HIGH-RESOLUTION STABLE ISOTOPE THERMOMETRY OF TACONIC STRAIN FRINGES

High-resolution oxygen isotope thermometry of a fibrous strain fringe from the Taconic Slate Belt shows a detailed record of temperature changes related to thrusting and thermal relaxation. Calcite+quartz strain fringes form around pyrite framboids in the footwall of the Bird Mountain fault, and have three distinct growth episodes (Goldstein et al., 2005). Primary fluid inclusions in quartz are aqueous and have T(h) that range from 110°C to 160°C and T(m) that range from 0° to -2.4°C. Millimeter-scale sampling by thin saw techniques yields Δ¹⁸O(qtz-cc)= 4.2 to 2.5‰ (n=12), which suggests a monotonic temperature increase from ca. 180 to 310°C during fringe growth (using the calibration of Sharp and Kirschner, 1994). Detailed study of this sample by Cameca IMS 1280 ion microprobe yields broadly similar results, but reveals intricate temperature variability. Oxygen isotope ratios were measured on adjacent calcite and quartz fibers by sputtering with a 20 KeV Cs+ primary beam focused to a 10 µm spot. Spot-to-spot precision on standard analyses averaged ±0.16‰ (1 sd). 114 qtz-cc pairs were analyzed in a 10 mm traverse parallel to fiber growth directions. Ion probe analysis confirms high temperatures at the end of fringe growth, but reveals oscillating temperatures during the first growth episode. In this growth zone initial temperatures were ca. 275°, dropped to 200°, rose back to 275°, and then fell to 210°C. We interpret these temperature highs as possibly reflecting episodes of thrusting along the Bird Mountain fault that heated the footwall by overthrusting more deeply buried rocks. Low-grade metamorphic tectonites are notoriously difficult targets for geothermometry, but stable isotope thermometry by ion microprobe allows detailed investigation of thermal histories at very high spatial resolution in these problematic rocks.