DOI: 10.5281/zenodo.17201424
title: PLANT DIVERSITY IN A MESOTHERMAL CLIMATE: INSIGHTS FROM THE RIO SAN PEDRO FORMATION (LATE EOCENE, SOUTH-CENTRAL CHILE, 39°S).
title: Analytical Method for U-Pb Geochronology and Geochronological Data

description: Analytical Method for U-Pb Geochronology and Geochronological Data

 

We conducted U-Pb geochronology on two samples. To determine the depositional ages of a volcanic rock and maximum depositional age, as well as the zircon provenance of a sedimentary rock, we analyzed more than 50 and over 100 zircon crystals per sample, respectively. The locations of the geographic samples are illustrated in Figure 1 of the main text.

Zircon concentrates from all samples were separated at the Zirchron LLC laboratory in Tucson, Arizona. Each sample was unpacked and pressure-washed with water to remove any debris and/or foreign materials. The sample rock fragments were introduced into the sample chamber of an Electro Pulse Disaggregator (EPD, Marx generator), where electrical pulses were applied at a 1-2 Hz repetition rate and discharges of approximately 220 kV for 15 minutes. The material, ranging in size from 500 µ to 25 µ, was subsequently processed using traditional techniques involving the Wilfley water table, Frantz paramagnetic separator, and a one-step (3.32 gm/cc) heavy liquid MEI separation. Zircons from the non-magnetic fraction were mounted together with standards in a 1-inch diameter epoxy puck and polished following standard laboratory procedures.

After performing cathodoluminescence imaging in a JEOL 8500F field emission electron microprobe, we conducted U-Pb analyses using laser ablation ICP-MS at Washington State University. We utilized an Analyte G2 193nm excimer laser connected to the Element2 high-resolution inductively coupled plasma mass spectrometer. For the U-Pb measurements, our methodology closely followed that of Chang et al. (2006), with the exception of using the 193nm laser instead of the 213nm laser. The laser parameters for the U-Pb analyses included a spot size ranging from 25 µm, a repetition rate of 10 Hz, with power of approximately 5 J/cm². He and Ar carrier gases delivered the sample aerosol to the plasma. Each analysis consisted of a short blank analysis followed by 250 sweeps through masses 202, 204, 206, 207, 208, 232, 235, and 238, taking approximately 30 seconds in total. Unknowns were run in sets of 10 analyses bracketed by standards. Data processing was conducted offline using the Iolite software (Paton et al., 2010). U and Th concentrations were monitored by comparison to the zircon standards 91500.  Plesovice (Sláma et al., 2008) was used as a primary standard and FC-1 (Paces and Miller, 1993), and Fish Canyon Tuff (Schmitz and Bowring, 2001) as secondary standards. Common Pb correction was applied using the 207Pb method (Williams, 1998). U-Pb diagrams and ages were calculated using Isoplot (Ludwig, 2003).

Interpreted ages are determined based on 206Pb/238U for grains younger than 1200 Ma and 206Pb/207Pb for grains older than 1200 Ma. Additionally, a soft filter of 30% was applied to rocks older than the Mesozoic. U-Pb zircon maximum deposition age errors are reported by calculating the quadratic sum of the analytical error and the total systematic error for the set of analyses (Valencia et al., 2005). Interpretations for detrital zircons are established using representative groups, which are those with three or more overlapping zircon ages (Gehrels et al., 2006). The maximum depositional age was calculated using the TuffZirc algorithm (Ludwig, 2003) from the youngest cluster ages (>6) that overlap within a 2σ error range. Zircons from the sandstone samples are clear, colorless, and exhibit various morphologies, primarily as long euhedral crystals. Minor proportions of subhedral to subrounded zircon crystals are also present. Zircons from the tuffs are clear, pink, long euhedral crystals with prominent bi-pyramidal terminations. Cathodoluminescence images reveal a straightforward growth history characterized by oscillatory to sector zoning.