Emplacement and cooling of the lower crust and upper mantle beneath two fast spreading ridge segments: A quantitative comparison of crustal gabbros and mantle peridotites from Oman Drilling Project site CMA-1 and IODP Expedition 345 - Hess Deep

Abstract:

The emplacement and formation of the lower crust at fast-spreading mid-ocean ridges is modeled by two proposed end-member scenarios: the gabbro glacier model, which assumes a mass of crystalline mush in a large crustal magma chamber, or the sheeted sill model, in which crust accretes by solidification of a series of sills continuously emplaced within the gabbroic section of the crust. These emplacement mechanisms are testable through the application of geothermometry and geospeedometry to gabbros and peridotites that formed beneath intermediate to fast spreading centers across the petrologic Moho. In a gabbro glacier scenario, hydrothermal circulation is limited to the uppermost crust, such that lower crust and mantle would be expected to cool conductively over long timescales, with a continuous decrease in cooling rate with depth. Conversely, a sheeted sill scenario would be supported by the observation of fast cooling independent of depth in the crustal column, due to the existence of a network of permeable fractures that enable circulation of sea water into the lower crust to cool the sheeted sills.

Here, we investigate and compare the cooling histories of peridotites and gabbros from two drilling projects: Exp345 – Hess Deep, and the Oman Drilling Project, both of which sampled the crust-mantle transition zone, enabling evaluation of the depth extent of hydrothermal circulation beneath the spreading centers. We characterized the major and trace element geochemistry of constituent minerals in 3 crustal and 4 mantle samples from Hess Deep drill site 895D and 4 crustal and 4 mantle samples from sites CM1 and CM2 from the Oman Drilling Project. Analysis of the Oman Drilling Project samples is ongoing, but results from Hess Deep suggest cooling at rates between 0.1 and 1 °C/year, from peak temperatures of ~1300 °C. Models for conductive cooling of oceanic lithosphere predict rates 2 orders of magnitude slower at the crust-mantle transition zone, while thermal models that invoke deep and efficient hydrothermal circulation predict rates consistent with our observations. We infer that hydrothermal cooling extended to or near the petrologic Moho beneath the East Pacific Rise, consistent with the sheeted sill model for crustal accretion.