Unrecognized complexities of metamorphism : crystallization kinetics, reaction affinity, and geochronology

dc.contributor.advisorKetcham, Richard Alan, 1965-en
dc.contributor.advisorCarlson, William, 1952-en
dc.contributor.committeeMemberGardner, James E.en
dc.contributor.committeeMemberCloos, Marken
dc.contributor.committeeMemberPattison, Daviden
dc.contributor.committeeMemberFarver, John R.en
dc.creatorKelly, Eric Daviden
dc.date.accessioned2012-01-27T20:13:56Zen
dc.date.available2012-01-27T20:13:56Zen
dc.date.issued2011-12en
dc.date.submittedDecember 2011en
dc.date.updated2012-01-27T20:14:31Zen
dc.descriptiontexten
dc.description.abstractUnrecognized metamorphic complexities can produce erroneous interpretations when using equilibrium thermodynamics and isotope geochronology. Universally employed methods for determining pressure-temperature conditions during regional metamorphism are based on the assumption of chemical equilibrium, and geochronology in metamorphic rocks can suffer from cryptic redistribution of isotopes. In this research, the scales of disequilibrium in regionally metamorphosed rocks and the effects of garnet resorption on Lu-Hf garnet ages were examined through numerical simulations of these processes. Concerning scales of disequilibrium, thirteen porphyroblastic datasets, previously measured using X-ray computed tomography, were examined by numerically simulating diffusion-controlled nucleation and growth of garnet while tracking chemical potential gradients to determine reaction affinity Ar (-[Delta]rG). Maximum nucleation rates are 10⁻¹³̇⁶-10⁻⁹̇⁸ nuclei cm⁻³ s⁻¹, interfacial energies are 0.004-0.14 J m⁻² assuming shape factors of 0.1-1.0, and Al intergranular diffusion (QD = 140 kJ/mol⁻¹) is 10⁻¹⁴̇⁴-10⁻¹¹̇¹ m² s⁻¹ at 600 °C. Limitations in determining crystallization kinetics arise from difficulties in constraining rock-specific properties (e.g., porosity and Al solubility). Ar at the time and location of nucleation is 0.4-5.9 kJ/mol⁻¹ of 12-oxygen garnet ([Delta]T = 4.0-62.0 °C) for the earliest nuclei, and 5.3-29.0 kJ/mol⁻¹ ([Delta]T = 50-125 °C) for nucleation at maximum Ar. The results demonstrate potential for delayed nucleation and metastability that can generate spurious interpretations. The timing of metamorphic events is also critical for understanding geologic history. In the Makhavinekh Lake Pluton aureole, Labrador, garnet resorption caused redistribution of Lu and loss of Hf from consumed rims, creating spuriously young ages. Garnet-ilmenite Lu-Hf geochronology using bulk separates yields apparent ages that young toward the contact from 1876 ± 21 Ma (4025 m) to 1396 ± 8 Ma (450 m). Toward the contact, garnet crystals are progressively more resorbed. Numerical modeling was used to test retention of Lu and loss of Hf during resorption as the dominant control on age. More resorption and Lu retention produce younger apparent ages (false ages). Application of the model to the aureole yields model ages from 1850 Ma to 1374 Ma, younging toward the contact. Thus, Lu-Hf geochronology applied to resorbed garnets requires careful examination of Lu zoning.en
dc.description.departmentEarth and Planetary Sciencesen
dc.format.mimetypeapplication/pdfen
dc.identifier.slug2152/ETD-UT-2011-12-4781en
dc.identifier.urihttp://hdl.handle.net/2152/ETD-UT-2011-12-4781en
dc.language.isoengen
dc.subjectDisequilibriumen
dc.subjectLu-Hf geochronologyen
dc.subjectGarneten
dc.subjectReaction affinityen
dc.subjectGarnet resorptionen
dc.subjectThermal oversteppingen
dc.subjectMetamorphismen
dc.subjectCrystalization kineticsen
dc.titleUnrecognized complexities of metamorphism : crystallization kinetics, reaction affinity, and geochronologyen
dc.type.genrethesisen
thesis.degree.departmentGeological Sciencesen
thesis.degree.disciplineGeological Sciencesen
thesis.degree.grantorUniversity of Texas at Austinen
thesis.degree.levelDoctoralen
thesis.degree.nameDoctor of Philosophyen

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