Metal Transport in Hydrothermal Vent Fluids Across an Eruption: 9°46'-9°52'N East Pacific Rise

Elements carried in hydrothermal fluids circulating within the oceanic crust constitute an important component of marine geochemical budgets and a significant mechanism for supporting unique chemosynthetic ecosystems along ridge axes. Temporal variabilities in hydrothermal fluid compositions are well documented and linked with proximity to magmatic activity [1]. In fast spreading ridges, such as the East Pacific Rise, eruptions can change hydrothermal fluid pathways, significantly and temporarily influencing metal transport if phase separation increases. Metal transport in fluids may be enhanced as well if hydrothermal fluid circulation occurs through freshly emplaced basalt. A well documented magmatic eruption in late 2005/early 2006 disrupted the hydrothermal system and offered the ideal opportunity to study how metal transport would be impacted and how long it would take for the hydrothermal metal transport to return to pre-eruptive rates. Accordingly, we have carried out analyses of transition metals across the eruptive cycle on a time series of fluids collected from 10 hydrothermal vent sites spanning ~8 km of ridge segment trending north-south along the axial summit trough. Hydrothermal fluids were sampled from the same vents in November 2004 (pre-eruptive) and then (post-eruptive) June 2006, November 2006, and December 2007. Analyses of Mn, Fe, Cu and Zn were carried out for all three hydrothermal fluid fractions: dissolved via flame atomic adsorption, filtered particulates and 'dregs' via HR-ICP-MS. Resulting data, coupled with existing data (exit temperature, major elemental, and modeled peak pressure and temperature), allow for the identification of the key factors influencing metal abundance in high temperature fluids. Briefly, phase separation was the most significant process influencing metal abundance in dissolved fluids. Exceptions to this generalization were for the immediate post-eruptive fluids issued from BioVent, the northernmost of the studied vents whose metal contents are also likely controlled by extensive water-rock interactions (as indicated by elevated H₂S contents). Along axis variations in chloride contents generally track in accordance with proximity to areas of greatest (or most recent) magma emplacement. Accordingly, along-axis variations in dissolved-fraction metal abundances were such that the highest dissolved loads were in areas significantly less impacted by the eruption (i.e. areas with minimal or no magmatic emplacement). As with chloride contents, the metal abundances in the dissolved fluids seemed to return to pre-eruptive levels within ~2 years following the eruption. [1] Von Damm, K. L. (2004), Geophys. Monogr. Ser.,148, pp. 285-304, AGU, Washington, DC.