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The LaPaz Icefield 04840 meteorite: Mineralogy, metamorphism, and origin of an amphibole- and biotite-bearing R chondrite

McCanta, M. C. and Treiman, A. H. and Dyar, M. D. and Alexander, C. M. O'D. and Rumble, D. and Essene, E. J. (2008) The LaPaz Icefield 04840 meteorite: Mineralogy, metamorphism, and origin of an amphibole- and biotite-bearing R chondrite. Geochimica et Cosmochimica Acta, 72 (23). pp. 5757-5780. ISSN 0016-7037. doi:10.1016/j.gca.2008.07.034.

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The R chondrite meteorite LaPaz Icefield (LAP) 04840 is unique among metamorphosed, non-carbonaceous chondrites in containing abundant OH-bearing silicate minerals: ~13% ferri-magnesiohornblende and ~0.4% phlogopite by volume. Other minerals include olivine (Fo(62)), orthopyroxene (En(69)Fs(30)Wo(1)), albite (An(8)Ab(90)Or(2)), magnetite, pyrrhotite, pentlandite, and apatite. Ferromagnesian minerals are rich in Fe3+ as determined by Mossbauer spectrometry and electron microprobe chemical analyses. Fe3+/Fe-tot values are olivine ≤ 5%, amphibole 80%, phlogopite 65%, and magnetite 42%. Mineral compositions are nearly constant across grains and the section, except for a small variability in amphibole compositions reflecting the edenite exchange couple (Na-A + Al-IV ⇌ (A)rectangle + Si). These mineral compositions, the absence of Fe-Ni metal, and the oxygen isotope data support its classification as an R (Rumuruti) chondrite. LAP 04840 is classified as petrologic grade 5, based oil the chemical homogeneity of its minerals, and the presence of distinctly marked chondrules and chondrule fragments in a fine-grained crystalline matrix. The mineral assemblage of LAP 04840 allows calculation of physical and chemical conditions at the peak of its metamorphism: T = 670 ± 60 degrees C from a amphibole-plagioclase thermometer; P-H2O between 250 and 500 bars as constrained by the assemblage phlogopite + orthopyroxene + olivine + feldspar and the absence of diopside; P-CO2 unconstrained: f(O2) at QFM + 0.5 log units; log(f(HF)/f(H2O))≈-5.8; log(f(HCl)/f(H2O)) ≈-3.3; and log(f(HCl)/f(HF)) ≈-2.6. The hydrogen in LAP 04840 is very heavy, an average delta D value of +3660 +/- 75% in the magnesiohornblende. Only a few known sources of hydrogen have such high δD and are suitable sources for LAP 04840: ordinary chondrite phyllosilicates (as in the Semarkona chondrite), and insoluble organic matter (IOM) in ordinary chondrites and CR chondrites. Hydrogen from the IOM could have been released by oxidation, and then reacted with an anhydrous R chondrite (at high temperature), but it is not clear whether this scenario is correct.

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Treiman, A. H.0000-0002-8073-2839
Dyar, M. D.0000-0003-4272-793X
Additional Information:Copyright © 2008 Elsevier. Received 7 February 2008; accepted in revised form 25 July 2008; available online 11 September 2008. The are grateful to the ANSMET program, the NASA Antarctic Meteorite Curator, and the Meteorite Working Group for allowing us access to this unique meteorite. Craig Schwandt and Anne Peslier (JSC/Jacobs Sverdrup) assisted with EMP analyses, and Matt Manon (U. Michigan) aided with thermochemical calculations. This work has benefited from discussions with J. Grossman, A. Rubin, G. Rossman, and T. McCoy. Critical reviews by A. Bischoff, E. Deloule, and A. Rubin are gratefully acknowledged. A.H.T. acknowledges support from the NASA Astrobiology Institute node at the NASA Ames Research Center, and NASA grant NNG06GH29G. C.M.O’D.A. was partially funded by NASA Astrobiology Institute node at the Carnegie Institution of Washington. Part of this work was done while M.C.M. was a Urey Post-Doctoral fellow at the LPI. Lunar and Planetary Institute Contribution #1419.
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Urey FellowshipUNSPECIFIED
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Lunar and Planetary Institute1419
Issue or Number:23
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ID Code:12551
Deposited By: Archive Administrator
Deposited On:16 Dec 2008 00:01
Last Modified:08 Nov 2021 22:29

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