The Effect of Deformation on Oxygen Isotope Exchange in Quartz and Feldspar and the Significance of Isotopic Temperatures in Mylonites

Abstract

Oxygen isotopic compositions of quartz and feldspar in greenschist‐grade mylonites from the Blue Ridge thrust and the Brevard zone in the southern Appalachians were analyzed by laser microprobe to examine the effect of deformation on isotopic behavior. In mylonites, texturally homogeneous polycrystalline quartz ribbons have a constant isotopic composition (δ18O = 12.9 ± 0.0‰, n = 3), whereas monocrystalline quartz ribbons, which display heterogeneous intercrystalline strain and only minor recrystallization, have variable δ18O values (11.6 ± 0.5‰, n = 5). Alkali feldspars in samples that contain fluid inclusion‐decorated microcracks, reflecting heterogeneous deformation, show a range in isotopic composition (8.8 to 10.2; mean = 9.4 ± 0.7‰, n = 3). In contrast, recrystallized myrmekite rims surrounding alkali feldspar augen in Brevard zone mylonites are isotopically heavier by about 1% (9.2 ± 0.1‰, n = 5) compared to the cores 8.3 ± 0.3‰, n = 4), reflecting isotopic homogenization during neocrystallization. Deformation mechanisms that result in heterogeneous strain on the grain scale (either crystal plastic or brittle) are associated with only partial isotopic homogenization, whereas deformation mechanisms that result in homogeneous strain (e.g., recrystallization, neocrystallization) are associated with isotopic homogenization on the grain scale. Agreement between measured quartz‐feldspar isotopic temperatures and calculated temperatures using a finite difference model indicates diffu‐sional exchange occurred between phases during closed system cooling, and that the measured temperatures in the mylonites are maximum temperatures for the deformation. The approximate agreement between measured temperatures in some mylonites and the calculated Dodson quartz closure temperatures indicates that isotopic exchange below Tc quartz was not substantial. The necessary conditions under which isotopic temperatures in mylonites correspond to the deformation temperature are outlined. On the basis of this study and reconsideration of older data, the onset of total dynamic recrystallization in quartz is estimated to be about 350°C in natural shear zones. Together with reaction weakening of feldspar observed in the mylonites, the temperature interval 350‐400°C is likely to be important for weakening of both quartz and feldspar in the continental crust.