Investigation of stable Cr isotopic fractionation during Earths silicate differentiation and reassessment of the accurate Bulk Silicate Earth delta53Cr value
Final Report Abstract
In this project, we first chose samples from two Archean komatiite-tholeiite suites, the 3.5- 3.22 Ga Barberton Greenstone Belt (BaGB) and the 2.9-2.7 Ga Belingwe Greenstone Belt (BeGB), to investigate the effects of magmatic processes on the melt’s Cr stable isotopic composition. The large range of MgO contents in the rock suites of the BaGB and BeGB enables the investigation of the effect of variable degrees of mineral crystallization on Cr isotopic variability. Fractional crystallization of Cr-bearing minerals, such as clinopyroxene and Cr-spinel, drives the residual melt towards lighter Cr isotopic compositions. In contrast, crystallization and accumulation of olivine has negligible effect on the Cr isotope compositions as shown by the invariant Cr isotope signature of komatiites. Komatiites are generated by very high degrees of partial mantle melting and thus serve as proxy for the stable Cr isotopic composition of bulk silicate Earth. Based on the average stable Cr isotopic composition of 21 komatiite samples, we estimate the δ53Cr value of bulk silicate Earth to be -0.12±0.06‰. This value agrees very well with previous estimates, paving the way for subsequent investigations of Cr isotopic variations of different terrestrial mantle reservoirs. To further investigate the effect of magmatic processes at modern terrestrial mantle conditions Cr and Fe isotopic compositions of basaltic glasses from the East- Pacific Rise, Pacific-Antarctic Ridge, and one sample from the Mid-Atlantic Ridge were measured. These samples represent normal mid-ocean ridge basalts depicting a typical depleted upper mantle signature in radiogenic Sr-Nd-Pb and trace element compositions. Magmatic differentiation involving a typical crystallization assemblage of plagioclase, olivine, clinopyroxene and Fe-Ti oxides induces no measurable Cr isotopic fractionation. Normal mid-ocean ridge basalts reveal a small range in δ53Cr between -0.278 and -0.186‰ with an average δ53Cr value of -0.237±0.050‰, significantly offset towards lighter isotopic composition compared to bulk silicate Earth. Forward modeling using a phase equilibria-based mantle melting model and published mineral-melt fractionation factors was applied to investigate Cr isotopic fractionation during partial melting of peridotite at mid-ocean ridge melting conditions. It shows that the difference in δ53Cr between a melt and its peridotite source (i.e. Δ53Crmelt-peridotite) is maximally -0.07‰ during partial melting in the spinel-stability field. Our data indicate the presence of a pyroxene-rich component enriched in light Cr isotopes in the melt source region and highlights stable Cr isotopes as a tracer for mantle source lithological heterogeneities. Finally, the Cr and Fe isotopic compositions of 45 ocean island -and continental intraplate basalts representing variable mantle sources, as revealed by Sr-Nd-Pb isotopic compositions, were determined. The observed trends between stable Cr isotopic compositions and indices of magmatic differentiation for most ocean island basalts are in conflict with expected trends for fractional crystallization based on published mineral-melt fractionation factors. This suggests that factors like temperature and pressure of melting, which are distinct for Archean vs. modern mantle melting regimes, do not control stable Cr isotopic fractionation. Instead, correlations of stable Cr isotopic signatures with radiogenic Sr and Nd isotope compositions in ocean island basalts indicate that the composition of the mantle source is a key mechanism controlling Cr isotope variability. Therein, samples displaying a HIMU- and PREMA-like mantle signature in radiogenic Sr-Nd-Pb isotope space show lighter Cr and heavier Fe isotopic compositions compared to bulk silicate Earth. This is tentatively explained by incorporation of recycled oceanic crust components carrying a midocean ridge-like stable Cr and Fe isotopic signature, and which are possibly present as pyroxenite lithologies. In contrast, ocean island basalts with EM2 isotopic affinities from French Polynesia show heavier Cr and lighter Fe isotopic signatures with increasing radiogenic Sr isotopic compositions.
Publications
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Stable Cr isotope fractionation during magmatic differentiation of 2.7 Ga komatiites from the Belingwe Greenstone Belt, Zimbabwe. Goldschmidt conference, Barcelona, Spain. 2019
Wagner, L. J., Kleinhanns, I. C., Hofmann, A. & Schoenberg, R.
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Stable Cr isotope fractionation during magmatic differentiation of 2.7 Ga komatiites, Belingwe Greenstone Belt, Zimbabwe. Deutsche Mineralogische Gesellschaft (DMG) – annual Meeting, Heidelberg, Germany. Poster
Wagner, L. J., Kleinhanns, I. C., Hofmann, A. & Schoenberg, R.
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Coupled stable chromium and iron isotopic fractionation tracing magmatic mineral crystallization in Archean komatiite-tholeiite suites. Chemical Geology, 576, 120121.
Wagner, Luise J.; Kleinhanns, Ilka C.; Weber, Nadja; Babechuk, Michael G.; Hofmann, Axel & Schoenberg, Ronny
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Exploring the selenium isotope record in the Archean mantle. Goldschmidt2021 abstracts, 2021. European Association of Geochemistry.
Varas-Reus, Maria; König, Stephan; Yierpan, Aierken; Wagner, Luise J.; Schoenberg, Ronny; Lorand, Jean-Pierre & Neubauer, Regina
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Stable Cr isotope compositions of modern silicate mantle reservoirs. Goldschmidt2021 abstracts, 2021. European Association of Geochemistry.
Wagner, Luise J.; Kleinhanns, Ilka; Bach, Wolfgang; König, Stephan; Wörner, Gerhard; Rouxel, Olivier & Schoenberg, Ronny
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Light stable Cr isotope compositions of mid-ocean ridge basalts: Implications for mantle source composition. Geochimica et Cosmochimica Acta, 353, 76-91.
Wagner, Luise J.; Kleinhanns, Ilka C.; Varas-Reus, Maria I.; Rosca, Carolina; König, Stephan; Bach, Wolfgang & Schoenberg, Ronny
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“The stable Cr isotope signatures of Earth’s Silicate reservoirs: probing the deep earth with time” within this project and acquired her doctoral title on the 05.05.2023. PhD
Luise Wagner
