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Stable isotope (δ18O, δD, δ11B) ratios of fault surface and shear zone minerals sampled from Marie Byrd Land in the West Antarctic rift system (WARS) provide opportunity to monitor potential fluid transport across multiple levels of the crust during active rifting. In the upper crust, high-angle brittle faults in the southern Ford Ranges display tourmaline-mineralized surfaces at Mt. Douglass, Mt. Dolber, and Lewissohn Nunatak. Tourmaline are strongly aligned with fault striae indicating mineralization during normal-oblique and strike-oblique displacement, with dilatancy allowing fluid infiltration of fault surfaces. Tourmaline’s refractory nature preserves isotopic compositions, which serve as a proxy for fluid sources and water-rock ratios. We compare tourmaline isotopic ratios with those of muscovite and quartz that occupy progressively deeper, kinematically linked fault-shear networks, and high-grade sillimanite-garnet-quartz±biotite associations, with the objective of characterizing potential fluid sources, relative depths of fluid interactions, and eventual estimation of volume of migrated fluids. Tourmaline δ18O values range from 9.1 and 10.4 ± 0.2 ‰ VSMOW (avg.= 9.8 ‰; st.dev. = 0.6), with intrasample reproducibility from 0.9 ‰ to 1.2‰, either as the result of variation in fluid sources or minor fluctuations in temperature during tourmaline formation. Quartz δ18O ratios range from 11.1 to 10.3 ± 0.2 ‰ (avg. =11.0‰; st.dev. = 0.64), with paired ∆Qtz-Tur values lower than quartz calculated to be in equilibrium with tourmaline at 450°C. Calculated qtz-tur temperatures exceed values reasonable for brittle crust (>700°C), indicating tourmaline grew rapidly or quartz has undergone subsolidus reequilibration. Fluids calculated to be in equilibrium with tourmaline at 450°C range from 8.2 to 9.5‰. Tourmaline 40Ar/39Ar geochronology in progress yields Early Cretaceous dates, indicating mineralization coincided with rifting onset. Very rapid development of the WARS and high thermal gradients during ENE- WSW transtension promoted upward movement of fluids in equilibrium with magmatic bodies or dehydrating metamorphic or sedimentary protolith. Tourmaline of Mt. Douglass and Mt. Dolber yield δD values of –60 and –64‰; these values confirm the role of fluids derived from mid crustal sources transported to the upper crust through fault-shear network. 

Brittle faults in the southern Ford Ranges of Marie Byrd Land, West Antarctica, provide unique opportunity to study fluid-rock interactions in the West Antarctic Rift System and the role of crustal fluids during regional-scale faulting. This fault array contains steep, NNW-striking, normal-oblique slip faults and sub-vertical WNW-ESE strike-oblique faults. \ Faults at Mt. Douglass, Mt. Dolber, and Lewissohn Nunatak display strongly aligned tourmaline, indicating syntectonic mineralization; surfaces in one location feature distinctive mirror surfaces, suggestive of formation during seismic slip. Tourmaline has been demonstrated to resist chemical and isotopic re-equilibration during even high-temperature metamorphism, and to maintain a record of conditions during formation, therefore oxygen isotope compositions of tourmaline and quartz pairs may elucidate crustal conditions (e.g. temperatures and fluid-rock ratios) and fluids sources. Analyzed tourmaline and quartz were separated from the upper ~2mm of the fault surfaces; host rocks are tourmaline-free. Tourmaline 18O ratios (n=4) fall within a range of +9.2 to +10.4 ± 0.1 ‰ VSMOW (average 9.7‰, StDev = 0.7). Paired quartz yield 18O values of +11.1 to +10.3 ± 0.1 ‰; ∆Qtz-Trm values between 1.3 and 2.0 may reflect an inability of quartz to equilibrate during tourmaline crystallization. Equilibrium between quartz and tourmaline would suggest temperatures of formation in excess of 550°C. Relative isotopic homogeneity between sites suggests similar fluid conditions were present across the region and supports field evidence for that the structures form a regional fault array. Geometric and kinematic relationships suggest a link to deeper level shears hosting muscovite, and sillimanite with garnet. On-going investigation includes O isotope analyses of these shears, as well as analysis of H and B isotopes in tourmaline, which will better characterize the relationship between the deeper crustal shears with the brittle fault array, and the fluid sources and metasomatic processes of regional fault systems. Furthermore, temporal constraints on tourmaline mineralization will establish whether faulting is associated with Cretaceous intracontinental extension of the West Antarctic rift system (Siddoway 2008) or a crustal response to Neogene mantle delamination beneath the South Pole region (Shen et al 2018). 

Outcrops of brittle faults are rare in Marie Byrd Land, West Antarctica, because fault damage zones commonly undergo enhanced erosion and form bedrock troughs occupied by glacier ice. Where exposures do exist, faults yield information about regional strain in the West Antarctic Rift System (WARS) and may host minerals that contain a record of the temperature and chemistry of fluids during regional-scale faulting. In MBL’s southern Ford Ranges, bordering Ross Sea, a distinctive fault array was sampled that hosts tourmaline and quartz, a mineral-pair that can provide temperature and composition of fault-associated fluids, using 18O. Host rocks are tourmaline-free. At three separate sites, fault surfaces display strongly aligned tourmaline, suggesting that mineralization occurred during tectonism. One site features highly polished, or mirrored, surfaces, a characteristic that may indicate tourmaline precipitation during seismic slip. The orientation and kinematics of the high angle faults are NNW-striking: normal-slip, and WNW-ESE striking: right-lateral strike-slip. The timing of mineralization is yet to be determined, but viable possibilities are that the faults formed during broad intracontinental extension during formation of Ross Embayment in the Cretaceous, or during development of deep, narrow basins beneath the RIS grounding zone, in the Neogene (newly detected, see Tankersley et al., this meeting). Once formed, tourmaline is resistant to chemical and isotopic re-equilibration, and therefore can retain a record of its conditions during formation. We used oxygen isotope compositions of tourmaline and quartz pairs to investigate temperatures, fluid-rock ratios, and fluid sources, with bearing on fault-localized flux of fluids and geothermal heat. Analyzed tourmaline and quartz were separated from the upper ~2mm of the fault surfaces, as well as quartz separated from host rock in the same hand samples. Tourmaline 18O ratios (n=4) fall within a range of +9.2 to +10.4 ± 0.1 ‰ VSMOW (average 9.7‰, StDev = 0.7). Paired quartz yield 18O values of +11.1 to +10.3 ± 0.1 ‰. Relative isotopic homogeneity between sites suggests similar fluid conditions were present across the region and supports field evidence for that the structures form a regional fault array. ∆Qtz-Trm values fall between 1.3 and 2.0, and 18O of quartz in faults closely resembles 18O of host rock quartz. We tentatively determine the water oxygen isotope ratio as greater than ~7.7 ‰. Plutonic-metamorphic associations in the immediate region, and comparisons with similar faults elsewhere (i.e. Isola d’Elba, Italy), suggest temperatures as high as 500°C for the fluids that circulated into the faults. The data are interpreted to show that brittle faults provided pathways for hot fluids derived from mid-crustal processes to make their way to shallow crustal depths. 18O values indicate magmatic and/or metamorphic fluid sources, with minor to no introduction of meteoric fluids. Tourmaline-quartz pairs did not attain equilibrium, likely due to tourmaline’s rapid crystallization. On-going investigation includes analysis of H and B isotopes in tourmaline, which will better characterize the relationship between fault-hosted and mid-crustal fluids.