Magnetic Structure of Fast-Spread Oceanic Crust at Pito Deep

Magnetic surveys at tectonic windows that expose magnetic polarity boundaries provide the unique opportunity to explore the pattern of magnetization variations within the oceanic crust and determine the spatially averaged magnetizations of source layers that contribute to marine magnetic anomalies. Here we investigate the C2An.2n/C2An.2r polarity boundary in the tectonic window of Pito Deep, which has exposed a cross-section through lavas, dikes, and the uppermost kilometer of gabbros at fast-spread ocean crust. Near-bottom magnetic anomaly surveys from two expeditions have been incorporated into a penalized least squares inversion method. The application of this method to magnetic data allows us to account for complex bathymetry and differing observation altitudes. When correlated with rock type, the magnetization solution shows median values of 4.4 2.7 A/m for lavas, 2.0 1.9 A/m for dikes, and 1.9 1.9 A/m for gabbros. On a regional scale, lavas and dikes have a different polarity of magnetization than the underlying gabbros. The geometry of the polarity boundary is compatible with a large (similar to 6 km) horizontal offset or very shallow dip of isotherms at the dike/gabbro boundary, and indicates that the zone of melt is significantly wider across axis than predicted from seismic tomography models that suggest pervasive cooling throughout the lower crust within a few kilometers of the spreading center. Plain Language Summary Over 30% of the Earth's ocean crust is created in fast-spread environments like the East Pacific Rise, but there is no consensus on how the lower portion of the crust is formed and cooled. This is in part because it is difficult to access lower crustal gabbroic samples formed at fast-spread ridges. Because ocean crustal rocks preserve a record of Earth's magnetic field at the time they cool, we can use magnetic data to help distinguish between different formation models. We use magnetic anomaly data in an exposed section of fast-spread crust in order to determine the pattern of magnetization variations in these rocks. This new method solves for crustal magnetization over a similar to 10-km wide survey area to determine a robust average magnetization of lavas, dikes, and gabbroic rocks. Our magnetizations agree with previous estimates for samples from fast-spread crust. The survey area also includes a magnetic reversal, and the pattern of magnetization can only be reproduced if there is a large horizontal offset in temperature between the dike and gabbroic rocks, or the gabbroic section cools very slowly (meaning shallow isotherms). This means temperatures remain hotter in the gabbroic section of lower ocean crust further off axis.