Carbon and Water Isotopic Compositions in Altered Oceanic Crust from the East Pacific Rise, ODP/IODP Hole 1256D: Implications for Volatile Budget and Speciation

The chemical composition of the oceanic crust is largely modified by interaction with seawater. During this alteration, the crust incorporates a significant amount of volatiles, mainly carbon and water, ultimately transferred to the mantle through subduction zones. While water bearing phases and fluxes are relatively well known, large uncertainties remain for carbon, and particularly whether condensed carbon-rich phases form during seawater-crust interactions. The aim of this study is to provide new insights into these interactions and try to estimate fluxes of carbon transferred from the seawater into the crust as well as the carbon sources in altered basalts. For this purpose we analyzed 19 samples (altered basalts, dolerites, and gabbros) cored in ODP/IODP Hole 1256D along a vertical profile ranging in depth from 296 down to 1490 mbsf. The step-heating extraction technique has been used to extract CO2 and water from potential various sources. The gases were analyzed by mass spectrometry for isotopic composition of carbon and hydrogen. Total water and CO2 contents vary from 0.32 to 2.14 wt% and from 564 to 2823 ppm, respectively; with δ13C values from -26.6 to -14.9‰ and δD values from -64.2 to -25.1‰. Compared with fresh MORB, both water and CO2 contents are significantly enriched, thus confirming previous conclusions that altered oceanic crust acts as a sink for these elements. δD variations are mainly controlled by temperature and equilibrium between hydrothermal fluids and secondary minerals. δ13C values could reflect variable contributions of carbonate and organic carbon. The very negative values suggest that the organic component is dominant for most samples. This is the first time that such carbon is identified in altered upper oceanic crust thus requiring a reassessment of global carbon budget and exchanges between mantle and surface reservoir.