Oxygen isotope variations of garnets and clinopyroxenes in a layered diamondiferous calcsilicate rock from Kokchetav Massif, Kazakhstan: a window into the geochemical nature of deeply subducted UHPM rocks

摘要

Calcsilicate and garnet-pyroxene rocks with dolomite and Mg-calcite matrices occur with UHPM diamondiferous biotite gneisses and schists of the Kokchetav Massif. The calcsilicates are characterized by high diamond grade, K-bearing diopside, and very high Mg-garnets (Mg# > 77) with variable Ca contents (Ca# = 42.5–80). A rare calcsilicate sample with alternating layers of different bulk compositions was selected for oxygen isotope and electron probe microanalysis of garnets and pyroxenes. A grain of fresh garnet with a brownish-yellow luminescent inner domain (Mg# 94) and a non-luminescent outer part (Mg# 88) was selected for in situ analysis of δ18O by ion microprobe (10 μm spot). The profile demonstrates a δ18O gradient of 1.5‰/200 μm, from 11.3 (rim) to 12.8‰ (core) VSMOW. Additional 2 mg samples of hand-picked garnet and clinopyroxene fragments from different parts of the same sample (selected by color and chemical differences) were analyzed for δ18O by laser fluorination, yielding even larger differences in δ18O: 6.3–10.6‰ in garnets and 6.1–8.1 in clinopyroxenes. The zonation in δ18O among grains of the same mineral in different lithologies may in part reflect initial heterogeneities of the finely layered sedimentary precursors. The δ18O values for the garnets are among the highest observed for UHP-origin (both for crustal or mantle rocks), confirming a sedimentary origin for these carbonate-bearing rocks, and ruling out a primitive mantle-derived protolith. Oxygen diffusion in garnet at peak metamorphism temperature (1,000°C) was arrested by rapid cooling.