Zircon LA-ICPMS U-Pb geochronology and trace element analysis, northern Prince Charles Mountains

Full description

Zircon grains were separated from hand-sized samples of rock using a combination of crushing, hand panning and magnetic separation and hand picking to obtain a clean separate. Zircons from metasedimentary samples were thermally annealed in a furnace at 900 °C in quartz crucibles to improve CL images and U–Pb data (see Table 1). Zircon grains were mounted in epoxy resin and polished to half grain thickness, before being photographed in transmitted and reflected light on an optical microscope. Zircon grains were then imaged using a Gatan Cathodoluminescence (CL) detector attached to a FEI Quanta 600 Scanning Electron Microscope (SEM) at Adelaide Microscopy, University of Adelaide, Australia. Analysis locations attempted to target all possible CL domains while avoiding visible cracks or inclusions. All U–Pb spot locations are provided in the attached CL mosaics for each sample.

Zircon U–Pb geochronology was done by Laser-Ablation Inductively-Coupled Plasma-Mass Spectrometry (LA-ICP-MS) at Adelaide Microscopy, using a RESOlution LR 193 nm Excimer laser coupled to an Agilent 7700s ICP–MS. Zircon were ablated at a frequency of 5 Hz with a spot size of 30 μm for zircon. Each analysis was preceded by five cleaning pulses followed by a pause prior to measurement. The acquisition time for Mounts 1 and 2 was 55 s, inclusive of 15 seconds of background measurement and 40 s of ablation. The acquisition time for Mounts 3–6 was 60 s, inclusive of 30 s of background measurement and 30 s of ablation. A list of the measured masses and their respective dwell times are provided in Table 2.

Zircon data were reduced using Iolite4 (Paton et al., 2010, 2011; Woodhead et al., 2007). Time-resolved signals were carefully inspected for evidence of common Pb, contamination or inclusions and if possible, these parts of the signal were avoided, or the analysis discarded. U–Pb data were corrected for instrument drift, mass bias and down hole fractionation using zircon standard GJ-1 (TIMS normalization data 207Pb/206Pb = 608.3 ± 4.3 Ma, 206Pb/238U = 600.7 ± 1.1 Ma and 207Pb/235U = 602.2 ± 1.0 Ma; Jackson et al., 2004). Data accuracy was monitored using repeated analysis of zircon standards Plešovice (206Pb/238U = 337.13 ± 0.37 Ma; Slama et al., 2008) and 91500 (207Pb/206Pb = 1065 Ma; Wiedenbeck et al., 1995), with standards measured after 15–20 unknowns. Detrital samples that had been thermally annealed were corrected using grains of the standards that had undergone the same thermal annealing process. Trace element data were processed and corrected using the glass NIST 610, with Zr as the internal standard. Zircon analyses were assumed to contain stochiometric Zr contents (43.14 wt%).

A geochronology summary document of sample analysis is also provided.

Lineage

Progress Code: completed

Notes

Purpose
To collect U-Pb isotopic data to constrain the age of zircons for 25 metasedimentary and (meta)igneous samples. Trace element compositions were also collected to better distinguish zircon populations.