Acfer 094, a uniquely primitive carbonaceous chondrite from the Sahara

An evolutionary system of mineralogy. Part III: Primary chondrule mineralogy (4566 to 4561 Ma)

Information-rich attributes of minerals reveal their physical, chemical, and biological modes of origin in the context of planetary evolution, and thus they provide the basis for an evolutionary system of mineralogy. Part III of this system considers the formation of 43 different primary crystalline and amorphous phases in chondrules, which are diverse igneous droplets that formed in environments with high dust/gas ratios during an interval of planetesimal accretion and differentiation between 4566 and 4561 Ma. Chondrule mineralogy is complex, with several generations of initial droplet formation via various proposed heating mechanisms, followed in many instances by multiple episodes of reheating and partial melting. Primary chondrule mineralogy thus reflects a dynamic stage of mineral evolution, when the diversity and distribution of natural condensed solids expanded significantly.

The 26Al-26Mg systematics of FeO-rich chondrules from Acfer 094: two chondrule generations distinct in age and oxygen isotope ratios

The 26Al-26Mg ages of FeO-rich (type II) chondrules from Acfer 094, one of the least thermally metamorphosed carbonaceous chondrites, were determined by SIMS analysis of plagioclase and olivine/pyroxene using a radio frequency (RF) plasma oxygen ion source. In combination with preexisting 26Al-26Mg ages of FeO-poor (type I) chondrules, the maximum range of formation ages recorded in chondrules from a single meteorite is determined to help provide constraints on models of material transport in the proto-planetary disk. We also report new SIMS oxygen three-isotope analyses of type II chondrules in Acfer 094. All but one of the plagioclase analyses show resolvable excesses in 26Mg and isochron regressions yield initial 26Al/27Al ratios of type II chondrules that range from (3.62 ± 0.86) × 10-6 to (9.3 ± 1.1) × 10-6, which translates to formation ages between 2.71 -0.22/+0.28 Ma and 1.75 -0.11/+0.12 Ma after CAI. This overall range is indistinguishable from that determined for type I chondrules in Acfer 094. The initial 26Al/27Al ratio of the oldest type II chondrule is resolved from that of all other type II chondrules in Acfer 094. Importantly, the oldest type I chondrule and the oldest type II chondrule in Acfer 094 possess within analytical error indistinguishable initial 26Al/27Al ratios and Δ17O values of ~0‰. Ages and oxygen isotope ratios clearly set these two chondrules apart from all other chondrules in Acfer 094. It is therefore conceivable that the formation region of these two chondrules differs from that of other chondrules and in turn suggests that Acfer 094 contains two distinct chondrule generations.

TEM foil preparation of sub-micrometre sized individual grains by focused ion beam technique

Analysis of presolar silicate grains provides new knowledge on interstellar and circumstellar environments and can be used to test models of the Galactic chemical evolution. However, structural information of these grains is rare because sample preparation for transmission electron microscopy is very difficult due to the small dimensions of these grains (<0.5 mum). With the use of the focused ion beam technique thin foils from these grains for transmission electron microscopy analysis can be prepared. Nevertheless, reaching the required precision of some tens of nanometres for the preparation of the transmission electron microscopy foil in the place of interest is not trivial. Furthermore, in the current samples, the grain of interest can only be identified by its different isotopic composition; i.e. there is no contrast difference in scanning electron microscopy or transmission electron microscopy images which allow the identification of the grain. Therefore, the grain has to be marked in some way before preparing the transmission electron microscopy foil. In the present paper, a method for transmission electron microscopy foil preparation of grains about 200 to 400 nm in diameter is presented. The method utilizes marking of the grain by Pt deposition and milling of holes to aid in the exact orientation of the transmission electron microscopy foil with respect to the grain. The proposed method will be explained in detail by using an example grain.

Volatile fractionation in the early solar system and chondrule/matrix complementarity

