⁴⁰Ar- ³⁹Ar age of Northwest Africa 091: More evidence for a link between L chondrites and fossil meteorites

Most ⁴⁰Ar- ³⁹Ar ages of L chondrites record an event at approximately 500Ma, indicating a large collisional impact at that time. However, there is a spread in ages from 400 to 600Ma 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±6Ma. This age coincides with Ordivician fossil meteorites that fell to Earth at 467±2Ma. 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 520Ma, 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±6Ma 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 800Ma. This earlier age coincides with ⁴⁰Ar- ³⁹Ar ages of some H and L chondrites, and lunar samples.