THE I-Xe SYSTEM IN LODRANITES SUGGESTS IMPACT-RELATED RAPID COOLING

Introduction: Lodranites are a class of primitive achondrites, in the same clan as acapulcoites – they may in fact represent a continuum [1, 2]. Lodranites are coarse-grained (540-700 µm) olivine-and pyrox-ene-rich rocks, depleted in troilite and plagioclase; acapulcoites are finer-grained (150-230 µm), with approximately chondritic abundances of olivine, pyrox-ene, plagioclase, metal and troilite [3, 4]. Rare, relict chondrules have been reported in several acapulcoites, confirming that their precursor material was chondritic [3, 5]. These differences suggest lodranites reached higher peak temperatures and so experienced higher degrees of partial melting than acapulcoites, with associated loss of some metal/sulfide. The larger grain size implies they cooled more slowly. It is tempting to view acapulcoites and lodranites as chondritic meteorites in which the continuum of thermal processing represented by variations in petrologic type is extended to the point of melt generation and, in the case of lodranites, partial extraction. However, among chondrites concentrations of primordial noble gases decrease strongly with increasing petrologic type [6], while acapulcoites and lodranites have high concentrations of primordial gases, much higher than those of more evolved achondrites [7, 8]. I-Xe data suggest closure of ordinary chondrites to Xe loss was increasingly late with increasing thermal processing, but phosphates in Acapulco closed to xenon loss ~30 to 50 Ma earlier than phosphates in H6 chondrites [9]. Experimental: Samples from three lodranites have been studied: Graves Nunataks 95209 (GRA 95209), Lewis Cliff 88280 (LEW 88280) and Gibson. GRA 95209 shows a number of properties intermediate between acapulcoites and lodranites, and may be termed a transitional lodranite. It is noteworthy that the smaller average grain size observed for GRA 95209 may indicate faster cooling than typical of lodranites in general. Samples were coarsely crushed and a magnetic separate produced using a hand magnet, prior to irradiation. Aliquots of Shallowater were also included in the irradiations to monitor the 127 I to 128 Xe conversion. After irradiation Xe isotope ratios were measured using the RELAX mass spectrometer [10, 11] for all the samples and the aliquots of Shallowater. In total 9 samples of GRA 95209, 5 samples of LEW 88280 and 2 samples of Gibson were analysed.