Résumé
The oceanic lithosphere cools as it spreads away from mid-ocean ridges, and subducts into the mantle at subduction zones. In the context of Earth's material cycle, quantitative investigation of the oceanic lithosphere is imperative to estimate material flux into the mantle. As a step toward reconstructing the oceanic lithosphere, we present a geochemical data set of mantle xenoliths from petit-spots in the northwestern Pacific. The petit-spots are remarkably small knoll ( nearly equal 1 km (super 3) in observable surface volume), formed in response to plate flexure developed in the outer trench swell. The petit-spot-borne mantle xenoliths provide us unique mantle materials void of chemical and thermal modifications by mantle plumes. 35 peridotite xenoliths, 20 lherzolites, 14 harzburgites, and 1 dunite were collected from petit-spot Sites A and B in the northwestern Pacific, using deep-submergence vehicle Shinkai 6500 during three expeditions of YK05-06, YK20-14S and YK21-07S. 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 chemical compositions. Rare-earth elements (REE) of clinopyroxene are evaluated with a one-dimension, steady-state, decompression melting model. The results indicate that nearly equal 9% fractional melting in the garnet-stable region is required before conventional fractional melting in the spinel-stable region. One spinel lherzolite is fertile in equilibrium with a source reservoir of MORB (i.e., DMM). Abyssal peridotites recovered from mid-ocean ridges are known to undergo melting from the garnet-stable region to the spinel-stable region. Thus, depleted spinel harzburgite layer is expected to be perched atop fertile spinel/garnet lherzolite layers as a melting column in the mid-ocean ridge. We present pressure-temperature estimates of the peridotite xenoliths and quantitatively reconstruct the geochemical stratigraphy of the oceanic lithospheric mantle and show that the petit-spot peridotite xenoliths substantiate such melting column, thus proving the presence of DMM beneath the depleted harzburgite layer in the mid-ocean ridge.