In this study we move beyond defining alteration sequences in CM chondrites towards understanding the relationship between modal mineralogy, the extent of aqueous alteration and O-isotope compositions

Benedix, G. K.

Bland, P. A.

Cressy, G

Franchi, I.

Greenwood, R. C.

Howard, K. T.

English

Open Research Online (The Open University)

Open Research OnlineThe Open University’s repository of research publicationsand other research outputsCorrelated modal mineralogy, aqueous alteration andoxygen isotope composition of CM ChondritesConference or Workshop ItemHow to cite:Howard, K. T.; Benedix, G. K.; Bland, P. A.; Greenwood, R. C.; Franchi, I. and Cressy, G (2010). Correlatedmodal mineralogy, aqueous alteration and oxygen isotope composition of CM Chondrites. In: 41st Lunar and PlanetaryScience Conference, 1-5 Mar 2010, Houston, Texas.For guidance on citations see FAQs.c© 2010 The AuthorsVersion: Accepted ManuscriptLink(s) to article on publisher’s website:http://www.lpi.usra.edu/meetings/lpsc2010/Copyright 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.ukCORRELATED MODAL MINERALOGY, AQUEOUS ALTERATION AND OXYGEN ISOTOPE COM-POSITION OF CM CHONDRITES. K. T. Howard1, G. K. Benedix1, P. A. Bland1,2, R.C. Greenwood3, I.A. Franchi3 and G. Cressey1. 1Impacts and Astromaterials Research Centre (IARC), The Natural History Museum, Mineralogy Department, London, SW7 5BD, United Kingdom. 2IARC, Dept. of Earth Sci. & Eng., Imperial Col-lege, London, SW7 2AZ, United Kingdom. 3 Planetary and Space Science Research Institute (PSSRI), Open Univer-sity, Milton Keynes, MK7 6AA, United Kingdom.  E-mail: kieren.howard@nhm.ac.uk  Introduction: CM carbonaceous chondrites repre-sent incompletely serpentinized fragments of primitive asteroids. They preserve a record of nebular and parent body processes evident in the range of anhydrous and hydrous mineral components of which they are com-posed. The CM chondrites are all dominated by fine grained, low-temperature, phyllosilicate from the ser-pentine family of minerals. These complexly inter-grown phyllosilicates form by hydration of high-temperature anhydrous phases in CMs, namely olivine and pyroxene. Partially hydrated olivines and pyrox-enes, mostly in chondrules, dominate the remaining mineralogy of CM samples with minor magnetite, sul-phide and carbonate [1]. While the origin of phyllosili-cate is clearly hydration of anhydrous silicates, the relationship between the remaining volumetrically minor anhydrous components and the amount of aque-ous alteration is not well understood. In studying these minor components to understand aqueous alteration, assumptions must be made. For this reason we define aqueous alteration by the ratio of phyllosilicate to an-hydrous silicate – since one forms from the other this is unambiguous. Previously such a scheme was impos-sible because the modal mineralogy of CM samples had not been defined beyond matrix:chondrule volume. This was simply a reflection of the limitations in defin-ing modal mineralogy in fine grained breccias by opti-cal methods.  By Position Sensitive Detector X-ray Diffraction (PSD-XRD) we have defined the modal mineralogy of CM chondrites spanning the entire petrographic range [1,2] and we add to this data set here. One of the advantages of a complete modal min-eralogy for phases present in abundances >1wt% is that it allows for evaluation of the relationship between mineral phases at a much broader scale than is possible in petrographic studies. Modal data also allows for examination of potential mineralogical controls on chemical and isotopic compositions and the evolution of isotopic systems. In this study we attempt to move beyond defining alteration sequences towards under-standing the relationship between CM mineralogy, aqueous alteration and O-isotope compositions.    Expected relationship between aqueous altera-tion and the O-isotope composition of CMs: It is generally accepted that O-isotope systematics in CM chondrites can be understood as reflecting interactions between isotopically “light” anhydrous silicate and isotopically “heavy” fluid reservoirs [3,4]. Therefore the expectation is that with increasing aqueous altera-tion, the oxygen isotope compositions of CMs should evolve towards heavier values [3,4,5]. This reflects incorporation of “heavy” fluid in phyllosilicate at the expense of “light” anhydrous silicates. Here we inte-grate modal and O-isotope data to test this model.  