AN EMPIRICAL ESTIMATE OF THE DIFFUSION RATE OF OXYGEN IN DIOPSIDE

The intracrystalline diffusion rate of oxygen in diopside was constrai ned based on natural isotopic variations from a granulite facies marbl e from Cascade Slide, Adirondacks (New York, USA). The oxygen isotope compositions of the diopsides, measured as a function of grain size, a re nearly constant (20.9 +/- 0.3 parts per thousand. vs. SMOW) over th e entire measured size range (0.3-3.2 mm diameter). The deltaO-18 valu es of the cores of calcite grains are 23.0 parts per thousand. Tempera ture estimates based on the DELTAO-18(calcite-diopside) are 800-degree s-C, in agreement with the highest previous thermometric estimates for these rocks. The lack of isotopic variation in the diopsides as a fun ction of grain size requires that the oxygen intracrystalline diffusio n rate in diopside from the Adirondack samples was very slow. The maxi mum diffusion rates (D800-degrees-C parallel to the c-axis) were calcu lated with an infinite reservoir model (IRM) and a finite reservoir mo del (FRM) that incorporates mineral modal abundances and initial isoto pic variations. For an assumed activation energy (Q) = 100 kJ/mol, the IRM diffusion rate estimate of 1.6 x 10(-20) cm2/s is two orders of m agnitude faster than from the FRM; at Q = 500 kJ/mol, the D800-degrees -C estimate for both methods is c. 5.6 x 10(-20) cm2/s. The present re sults require that a hydrothermal fluid significantly enhances the dif fusion rate of oxygen in diopside if previous data are correct. The de ltaO-18(SMOW) and deltaC-13(PDB) values of the calcite, measured in si tu with a CO2 laser, are 22.9 +/- 0.3, 0.1 +/- 0.3 parts per thousand. in the grain cores, 22.1 +/- 0.3, 0.2 +/- 0. 1 parts per thousand at the grain boundaries and 21.7 +/- 0.4, -0.6 +/- 0. 1 parts per thousan d. abutting diopside grains. The deltaO-18 and deltaC-13 values measur ed conventionally are: crystal cores, 22.96, -0.95 parts per thousand; abutting diopside grains, 22.38, -0.93 parts per thousand; bulk, 22.7 9, -0.95 parts per thousand. Use of the bulk deltaO-18(calcite) values for thermometry yields unreasonably high temperatures. The lower delt aO-18 values at the calcite grain boundaries are not due to retrograde diffusional exchange with the diopside, they are thought to be a resu lt of a late retrograde fluid infiltration.