Rock magnetic constraints on primary igneous features and hydrothermal alteration of MORBs along the South Atlantic ridge flanks

The South Atlantic Transect (SAT) ocean drilling expeditions (IODP Expeditions 390 & 393) recovered basaltic lavas formed between ~7 and 61 Ma along the western flank of the Mid-Atlantic Ridge at 31°S. Mid-ocean-ridge basalt (MORB) recovered during the SAT preserves primary magmatic characteristics and evidence of varying extents of reaction with seawater-derived hydrothermal fluids. This transect offers a unique opportunity for studying accretion of upper oceanic crust, off-axis hydrothermal processes over time, and the influence of rock alteration on long-term variations in seafloor magnetization.

Magnetic minerals in basaltic lava flows are known to reflect primary volcanic features, such as magma composition and emplacement style, and on- and off-axis hydrothermal processes.

In this study we performed detailed rock magnetic investigations to characterize the magnetic mineral assemblages and grain-size variations among fresh and altered basalts. Across the ridge flank (i.e., with increasing age), the magnetic properties highlight a strong dependence of magnetic mineral grain-sizes and composition on the nature of the volcanic units and their evolution during hydrothermal alteration. For example, fresh MORB displays Ti-rich titano-magnetite with finer and coarser grains in pillows and massive lava flows, respectively. Fluctuations in remanent magnetization and magnetic susceptibility intensities are also strictly dependent on primary textures and emplacement style.

Magnetic mineral compositions (e.g., changes in Ti-content) and grain-sizes vary across distinct types of alteration halos, with a general decrease in magnetization. As alteration evolves, the magnetic properties demonstrate a progressive oxidation of the primary titano-magnetite into titano-maghemite coupled with magnetic mineral grain-size reduction associated with various extents of groundmass and phenocryst replacement. Strongly altered basalts reveal a paramagnetic contribution related to the formation of secondary clays associated with Fe-oxyhydroxides (i.e., goethite).

Overall, the variation of magnetic properties across the South Atlantic ridge flanks provides constraints on the complex interplay of volcanic stratigraphy and the evolution of hydrothermal alteration as the upper oceanic crust ages, linking petrology with the long-term variation of marine magnetic anomalies.