Ingrid Herrwerth

Structure and thermal history of the H-chondrite parent asteroid revealed by thermochronometry

Our Solar System formed ∼4.6 billion years ago from the collapse of a dense core inside an interstellar molecular cloud. The subsequent formation of solid bodies took place rapidly. The period of <10 million years over which planetesimals were assembled can be investigated through the study of meteorites. Although some planetesimals differentiated and formed metallic cores like the larger terrestrial planets, the parent bodies of undifferentiated chondritic meteorites experienced comparatively mild thermal metamorphism that was insufficient to separate metal from silicate. There is debate about the nature of the heat source as well as the structure and cooling history of the parent bodies. Here we report a study of 244Pu fission-track and 40Ar–39Ar thermochronologies of unshocked H chondrites, which are presumed to have a common, single, parent body. We show that, after fast accretion, an internal heating source (most probably 26Al decay) resulted in a layered parent body that cooled relatively undisturbed: rocks in the outer shells reached lower maximum metamorphic temperatures and cooled faster than the more recrystallized and chemically equilibrated rocks from the centre, which needed ∼160 Myr to reach 390K.

Expansion velocity and temperatures of gas and ions measured in the coma of Comet Halley

In situ measurements of flow velocity and temperature in the inner coma were obtained from ram energy spectra of molecules and ions observed by the NMS experiment onboard the Giotto probe to comet Halley. Radial flow speed was (800 ± 50) m s−1 at distances from the nucleus between 1000 km and 4000 km. Whereas ions became stagnant just outside the contact surface (located near 4500 km), the outflow of water vapour even accelerated and was above 1 km s −1 at 30000 km. Ion temperatures were around 200 K inside the contact surface where the plasma is collisionally coupled to the gas; at this boundary ion temperature rises by about 1000 K and then further increases to reach about 5000 K at 15 000 km. Gas temperature remains below 300–500 K over this range of distances. A near zero volt spacecraft potential was inferred.

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.

Chondrules from Chainpur (LL-3): reduced parent rocks and vapor fractionation

Abstract Nineteen chondrules from the Chainpur (LL-3) chondrite were analyzed for their bulk lithophile element contents and mineral chemistries. Chondrule compositions are highly fractionated. The following compositional groups can be distinguished: (1) refractory (depleted in Na and K); (2) K-depleted(K≪CI, Na>CI); (3) approximately chondritic(Na/K∼CI, Na>CI); and (4) K-enriched(K≫CI, Na>CI). The most refractory chondrules are depleted in K (0.11 × CI abundances normalized to Sc), Na (0.26), Mn (0.52), and Cr (0.71). Therefore we conclude that the refractory group was mainly produced by vapor fractionation during the chondrule forming event. The majority of chondrules in enriched in Na (1.37–1.40 × CI) but fractionated in K which ranges from 0.4 × CI (K-depleted) to 2.84 × CI (K-enriched). For the fractionation of K from Na we suggest a separation mechanism via alkali sulfides. Uncorrelated fractionations of Ca, Mn, Cr, Eu, Yb, Sm give supporting evidence for such a mechanism. These findings imply that the pre-Chainpur matter went through highly reducing conditions (high H2/O2 and S2/O2 ratios) before it was sampled by the chondrule forming process. Most chondrules of our suite have similar fa contents in olivine (11–21) and are enriched in volatile elements. From this we conclude that these chondrules were formed under oxidizing conditions in a relatively dense atmosphere with high partial pressures of volatile elements (alkalis, halogens). A proto-planetary atmosphere could have provided these conditions. The fractionation process proposed should also be applicable to fractionations on a large scale like the Na/K and Mg/Si fractionations among chondrites.