Understanding Alteration of the Ocean Crust through Imaging Spectroscopy Measurements of the Oman Drilling Project Cores

Hydrothermal circulation plays an important rule in cooling the lower ocean crust and in geochemical transfer within the ocean crust and between the crust and ocean. However, it is difficult to study directly as no intact core has been drilled from the lavas to the lower ocean crust and, where core has been drilled, recovery is often low. The ICDP Oman Drilling Project drilled ~1.5 km of core in four boreholes in the ocean crust of the Samail Ophiolite, Oman, with ~100% recovery. We scanned the archive half of all drill core with a micro-imaging spectrometer (0.4-2.55 μm), providing >1 billion spectroscopic measurements of mineralogy at ~250 µm/pixel. Mineral detections are validated with x-ray diffraction from the Oman Drilling Project as well as our own SEM and electron microprobe analyses of samples. Imaging spectroscopy data highlight the importance of fault zones in altering and cooling the lower ocean crust with widespread amphibole and zeolites. The Al and Ti contents of some amphiboles measured on the microprobe are suggest formation temperatures of >600˚C, which suggests early, high temperature fluids. Spectroscopy and microprobe datasets also show higher amphibole Mg# in the lower ocean crust. The detection of zeolites throughout the major fault zones and prehnite and epidote in some provides evidence for their continued role as conduits for fluid circulation as the crust cooled. We also identify differences in the compositions of epidote/clinozoisite and zeolites with depth in the ocean crust with spectroscopy. Zeolites within the sheeted dikes typically have more Na (analcime, thomsonite) than the Ca-rich zeolites of the lower ocean crust (laumontite). Epidote in the sheeted dikes commonly has more Fe(III) than in the lower ocean crust. This provides further evidence for evolution of fluids away from seawater as they react with fluids in the ocean crust. Further work will quantitatively determine the geochemical exchanges and their contributions to seawater.