Magnetization during low‐temperature oxidation of seafloor basalts: No large scale chemical remagnetization

Abstract

Difficulty in removing the effects of grain size dependence have hampered interpretations of magnetic property changes during low‐temperature oxidation of titanomaghemite. For the present study, samples were collected perpendicular to cracks in seafloor basalts to produce subsamples of essentially constant grain size but varying degree of low‐temperature oxidation within a few centimeters. Coarse‐grained samples and those showing evidence of high‐temperature oxidation were excluded. Increasing Curie temperature was used as the most reliable measure of increasing maghemitization. Intensity of natural remanent magnetization (JNRM) consistently decreased with increasing low‐temperature oxidation indicating that any increase in JNRM with age of oceanic crust must be due to other factors. Coercive force (HC) increased until Curie temperatures reached 250°C and then, in samples exhibiting single‐domain behavior, decreased drastically. Because coercive force depended on degree of oxidation in this complex manner, chemical remanent magnetization (CRM) in even slightly oxidized grains should have a different coercivity than the original thermal remanent magnetization (TRM). Most Deep Sea Drilling Project samples in this study have experienced tectonic rotations or field reversals within 1.0 m.y. of extrusion. Thus, field‐controlled CRM, if present, should have different coercivities and a discrete direction and be discernible during alternating field (AF) demagnetization. Alternating field demagnetizations showed no evidence for significant acquisition of field‐controlled CRM. Viscous remanent magnetization (VRM) was the dominant secondary magnetization in samples with several components of remanence. In very fine‐grained basalts, VRM was difficult to remove by AF demagnetization and may be the source of increased dispersion in highly oxidized, oceanic basalts. Mechanisms which may, in part, be responsible for coercivity and VRM changes are (1) an increase in the critical size of domain transitions during low‐temperature oxidation, and (2) incipient phase separation into Fe‐rich and Ti‐rich areas, the Fe‐rich areas being superparamagnetic. The latter mechanism explains the increase in saturation magnetization (JS) and concomitant decrease in JNRM when Curie temperatures are above 360°C.