IN SITU OXYGEN ISOTOPE MEASUREMENTS BY SIMS REVEAL METEORIC WATER-ROCK INTERACTIONS IN DEEP-SEATED METAMORPHIC CORE COMPLEX FOOTWALL ROCKS

Water-rock interactions are an important means to weaken faults, especially metamorphic core complex (MCC) detachment faults. Oxygen isotopes are a potentially powerful geochemical tracer of water-rock interactions in MCCs because of the uniquely low δ¹⁸O signatures of meteoric water. We investigate intragrain-scale oxygen isotope variations in footwall rocks of two MCCs of the southwestern US, the Whipple and Buckskin-Rawhide. We show that oxygen isotope measurements by secondary ion mass spectrometry (SIMS) can detect subtle, microscale signatures of meteoric fluid-rock interactions in MCC rocks. Moreover, in situ SIMS measurements of oxygen isotopes can be spatially associated with deformation microstructures to determine the relative timing of meteoric fluid infiltration and weakening mechanisms during meteoric water-rock interactions.

Feldspar dominates the largely granitoid mylonites of the Whipple and Buckskin complex footwalls. Feldspar porphyroclasts in Whipple footwall rocks immediately adjacent (<1 m) to the main detachment fault record brittle followed by plastic deformation and pervasively lowered δ¹⁸O, suggesting that meteoric fluids were involved in both brittle crack-seal deformation and subsequent recrystallization of feldspars. These patterns are consistent with pervasive fluid-rock interactions through fracturing and solution processes near the detachment, followed by a transition to plastic processes, perhaps with decreasing strain rate. Feldspar porphyroclasts tens of meters below the detachment in both the Whipple and Buckskin complexes record a transition from brittle to plastic microstructures with low δ¹⁸O values occurring only in plastically recrystallized feldspar domains and neocrystallized feldspar. Thus, at greater depth, meteoric fluid infiltration occurred relatively later during deformation and was less intensive but may have caused a deformation mechanism switch in feldspar from primarily brittle fracturing to ductile creep. The associated grain size reduction and/or phase mixing would have significantly weakened the deeper footwall rocks, potentially enhancing strain localization and feeding back to increase strain rates and total deformation within the core complex detachment system.