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Quantifying Broken Zircon Grain Frequency during Mineral Separation: A Case Study from the Colorado River, Texas

Session: 37th Annual Undergraduate Research Exhibition Sponsored by Sigma Gamma Epsilon (Posters)

Presenting Author:

Rachel Kramer

Authors:

Kramer, Rachel¹, Wachob, Olivia², Malkowski, Matt³, Sylvester, Zoltan⁴

(1) Department of Applied Mathematics and Statistics, Colorado School of Mines, Golden, CO, USA; Department of Earth and Planetary Sciences, University of Texas at Austin, Austin, Texas, USA, (2) Department of Earth and Planetary Sciences, University of Texas at Austin, Austin, Texas, USA, (3) Department of Earth and Planetary Sciences, University of Texas at Austin, Austin, Texas, USA, (4) Bureau of Economic Geology, University of Texas at Austin, Austin, Texas, USA,

Abstract:

Zircons are uniquely suited to U-Pb geochronology, in part due to their resistance to chemical and mechanical weathering. However, intense processes to extract detrital zircons from a parent rock may be breaking zircons. Broken grains result in misleading size and age distributions. Previous studies acknowledge the risk of grain breakage but claim the breakage is not statistically significant (Larsen and Poldervaart, 1957). Our study examines whether pulverization of grains until all material passes through varying sieve sizes impact the frequency of broken zircons. Following Larsen and Poldervaart (1957), we hypothesize pulverization, independent of sieve size, would have no significant impact on the frequency and nature of broken zircons.

We tested this on modern river sand from the Colorado River near Austin, Texas, by randomly splitting three equal aliquots, which were sieved at three different grain sizes: 500, 355, and 250 µm. Two aliquots were pulverized in a disk mill until all material passed through a 355 µm and 250 µm sieve, respectively. The control sample at 500 µm bypassed pulverization. Zircons were separated from all samples via a zircon concentrating table, Frantz magnetic separator, and methyl iodide-based density separation. Entire zircon separates were then sprinkle-mounted and imaged under reflected light. Morphometric data for each grain was collected using segmenteverygrain, a discipline-specific semantic segmentation python module. Euhedral zircons from all samples were categorized as broken, non-broken, or equivocal.

Preliminary results show negligible difference in the proportion of broken versus unbroken grains between the 355 µm sieved sample and the control sample sieved at 500 µm. However, the sample sieved at 250 µm showed a fourfold increase in the proportion of zircon grains classified as broken. Given that relatively few zircons in the control sample are larger than 250 µm, we interpret this marked increase in broken grain abundance to be the result of increased rounds of pulverization in the disk mill. Thus, exploratory findings recommend future mineral separation techniques be modified to avoid pulverizing grains that pass through the 250 µm sieve. Future work aims to investigate age and volumetric patterns of zircon breakage using U-Pb geochronology and x-ray computed tomography.

Larsen, L.H., and Poldervaart, A., 1957, Measurement and distribution of zircons in some granitic rocks of magmatic origin: Mineralogical Magazine, v. 31, p. 544–564.