Franchi, I.A.

Greenwood, R.G.

Jambon, A.

English

Open Research Online (The Open University)

Open Research OnlineThe Open University’s repository of research publicationsand other research outputsOxygen isotope evidence for the origin of HEDs andAngritesConference or Workshop ItemHow to cite:Greenwood, R.G.; Franchi, I.A. and Jambon, A. (2003). Oxygen isotope evidence for the origin of HEDs andAngrites. In: 66th Annual Meteoritical Society Meeting, 28 Jul - 1 Aug 2003, Munster, Germany.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/metsoc2003/pdf/5178.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.ukOXYGEN ISOTOPE EVIDENCE FOR THE ORIGIN OF HEDs AND ANGRITES.  R C Greenwood1,I A Franchi1,A Jam-bon2 1PSSRI, Open University, Milton Keynes, MK7 6AA, U.K. 2Laboratoire MAGIE, Université Pierre et Marie Currie, CNRS UMR 7047 case 110, 4 place Jussieu, 75252 Paris Cedex 05 France.  Introduction:  HEDs and angrites represent distinct suites of mafic igneous rocks formed early in the history of the Solar System [1]. Despite their mineralogical and chemical differences both groups have similar O-isotope compositions [2]. To examine the origin of HEDS and angrites we have undertaken a high-precision study of their O-isotope systematics. Experimental Techniques: Oxygen isotope analyses were un-dertaken by infrared laser-assisted fluorination [3]. Samples were fused prior to fluorination. O2 was analyzed using a Micromass Prism III dual inlet mass spectrometer. Precision is ±0.04‰ for δ17O, ±0.08‰ for δ18O and ±0.025‰ for ∆17O [3]. Meteorites stud-ied: Diogenites: Bilanga (4), Johnstown (4), Shalka (2), Tatahouine (2), Eucrites: Pasamonte (5), Juvinas (2), Padvarnika (2), Sioux County (2), Stannern (2), Moore County (2), Howardites: Kapoeta (2), Pavlovka (2), Molteno (2), Angrites: Angra dos Ries (4), LEW 86010*(2), D’Orbigny*(2), NWA 1296* (2). (all falls except as indi-cated by an asterisk, number of replicate analyses in brackets). Results: Angrites and HEDs plot as distinct groups (Fig 1) and define single mass fractionation lines  (∆17O = –0.056±0.007 for angrites and –0.220±0.012 for HEDs). Previous studies have indi-cated that Angra dos Reis has an O-isotope composition indistin-guishable from HEDs [2]. Using two distinct sub-samples we have found it to have a ∆17O value of –0.065±0.009 and plots in the an-grite field (Fig. 1). Eucrites and diogenites plot at either end of the HED array (Fig. 1), while howardites occupy a more central posi-tion. The large error bars for δ18O reflect the inhomogeneous, coarse-grained nature of HEDs, with diogenites having grain sizes in excess of 5cm [1].  Discussion:  Experimental work indicates that either an angritic or eucritic melt can be produced from a chondritic parent simply by partial melting under differing conditions of oxygen fugacity [1]. Thus a single asteroid might be capable of producing both the HEDs and angrites. However, O-isotope data indicates that this is highly unlikely and that both suites must come from distinct parent bodies. Furthermore, as both fall on well-defined mass fractionation lines only a single parent body per group is required. Finally, in contrast to recent suggestions [4], O-isotope data presented here clearly demonstrates that Angra dos Ries is an angrite.   References: [1] Mittlefehldt D. W. et al (1998) in Planetary Materials, Papike J.J.(ed). [2] Clayton R. N. and Mayeda T.K. (1996) GCA, 60, 1999-2017. [3] Miller M.F. et al. (1999)  Rapid Commun. Mass Spectrom. 13, 1211-1217. [4] Mittlefehldt et al. (2002) Meteorit. Planet. Sci.,37,345-369. 66th Annual Meteoritical Society Meeting (2003) 5178.pdf

Oxygen isotope evidence for the origin of HEDs and Angrites

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Oxygen isotope evidence for the origin of HEDs and Angrites

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