Field samples of olivine-rich rocks are nearly always serpentinized-commonly to completion- but, paradoxically, their intrinsic porosity and permeability are diminishingly low. Serpentinization reactions occur through a coupled process of fluid infiltration, volumetric expansion, and reaction-driven fracturing. Pores and reactive surface area generated during this process are the primary pathways for fluid infiltration into and reaction with serpentinizing rocks, but the size and distribution of these pores and surface area have not yet been described. Here, we utilize neutron scattering techniques to present the first measurements of the evolution of pore size and specific surface area distribution in partially serpentinized rocks. Samples were obtained from the ca. 2 Ma Atlantis Massif oceanic core complex located offaxis of the Mid-Atlantic Ridge and an olivine-rich outcrop of the ca. 1.1 Ga Duluth Complex of the North American Mid-Continent Rift. Our measurements and analyses demonstrate that serpentine and accessory phases form with their own, inherent porosity, which accommodates the bulk of diffusive fluid flow during serpentinization and thereby permits continued serpentinization after voluminous serpentine minerals fill reaction-generated porosity.
Bibliographical noteFunding Information:
Funding was provided by the U.S. Department of Energy and the U.S. National Science Foundation(NSF)under grants EE0002764 and 1426695, respectively, and a University of Minnesota(UMN)Doctoral Dissertation Fellowship and Geological Society of America research grant to Tutolo. Saar thanks the George and Orpha Gibson Endowment for supporting the Hydrogeology and Geofluids research group at UMN and this project. Jim Miller and John Green are thanked for field assistance. This work utilized samples and data provided by the IODP.(U)SANS instrumentation is supported in part by the NSF under agreement DMR-0944662. Any mention of commercial products is for information only; it does not imply recommendation or endorsement by the National Institute of Standards and Technology. Oliver Pl?mper, Bob Lowell, and Norman Sleep are thanked for their helpful reviews.
© 2016 Geological Society of America.