Abstract
Processes for formation, cooling, and altering Earth's ocean crust are not yet completely understood due to challenges in access and sampling. Here, we use contiguous micro‐imaging infrared spectroscopy to develop complete‐core maps of mineral occurrence and investigate spatial patterns in the hydrothermal alteration of 1.2 km of oceanic crust recovered from Oman Drilling Project Holes GT1A, GT2A, and GT3A drilled in the Samail Ophiolite, Oman. The imaging spectrometer shortwave infrared sensor measured reflectance of light at wavelengths 1.0–2.6 μm at 250–260 μm/pixel, resulting in >1 billion independent measurements. We map distributions of nine key primary and secondary minerals/mineral groups—clinopyroxene, amphibole, calcite, chlorite, epidote, gypsum, kaolinite/montmorillonite, prehnite, and zeolite—and find differences in their spatial occurrences and pervasiveness. Accuracy of spectral mapping of occurrence is 68%–100%, established using X‐ray diffraction measurements from the core description. The sheeted dikes and gabbros of upper oceanic crust Hole GT3A show more pervasive alteration and alteration dominated by chlorite, amphibole, and epidote. The foliated/layered gabbros of GT2A from intermediate crustal depths have similarly widespread chlorite but more zeolite and little amphibole and epidote. The layered gabbros of the lower oceanic crust (GT1A) have remnant pyroxene and 2X less chlorite, but alteration is extensive within and surrounding major fault zones with widespread occurrences of amphibole. The results indicate greater distribution of higher temperature alteration minerals in the upper oceanic crust relative to deeper gabbros and highlight the importance of fault zones in hydrothermal convection in the lower ocean crust.
Highlights
The ocean crust comprises the majority of Earth’s crust, yet significant questions remain in our understanding of how this basaltic and gabbroic ocean crust forms and the extent and distribution of chemical alteration from reactions with seawater
We used micro-imaging spectroscopy of 1.2 km of this core to systematically acquire more than 1 billion measurements of mineralogy of the sheeted dikes and dike-gabbro transition in the upper oceanic crust
GT3A), the foliated to layered gabbros at intermediate depth in the oceanic crust (Hole GT2A), and the layered gabbros with major fault zones in the lower oceanic crust (Hole GT1A)
Summary
The ocean crust comprises the majority of Earth’s crust, yet significant questions remain in our understanding of how this basaltic and gabbroic ocean crust forms and the extent and distribution of chemical alteration from reactions with seawater. One challenge is the difficulty in collecting representative samples of the heterogeneous ocean crust. A second means of access is collection from ophiolites, sub-aerially exposed ancient blocks where ocean crust has been obducted onto land. Ophiolites mostly form in supra-subduction zone settings and results might not be directly applicable to processes occurring in the major ocean basins, the broad thermal structure is comparable. Outstanding questions remain about how the ocean crust formed and cooled
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