The composition of the incipient partial melt of garnet peridotite at 3 GPa and the origin of OIB
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Peridotite
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The Journal of the Japanese Association of Mineralogists Petrologists and Economic Geologists (1968)
On the basis of Fe2+-Mg partition between coehisting garnet and clinopyroxene of eclogites in the Higasi-akaisi peridotite mass, both from the Gongen valley and from the main adit of the Akaisi mine, it is suggested that the crystallization temperatnre of the eclogites in this peridotite mass corresponds to that of lower amphibolite fades. The temperature thus estimated appears to be higher than that of surrounding metabasites, and eclogites enclosed therein. The chemical compositions of six pairs of gamet-clinopyroxene assemblage, including the new analyses of three pairs, are described in Table 1.
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Experimentally at 7 GPa phase relations in two sections of the system garnet peridotite–eclogite– carbonatite are studied in connection with the problem of physico-chemical conditions of differentiation of the upper mantle ultrabasic-basic magmas and formation of continuous series of peridotite–eclogite rocks as well as the syngenesis of diamond and primary inclusions of peridotitic and eclogitic parageneses. Diagrams of equilibrium and fractional crystallization of the boundary silicate multicomponent system peridotite–eclogite are constructed. As a result, a new effect of “peridotite–eclogite tunnel” is established. The tunnel provides formation of the continuous series of peridotite–eclogite rocks of the Earth’s upper mantle. Also, diagrams of equilibrium and fractional crystallization for the polythermal section peridotite30carbonatite70–eclogite35carbonatite65 are constructed. As a result, a combined action of the effects of peridotite–eclogite tunnel and carbonatization of peridotitic magnesian phases is recognized. The effects provide consecutive formation of the phases of peridotitic and eclogitic parageneses in the natural processes of diamond origin.
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Subduction of oceanic crust buries an average thickness of 300–500 m of sediment that eventually dehydrates or partially melts. Progressive release of fluid/melt metasomatizes the fore-arc mantle, forming serpentinite at low temperatures and phlogopite-bearing pyroxenite where slab surface reaches 700–900 °C. This is sufficiently high to partially melt subducted sediments before they approach the depths where arc magmas are formed. Here, we present experiments on reactions between melts of subducted sediments and peridotite at 2–6 GPa/750–1100 °C, which correspond to the surface of a subducting slab. The reaction of volatile-bearing partial melts derived from sediments with depleted peridotite leads to separation of elements and a layered arrangement of metasomatic phases, with layers consisting of orthopyroxene, mica-pyroxenite, and clinopyroxenite. The selective incorporation of elements in these metasomatic layers closely resembles chemical patterns found in K-rich magmas. Trace elements were imaged using LA-ICP-TOFMS, which is applied here to investigate the distribution of trace elements within the metasomatic layers. Experiments of different duration enabled estimates of the growth of the metasomatic front, which ranges from 1–5 m/ky. These experiments explain the low contents of high-field strength elements in arc magmas as being due to their loss during melting of sedimentary materials in the fore-arc.
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