Contributions to Mineralogy and Petrology. Department of Geology, University of California Davis, Davis, CA, USA

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1 Magma mixing and the generation of isotopically juvenile silicic magma at Yellowstone caldera inferred from coupling 238 U- 230 Th ages with trace elements and Hf and O isotopes in zircon and Pb isotopes in sanidine Contributions to Mineralogy and Petrology Mark E. Stelten a, Kari M. Cooper a, Jorge A. Vazquez b, Mary R. Reid c, Gry H. Barfod a, Josh Wimpenny a, Qing-zhu Yin a a Department of Geology, University of California Davis, Davis, CA, USA b U.S. Geological Survey, Menlo Park, CA, USA c School of Earth Sciences and Environmental Sustainability, Northern Arizona University, Flagstaff, AZ, USA Corresponding author: Mark E. Stelten address: mestelten@ucdavis.edu Supplementary Material A1: details of analytical method and data reduction Zircon SHRIMP-RG and CAMECA ims 1270 trace-element analyses and 238 U- 230 Th dating All zircons from the Hayden Valley flow (HVF), most zircons from the Solfatara Plateau flow (SPF), and one zircon from the Gibbon River flow (GRF) were analyzed for 238 U- 230 Th ages and/or traceelement concentrations using the Stanford-USGS SHRIMP-RG ion microprobe located at Stanford University (Online Resource C2). Some zircons from the SPF and GRF that were previously analyzed for 238 U- 230 Th ages by Vazquez and Reid (2002) were analyzed for trace element concentrations using the CAMECA ims 1270 at UCLA. The analytical setup for all SHRIMP-RG 238 U- 230 Th age analyses and trace-element analyses is the same as that described in Stelten and Cooper (2012). Additional details on zircon trace-element and 238 U- 230 Th age analyses are included below. Trace-element analyses Trace element analyses using the CAMECA ims 1270 followed analytical protocol described in Schmitt and Vazquez (2006) and Reid et al. (2011). SHRIMP-RG analyses were conducted with a ~1.5

2 na primary beam of O - 2 focused into a ~20 μm diameter spot. Concentrations are calculated relative to an in-house zircon standard ( MAD ) with an age of ca. 555 Ma; concentration values are published by Barth and Wooden (2010). Trace-element concentrations are derived from 28 Si + -normalized secondary ion yields relative to those measured for the standard. Estimated uncertainties based on the sessionspecific reproducibility (percent 2 standard deviation) of the MAD standard are: Sc and Yb <5 %; P, Ti, Y, Dy, Er, Tm, Lu, Hf, and U <8 %; Nb, Ce, Ho, Tb, and Th <10 %; Gd <11 %; Nd and Sm <16 %; Eu <20 %; B, F, and Fe <30%; Li, Be, and La are %. Comparison of SHRIMP-RG and CAMECA ims 1270 trace-element results Eleven SPF zircons and one GRF zircon analyzed for trace-element concentrations at UCLA using a CAMECA ims 1270 ion microprobe were re-analyzed for trace elements (in an immediately adjacent spot when possible) at Stanford using the SHRIMP-RG in order to test for inter-laboratory bias in the measurements. Results from the analyses using the two instruments are commonly outside of the reported 2σ uncertainty of each other; however, the SPF and GRF zircons typically display intragrain trace-element zoning (even when only one instrument is used) and the analysis spots being compared are not in the exact same location within the grain. Thus, it is likely that this slight scatter is geological rather than analytical in origin. Importantly, for all 12 grains analyzed using both the SHRIMP-RG and CAMECA ims 1270, the different analyses of the same grain using the two instruments fall within the same trace-element population. This indicates that no systematic bias in trace-element population classification will result from the use of different instrumentation. SHRIMP-RG 238 U- 230 Th analyses 238 U- 230 Th analyses were conducted using an na primary beam of O 2 - focused into a μm diameter spot. Analyses were conducted at a mass resolution of ~8000 (10% peak height), which is sufficient to resolve potential interferences. U and Th activities are calculated from measured UO + and ThO + yields and corrected for relative ionization during ion microprobe analyses (e.g., Reid et al., 1997;

