METEORIC WATER INFILTRATION IN SKALLEN MARBLES, EAST ANTARCTICA : OXYGEN ISOTOPIC EVIDENCE

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We report here extremely large millimeter scale oxygen isotopic heterogeneity in calcites from a Skallen marble specimen. The δ^<18>O of calcite cores and most of the calcite grain boundaries are grouped around 17‰ while two analyses from the calcite-phlogopite grain boundary show depletion to -1‰. A corresponding δ^<13>C enrichment (only 0.7‰) is also observed. The δ^<13>C of graphite does not show any heterogeneity. The homogeneity of δ^<18>O and δ^<13>C of calcite (except for the local depletion) and δ^<13>C of graphite indicate that the marbles evolved through prograde and peak metamorphism with possible internal fluid buffering. The variations observed in carbon and oxygen isotopes can be ascribed to devolatilization reactions. The peak metamorphic temperature estimate by carbon isotope thermometry gave 760℃, consistent with regional peak metamorphic conditions. The local occurrence of isotopic heterogeneity on the calcite-phlogopite boundary suggests that there was meteoric water infiltration during the late stages of metamorphic history. Millimeter scale gigantic oxygen isotopic heterogeneity is observed for the first time in metamorphic rocks. The mechanism of isotope exchange invloved is unclear in the present study, but may indicate a process of fluid flow through preferential grain boundaries and by solution-reprecipitation or diffusion.
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