On the re-dissolution of subsurface hydrothermal deposits at 9°50'N East Pacific Rise: Implications from geochemical studies of high- and low-temperature fluids

Hydrothermal fluids circulated through mid-oceanic ridges provide the necessary chemical energy for biological reactions in the thriving deep biosphere (1). In order to refine hydrothermal flux estimates and determine the implications for macro- and microbiological communities, the processes controlling chemical composition of hydrothermal fluids must first be better understood. Low-temperature hydrothermal fluids, generated when high-temperature fluids mix with seawater during their ascent to the seafloor, have long been considered to be simple dilutions of high-temperature fluids with ambient seawater in the upper oceanic crust (2). However, the degree to which fluids undergo compositional changes due to further reaction in the subsurface prior to their expulsion at the seafloor has not been quantified. Potential processes that alter fluid chemistry may include the re-dissolution of previously precipitated minerals such as anhydrite and metal sulfides. One implication of these and possibly other undocumented mineral reactions is that the hydrothermal flux component of elemental budgets, specifically Ca, Sr, SO4, Fe, Mn, Cu and Zn, must be reinterpreted since these elements transit through the system via temporary sinks. Furthermore, these reactive processes influence trace metal concentrations in low-temperature fluids, thereby affecting the diversity and distribution of macro- and microbiological communities living on or within the seafloor at hydrothermal areas. The Ridge2000 Integrated Study Site at 9°50'N EPR contains an area in which two high-temperature vents (Ty and Io) formed from a low-temperature site (BM82), which remained active, thereby providing a unique opportunity to investigate the simple dilution model. Preliminary time series chemical data for Ty and Io spanning April 1991 through December 2007 support the hypothesis of re-dissolution of metal sulfide deposits. The means of end member Fe and Mn for Ty (4650 μmol/kg in Fe, 1500 μmol/kg in Mn) and Io (5030 μmol/kg in Fe, 1350 μmol/kg in Mn) are higher than the mean of end members for all high-temperature vents at 9-10°N EPR (3530 μmol/kg in Fe, 828 μmol/kg in Mn). In June 2006 Ca, Sr and SO4 data demonstrate that the adjacent low-temperature flow may have been re- dissolving previously precipitated anhydrite. Acquisition of trace Fe, Mn, Cu, and Zn concentrations in the BM82 fluids, as well as in the accompanying Ty and Io filter and dreg digests, are underway via high resolution inductively coupled plasma mass spectrometry. Preliminary data for these metals in the BM82 fluids range from 3.66-53.8 μmol/kg in Mn, 3.55-13.0 μmol/kg in Fe, 70.0-115 nmol/kg in Cu, and 125-935 nmol/kg in Zn. These forthcoming elemental data, coupled with geochemical and mass balance modeling, will further constrain relationships between adjacent high- and low-temperature vent areas. (1) Von Damm and Lilley, AGU Mono., 2004. (2) Edmond et al., Earth Planet Sci. Lett., 1979ab.