Calcium isotope sources and fractionation during melt-rock interaction in the lithospheric mantle: Evidence from pyroxenites, wehrlites, and eclogites

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 delta Ca-44/40 in clinopyroxenes (cpx) are similar to 0.3 parts per thousand 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 delta Ca-44/40 in the pyroxenite veins (cumulates from mafic melts) and their host peridotites from Tariat in Mongolia range from 0.88 parts per thousand to 1.05 parts per thousand and are close to the Bulk Silicate Earth (BSE) value (0.95 +/- 0.05 parts per thousand). 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 delta(44/40) Ca (0.65 parts per thousand-0.87 parts per thousand) in the Tok peridotites are negatively correlated with Ca/Al, (Gd/Yb)(N) and positively with Mg# (indices of melt-rock reaction degrees). The delta Ca-44/40 ratios in the LW series rocks are lighter than the BSE estimate, and similar to delta Ca-44/40 in their host basalt and an olivine-cpx cumulate (0.70 parts per thousand-0.76 parts per thousand). 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 delta Ca-44/40 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.