Refractory inclusions are characteristic components in carbonaceous chondrites. Their mineralogy is dominated by refractory oxides and silicates like corundum, perovskite, spinel, hibonite, melilite, and Ca-pyroxene, which are predicted to be the first phases to have condensed from the cooling solar nebula. Allowing insights into processes occurring in the early solar system, CAIs in carbonaceous and ordinary chondrites were studied in great detail, whereas only a few refractory inclusions were found and studied in micrometeorites (e.g. Kurat et al., 1994, Hoppe et al., 1994). Because refractory inclusions are characteristic constituents of carbonaceous chondrites they can give informations about the relationship between micrometeorites and meteorites.
Polished sections of two Antarctic micrometeorites (BI54B3-31 and BI54B4-17) were studied using a JEOL 840A analytical scanning electron microscope equipped with EDS-detector. Trace element abundances and oxygen isotopic compositions have been determined in-situ with a modified Cameca IMS 3f ion microprobe at the University of Bern following the procedure described by Zinner and Crozaz (1986). The oxygen isotopic analyses were made with a 10-50 pA Cs+ primary beam of 3-5 µm in diameter and negative secondary ions.
Particle BI54B4-17 is a 100 µm sized, irregular shaped porous micrometeorite mainly consisting of a fine-grained matrix of dehydrated former phyllosilicates. Due to atmospheric entry heating most of the phyllosilicates were converted into forsteritic olivines and enstatitic pyroxenes with grain-sizes below the resolution of the SEM. Two perovskites surrounded by thin ilmenite rims are enclosed in the matrix. The largest (5 µm) grain contains up to 1.3 wt% FeO and was used for trace element analyses.
The REE pattern of the perovskite is volatility-fractionated having low abundances (less than 10 x CI) of the highly refractory rare earth elements Gd to Er and Lu. The less refractory REEs La to Sm and Tm and Yb are 100-200 x CI, whilst Eu has an enrichment factor of < 26 x CI. In general, the pattern is closely related to the Allende Group II pattern as described by Mason and Taylor (Martin and Mason, 1974). Variations from the ideal Group II pattern are due to condensation at lower temperature, at which Eu and Yb condense to various degrees.
Particle BI54B3-31 is a ~ 80 µm sized porous micrometeorite consisting of two areas different in grain size and mineralogy. The small fine-grained area consists of orthopyroxene (Fs 13.4), whereas fassaite is the main mineral within the coarse-grained area. The REE pattern obtained for the fassaite in micrometeorite BI54B3-31 is an unfractionated, relatively smooth pattern (25-40 x CI) with substantial Eu and Gd depletions (< 4 and <7 x CI, respectively). The pattern is related to the Allende Group III pattern, although a negative Yb anomaly is missing. The orthopyroxene shows an unique REE pattern. The less refractory REEs La to Sm and Ho to Yb are enriched by a factor of 10-20 x CI. Europium and Dy have CI-normalized enrichment factors of <9, whereas Gd and Lu are strongly and Tb is slightly depleted (<3.5 x CI). The CI-normalized abundance sequence of the depletion is Gd<Lu<Tb. Such pattern can only be formed under highly reducing conditions, which are necessary for the formation of constituents in enstatite chondrites (Lodders and Fegley, 1993). Fassaite and orthopyroxene show oxygen isotopic compositions that plot close to the terrestrial fractionation line. Based on this and previous studies on relict minerals in micrometeorites (Beckerling and Bischoff, 1995) we suggest that most of refractory object-bearing micrometeorites should have a relationship to carbonaceous chondrites.
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