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Clumped Isotope Thermometry Of Calcite And Dolomite In A Contact Metamorphic Environment

Lloyd, Max K. and Eiler, John M. and Nabelek, Peter I. (2017) Clumped Isotope Thermometry Of Calcite And Dolomite In A Contact Metamorphic Environment. Geochimica et Cosmochimica Acta, 197 . pp. 323-344. ISSN 0016-7037. doi:10.1016/j.gca.2016.10.037.

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Clumped isotope compositions of slowly-cooled calcite and dolomite marbles record apparent equilibrium temperatures of roughly 150-200 °C and 300-350 °C, respectively. Because clumped isotope compositions are sensitive to the details of T-t path within these intervals, measurements of the Δ_(47) values of coexisting calcite and dolomite can place new constraints on thermal history of low-grade metamorphic rocks over a large portion of the upper crust (from ∼5 to ∼15 km depth). We studied the clumped isotope geochemistry of coexisting calcite and dolomite in marbles from the Notch Peak contact metamorphic aureole, Utah. Here, flat-lying limestones were intruded by a pluton, producing a regular, zoned metamorphic aureole. Calcite Δ_(47) temperatures are uniform, 156 ± 12 ˚C (2σ s.e.), across rocks varying from high-grade marbles that exceeded 500 °C to nominally unmetamorphosed limestones >5 km from the intrusion. This result appears to require that the temperature far from the pluton was close to this value; an ambient temperature just 20 ˚C lower would not have permitted substantial re-equilibration, and should have preserved depositional or early diagenetic Δ_(47) values several km from the pluton. Combining this result with depth constraints from overlying strata suggests the country rock here had an average regional geotherm of 22.3–27.4 ˚C/km from the late Jurassic Period until at least the middle Paleogene Period. Dolomite Δ_(47) in all samples above the talc+tremolite-in isograd record apparent equilibrium temperatures of 328^(+13)_(-12) °C (1σ s.e.), consistent with the apparent equilibrium blocking temperature we expect for cooling from peak metamorphic conditions. At greater distances, dolomite Δ_(47) records temperatures of peak (anchi)metamorphism or pre-metamorphic diagenetic conditions. The interface between these domains is the location of the 330 ˚C isotherm associated with intrusion. Multiple-phase clumped isotope measurements are complemented by bulk δ^(13)C and δ^(18)O dolomite-calcite thermometry. These isotopic exchange thermometers are largely consistent with peak temperatures in all samples within 4 km of the contact, indicating that metamorphic recrystallization can occur even in samples too low-grade to produce growth of conventional metamorphic index minerals (i.e., talc and tremolite). Altogether, this work demonstrates the potential of these methods to quantify the conditions of metamorphism at sub-greenschist facies.

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Lloyd, Max K.0000-0001-9367-2698
Additional Information:© 2016 Elsevier B.V. Received 23 March 2016, Revised 19 October 2016, Accepted 23 October 2016, Available online 29 October 2016. We would like to thank Julita Penido for her assistance with sample collection, Daniel Stolper for his instruction in modeling solid-state calcite reordering, and Nami Kitchen and Kristin Bergmann for their assistance with analytical techniques. George Rossman provided an additional sample and helpful discussion that strengthened this paper. Theodore Labotka and two anonymous reviewers provided thoughtful suggestions that greatly improved the quality of this paper. This research was funded by the Petrology and Geochemistry Program of the United States National Science Foundation (EAR-1322058).
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Official Citation:Max K. Lloyd, John M. Eiler, Peter I. Nabelek, Clumped isotope thermometry of calcite and dolomite in a contact metamorphic environment, Geochimica et Cosmochimica Acta, Volume 197, 15 January 2017, Pages 323-344, ISSN 0016-7037, (//
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:71737
Deposited By: Tony Diaz
Deposited On:04 Nov 2016 17:04
Last Modified:11 Nov 2021 04:50

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