Matrix and whole-rock fractionations in the Acfer 094 type 3.0 ungrouped carbonaceous chondrite
نویسندگان
چکیده
We used the electron microprobe to study matrix in the ungrouped type 3.0 carbonaceous chondrite Acfer 094 using 7 · 7-point, focused-beam arrays; data points attributable to mineral clasts were discarded. The grid areas show resolvable differences in composition, but differences are less pronounced than we observed in studies of CR2 LaPaz Icefield (LAP) 02342 (Wasson and Rubin [2009]) and CO3.0 Allan Hills A77307 (Brearley [1993]). A key question is why Acfer shows an anomalously uniform composition of matrix compared with these other carbonaceous chondrites. Both whole-rock and matrix samples of Acfer 094 show enhancements of Ca and K; it appears that these reflect contamination during hot desert weathering. By contrast, the whole-rock abundance of Na is low. Although weathering effects are responsible for some fractionations, it appears that nebular effects are also resolvable in matrix compositions in Acfer 094. As with LAP 02342, we infer that the observed differences among different areas were inherited from the solar nebula and may have been carried by porous chondrules that experienced low (about 20%) degrees of melting. Acfer 094 has been comminuted by one or more impact events that may also have caused volatile loss. Thus, despite preserving evidence (e.g., an exceptionally high content of presolar SiC) implying a high degree of pristinity, Acfer 094 is far from pristine in other respects. This evidence of comminution and an O-isotopic composition similar to values measured in metamorphosed CM chondrites suggest that Acfer was hydrated before being outgassed by the inferred impact event. Convection within the plume associated with the impact event probably also contributed to the homogenization of the Acfer 094 matrix.
منابع مشابه
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 di...
متن کاملEvolution of protoplanetary disk inferred from Al chronology of individual chondrules
We review the Al ages of chondrules in various type 3.0 chondrites. The Al ages of chondrules are 1–3 Myr after calcium-aluminum-rich inclusion (CAI) for LL3.0, CO3.0, and Acfer 094 (Ungrouped C 3.0). Available data for chondrules in CR chondrites indicate that many chondrules are relatively younger (‡3 Myr), although data from chondrules in CR3.0 are not yet available to confirm their younger ...
متن کاملAl-mg Dating of Ca-al-rich Inclusions in Acfer 094 Chondrite
Introduction: Acfer 094 is one of the most primitive chondrites, which appears to have escaped thermal metamorphism above 300°C [1] and can potentially provide important clues to primary, undisturbed by parent body processes, abundances of short-lived ra-dionuclides in its components. Krot et al. [2] have recently described mineralogy and petrography of the Al-rich chondrules and refractory inc...
متن کاملDiscovery of ancient silicate stardust in a meteorite.
We have discovered nine presolar silicate grains from the carbonaceous chondrite Acfer 094. Their anomalous oxygen isotopic compositions indicate formation in the atmospheres of evolved stars. Two grains are identified as pyroxene, two as olivine, one as a glass with embedded metal and sulfides (GEMS), and one as an Al-rich silicate. One grain is enriched in 26Mg, which is attributed to the rad...
متن کامل