Characterizing Hydration of the Basaltic/Gabbroic Oceanic Crust with Microimaging Spectroscopy of ICDP Oman Drilling Project Cores

Abstract:

The ICDP Oman Drilling Project cored 3.2 km of rock from nine boreholes within the Oman ophiolite. Our work examines the basaltic/gabbroic core sections to understand hydrothermal fluid penetration into the ocean crust and the extent of fluid-rock interaction. In particular, we are examining the spatial patterns of hydration and mineralization to distinguish between the gabbro glacier and sheeted sill models for formation and cooling of the crust. The archive half of the core was scanned with a visible-shortwave infrared imaging spectrometer at spatial resolution ~250 μm/pixel and wavelengths 0.4 – 2.6 μm.

To track hydration, we use the depth of the 1.9-μm absorption with 162 independent geochemical measurements of loss on ignition (LOI), both proxies for H₂O, to classify H₂O values in the core into low (<1.7 ± 0.9%), medium (2.6 ± 1.3%), and high (>5.4 ± 1.2%) groups. Initial work shows that borehole GT2 (foliated, layered gabbros) exhibits the highest degrees of hydration, with ~11% of this borehole classified in the high hydration group, ~45% medium, and ~44% low; it also exhibits the highest mean LOI values. Deeper borehole GT1 (layered gabbros) showed lower hydration values (11% high hydration, 66% low hydration). Hydration values are lowest in shallowest borehole GT3 (sheeted dykes, vari-textured gabbros), with 77% having 1.9 μm depths classified as low hydration. Additionally, we compared hydration values to the core mineralogy and found that samples in the high hydration group contain alteration minerals, e.g. chlorite, prehnite, and zeolite, and lack pyroxene, indicating that higher hydration values correlate with minerals indicative of hydrothermal alteration. Collectively, these patterns suggest hydrothermal alteration is producing higher volumes of minerals with >3% H₂O (i.e. chlorite, zeolite) in GT2. We will present hydration and alteration trends in the full length of the core, with case studies of areas of varying 1.9 μm depth and mineralogy. Beyond advancing our understanding of ocean crust formation on Earth, similar rocks have been found on Mars, and the Mars 2020 rover will have a spectrometer covering most of this wavelength range. Lessons learned in deciphering hydration and alteration mineral assemblages in mafic lithologies are relevant to planetary exploration.