Primitive meteorites: an attempt towards unification

Abstract

A model is developed that recognizes genetic links between almost all meteorites. The model is based on processes that were active in the solar nebula. The most important processes identified are: (1) early part to total evaporation of presolar matter; (2) recondensation beginning with olivine followed by other phases; (3) aggregation of the condensates to millimetre-sized objects during condensation; (4) partial or total compaction of early aggregates by continuing condensation preferentially utilizing the aggregate’s pore space; (5) a second heating event leading to sintering and partial melting of the aggregates (chondrule formation) and mild vapour fractionation; (6) vapour—solid exchange reactions introducing Fe 2+ (and other elements) into the silicates (‘equilibration’ via metasomatism) and forming FeS via S-metasomatism; (7) depending on conditions, centimetre-sized aggregates of early aggregates form either before or while processes (4)-(7) are active, or later. This process can continue to form decimetre-sized aggregates; (8) accretion takes place at very low temperatures with condensation of volatile elements still continuing; (9) some matter experienced H 2 O- and CO 2 -metasomatism (formation of phyllosilicates and carbonates) and almost total oxidation before accretion (Cl, C2). The typical early condensate is olivine, which displays different growth features depending on physical conditions. Common are ‘barred’ olivines (greater than 200 pm), huge (100 pm) subparallel stacks of olivine platelets and small stacks (10-50 pm) of similar platelets. These olivines have highly defective lattices and contain large amounts of minor elements (including refractory lithophile elements). Depending on growth rate and particle abundance, primitive olivines aggregate to millimetre-sized objects (the protoliths of chondrite constituents) or centimetre-sized aggregates. Formation of large aggregates apparently takes place when the condensation rate is high. This quickly leads to compaction of the aggregates and highly reduced pore space. These aggregates will ultimately end up in ureilites and pallasites. Slower growth rates produce smaller aggregates (millimetre-sized) with abundant pore space which serves as a cold trap for the condensation of the volatile elements. Olivine is partly converted into pyroxene by reaction with the vapour (formation of poikilitic pyroxenes). Pyroxene and olivine abundances will vary depending on reaction time and other parameters. The pore space will also be highly variable. Moderately volatile elements condense into this pore space with their abundance governed by the space available. In this manner, the protoliths for chondrules formed. The chemical fractionations observed in meteoritic constituents can be accounted for mostly by physical parameters: (1) the main condensing phase and the pore space provided by the aggregates; (2) the timing of formation of aggregates of aggregates and of the closure of pore space (physical isolation of aggregate interiors); (3) reprocessing of aggregates in a high temperature event; (4) metasomatic exchange reactions between solid aggregates and vapour; (5) isolation of aggregates from vapour via shielding by condensates (e.g. metal) or physical removal; (6) timing and rate of final accretion; (7) degree of mixing with solar and presolar dust.