Franchi, I.A.

Greenwood, R.C.

Ruzicka, A.M.

Schepker, K.L.

English

Open Research Online (The Open University)

Open Research OnlineThe Open University’s repository of research publicationsand other research outputsCombined chemical-oxygen isotope study of largeigneous inclusions in ordinary chondritesConference or Workshop ItemHow to cite:Ruzicka, A.M.; Schepker, K.L.; Greenwood, R.C. and Franchi, I.A. (2016). Combined chemical-oxygen isotopestudy of large igneous inclusions in ordinary chondrites. In: 47th Lunar and Planetary Science Conference, 21-25 Mar2016, Houston, Texas.For guidance on citations see FAQs.c© [not recorded]Version: Version of RecordCopyright 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.ukCOMBINED CHEMICAL-OXYGEN ISOTOPE STUDY OF LARGE IGNEOUS INCLUSIONS IN ORDINARY CHONDRITES.  A. M. Ruzicka1, K. L. Schepker1, R.C. Greenwood2, and I.A. Franchi2. 1Cascadia Meteorite Laboratory, Portland State University, Dept. of Geology, 1721 SW Broadway, Portland, OR 97201 U.S.A. (ruzickaa@pdx.edu), 2PSSRI, Open University, Walton Hall, Milton Keynes MK7 6AA, UK.    Introduction: Large igneous-textured inclusions poor in metal and sulfide occur in ~4% of ordinary chondrites [1] but are otherwise diverse, suggesting various formation mechanisms [2,3]. Recent work on the petrology of 29 inclusions suggested that they can be subdivided into different bulk chemical groups, with no evidence that any were produced by igneous differentiation [4]. This is in contrast to other inclu-sions that could have formed by differentiation [5]. Here we expand the geochemical database to 41 inclu-sions and report on the oxygen isotopic compositions of 12. Our results have important implications for the origins of such inclusions.  Chemical groups: Bulk chemical compositions of inclusions reconstructed from modal and phase com-position data (SEM + EMPA) confirm earlier work [4] that inclusions comprise a few basic chemical groups, none of which correspond to that expected for igneous differentiation. The groups include: vapor-fractionated (Vfr, n=18 examples), unfractionated (Unfr, n=13), K-enriched but otherwise unfractionated (Unfr+K, n=4), feldspar-rich (FldR, n=4), and unique (n=2). Examples of each group can be found in both less and more metamorphosed chondrites, although Unfr is more prevalent in type 5 and 6 (9 of 13 examples) and Vfr in type 3 and 4 chondrites (17 of 18).  The unfractionated (Unfr) group has lithophile ele-ment abundances similar to ordinary chondrites and depletions in Ni, Fe and S (Fig. 1a). This composition is best explained by the melting of chondrite accompa-nied by metal and sulfide loss. Five such inclusions are located near coarse metal-sulfide nodules that could have separated from the inclusions during in situ melt-ing. Unfr inclusions sometimes show evidence for brecciation while partly molten, consisting of obvi-ously brecciated olivine microphenocrysts or brecci-ated regions of olivine + mesostasis embedded in in-clusion mesostasis that is not brecciated. Some or all Unfr inclusions could have formed by shock melting of chondrites. The Unfr+K group (inclusions 869-I1, 8231-I1, MET-I2, Dim-I1) is chemically and texturally similar to Unfr but distinguished by variable and sometimes large enrichments in K (3.5-14.5 x CI chondrite) (Fig. 1b). This composition resembles the silicate portion of impact melt in some ordinary chondrite melt rocks and melt breccias, which also are enriched in K (2.3-12.7 x CI) (Fig. 1b). K-enriched impact melt was previously noted for Chico [6]; inclusion 869-I1 in Northwest Africa 869 texturally resembles impact melt rock iden-tified elsewhere in the meteorite [7]. Although the rea-son for K enrichment in these melts is uncertain, inclu-sions in the Unfr+K group probably formed by impact melting.  Fig. 1. CI-normalized abundances (mean and ±1σ) of 3 inclusion types compared to mean O chondrite com-position [8]; impact melt composition in (b) is based on the average of 2 L melt rocks and 2 L melt breccias.   