Studies of Minor Phases in Primitive Chondrite Matrix

Introduction: Fine-grained matrix is a major (and thermally the most primitive) component in chondrites. Analyses of chondrite matrix [1] reveal that it does not have a primordial CI-like composition, showing instead a consistent depletion compared to solar in all chondrite groups (except CI). This suggests that volatile depletion was an early event, possibly involving cooling of a hot inner disk [1,2]. Understanding the mineralogy of the trace and minor element carrier phases in pristine chondrite matrix would constrain a number of aspects of the early Solar System environment , and inform our choice of model for volatile depletion. However, matrix is rarely pristine. Fine-grained materials experienced variable aqueous, thermal , and impact processing following accretion. Studies of trace element carriers in pristine anhydrous IDPs overcome this problem [3,4], but may not resolve the issue of volatile depletion in the inner Solar System. In ongoing work [5], we are investigating matrix mineralogy in texturally primitive chondrites: Vigarano (CV3red) and Acfer 094. Acfer 094 in particular is an extremely primitive rock, containing presolar silicates [6,7]: if presolar silicates survived asteroidal processing , later condensed materials should also have survived. The observed mineralogy is compared to that predicted from equilibrium condensation models [8]. It has been proposed that Acfer 094 is a CM3, although C and N isotopic compositions appear to exclude this [9], and O isotopes suggest an affinity to the CO subgroup [10]. Acfer 094 is generally considered a unique carbonaceous chondrite. We report here on a study of minor phases in the matrices of Vigarano and Acfer 094. Methods: Samples were mapped and analysed using a JEOL 5900 LV scanning electron microscope (SEM), fitted with an Oxford Instruments INCA energy dispersive X-ray spectrometer (EDS). Point analyses employed a 2nA beam current at 20kV accelerating voltage, 10mm specimen working distance and 6 cm EDS working distance. The key areas were then imaged using Leo 1455 VP Scanning Electron Microscope (SEM) with Oxford Instruments INCA energy dispersive X-ray analysis system. The instrument was operated at an accelerating voltage of 20 kV and a beam current of 1 nA. In both samples the entire sample was mapped and montaged before searching for areas of interest – typically montaged maps are in the