RESEARCH ARTICLE Coexisting Discrete Bodies of Rhyolite and Punctuated 10.1029/2019GC008321 Volcanism Characterize Yellowstone's Post‐Lava Key Points: • Zircons from Yellowstone's Upper Creek Tuff Caldera Evolution ‐ Basin Member rhyolites yield U Pb Christy B. Till1 , Jorge A. Vazquez2 , Mark E. Stelten2 , Hannah I. Shamloo1 , dates defining crystallization 1,3 populations at ~750–550 and and Jamie S. Shaffer ~350–250 ka 1 2 • Discrete bodies of magma School of Earth and Space Exploration, Arizona State University, Tempe, AZ, USA, U.S. Geological Survey, Menlo Park, characterized the subvolcanic CA, USA, 3Now at Arizona State Geological Sciences, Now at New Mexico State University, Las Cruces, NM, USA system during the Upper Basin Member period and during storage of the Lava Creek Tuff ‐ 206 238 • Abstract Ion microprobe Pb/ U geochronology and trace element geochemistry of the unpolished The geochemical and isotopic ‐ evolution of Yellowstone's rims and sectioned interiors of zircons from Yellowstone caldera's oldest post caldera lavas provide post‐caldera rhyolites suggests a insight into the magmatic system during the prelude and aftermath of the caldera‐forming Lava Creek shift in the magmatic supereruption. The post‐caldera lavas compose the Upper Basin Member of the Plateau Rhyolite and fall assimilation/recharge ratio into two groups based on zircon crystallization age: early lavas with zircon ages between ~750 and 550 ka ‐ Supporting Information: and late lavas with zircon ages between ~350 and 250 ka. Zircons from the early erupted East Biscuit Basin • Supporting Information S1 flow yield U‐Pb dates and trace element compositions, which when considered with the Pb isotopic • Table S1 compositions of their coexisting feldspars and pyroxenes, point to an isotopically distinct parental melt • Table S2 • Table S3 present during crystallization of the Lava Creek magma but untapped by the supereruption. Distinct zircon crystallization ages and Pb‐isotope compositions of major minerals between the early and late Upper Basin Member groups suggest contrasting sources in the magma reservoir. As proxies for melt evolution, Correspondence to: the zircons indicate that Yellowstone's post‐caldera rhyolites became more evolved between mid‐ to C. B. Till, late‐Pleistocene time, during the same interval that melting of hydrothermally altered wall rock and [email protected] recharge by new silicic magmas changed in their relative roles. The results from this study indicate that discrete and ephemeral bodies of silicic magma, at times within a mush dominated reservoir and including Citation: during the prelude to the Lava Creek eruption, have characterized Yellowstone's subvolcanic reservoir. Till, C. B., Vazquez, J. A., Stelten, M. E., Shamloo, H. I., & Shaffer, J. S. (2019). Coexisting discrete bodies of rhyolite and punctuated volcanism characterize 1. Introduction Yellowstone's post‐Lava Creek Tuff caldera evolution. Geochemistry, The volcanic history of Yellowstone caldera after the circa 630‐ka eruption (Matthews et al., 2015) of the Geophysics, Geosystems, 20. https://doi. org/10.1029/2019GC008321 Lava Creek Tuff (LCT) includes episodes of rhyolitic eruptions with intervals of volcanic repose on the order of 104–105 years (Christiansen et al., 2007). At least 23 eruptions of mostly effusive rhyolites have occurred Received 12 MAR 2019 during three apparent intervals (Figure 1) since the Lava Creek eruption, with individual flow unit volumes Accepted 28 JUN 2019 of 2–150 km3 for a cumulative volume of >500 km3 (Christiansen, 2001). Together, these post‐caldera rhyo- Accepted article online 11 JUL 2019 lites track the magmatic evolution of the Yellowstone magmatic system (e.g., Befus & Gardner, 2016; Bindeman et al., 2008; Hildreth et al., 1984, 1991; Girard & Stix, 2009, 2010; Pritchard & Larson, 2012; Stelten et al., 2013, 2015, 2017). A rich body of work on the physical and geochemical nature of silicic mag- matic systems (see reviews by Bachmann & Huber, 2016; Cashman & Sparks, 2013; Cashman & Giordano, 2014; Lipman & Bachmann, 2015; de Silva & Gregg, 2014) has revealed that silicic magma reservoirs at cal- dera volcanoes are variable mixtures of melt, fluid, and crystals that are formed and assembled over multiple levels in the crust, and these phases may evolve over time through complex interactions with wall rocks and additions of new magma. A shared conclusion is that silicic magmas spend the majority of their lifetimes as near‐solidus high‐crystallinity bodies within the upper crust, where the high percentage of crystals relative to silicate liquid (referred to as a “crystal mush”) limits their mobility and in turn their eruptibility (e.g., Bachmann & Bergantz, 2004; Cooper & Kent, 2014; Gualda et al., 2012; Hildreth, 2004; Huber et al., 2009; Miller & Wark, 2008). Outstanding questions remain, including (1) What is the architecture of subvolcanic reservoirs including the spatial relation and interconnectivity of crystal‐rich and crystal‐poor magma domains? (2) What are the timescales for storage and geochemical evolution of silicic magma bodies asso- ciated with the large caldera‐forming, as well as smaller effusive eruptions? and (3) What are the relative ‐ ©2019. American Geophysical Union. roles of recharge, crystallization, and assimilation during the long term evolution of a caldera volcano, All Rights Reserved. and how are these factors reflected in the crystal and melt record of magmatic‐volcanic evolution? This TILL ET AL. 1 Geochemistry, Geophysics, Geosystems 10.1029/2019GC008321 study addresses these questions at Yellowstone by combining zircon U‐Pb geochronology and trace element geochemistry, the Pb‐isotope composi- tion of pyroxenes, feldspars, and glasses, as well as the petrologic con- straints from past studies. 