Volatile fractionation in the early solar system and chondrule/matrix complementarity

Abstract

Bulk chondritic meteorites and terrestrial planets show a monotonic depletion in moderately volatile and volatile elements relative to the Sun's photosphere and CI carbonaceous chondrites. Although volatile depletion was the most fundamental chemical process affecting the inner solar nebula, debate continues as to its cause. Carbonaceous chondrites are the most primitive rocks available to us, and fine-grained, volatile-rich matrix is the most primitive component in these rocks. Several volatile depletion models posit a pristine matrix, with uniform CI-like chemistry across the different chondrite groups. To understand the nature of volatile fractionation, we studied minor and trace element abundances in fine-grained matrices of a variety of carbonaceous chondrites. We find that matrix trace element abundances are characteristic for a given chondrite group; they are depleted relative to CI chondrites, but are enriched relative to bulk compositions of their parent meteorites, particularly in volatile siderophile and chalcophile elements. This enrichment produces a highly nonmonotonic trace element pattern that requires a complementary depletion in chondrule compositions to achieve a monotonic bulk. We infer that carbonaceous chondrite matrices are not pristine: they formed from a material reservoir that was already depleted in volatile and moderately volatile elements. Additional thermal processing occurred during chondrule formation, with exchange of volatile siderophile and chalcophile elements between chondrules and matrix. This chemical complementarity shows that these chondritic components formed in the same nebula region.carbonaceous chondrite ͉ chondrule formation ͉ volatile depletion C arbonaceous chondrites were first recognized as primitive samples of the early solar nebula in the 1960s, when it became apparent that a group of these meteorites contained moderately volatile elements (condensing between Ϸ1,350 K and 650 K) and volatile elements (condensing at Ͻ650 K) in similar abundance to the solar photosphere (1). These elements are severely depleted in the terrestrial planets and in most other meteorite groups. Chondrites are constructed from chondrules, the igneous products of transient heating events, and Ca-Al-rich refractory inclusions (CAIs). These high-temperature components are embedded in a volatile-rich, fine-grained, mineralogically complex matrix, which is a host for presolar grains. In addition to the different compositional groups (e.g., CV, CO, CR, CM, and CI), carbonaceous chondrites are subdivided from types 1 to 6 based on the degree of secondary aqueous and thermal processing that they have experienced, type 3 being the most primitive, with aqueous alteration increasing to type 1, and types 3 to 6 showing increasing thermal alteration. CI1 chondrites, aqueously altered, and composed almost entirely of matrix, have compositions indistinguishable from solar. Other carbonaceous meteorites, containing less matrix (and more chondrules), are depleted in volatiles to varying degrees.The var...

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Tungsten isotopic constraints on the age and origin of chondrules

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