Present address: School of Earth and Space Exploration, Arizona State University, 781 E. Terrace Rd., Tempe, Arizona 85287, USA.
40Ar-39Ar age of Northwest Africa 091: More evidence for a link between L chondrites and fossil meteorites
Article first published online: 23 JUL 2012
© The Meteoritical Society, 2012
Meteoritics & Planetary Science
Volume 47, Issue 8, pages 1324–1335, August 2012
How to Cite
WEIRICH, J. R., SWINDLE, T. D. and ISACHSEN, C. E. (2012), 40Ar-39Ar age of Northwest Africa 091: More evidence for a link between L chondrites and fossil meteorites. Meteoritics & Planetary Science, 47: 1324–1335. doi: 10.1111/j.1945-5100.2012.01397.x
- Issue published online: 15 AUG 2012
- Article first published online: 23 JUL 2012
- (Received 11 April 2012; revision accepted 19 June 2012)
Abstract— Most 40Ar-39Ar ages of L chondrites record an event at approximately 500 Ma, indicating a large collisional impact at that time. However, there is a spread in ages from 400 to 600 Ma in these meteorites that is greater than the analytical uncertainty. Identification of, and correction for, trapped Ar in a few L chondrites has given an age of 470 ± 6 Ma. This age coincides with Ordivician fossil meteorites that fell to Earth at 467 ± 2 Ma. As these fossil meteorites were originally L chondrites, the apparent conclusion is that a large impact sent a flood of L chondrite material to Earth, while material that remained on the L chondrite parent body was strongly heated and reset. We have reduced 40Ar-39Ar data for Northwest Africa 091 using various techniques that appear in the literature, including identification and subtraction of trapped Ar. These techniques give a range of ages from 455 to 520 Ma, and show the importance of making accurate corrections. By using the most straightforward technique to identify and remove a trapped Ar component (which is neither terrestrial nor primordial), an 40Ar-39Ar age of 475 ± 6 Ma is found for Northwest Africa 091, showing a temporal link to fossil meteorites. In addition, high temperature releases of Northwest Africa 091 contain evidence for a second trapped component, and subtraction of this component indicates a possible second collisional impact at approximately 800 Ma. This earlier age coincides with 40Ar-39Ar ages of some H and L chondrites, and lunar samples.