Alkaline Fluids and Reduced Volatiles in Modern Serpentinizing Environments
Abstract
The alteration of mantle rocks during serpentinization is a fundamental process that has significant geochemical and biological importance in marine systems, subduction zone processes, and present-day weathering of mantle rocks on land. Here we present a comparative geochemical and isotopic study of alkaline fluids and carbonate precipitates at Lost City (Atlantis Massif, Mid-Atlantic Ridge) with high alkaline, Ca-OH springs and carbonate deposits associated with present-day serpentinization in peridotites of Liguria (Northern Italy). The low temperature hydrothermal system at Lost City is characterized by carbonate-brucite structures that are deposited from high pH fluids (9-11) with elevated hydrogen and methane contents resulting from serpentinization processes at depth (Kelley et al., 2005). In Liguria, fluids originating from deep aquifers circulate through serpentinites of the Gruppo di Voltri and produce Mg-rich to Ca- or Na-rich, alkaline waters (Bruni et al., 2002; Cipolli et al., 2004). The Ca-OH springs in Liguria are characterized by pH of 11-12, high Ca concentrations, varying sulfur-contents and negligible bicarbonate or carbonate concentrations. Dissolved methane concentrations measured in 2009 vary from 10 to 723 μmol/l, whereas hydrogen contents are relatively low. The concentrations of methane have remained constant for the past 10 years, which suggests relatively stable rates of serpentinization. The Ca-OH springs are less abundant than Mg-bicarbonate-springs, but are significant in terms of methane fluxes and the implied potential for CO2 uptake from the atmosphere. Similar to the Lost City system, methane in the Ligurian springs is enriched in 13C and points to an origin derived from mantle carbon. Here we discuss the links between the inorganic reactions during serpentinization, cycling of carbon and sulfur, and microbial activity in these high pH systems. The production of reduced volatiles and sulfur during serpentinization is an important process in sustaining diverse microbial communities in hot/cold spring environments devoid of volcanic/magmatic activity and in the subsurface - and thus has important consequences for the existence of a deep hydrogen-based biosphere in marine and terrestrial settings. References: Cipolli et al., 2004, Appl. Geochem., 19: 787-802; Bruni et al., 2002, Appl. Geochem., 17:455-474; Kelley et al., 2005, Science, 307(5714): 1428-1434.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2009
- Bibcode:
- 2009AGUFM.V43B2256F
- Keywords:
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- 0428 BIOGEOSCIENCES / Carbon cycling;
- 1030 GEOCHEMISTRY / Geochemical cycles;
- 3017 MARINE GEOLOGY AND GEOPHYSICS / Hydrothermal systems;
- 3035 MARINE GEOLOGY AND GEOPHYSICS / Midocean ridge processes