Inclusions in the Vfr group show evidence for a vapor-fractionation process, as lithophile element abundances trend with volatility (Fig. 1c). Six of the 18 inclusions are droplets and could be called megachondrules, but both the droplets and non-2230.pdf47th Lunar and Planetary Science Conference (2016)droplets have similar compositions. Compared to other inclusion melts, Vfr melts were evidently more ex-posed to a space environment (to facilitate evaporation or condensation) and/or were more thoroughly heated (to facilitate evaporation). These inclusions could have formed as dispersed melts analogous to chondrules.  Oxygen isotope composition: 1.2-10.2 mg sam-ples of inclusions were mechanically separated and 1-2 mg subsamples were analyzed by laser fluorination at Open University [9] with system precision (2σ) for δ17O, δ18O and ∆17O of ±0.05 ‰, ±0.09 ‰, and ±0.02 ‰, respectively. Results are shown compared to type 4-6 H-L-LL chondrites [10] in Fig. 2. Inclusions span a range in Δ17O (~0.1 to 1.4 ‰) and δ18O (3.8-7.8 ‰) values, broader than but overlapping the H-L-LL fields. There is only a partial correspondence between in-clusion and host oxygen isotope composition and Fe-Mg equilibration state (Fig. 2a). Inclusions with equili-brated Fe-Mg tend to lie close to the H-L-LL fields, but not always within the appropriate host composi-tion. For example, 8645-I1 (host Northwest Africa 8645, L5) has L-like silicate compositions but Δ17O values far from L, at the upper end of LL (Fig. 2a). Conceivably 8645-I1 was metamorphosed in situ and equilibrated in Fe-Mg, but preserved an oxygen com-position different than the host reflecting a different formation reservoir. Inclusion 7871-I1 has L-like sili-cate compositions but fits better with the H-group oxy-gen field. Finally, most inclusions with unequilibrated Fe-Mg have L and LL hosts and lie well outside of the L and LL fields, with Δ17O values between H chon-drites and the terrestrial fractionation (TF) line (Fig. 2a).   The relationship between inclusion chemical group and oxygen isotope composition is shown in Fig. 2b. Unfr and Unfr+K inclusions mostly have O-isotopic compositions that resemble H-L-LL chon-drites, consistent with them being impact melts of such chondrites. Specifically, the oxygen data support the interpretation that inclusions 869-I1 and 8231-I1 are shock melts of L and H chondrite, respectively; and that inclusion 4859-I18 is a shock melt of LL chon-drite (consistent with previous interpretations of igne-ous material in Northwest Africa 4859 [11]). Inclu-sions 8645-I1 and 7871-I1 could be shock melts of LL and H chondrites, respectively, though both were in-corporated in L chondrite. Inclusion MET-I2 has a low Δ17O value (Fig. 2b), and could have formed by impact melting of a separate, low-Δ17O parent body. Lastly, the Vfr inclusions all have generally low Δ17O values outside the main H-L-LL fields (Fig. 2b). Potentially all are megachondrules. Although the oxygen isotopic compositions of chondrules from ordinary chondrites are more diverse than type 4-6 H-L-LL [10], there are not many with the low Δ17O values we have found for Vfr inclusions. Why all such inclusions (so far) have low Δ17O is not clear, but suggests a distinctive proc-ess or provenance, possibly exchange with nebular gas of distinct O-isotope composition.    References: [1] Bridges J.C. and Hutchison R. (1997) MAPS 32, 389-394. [2] Ruzicka A. et al. (1998) GCA 62, 1419-1442. [3] Ruzicka A. et al. (2000) Ant-arctic Meteorite Res. 13, 19-38. [4] Armstrong K. and Ruzicka A.M. (2015) 46th LPS, Abstract #1572. [5] Ruzicka A. et al. (2012) MAPS 47, 1809-1829. [6] Norman M.D. and Mittlefehldt D.W. (2002) MAPS 37, 329-344. [7] Metzler K. et al. (2011) MAPS 46, 652-680. [8] Jarosewich E. (1990) Meteoritics 25, 323-337. [9] Miller M.F. et al. (1999) Rapid Comm. Mass Spectrm. 13, 1211-1217. [10] Clayton R.N. et al. (1991) GCA 55, 2317-2377. [11] Jamsja N. and Ruzicka A. (2010) MAPS 45, 828-849.   Fig. 2 Oxygen isotopic compositions of inclusions compared to type 4-6 H, L, LL chondrites [10].  2230.pdf47th Lunar and Planetary Science Conference (2016)

Combined chemical-oxygen isotope study of large igneous inclusions in ordinary chondrites

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