Coupled stable chromium and iron isotopic fractionation tracing magmatic mineral crystallization in Archean komatiite-tholeiite suites

Abstract

Chromium exists in two oxidation states Cr2+ and Cr3+ during high-temperature magmatic processes and changes in Cr redox are often associated with stable isotopic fractionation. Thus, the stable chromium isotope compositions of mantle-derived magmas bear the potential to trace the oxidation states of their mantle sources as well as any post melting changes in Cr redox (e.g. during magmatic differentiation), in a manner similar to magmatic stable Fe isotopic fractionation. However, these stable isotope fractionation effects are less understood for Cr relative to Fe. Komatiites and tholeiitic basalts represent a wide range of mantle-derived partial melts with variable fractional crystallization of phases with different affinity for Cr2+ and Cr3+. Thus, they offer potential archives to better understand high-temperature Cr isotope fractionation processes and mantle redox.Here, we report major and trace elements as well as coupled stable Cr and Fe isotope compositions of two well-characterized Archean komatiite-tholeiite suites from the Barberton Greenstone Belt, South Africa and Eswatini, and Belingwe Greenstone Belt, Zimbabwe. The sample suites range in MgO concentrations from 3.85 to 34.33 wt%, which allows investigation of the impact of large degrees of magmatic differentiation in a komatiite-basalt system. Whole-rock δ53/52CrSRM979 and δ56/54FeIRMM014 values range from −0.390 ± 0.016 to −0.061 ± 0.016‰ and −0.014 ± 0.018 to +0.192 ± 0.018‰, respectively. The komatiites have a very narrow range in their Cr isotopic composition with an average δ53/52CrSRM979 value of −0.122 ± 0.050‰ (2SD; n = 21), which supports previous estimates of the bulk silicate Earth δ53/52CrSRM979 value. However, high-Mg tholeiites and basaltic andesites exhibit significantly lighter δ53/52CrSRM979 and heavier δ56/54FeIRMM014 values than komatiites. These variations can be linked to crystallization and accumulation of mineral phases observed from fractionation trends of the two Archean komatiite-tholeiite suites. In detail, during crystallization and accumulation of olivine the Cr isotope compositions of komatiites stay invariant, whereas at the onset of Cr-bearing spinel and pyroxene crystallization the Cr isotope signatures of komatiitic basalts, high-Mg tholeiites and basaltic andesites become progressively lighter, which is attributed to the preferential incorporation of isotopically heavier Cr3+ in these mineral phases. The gradual increase of δ56/54FeIRMM014 with increasing magmatic differentiation, does not allow identifying the crystallization of particular mineral phases using Fe isotopes alone. Ultimately, this study demonstrates the power of combining stable Cr and Fe isotopic analyses to examine the effects of fractional crystallization on modifying melt source values and thus to ensure accurate mantle redox estimations.