Publication Date:
2019
Description:
Abstract
Rutile (TiO2) is a common U‐Th‐bearing accessory mineral in metamorphic and, to a lesser extent, igneous rocks. Its development as a (U‐Th)/He thermochronometer would diversify the lithologies dateable with the (U‐Th)/He technique. We report (U‐Th)/He dates for rutile with crystallization ages that vary from Cretaceous to Proterozoic from 8 samples with independent constraints on their thermal history to empirically calibrate the temperature sensitivity of the rutile (U‐Th)/He (RHe) system. The results document an analytical challenge for the Proterozoic rutile, where degassing at laser powers typical for zircon volatilized the parent isotopes while lower laser powers failed to consistently and completely extract all 4He from the crystals. In contrast, the 6 Phanerozoic samples were easily degassed without U‐Th loss. RHe dates for 5 of these samples are fairly reproducible (6‐22% dispersion) or yield positive RHe date‐eU correlations. RHe dates are older than or overlap with apatite (U‐Th)/He dates available for four of these samples and are younger than zircon (U‐Th)/He dates available for two of these samples. This implies that the He closure temperature (Tc) for these Phanerozoic rutile is between apatite (~70 °C) and lower‐damage zircon (~200°C). 4He diffusion experiment data are consistent with anisotropic He diffusion, suggest differing retentivities for the analyzed rutile, and yield Tc estimates of ~155‐159°C for diffusion parallel to the c‐axis (at a 10°C/Ma cooling rate). These results, the degassing patterns for Phanerozoic versus Proterozoic rutile, and the positive RHe date‐eU correlations in two samples imply that radiation damage increases rutile He retentivity.
Electronic ISSN:
1525-2027
Topics:
Chemistry and Pharmacology
,
Geosciences
,
Physics
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