Incremental heating of Bishop Tuff sanidine reveals preeruptive radiogenic Ar and rapid remobilization from cold storage Nathan L. Andersena,1,2, Brian R. Jichaa, Brad S. Singera,1, and Wes Hildrethb aDepartment of Geoscience, University of Wisconsin–Madison, Madison, WI 53706; and bUnited States Geological Survey, Menlo Park, CA 94025 Edited by Kenneth A. Farley, California Institute of Technology, Pasadena, CA, and approved August 30, 2017 (received for review June 5, 2017) Accurate and precise ages of large silicic eruptions are critical to sensitivity of multicollector noble gas mass spectrometers relative to calibrating the geologic timescale and gauging the tempo of older, single-collector instruments now yields dates that are nearly changes in climate, biologic evolution, and magmatic processes an order of magnitude more precise. These high-precision dates throughout Earth history. The conventional approach to dating reveal intracrystal and intercrystal heterogeneities that require in- these eruptive products using the 40Ar/39Ar method is to fuse doz- terpretation and complicate the assignment of an eruption age (9– ens of individual feldspar crystals. However, dispersion of fusion 11). However, the relative contributions of the sources of these dates is common and interpretation is complicated by increasingly perturbations and their implications for magma dynamics are not precise data obtained via multicollector mass spectrometry. Incre- commonly explored. mental heating of 49 individual Bishop Tuff (BT) sanidine crystals This enhanced analytical resolution also allows for the incremental produces 40Ar/39Ar dates with reduced dispersion, yet we find a 16- heating of single young sanidine crystals that can reduce the overall dispersion of the dataset and discriminate between subpopulations of ky range of plateau dates that is not attributable to excess Ar. We – interpret this dispersion to reflect cooling of the magma reservoir dates that are convoluted by low-precision techniques (10 14). We margins below ∼475 °C, accumulation of radiogenic Ar, and rapid applied an incremental heating multicollector mass spectrometry preeruption remobilization. Accordingly, these data elucidate the (IH-MCMS) procedure (11) (see Supporting Information for details) recycling of subsolidus material into voluminous rhyolite magma to dating single sanidine from the Bishop Tuff (BT), an extensively reservoirs and the effect of preeruptive magmatic processes on studied middle Pleistocene rhyolitic fall and ignimbrite deposit 40 39 erupted from Long Valley Caldera, CA (Fig. 1). The proximity of the the Ar/ Ar system. The youngest sanidine dates, likely the most normally magnetized BT to the Matuyama−Brunhes magnetic po- representative of the BT eruption age, yield a weighted mean of larity reversal makes it an important middle Pleistocene stratigraphic 764.8 ± 0.3/0.6 ka (2σ analytical/full uncertainty) indicating eruption marker in the western United States (15), and models of its mag- only ∼7 ky following the Matuyama−Brunhes magnetic polarity matic evolution have shaped the current understanding of the dy- reversal. Single-crystal incremental heating provides leverage with 40 39 namics of voluminous silicic magma systems (16). which to interpret complex populations of Ar/ Ar sanidine and The 40Ar/39Ar age of the BT eruption has recently been con- U-Pb zircon dates and a substantially improved capability to resolve troversial due to proposed ages, 776.4 ka to 780.0 ka (17, 18), that the timing and causal relationship of events in the geologic record. are older than the youngest zircon dates (19, 20). Subsequent 40Ar/39Ar | geochronology | Bishop Tuff | magma reservoir Significance ccurate, high-precision geochronology of volcanic ash de- Recent improvements in analytical and microsampling tech- Aposits is essential to determine the timing of magnetic, niques for multiple geochronometers have resulted in datasets tectonic, biologic, and climate events and the rates of surficial and with unprecedented temporal and spatial resolution. These ad- deep Earth processes. It is indispensable for establishing causal re- EARTH, ATMOSPHERIC, vances are accompanied by the discovery of crystal- and outcrop- AND PLANETARY SCIENCES lationships between physical and biologic processes that occur over scale complexities previously obscured by low analytical pre- only centuries to millennia (1). Moreover, the crystallization histo- cision. Single-crystal incremental heating resolves subtle, intra- ries revealed by high-precision zircon and sanidine dates establish a crystal isotopic heterogeneity, allowing for more-accurate 40Ar/39Ar tempo for the dynamics of crustal magmatism and triggering of eruption ages. The eruption ages