Revisiting 26 Al‐ 26 Mg systematics of plagioclase in H4 chondrites

• Published in: Meteoritics & Planetary Science
• Main Authors: Telus, M., Huss, G. R., Nagashima, K., Ogliore, R. C.
• Other Authors: National Aeronautics and Space Administration
• Format: Article in Journal/Newspaper
• Language: English
• Published: Wiley 2014
• Subjects: Space and Planetary Science; Geophysics; Marguerite; Beaver Creek
• Online Access: http://dx.doi.org/10.1111/maps.12304; https://api.wiley.com/onlinelibrary/tdm/v1/articles/10.1111%2Fmaps.12304; https://onlinelibrary.wiley.com/doi/pdf/10.1111/maps.12304

Abstract Zinner and Göpel ( , ) found clear evidence for the former presence of 26 Al in the H4 chondrites Ste. Marguerite and Forest Vale. They assumed that the 26 Al‐ 26 Mg systematics of these chondrites date “metamorphic cooling of the H4 parent body.” Plagioclase in these chondrites can have very high Al/Mg ratios and low Mg concentrations, making these ion probe analyses susceptible to ratio bias, which is inversely proportional to the number of counts of the denominator isotope (Ogliore et al. ). Zinner and Göpel ( ) used the mean of the ratios to calculate the isotope ratios, which exacerbates this problem. We analyzed the Al/Mg ratios and Mg isotopic compositions of plagioclase grains in thin sections of Ste. Marguerite, Forest Vale, Beaver Creek, and Sena to evaluate the possible influence of ratio bias on the published initial 26 Al/ 27 Al ratios for these meteorites. We calculated the isotope ratios using total counts, a less biased method of calculating isotope ratios. The results from our analyses are consistent with those from Zinner and Göpel ( ), indicating that ratio bias does not significantly affect 26 Al‐ 26 Mg results for plagioclase in these chondrites. Ste. Marguerite has a clear isochron with an initial 26 Al/ 27 Al ratio indicating that it cooled to below 450 °C 5.2 ± 0.2 Myr after CAI s. The isochrons for Forest Vale and Beaver Creek also show clear evidence that 26 Al was alive when they cooled, but the initial 26 Al/ 27 Al ratios are not well constrained. Sena does not show evidence that 26 Al was alive when it cooled to below the Al‐Mg closure temperature. Given that metallographic cooling rates for Ste. Marguerite, Forest Vale, and Beaver Creek are atypical (>5000 °C/Myr at 500 °C) compared with most H4s, including Sena, which have cooling rates of 10–50 °C/Myr at 500 °C (Scott et al. ), we conclude that the Al‐Mg systematics for Ste. Marguerite, Forest Vale, and Beaver Creek are the result of impact excavation of these chondrites and cooling at the surface of the parent ...