Young (61 Ma) unaltered picrites from Baffin Island, northeast Canada, possess some of the highest 3He/4He (up to 50 Ra) seen on Earth, and provide a unique opportunity to study primordial mantle ...that has escaped subsequent chemical modification. These high-degree partial melts also record anomalously high 182W/184W ratios, but their Sr-Nd-Hf-Pb isotopic compositions (including 142Nd) are indistinguishable from those of North Atlantic mid-ocean ridge basalts. New high precision Fe and Zn stable isotope analyses of Baffin Island picrites show limited variability with δ56Fe ranging from −0.03‰ to 0.13‰ and δ66Zn varying from 0.18‰ to 0.28‰. However, a clear inflection is seen in both sets of isotope data around the composition of the parental melt (MgO = 21 wt%; δ56Fe = 0.08 ± 0.04‰; and δ66Zn = 0.24 ± 0.03‰), with two diverging trends interpreted to reflect the crystallisation of olivine and spinel in low-MgO samples and the accumulation of olivine at higher MgO. Olivine mineral separates are significantly isotopically lighter than their corresponding whole rocks (δ56Fe ≥ −0.62‰ and δ66Zn ≥ −0.22‰), with analyses of individual olivine phenocrysts having extremely variable Fe isotope compositions (δ56Fe = −0.01‰ to −0.80‰). By carrying out modelling in three-isotope space, we show that the very negative Fe isotope compositions of olivine phenocryst are the result of kinetic isotope fractionation from disequilibrium diffusional processes. An excellent correlation is observed between δ56Fe and δ66Zn, demonstrating that Zn isotopes are fractionated by the same processes as Fe in simple systems dominated by magmatic olivine. The incompatible behaviour of Cu during magmatic evolution is consistent with the sulfide-undersaturated nature of these melts. Consequently Zn behaves as a purely lithophile element, and estimates of the bulk Earth Zn isotope composition based on Baffin Island should therefore be robust. The ancient undegassed lower mantle sampled at Baffin Island possesses a δ56Fe value that is within error of previous estimates of bulk mantle δ56Fe, however, our estimate of the Baffin mantle δ66Zn (0.20 ± 0.03‰) is significantly lower than some previous estimates. Comparison of our new data with those for Archean and Proterozoic komatiites is consistent with the Fe and Zn isotope composition of the mantle remaining constant from at least 3 Ga to the present day. By focusing on large-degree partial melts (e.g. komatiites and picrites) we are potenitally biasing our record to samples that will inevitably have interacted with, entrained and melted the ambient shallow mantle during ascent. For a major element such as Fe, that will continuosly participate in melting as it rises through the mantle, the final isotopic compositon of the magama will be a weighted average of the complete melting column. Thus it is unsuprising that minimal Fe isotope variations are seen between localities. In contrast, the unique geochemical signatures (e.g. He and W) displayed by the Baffin Island picrites are inferred to solely originate from the lowermost mantle and will be continuously diluted upon magma ascent.
Arc lavas display elevated Fe3+/ΣFe ratios relative to MORB. One mechanism to explain this is the mobilization and transfer of oxidized or oxidizing components from the subducting slab to the mantle ...wedge. Here we use iron and zinc isotopes, which are fractionated upon complexation by sulfide, chloride, and carbonate ligands, to remark on the chemistry and oxidation state of fluids released during prograde metamorphism of subducted oceanic crust. We present data for metagabbros and metabasalts from the Chenaillet massif, Queyras complex, and the Zermatt‐Saas ophiolite (Western European Alps), which have been metamorphosed at typical subduction zone P‐T conditions and preserve their prograde metamorphic history. There is no systematic, detectable fractionation of either Fe or Zn isotopes across metamorphic facies, rather the isotope composition of the eclogites overlaps with published data for MORB. The lack of resolvable Fe isotope fractionation with increasing prograde metamorphism likely reflects the mass balance of the system, and in this scenario Fe mobility is not traceable with Fe isotopes. Given that Zn isotopes are fractionated by S‐bearing and C‐bearing fluids, this suggests that relatively small amounts of Zn are mobilized from the mafic lithologies in within these types of dehydration fluids. Conversely, metagabbros from the Queyras that are in proximity to metasediments display a significant Fe isotope fractionation. The covariation of δ56Fe of these samples with selected fluid mobile elements suggests the infiltration of sediment derived fluids with an isotopically light signature during subduction.
