26Al, 244Pu, 50Ti, REE, and trace element abundances in hibonite grains from CM and CV meteorites

Refractory inclusions with unusual chemical compositions from the Vigarano carbonaceous chondrite

“…If, during accretion of inclusion precursors, phases containing distinct ratios of refractory lithophiles were sampled in different proportions from those that existed in the nebula, the fractionations seen among refractory lithophiles in individual inclusions could have arisen. Although the activity coefficients necessary for calculating the distribution of all refractory lithophiles among ah possible host phases during condensation are unknown, it has been inferred that Sr was carried into the inclusions predominantly in melihte, Ba in an unknown phase other than melilite (GROSSMAN et al, 1977) and REEs, SC, Th, Zr, Ta, and Hf in one or more of hibonite, perovskite, and zirconium oxide (DAVIS et al, 1978;FAHEY et al, 1987;HINTON et al, 1988). Although a given pair of refractory lithophiles may be fractionated from one another by relatively large amounts in individual Group I or modified Group I inclusions compared to Cl chondrites, the fractionation between the same pair of elements is always less than -20% when the compositions of large numbers of such inclusions are averaged.…”

Refractory Phases in Primitive Meteorites Devoid of [TSUP]26[/TSUP]A[CLC]l[/CLC] and [TSUP]41[/TSUP]C[CLC]a[/CLC]:Representative Samples of First Solar System Solids?

“…In fact, corundum and hibonite are predicted to be the first solid phases to condense in the solar nebula (Grossman 1980). The nearly hundred-fold enrichment of refractory trace and rare-earth element abundances relative to carbonaceous chondrites in the hibonites devoid of 26 Al and 41 Ca (Fahey et al 1987;Ireland et al 1988;Sahijpal et al 1998aSahijpal et al , 1998b) also indicate them to be some of the first nebular condensates that preferentially incorporated these refractory elements. As noted earlier, the enrichment of the neutron-rich isotopes ( 48 Ca and 50 Ti) seen in the hibonites also follows a certain trend; the hibonites devoid of 26 Al and 41 Ca show larger enrichments compared to hibonites with canonical 26 Al and 41 Ca abundances.…”

“…In particular, the requirement of centimeter-sized interstellar clumps is very difficult to accommodate within our current understanding of the mechanism of formation and growth of interstellar/circumstellar grains. We also note that some of the characteristics of individual hibonites devoid of 26 Al and 41 Ca, particularly their trace element abundances, suggest their precursor to be nebular material of solar composition rather than some exotic presolar material (Fahey et al 1987;Ireland et al 1988;Sahijpal et al 1998aSahijpal et al , 1998b. In the following, we argue that the hibonite, corundum, and the FUN-type CAIs devoid of 26 Al or with a wide range of initial 26 Al/ 27 Al ratios are some of the first solids to form in the solar system, and the absence or lower abundance of 26 Al in these objects could be a natural consequence of their formation in the inner region of the collapsing protosolar cloud prior to the arrival of the freshly synthesized short-lived nuclides injected from a stellar source.…”

“…5 # 10 of 26 Al often show enrichment in the abundance of the neutronrich Ca and Ti isotopes ( 48 Ca and 50 Ti) and are also characterized by high abundances of refractory trace elements compared to the average solar system (carbonaceous chondrite) values (Fahey et al 1987;Ireland, Fahey, & Zinner 1988;Sahijpal, Davis, & Goswami 1998a, 1998b. The second group of objects, the FUN (fractionation and unknown nuclear effects) inclusions, show highly mass-fractionated isotopic compositions as well as a variety of nucleogenic stable isotope anomalies (see, e.g., Lee 1988;Ireland et al 1992).…”

“…Late formation of these objects, after partial or complete decay of 26 Al, would imply a time interval of several million years between their formation and those of CAIs with canonical 26 Al abundance. In some cases, e.g., the HAL hibonite from Allende meteorite with an initial 26 Al/ 27 Al ratio of ∼ (Fahey et al 1987),…”