb/ U, Sr/Nd, positive Eu anomalies and high d 7 Li ($6.9&), providing evidence of a cumulate-enriched source that could be part of an ancient altered ocean floor slab. Basalts from Jan Mayen are characterized by large degrees of enrichment in incompatible trace elements typical of EM-like basalts but have homogeneous d 7 Li typical of depleted mantle (3.9-4.7&) providing evidence for a third mantle source in the North Atlantic. It appears that oceanic basalts can display a wide range in isotope and trace element compositions associated with recycled components whilst exhibiting no sign of modern surface-altered slab or wedge material from the Li isotope composition.
Chemical Geology, 2011
Most geochemical variability in MOR basalts is consistent with low-to moderate-pressure fractional crystallization of various mantle-derived parental melts. However, our geochemical data from MOR highsilica glasses, including new volatile and oxygen isotope data, suggest that assimilation of altered crustal material plays a significant role in the petrogenesis of dacites and may be important in the formation of basaltic lavas at MOR in general. MOR high-silica andesites and dacites from diverse areas show remarkably similar major element trends, incompatible trace element enrichments, and isotopic signatures suggesting similar processes control their chemistry. In particular, very high Cl and elevated H 2 O concentrations and relatively light oxygen isotope ratios (~5.8‰ vs. expected values of~6.8‰) in fresh dacite glasses can be explained by contamination of magmas from a component of ocean crust altered by hydrothermal fluids. Crystallization of silicate phases and Fe-oxides causes an increase in δ 18 O in residual magma, but assimilation of material initially altered at high temperatures results in lower δ 18 O values. The observed geochemical signatures can be explained by extreme fractional crystallization of a MOR basalt parent combined with partial melting and assimilation (AFC) of amphibole-bearing altered oceanic crust. The MOR dacitic lavas do not appear to be simply the extrusive equivalent of oceanic plagiogranites. The combination of partial melting and assimilation produces a distinct geochemical signature that includes higher incompatible trace element abundances and distinct trace element ratios relative to those observed in plagiogranites.