Origin of robust multicomponent remanence from oceanic gabbros in Pito Deep

As oceanic crust cools, it acquires a signal of Earths magnetic field in the form of thermoremanent magnetization. Pito Deep (23S, 112W) off the East Pacific Rise, is one of two well documented exposures of fast spread oceanic crust including the uppermost gabbros. Previous studies of magnetic remanence in Pito Deep gabbros have shown that they preserve magnetizations spanning multiple polarity intervals, indicating slow cooling over at least 100kyr. If the stable remanence of these gabbros is carried by sufficiently fine grained magnetite, their behavior during thermal demagnetization can ultimately provide quantitative information about the magnetization and cooling history of the lower oceanic crust. Here we show that silicate hosted magnetite (fine grained, single domain) is a primary contributor to the remanence of the lower crustal gabbros of Pito Deep. Thin sections reveal little, if any, discrete magnetite grains that could contribute to a reliable magnetic signal. Nearly half the samples that underwent thermal demagnetization have maximum unblocking temperature above 550°C, with little to no demagnetization occurring before 450°C, indicating a stable remanence. In addition, the high coercivities evident from alternating field demagnetization (40% with median destructive fields over 60mT) are consistent with single domain magnetite being a dominant carrier of magnetic remanence in our samples. Further exploration using photomicrographs revealed the presence of opaque, oriented inclusions within many of the pyroxene and plagioclase crystals. Successful first order reversal curves from non-ferromagnetic pyroxene grains yielded a strong central ridge, typical for single domain ferromagnets. Orthogonal measurements of outer hysteresis loops of pyroxenes revealed an anisotropic signal, consistent with the presence of crystallographically oriented magnetite. These silicate hosted magnetite inclusions are isolated from the effects of chemical alteration and oxidation by their host grain, making them very stable recorders of magnetic remanence. The fine grained silicate hosted magnetites in Pito Deep gabbros provide a robust record of magnetic polarity that can be applied to crustal cooling models.