Collisional modification of the acapulcoite/lodranite parent body revealed by the iodine-xenon system in lodranites

Abstract

The I-Xe system of three lodranites has been investigated. Samples of Gibson yielded no isochrons, and late model ages are attributed to late addition of iodine. Two metal and one silicate separate from the transitional lodranite Graves Nunataks (GRA) 95209 gave ages that are consistent with each other and with the literature I-Xe age of Acapulco feldspar. These yield a mean closure age 4.19 +/- 0.53 Ma after the Shallowater enstatite reference age (4562.3 +/- 0.4 Ma). Such identical I-Xe ages from distinct phases imply that the parent material underwent a period of rapid cooling, the absolute age of this event being 4558.1 +/- 0.7 Ma. Such rapid cooling indicates an increase in the rate at which heat could be conducted away, requiring a significant modification of the parent body. We suggest the parent body was modified by an impact at or close to the time recorded by the I-Xe system. An age of 10.4 +/- 2.3 Ma after Shallowater has been determined for one whole-rock sample of Lewis Cliff (LEW) 88280. Since the release pattern is similar to that of GRA 95209 this hints that the larger grain size of this sample may reflect slower cooling due to deeper post impact burial.