Hunting the Datable Garnet using the LA‐ICP‐MS U‐Pb Method: Predicting Garnet U Concentration, based on Major and Minor Elements

Abstract

Garnet occurs in a wide range of rock types, from mantle peridotites to granites, from eclogites to skarns. In recent years, garnet LA‐ICP‐MS (Laser Ablation Inductively Coupled Plasma Mass Spectrometry) U‐Pb dating has provided a powerful solution for retrieving the ages of rock formations, but successful dating is often prohibited by the low concentration of U. However, the concentration of U, a trace element of garnet, is unknown prior to the LA‐ICP‐MS analysis. In this study, we propose that the U concentration in garnet can be predicted by the contents of major and minor elements, which can be quantitatively obtained by EPMA (electron probe microanalysis). Using a supervised machine learning method (neural network), a model is trained to discriminate U‐rich (>2 ppm) and U‐poor (<2 ppm) garnets, based on EPMA results. Results of cross validation shows that the model has an average accuracy of ∼92% and is a powerful tool in detecting datable U‐rich garnet. To facilitate the use of the discriminator, it is programmed as a stand‐alone Microsoft Excel spreadsheet (HighUGarnet) and users directly paste the molar proportions of garnet end members into it and obtain the discrimination result.

Uranium–lead geochronology applied to pyrope garnet with very low concentrations of uranium

“…This finding contradicts recent meta-analysis (i.e. analysis of aggregated data from several studies) of garnet that considered pyrope garnet and garnet from peridotites as poor candidates for U-Pb dating, based on low U concentrations (Deng et al 2022). This may be explained, as the metaanalysis of Deng et al (2022) did not account for high U/Pb, only considering the concentration of U.…”

“…analysis of aggregated data from several studies) of garnet that considered pyrope garnet and garnet from peridotites as poor candidates for U–Pb dating, based on low U concentrations (Deng et al 2022). This may be explained, as the meta-analysis of Deng et al (2022) did not account for high U/Pb, only considering the concentration of U. In most garnet in this study, a majority of Pb in the analysed crystals is radiogenic and many garnet analyses are near-concordant.…”

Detrital Garnet Geochronology by In Situ U‐Pb and Lu‐Hf Analysis: A Case Study From the European Alps

“…For example, pyroperich mantle garnet may be more amenable to U-Pb dating as bulk Pb is extremely low in such rocks (e.g., O'Sullivan et al, 2023). Garnet U content is known to correlate with composition (Deng et al, 2022), but it is the initial U/Pb c ratio which determines whether an acceptable age can be obtained. In addition, a detailed investigation of potential relationships between source lithology and amenability to dating requires a bedrock rather than a detrital data set.…”

Trace Elements of Multi‐stage Minerals and Titanite U‐Pb Dating for the Gneisses from Liansan Island, Sulu UHPM Belt

“…Different origins of garnet may occur on the same grain, with different metamorphic dehydration or partial melting conditions being recorded. Therefore, garnet plays an important role in revealing the evolution of subduction zone processes, including pressure and temperature conditions (Hickmott and Spear, 1992;Xia et al, 2016;Xia and Zhou, 2017;Deng et al, 2022;He et al, 2022), along with the duration of the metasomatism, deformation and anatexis (e.g., Storey and Prior, 2005;Caddick et al, 2010;Pollington and Baxter, 2010;Caddick and Kohn, 2013;Angiboust et al, 2014).…”