The reaction of seawater with exposed ultramafic mantle and lower crustal rocks at slow and ultra-slow spreading mid-ocean ridges (a process called serpentinization) has important implications for the chemistry of the oceans, as well as the mechanical and magnetic properties of the seafloor. Unraveling the geochemical consequences of these reactions is key to furthering our understanding of global geochemical cycles, particularly of water and carbon dioxide. Serpentinization is associated with strongly reducing conditions that lead to the generation of free hydrogen gas, and fluids with a low pH; and the Hydrogen and methane released during serpentinization support microbial communities that form the base of the food web in unique and complex ecosystems. Despite the importance of the serpentinization process in changing the physical and chemical nature of the oceanic lithosphere, the reaction pathways of serpentinization are poorly understood and much more complicated than previously anticipated. They are chiefly controlled by the temperature of alteration, fluid flux, and initial rock composition, and hence are highly variable. This research involves a comparative analytical and modeling study of the mineralogy and geochemistry of serpentinized peridotites from different deep-sea environments, including mid-ocean ridges, continental rifted margins, and the forearc mantle of subduction zones. A combination of traditional microprobe and cutting edge synchrotron techniques will be used on Ocean Drilling Program drill cores to determine the textural associations and chemical compositions of secondary minerals. Results with be compared with thermodynamic models to gain deeper insights into the formation conditions and reaction pathways during serpentinization. Broader impacts of the work include support of an early career researcher with no prior NSF support and train two undergraduate students from Bridgewater State College in Massachusetts. It also involves use of national lab facilities at the Advanced Light Source at Lawrence Berkeley Lab in California.
In recent years, it has been suggested that the reaction of mantle rocks with seawater, a process referred to as ‘serpentinization’, has direct implications for the potential of life in the deep sub-seafloor. This is mainly because hydrogen, a byproduct of serpentinization, is liberated in substantial amounts. However, petrophysical data reveal that serpentinization pathways are incredibly variable, and their influence on hydrogen generation remains poorly understood. To gain deeper insights into these reaction pathways, we have examined various serpentinization systems in a range of seafloor settings along active and passive plate margins. Moreover, we have used thermodynamic constraints to develop conceptual models for serpentinization of a wide range of ferromagnesian rock types, including ones that were present on early Earth, Fe-rich varieties present on Mars and Ceres, and mantle rocks exposed at the seafloor. Our results suggest that serpentinization reactions are influenced by changes in temperature, rock composition, and by the amount of water available. One of the most remarkable features of serpentinization, however, is the formation of reducing fluids strongly enriched in hydrogen. Our results highlight the key role the distribution of iron among secondary minerals plays in controlling hydrogen concentrations during serpentinization. We have demonstrated that hydrogen generation is most efficient when magnetite (iron oxide) forms at temperatures between ca. 330°C for serpentinization of mantle rocks to less than 180°C for serpentinization of Fe-rich equivalents, such as in meteorites. Spectroscopic, magnetic, thermodynamic, and isotopic constraints also indicate that Fe(III)-rich serpentine can generate abundant hydrogen sufficient to stabilize native Ni-Fe within the temperature limits of life without making hardly any magnetite. The results of this study suggest that serpentinization of Fe-bearing olivine, the main constituent of mantle rock, is thermodynamically feasible and will result in the formation of hydrogen virtually everywhere olivine and water interact – from marine to continental settings, and possibly on other planets like Mars. Four students (two female, one male, and one male with a learning disability) have been involved in this project. Results of this project were presented at the Frontiers in Earth Surface System Interactions meeting (Yale University), during the Goldschmidt Conferences in 2011 (Prague) and 2012 (Montreal), at the Geological and Environmental Science Department Seminar at Stanford University, at the AGU 2011 Fall Meeting (San Francisco), and the EGU 2013 conference (Vienna).
