Kr-Kr AGE OF THE VACA MUERTA MESOSIDERITE: ON THEIR PARENT BODY

Introduction: Vaca Muerta is classified as a mesosiderite, a breccia of approximately equal mixture of silicate and metal [1]. The mesosiderite includes several types of igneous pebbles [2], of which various textures suggest collision event(s) between differentiated asteroidal bodies for its formation. The unique textures have been attracting interests to investigate formation mechanisms and histories of Vaca Muerta mesosiderite. Since the formation process of mesosiderite is complicated, multi-stage cosmic-ray irradiation is also conceivable. A portion which had existed on surface layer (< 3 m) of the parent body would show evidence for cosmic-ray exposure. We utilized 81 Kr-Kr method to investigate different histories of cosmic-ray exposure (CRE) among the different rock types from the Vaca Muerta. Sample and Experimental: We measured two chips of different lithologies taken from a Vaca Muerta sample; a general metal-rich brecciated rock (VM-B) and a coarse grained gabbroic pebble (VM-P) which resembles polygenic and monogenic cu-mulates [2]. Abundances and isotopic ratios of noble gases were measured by using stepwise heating method with the modified-VG5400 (MS-II) mass spectrometer at the University of Tokyo. Duplicate measurements of 81 Kr-Kr ages were carried out on each sample to confirm the obtained ages. Results and Discussion: The production rate ratio 81 Kr/ 83 Kr for the samples were determined using a least squares fitting to the Kr isotopic data obtained by the step-wise heating method. Resulted 81 Kr-Kr ages for the VM-B and VM-P samples are 141 ± 9 and 167 ± 9 Myr, respectively. 36 Cl-36 Ar CRE age of 138 ± 11 Myr [3] was reported for metal phase (VM-M) from the Vaca Muerta. The CRE age of VM-P is longer than those for VM-B and VM-M, indicating that the VM-P had been irradiated in addition to the space irradiation, ~140 Myr, during transit from the parent body to the Earth as observed for VM-B and VM-M. The difference of CRE ages among the samples reveals that VM-P was irradiated by cosmic-rays on its parent body, on which only VM-P was present on the surface. This result is consistent with the model calculation for its temperature evolution by Stewart et al. [4]. They mentioned burial depths greater than ~20 m for metal or ~1 m for regolith which corresponds to the VM-P sample. A precursor of VM-P had been located in shallower depth than VM-M and VM-B. It is considered reasonable that a mixing of VM-B and …