OXYGEN ISOTOPE RATIOS OF QUARTZ INCLUSIONS IN GARNET AND IMPLICATIONS FOR MINERAL PAIR THERMOMETRY

Minerals can armor inclusions from effects of metamorphic reactions and secondary alteration. SIMS analysis of oxygen isotope ratios provides a test of such preservation by comparing the δ¹⁸O values of quartz grains included in garnets and quartz in the matrix. Numerous processes predicted to alter the d¹⁸O of matrix quartz after crystallization of garnets include: mineral reactions, changing T, retrogression, and fluid flow. Granulite-facies migmatites from Paleoproterozoic metasediments in the Ivanpah Mountains, southeastern California, contain abundant garnets that host numerous quartz inclusions from 1 to 100's of microns in diameter. The samples chosen display little to no visible alteration or retrogression, but sparse carbonatite dikes related to the 1.4 Ga Mountain Pass carbonatite complex are found in the sampled area. All quartz grains were imaged in CL to identify zoning and irregularities prior to SIMS analysis. In one sample, quartz inclusions in garnet produced average δ¹⁸O values of 12.4 ± 0.3‰ (2SD), while the matrix quartz yielded an average δ¹⁸O value of 13.2 ± 0.4‰ (2SD). No correlations between size of the inclusion or distance to cracks in the host garnet and the oxygen isotopic ratio has been noted. Individual analyses of quartz in the matrix range in oxygen isotopic composition from 12.9 - 13.6 ± 0.3‰, and are more variable than quartz included in garnets, which could suggest varying degrees of isotopic exchange during the intrusion of the Mountain Pass carbonatite complex. Alternatively, diffusive exchange with feldspars and mica during slow cooling could affect of d¹⁸O of matrix quartz , but not quartz armored by garnet, which has very slow oxygen diffusion. Hand-picked quartz from a crushed portion of the sample was analyzed by laser fluorination yielded a δ¹⁸O value of 12.94‰ ± 0.1, suggesting that dominantly matrix quartz was analyzed. These preliminary results suggest that in situ SIMS analysis of quartz included in garnet can preserve δ¹⁸O values from the time of garnet growth, and can be combined with the δ¹⁸O values of host garnet to produce reliable mineral-pair oxygen isotope thermometry.