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Phosphate minerals in LL chondrites: A record of the action of fluids during metamorphism on ordinary chondrite parent bodies

Jones, Rhian H. and McCubbin, Francis M. and Dreeland, Linda and Guan, Yunbin and Burger, Paul V. and Shearer, Charles K. (2014) Phosphate minerals in LL chondrites: A record of the action of fluids during metamorphism on ordinary chondrite parent bodies. Geochimica et Cosmochimica Acta, 132 . pp. 120-140. ISSN 0016-7037.

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Ordinary chondrites contain two phosphate minerals, merrillite and chlorapatite, both of which are secondary minerals that developed in response to metamorphism on the chondrite parent bodies. We have studied the phosphate mineralogy of four LL chondrites, of petrologic types 3.9–6, in order to determine the petrogenesis of the two minerals and interpret the conditions under which they formed. Characterization of merrillite and apatite includes textural observations, mineral compositions determined by electron probe microanalysis, and ion microprobe analyses of trace element and volatile anion elemental abundances. Initial formation of phosphate minerals during mild metamorphism, to petrologic type 4 conditions, resulted in oxidation of P that was originally incorporated in metal, and growth of merrillite as inclusions within metal grains. Subsequent development of both phosphate minerals occurred in response to diffusional equilibration, possible precipitation from fluids as well as replacement reactions resulting from interactions with fluids. Porosity and vein-filling textures in both merrillite and chlorapatite, as well as textures indicating replacement of merrillite by chlorapatite, support a model in which fluid played a significant role and suggest an interface-coupled dissolution–reprecipitation mechanism during metasomatism. Some associations of phosphate minerals with chromite-plagioclase assemblages suggest that phosphate minerals could also be related to impact processes, either as precipitation from an impact melt or as a result of interactions with a fluid or vapor derived from an impact melt. Fluid compositions may have been water-bearing initially, at low temperatures of metamorphism, but later evolved to become halogen-rich and very dry. Late-stage halogen-rich fluids that dominated during cooling of LL chondrite material may have been derived from vaporization of partial melts in the interior of the parent body. Overall, the LL chondrite parent body underwent a complex chemical evolution, in which metasomatism played a significant role.

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Guan, Yunbin0000-0002-7636-3735
Additional Information:© 2014 Elsevier Ltd. Received 3 June 2013; accepted in revised form 22 January 2014; available online 19 February 2014. The authors would like to thank A. Rubin and E. Scott for helpful reviews, Michael Spilde for help with electron microprobe analyses, and Adrian Brearley for many discussions. Electron microprobe and SEM work was carried out in the Electron Microbeam Analysis Facility, Department of Earth and Planetary Sciences and Institute of Meteoritics, University of New Mexico. The work was partially funded by NASA grants NNX09AG42G and NNX12AH61G (R.J.), NNX11AG76G (F.M.), the Moore foundation for support of the Caltech Microanalysis Center (Y.G.) and NNX10AI77G and NNX13AH85G (C.S.). Associate editor: Yuri Amelin.
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Gordon and Betty Moore FoundationUNSPECIFIED
Record Number:CaltechAUTHORS:20140527-102509954
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Official Citation:Rhian H. Jones, Francis M. McCubbin, Linda Dreeland, Yunbin Guan, Paul V. Burger, Charles K. Shearer, Phosphate minerals in LL chondrites: A record of the action of fluids during metamorphism on ordinary chondrite parent bodies, Geochimica et Cosmochimica Acta, Volume 132, 1 May 2014, Pages 120-140, ISSN 0016-7037, (
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:45917
Deposited By: Ruth Sustaita
Deposited On:27 May 2014 17:53
Last Modified:26 Nov 2019 23:09

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