Microscale isotopic zoning in calcite and graphite crystals in marble

Abstract

Chemical and isotopic zoning in minerals is a clear indication of disequilibrium and provides an important key for elucidating metamorphic processes. Here I present carbon and oxygen isotope data from calcite and graphite crystals in a marble from the Hida metamorphic belt, central Japan, showing isotopic zoning within individual crystals. Within 100μm of the calcite–graphite grain boundary the 13C contents gradually decrease from the constant values of the crystal core to values ˜1‰ lower; 18O contents in calcite decrease by up to 7‰ within 200–300 (Jim of the boundary. At a calcite–calcite grain boundary, similar patterns of 13C and 18O depletions were found, whereas the 13C contents of graphites remained constant except in a narrow zone at the calcite contact. These isotopic patterns are interpreted as reflecting isotopic exchange with metamorphic fluids that were relatively depleted in 13C and 18O. These results strongly suggest that the most accurate geothermometry will be obtained if only the cores of crystals are used, and that isotopic zoning may provide another way to elucidate polymetamorphic processes.