Multiphase inclusions in peritectic garnet from granulites of the Athabasca granulite terrane (Canada): Evidence of carbon recycling during Neoarchean crustal melting

In the last decade, the study of fluid and melt inclusions in partially melted rocks has become a key tool to acquire unprecedented information about crustal anatectic processes. In this study we report the results of the microstructural and microchemical investigation on multiphase inclusions trapped within peritectic garnet of a Neoarchean felsic granulite from the Upper Deck domain of the Athabasca granulite terrane, Canada. Inclusions have been studied by SEM-EDS, FIB-SEM serial sectioning and Raman microspectroscopy, and classified in terms of size and hosted phases. Type I multiphase inclusions are small (≤15 μm), primary in origin, and do not show evidence of decrepitation. Their multiphase assemblage is made of ferroan magnesite, quartz and graphite in association with minor amounts of corundum, pyrophyllite and Zn-spinel. Calcite, dolomite and zinc-bearing sulphide may also be present. The coexistence of quartz and corundum in these inclusions is interpreted as the product of metastable growth within pores of extremely small size. The fluid phase of Type I inclusions, always present in amounts >40 vol%, is CO2-rich (96.5 mol%) with traces of N2 (3.3 mol%) and CH4 (0.2 mol%). These carbon-rich Type I inclusions coexist in the same cluster with primary melt inclusions (nanogranitoids; Type II). These are large (up to 50 μm) and composed of K-feldspar, quartz and plagioclase with minor amounts of graphite, biotite and aluminosilicate. Because nanogranitoids are droplets of anatectic silicate melt formed and trapped during incongruent melting of crustal rocks, the coexistence of Type I multiphase inclusions proves the presence of a carbon-rich fluid during the Neoarchean anatexis (800–950 °C, 0.6–1.4 GPa) of this portion of continental crust, in a likely situation of melt/fluid immiscibility. According to phase equilibria modelling, the uncommon multiphase assemblage within Type I inclusions is here interpreted as the result of a post-entrapment carbonation reaction between an original CO2-bearing fluid and the garnet host during rock cooling from UHT conditions.