2. Geologic Setting 2.1. Temporal Evolution The Yellowstone Plateau volcanic field has generated three caldera‐ forming eruptions over the past 2.1 Ma (Christiansen, 2001), the most recent of which produced the >1,000‐km3 LCT at 631.3 ± 4.3 ka (2σ; Matthews et al., 2015). The LCT ignimbrite around Yellowstone caldera is composed of two members (LCT‐A and LCT‐B) that represent two pyr- oclastic phases indistinguishable in age but separated by a cooling break (Christiansen, 2001). A recent study concluded that LCT may have been preceded by two additional tuff units that erupted shortly before members A and B, although the volumes of these new units are unclear (Wilson et al., 2018). Following eruption of the LCT, post‐caldera rhyolites were Figure 1. Summary of eruption ages for the second and third caldera cycles erupted over three apparent intervals (~630–450; ~260; and ~170–75 ka) on the Yellowstone Plateau. Stratigraphic order is from Christiansen et al. (2007). Colored boxes represent eruption age with 2σ error. Eruption ages of mostly effusive intracaldera volcanism (Christiansen, 2001; 40 39 ‐ are based on Ar/ Ar geochronology (Christiansen et al., 2007; Matthews Christiansen et al., 2007). Together with some extra caldera rhyolites, et al., 2015; Stelten et al., 2015; Stelten et al., 2018), with the exception of the the post‐caldera lavas from these intervals compose the formal Plateau North Biscuit Basin and East Biscuit Basin flows, which are based on the Rhyolite, with the rhyolites from the early and middle episodes compos- ‐ youngest U Pb crystallization age from zircon rims (this study). Yellow ing the Upper Basin Member (UBM) and the rhyolites of the youngest epi- boxes denote Central Plateau Member (CPM) units. Early and late divisions of Upper Basin Member (UBM) units are denoted by light and dark green sode composing the Central Plateau Member (CPM; Christiansen & colors, respectively. The pre‐caldera Lewis Canyon‐Mount Jackson units are Blank, 1972; Figure 1). The early UBM rhyolites are exposed in the center denoted in teal and the second caldera Island Park units in black. The and eastern margins of the caldera (Figure 2) and yield 40Ar/39Ar dates caldera‐forming units are denoted in red. Unit abbreviations are as follows: between 527 ± 28 and 489 ± 20 ka (Figure 1; 2σ uncertainties, data of MBB (Middle Biscuit Basin), NBB: (North Biscuit Basin), EBB (East Gansecki et al., 1996, recalculated to the Fish Canyon sanidine monitor Biscuit Basin), LCT (Lava Creek Tuff), MFT (Mesa Falls Tuff). SCL (Scaup δ18 Lake), and SBB (South Biscuit Basin). at 28.17 Ma). These early UBM rhyolites are characterized by O values as low as ~1‰ (Bindeman et al., 2008; Bindeman & Valley, 2001; Hildreth et al., 1984; Pritchard & Larson, 2012). Recently, Wilson et al. (2018) pro- posed a revised caldera margin based on newly recognized LCT exposures, which may mean that early UBM rhyolites near Sour Creek dome erupted from vents just outside of Yellowstone caldera. After a circa 220‐kyr hiatus (Figure 1), the late UBM rhyolites made up of the South Biscuit Basin (SBB) flow (255 ± 22 ka; Bindeman et al., 2008) and Scaup Lake (SCL) flow (262 ± 26 ka; Christiansen et al., 2007) were erupted near the Mallard Lake resurgent dome (Figure S1 in the supporting information). Although the two intervals of UBM volcanism erupted rhyolites with similar plagioclase‐rich mineralogy (Christiansen & Blank, 1972), the early and late groups of UBM rhyolites differ in their radiogenic (e.g., Pritchard & Larson, 2012; Stelten et al., 2013, 2015) and oxygen isotopic (Bindeman & Valley, 2001; Hildreth et al., 1984) compositions. This study focuses on the early and late UBM flows erupted in the center portion of Yellowstone caldera. 2.2. Divisions of the Biscuit Basin Rhyolite The Biscuit Basin flow was originally defined as a single geologic unit and placed in Yellowstone's volcanic stratigraphy by Christiansen and Blank (1972). Largely concealed by younger lavas, Hildreth et al. (1984) estimated a minimum eruptive volume of 2.5 km3 (Figure 2). The unit's pervasive perlitic texture was inter- preted to reflect emplacement into a caldera lake (Christiansen, 2001; Hildreth et al., 1984). Christiansen (2001) recognized the Biscuit Basin rhyolite as one of the least silicic (~72 wt.% SiO2) at Yellowstone, with higher CaO and ΣFeO, and suggested that the unit might be several different flows despite its limited geographic extent.