Stability of osumilite coexisting with spinel solid solution in metapelitic granulites at high oxygen fugacity
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Kaushik Das
,
Somnath Dasgupta
Abstract
Experiments were carried out in the quartz-saturated part of the system KFMASH at hematitemagnetite buffer conditions in a piston-cylinder apparatus using synthetic biotite, K-feldspar, quartz, and sillimanite80-quartz20 gel. Natural garnet and sillimanite were used as seeds. Three bulk compositions were used: XMg = 0.81, 0.72, and 0.53, and the experiments were vapor absent. In the bulk composition with the highest XMg, dehydration-melting of biotite produced osumilite coexisting with spinel solid solution within the P-T window 7-8.5 kbar and 850-1000 °C. However, this mineral pair coexisted with cordierite at P < 7.5 kbar, and with sillimanite above 7.5 kbar. There is no appreciable change in the lower thermal stability of osumilite with lowering of XMg in the bulk composition, but both the upper pressure and upper thermal stabilities are reduced. With increasing pressure, osumilite + spinel breaks down to an orthopyroxene + sillimanite assemblage. There is no overlap of the stability fields of osumilite and sapphirine. Rather, sapphirine-bearing assemblages are produced at the expense of osumilite with increasing temperature. Sapphirine, however, did not appear in the bulk composition having XMg = 0.53. Instead, a wide stability field of spinel + cordierite + orthopyroxene + sillimanite is noted at P > 7 kbar and T > 950 °C. Garnet + orthopyroxene + sillimanite become stable at lower pressures with decreasing XMg in the bulk composition. The effects of additional components in natural assemblages, such as Zn in spinel and F and Ti in biotite, on the KFMASH equilibria are evaluated qualitatively. The experimental results were applied to six high-grade terranes, where osumilite + spinel coexistence at peak metamorphic conditions has been reported. The deduced P-T window for the osumilite + spinel association is entirely consistent with independent PT estimates for the natural occurrences. It is argued that osumilite + spinel assemblage should be characteristic of metamorphism of highly magnesian pelites at high fO₂ along a prograde path of high dT/dP.
© 2015 by Walter de Gruyter Berlin/Boston
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Articles in the same Issue
- First-principles modeling of the infrared spectrum of kaolinite
- Determination of the limiting fictive temperature of silicate glasses from calorimetric and dilatometric methods: Application to low-temperature liquid volume measurements
- Enthalpies of formation of tremolite and talc by high-temperature solution calorimetry – a consistent picture
- Evidence for an I2/a to Imab phase transition in the silica polymorph moganite at ~570 K
- Thermal decomposition of rhombohedral KClO3 from 29–76 kilobars and implications for the molar volume of fluid oxygen at high pressures
- High-pressure behavior of clinochlore
- Structure and elasticity of wadsleyite at high pressures
- Determination of the fluid–absent solidus and supersolidus phase relationships of MORB-derived amphibolites in the range 4–14 kbar
- F-rich phlogopite stability in ultra-high-temperature metapelites from the Napier Complex, East Antarctica
- Instability of Al2SiO5 “triple-point” assemblages in muscovite+biotite+quartz-bearing metapelites, with implications
- Stability of osumilite coexisting with spinel solid solution in metapelitic granulites at high oxygen fugacity
- Geikielite exsolution in spinel
- Aeromagnetic anomalies, magnetic petrology, and rock magnetism of hemo-ilmenite- and magnetite-rich cumulate rocks from the Sokndal Region, South Rogaland, Norway
- Minor element chemistry of hemo-ilmenite and magnetite in cumulate rocks from the Sokndal Region, South Rogaland, Norway
- Crystal structure analysis of synthetic Ca4Fe1.5Al17.67O32: A high-pressure, spinel-related phase
- Crystal structure of phase X, a high pressure alkali-rich hydrous silicate and its anhydrous equivalent
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- Description and crystal structure of turtmannite, a new mineral with a 68 Å period related to mcgovernite
- The crystal structure of low melanophlogite
- Crystal structures of Na and K aluminate mullites
- From mastodon ivory to gemstone: The origin of turquoise color in odontolite
- Letters. Elasticity of single-crystal calcite and rhodochrosite by Brillouin spectroscopy
- Ikaite, CaCO3·6H2O: Cold comfort for glendonites as paleothermometers
