Garnet pyroxenite lens within Ugelvik layered garnet peridotite
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Peridotite
Enstatite
Diopside
Experimental data are reported for the melting of komatiite, peridotite, and chondrite compositions in the pressure range 5–16.5 GPa. All experiments were run using the multiple‐anvil apparatus facilities at Nagoya and Stony Brook. Equilibrium between coexisting crystals and liquid is demonstrated to occur in less than 3 min in the 2100°C range. The anhydrous solidus in CaO‐MgO‐Al 2 O 3 ‐SiO 2 has been calibrated and is shown to be about 100° higher than that for naturally occurring peridotite (KLB1). All melting curves have positive dT / dP . The effect of pressure is to expand the crystallization field of garnet at the expense of all other phases, resulting in a change in the liquidus phase from olivine to garnet at high pressures. The melting of rocks which contain the four crystalline phases olivine, orthopyroxene, clinopyroxene, and garnet is restricted to enstatite‐rich compositions such as chondrite. For these it is demonstrated that melting is peritectic, rather than eutectic, and takes the form L+Opx = Ol+Cpx+Gt. Partial melting yields liquids with the following properties: 5 GPa for komatiite; and 10–15 GPa for liquid peridotite with about 40% MgO, but one that is unlike mantle peridotite in that it is distinctly enriched in silica. These results provide a test and refutation of the model that upper mantle peridotite originated by direct initial melting of a chondritic mantle (Herzberg and O'Hara, 1985). Unlike chondrite, partial melting of peridotite does not usually involve orthopyroxene. Instead, it occurs by the generation of ultrabasic liquids along a cotectic involving L+Ol+Cpx+Gt. Although the thermal and compositional characteristics of this cotectic have not been fully calibrated, it is very likely that it will degenerate into a thermal minimum (L+Ol+Cpx+Gt), compositionally similar to komatiite at 5 GPa and mantle peridotite at 10–15 GPa. Peridotite liquids that occupy a thermal minimum can be derived from those formed from the melting of chondrite by removal of orthopyroxene, followed by fractional crystallization of olivine, clinopyroxene, and garnet. The possibility exists that the thermal minimum is compositionally identical to mantle peridotite in the 10–15 GPa range. If this can be confirmed by experiment, the upper mantle can be understood as having originated by the fractional crystallization of peridotite liquids in a large‐scale differentiation event, consistent with magma ocean models for an early Earth.
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Metasomatism
Diopside
Enstatite
Xenolith
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Peridotite
Chromitite
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Data determined using a multianvil press are given for liquidus phase relations in the system diopside – forsterite – enstatite (CaMgSi2O6 – Mg2SiO4 – MgSiO3) and extend for a limited distance into the larger ternary system diopside – forsterite – quartz at 5.1 GPa. In the system diopside–enstatite, which cuts across the system diopside – forsterite – quartz and divides it into t wo smaller ternary systems, the peritectic between clinopyroxene and orthopyroxene occurs at Di 43En57 and is defined by the reaction 100 cpx = 64 opx + 36 liq. An azeotropic minimum occurs on the clinopyroxene liquidus at Di 67En33. In the ternary system diopside – forsterite – enstatite, a peritectic occurs at Di 43Fo46Qtz11 and is defined by the reaction 69 opx + 31 liq = 95 cpx + 5 fo (wt.%). These ternary phase relations can be used to model three important aspects of the volatile-free phase relations and mel ting behavior of natural peridotite with a high degree of accuracy. (1) Olivine, orthopyroxene, and clinopyroxene occur at the solid us at 2 GPa, but orthopyroxene is absent at the solidus at 5.1 GPa. (2) During equilibrium melting at 5.l GPa, orthopyroxene appea rs just above the solidus and then disappears again at higher temperatures. At both 2 and 5.1 GPa, the proportions of phases in th e ternary system at various degrees of melting are close to the proportions observed in the melting of natural peridotite by Walt er (1998). (3) As pressure increases, MgO increases and SiO 2 decreases in initial melts.
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We present petrography, mineralogy, and thermobarometry for 53 mantle-derived xenoliths from the Muskox kimberlite pipe in the northern Slave craton. The xenolith suite includes 23% coarse peridotite, 9% porphyroclastic peridotite, 60% websterite, and 8% orthopyroxenite. Samples primarily comprise forsteritic olivine (Fo 89–94), enstatite (En 89–94), Cr-diopside, Cr-pyrope garnet, and chromite spinel. Coarse peridotites, porphyroclastic peridotites, and pyroxenites equilibrated at 650–1220 °C and 23–63 kbar (1 kbar = 100 MPa), 1200–1350 °C and 57–70 kbar, and 1030–1230 °C and 50–63 kbar, respectively. The Muskox xenoliths differ from xenoliths in the neighboring and contemporaneous Jericho kimberlite by their higher levels of depletion, the presence of a shallow zone of metasomatism in the spinel peridotite field, a higher proportion of pyroxenites at the base of the mantle column, higher Cr 2 O 3 in all pyroxenite minerals, and weaker deformation in the Muskox mantle. We interpret these contrasts as representing small-scale heterogeneities in the bulk composition of the mantle, as well as the local effects of interaction between metasomatizing fluid and mantle wall rocks. We suggest that asthenosphere-derived pre-kimberlitic melts and fluids percolated less effectively through the less permeable Muskox mantle, resulting in lower degrees of hydrous weakening, strain, and fertilization of the peridotitic mantle. Fluids tended to concentrate and pool in the deep mantle, causing partial melting and formation of abundant pyroxenites.
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Large-ion lithophile elements (LILE)-enriched chromium titanates of the magnetoplumbite (AM12O19) and crichtonite (ABC18T2O38) groups have been recognized as abundant inclusions in orthopyroxene grains in a mantle-derived xenolith from the Udachnaya-East kimberlite pipe, Daldyn field, Siberian craton. The studied xenolith consists of three parts: an orthopyroxenite, a garnet clinopyroxenite, and a garnet-orthopyroxene intermediate domain between the two. Within the host enstatite (Mg# 92.6) in the orthopyroxenitic part of the sample titanate inclusions are associated with Cr-spinel, diopside, rutile, Mg-Cr-ilmenite, and pentlandite. Crichtonite-group minerals also occur as acicular inclusions in pyrope grains of the intermediate domain adjacent to the orthopyroxenite, as well as in interstitial to enstatite oxide intergrowths together with Cr-spinel, rutile, and ilmenite.
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Phlogopite
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Peridotite
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Mantle plume
Radiogenic nuclide
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