Search

The Hurricane fault is a ∼250 km long, west-dipping, segmented normal fault zone located along the transition between the Colorado Plateau and the Basin and Range tectonic provinces in the western USA. Extensive evidence of fault–fluid interaction includes calcite mineralization and veining. Calcite vein carbon (δ13 C VPDB) and oxygen (δ18 O VPDB) stable isotope ratios range from -4.5 ‰ to 3.8 ‰ and from -22.1 ‰ to -1.1 ‰, respectively. Fluid inclusion microthermometry constrains paleofluid temperatures and salinities from 45 to 160 ∘ C and from 1.4 wt % to 11.0 wt % as NaCl, respectively. These data suggest mixing between two primary fluid sources, including infiltrating meteoric water (70±10 ∘ C, ∼1.5 wt % NaCl, δ18 O VSMOW ∼-10 ‰) and sedimentary brine (100±25 ∘ C, ∼11 wt % NaCl, δ18 O VSMOW ∼ 5 ‰). Interpreted carbon sources include crustal- or magmatic-derived CO 2 , carbonate bedrock, and hydrocarbons. Uranium–thorium (U–Th) dates from five calcite vein samples indicate punctuated fluid flow and fracture healing at 539±10.8 (1σ), 287.9±5.8 , 86.2±1.7 , and 86.0±0.2 ka in the upper 500 m of the crust. Collectively, data predominantly from the footwall damage zone imply that the Hurricane fault imparts a strong influence on the regional flow of crustal fluids and that the formation of veins in the shallow parts of the fault damage zone has important implications for the evolution of fault strength and permeability. [ABSTRACT FROM AUTHOR]

The Hurricane Fault is a ~250-km-long, west-dipping normal fault located along the transition between the Colorado Plateau and Basin and Range tectonic provinces in the western U.S. Extensive evidence of fluid-fault interaction, including calcite mineralization and veining, occur in the footwall damage zone. Calcite vein carbon (δ13CVPDB) and oxygen (δ18OVPDB) stable isotope ratios range from -4.5 to 3.8 ‰ and -22.1 to -1.1 ‰, respectively. Fluid inclusion microthermometry constrain paleofluid temperatures and salinities from 45-160 °C and 1.4-11.0 wt % as NaCl, respectively. These data identify mixing between two primary fluid sources including infiltrating meteoric groundwater (70 ± 10 °C, ~1.5 wt % NaCl, δ18OSMOW ~-10 ‰) and sedimentary brine (100 ± 25 °C, ~11 wt % NaCl, δ18OSMOW ~5 ‰). Interpreted carbon sources include crustal- or magmatic-derived CO2, carbonate bedrock, and hydrocarbons. U-Th dates from 5 calcite vein samples indicates punctuated fluid-flow and fracture healing at 539 ± 10.8, 287.9 ± 5.8, 86.2 ± 1.7, and 86.0 ± 0.2 ka in the upper 300 m of the crust. Collectively, the data imply that the Hurricane Fault imparts a strong influence on regional flow of crustal fluids, and that the formation of veins in the shallow parts of the fault damage zone has important implications for the evolution of fault strength and permeability. [ABSTRACT FROM AUTHOR]

Gas-rich hot springs throughout the Peruvian Andes contain a surprising contribution of mantle and crustal volatiles (CO 2 and N 2) despite being located along a volcanic gap associated with modern flat-slab subduction. Similar mantle and crustal volatile degassing is observed in springs from the backarc region of the northern Altiplano Plateau which experienced flat-slab subduction in the Oligocene. We constrain the sources of deeply-derived volatiles using C, N, O and H stable isotope analyses alongside previously reported helium isotope (3He/4He) and gas abundance data. δ18O and δ2H values of spring waters are consistent with regional meteoric water, with deviations towards higher values due to fluid mixing and high-temperature fluid-rock interaction. δ15N values of N 2 gas range from −0.5 to +4.4‰ (vs AIR), indicating contributions of mantle and crustal (sedimentary) nitrogen. δ13C values of dissolved inorganic carbon (DIC) and CO 2 gas from bubbling springs are used to model initial δ13C values before degassing, with resulting values ranging from −13.6 to −0.3‰ (vs VPDB) and an average value of −6.9‰. DIC concentrations range from 1 to 57 mmol/kg, of which carbonate dissolution accounts for only a small fraction (average of 15%). The remaining carbon (1 to 44 mmol/kg) is derived from a mixture of deep CO 2 sources with endmember δ13C values of −7‰ and −14‰ which, in light of relationships with 3He/4He ratios, are interpreted to be the subcontinental lithospheric mantle (SCLM) and metamorphic crustal carbon, respectively. These results are consistent with previous interpretations that slab-to-lithosphere fluid transfer is mobilizing volatiles from the SCLM to the continental crust above the flat slab and confirms that mantle CO 2 is migrating with mantle helium. Metamorphic CO 2 and N 2 are mobilized from the continental crust and entrained by mantle fluids on their way to the surface. In the backarc region, similar geochemical patterns suggest that volatiles are released by dehydration and/or partial melting of previously hydrated lithosphere. We estimate a CO 2 flux of ~3 × 108 mol yr−1 (34 t d−1) from 45 springs with available discharge values. If scaled to all reported thermal springs in Peru, then the total CO 2 emissions from Peruvian thermal springs may approach 2 × 109 mol yr−1 (203 t d−1), or ~ 0.2% of global emissions from subaerial volcanism. Regional CO 2 emissions may be 10 to 100 times greater when considering CO 2 diffusely lost along fault zones or temporarily dissolved near-surface aquifers. These results demonstrate that flat-slab subduction leads to an efficient and unexpected transfer of mantle and crustal volatiles to Earth's surface, and more generally, that deep volatile fluxes in subduction zones are not limited to active volcanism. • Mantle and crustal degassing observed in the volcanic gap above a modern flat slab. • CO 2 and N 2 in hot springs show isotopic evidence for mantle and crustal sources. • Point source CO 2 flux from Peruvian hot springs is estimated to be 2 × 109 mol yr−1. • Mantle volatiles are mobilized by metasomatism above the dehydrating Nazca slab. • Dehydration and/or melting facilitates degassing of modern backarc lithosphere. [ABSTRACT FROM AUTHOR]