^1,2,3Philipp R. Heck,^1,2,3Jennika Greer,^1,2,3Levke Kööp,⁴Reto Trappitsch,^5,6Frank Gyngard,⁷Henner Busemann,⁷Colin Madeg,⁸Janaína N. Ávila,^1,2,3,9Andrew M. Davis,⁷Rainer Wieler
Proceedings of the National Academy of Sciences of the United States of America (in Press) Link to Article [https://doi.org/10.1073/pnas.1904573117]
¹Robert A. Pritzker Center for Meteoritics and Polar Studies, The Field Museum of Natural History, Chicago, IL 60605;
²Chicago Center for Cosmochemistry, The University of Chicago, Chicago, IL 60637;
³Department of the Geophysical Sciences, The University of Chicago, Chicago, IL 60637;
⁴Nuclear and Chemical Sciences Division, Lawrence Livermore National Laboratory, Livermore, CA 94550;
⁵Physics Department, Washington University, St. Louis, MO 63130;
⁶Center for NanoImaging, Harvard Medical School, Cambridge, MA 02139;
⁷Institute of Geochemistry and Petrology, ETH Zürich, 8092 Zürich, Switzerland;
⁸Research School of Earth Sciences, The Australian National University, Canberra, ACT 2601, Australia;
⁹Enrico Fermi Institute, The University of Chicago, Chicago, IL 60637

We determined interstellar cosmic ray exposure ages of 40 large presolar silicon carbide grains extracted from the Murchison CM2 meteorite. Our ages, based on cosmogenic Ne-21, range from 3.9 ± 1.6 Ma to ∼3 ± 2 Ga before the start of the Solar System ∼4.6 Ga ago. A majority of the grains have interstellar lifetimes of <300 Ma, which is shorter than theoretical estimates for large grains. These grains condensed in outflows of asymptotic giant branch stars <4.9 Ga ago that possibly formed during an episode of enhanced star formation ∼7 Ga ago. A minority of the grains have ages >1 Ga. Longer lifetimes are expected for large grains. We determined that at least 12 of the analyzed grains were parts of aggregates in the interstellar medium: The large difference in nuclear recoil loss of cosmic ray spallation products ³He and ²¹Ne enabled us to estimate that the irradiated objects in the interstellar medium were up to 30 times larger than the analyzed grains. Furthermore, we estimate that the majority of the grains acquired the bulk of their cosmogenic nuclides in the interstellar medium and not by exposure to an enhanced particle flux of the early active sun.