Thermodynamics of pyroxene solid solution: Synthesis of theory and experiments.
Item
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Title
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Thermodynamics of pyroxene solid solution: Synthesis of theory and experiments.
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Identifier
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AAI9000069
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identifier
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9000069
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Creator
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Sykes, Julia Ann.
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Contributor
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Adviser: Surendra K. Saxena
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Date
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1989
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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 | Mineralogy | Geology
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Abstract
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Theoretical modeling and experimental studies were undertaken to study the subsolidus relations in pyroxenes. Intercrystalline (two-pyroxene) and intracrystalline (orthopyroxene) reactions were used to develop graphical geothermometers and determine thermal histories.;Experimental phase equilibrium data on coexisting pyroxenes (700 to 1400 {dollar}\sp\circ{dollar}C) in the pyroxene quadrilateral have been used to model the solvus resulting in the following set of thermochemical data and simple mixture parameters (W{dollar}\sb{lcub}\rm ij{rcub}{dollar}). Standard (T=298.15K) enthalpy and entropy of formation from the elements for metastable orthohedenbergite are {dollar}-{dollar}1416.8 kJ and 84.88 J/m/K respectively. The heat capacity is given by 114.67 + 17.09E-3 T {dollar}-{dollar} 31.40E5 T{dollar}\sp{lcub}-2{rcub}{dollar}. The W{dollar}\sb{lcub}\rm ij{rcub}{dollar} data are: (Enstatite = 1, diopside = 2, ferrosilite = 3, hedenbergite = 4) Opx: W{dollar}\sb{12}{dollar} = W{dollar}\sb{lcub}21{rcub}{dollar} 25; W{dollar}\sb{13}{dollar} = (13.1{dollar}-{dollar}0.015 T); W{dollar}\sb{lcub}31{rcub}{dollar} = (3.37 {dollar}-{dollar} 0.005 T); W{dollar}\sb{lcub}23{rcub}{dollar} = 20; W{dollar}\sb{lcub}32{rcub}{dollar} = 16; W{dollar}\sb{lcub}24{rcub}{dollar} = 5; W{dollar}\sb{lcub}42{rcub}{dollar} = 7; W{dollar}\sb{lcub}34{rcub}{dollar} = 15; W{dollar}\sb{lcub}43{rcub}{dollar} = 15; W{dollar}\sb{14}{dollar} =60; W{dollar}\sb{lcub}41{rcub}{dollar} = 30-0.00834 T; Cpx: W{dollar}\sb{12}{dollar} =(25.484 + 0.0812 P); W{dollar}\sb{lcub}21{rcub}{dollar} = (31.216 {dollar}-{dollar} 0.0061 P); W{dollar}\sb{lcub}31{rcub}{dollar} = W{dollar}\sb{13}{dollar} =0; W14 = (9.3 {dollar}-{dollar} 0.045 T); W{dollar}\sb{lcub}41{rcub}{dollar} = ({dollar}-{dollar}20.0 + 0.028 T); W{dollar}\sb{lcub}23{rcub}{dollar} = 24; W{dollar}\sb{lcub}32{rcub}{dollar} = 15; W{dollar}\sb{lcub}24{rcub}{dollar} = 12; W{dollar}\sb{lcub}42{rcub}{dollar} = 12; W{dollar}\sb{lcub}34{rcub}{dollar} = (16.41 + 0.00592 P); W{dollar}\sb{lcub}43{rcub}{dollar} = (20.697 {dollar}-{dollar} 0.00235 P).;Two graphical geothermometers have been constructed based on the hypothesis that Fe{dollar}\sp{lcub}2+{rcub}{dollar}-Mg exchange has a lower closure temperature than the Ca-transfer. Quenched igneous rocks yielded similar temperatures for both geothermometers; plutonic rocks showed a lower closure temperature for the Fe{dollar}\sp{lcub}2+{rcub}{dollar}-Mg exchange; metamorphic rocks agreed with other geothermometers but gave mixed temperature relations.;Single crystal X-ray techniques were used to study isothermal Fe{dollar}\sp{lcub}2+{rcub}{dollar}-Mg disordering in two orthopyroxene samples, Fs62 and Fs82, at temperatures 625, 675, 725{dollar}\sp\circ{dollar}C and 525, 575 and 625{dollar}\sp\circ{dollar}C respectively. The structural data provides an internal constraint on the degree of ordering. Disordering rate constants yielded activation energies of 49 and 51 kcal/mol respectively.;Time-temperature-transformation (TTT) diagrams have been calculated for the extent of ordering in the natural samples (Fs62; Fs82), both from granulitic terranes. Calculated cooling rates are 140 and 11{dollar}\sp\circ{dollar}/million years respectively; the latter is within geophysical expectations. Errors introduced and propagated by this method are estimated and it is believed these cooling rates can never be better than "order of magnitude"; equivalent to many geophysical estimates.
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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.
