P079-0005
Hydrothermal Alteration and Mineralogy of the Basaltic/Gabbroic Ocean Crust: Insights from Microimaging Spectroscopy of the Oman Drilling Project Cores

Wednesday, 16 December 2020
Poster
Rebecca N Greenberger1, Michelle Harris2, Bethany L Ehlmann3,4, Molly Crotteau1, Peter B Kelemen5, Craig E Manning6, Damon A H Teagle7 and The Oman Drilling Project Science Party, (1)California Institute of Technology, Pasadena, CA, United States, (2)University of Southampton, Southampton, United Kingdom, (3)California Institute of Technology, Division of Geological and Planetary Sciences, Pasadena, CA, United States, (4)NASA Jet Propulsion Laboratory, Pasadena, CA, United States, (5)Lamont Doherty Earth Observatory, Columbia University, Palisades, NY, United States, (6)Univ California Los Angeles, Los Angeles, CA, United States, (7)University of Southampton, School of Ocean & Earth Science, National Oceanography Centre Southampton, Southampton, United Kingdom
Abstract:
The Oman Drilling Project (OmanDP) recovered 3.2 km of core from nine boreholes in the ocean crust and upper mantle of the Oman ophiolite. We scanned the archive half of the core with an imaging spectrometer system measuring reflected light at 0.4-2.6 µm with a spatial resolution of ~250 µm/pixel to determine the distribution of hydrothermal minerals. Combining billions of measurements of mineralogy from infrared spectroscopy with ~100% core recovery, our goal is to characterize hydrothermal alteration and identify cooling mechanisms of the ocean crust, focusing on OmanDP Holes GT1A, GT2A, and GT3A.

Minerals identified via spectroscopy include clinopyroxene, amphibole, chlorite, epidote, prehnite, zeolites, gypsum, and calcite. The sheeted dikes and gabbros of GT3A (upper crust) appear to be the least hydrated (Crotteau et al., this meeting) but also have the least abundant pyroxene. Chlorite, amphibole, and epidote are common, and the position of the ~1.55 µm absorption in epidote indicates that GT3A epidote has the highest Fe3+/Al of all boreholes. Chlorite and zeolite are the most abundant hydrated secondary minerals in GT2A (middle crust; foliated and layered gabbros). Lower crustal fault zones of GT1A have pervasive amphibole throughout the matrix and veins at >150 m depth, and the Mg# of amphibole within the fault zones is among the highest in these drill cores. These fault zones are also some of the most hydrated crust measured (Crotteau et al.). In contrast, amphibole is less abundant spatially and occurs in less hydrated crust at depths <150 m in GT1A and in all of GT2A.

Laboratory measurements for validation are ongoing, though results generally agree with OmanDP core description XRD. If the particular amphibole mineral in the fault zones formed at high temperature, our results would suggest that fault zones play an important role in early convective hydrothermal cooling of the lower ocean crust (e.g., Harris et al., 2017). The relationship between epidote composition and formation conditions is complicated (Bird and Spieler, 2004) but may be related to redox differences in the sheeted dikes versus deeper within the crust. We will present downhole trends in mineralogy and mineral chemistry and discuss the implications for the extent and temperatures of hydrothermal alteration and cooling of the crust.

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