​40Ar/39Ar geochronology of terrestrial pyroxene. (1st June 2018)

• Record Type: Journal Article
• Authors: Ware, Bryant; Jourdan, Fred
• Is Part Of: Geochimica et cosmochimica acta. Volume 230(2018)
• Page Start: 112
• Page End: 136
• Publication Date: 2018-06-01
• Subjects: 40Ar/39Ar -- Geochronology -- Thermochronology -- Pyroxene -- Large igneous provinces; Geochemistry -- Periodicals; Meteorites -- Periodicals; Géochimie -- Périodiques; Météorites -- Périodiques; Geochemie; Astrochemie; Electronic journals; 551.905
• DOI: 10.1016/j.gca.2018.04.002
• Languages: English
• ISSNs: 0016-7037

Abstract

Abstract: Geochronological techniques such as U/Pb in zircon and baddeleyite and 40 Ar/ 39 Ar on a vast range of minerals, including sanidine, plagioclase, and biotite, provide means to date an array of different geologic processes. Many of these minerals, however, are not always present in a given rock, or can be altered by secondary processes (e.g. plagioclase in mafic rocks) limiting our ability to derive an isotopic age. Pyroxene is a primary rock forming mineral for both mafic and ultramafic rocks and is resistant to alteration process but attempts to date this phase with 40 Ar/ 39 Ar has been met with little success so far. In this study, we analyzed pyroxene crystals from two different Large Igneous Provinces using a multi-collector noble gas mass spectrometer (ARGUS VI) since those machines have been shown to significantly improve analytical precision compared to the previous single-collector instruments. We obtain geologically meaningful and relatively precise 40 Ar/ 39 Ar plateau ages ranging from 184.6 ± 3.9 to 182.4 ± 0.8 Ma (2 σ uncertainties of ±1.8–0.4%) and 506.3 ± 3.4 Ma for Tasmanian and Kalkarindji dolerites, respectively. Those data are indistinguishable from new and/or published U-Pb and 40 Ar/ 39 Ar plagioclase ages showing that 40 Ar/ 39 Ar dating of pyroxene is a suitable geochronological tool. Scrutinizing the analytical results of the pyroxene analyses as well as comparing them to the analytical result from plagioclase of the same samples indicate pure pyroxene was dated. Numerical models of argon diffusion in plagioclase and pyroxene support these observations. However, we found that the viability of 40 Ar/ 39 Ar dating approach of pyroxene can be affected by irradiation-induced recoil redistribution between thin pyroxene exsolution lamellae and the main pyroxene crystal, hence requiring careful petrographic observations before analysis. Finally, diffusion modeling show that 40 Ar/ 39 Ar of pyroxene can be used as a powerful tool to date the formation age of mafic rocks affected by greenschist metamorphism and will likely play an important role in high temperature thermochronology.