We report results from a 40Ar/39Ar sanidine and CA-TIMS 238U/206Pb zircon dating study of eruption and crystal residence timescales of the Alder Creek Rhyolite (ACR), California, extruded during the Cobb Mountain normal-polarity subchron (C1r.2n). A 40Ar/39Ar ACR sanidine date of 1.1850 ± 0.0016 Ma (2σ external uncertainty), determined relative to the astronomically dated A1 tephra sanidine, is interpreted as the ACR eruption age. This age is supported by CA-TIMS U–Pb zircon dating, guided by LA-ICPMS trace element analyses, titanium-in-zircon (TiZR) thermometry, and cathodoluminescence (CL) imaging. Using these data, two compositionally distinct zircon populations were revealed. “Pre-ACR” Group B zircon exhibit oscillatory zoning, large positive Ce and negative Eu anomalies, high incompatible trace element contents, TiZR temperatures of 650 °C–750 °C, and Th/U disequilibrium corrected 238U/206Pb dates of 1.38–1.24 Ma. These crystals are interpreted as antecrysts inherited from earlier intrusives in the Geysers–Cobb Mountain magma source region. “ACR-related” Group A zircon, present as discrete grains and overgrowths on Group B zircon, display less intense CL with diffuse zoning, and have less pronounced positive Ce and negative Eu anomalies, lower incompatible trace element contents, higher TiZR temperatures that range up to 840 °C, and significantly younger dates. The youngest Group A dates yield a weighted mean of 1.1978 ± 0.0046 Ma (2σ, including systematic uncertainties) that is interpreted as the mean age of zircon crystallization prior to eruption. The 13 ± 5 ka offset between the 40Ar/39Ar and 238U/206Pb dates can be attributed to zircon magma residence time. Recognition of a young population of ACR zircon is consistent with the 40Ar/39Ar eruption age, which coincides with the astronomical age estimate for the Cobb Mountain subchron determined by correlating the oxygen isotope record of the giant piston core MD972143 to the La93(1,1) orbital solution. On the basis of independent radio-isotopic and orbital forcing results, we propose the refined age of 1.1850 ± 0.0016 Ma (2σ external uncertainty) for the Quaternary ACR 40Ar/39Ar sanidine standard.
- Cobb Mountain