Isotope disequilibrium during partial melting of metasedimentary rocks

Abstract

Abstract The chemical composition of partial melts is buffered by the rock forming minerals that participate in the melting reactions and by the solubility of accessory phases in these melts. Both depend on the melting conditions. The chemical composition of restite depends on the restite mineralogy and may be highly variable. Partial melts and restite show contrasting Sr and Nd isotopic compositions if the various mineral phases of the protolith have developed contrasting Sr and Nd isotopic compositions by radiogenic in situ ingrowth, which depends on the parent-to-daughter ratios and time. We present chemical and isotope data from different leucosome-restite pairs that differ by 8 to 68 eSr-units and 0.2 to 6.2 eNd-units, respectively. As the Sr and Nd isotopic compositions of the partial melts show little variation for comparable protoliths, the difference in Sr and Nd isotopic composition is related to the restites, whose variable compositions largely reflect variations in the mineral assemblages. We present Li and B concentration and isotope data from a cm-scale profile across the contact between leucosome and restite. Lithium has systematically lower concentrations and higher δ7Li in the leucosome than in the restite. There is a sharp change in δ7Li at the contact between restite and leucosome. In contrast, B shows only minor variations in concentration and isotopic composition across the contact zone. Our data indicate that (i) the Li and B isotopic compositions of the leucosome are controlled by different minerals (ii) isotope fractionation of Li and B by diffusion processes is of subordinate importance on the handspecimen scale, even at magmatic conditions.