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

G.J. Wasserburg I.D. Hutcheon J. Aléon 1 E.C. Ramon A.N. Krot K. Nagashima A.J. Brearley
1 CSNSM AS
CSNSM - Centre de Spectrométrie Nucléaire et de Spectrométrie de Masse
Abstract : 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 H2O. 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 26Al and 36Cl originally present, 26Al-26Mg and 36Cl-36S isotopic systematics in sodalite were investigated. Al3509 shows no evidence of radiogenic 26Mg*, establishing an upper limit of the initial 26Al/27Al ratio of 3106. All sodalite grains measured show large but variable excesses of 36S, which, however, do not correlate with 35Cl/34S ratio. If these excesses are due to decay of 36Cl, local redistribution of radiogenic 36S* after 36Cl 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 Δ17O not, vert, similar 3‰. We infer that internal heating of planetesimals by 26Al 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 36Cl in Allende. The problem of 36Cl production by solar energetic particle (SEP) bombardment and the possibility that 36Cl and 41Ca might be the product of neutron capture resulting from SEP bombardment of protoplanetary surfaces are discussed. This hypothesis can be tested comparing inferred "initial" 36Cl with neutron fluences measured on the same samples and on phases showing 36S* by Sm and Gd isotopic measurements.
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Submitted on : Monday, June 20, 2011 - 2:52:54 PM
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G.J. Wasserburg, I.D. Hutcheon, J. Aléon, E.C. Ramon, A.N. Krot, et al.. Extremely Na- and Cl-rich chondrule from the CV3 carbonaceous chondrite Allende. Geochimica et Cosmochimica Acta, Elsevier, 2011, 75, pp.4752-4770. ⟨10.1016/j.gca.2011.06.004⟩. ⟨in2p3-00601783⟩

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