ION MICROPROBE STUDY OF THE REDISTRIBUTION OF CARBON AND OXYGEN ISOTOPES AMONG MINERALS DURING METAMORPHIC INFILTRATION-DRIVEN DECARBONATION REACTIONS

The traditional model for redistribution of O and C isotopes among minerals and fluid during metamorphic decarbonation reactions assumes reactants and products are uniform in composition and at local equilibrium. An alternative model assumes equilibrium among products with uniform composition but not between reactants and products. Ion microprobe analyses (~10 micron spot) of minerals in mm-size domains of rocks with arrested reaction directly test these assumptions at a grain-size scale. Five samples from 3 contact aureoles were examined. Four exhibit arrested formation of Fo + Cal from Dol and aqueous SiO2; 1 exhibits arrested formation of Per (now Brc) + Cal from Dol. Data are reported only for domains with carbonates and silicates virtually free of retrograde alteration. One Fo-bearing sample almost conforms to the traditional model. Values of d13CCal (–0.2±0.3‰), d13CDol (–0.4±0.3‰), d18OCal (21.3±0.2‰), d18ODol (21.2±0.2‰), and d18OFo (18.8±0.2‰) are uniform. C and O isotope fractionation between Cal and Dol is consistent with equilibrium; D18OCal-Fo, however, is ~1‰ less than equilibrium. A second Fo-bearing sample almost conforms to the alternative model. d18OCal (10.7±0.1‰) and d18OFo (6.8±0.2‰) are uniform and consistent with equilibrium, while d18ODol (range 9.0-19.1, average 12.7‰) is highly variable and out of equilibrium with Cal and Fo. d13CCal (–2.6 to –4.8, –3.5‰), however, is not uniform (nor is d13CDol = –1.4 to +2.3, 0.3‰). The other 2 Fo-bearing samples conform to neither model. d13CCal (–1.2±0.5, –1.4±0.2‰) and d13CDol (–0.9±0.5, –1.4±0.2‰) in both samples are uniform and consistent with equilibrium. d18ODol (18.9±0.1, 17.4±0.2‰) is uniform in both samples. Values of d18OCal (18.2-19.9, 19.2‰; 15.9-18.2, 17.2‰) and d18OFo (9.8-17.2, 12.4‰; 12.1-15.0, 12.9‰), however, are variable and inconsistent with equilibrium. The isotopic behavior of Fo-bearing samples correlates with Fo grain number density and morphology. The Brc-bearing sample also conforms to neither model. d13CCal (1.6±0.3‰) and d13CDol (1.9±0.3) are uniform and consistent with equilibrium, and d18ODol is uniform (23.3±0.1‰). d18OCal (19.9-23.6, 22.0‰), however, is variable. Small-scale isotopic variability and disequilibrium imply short reaction times and call for further in situ analysis and new models.