Rock Magnetic Characterization Through an Intact Sequence of Oceanic Crust, IODP Hole 1256D

Abstract

Coring at Site 1256 (6.736◦N, 91.934◦W, 3635 m water depth) during Ocean Drilling Program (ODP) Leg 206 and Integrated Ocean Drilling Program (IODP) Expeditions 309 and 312 successfully sampled a complete section of in situ oceanic crust, including sediments of Seismic Layer 1, lavas and dikes of Layer 2, and the uppermost gabbros of Layer 3. The crust at this site was generated by superfast seafloor spreading (>200 mm/yr full spreading rate) along the East Pacific Rise some 15 Ma ago. One goal of drilling a complete oceanic crust section is to determine the source of marine magnetic anomalies. For crust generated by fast seafloor spreading, is the signal dominated by the upper extrusive layer, do the sheeted dikes play any role, how significant is the magnetic signal from gabbros relative to that at slow spreading centers and what is the timing of acquisition of the magnetization? To address these questions, we have made a comprehensive set of rock magnetic and paleomagnetic measurements that extend through the igneous interval. Continuous downhole variations in magnetic grain size, coercivity, mass-normalized susceptibility, Curie temperatures, and composition have been mapped. Compositionally, we have found that the iron oxides vary from being titanium-rich titanomagnetite (TM60), which are commonly partially oxidized to titanomaghemites, to titanium-poor magnetite as determined semi-quantitatively from Curie temperature analyses and microscopy studies. Skeletal titanomagnetite with varying degrees of alteration is the most common magnetic mineral throughout the section and is often bordered by large iron sulfide grains. The low-Ti magnetite or stoichiometric magnetite is present mainly in the dikes and gabbros and is associated with higher Curie temperatures (550◦C to near 580◦C) and higher coercivities than in the extrusive section. Magnetic grain sizes predominantly fall in the pseudo single domain (PSD) grain size region on Day diagrams, with only a small numbers of samples falling within the single domain (SD) or multi-domain (MD) regions. Overall the magnetic properties of this hole are strongly influenced by post-emplacement alteration, particularly the lower part of the section from the gabbros up into the transition zone. Some of the more prominent features of the rock magnetic data are the gradual increase in Curie temperatures with depth from about 200–350◦C at the top of the extrusives to about 425◦C just above the transition zone, the more variable Curie temperatures and less variable susceptibility and coercivity of remanence in the upper half of the extrusives relative to the lower half the near constant composition (x = 0.6) and oxidation (z = 0.6) of the iron oxide grains (>5νm) in the extrusives (Chapter 12 this volume), the highly irreversible nature of thermomagnetic curves in the extrusives, in which the cooling curve has Curie temperatures higher (generally > 500◦C) than indicated by the heating curve, the abrupt change in rock magnetic properties across the transition zone, particularly the Curie temperature., a somewhat finer grain size and increased intensity in the sheeted dike zone relative to the extrusives and gabbros, and the nearly constant Curie temperatures (530 and 585◦C) for the dikes and gabbros.