(U-TH)/HE GEOCHRONOMETRY OF PHENOCRYSTIC AND XENOCRYSTIC SILICATE AND OXIDE PHASES FROM VOLCANIC ROCKS

Recent studies have reported (U-Th)/He data from U- and Th bearing accessory phases such as apatite, zircon, and titanite from well-established age standards and volcanic rocks of known age. These published and many unpublished age standard data have demonstrated the feasibility of (U-Th)/He dating to reliably constrain the timing of eruption and the subsequent thermal history of volcanic rocks. Detailed case studies of Oligocene to Pleistocene rhyolites and andesites from western Nevada were used to investigate the reliability of zircon and titanite (U-Th)/He dating of volcanic rocks as shown by reproducibility and intercalibration with new and published 40Ar/39Ar ages. The large data set, however, also exhibit potential pitfalls complicating age determinations, such as incorporation of partially reset or unreset xenocrystic zircon and titanite and partial to total resetting by overlying volcanic units or younger hydrothermal alteration. Zircon and titanite have the advantage of being little affected by chemical alteration and weathering that often characterize K-rich phases used for K-Ar or 40Ar/39Ar chronometry. Extensive (U-Th/He dating of both pheno- and xenocrystic oxides and silicates from kimberlitc and lamproitic rocks in eastern Kansas shows that (U-Th)/He dating can reliably constrain the emplacement age of rocks that are often difficult to date by 40Ar/39Ar or U/Pb due to excess Ar and common Pb, respectively. (U-Th)/He dating of zircon, titanite, rutile, perovskite, garnet, and clinopyroxene yield an emplacement age for the Kansas kimberlites of ~105 Ma, which is supported by independent Rb-Sr data on phlogopite. (U-Th)/He geochronometry of silicates and oxides from crustal and mantle xenoliths in basaltic and kimberlitic rocks also shows great potential for dating volcanic rocks. Preliminary data from mantle xenoliths from the Oligocene Sullivan Butte Latite (Chino Valley) and granitic xenoliths from a Pliocene basalt in eastern California demonstrate that (U-Th)/He thermochronometry provides reliable age constraints on these specific volcanic rocks. In summary, these data prove that (U-Th)He dating can yield reliable age constraints on volcanic rocks using phenocrystic and xenocrystic silicate and oxide phases from felsic to intermediate composition volcanic rocks and xenoliths from basaltic to kimberlitic rocks.