Key Points
Iron and zinc stable isotope and elemental data are presented for a prograde suite of metabasalts and metagabbros from Western Alpine ophiolite complexes
Bulk rock δ56Fe and δ66Zn do not vary across metamorphic facies and the eclogitic samples show a MORB‐like isotope composition
Blueschist facies metagabbros preserve evidence for infiltration of sediment derived fluids that impart a light δ56Fe isotope composition to the gabbro
Techniques for the determination of radiogenic strontium (87Sr/86Sr) and neodymium (143Nd/144Nd) isotope ratios by Thermal Ionisation Mass Spectrometry (TIMS) are presented in this study. We have ...developed a 5 lines acquisition routine for both elements, taking advantage of the 16 Faraday cups available on the Nu Instruments TIMS in conjunction with an efficient zoom lens system. This allows increased flexibility regarding the number of lines in a multidynamic acquisition routine whilst maintaining optimal peak alignment and peak shape for every measurement line despite the fact that the detectors are not movable. The long-term reproducibility obtained for our Sr (NBS 987) and Nd (Rennes-Ames) standard solutions gives a (87Sr/86Sr)multidyn of 0.7102467 ± 0.0000043 (6.0 ppm, 2 s.d., n = 38) and a (143Nd/144Nd)multidyn of 0.5119537 ± 0.0000022 (4.2 ppm, 2 s.d., n = 31), respectively. We also report (87Sr/86Sr)multidyn and (143Nd/144Nd)multidyn for a set of terrestrial rock standards (including 5 basalts (BCR-2, BHVO-2, BIR-1, Be-N and BR-24), one rhyolite (RGM-1) and one andesite (AGV-2)) with the same, or greater, level of precision as for the pure standards solutions. This level of precision is 2 to 3 times better than the literature data for Sr isotope ratios, and comparable to the latest high precision data reported for Nd isotope ratios. This is the first paper reporting such high precisions for radiogenic Sr and Nd data generated using a TIMS from Nu Instruments.
High precision Sr and Nd isotopic measurements are of key importance in the field of Earth Sciences because they are powerful tools to trace sources of materials and to date them. Here, we report the first high-precision data for 87Sr/86Sr and 143Nd/144Nd (external error of 5–6 ppm for Sr and 4–5 ppm for Nd) measured using a Nu Instruments TIMS. Such precisions will open new opportunities in fields such as geochemistry, geochronology, cosmochemistry, archaeology or forensics.
•First estimate of the δ94/90ZrIPGP-Zr of the UCC using diverse sedimentary samples.•No measurable Zr isotope fractionation due to chemical weathering and mineral sorting.•No evidence for Zr isotope ...fractionation of the UCC through space and time.
The stable isotopic composition of insoluble, refractory elements such as titanium (Ti) or zirconium (Zr), which are modified by magmatic differentiation but, a priori, are poorly affected by weathering or diagenesis, serve as powerful potential proxies to reconstruct the compositional evolution of the continental crust. Here we present the evolution of the Zr stable isotopic compositions (δ94/90ZrIPGP-Zr, per mille deviation of 94Zr/90Zr from IPGP-Zr standard) of the continental crust through time, using 38 sedimentary samples from the upper continental crust (UCC), including 12 Holocene loesses from the Chinese Loess Plateau and Xinjiang, three oceanic sediments from the sea floor outboard of the Lesser Antilles island arc and 23 glacial diamictite composites with depositional ages ranging from ∼ 2.9 Ga to 0.3 Ga from South Africa, South America, Canada, USA and China. The samples show limited Zr isotopic variations with δ94/90ZrIPGP-Zr values of 0.043‰ - 0.109‰ for loess; 0.069‰ - 0.083‰ for oceanic sediments and 0.031‰ - 0.118‰ for glacial diamictites; their Zr-weighted average values are, 0.081 ± 0.044‰ (2SD, n = 12), 0.073 ± 0.015‰ (2SD, n = 3) and 0.078 ± 0.047‰ (2SD, n = 23), respectively. The isotopic similarity among loess, oceanic sediments and glacial diamictites, suggests that zircon enrichment effects previously documented in some sedimentary samples have not biased the Zr isotope compositions of these sedimentary rocks from their source rocks. Two groups with or without Zr enrichment have similar average δ94/90ZrIPGP-Zr values (0.075 ± 0.040‰ and 0.080 ± 0.046‰). There is no correlation between Zr isotope compositions and any proxy of chemical weathering (e.g., Al2O3/SiO2, Fe2O3/SiO2, CIA, K2O/Al2O3 and δ7Li). The δ94/90ZrIPGP-Zr values are quite constant for these sedimentary samples regardless of their depositional ages and locations. Therefore, the UCC appears to have had a constant Zr isotopic composition between 3 Ga and present, and is homogeneous at a large scale. Combining data for sedimentary reference materials from the literature and the sedimentary rocks in this study, we suggest a Zr-weighted δ94/90ZrIPGP-Zr value of 0.077 ± 0.058‰ (2SD, n = 44) for the UCC, which is statistically distinct (t test, p value = 2.88 × 10−10) and higher than that of the mantle (0.040 ± 0.044‰, n = 72). Combining the δ94/90ZrIPGP-Zr values of different terrestrial reservoirs, the δ94/90ZrIPGP-Zr of the BSE and bulk Earth is constrained to be 0.041 ± 0.041‰.
