Alteration of Magnetic Minerals in 2017 Basaltic Tuff Drill Cores from Surtsey Volcano, Iceland, 50 Years After Eruption

Abstract:

Oceanic basalt is one of our principal archives for studying the Earth’s magnetic field and the magnetic properties of these rocks are a keystone of plate tectonic theory. Low-temperature oxidation (maghemitization) is a ubiquitous alteration mechanism that can change and even corrupt the magnetic properties of remanence carrying iron oxides in oceanic basalt. This study investigates drill cores acquired from Surtsey volcano, Iceland, by the ICDP SUSTAIN drilling project in 2017, 50 years after eruptions terminated. The tempo of natural magnetic mineral alteration in the low-temperature hydrothermal system (25–142 °C) is situated within the context of volcanic glass alteration in seven structural and alteration zones that span the subaerial, submarine, and sub-seafloor deposits of the Surtur vent at Surtsey volcano. Thin section scans show alteration features of the basaltic tuff; low-temperature remanence behavior and the temperature dependence of magnetic susceptibility above room temperature characterize the composition of the magnetic remanence carriers; and first-order reversal curve diagrams diagnose the magnetic domain state. Near the cool pre-eruptive seafloor at 181 m below surface, in a zone of least alteration, uniaxial and cubic stable single domain magnetite and maghemite grains within fresh-to-very weakly-altered volcanic glass pyroclasts are the principal remanence carriers. Elsewhere, vesicular glass pyroclasts with palagonitized rinds and pervasive development of nano-crystalline clay mineral and zeolites indicate higher rates of alteration. The magnetic minerals in these zones are cubic to isotropic stable single domain titanomagnetite, titanomaghemite, or both. Our results suggest that the magnetic properties of magnetic remanence carrying iron oxides in oceanic basalts can change over a few decades through fluid-rock interactions and elevated temperatures as they undergo alteration through maghemitization. Maghemite-induced cracking of the magnetite crystal surface may provide conduits into the more reduced cores of each iron oxide grain, thereby accelerating the redistribution of Fe²⁺ through very fine-scale fluid-rock interactions.