Chemical and thermal state of oceanic lithosphere reconstructed by petit-spot mantle xenoliths from the northwestern Pacific

The oceanic lithosphere cools as it spreads away from the mid-ocean ridges, and subducts into the mantle at the subduction zones. In the context of Earth’s material cycle, quantitative chemical and thermal state of the oceanic lithosphere is desired to be estimated to elucidate material flux into the mantle. As a step toward reconstructing the chemical and thermal state of oceanic lithosphere, we present geochemical data set of mantle xenoliths from petti-spots in the northwestern Pacific, where no seismic anomaly is imaged. The petit-spot-borne mantle xenoliths provide us unique chemical and thermal information avoiding modifications derived from the mantle plumes.

The petit-spot mantle xenoliths include lherzolites, harzburgites, and dunites collected at petit-spot Sites A and B in the northwestern Pacific, using deep-submergence vehicle Shinkai 6500 during four expeditions of YK05-06, YK20-14S, YK21-07S, and YK24-10S. They are small in size ranging from 1 to 5 cm in diameter, except for a lherzolite with 15 cm-long diameter. The peridotites show variation in terms of the presence of spinel and garnet, and degree of melt depletion. Some of the peridotites include fine-grained mineral aggregates, which are broken-down products after pyrope-rich garnets considering their average bulk chemical compositions. Rare-earth elements (REE) of clinopyroxene are evaluated with a one-dimensional, steady-state, decompressional melting model. The results indicate that fractional melting in the garnet-stable region is required before conventional fractional melting in the spinel-stable region. Geothermobarometric pressure-temperature estimation results indicate that the peridotite xenoliths were derived from ~2.5 GPa where asthenosphere/lithosphere boundary is expected based on the geophysical investigations.

Abyssal peridotites recovered from the mid-ocean ridges are known to undergo melting from the garnet-stable region to the spinel-stable region. Thus, depleted spinel dunite/harzburgite layer is expected to be perched atop fertile spinel/garnet harzburgite-lherzolite layers as a melting column in the mid-ocean ridge. Since the petit-spot peridotite xenoliths cover a long range of the oceanic stratigraphy deep down to the lithosphere/asthenosphere boundary, we present more detailed chemical and thermal state of the whole oceanic lithosphere in the presentation. In addition, we attempt to present a perspective vision for future petit-spot drilling.