Melting of mantle minerals at high pressures: Experimental study and thermodynamic evaluation.
Item
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Title
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Melting of mantle minerals at high pressures: Experimental study and thermodynamic evaluation.
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Identifier
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AAI9304750
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identifier
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9304750
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Creator
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Zhang, Jianzhong.
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Contributor
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Adviser: S. K. Saxena
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Date
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1992
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Language
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English
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Publisher
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City University of New York.
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Subject
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Geochemistry | Geophysics | Mineralogy
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Abstract
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Melting and subsolidus relations of mantle minerals have been experimentally determined for pyrope {dollar}\rm Mg\sb3Al\sb2Si\sb3O\sb{12}{dollar} between 10 and 16 GPa, SiO{dollar}\sb2{dollar} polymorphs (coesite and stishovite) between 9 and 14 GPa, and brucite Mg(OH){dollar}\sb2{dollar} between 6 and 22 GPa using a split-sphere multianvil apparatus. Compared with the previous studies, the present determinations provide several new features which have not been reported earlier or predicted theoretically. These results are: (1) The melting curve of pyrope shows an increase in dT/dP slope at pressures above 10 GPa, which is considerably different from that of other mantle minerals; (2) The melting temperatures of coesite are independent of pressure between 11 and 13.5 GPa. The predicted melting curve of coesite that showed a temperature maximum around 8 GPa and a large negative slope at higher pressures has not been confirmed. (3) The coesite-stishovite-liquid triple point is bracketed between 13.5 and 13.8 GPa at 2800{dollar}\sp\circ{dollar}C higher than the previous estimation. (4) Stishovite is melted for the first time through a static experiment. The melting temperature at 14 GPa is 2850{dollar}\sp\circ{dollar}C, and is 300{dollar}\sp\circ{dollar}C higher than those obtained from shock wave measurements at 16 to 30 GPa; (5) The dT/dP slope of the coesite-stishovite transition determined from 2000 to 2800{dollar}\sp\circ{dollar}C is two and half times smaller than indicated by the previous studies below 1400{dollar}\sp\circ{dollar}C; (6) The dehydration of brucite is terminated at a pressure between 8 and 10 GPa with a melting curve arising from it. The melting of brucite is reported for the first time and the melting temperatures are found independent of pressures between 12 and 22 GPa.;The experimental results have been thermodynamically evaluated. The procedure includes the selection of thermodynamic data on the related phases, extrapolation of heat capacity for solids beyond the melting temperatures, a modification of third order Birch-Murnghan equation at high temperature, and evaluation of data on liquid phases through analysis of the melting curves.
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Type
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dissertation
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Source
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PQT Legacy CUNY.xlsx
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degree
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Ph.D.
