Abstract
To investigate the effects of melt metasomatism on Ca isotope fractionation in the upper mantle, we report data for xenoliths of mantle rocks including composite peridotites (lherzolites and coexisting pyroxenite veins), wehrlites as well as eclogites and their minerals. The δ44/40Ca in clinopyroxenes (cpx) are ~0.3‰ lower than in coexisting garnets in eclogites from the Udachnaya kimberlite in the central Siberian craton, likely due to equilibrium isotope fractionation controlled by mineral structure. The δ44/40Ca in the pyroxenite veins (cumulates from mafic melts) and their host peridotites from Tariat in Mongolia range from 0.88‰ to 1.05‰ and are close to the Bulk Silicate Earth (BSE) value (0.95 ± 0.05‰). Calcium-Fe-rich low-orthopyroxene lherzolites and wehrlites (LW series) from Tok in the SE Siberian craton were formed by reaction of lherzolites and harzburgites with percolating silica-undersaturated melts. The δ44/40Ca (0.65‰–0.87‰) in the Tok peridotites are negatively correlated with Ca/Al, (Gd/Yb)N and positively with Mg# (indices of melt-rock reaction degrees). The δ44/40Ca ratios in the LW series rocks are lighter than the BSE estimate, and similar to δ44/40Ca in their host basalt and an olivine-cpx cumulate (0.70‰–0.76‰). These observations on the Tok suite can be explained by reaction of refractory peridotite protoliths with melts derived from mixed eclogite and peridotite sources. Overall, the δ44/40Ca variations in eclogites, wehrlites, and various rocks related to silicate melts in the mantle, demonstrate that melt metasomatism plays an important role in producing mantle Ca isotope heterogeneity. The data for coexisting garnet and cpx in eclogites outline a potential for garnet to affect Ca isotope inter-mineral and solid-melt fractionation in the mantle.