ASSESSING ALTERATION AND RHEOLOGY OF THE OCEANIC LITHOSPHERE IN THE FIELD: MAGNETIC SUSCEPTIBILITY AND SERPENTINIZATION OF CRITICAL MANTLE-CRUSTAL SECTIONS

The rheology of oceanic lithosphere is profoundly affected throughout its section by water activity. Distinguishing magmatic, metasomatic, metamorphic, tectonic, and weathering hydration signatures can be challenging but offers insights into global plate kinematics, magma petrogenesis, the secular variation of tectonic styles and crustal growth, and biogeochemical cycles of carbon, sulfur, iron, and oxygen. Magnetic Susceptibility (MS) is a rapid (seconds per measurement), field-portable, sensitive (regularly per mil, commonly ppm or better) and quantitative tool in the field geologist’s arsenal with power to address these processes and problems.

Easy utility employing MS is balanced against interpretive complications. While iron spinels dominate the MS signal of all oceanic lithospheric rocks, these minerals are common products of hydrous reactions (broadly, serpentinization) in a variety of settings, and the potential sources of oceanic lithosphere hydration are myriad. These include mineral water of primary magmatic origin; near-axis hydrothermal alteration of the upper crustal section; seawater infiltration into fracture zones and along crustal-scale detachments; and connate water remobilized during subduction. For ophiolitic sections exposed on modern continents, both geologic and meteoric waters might be focused along obduction-related fault surfaces or advected through the bulk section during retrogression. Post-emplacement orogenic metamorphic overprints and even surficial weathering may transpose earlier patterns of hydration suggested by MS mapping.

In order to test the interpretive power and demonstrate the caveats of magnetic susceptibility mapping in ophiolites, we present a detailed field-based study of the Shetlands ophiolite, supported by select sections from other British Isles ophiolites; a petrological “Moho” in the Alpine Insubric Zone; and select units from the Semail ophiolite.