A fertile harzburgite–garnet lherzolite transition: possible inferences for the roles of strain and metasomatism in upper mantle peridotites
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Keywords:
Enstatite
Diopside
Pyrope
Xenolith
Metasomatism
Peridotite
Trace element
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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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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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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A large (17 cm) single crystal of enstatite (En88) contains Mg-ilmenite (MgO ∼ 12 wt%) and inclusions of polyphase garnets. The major garnet is orange in color and is predominantly pyrope-almandine (Py71Al21Gr8) whereas inside this garnet is a second one that is pink in color, and Cr-rich (Cr-Py33Py28Al19Gr20). This Cr-rich garnet itself contians inclusions of Cr-diopside, chromite and ilmenite. The enclosing orange almandine garnet at the boundary with the pink Cr-rich garnet contain rounded Mg-ilmenites together with a string of elongated olivines (Fo86) that are in optical continuity. Also present in the same zone is diopside. Within the enstatite host and in close association with the polyphase garnet assemblage are calcite, Ti-phlogopite and serpentine. Although it is conceivable the Cr-rich garnet and related phases represent the remnant of an earlier pre-existing rock that underwent considerable partial melting it must also be considered that the second garnet may be metamorphic in origin. However, irrespective of the origin, based on orthopyroxene geothermometry and A12O3 content possible P and T of equilibration are in the region of 60kb and 1200°C.
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Chondrule
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