K. Nagel

An ultra-refractory inclusion from the Ornans carbonaceous chondrite

A small refractory inclusion (∼1 μg) was found in the Ornans carbonaceous chondrite. The inclusion is extremely enriched in the most refractory REE (Lu, Er, Ho), the more volatile light REE having more than two orders of magnitude lower concentrations. Refractory metals are fractionated in a similar way; high abundances of the most refractory metals (Os, Re), low content of more volatile metals such as Ru and Pt. Gas-solid equilibrium at high temperatures can easily account for the observed relative abundances of refractory elements. Independent temperature estimates from REE abundances and refractory metal contents suggest an equilibrium temperature of about 1670°K (at 10−3 atm). The trace element pattern of the inclusion is complementary to the pattern of fine-grained Allende inclusions. The inclusion may either represent a first condensate from a cooling gas of approximately solar composition, or the very last residue from an evaporation process. Only the low contents of W and Mo do not fit the pattern of refractory elements predicted from condensation calculations. Both elements, however, readily form volatile oxides. Therefore, this depletion could be taken as indication of an evaporative origin of the inclusion. The metal phase consists of small Os-rich grains in a comparatively Os-poor matrix. Grains and matrix appear to be a mechanical mixture. This textural evidence cannot be easily reconciled with an evaporative origin. A condensation origin of the inclusion would require that W and Mo were lost at a later time in a more oxidising environment, without affecting the metallic Ta found in some of the metal grains.

A calcium-aluminum-rich inclusion from the Essebi (CM2) chondrite: Evidence for captured spinel-hibonite spherules and for an ultra-refractory rimming sequence

Abstract An unusual refractory inclusion was discovered in the Essebi (CM2) chondrite. The inclusion has high concentrations of refractory lithophile and siderophile elements, with strong enrichments of the most refractory elements (Lu, Sc, Hf) in one part of the inclusion. The inclusion consists of a melilite-rich core partially surrounded by a very refractory rim, which in turn is covered by a mantle enriched in Si, Al, Fe, Mn, Cl, and S. Melilite compositions vary from Ak 10–26 in the core to Ak 0–12 in the rim. The core contains several complex framboids consisting of spinel + fassaite or spinel + hibonite. These framboids are probably spherules of the types reported from Murchison, and were possibly captured by the molten inclusion before it solidified. These findings may indicate a genetic link between framboids in CAIs and the spinel-rich spherules in Murchison. The rim sequence consists of the following five layers from inside to outside: 1. (1) hibonite + spinel + melilite; 2. (2) hibonite + perovskite + spinel + melilite (melilite in this layer is altered to sodalite + nepheline + calcite); 3. (3) a hibonite-corundum solid solution; 4. (4) a spinel — corundum solid solution (as much as 0.1 mole fraction Al 2.67 O 4 ); 5. (5) Sc-rich fassaite (as much as 6.2 wt.% Sc 2 O 3 ) + spinel − Al 2 O 3 solid solution + Al-rich diopside. Pt-rich metal crystals occur only in the rim. Some of these metal crystals contain minor amounts of Zr and Hf, indicating that they formed under reducing conditions. The bulk mineral chemistry and texture of the Essebi inclusion indicates a complex history involving capture of solid spinel and spinel — hibonite spherules similar to the Murchison type by a refractory melt droplet, solidification of the droplet without obliteration of the spherule textures, transportation of the composite object to hotter regions of the solar nebula where the very refractory rim condensed, alteration of melilite to sodalite + nepheline + minor calcite, and finally burial in the Essebi parent body. The Cl-, S-bearing mantle probably formed by reactions between the inclusion and liquid and gaseous phases in the parent body.