Fara N. Lindsay^a, Gregory F. Herzog^a, Jisun Park^a,b, Jeremy S. Delaney^a, Brent Turrin^c, Carl C. Swisher III^c

^aDepartment of Chemistry and Chemical Biology Rutgers University 610 Taylor Rd. Piscataway, NJ 8854
^bLunar and Planetary Institute 3600 Bay Area Blvd. Houston, TX 77058
^cDepartment of Earth and Planetary Sciences Rutgers University 610 Taylor Rd. Piscataway, NJ 08854

⁴⁰Ar/³⁹Ar ages of single feldspar grains from the paired meteorites Graves Nunatak 06128 (GRA8; 8 grains) and 06129 (GRA9; 26 grains) are presented. Plateau ages (⩾70% of the ³⁹Ar released) ranged from 4000 Ma to 4600 Ma with an average 1-σ uncertainty of ± 90 Ma. The most precise ages obtained were 4267±17 Ma for a grain from GRA8 and 4437±19 Ma and 4321±18 Ma for two grains from GRA9. Isotope correlation diagrams yield less precise ages ranging from 3800 Ma to 5200 Ma with an average 1-σ uncertainty of 250 Ma; they indicate a negligible trapped component. Plateau ages, integrated total fusion ages, and isochron ages are internally concordant at the 95% confidence level.
The distribution of the plateau ages for GRA9 is bimodal with peaks at 4400 and 4300 Ma. In contrast, the plateau age distribution for GRA8 peaks at about 4260 Ma with broad wings extending toward younger and older ages. To explain the distributions of grain ages we prefer a scenario that includes a major post-formation event about 4400 Ma ago and a later melt intrusion event that heated GRA8 more than some parts of GRA9.

Reference
Lindsay FN, Herzog GF, Park J, Delaney JS, Turrin B and Swisher III CC (in press) 40Ar/39Ar dating of microgram feldspar grains from the paired feldspathic achondrites GRA 06128 and 06129. Geochimica et Cosmochimica Acta
[doi:10.1016/j.gca.2013.12.023]
Copyright Elsevier

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