Rutile (U-Th)/he Thermochronology

Robinson, Katherine H.

Graduate Thesis Or Dissertation

Rutile (U-Th)/he Thermochronology Public Deposited

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Citeable URL: https://scholar.colorado.edu/concern/graduate_thesis_or_dissertations/nz805z99q

Abstract

Rutile is a relatively common phase in metamorphic and to a lesser extent igneous rocks. Its development as a (U-Th)/He thermochronometer would allow its application to decipher thermal histories in a broader range of lithologies than currently accessible to the technique. This study aims to determine the temperature sensitivity of He diffusion in rutile and evaluate the utility of rutile as a (U-Th)/He thermochronometer. (U-Th)/He analyses were acquired on individual rutile grains with well-constrained Phanerozoic crystallization ages from samples with independent thermochronologic constraints. These samples include a mafic granulite from Fiordland, New Zealand, three South African kimberlites, a schist from the East Humboldt Range in Nevada, USA, and an Appalachian eclogite. Data were also obtained for rutile from Proterozoic samples, but an analytical problem was encountered for these grains in which higher laser degassing powers (≥10A) volatilized the parent isotopes, while lower laser power (6A) failed to consistently and completely extract all He gas from the crystal. Therefore, interpretations in this study focus on the data from the six younger samples. RHe dates for 5 of these samples are either fairly reproducible (6-22% sample standard deviation) or yield a positive RHe date-eU correlation. RHe dates are older than or overlap with apatite (U-Th)/He dates that are available for four of these samples but are younger than zircon (U-Th)/He dates available for two of these samples. These results imply that the He closure temperature (Tc) of Phanerozoic rutile is between that of apatite (~70 °C) and lower-damage zircon (~185°C). The sixth sample yields more dispersed results, which may be explained by titanite overgrowths imaged in BSE that biased the sample dates. One 4He diffusion experiment on rutile from the Ordovician eclogite sample suggests a Tc of ~157°C, consistent with the empirically-derived Tc above, although additional diffusion experiments on Proterozoic rutile yielded inconsistent results. Rutile degassing patterns for Phanerozoic versus Proterozoic rutile, as well as the positive RHe date-eU correlation in one sample, imply that radiation damage may increase rutile He retentivity.

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