Temperatures and Timescales of Crystallization in the “Mush Zone” Beneath Fast‐Spreading Ocean Ridges

Abstract

AbstractQuantifying the timescales of magma solidification and eruption beneath mid‐ocean ridges is critical to understanding how melts are emplaced and crystallized, and how long these magmas and their crystals remain in a semi‐molten “mush‐like” state. Here, we present a new method to quantify the crystallization and solidification temperatures of individual gabbros in the lower crust of the Wadi Tayin massif (Samail ophiolite). Cotectic crystallization temperatures of olivine + plagioclase + clinopyroxene are constrained by our clinopyroxene Mg# thermometer, and supported by clinopyroxene‐olivine Fe‐Mg equilibration temperatures. We find that lower crustal gabbros reach the cotectic at approximately 1,160 ± 16°C, and are fully solidified between 781 and 1011°C, depending on cooling rate. Our results show that lower crustal gabbros spend, on average, 7,500–11,500 years in a partially molten state with no systematic change in duration with depth in the crust. Samples with very fast cooling rates result in solidification only a few tens of meters off‐axis from their emplacement location and likely indicate solidification near the distal edge of the on‐axis melt lens or in off‐axis melt lenses. Incorporating a uniform 1 km wide emplacement region directly on‐axis, as inferred geophysically, and a half‐spreading rate of 55 mm/yr, our results suggest that the Mush Zone at fast‐spreading ridges remains partially molten for 0–3 km away from the ridge axis, but up to 4–7 km considering all possible sources of uncertainty. Both geophysical and geochemical estimates of the Mush Zone are inconsistent with conductive cooling of the lower oceanic crust.