ION MICROPROBE EVIDENCE FOR THE MECHANISMS OF STABLE-ISOTOPE RETROGRESSION IN HIGH-GRADE METAMORPHIC ROCKS

Retrograde interdiffusion is widely proposed as the dominant factor in producing the stable isotopic fractionation among minerals in slowly cooled igneous and metamorphic rocks. Mineral zonation consistent with interdiffusion of stable isotopes has never been directly observed, h owever, leaving doubt as to the mechanism responsible for the bulk-min eral isotopic compositions commonly measured. Ion microprobe analyses of oxygen isotope ratios in magnetite were combined with conventional bulk mineral analyses and diffusion modeling to document the relations hip between mineral zonation and the mechanism of retrogression inferr ed from bulk mineral data. Two samples of magnetite-bearing, quartzo-f eldspathic Lyon Mountain gneiss from the Adirondack mountains, N.Y. we re studied in detail. Conventional stable isotope analysis of both sam ples indicates that isotopic thermometers are discordant and were rese t by as much as 200 degrees C from the estimated peak temperature of 7 50 degrees C. The relative order of apparent temperatures recorded by various thermometers differs between the two samples, however, with T- gtz-fsp much greater than T-mt-qtz and T-mt-fsp in one sample and T-qt z-fsp < T-mt-qtz and T-mt-fsp in the other. Diffusion modeling using t he Fast Grain Boundary model shows that the former pattern of apparent temperatures is consistent with closed system interdiffusion during c ooling, whereas the latter is not. The modeling predicts that 0.5 mm d iameter magnetite grains common to this rock type will contain isotopi c zonation of 1 parts per thousand (rims lower in delta(18)O than core s), and that the cores of smaller (0.1 mm) grains will be similarly lo wer than to the cores of large (0.5 mm) grains. Ion microprobe analysi s reveals that the zoning patterns of magnetite grains from the first sample contain clear core to rim zonation in multiple grains (Delta co re-rim = 1.1 +/- 0.4 parts per thousand) and predicted grain-size vs c ore composition variations, consistent with diffusion-controlled reset ting of bulk mineral fractionations. In contrast, the second sample sh ows irregular inter- and intra-granular variations over an 8 parts per thousand range, consistent with open system alteration. These results provide direct documentation of the importance of interdiffusion in a ffecting stable isotope distributions in slowly cooled rocks. The corr elations of bulk-mineral resetting with zonation show that bulk minera l data, when interpreted with detailed modeling, can be used to determ inate what processes controlling retrogression.