Zirconium plays a major role in geochemistry as it is the major cation of zircons - the oldest preserved minerals on Earth. While Zr isotopic anomalies in meteorites have been widely studied, mass ...dependant Zr stable isotope fractionation during geological processing has been untouched. Here, we report Zr stable isotopic data for terrestrial igneous rocks and present a novel method for the determination of Zr stable isotope ratios within natural geological materials using ion exchange, double-spike, multiple-collector inductively coupled mass spectrometry (MC-ICPMS). Zirconium is isolated from the rock matrix via a chromatographic separation protocol using a first pass column with AG1-X8 anion exchange resin, and a second pass column containing Eichrom® DGA resin. A 91Zr–96Zr double-spike was created from enriched single 91Zr and 96Zr isotope spikes. Samples were combined with the Zr double-spike at a 43:57 spike:sample Zr ratio, prior to dissolution and column chemistry. After column chemistry the purified sample solutions were analysed on a Thermo Scientific Neptune Plus MC-ICPMS and the data was reduced using IsoSpike, with the final Zr isotope data being reported as the per mil deviation of the 94Zr/90Zr from the IPGP-Zr standard (δ94/90ZrIPGP-Zr). The δ94/90ZrIPGP-Zr of six igneous standard reference materials: two basalts (BHVO-2 and JB-2), one andesite (AGV-2), two granites (GA and GS-N) and a serpentinite (UB-N) as well as one individual zircon grain (Plešovice zircon), are presented using this method. Sample measurements are presented with an analytical uncertainty of ±~0.05‰ (2sd) for δ94/90ZrIPGP-Zr and these rocks exhibit isotopic variations of ~0.15‰ for δ94/90ZrIPGP-Zr. These results demonstrate that natural variations of Zr isotopes occur within terrestrial igneous rocks, and are resolvable with this method. Finally the variation of δ94/90ZrIPGP-Zr values observed within the magmatic rock reference materials is correlated (R2 = 0.78; n = 5) with SiO2, suggesting that Zr isotopes could serve as a sensitive tracer of magmatic processes.
•First comprehensive study of Zr stable isotopes in terrestrial basalts and their igneous differentiates.•Undifferentiated basaltic samples display limited Zr isotope compositions, suggesting limited ...Zr isotope heterogeneity within the terrestrial mantle.•N-MORBs that sample depleted MORB mantle have variable Zr isotope compositions relative to OIB or T- and E-MORB.•Significant Zr isotope variation in differentiated igneous rocks (i.e., >65 wt% SiO2) interpreted to reflect crystallisation of isotopically light zircon.•Estimated primitive mantle δ94/90ZrIPGP-Zr is 0.048 ± 0.032‰; 2 sd, n = 48.
High-precision double-spike Zr stable isotope measurements (expressed as δ94/90ZrIPGP-Zr, the permil deviation of the 94Zr/90Zr ratio from the IPGP-Zr standard) are presented for a range of ocean island basalts (OIB) and mid-ocean ridge basalts (MORB) to examine mass-dependent isotopic variations of zirconium in Earth. Ocean island basalt samples, spanning a range of radiogenic isotopic flavours (HIMU, EM) show a limited range in δ94/90ZrIPGP-Zr (0.046 ± 0.037‰; 2sd, n = 13). Similarly, MORB samples with chondrite-normalized La/Sm of >0.7 show a limited range in δ94/90ZrIPGP-Zr (0.053 ± 0.040‰; 2sd, n = 8). In contrast, basaltic lavas from mantle sources that have undergone significant melt depletion, such as depleted normal MORB (N-MORB) show resolvable variations in δ94/90ZrIPGP-Zr, from −0.045 ± 0.018 to 0.074 ± 0.023‰. Highly evolved igneous differentiates (>65 wt% SiO2) from Hekla volcano in Iceland are isotopically heavier than less evolved igneous rocks, up to 0.53‰. These results suggest that both mantle melt depletion and extreme magmatic differentiation leads to resolvable mass-dependent Zr isotope fractionation. We find that this isotopic fractionation is most likely driven by incorporation of light isotopes of Zr within the 8-fold coordinated sites of zircons, driving residual melts, with a lower coordination chemistry, towards heavier values. Using a Rayleigh fractionation model, we suggest a αzircon-melt of 0.9995 based on the whole rock δ94/90ZrIPGP-Zr values of the samples from Hekla volcano (Iceland). Zirconium isotopic fractionation during melt-depletion of the mantle is less well-constrained, but may result from incongruent melting and incorporation of isotopically light Zr in the 8-fold coordinated M2 site of orthopyroxene. Based on these observations lavas originating from the effect of melt extraction from a depleted mantle source (N-MORB) or that underwent zircon saturation (SiO2 > 65 wt%) are removed from the dataset to give an estimate of the primitive mantle Zr isotope composition of 0.048 ± 0.032‰; 2sd, n = 48. These data show that major controls on Zr fractionation in the Earth result from partial melt extraction in the mantle and by zircon fractionation in differentiated melts. Conversely, fertile mantle is homogenous with respect to Zr isotopes. Zirconium mass-dependent fractionation effects can therefore be used to trace large-scale mantle melt depletion events and the effects of felsic crust formation.
