It is generally assumed that the seawater-derived fluids that feed black smoker vent fields on the seafloor are discharged vertically from depths of similar to 1-3 km. We present new oxygen and strontium isotope data that show that fluids at black smoker temperatures of 300-400 degrees C were focused along a low-angle detachment fault at 15 degrees 45'N near the Mid-Atlantic Ridge. Isotopic alteration is the most extreme ever reported from oceanic rocks altered at similar temperatures, indicating intensely focused fluid flow both in recharge and discharge parts of the hydrothermal system. Rare earth element mobility in the fault rocks demonstrates isotopic alteration by evolved hydrothermal fluids, not conductively heated seawater. The fault zone protolith was predominantly ultramafic, but also included mafic rocks, with metasomatic alteration to talc-tremolite-chlorite schists resulting mainly from chemical exchange between these lithologies during fluid flow. Fluids in equilibrium with this assemblage would be similar to ultramafic-hosted black smoker fluids. We present a new model in which hydrothermal circulation around detachment faults evolves from basalt hosted (TAG type), to footwall ultramafic hosted (Rainbow type), to low-temperature ultramafic hosted (Lost City type). Key features of our model are the intrusion of gabbro bodies immediately below the detachment to provide a heat source for circulation, and focusing of fluid flow into the detachment fault to allow venting away from the neovolcanic axis.
GeologyISI Document Delivery No.: 219RRTimes Cited: 0Cited Reference Count: 33Cited References:ALT JC, 1996, P ODP SCI RESULTS, V148, P57BACH W, 1996, P ODP SCI RESULTS, V148, P39BICKLE MJ, 1992, EARTH PLANET SC LETT, V113, P219BOSCHI C, 2006, GEOCHEM GEOPHY GEOSY, V7, ARTN Q01004CANN JR, 1997, NATURE, V385, P329CHACKO T, 2004, REV MINERAL GEOCHEM, V43, P1COLE DR, 1999, GEOCHIM COSMOCHIM AC, V63, P449DEMARTIN BJ, 2007, GEOLOGY, P711, DOI 10.1130/G23718A.1DOUGLASS J, 2000, CHEM GEOL, V163, P1DOUVILLE E, 2002, CHEM GEOL, V184, P37ESCARTIN J, 2003, GEOCHEM GEOPHY GEOSY, V4, ARTN 1067FRUHGREEN GL, 2003, SCIENCE, V301, P495GERMAN CR, 2004, GEOPH MONOG SERIES, V148, P1, DOI 10.1029/148GM01GRACIA E, 2000, EARTH PLANET SC LETT, V177, P89HART SR, 1999, GEOCHIM COSMOCHIM AC, V63, P4059HUMPHRIS SE, 1998, P ODP SCI RESULTS, V158, P255HUMPHRIS SE, 2000, J GEOPHYS RES-SOL EA, V105, P28477IIDEFONSE B, 2007, GEOLOGY, V35, P623KELEMEN PB, 2004, P OC DRILL PROGR IN, V209MACLEOD CJ, 2002, GEOLOGY, V30, P879MCCAIG AM, 1997, GEOCHEMISTRY VOLATIL, P227SAVIN SM, 1988, REV MINERAL, V19, P189SCHIFFMAN P, 1988, J GEOPHYS RES, V93, P4612SCHROEDER T, 2004, GEOCHEM GEOPHY GEOSY, V5, ARTN Q11007SEYFRIED WE, 2004, GEOPH MONOG SERIES, V148, P267, DOI 10.1029/148GM11SHANKS WC, 2001, REV MINERAL GEOCHEM, V43, P469SINHA MC, 2004, GEOPH MONOG SERIES, V148, P19, DOI 10.1029/148GM02SMITH DK, 2006, NATURE, V442, P440, DOI 10.1038/nature04950TEAGLE DAH, 1998, CHEM GEOL, V149, P1TEAGLE DAH, 2003, EARTH PLANET SC LETT, V210, P81, DOI10.1016/S0012-821X(03)00126-2TEAGLE DAH, 2004, ECON GEOL BULL SOC, V99, P561TIVEY MA, 2003, J GEOPHYS RES-SOL EA, V108, ARTN 2277TUCHOLKE BE, 1998, J GEOPHYS RES-SOL EA, V103, P9857