Mantle-derived helium in hot springs of the cordillera Blanca, Peru: Implications for mantle-to-crust fluid transfer in a flat-slab subduction setting, ABSTRACT Fault-controlled hot springs in the Cordillera Blanca, Peru provide geochemical evidence of mantle-derived fluids in a modern flat-slab subduction setting. The Cordillera Blanca is a ~200 km-long mountain range contains the highest peaks in the Peruvian Andes, located in an amagmatic reach of the Andean arc. The Cordillera Blanca detachment defines the southwestern edge of the range and records a progression of top-down-to-the-west ductile shear to brittle normal faulting since ~5 Ma. Hot springs, recording temperatures up to 78 °C, issue along this fault zone and are CO 2 -rich, near neutral, alkaline-chloride to alkaline-carbonate waters, with elevated trace metal contents including arsenic (≤11 ppm). Water  18 O SMOW (-14.2 to -4.9 ‰) and D SMOW (-106.2 to -74.3 ‰), trends in elemental chemistry, and cation geothermometry collectively demonstrate mixing of hot (200-260°C) saline fluid with cold meteoric water along the fault. Helium isotope ratios ( 3 He/ 4 He) for dissolved gases in the waters range from 0.62 to 1.98 R A (where R A = air 3 He/ 4 He), indicating the presence of up to 25% mantle-derived helium.Given the long duration since and large distance to active magmatism in the region, and the possible presence of a tear in the flat slab south of the Cordillera Blanca, we suggest that mantle helium may originate from asthenosphere entering the slab tear, or from the continental mantlelithosphere, mobilized by metasomatic fluids derived from slab dehydration.

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Strain localization across the brittle-ductile transition is a fundamental process in accommodating tectonic movement in the mid-crust. The tectonically active Cordillera Blanca shear zone (CBSZ), a ∼200-km-long normal-sense shear zone situated within the footwall of a discrete syn-convergent extensional fault in the Peruvian Andes, is an excellent field laboratory to explore this transition. Field and microscopic observations indicate consistent top-down-to-the-southwest sense of shear and a sequence of tectonites ranging from undeformed granodiorite through mylonite and ultimately fault breccia along the detachment.Using microstructural analysis, two-feldspar and Ti-in-quartz (TitaniQ) thermometry, recrystallized quartz paleopiezometry, and analysis of quartz crystallographic preferred orientations, we evaluate the deformation conditions and mechanisms in quartz and feldspar across the CBSZ. Deformation temperatures derived from asymmetric strain-induced myrmekite in a subset of tectonite samples are 410 ± 30 to 470 ± 36 °C, consistent with TitaniQ temperatures of 450 ± 60 to 490 ± 33 °C and temperatures >400 °C estimated from microstructural criteria. Brittle fabrics overprint ductile fabrics within ∼150 m of the detachment that indicate that deformation continued to lower-temperature (∼280–400 °C) and/or higher-strain-rate conditions prior to the onset of pervasive brittle deformation. Initial deformation occurred via high-temperature fracturing and dissolution-precipitation in feldspar. Continued subsolidus deformation resulted in either layering of mylonites into monophase quartz and fine-grained polyphase domains oriented subparallel to macroscopic foliation or the interconnection of recrystallized quartz networks oriented obliquely to macroscopic foliation. The transition to quartz-controlled rheology occurred at temperatures near ∼500 °C and at a differential stress of ∼16.5 MPa. Deformation within the CBSZ occurred predominantly above ∼400 °C and at stresses up to ∼71.4 MPa prior to the onset of brittle deformation.

Radiotherapy and chemotherapy are effective cancer treatments due to their ability to generate DNA damage. The major lethal lesion is the DNA double-strand break (DSB). Human cells predominantly repair DSBs by non-homologous end joining (NHEJ), which requires Ku70, Ku80, DNA-PKcs, DNA ligase IV and accessory proteins. Repair is initiated by the binding of the Ku heterodimer at the ends of the DSB and this recruits DNA-PKcs, which initiates damage signaling and functions in repair. NHEJ also exists in certain types of bacteria that have dormant phases in their life cycle. The Mycobacterium tuberculosis Ku (Mt-Ku) resembles the DNA-binding domain of human Ku but does not have the N- and C-terminal domains of Ku70/80 that have been implicated in binding mammalian NHEJ repair proteins. The aim of this work was to determine whether Mt-Ku could be used as a tool to bind DSBs in mammalian cells and sensitize cells to DNA damage. We generated a fusion protein (KuEnls) of Mt-Ku, EGFP and a nuclear localization signal that is able to perform bacterial NHEJ and hence bind DSBs. Using transient transfection, we demonstrated that KuEnls is able to bind laser damage in the nucleus of Ku80-deficient cells within 10 sec and remains bound for up to 2 h. The Mt-Ku fusion protein was over-expressed in U2OS cells and this increased the sensitivity of the cells to bleomycin sulfate. Hydrogen peroxide and UV radiation do not predominantly produce DSBs and there was little or no change in sensitivity to these agents. Since in vitro studies were unable to detect binding of Mt-Ku to DNA-PKcs or human Ku70/80, this work suggests that KuEnls sensitizes cells by binding DSBs, preventing human NHEJ. This study indicates that blocking or decreasing the binding of human Ku to DSBs could be a method for enhancing existing cancer treatments.

The Cordillera Blanca detachment in the highest elevations of the Peruvian Andes has been accommodating synconvergent extension since the late Miocene. Stable isotope analysis of synkinematic mica from its exhumed footwall shear zone provides new constraints on deep meteoric-hydrothermal circulation during ductile deformation and regional paleoelevation. Muscovite and biotite that deformed and/or grew synkinematically in the shear zone have δ2H values of −131‰ to −58‰ and −149‰ to −98‰ (versus Vienna standard mean ocean water, VSMOW), respectively. The δ2H value difference between coexisting muscovite and biotite is consistent with equilibrium fractionation of the same fluid at the same temperature. Calculated δ2H values of water (−107‰ to −78‰) in equilibrium with these micas are indistinguishable from those of present-day, deeply circulated (9–11 km) hot spring waters emanating from the fault. Such low-δ2H fluids indicate circulation of meteoric water to the depths of the brittle-ductile transition that cannot be explained by other mechanisms. Average recharge paleoelevation for water entering the shear zone based on hydrogen isotopes was 3400 + 500/–700 m (1σ). This is near, but ~500 m below, the present-day mean elevation of the catchments feeding modern hot springs of 3965 ± 880 m, and ~700 m below the 4200 + 700/–900 m mean recharge elevation derived from δ2H values of modern surface and thermal water. The consistency between modern and ancient fault-assisted hydrothermal systems and elevation suggests that high topography, steep relief, and meteoric-hydrothermal circulation have persisted throughout the history of the Cordillera Blanca detachment system.