Extremely Na- and Cl-rich chondrule from the CV3 carbonaceous chondrite Allende

摘要

We report on a study of Al3509, a large Na- and Cl-rich, radially-zoned object from the oxidized CV carbonaceous chondrite Allende. Al3509 consists of fine-grained ferroan olivine, ferroan Al-diopside, nepheline, sodalite, and andradite, and is crosscut by numerous veins of nepheline, sodalite, and ferroan Al-diopside. Some poorly-characterized phases of fine-grained material are also present; these phases contain no significant H_2O. The minerals listed above are commonly found in Allende CAIs and chondrules and are attributed to late-stage iron-alkali-halogen metasomatic alteration of primary high-temperature minerals. Textural observations indicate that Al3509 is an igneous object. However, no residual crystals that might be relicts of pre-existing CAI or chondrule minerals were identified. To establish the levels of ~(26)Al and ~(36)Cl originally present, ~(26)Al-~(26)Mg and ~(36)Cl-~(36)S isotopic systematics in sodalite were investigated. Al3509 shows no evidence of radiogenic ~(26)Mg*, establishing an upper limit of the initial ~(26)Al/~(27)Al ratio of 3×10~(-6). All sodalite grains measured show large but variable excesses of ~(36)S, which, however, do not correlate with ~(35)Cl/~(34)S ratio. If these excesses are due to decay of 36Cl, local redistribution of radiogenic ~(36)S* after ~(36)Cl had decayed is required. The oxygen-isotope pattern in Al3509 is the same as found in secondary minerals resulting from iron-alkali-halogen metasomatic alteration of Allende CAIs and chondrules and in melilite and anorthite of most CAIs in Allende. The oxygen-isotope data suggest that the secondary minerals precipitated from or equilibrated with a fluid of similar oxygen-isotope composition. These observations suggest that the formation of Al3509 and alteration products in CAIs and chondrules in Allende requires a very similar fluid phase, greatly enriched in volatiles (e.g., Na and Cl) and with Δ~(17)O ~ -3‰. We infer that internal heating of planetesimals by ~(26)Al would efficiently transfer volatiles to their outer portions and enhance the formation of volatile-enriched minerals there. We conclude that the site for the production of Na- and Cl-rich fluids responsible for the formation of Al3509 and the alteration of the Allende CAIs and chondrules must have been on a protoplanetary body prior to incorporation into the Allende meteorite. Galactic cosmic rays cannot be the source of the inferred initial ~(36)Cl in Allende. The problem of ~(36)Cl production by solar energetic particle (SEP) bombardment and the possibility that ~(36)Cl and ~(41)Ca might be the product of neutron capture resulting from SEP bombardment of protoplanetary surfaces are discussed. This hypothesis can be tested comparing inferred " initial" ~(36)Cl with neutron fluencies measured on the same samples and on phases showing ~(36)S* by Sm and Gd isotopic measurements.