Solar nebula liquid condensates

Chondrules are the main building blocks of chondrites, the oldest and most primitive rocks of the solar system. In principle, chondrules formed by aggregation of mostly solid precursors which were subsequently melted to form droplets which solidified under quick cooling conditions. In spite of originating from solid precursors, chondrules commonly do not show geochemical trace element fractionations which have to be expected. They rather are geochemically unfractionated but commonly are cosmochemically fractionated. The latter is manifested in elemental ambundances commonly changing smoothly with the cosmochemical volatility of the elements. A peculiar class of chondrules, present in all chondrites, comprises radiating pyroxene (RP) and radiating olivine- pyroxene (ROP) chondrules. They have crypto-crystalline to fibrous textures and resemble chilled liquids. The few chondrules of that class analyzed so far have mostly non-fractionated refractory lithophile element abundances but are depleted in volatile lithophile elements and refractory siderophile elements. These features suggest that they could have formed as liquid droplets directly by condensation from the solar nebular. However, the canonical solar nebula models do not provide the necessary conditions which need 100-1000-fold increase of the partial pressure of the condensible elements. This study is designed to clearly establish the origin of RP and ROP chondrules and constrain their formation conditions. Utilization of modern micro-analytical methods shall ensure that well-founded conclusions could be drawn.

Chondrules are a major component of chondrites, the most common meteorites, very old (about 4,6 Ga) rocks not known on Earth and which have a chemical composition that indicates that they originated in the solar nebula: the chemical composition of all chondrites is similar to that of the sun minus their non-condensable gases (H, rare gases). Chondritic rocks are chaotic, the components of which have highly variable chemical, mineralogical and isotopic compositions and obviously have individual histories. Chondrules are one of them, are enigmatic, round objects, which obviously were at least in part liquid droplets, which cooled quickly and crystallized to different degrees ("fiery rain" of Sorby, ca. 1870). Their chemical, mineralogical and isotopical messages are as chaotic as those of the rocks they are part of. Consequently, we still don`t know, how they formed and no proper model exists yet for their origin(s) and formation. Two major problems are unsolved: 1) where do the many components come from which need to mixed into a liquid droplet? and 2) where in the solar nebula and how could a liquid silicate be formed? The popular model for the solar nebula all predict only very low pressures (0,000x atm.) and do not allow liquids to be stable. Consequently, a large number of chondrule formation models exist, some of them become popular from time to time, but not better, and altogether remain non-satisfying. This, of course, is also a consequence of us being terretrial petrologists/geochemists who tend to forget, that the solar nebula cannot be like our Earth (or similar to how she was in the past): processes we are familiar with are not the only ones who can creat chondrules. In order to check the various chondrule formation models, a total of 114 micro-objects from 15 different chondrites were selected for detailed studies. Detailed petrographic studies and mineralogical analyses, carried out by optical microscopy, electron microscopy and electron microprobe. The results suggest that these objects represent quickly quenched liquids and that each object reflects a very complex and unique evolution history. Bulk major element contents as well as bulk lithophile (rock-loving) trace element contents were determined by laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). It turned out that many elements are present in unfractionated (=solar) ratios to each other (e.g., CaO/Al2 O3 , Yb/Ce as well as Sc/Yb). The mostly unfractionated refractory (=not easy to be evaporated) lithophile trace element abundance patterns are present in combination with fractionated Si/Mg ratios. These features cannot be produced by sampling and melting of solid precursors, as the most popular chondrule formation model requires. They do, however, support an origin by condensation from the solar nebula. Moderately volatile and volatile elements (e.g., V, Cr, Mn, Fe as well as K and Na) seem to have been added to the objects by metasomatic processes (exchange of elements with the solar nebula). This process was successful to very different degrees for individual chondrules and elements leading to the apparently chaotic, non-correlated elemental abundances, as they are observed. These metasomatic processes have taken place mainly after chondrule formation. Full equilibrium condensation calculations, performed to test if pyroxene-dominated chondrules could have formed directly by crystallization of droplets formed in the cooling nebular gases demonstrate the possibility that pyroxene- dominated chondrules with all the features observed can be produced as liquid condensates in Si/Mg fractionated nebular gases with a dust/gas ratio enriched 800 x CI and removing 72 % of the original Mg as forsterite from a system with solar composition. Thus, our detailed investigations of pyroxene-dominated objects from different chondrites indicate that condensation of liquids from vapor is a possible chondrule formation process. This process alone is capable of producing a large spectrum of chemical compositions, however, typically with unfractionated RLE abundances. Late stage and subsolidus metasomatic events probably played an important role and furthered the compositional diversity of chondrules and related objects. Histories of individual objects differ in detail from each other and clearly indicate individual formation and processing. The very different records of the individual travelers reflect the chaotic situation in the solar nebula.