Calcium isotopic fractionation in mantle peridotites by melting and metasomatism and Ca isotope composition of the Bulk Silicate Earth

摘要

To better constrain the Ca isotopic composition of the Bulk Silicate Earth (BSE) and explore the Ca isotope fractionation in the mantle, we determined the Ca isotopic composition of 28 peridotite xenoliths from Mongolia, southern Siberia and the Siberian craton. The samples are divided in three chemical groups: (1) fertile, unmetasomatized lherzolites (3.7-4.7 wt.% Al2O3); (2) moderately melt depleted peridotites (1.3-3.0 wt.% Al2O3) with no or very limited metasomatism (LREE-depleted cpx); (3) strongly metasomatized peridotites (LREE-enriched cpx and bulk rock) further divided in subgroups 3a (harzburgites, 0.1-1.0% Al2O3) and 3b (fertile Iherzolites, 3.9-4.3% Al2O3). In Group 1, delta Ca-44/40 of fertile spinel and garnet peridotites, which experienced little or no melting and metasomatism, show a limited variation from 0.90 to 0.99 parts per thousand (relative to SRM 915a) and an average of 0.94 +/- 0.05 parts per thousand (2SD, n =14), which defines the Ca isotopic composition of the BSE. In Group 2, the delta Ca-44/40 is the highest for three rocks with the lowest Al2O3, i.e. the greatest melt extraction degrees (average 1.06 +/- 0.04 parts per thousand, i.e. similar to 0.1 parts per thousand heavier than the BSE estimate). Simple modeling of modal melting shows that partial melting of the BSE with 10(3) In alpha(peridonte-melt) ranging from 0.10 to 0.25 can explain the Group 2 data. By contrast, delta Ca-44/40 in eight out of nine metasomatized Group 3 peridotites are lower than the BSE estimate. The Group 3a harzburgites show the greatest delta Ca-44/40 variation range (0.25-0.96 parts per thousand), with delta Ca-44/40 positively correlated with CaO and negatively correlated with Ce/Eu. Chemical evidence suggests that the residual, melt-depleted, low-Ca protoliths of the Group 3a harzburgites were metasomatized, likely by carbonate rich melts/fluids. We argue that such fluids may have low (<= 0.25 parts per thousand) delta Ca-44/40 either because they contain recycled crustal components or because Ca isotopes, similar to trace elements and their ratios, may be fractionated by kinetic and/or chromatographic effects of melt percolation in the mantle. The delta Ca-44/40 in Group 3b lherzolites (0.83-0.89 parts per thousand) are lower than in the BSE as well, but the effects of metasomatism on delta Ca-44/40 are smaller, possibly because of the high Ca contents in their protoliths and/or smaller delta Ca-44/40 differences between the protoliths and metasomatic agents. The BSE estimates based on fertile peridotites in this study fall in the delta Ca-44/40 ranges for oceanic and continental basalts, various meteorites (achondrites; carbonaceous, ordinary and enstatite chondrites), Mars, and the Moon. These results provide benchmarks for the application of Ca isotopes to planet formation, mantle evolution, and crustal recycling. (C) 2017 Elsevier B.V. All rights reserved.