T82 What’s the Cosmognosis? Recent Advances in Understanding Earth and Planetary Processes with Cosmogenic Nuclides

Monday, 23 September 2024
8:00 AM - 12:00 PM
212B

ASSESSING INCLUSIONS AND Li CONCENTRATIONS IN MAGNETITE FOR COSMOGENIC 3He DATING

Paper number: T82-3
Program: Technical Sessions
Time: 8:20 AM – 8:35 AM

Authors

Abstract

Magnetite is a ubiquitous mineral that is highly retentive to helium and, therefore, a potential target phase for cosmogenic 3He exposure studies. However, magnetite grains frequently contain mineral and fluid inclusions, which can increase 3He concentrations and lead to erroneous results. Previous studies have shown that using micro computed tomography (microCT) pre-screening to select only grains without inclusions for geochronologic applications can significantly improve the resulting data. This study evaluates the influence of lithology and geologic environments on the number of inclusions as well as Li concentrations to inform sampling strategies. We surveyed >3000 magnetite grains from a range of lithologies, including felsic to ultramafic volcanic and intrusive igneous rocks, as well as hydrothermal and detrital magnetite. Grains were characterized by type, number, and volume fraction of inclusions using 3D microCT and this data was compared to 2D SEM pre-screening. MicroCT pre-screening detected more inclusions than SEM and is thus a more reliable method. We also measured Li concentrations in a number of magnetite grains because nucleogenic 3He can be produced from 6Li. This process increases the overall 3He concentration and can lead to erroneous results if not taken into account. Magnetite grains from (ultra-)mafic rocks are generally more suitable for 3He studies since they contain few inclusions and have low Li concentrations. Magnetite grains from felsic rocks, however, have higher Li concentrations and more inclusions than those from mafic lithologies, making them potentially more problematic for cosmogenic 3He applications. We discuss study design and sampling strategies to optimize the selection of magnetite grains for cosmogenic 3He studies, including in-situ exposure ages and catchment-averaged erosion rates.

Geological Society of America Abstracts with Programs. Vol. 56, No. 5

doi: 10.1130/abs/2024AM-403808

© Copyright 2024 The Geological Society of America (GSA), all rights reserved. Permission is hereby granted to the author(s) of this abstract to reproduce and distribute it freely, for noncommercial purposes. Permission is hereby granted to any individual scientist to download a single copy of this electronic file and reproduce up to 20 paper copies for noncommercial purposes advancing science and education, including classroom use, providing all reproductions include the complete content shown here, including the author information. All other forms of reproduction and/or transmittal are prohibited without written permission from GSA Copyright Permissions.