Dating young basalt eruptions by (U-Th)/He on xenolithic zircons

Abstract

Accurate ages for young (e.g., Pleistocene) volcanic eruptions are important for geomorphic, tectonic, climatic, and hazard studies. Existing techniques can be time-consuming and expensive when many ages are needed, and in the case of K/Ar and 40Ar/39Ar dating, extraneous Ar often can limit precision, especially for continental basalts erupted through old lithosphere. We present a new technique for dating young basaltic eruptions by (U-Th)/He dating of zircons (ZHe) from crustal xenoliths. Single-crystal ZHe dates generally have lower precision than typical 40Ar/39Ar dates, but can be determined relatively easily on multiple replicate grain aliquots. We dated zircons from xenoliths from four volcanic centers in western North America: Little Bear Mountain, British Columbia (157 ± 3.5 [2.2%] ka weighted 95% confidence interval [CI], mean square of weighted deviates [MSWD] = 1.7) and Prindle Volcano, Alaska (176 ± 16 [8.9%] ka, MSWD = 13), in the northern Cordilleran volcanic province, and Fish Springs (273 ± 23 [8.6%], MSWD = 43) and Oak Creek (179 ± 8.1 [4.5%] ka, MSWD = 12), in the Big Pine Volcanic Field, California. All ZHe ages are either equivalent to or younger than previously determined K/Ar or 40Ar/39Ar ages, indicating the possibility of inherited 40Ar in some of the previous measurements. Zircons from upper crustal xenoliths in the Oak Creek and Fish Springs vents show poorer reproducibility and multiple apparent age distribution peaks, consistent with either intracrystalline U-Th zonation or <99.99% He degassing (assuming ca. 100 Ma pre-entrainment ZHe ages) of some zircons during magmatic entrainment. Removal of clear outliers in the older age-distribution peaks of the upper crustal xenoliths, most of which have extremely high U compared to other zircons of the same xenolith, improve the reproducibilities of Fish Springs to 4.7% (95% CI, MSWD = 4.8) and Oak Creek to 3.4% (95% CI, MSWD = 6.2). Coupled thermal and He diffusion modeling using appropriate xenolith sizes and magma temperatures and assuming published diffusion kinetics for zircon indicate that incomplete He degassing would require entrainment times <1 h. However, the observation of extremely high U in most zircons with older ages raises the possibility that zircons with high radiation dosages may have more retentive He diffusion characteristics.