Remarkably uniform oxygen isotope systematics for co-existing pairs of gem-spinel and calcite in marble, with special reference to Vietnamese deposits

Oxygen isotope systematics for co-existing pairs of gem-spinel and calcite in marble from Vietnam and other worldwide deposits have been determined in order to characterize the O-isotope fractionation between calcite and spinel. In Vietnam, the Δ18Occ–sp (=3.7±0.1‰ for six samples from the An Phu and Cong Troi deposits) is remarkably constant. The combination of these data with those obtained on calcite–spinel pairs of Paigutan (Nepal, n=2), Ipanko (Tanzania, n=1), and Mogok (Myanmar,=2) are also consistent with an overall Δ18Occ–sp of 3.6±0.3‰ for all the spinel samples (n=11). The straight line correlation δ18Occ=0.96 δ18Osp+4.4 is excellent despite their worldwide geographic spread. The increment method of calculating oxygen isotope fractionation gave a geologically unreasonable temperature of formation for both minerals at 1374°C when compared to temperatures obtained by mineral assemblage equilibrium of these marble type deposits, between 610 and 750°C. The constant Δ18Occ–sp reflects a constant temperature for this amphibolite facies assemblage, whose current best estimate is calculated at 620±40°C, but unquantified uncertainties remain.