Baker, L.

Franchi, I.A.

Morris, A.A.

Wright, I.P.

English

Open Research Online (The Open University)

Open Research OnlineThe Open University’s repository of research publicationsand other research outputsDetermining the effects of aqueous alteration on thedistribution of oxygen isotopes in carbonaceouschondritesConference or Workshop ItemHow to cite:Morris, A.A.; Baker, L.; Franchi, I.A. and Wright, I.P. (2005). Determining the effects of aqueous alterationon the distribution of oxygen isotopes in carbonaceous chondrites. In: Workshop on Oxygen in Asteroids andMeteorites, 2-3 Jun 2005, Flagstaff, Arizona, USA.For guidance on citations see FAQs.c© [not recorded]Version: [not recorded]Link(s) to article on publisher’s website:http://www.lpi.usra.edu/meetings/am2005/pdf/7008.pdfCopyright and Moral Rights for the articles on this site are retained by the individual authors and/or other copyrightowners. For more information on Open Research Online’s data policy on reuse of materials please consult the policiespage.oro.open.ac.ukDETERMINING THE EFFECTS OF AQUEOUS ALTERATION ON THE DISTRIBUTION OF OXYGEN ISOTOPES IN CARBONACEOUS CHONDRITES.  A. A. Morris, L. Baker, I.A. Franchi and I.P. Wright. Planetary and Space Science Research Institute, The Open University, Milton Keynes, MK7 6AA (a.a.morris@open.ac.uk)   Introduction: Carbonaceous chondrites play a key role in determining the history of the early Solar Sys-tem; a knowledge of the oxygen isotope variation within these materials will not only help in our under-standing of the origin and evolution of those meteor-ites, but also the early Solar System reservoirs from which they formed [1]. However, several problems present themselves when trying to interpret early Solar System history. One is identifying the isotopic signature of oxygen from specific component compounds in a complex mixture of materials that existed in the solar nebula, and deconvoluting this from signatures developed dur-ing parent body processing (e.g. aqueous alteration, thermal metamorphism). Only then is it possible to develop an  understanding of the nature and conditions of these secondary processes. Background: Of all the meteorite groups the car-bonaceous chondrites (particularly the CI, CM and CR meteorites) have the most diverse range of high and low temperature mineral phases, and display the wid-est range of oxygen-isotopic compositions [1,2]. Volumetrically, much of these meteorites consists of a very fine grained matrix material composed largely of low temperature alteration minerals but intimately mixed with a fine grained fraction of the anhydrous, isotopically distinct phases [3]. A full understanding of the oxygen-isotopic systematics between these differ-ent phases is required in order to constrain the proc-esses that led to their formation [3]. However most previous oxygen isotope studies have concentrated on the silicate portions which in the case of the matrix means studying complex mixtures [3,4]. Aqueous re-actions can not only promote chemical redistribution and changes in petrological relationships but also gen-erate large isotopic fractionation of oxygen isotopes [5].  Models to account for the variations in oxygen-isotopic compositions in carbonaceous chondrites e.g. [6,7] generally involve mixing of a solid anhydrous silicate component, with an isotopic composition en-riched in 16O, with liquid water depleted in 16O along with mass-dependent isotopic fraction between various phases at low temperature [3]. Isotopic evolution of the fluids involved in these reactions would be ex-pected to lead to significant isotopic heterogeneity within the components e.g. [7]. Recent studies have shown that structural water (OH)- can be extracted from phyllosilicate components in the CM meteorites by incremental heating,  and used for oxygen isotope measurements. This also provides the ability to resolve oxygen from a number of differ-ent components within the complex mixture of miner-als in the matrix of these meteorites[3,4]. The (OH)- from the CM phyllosilicates is found to have a similar ∆17O to co-existing carbonates, analysed by [8] indi-cating that they formed from the same fluid reservoir, and may infact have formed in isotopic equilibrium. We are employing a number of approaches to bet-ter determine the distribution of oxygen isotopic com-ponents within these complex meteorites. Stepped heating extraction to liberate water and structural (OH)- permits isotopic measurements of resolved com-ponents [4], the identification of which is being estab-lished by a detailed study of the thermal release tem-perature of water (OH)- components from meteorites and a range of terrestrial analogues. Artifical weather-ing experiments using isotopically labeled water are being used to trace the location of different oxygen reservoirs as alteration proceeds.  Such data may then be used to constrain tempera-ture of equilibrium between the phyllosilicates and carbonates minerals and water-rock ratios involved; important constraints for evolution models of carbona-ceous chondrites [6,7].  Summary: The initial stepped heating experiments for carbonaceous chondrites revealed considerable variation in δ18O [3]. Much of our new work is aimed at determining the extent of isotopic fractionation as-sociated with the formation of the common phyllosili-cates found in meteorites, and identifying the specific minerals liberating (OH)- at each temperature. This will be used ultimately to better constrain the isotopic composition of the fluids involved in the al-teration process.  [1] Clayton R. N. & Mayeda T. K. (1999) GCA, 63, 2089–2104. [2] Brearley A. J. & Jones R. H. (1998) Planetary Materials, 36, 3-01-3-370. [3] Franchi. et al (2001) Phil. Trans. R. Soc. Lond. A 359, 2019–2035. [4] Baker. et al. (2002) Analyl. Chem. 74 1665-1673 . [5] Zolensky M. & McSween H. Y. (1988) Meteorites and the Early Solar System, 114-144. [6] Clayton R. N. & Mayeda T. K. (1984) EPSL, 67, 151-161. [7] Young E. D. et al. (1999) Science, 286, 452-455. [8] Benedix. et al. (2000) LPSC XXX1 Abs #1840 Oxygen in Asteroids and Meteorites 2005 7008.pdf

Determining the effects of aqueous alteration on the distribution of oxygen isotopes in carbonaceous chondrites

http://oro.open.ac.uk/8480/1/DETERMINING_THE_EFFECTS_OF_AQUEOUS_ALTERATION_ON_THE_DISTRIBUTION_OF_OXYGEN_ISOTOPES_IN_CARBONACEOUS_CHONDRITES.pdf

Determining the effects of aqueous alteration on the distribution of oxygen isotopes in carbonaceous chondrites

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