Calcium isotope fractionation during melt-rock interaction in the lithospheric mantle

Calcium is a major component of the Earth's mantle, which plays an important role during its melting and metasomatism. The behavior of Ca isotopes during these processes remains poorly constrained, but it has a potential to shed new light on mantle evolution. We determined Ca isotope compositions in two suites of well studied mantle xenoliths with distinct types and degrees of melt-rock interaction. The xenoliths from Tok at the SE margin of the Siberian craton range from lherzolites and harzburgites to wehrlites produced by reaction with evolved Ca-Fe rich silicate melts. The xenoliths from Tariat in Mongolia are lherzolites crosscut by pyroxenite veins. We also analyzed three eclogites from the Udachnaya kimberlite in the central Siberian craton, and their clinopyroxene (cpx) and garnet separates.

Calcium isotope measurements were conducted by Triton-TIMS using double spike method at the Guangzhou Institue of Geochemistry, CAS. The data are reported as δ^44/40Ca (relative to NIST SRM 915a). Results of the reference samples are consistent with those from other laboratories.

δ^44/40Ca of the cpx in the eclogite xenoliths are 0.3‰ lighter than of coexisting garnets, which we attribute to equilibrium isotope fractionation controlled by mineral structure. The δ^44/40Ca in fertile and refractory peridotites with little or no effects of metasomatism range from 0.86 to 0.97‰ and are correlated with melting indices (e.g., Mg# and CaO wt.%) confirming the effects of partial melting on Ca isotopes [1]. Wehrlites from Tok produced by advanced melt-rock reaction show lighter δ^44/40Ca from 0.65 to 0.87‰ negatively correlated with (Gd/Yb)_N. The alkali basalts hosting the Tok xenoliths have low δ^44/40Ca as well (0.68‰). We argue that metasomatic melts with low δ^44/40Ca and low HREE can be produced by partial melting of eclogites, and then impart their Ca isotope signatures on strongly metasomatized mantle rocks. By contrast, the δ^44/40Ca in the veined Tariat peridotites are close to BSE estimates and may be linked to intrusion and solidification of melts derived from the DMM-type mantle.

[1] Kang et al (2017) EPSL 474.