09:45 〜 10:05
[BCG06-04] Geochemical stratigraphy of oceanic lithosphere reconstructed by spinel/garnet peridotite xenoliths from petit-spots in the northwestern Pacific
★Invited Papers
キーワード：プチスポット、マントル、リソスフェア-アセノスフェア境界、DMM、東北沖
The oceanic lithosphere cools as it spreads away from mid-ocean ridges, and returns to the mantle at the subduction zones. In the context of Earth’s material cycle, quantitative visualization of the oceanic lithosphere is imperative to estimate material flux into the mantle. Whereas seismic velocity gradients can illustrate coherent lithosphere over weak asthenosphere, chemical layers stratified over the lithosphere cannot be specified. Mantle fragments found as xenoliths embedded in volcanic rocks in hotspots are noble way to reconstruct geochemical stratigraphy of the oceanic lithosphere deep into the base of the oceanic lithosphere. Yet, hot mantle plumes thermally and chemically affect the above oceanic lithosphere when they impinge on the base of the oceanic lithosphere, creating a bottleneck in specifying geochemical stratigraphy of “normal” oceanic lithosphere. As a step toward resolving this issue, we present a geochemical data of mantle xenoliths from petit-spots in the northwestern Pacific, where no mantle plume is imaged. The petit-spots are remarkably small knoll (~ 1 km3 in observable surface volume), formed in response to plate flexure developed in the outer trench swell. The petit-spot-borne mantle xenoliths are thus unique mantle materials devoid of chemical and thermal modifications from mantle plumes.
25 peridotite xenoliths 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 samples show variation in terms of the presence of spinel and garnet with melt depletion from harzburgite to lherzolite. The peridotite xenoliths are small in size ranging from 1 to 3 cm in diameter, except for a garnet lherzolite with 15 cm-long diameter. Some of the peridotites include fine-grained mineral aggregates, which are broken-down products after pyrope-rich garnets considering their average chemical compositions. The garnet lherzolites and spinel lherzolites are fertile, whereas spinel harzburgites are depleted in melt components. We applied a mantle melting model using rare-earth elements (REE) in clinopyroxenes of the spinel harzburgites and spinel lherzolites. The results indicate that a few percent of fractional melting in the garnet-stable region is required before conventional fractional melting in the spinel-stable region. One spinel lherzolite is exceptionally fertile in equilibrium with a source reservoir of MORB (i.e., DMM).
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 harzburgite layer is expected to be perched atop fertile spinel/garnet lherzolite layers in hypothetical melting column in the mid-ocean ridge. The peridotite xenoliths from the petit-spots substantiate this melting column, and corroborate the presence of DMM beneath the depleted harzburgite layer in the mid-ocean ridge. We will present pressure–temperature estimates on the peridotite xenoliths and quantitatively reconstruct the geochemical stratigraphy of the oceanic lithosphere.
25 peridotite xenoliths 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 samples show variation in terms of the presence of spinel and garnet with melt depletion from harzburgite to lherzolite. The peridotite xenoliths are small in size ranging from 1 to 3 cm in diameter, except for a garnet lherzolite with 15 cm-long diameter. Some of the peridotites include fine-grained mineral aggregates, which are broken-down products after pyrope-rich garnets considering their average chemical compositions. The garnet lherzolites and spinel lherzolites are fertile, whereas spinel harzburgites are depleted in melt components. We applied a mantle melting model using rare-earth elements (REE) in clinopyroxenes of the spinel harzburgites and spinel lherzolites. The results indicate that a few percent of fractional melting in the garnet-stable region is required before conventional fractional melting in the spinel-stable region. One spinel lherzolite is exceptionally fertile in equilibrium with a source reservoir of MORB (i.e., DMM).
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 harzburgite layer is expected to be perched atop fertile spinel/garnet lherzolite layers in hypothetical melting column in the mid-ocean ridge. The peridotite xenoliths from the petit-spots substantiate this melting column, and corroborate the presence of DMM beneath the depleted harzburgite layer in the mid-ocean ridge. We will present pressure–temperature estimates on the peridotite xenoliths and quantitatively reconstruct the geochemical stratigraphy of the oceanic lithosphere.