Thermodynamics and kinetics of Fe²⁺-Mg exchange between orthopyroxene and spinel: Experimental calibrations and applications

Persistent Link:
http://hdl.handle.net/10150/289170
Title:
Thermodynamics and kinetics of Fe²⁺-Mg exchange between orthopyroxene and spinel: Experimental calibrations and applications
Author:
Liermann, Hanns-Peter
Issue Date:
2000
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 variation of equilibrium Fe²⁺-Mg fractionation between orthopyroxene and spinel as a function of temperature was experimentally determined in the range from 1250-850°C and 0.8-1.3 GPa. The effect of chromium on the equilibrium fractionation was constrained by limited experiments at 1000°C and 1.2 GPa. The experimental data can be cast into a thermometric expression as: T(K) = [(1373±155)+121· P(GPa)+(2558±181)·Xˢᵖ(Cr3+)]/[ln K(D) + (0.55± 0.13)] where K(D) = (Fe²⁺/Mg)ˢᵖ/(Fe²⁺/Mg)ᴼᵖˣ. The self-diffusion of Fe²⁺ and Mg in spinel was determined from binary diffusion couple experiments at 1325-950°C and 2.0 GPa. These experiments yield for Fe²⁺: Dₒ = (1.8 ± 2.8) x 10⁻⁵ cm²/sec and Q = (198 ± 19) kJ/mol; and for Mg: Dₒ = (1.9 ± 1.4) x 10-5 cm²/sec and Q = (202 ± 8) kJ/mol. The above data were applied to compositional profiles of coexisting orthopyroxenes and spinels in diogenite samples from the Antarctic meteorite collection to constrain their thermal histories. The modeling suggests that diogenites cooled to a temperature of ≥ 885°C at a rate of ≤ 3800°C/Ma at 1000°C. Modeling of the compositional zoning of spinel indicates that it cooled at ∼(6-20) x 10³ °C/Ma at 700°C corresponding to a burial of ∼65-130 m within a 100-200 m thick regolith blanket. This supports the idea that the samples were ejected and buried within a blanket of regolith material after initial cooling from the magmatic stage. Application of the diffusion data for spinel to the cooling rates of terrestrial ultrabasic rocks suggests that Ozawa (1984) cooling rates derived from the modeling of compositional profiles between olivine and spinel should be revised downwards by approximately two orders of magnitude.
Type:
text; Dissertation-Reproduction (electronic)
Keywords:
Geology.; Geochemistry.
Degree Name:
Ph.D.
Degree Level:
doctoral
Degree Program:
Graduate College; Earth Sciences
Degree Grantor:
University of Arizona
Advisor:
Ganguly, Jibamitra

Full metadata record

DC FieldValue Language
dc.language.isoen_USen_US
dc.titleThermodynamics and kinetics of Fe²⁺-Mg exchange between orthopyroxene and spinel: Experimental calibrations and applicationsen_US
dc.creatorLiermann, Hanns-Peteren_US
dc.contributor.authorLiermann, Hanns-Peteren_US
dc.date.issued2000en_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 variation of equilibrium Fe²⁺-Mg fractionation between orthopyroxene and spinel as a function of temperature was experimentally determined in the range from 1250-850°C and 0.8-1.3 GPa. The effect of chromium on the equilibrium fractionation was constrained by limited experiments at 1000°C and 1.2 GPa. The experimental data can be cast into a thermometric expression as: T(K) = [(1373±155)+121· P(GPa)+(2558±181)·Xˢᵖ(Cr3+)]/[ln K(D) + (0.55± 0.13)] where K(D) = (Fe²⁺/Mg)ˢᵖ/(Fe²⁺/Mg)ᴼᵖˣ. The self-diffusion of Fe²⁺ and Mg in spinel was determined from binary diffusion couple experiments at 1325-950°C and 2.0 GPa. These experiments yield for Fe²⁺: Dₒ = (1.8 ± 2.8) x 10⁻⁵ cm²/sec and Q = (198 ± 19) kJ/mol; and for Mg: Dₒ = (1.9 ± 1.4) x 10-5 cm²/sec and Q = (202 ± 8) kJ/mol. The above data were applied to compositional profiles of coexisting orthopyroxenes and spinels in diogenite samples from the Antarctic meteorite collection to constrain their thermal histories. The modeling suggests that diogenites cooled to a temperature of ≥ 885°C at a rate of ≤ 3800°C/Ma at 1000°C. Modeling of the compositional zoning of spinel indicates that it cooled at ∼(6-20) x 10³ °C/Ma at 700°C corresponding to a burial of ∼65-130 m within a 100-200 m thick regolith blanket. This supports the idea that the samples were ejected and buried within a blanket of regolith material after initial cooling from the magmatic stage. Application of the diffusion data for spinel to the cooling rates of terrestrial ultrabasic rocks suggests that Ozawa (1984) cooling rates derived from the modeling of compositional profiles between olivine and spinel should be revised downwards by approximately two orders of magnitude.en_US
dc.typetexten_US
dc.typeDissertation-Reproduction (electronic)en_US
dc.subjectGeology.en_US
dc.subjectGeochemistry.en_US
thesis.degree.namePh.D.en_US
thesis.degree.leveldoctoralen_US
thesis.degree.disciplineGraduate Collegeen_US
thesis.degree.disciplineEarth Sciencesen_US
thesis.degree.grantorUniversity of Arizonaen_US
dc.contributor.advisorGanguly, Jibamitraen_US
dc.identifier.proquest9983877en_US
dc.identifier.bibrecord.b4082455xen_US
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