•The new Comoros volcano, Fani Maoré, has exceptionally high Ba and low Pb contents.•Fani Maoré mantle source includes a highly altered recycled oceanic crust.•The carbonated source is rich in Ba and ...sulfur.•The Comoros Archipelago and East African Rift share a common deep mantle source.•A link is established between carbonatites and HIMU islands.
Intraplate volcanism provides remarkable insight into diverse sources in the mantle because its source can be quite shallow, or as deep as the core-mantle boundary, and its origin can be as diverse as recycled crustal material or undifferentiated mantle. While geophysical approaches can in some cases locate the source of magmas, a geochemical approach is necessary to characterize both the nature of the source and the way it melts to produce the erupted lavas.
Here we present geochemical and isotopic data obtained on a new submarine volcano (Fani Maoré) that was discovered in 2019 next to Mayotte Island in the Comoros. The radiogenic isotope data are remarkably uniform at subdued values intermediate between HIMU and EM1 compositions but trace element contents are unusual with a marked enrichment in Ba (Ba/Th ≈ 370 compared with the ocean island basalt (OIB) average of 100) and depletion in Pb (Ce/Pb ≈ 70 versus 25 for average OIB). This unique data set suggests that the basanites formed by melting of a carbonated mantle source that was highly enriched in Ba and volatiles. A similar source is also present under the East African Rift where contemporaneous basanite and carbonatite eruptions are known. This establishes a possible link between the volcanic activity of the Comoros and the East African Rift zone. More generally, it demonstrates that carbonated sources are more common in the mantle than previously thought and can be traced using trace element geochemistry. Other volcanoes in the world carry similar characteristics and we suggest that carbonated mantle sources explain the geochemical peculiarities of not only Fani Maoré in the Comoros but also those of Cape Verde volcanics and more generally those of many HIMU-like OIBs, in particular the so-called ‘young HIMU’ OIBs.
Lavas produced at subduction zones represent the integration of both source heterogeneity and an array of crustal processes, such as: differentiation; mixing; homogenisation; assimilation. Therefore, ...unravelling the relative contribution of the sub-arc mantle source versus these crustal processes is difficult when using the amalgamated end products in isolation. In contrast, plutonic xenoliths provide a complementary record of the deeper roots of the magmatic plumbing system and provide a unique record of the true chemical diversity of arc crust. Here, we present the δ
56
Fe record from well characterised plutonic xenoliths from two distinct volcanic centres in the Lesser Antilles volcanic arc–the islands of Martinique and Statia. The primary objective of this study is to test if the Fe isotope systematics of arc lavas are controlled by sub-arc mantle inputs or during subsequent differentiation processes during a magma’s journey through volcanic arc crust. The Fe isotopic record, coupled to petrology, trace element chemistry and radiogenic isotopes of plutonic xenoliths from the two islands reveal a hidden crustal reservoir of heavy Fe that previously hasn’t been considered. Iron isotopes are decoupled from radiogenic isotopes, suggesting that crustal and/or sediment assimilation does not control the Fe systematics of arc magmas. In contrast to arc lavas, the cumulates from both islands record MORB-like δ
56
Fe values. In Statia, δ
56
Fe decreases with major and trace element indicators of differentiation (SiO
2,
Na
2
O + K
2
O, Eu/Eu*, Dy/Yb), consistent with fractionating mineral assemblages along a line of liquid descent. In Martinique, δ
56
Fe shows no clear relationship with most indicators of differentiation (apart from Dy/Yb), suggesting that the δ
56
Fe signature of the plutonic xenoliths has been overprinted by later stage processes, such as percolating reactive melts. Together, these data suggest that magmatic processes within the sub-arc crust overprint any source variation of the sub-arc mantle and that a light Fe source is not a requirement to produce the light Fe isotopic compositions recorded in volcanic arc lavas. Therefore, whenever possible, the complimentary plutonic record should be considered in isotopic studies to understand the relative control of the mantle source versus magmatic processes in the crust.
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