DOI: 10.5880/fidgeo.2021.027
title: Detrital zircon (U-Th)/He thermochronometry data from the Leones Valley, Patagonian Andes

description: The data presented here were produced to study glacial and glacio-fluvial catchment erosion using 'tracer thermochronology' where detrital downstream samples can be used to infer the source elevation sectors of sediments when integrated with known surface bedrock ages from the catchment. For the first time, our study used the zircon (U-Th)/He (ZHe) method as tracer thermochronometer. The samples come from the Leones Valley at the northeastern flank of the Northern Patagonian Icefield, Chile (46.7° S) This data set comprises ZHe analytical results from (i) six detrital samples of different depositional age and grain size (622 single-grain analyses in total), and (ii) two previously analyzed (Andrić-Tomašević et al., 2021) bedrock samples (22 single-grain analyses in total), as well as grain size measurements and lithology identification of two of the detrital samples (two pebble samples with 262 and 211 pebbles, respectively). Data are provided in 10 tab-delimited text files. The full description of the data and methods is provided in the data description file.
description: Six detrital samples were collected along ~19 km of the Leones Valley at the northeastern flank of the Northern Patagonian Icefield, Chile. Sample coordinates are presented in Table 1. Samples include one sand- to pebble-sized sample from the ~2.5–1.1 ka (Harrison et al., 2008) Leones terminal moraine that dams Lago Leones, four modern trunk river samples from ~7.5 km and ~19 km downstream of the moraine, where at each location a sand and a pebbles sample was collected separately, and one modern tributary river sand sample from ~13.5 km downstream of the moraine. The moraine sample is a mixture of mainly very fine to coarse sand and granules with some fine to coarse pebbles (grain sizes according to the classification of Wentworth, 1922) from four locations at the lakeward flank of the ~135-m-high and 2-km-wide moraine. The sample material was collected from beneath coarser material at the surface of the moraine and was in total ~16 kg. Sand and pebble samples of the modern river were collected as mixtures from several locations along tens of meters of point bars or sand/pebble bars within the river. Sand samples were ~8 kg each and the two pebble samples contained 211 and 262 individual pebbles, respectively, of ~2–4 cm diameter (Table S1). The pebble samples are representative of the pebble lithologies present at each sampling location, but not of the pebble grain sizes present at each location. The percentage of pebble lithologies present was estimated and then pebbles of the same size range were collected one-by-one. We did not conduct point-counting. Sampling Measurements of pebble size and lithology identification Pebbles were measured along three axes (shortest, intermediate, longest) with a caliper, then their lithology was identified where possible. Data can be found in Table S1. Zircon (U-Th)/He thermochronometry The bulk moraine sample was processed for mineral separation by crushing, milling, and sieving to the 63–250 µm grain size fraction before density and magnetic separation at the University of Potsdam, Germany. The modern river sand samples were sieved to the 63–250 µm fraction before density and magnetic mineral separations at the University of Tübingen, Germany. After the measurements of pebble size and lithology identification, each pebble sample was crushed as bulk sample and sieved to the 63–250 µm fraction before density and magnetic mineral separation at the University of Tübingen. All samples' mineral separates were picked for suitable zircons at 256X magnification under reflected and transmitted light at a binocular microscope at the University of Tübingen. Selection criteria for bedrock zircons were their transparency, no or only few small inclusions, no fractures or broken parts, idiomorphic crystal habit, grain diameters of &gt;80 µm, and similar size of crystals for each sample. Zircon quality and abundance was high in bedrock samples. Zircon selection in detrital samples aims at selecting a representative zircon population for measurements to avoid bias. We picked ~100 grains of representative sizes, crystal habits, and colors of each sample. Zircon abundance and quality was high in all detrital samples. Selected zircons were individually packed in niobium tubes and measured in an Alphachron™ helium line at the University of Tübingen. Subsequently, concentrations of uranium and thorium were measured by isotope dilution inductively-coupled plasma mass spectrometry (ID-ICP-MS) at the University of Tübingen. For this, zircons were first spiked with a 233U and 230 Th spike solution, dried, and then digested in a two-step high-pressure digestion procedure. Final solutions of 5% HNO3 + 0.5% HF were measured with a Thermo Fisher Scientific iCAP Qc quadrupole ICP-MS. Analytical procedures were developed by Stübner et al. (2016) and analytical details and instrument settings are reported in their supplementary material. Alpha-ejection correction (Ft-correction) of helium measurements was performed after Glotzbach et al. (2019) and ZHe age calculations followed Meesters and Dunai (2005). Grain masses and sphere-equivalent radii (ser) were determined from numerically determined grain geometries (after Glotzbach et al., 2019) and assumed densities (see description of data tables).