Zircon U-Pb geochronology and trace element dataset from the Southern Rocky Mountain Volcanic Field, Colorado, USA
This contribution provides in-situ LA-ICP-MS U-Pb ages and trace element determinations of zircons from dacitic to rhyolitic lavas, ignimbrites and intrusions in the Southern Rocky Mountain Volcanic Field (SRMVF) in Colorado, USA. The data record a period of intense magmatic activity in the Oligocene-early Miocene (~37-22 Ma) which gave rise to some of the largest explosive ignimbrites in the geological record (e.g. the Fish Canyon Tuff). Age data are drift corrected, but not corrected for radiation dosage or Th disequilibrium, in order to allow users to apply their own algorithms. Xenocrysts (much older crystals up to 2 Ga from the Proterozoic basement) are included in this record.
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Samples were collected during multiple field campaigns in southwest Colorado, USA. Rock samples were crushed using a SELFRAG high voltage fragmentation device at ETH Zürich and zircons were separated using methylene iodide using standard protocols. A subset of zircons were pre-treated either by thermal annealing for 48 hr at 850 °C, or by thermal annealing combined with chemical abrasion (CA) . Zircons were mounted in epoxy and polished to a 1 μm finish using a combination of SiC grinding paper (p2500 grit) and diamond suspension (6 μm, 3 μm and 1 μm, sequentially). Reference materials GJ-1 (601.86 ± 0.37 Ma; [3, 4]) was used as a calibration reference material (CRM). Validation reference materials (VRM’s) include: AusZ7-1 (~38.9 Ma; ; AusZ7-5 (2.4082 Ma ± 0.0022; ; OD-3 (33.0 ± 0.1 Ma; ); Plešovice (337.15 Ma; [4, 8]); Mud Tank (731.65 ± 0.49; [4, 9]); Temora2 (416.78 ± 0.33 Ma; ) and 91500 (1063.51 ± 0.39 Ma; [4, 11]). LA-ICP-MS instrumentation and data reduction Data were collected using an ASI Resolution 193 nm ArF laser and a Laurin Technic S155 constant geometry 2-volume ablation cell, connected to a Thermo Element XR sector field ICP-MS. Ablation was performed under a pure He atmosphere (0.7 l/min), after which the ablated aerosol was mixed with Ar in the ablation funnel and homogenized in a signal smoothing device prior to ionization in the plasma. More analytical parameters, including spot size, laser fluence and repetition rate, can be found in Table 2. Data were reduced using VizualAge (Petrus and Kamber 2012) on the Iolite platform (v2.5; Paton et al. (2011)). Hg, U, Th and Pb were measured for geochronology, and the data reduction scheme for U-Pb dating consists of: (1) baseline subtraction of raw counts on masses 202, 204, 206, 207, 208, 232, 235 and 238; (2) calculation of U/Pb ratios by the mean-of-ratios method; (3) downhole fractionation by cubic or exponential spline, as relevant ; (4) instrumental drift correction; and (5) normalization to the ID-TIMS U/Pb ratio of the chemically-abraded (CA) or non-abraded CRM (for CA and non-CA samples, respectively). Trace element data reduction was performed separately using the TraceElements.ipf routine in VizualAge, including slightly modified steps: (1) background subtraction; (2) calculation of trace element : Si ratios by a means-of-ratios method; (3) instrumental drift correction and normalization to NIST-612 synthetic glass using Si as an internal reference (15% Si by wt.).