268-3 New Method for Analyzing Rb-Sr Isotopes in Whole-rock Glasses Using Laser Ablation Inductively Coupled Plasma Mass Spectrometry with Chemical Separation Techniques

Session: Old and the New, Long and the Short: Perspectives on Integration of Timescales of Magmatic Processes: Special Session Related to MGPV Awards to Madison Myers and Anita Grunder (Posters)

Presenting Author:

Authors:

Abstract:

Rb-Sr isotope compositions are useful for deciphering processes that create and evolve a range of rock types, elucidating information about magma sources, assimilant character, and age of formation or metamorphism. In particular, Rb-Sr dating is a well-established system for determining ages of cogenetic suites of igneous and metamorphic rocks. In situ analytical techniques were not previously available for the Rb-Sr system because beta decay has inherent parent on daughter interferences. Seminal work by Zack and Hogmalm (2016) set the stage for advancement of in situ mineral phase Rb-Sr isotope analysis via laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). This method is now well-established and typically calibrated using known values from whole-rock and synthetic glasses; however, there has been limited analysis employing whole-rock glass in the Rb-Sr system.

            We have devised a new protocol for analyzing Rb-Sr isotopes in whole-rock glasses using an Agilent 8900 triple quadrupole ICP-MS with a laser ablation system, modeled after the in situ mineral method of Zack and Hogmalm and subsequent workers (e.g. Hogmalm et al. 2017; Rosel and Zack, 2021; Cruz-Uribe et al. 2023), while optimizing parameters to achieve high sensitivity and reproducibility with our system. To eliminate ⁸⁷Rb interference on ⁸⁷Sr, we introduce SF₆ gas into the reaction cell to produce SrF⁺, while Rb remains unreacted. We then measure the fluorinated product ions of ⁸⁶Sr, ⁸⁷Sr, and ⁸⁸Sr and non-reacted ions of ⁸⁵Rb and ⁸⁷Rb. Repeated trials conducted on USGS standards AGV-2 and BCR-2 glasses (n = 55) produce an average ⁸⁷Sr/⁸⁶Sr of 0.70396 and 0.70491, respectively, within uncertainties of GeoReM preferred values (0.703992 +/-0.000033 and 0.70492 +/- 0.00055). Day to day runs (n = 15) yield values within GeoReM preferred and internal errors for both USGS standards (2SE). We have also analyzed several samples of Sierra Nevada granitoids with published values of Sr isotopes from Thermal Ionization Mass Spectrometry (TIMS), using AGV-2 and BCR-2 standards as calibrants. Results from day to day runs (n = 9) of the granitoid samples are reproducible, compared to published TIMS values, within internal error (2SE) as well. This technique has the potential to reduce cost and sample preparation time over dissolution/ion chromatography for TIMS, increasing the accessibility of Rb-Sr isotope analysis and dating to a wider range of users.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-10902

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