Widespread melt percolation in the subcontinental mantle: Perspectives from chromium stable isotopes

Abstract

Peridotites xenoliths from the subcontinental mantle exhibit a notably broad range of chromium isotope compositions in the literature, with refractory harzburgites demonstrating slightly higher δ⁵³Cr values (mean = −0.04 ± 0.06 ‰, 2SE, N = 49) compared to fertile lherzolites (mean = −0.12 ± 0.03 ‰, 2SE, N = 80). Such discrepancy cannot be explained by the partial melting of lherzolite. To better understand the observation, we analyzed the Cr isotope compositions of twenty-two lherzolites and fifteen harzburgites in Cenozoic basalts from eastern China. Taking the literature data together, the lherzolites yield an average δ⁵³Cr of −0.12 ± 0.05 ‰ (2SE, N = 99), similar to that estimated for the BSE (−0.12 ± 0.04 ‰, 2SD). Moreover, their δ⁵³Cr show no correlations with indicators of melt extraction (such as Al₂O₃ and CaO in whole-rock) or metasomatism (such as (La/Yb)_N in whole-rock and Ca/Al ratios in clinopyroxene), suggesting negligible effects of partial melting or metasomatism. Therefore, the Cr isotopic composition of the studied lherzolites may represent the original mantle signal. In contrast, the harzburgite display significantly higher δ⁵³Cr values (average − 0.04 ± 0.05 ‰, 2SE, N = 60) compared to the lherzolites. They also show no correlation between δ⁵³Cr values and melt extraction indices (such as Al₂O₃ and CaO concentrations of in whole-rocks) or metasomatism indicators (such as (La/Yb)_N in whole-rock and Ca/Al ratios in clinopyroxene). Chromium isotope fractionation during batch or fractional melting can only induce <0.01 ‰ Cr isotope variation, and thus cannot explain the elevated δ⁵³Cr values observed in harzburgites. Instead, kinetic isotope fractionation during melt percolation is more likely responsible for causing such Cr isotope offset between lherzolites and harzburgites in the subcontinental mantle. The isotopically heavy Cr pool harzburgites completements the isotopically light Cr pool in pyroxenite recognized previously.