Experimental studies of iron-magnesium order-disorder in orthopyroxene: Equilibrium, kinetics, and applications

Persistent Link:
http://hdl.handle.net/10150/289990
Title:
Experimental studies of iron-magnesium order-disorder in orthopyroxene: Equilibrium, kinetics, and applications
Author:
Stimpfl, Marilena
Issue Date:
2003
Publisher:
The University of Arizona.
Rights:
Copyright © is held by the author. Digital access to this material is made possible by the University Libraries, University of Arizona. Further transmission, reproduction or presentation (such as public display or performance) of protected items is prohibited except with permission of the author.
Abstract:
The thermodynamic and the kinetics of the Fe-Mg order-disorder process in orthopyroxene were studied by means of thermal annealing experiments at fix fO₂ condition, and single crystal X-raydiffraction to determine the site occupancies. The behavior of Mn on the equilibrium fractionation of Fe-Mg was studied by a series of annealing experiments using a naturally occurring Mn-rich orthopyroxene (donpeacorite). It was shown that, although Mn and Fe preferentially order to the M2 site relative to Mg, Mn has a significantly stronger M2 site preference than Fe. This result implies that when computing the site fractionation for Fe-Mg in Fe-rich Mn-poor orthopyroxene, the small amount of Mn present in the structure should always be totally ordered in the M2 site. The kinetics of the order-disorder reaction was studied as a function of temperature, composition, and fO₂. The results are compatible with the theoretical predicted variation of the rate constant as (fO₂)¹/⁶. The temperature and compositional dependence of the disordering rate constant, K⁺, can be expressed as: ln K⁺(Ord) = - [(36420)/T(K)] min⁻¹ + 29.03 + 4162(XFe) where XFe is the Fe molar fraction of the sample. The thermodynamic and kinetics data of Fe-Mg order-disorder in orthopyroxene permit retirval of cooling rates natural orthopyroxene crystals from their quenched ordering states around the closure temperature (T(C)) of ordering. Thus, I have applied the data to Central Gneiss Complex, British Columbia, and to a diogenite-meteorite which is believed to have originated on Vesta, to constrain their cooling rates. For the Central Gneiss Complex the cooling rate was found to be ∼10-15°C/My at T(C) ∼290°C. This cooling rate is in excellent agreement with that constrained by geochronological data, and implies an exhumation velocity of ∼0.2mm/y. The thermal history inferred for the diogenite-meteorite GRO95555 suggests that the sample underwent very fast cooling at two different rates: a faster cooling at ∼400°C/year as retrieved from modeling of the compositional zoning the orthopyroxene-spinel couple, followed by a slower cooling at ∼5°C/1000y as obtained from the modeling of the observed quenched state in the orthopyroxene. The implication of these rapid but constraining cooling rates on the excavation and burial of the sample in its parent body has been discussed.
Type:
text; Dissertation-Reproduction (electronic)
Keywords:
Geology.; Mineralogy.
Degree Name:
Ph.D.
Degree Level:
doctoral
Degree Program:
Graduate College; Geoscience
Degree Grantor:
University of Arizona
Advisor:
Ganguly, Jibamitra

Full metadata record

DC FieldValue Language
dc.language.isoen_USen_US
dc.titleExperimental studies of iron-magnesium order-disorder in orthopyroxene: Equilibrium, kinetics, and applicationsen_US
dc.creatorStimpfl, Marilenaen_US
dc.contributor.authorStimpfl, Marilenaen_US
dc.date.issued2003en_US
dc.publisherThe University of Arizona.en_US
dc.rightsCopyright © is held by the author. Digital access to this material is made possible by the University Libraries, University of Arizona. Further transmission, reproduction or presentation (such as public display or performance) of protected items is prohibited except with permission of the author.en_US
dc.description.abstractThe thermodynamic and the kinetics of the Fe-Mg order-disorder process in orthopyroxene were studied by means of thermal annealing experiments at fix fO₂ condition, and single crystal X-raydiffraction to determine the site occupancies. The behavior of Mn on the equilibrium fractionation of Fe-Mg was studied by a series of annealing experiments using a naturally occurring Mn-rich orthopyroxene (donpeacorite). It was shown that, although Mn and Fe preferentially order to the M2 site relative to Mg, Mn has a significantly stronger M2 site preference than Fe. This result implies that when computing the site fractionation for Fe-Mg in Fe-rich Mn-poor orthopyroxene, the small amount of Mn present in the structure should always be totally ordered in the M2 site. The kinetics of the order-disorder reaction was studied as a function of temperature, composition, and fO₂. The results are compatible with the theoretical predicted variation of the rate constant as (fO₂)¹/⁶. The temperature and compositional dependence of the disordering rate constant, K⁺, can be expressed as: ln K⁺(Ord) = - [(36420)/T(K)] min⁻¹ + 29.03 + 4162(XFe) where XFe is the Fe molar fraction of the sample. The thermodynamic and kinetics data of Fe-Mg order-disorder in orthopyroxene permit retirval of cooling rates natural orthopyroxene crystals from their quenched ordering states around the closure temperature (T(C)) of ordering. Thus, I have applied the data to Central Gneiss Complex, British Columbia, and to a diogenite-meteorite which is believed to have originated on Vesta, to constrain their cooling rates. For the Central Gneiss Complex the cooling rate was found to be ∼10-15°C/My at T(C) ∼290°C. This cooling rate is in excellent agreement with that constrained by geochronological data, and implies an exhumation velocity of ∼0.2mm/y. The thermal history inferred for the diogenite-meteorite GRO95555 suggests that the sample underwent very fast cooling at two different rates: a faster cooling at ∼400°C/year as retrieved from modeling of the compositional zoning the orthopyroxene-spinel couple, followed by a slower cooling at ∼5°C/1000y as obtained from the modeling of the observed quenched state in the orthopyroxene. The implication of these rapid but constraining cooling rates on the excavation and burial of the sample in its parent body has been discussed.en_US
dc.typetexten_US
dc.typeDissertation-Reproduction (electronic)en_US
dc.subjectGeology.en_US
dc.subjectMineralogy.en_US
thesis.degree.namePh.D.en_US
thesis.degree.leveldoctoralen_US
thesis.degree.disciplineGraduate Collegeen_US
thesis.degree.disciplineGeoscienceen_US
thesis.degree.grantorUniversity of Arizonaen_US
dc.contributor.advisorGanguly, Jibamitraen_US
dc.identifier.proquest3108958en_US
dc.identifier.bibrecord.b44830737en_US
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