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

摘要

The I-Xe system of three lodranites has been investigated. Samples of Gibson yielded noisochrons, and late model ages are attributed to late addition of iodine. Two metal and one silicateseparate from the transitional lodranite Graves Nunataks (GRA) 95209 gave ages that are consistentwith each other and with the literature I-Xe age of Acapulco feldspar. These yield a mean closure age4.19 ± 0.53 Ma after the Shallowater enstatite reference age (4562.3 ± 0.4 Ma). Such identical I-Xeages from distinct phases imply that the parent material underwent a period of rapid cooling, theabsolute age of this event being 4558.1 ± 0.7 Ma. Such rapid cooling indicates an increase in the rateat which heat could be conducted away, requiring a significant modification of the parent body. Wesuggest 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 ofLewis Cliff (LEW) 88280. Since the release pattern is similar to that of GRA 95209 this hints that thelarger grain size of this sample may reflect slower cooling due to deeper post impact burial.