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Northeastern Section - 51st Annual Meeting - 2016

Paper No. 2-5
Presentation Time: 9:30 AM

OXYGEN ISOTOPE SPEEDOMETRY USING QUARTZ INCLUSIONS IN GARNET AND MATRIX QUARTZ


QUINN, Ryan Joel, Department of Geoscience, University of Wisconsin - Madison, 1215 W. Dayton St., Madison, WI 53706, KITAJIMA, Kouki, Department of Geoscience, University of Wisconsin-Madison, Madison, WI 53706, SPICUZZA, Michael J., Department of Geoscience, University of Wisconsin-Madison, 1215 W. Dayton St, Madison, WI 53706, NAKASHIMA, Daisuke, Department of Earth and Planetary Material Sciences, Tohoku University, Aoba, Sendai, Miyagi, 980-8578, Japan; Department of Geoscience, University of Wisconsin - Madison, 1215 W. Dayton St., Madison, WI 53706 and VALLEY, John W., Department of Geoscience, University of Wisconsin-Madison, 1215 West Dayton Street, Madison, WI 53706, rquinn2@wisc.edu

Oxygen isotope ratios of quartz inclusions (20-800µm dia) within garnets (1-4mm dia) from the Adirondack Mountains, NY, were analyzed by ion microprobe (SIMS) and compared to δ18O of matrix quartz to develop an in situ method of oxygen isotope speedometry. Primary quartz inclusions (PQI) are typically lower in δ18O than matrix quartz (MQ) and average measured δ18O(PQI) values for 8 of the 12 samples are significantly different than δ18O(MQ) at 95% confidence.

Values of Δ18O(MQ-PQI) develop due to oxygen exchange between matrix quartz and other matrix minerals (e.g. micas, oxides, and feldspars) during cooling; whereas PQI are armored by garnet (slow O-diffusion), have no exchange partner, and thus retain peak Δ18O(Qz-Grt) equilibrium. Interpreting these differences between PQI and MQ assumes a closed system with respect to externally derived fluid and that garnets didn’t leak. Disequilibrium between δ18O(PQI) and δ18O(MQ) is predicted by the Fast Grain Boundary (FGB) diffusion model, which describes interdiffusion of stable isotopes between matrix minerals in response to temperature changes.

A consequence of the FGB calculations is that the diffusional resetting of matrix quartz is proportional to the peak temperature and diffusion rate of oxygen in quartz; and inversely proportional to the ratio of oxygen in Qz/(micas + feldspars + oxides), grain size of MQ, and cooling rate.

Petrographic microscopy was used to estimate grain sizes and mineral proportions. Experimental diffusion data for oxygen in α-quartz were used from Farver and Yund (1991). Calculations of Δ18O(PQI-Grt) range from 2.65 - 3.24‰, which yield temperatures of 640-740 ± 50 °C (A(Qz-Gt) = 2.71). Cooling rate can thus be estimated by adjusting the cooling rate input in the FGB code until the calculated change in δ18O(Qz) after cooling matches the measured value of Δ18O(MQ-PQI) in natural samples.

The average values of Δ18O(MQ-PQI) for each hand sample vary from -0.04 - 0.80‰. The three largest values (0.51 - 0.80‰) were measuerd from a amphibolite-facies gneisses from the Lowlands and corresponds to a cooling rates of 0.5 - 1 °C/Myr. Samples from the Highlands have smaller Δ18O(MQ-PQI) values (-0.04 - 0.46‰) corresponding to 1 to >10 °C/Myr. Alternatively, regional differences in Δ18O(MQ-PQI) could be due to lower f(H2O) during cooling in the Highlands.

Session No. 2

S2. Timing and Nature of Deformation in the Adirondack Mountains I
Monday, 21 March 2016: 8:00 AM-12:00 PM
Meeting Room 6 (Empire State Plaza Convention Center)
Geological Society of America Abstracts with Programs. Vol. 48, No. 2
doi: 10.1130/abs/2016NE-272784
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