3 Schmitt, 2011). Relative sensitivity of UO + and ThO + ionization during SHRIMP-RG analyses was determined by repeated analysis of the MAD standard with session-specific uncertainties (~ 1.5%) propagated into calculated ( 238 U)/( 232 Th). During this analytical session, the background was undercounted for the SPF and HVF zircon analyses requiring normalization of 230 Th 16 O + / 232 Th 16 O + using a secondary secular equilibrium standard (ca. 1.2 Ga zircon) with U concentrations similar to those for the SPF and HVF zircons. An uncertainty of 7% in the normalization factor was included in those age calculations, based on the reproducibility of 230 Th/ 238 U in the secondary standard. Due to low U concentrations (< 200 ppm) and 230 Th 16 O + intensities at background levels for some SPF and HVF zircon, only analyses in which the count rate of 230 Th 16 O + is more than double the background intensity are considered in this study. In subsequent sessions this problem has been resolved by filtering low energy secondary ions using an energy selection slit set to sample ions with energies greater than +40 ev of the accelerating voltage, which serves to significantly increase signal/background ratios. Zircon LA-MC-ICMS Hf isotopic analyses Hafnium isotope compositions were measured using a 193nm excimer laser ablation system in conjunction with a Thermo Neptune Plus multiple-collector ICPMS at UC Davis. The MC-ICPMS was initially tuned in solution mode (low resolution) prior to attaching the laser. A 20 ppb solution of JMC- 475 typically yields an intensity of ~8V of 180 Hf. After maximizing the signal intensity of 180 Hf using JMC-475, the laser was connected and tuned for maximum sensitivity and stability. LA-MC-ICPMS analyses were performed using a ~53 µm diameter spot size, 8 Hz pulse rate, and 25% laser energy. Intensities were measured simultaneously on Faraday cups with Ω resistors. The cup configuration consists of 177 Hf on the center cup, with 172 Yb, 173 Yb, 175 Lu and 176 Hf measured in the low mass cups, and 178 Hf, 179 Hf and 180 Hf measured in the high mass cups. Each zircon analysis consisted of 200 measurements with an integration time of 0.25 s. On-peak baselines were measured at the start of each

4 analytical session and were automatically subtracted from the measured data by the Neptune software. Isotope ratios were corrected for instrumental mass bias by applying an exponential correction assuming a constant value for 179 Hf/ 177 Hf = Interferences of 176 Yb and 176 Lu on 176 Hf were corrected offline assuming exponential mass bias and using 172 Yb/ 173 Yb = , 176 Yb/ 172 Yb = , and 176 Lu/ 175 Lu = (Vervoort et al. 2004). Additional technical details and information about data reduction can be found in Tollstrup et al. (2012). A combination of the Temora, 91500, and GJ1 zircon standards were analyzed after every 5-10 unknowns to monitor reproducibility. Uncertainties calculated for unknown zircons are based on the 2σ reproducibility of the zircon standards run during the same day. The combination of the GJ1, 91500, and Temora zircon standards allows us to assess the quality of the Hf isotopic data over a range of zircon Yb/Hf that is similar to what is observed in the Yellowstone zircons (i.e., to test the Yb/Lu interference correction). These data demonstrate that there is no systematic deviation from the true value over the range of Yb/Hf observed in the zircon standards. For unknown zircons, only Hf isotopic analyses with Yb/Hf within the range of the GJ1, 91500, and Temora zircon analyses are considered in the text (to ensure an accurate Yb interference correction; Fisher et al., 2011). Over the two analytical sessions we obtained mean 176 Hf/ 177 Hf values of ± 10 (2 SE) for GJ1, ± 21 (2 SE) for 91500, and ± 13 (2 SE) for Temora. These averages are within error of the Hf isotopic compositions of the same zircon standards measured in solution mode: ± 5 (2 SE) for GJ1 (Morel et al., 2008), ± 8 (2 SE) for (Blichert-Toft, 2008), and ± 5 (2 SE) for Temora (Woodhead et al., 2004). LA-MC-ICPMS Pb isotopic analyses Pb isotopic analyses performed on SPF and HVF sanidines and glasses were conducted using a Nu Plasma MC-ICPMS coupled with a New Wave Research Nd: YAG deep UV (213nm) laser ablation

5 system housed in the Interdisciplinary Center for Plasma Mass Spectrometry at UC Davis. The analytical setup used for Pb isotopic analyses is that same as that used in Kent (2008). Operating conditions used for LA-MC-ICPMS analyses are as follows: 90% laser energy, 100 µm spot size, 20 Hz pulse rate, and 25 second integration time. All isotopes were measured on Faraday detectors including 204 Pb and 202 Hg. Prior to analysis we measured an on-peak background for 30 seconds and subtracted the background from the beam intensities measured during ablation. We correct for the interference of 204 Hg derived from the gas and the sample in the same manner as described in Kent (2008). We calculate mass bias externally using Pb isotope ratios measured for bracketing NIST 610 analyses and a value of for 208 Pb/ 206 Pb (Baker et al., 2004). For all analytical sessions we used the following procedure to account for mass bias and to monitor the accuracy and reproducibility of our measurements: 1) measure NIST 610 to calculate mass bias, 2) analyze either NIST 612 or StHs6/80-G (MPI-DING glass) as a check standard, 3) perform 5-8 unknown analyses, 4) analyze either NIST 612 or StHs6/80-G (MPI-DING glass) as a check standard, and 5) measure NIST 610 to calculate mass bias. We would then average the fractionation factors for the bracketing NIST 610 analyses and use this average fractionation factor when calculating the composition of the unknown analyses (using an exponential law to correct for mass bias). To account for possible day-to-day bias between analytical sessions, we normalized all Pb isotopic compositions using the massbias corrected NIST 612 values and the true Pb isotopic composition of NIST 612 reported in Baker et al. (2004). This normalization has a very minor effect on the reported Pb isotopic compositions because the average measured NIST 612 values on a given day are within error of the true values, and are within error of the average NIST 612 values for all other analytical sessions.

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