TARIM Calcite: A New Benchmark for Laser ICP-MS In Situ Calcite U–Pb Dating

Calcite, known for its utility in uranium-lead (U–Pb) dating through laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS), has faced a challenge in finding suitable reference materials. A study conducted by researchers at China University of Geosciences in Beijing, China, the University of Toronto, Ontario, Canada, and Sichuan Chuangyuan Weipu Analytical Technology Co., Chengdu, China, introduces TARIM calcite as a potential solution to this limitation (1).

calcite mineral cyan macro texture | Image Credit: © Alexander Potapov - stock.adobe.com

Calcite plays a crucial role in U–Pb dating by serving as a common accessory mineral that can incorporate uranium during its formation. When calcite forms, it may contain trace amounts of uranium, including the radioactive isotope uranium-238. Over time, as uranium undergoes radioactive decay to lead, the ratio of uranium to lead isotopes in calcite crystals can be measured, providing valuable information about the age of the geological samples in which calcite is found.

TARIM—also the name of a basin in the Xinjiang region of northwestern China—characterized through electronic probe microanalysis (EPMA), exhibits homogeneity in its low-magnesium (Mg) calcite composition. LA-ICP-MS trace elemental analyses and mapping results confirm the relative homogeneity of TARIM, with an appropriate U content (mean value = 0.48 μg/g) suitable for U–Pb isotopic dating.

The U–Pb analysis performed in this study, which was published in the Journal of Analytical Atomic Spectrometry, employed isotope dilution-thermal ionization mass spectrometry (ID-TIMS) methods and yielded a lower intercept age of 208.5 ± 0.6 Ma (2S, mean squared weighted deviation [MSWD] = 1.04). Further validation through 515 LA-ICP-MS U–Pb isotope analyses on random calcite pieces consistently produced ages with a lower intercept age of 208.0 ± 0.4/3.2 Ma (2S, MSWD = 3.0), aligning with the ID-TIMS age.

In the context of U–Pb dating, the intercept age refers to the age calculated based on the intersection point, or intercept, of the discordia line with the concordia line on a concordia diagram. The concordia diagram is a graphical representation used in radiometric dating to assess the reliability of age determinations. The intercept age provides an estimate of the time at which the minerals in a sample last experienced a common lead-loss event, helping researchers constrain the geological history of the sample.

The significance of TARIM calcite lies in its potential application as a reference material for matrix-matched calibration or as a monitoring standard for calcite LA-ICP-MS U–Pb dating. This study helps to bridge a critical gap in the availability of suitable reference materials for this analytical technique, offering researchers a reliable benchmark for enhancing the precision and accuracy of in situ U–Pb dating using calcite.

This discovery may also open up avenues for improved methodologies in geological research, where U–Pb dating plays a crucial role in understanding various geological phenomena. The TARIM calcite's homogeneous composition and consistent U–Pb isotopic dating results position it as a valuable asset in advancing the reliability of LA-ICP-MS analyses in geological studies.

In conclusion, if indeed TARIM calcite emerges as a promising addition to the toolkit of researchers involved in U–Pb dating using LA-ICP-MS, its apparent suitability as a reference material would bring much-needed precision to in situ analyses, contributing to the refinement of geological timelines and enhancing our understanding of Earth's history.

This article was written with the help of artificial intelligence and has been edited to ensure accuracy and clarity. You can read more about our policy for using AI here.

Reference

(1) Zhang, L.- L.; Zhu, D.- C.; Xie, J.- C.; et al. TARIM Calcite: A Potential Reference Material for Laser ICP-MS In Situ Calcite U–Pb Dating. J. Anal. At. Spectrom. 2023, 11, 2302–2312. DOI: 10.1039/D3JA00222E