Geochemistry and Evolution of the Earth’s Mantle at the Hess Deep and the Mado Megamullion

Mantle peridotites have been widely sampled in the ocean basins. These are considered residues of the melting process that ultimately results in the formation of the oceanic crust. However, the exact mechanism for their evolution has not been comprehensively understood. Plagioclase-bearing mantle rocks, for example, may be formed by fractional crystallization, melt-rock reaction and assimilation in the crust and shallow mantle. Nearly all mid-ocean ridge basalts (MORB) are believed to have at least some melt-rock reaction process in their evolutionary history. Yet, there is a lack of direct evidence that captures the reaction textures or intermediate compositions of the melt-rock reaction process. Spreading centers and oceanic core complexes (OCC) provide a window to understand melt evolution processes in the lower crust and upper mantle. In this study we address the ongoing debate about the mechanism of melt transport in the mantle by textural and chemical analysis (major and trace elements) of samples from two unique tectonic regimes; a. the Hess Deep (fast spreading center in the Equatorial Pacific Ocean) and the Mado Megamullion (an OCC in the Shikoku back-arc basin). Magnetic anomaly data suggested that spreading in the Shikoku basin ceased after 15 Ma although the age of termination of spreading has not been constrained from geochronologic evidences until now. We integrated the model results with the natural samples to quantify the evolution of the peridotites. We dated zircons from amphibole-chlorite bearing veins that crosscut the peridotites of the Mado Megamullion. The U-Pb age of 13.37 ± 0.24 Ma and the trace and rare earth element (REE) chemistry of the zircons and the amphiboles suggests the possibility that magmatic activity and back-arc spreading at the Shikoku basin continued till 13 Ma. We could model the elevated TiO2 content in melt-reacted plagioclase-bearing peridotites from Hess Deep through assimilation fractional crystallization (AFC) process which could not be modelled by fractional crystallization process alone and has remained a topic of debate. Our results also show that back-arc basin peridotites at Mado Megamullion appear to have a unique petrographic and geochemical character that is distinct from those mid-ocean ridges or fore-arcs.