The onset of metamorphism in ordinary and carbonaceous chondrites

Abstract

Received May 18, 2004; revision accepted November 3, 2004) Abstract-Ordinary and carbonaceous chondrites of the lowest petrologic types were surveyed by Xray mapping techniques. A variety of metamorphic effects were noted and subjected to detailed analysis using electron microprobe, scanning electron microscopy (SEM), transmission electron microscopy (TEM), and cathodoluminescence (CL) methods. The matrix in the least metamorphosed chondrites is rich in S and Na. The S is lost from the matrix at the earliest stages of metamorphism due to coalescence of minute grains. Progressive heating also results in the loss of sulfides from chondrule rims and increases sulfide abundances in coarse matrix assemblages as well as inside chondrules. Alkalis initially leave the matrix and enter chondrules during early metamorphism. Feldspar subsequently nucleates in the matrix and Na re-enters from chondrules. These metamorphic trends can be used to refine classification schemes for chondrites. Cr distributions in olivine are a highly effective tool for assigning petrologic types to the most primitive meteorites and can be used to subdivide types 3.0 and 3.1 into types 3.00 through 3.15. On this basis, the most primitive ordinary chondrite known is Semarkona, although even this meteorite has experienced a small amount of metamorphism. Allan Hills (ALH) A77307 is the least metamorphosed CO chondrite and shares many properties with the ungrouped carbonaceous chondrite Acfer 094. Analytical problems are significant for glasses in type II chondrules, as Na is easily lost during microprobe analysis. As a result, existing schemes for chondrule classification that are based on the alkali content of glasses need to be revised.

Radial transport in the solar nebula: Implications for moderately volatile element depletions in chondritic meteorites

“…Specifically, Palme et al (1988) and Palme and Boynton (1993) identified a number of reasons why the two-component model failed to explain the observed trend, including the fact that chondrules contain a significant amount of volatiles and therefore are not "volatile-depleted" as required by the Anders model (although parent body effects may have played a role [e.g., Grossman and Brearley 2005]). Palme (2001) also argued that the Rb/Sr inventory of chondritic meteorites is inconsistent with the molecular cloud inheritance theory, though Yin (2005) outlines counter arguments.…”

Mn−Cr ages of Fe-rich olivine in two Rumuruti (R) chondrites

“…In contrast, Cr concentrations in olivine are quite variable. Grossman and Brearley (2005) made a detailed study of Cr in olivine in ordinary and CO chondrites and concluded that Cr diffused out of olivine at very low temperatures. Since the Cr diffusion rate in olivine is not fast compared with those of other cations (Ito and Ganguly, 2006), the Cr loss was probably promoted by the presence of water.…”

Diffusion of Oxygen Isotopes in Thermally Evolving Planetesimals and Size Ranges of Presolar Silicate Grains

“…Petrologic type 3 chondrites could be subdivided into types 3.0 to 3.9 based on their characteristics of thermoluminescence and compositions of minerals (Sears et al 1980;Scott & Jones 1990;Scott et al 1994;Grossman & Brearley 2005;Bonal et al 2006). It is suggested that type 3.0 and 3.1 chondrites experienced lower peak temperatures of ∼ 200 -400…”