Chromium isotope variations (δ53/52Cr) in mantle-derived sources and their weathering products: Implications for environmental studies and the evolution of δ53/52Cr in the Earth’s mantle over geologic time

0399326 - GLÚ 2014 GB eng J - Článek v odborném periodiku
Farkaš, J. - Chrastný, V. - Novák, M. - Čadková, E. - Pašava, J. - Chakrabarti, R. - Jacobsen, S. B. - Ackerman, Lukáš - Bullen, T. D.
Chromium isotope variations (δ53/52Cr) in mantle-derived sources and their weathering products: Implications for environmental studies and the evolution of δ53/52Cr in the Earth’s mantle over geologic time.
Geochimica et Cosmochimica Acta. Roč. 123, 15 December 2013 (2013), s. 74-92. ISSN 0016-7037. E-ISSN 1872-9533
Klíčová slova: chemical weathering * chromite * chromium * isotopic composition * isotopic fractionation * mantle chemistry * mantle source * serpentine * serpentinization * ultramafic rock
Kód oboru RIV: DD - Geochemie
Impakt faktor: 4.250, rok: 2013

Here we report chromium isotope compositions, expressed as δ53/52Cr in per mil (>) relative to NIST 979, measured in selected Cr-rich minerals and rocks formed by the primary magmatic as well as the secondary metamorphic and weathering processes. The main objectives of this study were: (i) to further constrain the isotope composition of the Earth's mantle Cr inventory and its possible variation during geological history, based on the analysis of globally distributed and stratigraphically constrained mantle-derived chromites; and (ii) to investigate the magnitude and systematics of Cr isotope fractionation during oxidative weathering and secondary alteration (i.e., hydration, serpentinization) of the magmatic Cr sources. Specifically, we analyzed δ53/52Cr in a set of globally distributed mantle-derived chromites (FeMgCr2O4, n=30) collected from various locations in Europe, Asia, Africa and South America, and our results confirm that a chromite-hosted Earth's mantle Cr inventory is uniform at -0.079±0.129> (2SD), which we named here as a 'canonical' mantle δ53/52Cr signature. Furthermore our dataset of stratigraphically constrained chromites, whose crystallization ages cover most of the Earth's geological history, indicate that the bulk Cr isotope composition of the chromite-hosted mantle inventory has remained uniform, within about ±0.100>, since at least the Early Archean times (~3500 million years ago, Ma).To investigate the systematics of Cr isotope fractionation associated with alteration processes we analyzed a number of secondary Cr-rich minerals and variably altered ultramafic rocks (i.e., serpentinized harzburgites, lherzolites) that revealed large positive δ53/52Cr anomalies that are systematically shifted to higher values with an increasing degree of alteration and serpentinization. The degree of aqueous alteration and serpentinization was quantified by the abundances of fluid-mobile (Rb, K) elements, and by the Loss On Ignition (LOI) parameter, which determines the amount of structurally bound water (OH/H2O) present in secondary hydrated minerals like serpentine. Overall, we observed that altered ultramafic rocks that yielded the highest LOI values, and the lowest amounts of fluid mobile elements, also yielded the heaviest δ53/52Cr signatures. Therefore, we conclude that secondary alteration (i.e., hydration, serpentinization) of ultramafic rocks in near-surface oxidative environments tend to shift the bulk Cr isotope composition of the weathered products to isotopically heavier values, pointing to a dynamic redox cycling of Cr in the Earth's crustal and near-surface environments. Hence, if validated by future studies, this would suggest that Cr isotopes could be used to trace the recycling of altered oceanic lithosphere through subduction zones, and to detect the sources of dehydrated and previously serpentinized oceanic crust carrying 'heavy' δ53/52Cr signatures in island arc systems.Finally, the fact that the geogenic Cr sources may locally exhibit anomalous (non-canonical) δ53/52Cr signatures has also implications for environmental studies that use δ53/52Cr as a tracer to quantify the amount of the hexavalent Cr reduction in waters.
Trvalý link: http://hdl.handle.net/11104/0226649