Peridotite and pyroxenite xenoliths from the Muskox kimberlite, northern Slave craton, Canada
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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.Keywords:
Peridotite
Xenolith
Metasomatism
Enstatite
Asthenosphere
Amphibole
Pyrope
A possible age of 745 ± 100m.y. (using 50b.y. = half life of 87Rb) has been determined for pyrope from the Stockdale kimberlite assuming an upper mantle initial 87Sr/86Sr ratio of 0.702 ± 0.001. Despite the large error associated with this age it clearly demonstrates that the pyrope crystallized long before its final emplacement which is set at ≤ 100 m.y. Allowance for crustal and mantle contamination and other sources of error have been carefully considered in this determination.
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Abstract The author studied the grain size, shape, colour, altered coat, mineral species, chemical composition, end‐ member components and infrared spectra of clinopyroxenes occurring as megacryst, macrocryst and groundmass minerals, intergrowths with pyrope and ilmenite and minerals in deep‐seated xenoliths and inclusions in diamonds in kimberlites of China. The clinopyroxenes under study were compared with megacryst clinopyroxenes in basalts and minerals in their deep‐seated xenoliths and clinopyroxenes in lamproites and minettes. The coexisting clinopyroxene‐pyrope pair was studied. Besides the author also studied the origin of clinopyroxenes in kimberlites, P‐T conditions for their formation and their reflected tectonic environments of the kimberlite formation. He suggests that this mineral is an indicator for diamond exploration.
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Pyrope
Ilmenite
Hadean
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Garnet, diopside and Mg-ilmenite have been analyzed from the Vargem diatreme, near Coromandel, State of Minas Gerais, and conlirmed to be comparable to similar minerals from kimberlites in South Africa, Siberia and the United States. Similar minerals occur downstream from the Vargem kimberlite in placer deposits in which diamond is found. The Redondão diatreme in southwest State of Piaul also contairs kimberlitic garnets as well as several xenoliths of crustal and mantle rocks. One xenolith, although extensively serpentinized, was originally a garnetJherzolite comparable in texture and garnet chemistry to those from kimberlites in Southern Africa.
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Ilmenite
Diopside
Country rock
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Pyrope
Xenolith
Mahalanobis distance
Lithology
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Xenolith
Peridotite
Mineral resource classification
Phlogopite
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Cluster analysis of 458 pyroxenes from kimberlites, associated xenoliths and diamonds has allowed recognition of 5 chemically distinct orthopyroxene groups and 10 distinct clinopyroxene groups from the $$TiO_{2}, Al_{2}O_{3}, Cr_{2}O_{3}$$, FeO, MgO, CaO, and $$Na_{2}O$$ contents. Names assigned to these groups convey their distinctive chemical features. Because many groups contain cases from both kimberlite and xenoliths, some kimberlite pyroxenes may derive from fragmented xenoliths. However from size alone, large discrete orthopyroxene crystals, discrete sub-calcic diopside nodules and low-Cr diopsides intergrown with ilmenite are apparently not xenolithic; nor are the minute diopside crystals growing in the kimberlite matrix. Pyroxene inclusions in diamonds and pyroxenes coexisting with diamond in eclogite and peridotite xenoliths range widely in chemical composition.
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Diopside
Pyroxene
Ilmenite
Peridotite
Pyrope
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Melting temperatures of the assemblage pyroxene + garnet on the enstatite‐pyrope join were experimentally determined at 11 different pressures between 80 and 152 kbar with a split‐sphere anvil apparatus (USSA‐2000). The compositions of pyroxene, garnet, and melt coexisting along the solidus were also determined. Comparison with previous data at lower temperatures revealed a large temperature dependence of the compositions of garnet coexisting with pyroxene. It is proposed that the observed variation in the garnet composition is caused by disorder in garnet. The obtained alumina contents of orthopyroxene coexisting with garnet on the solidus at 80–110 kbar contributed to an improvement of the garnet peridotite thermobarometry. An internally consistent set of thermodynamic parameters was derived, which allows calculation of the temperature‐pressure phase diagram for the enstatite‐pyrope join at 20–260 kbar.
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Enstatite
Solidus
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Peridotite
Diopside
Grossular
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Xenolith
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Solidus
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Enstatite
Pyrope
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Pyrope
Enstatite
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