of widespread volcanic ash large, caldera-forming eruptions (2, 3). Voluminous ash fall deposits deposits are critical for calibrating the geologic timescale, and from explosive silicic eruptions are also important chronostrati- thus their accuracy has substantial implications for the geologic, graphic markers. Their wide dispersal allows for intercalibration biologic, and global climate records. Complex distribution of among radioisotopic, geomagnetic, and astrochronologic timescales 40Ar/39Ar dates in the deposits of supervolcanic eruptions re- and correlation of the marine and terrestrial records (1, 4, 5). quires rethinking the magmatic processes and their effect on the 40 39 The Ar/ Ar method is among the most commonly employed 40Ar/39Ar system, specifically the extent of cooling and remobi- techniques to determine the eruption ages of these deposits, typi- lization during the decades to centuries preceding these events. cally by fusing dozens of individual sanidine crystals. We use the term “date” when referring to time calculated using the radiogenic Author contributions: N.L.A., B.R.J., and B.S.S. designed research; N.L.A. and B.R.J. per- parent–daughter ratios measured in a single crystal. An “age” refers formed research; N.L.A., B.R.J., and B.S.S. analyzed data; N.L.A., B.R.J., B.S.S., and W.H. to the geologic significance of a date, or group of dates, and as in wrote the paper; and B.S.S. and W.H. performed field work. this study may require interpretation of large sets of dates from a The authors declare no conflict of interest. common rock or deposit (6). Owing to rapid diffusion of Ar at This article is a PNAS Direct Submission. 40 39 magmatic temperatures, Ar/ Ar dates are commonly interpreted Published under the PNAS license. as eruption ages without the ambiguity of protracted crystallization 1To whom correspondence may be addressed. Email: [email protected] or intervals recorded by U-Pb dates of accessory phases (1). However, [email protected]. dispersion of the nominal dates produced by sanidine fusion analysis 2Present address: School of Earth and Atmospheric Sciences, Georgia Institute of Technol- is common and typically attributed to xenocrysts, nonradiogenic Ar, ogy, Atlanta, GA 30332. or Ar loss. Filtering and pooling many low-precision dates may yield This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10. a statistically valid weighted mean age (7, 8). However, the increased 1073/pnas.1709581114/-/DCSupplemental. www.pnas.org/cgi/doi/10.1073/pnas.1709581114 PNAS | November 21, 2017 | vol. 114 | no. 47 | 12407–12412 Downloaded by guest on October 1, 2021 isotope dilution thermal ionization mass spectrometry (ID- variation of sanidine 40Ar/39Ar age with crystal size. Similarly, TIMS) and laser ablation inductively coupled plasma mass despite the well-documented compositional and thermal zoning of spectrometry (LA-ICP-MS) studies of BT zircon show that the the BT magma reservoir (16), there is no stratigraphic gradient in geologic uncertainties of the initial zircon U-series disequilib- sanidine incremental heating dates (Fig. 2). Each sample contains rium are not sufficient to produce this discordance between the a coeval population of dates at ca. 765 ka; however, crystals that 40Ar/39Ar and U-Pb systems (21, 22). The older 40Ar/39Ar yield older plateau dates are more abundant in the ignimbrites ages for the BT are calculated relative to the age of the Alder than in the fall units. Creek sanidine standard (ACs) proposed by Renne et al. (17). Several recent studies suggest the age of the ACs is >1% younger Sources of Age Dispersion than previous estimates (11, 23, 24), thereby bringing all recent The 16-ky spread in the IH-MCMS plateau dates exceeds that 40Ar/39Ar and U-Pb BT ages into agreement. predicted by the analytical uncertainties. A weighted mean of all However, all BT 40Ar/39Ar single-crystal fusion datasets exhibit 49 plateau dates has a mean square weighted deviation (MSWD; a significant range, 40 ky to 420 ky, between the oldest and i.e., reduced χ2 statistic) of 12.5. However, this 16-ky dispersion is youngest dates (7, 8, 18). We present high-precision IH-MCMS 2.5to25timeslessthanthatofrecent,“high-precision” single- dates to better characterize the BT sanidine population and crystal fusion datasets (7, 8, 18). Moreover, the youngest 70% of thereby improve the accuracy and precision of the eruption age. the plateau dates comprise a range of only 5.4 ky, indicating most Moreover, these data provide leverage with which to investigate of the 16 ky spread is produced by a subordinate crystal population. the source and geologic significance of scattered 40Ar/39Ar sani- The accuracy of a 40Ar/39Ar eruption age depends on isolating dine dates that require