Abstract
Hosted in basaltic substrate on the ultra-slow spreading Mid-Cayman Rise, the Piccard hydrothermal field is the deepest currently known seafloor hot-spring (4957–4987 m). Due to its great depth, the Piccard site is an excellent natural system for investigating the influence of extreme pressure on the formation of submarine vent fluids. To investigate the role of rock composition and deep circulation conditions on fluid chemistry, the abundance and isotopic composition of organic, inorganic, and dissolved volatile species in high temperature vent fluids at Piccard were examined in samples collected in 2012 and 2013.Fluids from the Beebe Vents and Beebe Woods black smokers vent at a maximum temperature of 398 °C at the seafloor, however several lines of evidence derived from inorganic chemistry (Cl, SiO2, Ca, Br, Fe, Cu, Mn) support fluid formation at much higher temperatures in the subsurface. These high temperatures, potentially in excess of 500 °C, are attainable due to the great depth of the system. Our data indicate that a single deep-rooted source fluid feeds high temperature vents across the entire Piccard field. High temperature Piccard fluid H2 abundances (19.9 mM) are even higher than those observed in many ultramafic-influenced systems, such as the Rainbow (16 mM) and the Von Damm hydrothermal fields (18.2 mM). In the case of Piccard, however, these extremely high H2 abundances can be generated from fluid-basalt reaction occurring at very high temperatures.Magmatic and thermogenic sources of carbon in the high temperature black smoker vents are described. Dissolved ΣCO2 is likely of magmatic origin, CH4 may originate from a combination of thermogenic sources and leaching of abiotic CH4 from mineral-hosted fluid inclusions, and CO abundances are at equilibrium with the water–gas shift reaction. Longer-chained n-alkanes (C2H6, C3H8, n-C4H10, i-C4H10) may derive from thermal alteration of dissolved and particulate organic carbon sourced from the original seawater source, entrainment of microbial ecosystems peripheral to high temperature venting, and/or abiotic mantle sources. Dissolved ΣHCOOH in the Beebe Woods fluid is consistent with thermodynamic equilibrium for abiotic production via ΣCO2 reduction with H2 at 354 °C measured temperature. A lack of ΣHCOOH in the relatively higher temperature 398 °C Beebe Vent fluids demonstrates the temperature sensitivity of this equilibrium.Abundant basaltic seafloor outcrops and the axial location of the vent field, along with multiple lines of geochemical evidence, support extremely high temperature fluid-rock reaction with mafic substrate as the dominant control on Piccard fluid chemistry. These results expand the known diversity of vent fluid composition, with implications for supporting microbiological life in both the modern and ancient ocean.
Highlights
Located in the basaltic neovolcanic zone at 4957–4987 m depth on the Mid-Cayman Rise, the Piccard vent field is Earth’s deepest known mid-ocean ridge hydrothermal system (Beaulieu et al, 2013)
Fluids at Beebe Vents 1, 3, and 5 and Beebe Woods sampled in 2012 are characterized by nearly identical endmember concentrations of Cl, SiO2, SO4, Na, Br, Ca, Sr, K, Li, and Rb, suggesting that the high temperature regions of the Piccard vent field are fed by a common source fluid
Vent fluids circulating at Piccard are subject to exceptionally high pressures, relative to all other oceanic ridgecrest hydrothermal systems, due to their setting on the world’s deepest mid-ocean ridge crest, the Mid-Cayman Rise
Summary
Located in the basaltic neovolcanic zone at 4957–4987 m depth on the Mid-Cayman Rise, the Piccard vent field is Earth’s deepest known mid-ocean ridge hydrothermal system (Beaulieu et al, 2013). The chemical diversity expressed in fluids venting from axial hot-springs is reflective of a broad spectrum of physical, chemical, and biological processes that transform the seawater source fluid during convective circulation through the oceanic crust (Von Damm, 1995; Alt, 1995; Von Damm and Lilley, 2004; Butterfield et al, 2004). Due to the much greater axial depth at Piccard, the two-phase boundary at the seafloor occurs at 483 °C (Bischoff and Rosenbauer, 1988) creating the potential for circulating fluids to attain substantially higher temperatures in the oceanic lithosphere relative to hydrothermal fluids from shallower portions of the global mid-ocean ridge system
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