AN IN SITU OXYGEN ISOTOPE RECORD IN FELDSPAR PORPHYROCLASTS TO UNDERSTAND FLUID-DEFORMATION FEEDBACKS IN METAMORPHIC CORE COMPLEXES (Invited Presentation)

Fluids can drive both mass flux and thermal flux along detachment faults. Given that oxygen isotopes are sensitive to variation in temperature, time, and water composition during water-rock interactions, patterns in δ¹⁸O within extensional detachment fault systems can record details of fault-controlled hydrothermal activity and fluid-assisted deformation. We applied in situ measurements of δ¹⁸O by SIMS to K-feldspar and plagioclase porphyroclasts in quartzofeldspathic mylonite samples collected from three different structural depths below the Whipple Mountain metamorphic core complex detachment fault (WDF). Individual porphyroclasts record multiple crosscutting brittle and ductile deformation microstructures, which can be targeted by SIMS for δ¹⁸O signatures of deformation-associated fluids.

Feldspar SIMS profiles show large intragrain δ¹⁸O oscillations of several permil. Immediately below the detachment fault (<1 m), feldspar δ¹⁸O profiles are dominated by irregular fluctuations between 2 – 6 ‰. Deeper beneath the WDF (40 m), feldspar profiles show smaller δ¹⁸O fluctuations superimposed on a systematic core-to-rim δ¹⁸O zoning from 9 – 4 ‰. In both cases, the δ¹⁸O patterns cannot be explained by closed-system diffusive exchange during cooling alone. Microstructural characterization by cathodoluminescence and electron backscatter diffraction shows that feldspars immediately below the WDF record brittle crack-seal deformation followed by plastic recrystallization and then a return to brittle crack-seal behavior. Low and variable δ¹⁸O throughout the grains indicates exchange with meteoric fluids during both brittle and ductile deformation. Feldspar porphyroclasts farther below the WDF record initial brittle crack-seal deformation giving way to plastic recrystallization, with low-δ¹⁸O meteoric fluid interactions recorded only by ductile microstructures. Together, these observations suggest that fluid infiltration tended to promote ductile deformation in the WDF footwall but that brittle behavior recurred near the main detachment surface, likely due to transient changes in strain rate or fluid pressure. Thus, in situ SIMS studies of feldspar porphyroclasts provide windows into the microscale dynamics of brittle-ductile transition in midcrustal fault systems.