Bulk chondritic meteorites and terrestrial planets show a monotonic depletion in moderately volatile and volatile elements relative to the Sun's photosphere and CI carbonaceous chondrites. Although volatile depletion was the most fundamental chemical process affecting the inner solar nebula, debate continues as to its cause. Carbonaceous chondrites are the most primitive rocks available to us, and fine-grained, volatile-rich matrix is the most primitive component in these rocks. Several volatile depletion models posit a pristine matrix, with uniform CI-like chemistry across the different chondrite groups. To understand the nature of volatile fractionation, we studied minor and trace element abundances in fine-grained matrices of a variety of carbonaceous chondrites. We find that matrix trace element abundances are characteristic for a given chondrite group; they are depleted relative to CI chondrites, but are enriched relative to bulk compositions of their parent meteorites, particularly in volatile siderophile and chalcophile elements. This enrichment produces a highly nonmonotonic trace element pattern that requires a complementary depletion in chondrule compositions to achieve a monotonic bulk. We infer that carbonaceous chondrite matrices are not pristine: they formed from a material reservoir that was already depleted in volatile and moderately volatile elements. Additional thermal processing occurred during chondrule formation, with exchange of volatile siderophile and chalcophile elements between chondrules and matrix. This chemical complementarity shows that these chondritic components formed in the same nebula region.

Stardust silicates from primitive meteorites

Primitive chondritic meteorites contain material (presolar grains), at the level of a few parts per million, that predates the formation of our Solar System. Astronomical observations and the chemical composition of the Sun both suggest that silicates must have been the dominant solids in the protoplanetary disk from which the planets of the Solar System formed, but no presolar silicates have been identified in chondrites. Here we report the in situ discovery of presolar silicate grains 0.1-1 microm in size in the matrices of two primitive carbonaceous chondrites. These grains are highly enriched in 17O (delta17O(SMOW) > 100-400 per thousand ), but have solar silicon isotopic compositions within analytical uncertainties, suggesting an origin in an oxygen-rich red giant or an asymptotic giant branch star. The estimated abundance of these presolar silicates (3-30 parts per million) is higher than reported for other types of presolar grains in meteorites, consistent with their ubiquity in the early Solar System, but is about two orders of magnitude lower than their abundance in anhydrous interplanetary dust particles. This result is best explained by the destruction of silicates during high-temperature processing in the solar nebula.

The primitive matrix components of the unique carbonaceous chondrite Acfer 094: a TEM study

The mineralogical and chemical characteristics of the fine-grained matrix (< or = 3 micrometers) of the unique primitive carbonaceous chondrite Acfer 094 have been investigated in detail by scanning electron microscopy (SEM) and analytical transmission electron microscopy (ATEM). Generally, the fine-grained matrix represents a highly unequilibrated assemblage of an amorphous material, small forsteritic olivines (200-300 nm), low Ca-pyroxenes (300-400 nm), and Fe,Ni-sulfides (100-300 nm). The matrix is basically unaffected by secondary processes. Only minor amounts of serpentine and ferrihydrite, as products of hydrous alteration, are present. Texturally, the amorphous material acts as a groundmass to olivines, pyroxenes, and sulfides, mostly exhibiting rounded or elongated morphologies. Only very few clastic mineral grains have been found. The texture and chemical composition of the amorphous material are consistent with an origin by disequilibrium condensation in either the cooling solar nebula or a circumstellar environment. As such, the amorphous material may be considered as a possible precursor of matrix materials in other types of chondrites. The non-clastic matrix olivines (Fo98-99) and pyroxenes (En97-100) are suggested to have formed either by condensation in the solar nebula under highly oxidizing conditions or by recrystallization from the amorphous material. The formation of these grains by fragmentation of chondrule components is unlikely due to chemical and microstructural reasons. Rapid cooling caused the observed intergrowths of clino/orthoenstatite in the Mg-rich matrix pyroxenes. Although some similarities exist comparing the fine-grained matrix of Acfer 094 with the matrices of the unequilibrated CO3 chondrite ALHA77307 and the unique type 3 chondrite Kakangari, Acfer 094 remains unique. Since it contains the highest measured concentrations of circumstellar SiC and the second highest of diamond (highest is Orgueil), it seems reasonable to suggested that at least parts of the amorphous material in the fine-grained matrix may be of circumstellar origin.

Corundum, rutile, periclase, and CaO in Ca,Al-rich inclusions from carbonaceous chondrites

Four calcium,aluminum-rich inclusions from four carbonaceous chondrites-Allende, Acfer 082, Acfer 086, and Acfer 094-were studied by transmission electron microscopy. All inclusions contained at least two of the oxides periclase (MgO), rutile (TiO2), calcium oxide (CaO), and corundum (Al2O3). The oxides (50 to 200 nanometers in size) were found inside and at grain boundaries of the constituent minerals of the inclusions. Determining how these oxides formed may provide insight about condensation processes in the early solar nebula and the origin of refractory inclusions in chondrites. Formation of these oxides by exsolution is considered unlikely. An origin by kinetically controlled condensation appears more probable.