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

Wasserburg, G. J. and Hutcheon, I. D. and Aléon, J. and Ramon, E. C. and Krot, A. N. and Nagashima, K. and Brearley, A. J. (2011) Extremely Na- and Cl-rich chondrule from the CV3 carbonaceous chondrite Allende. Geochimica et Cosmochimica Acta, 75 (17). pp. 4752-4770. ISSN 0016-7037.

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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 ^(36)Cl, 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.

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Additional Information:© 2011 Elsevier Ltd. Received 28 November 2010; accepted in revised form 10 May 2011; available online 17 June 2011. We acknowledge discussions with Lars Borg and Meenakshi Wadhwa. The constructive reviews by Makoto Kimura, Roger Hewins and Greg Herzog are appreciated. This work was supported by NASA Grants NAG5-10610 and NNX07AI81G (A.N. Krot, P.I.), NAG5-4212 (K. Keil, P.I.), NNG06GG37G (A.J. Brearley, P.I.) and NNH04AB47I (I.D. Hutcheon, P.I.) and by the Glenn Seaborg Institute. This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344. This is Hawaii Institute of Geophysics and Planetology Publication No. 8210 and School of Ocean and Earth Science and Technology Publication No. 8212. G.J. Wasserburg acknowledges support by a NASA Cosmochemistry RTOP to J. Nuth, at GSFC, and by the Epsilon Foundation.
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Department of Energy (DOE) Lawrence Livermore National LaboratoryDE-AC52-07NA27344
Glenn Seaborg InstituteUNSPECIFIED
Epsilon FoundationUNSPECIFIED
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Other Numbering System NameOther Numbering System ID
Hawaii Institute of Geophysics and Planetology Publication8210
School of Ocean and Earth Science and Technology Publication8212
Issue or Number:17
Record Number:CaltechAUTHORS:20111005-101306666
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Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:26593
Deposited By: Jason Perez
Deposited On:06 Oct 2011 20:20
Last Modified:03 Oct 2019 03:16

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