Improvements to Argon-Argon Dating of Extraterrestrial Materials

Abstract

The source of potassium and argon in ordinary chondrites is determined by comparing the argon activation energies of feldspar, pyroxene, and olivine with that of the L chondrites Chico and North West Africa (NWA) 091. In addition, shock pressures of 29 to 55.8 GPa are shown to lower the activation energy of feldspar. Comparable shock pressures lowers the activation energy of pyroxene outside of error, but the variability of this value, even among unshocked samples, makes a clear distinction difficult. The effect of shock on olivine has not been investigated, by myself or others. Like many ordinary chondrites, Chico and NWA 091 have two major releases of argon, one at low temperature, and the other at high temperature. The low temperature release of Chico contains two releases, which match the activation energies of shocked and unshocked feldspar. The low temperature release of NWA 091 only contains a single release, which matches shocked feldspar. The high temperature release of both Chico and NWA 091 has an activation energy that is similar to pyroxene, but not olivine. A potassium mass balance of Chico shows that all the potassium in the meteorite is contained in feldspar, and Raman spectroscopy shows this feldspar has not been converted into a high pressure phase, indicating the high temperature release is inclusions in a high temperature mineral. This mineral is probably pyroxene based upon the activation energy, though thin sections provide evidence that feldspar is more closely associated with olivine. NWA 091 exhibits multiple isochrons, showing the presence of two nonprimordial and (probably) non-terrestrial trapped components of argon. The removal of these trapped components reveals a thermal event produced by a collisional impact on the L chondrite parent body at 475 ± 6 Ma (which supports a link between L chondrites and Ordovician fossil meteorites), as well as a similar event at ~800 Ma (which, combined with similar ages on other Solar System objects, suggests an increased impact flux at that time). Chico did not exhibit an isochron, and the age data for Chico is not reported.