Samples and methods: We have determined mo-dal mineralogy by PSD-XRD and a pattern stripping technique using mineral standards [1,2,6,9]. Lack of a tochilinite standard prevents determination of its abun-dance. Every modal determination represents an analy-sis of 180mm3 (~0.2g). Here we present modal data for the following samples (values in parenthesis equal number of 0.2g samples analysed). CM2 falls: Mighei (3), Murray, Murchison, Nogoya (2), Cold Bokkeveld (2). CM2 finds: QUE97990; ALHA81002; Y791198; QUE93005 (2); CM1/2: ALH83100; CM1: MET01070 (2); SCO06043.  C2/ung: Essebi. We have determined the O-isotope compositions of the following samples that had not previously been analysed: ALHA81002; MET01070; QUE97990; Y79119. All analyses were performed by laser fluori-nation following the method and in the same lab as described in [7].  Remaining oxygen isotope values are taken from the literature [3,4,8]  Results: The maximum volume of anhydrous sili-cate (olivine+pyroxene) in CM chondrites is ~30%. In all CM samples Mg-rich olvine is the dominant anhy-drous phase with pyroxenes always comprising <10%.  This reflects the fact that, even though pyroxenes may be less reactive than olivine during aqueous alteration, the initial abundance of pyroxene was much less than olivine - as for all the carbonaceous chondrites. CM1 samples may contain as little as 6% olivine, approxi-mately the same as for Orgueil (CI) [9]. Regarding this point, it is important to note that TEM and XRD stud-ies show CI phyllosilicate is different to the CMs in containing saponite as well as serpentines [9,10].  Therefore it is difficult to define a mineralogic contin-uum between CM and CI chondrites, which relates only to the extent of aqueous alteration, because the process involved was fundamentally different on the two parent bodies.  At this stage we do not attempt to define  polytypes in CM serpentines beyond a relatively crystalline com-ponent producing high-intensity peaks at 12° and 25° (2θ CuKα) and a much more fine grained component corresponding to broad hk reflections. TEM studies [11,12] show the crystalline phases to be Fe-rich ser-1595.pdf41st Lunar and Planetary Science Conference (2010)pentines generally larger in grainsize (up to 2500 nm) than the intergrown, more Mg-rich serpentines that may be cylindrical or fibrous and smaller than 20 nm. These Mg-rich serpentines correspond to the broad reflections in XRD patterns that are typical of fine grainsize phyllosilicate present as platy crystals con-sisting of only a few structural layers along [001]. For this study, the distinction between polytypes is not significant as we define alteration simply by the ratio of total phyllosilicate to anhydrous phases. The modal abundance of phyllosilicate ranges  up to 90% in CM1 samples and averages 75% in CM2 samples. The ratio of phyllosilicate/anhydrous silicate varies from 2-6 in CM2 samples and 9-13 in CM1/2 and CM1 samples. Oxygen isotope data from this and previous studies [3,4,8] are plotted in Fig. 1. In CM2 falls we observe a clear trend whereby as the phyllosilicate/anhydrous silicate ratio increases (increasing hydration), oxygen isotope compositions become steadily heavier. This is as predicted [3,4] and likely reflects equilibration of phyllosilicate with isotopically heavy fluid during hy-dration. The least altered CM2 sample QUE97990 plots in the CV/CK field.  On petrographic grounds (e.g. chondrule size) the CVs and CKs are unlikely to be the anhydrous precursors of CMs, which are more often related to COs [3,4].  QUE97790 plots on the CM mixing line of [4], close to the COs and well away from other CM2 samples. It therefore appears to be an important link between these two groups. An unex-pected finding is that more altered CM1/2 and CM1 samples are isotopically lighter than the CM2 falls and most Antarctic CM2 finds. This unexpected observa-tion is apparent in our analyses but also in existing literature analyses of CM1/2 and CM1 samples [3,4,8]. O-isotopes in carbonates from CM2 samples also ap-pear to become progressively lighter with increasing alteration [13]. The modal abundance of carbonate is very similar (<2vol%) in CM2 and CM1 samples and is not likely to be controlling O-isotope variations. Clearly these data require explanation, given that they appear to directly contradict existing models [3,4] for the evolution of O-isotope compositions in CMs during aqueous alteration Discussion: The paradoxical observation where more altered CM1 meteorites are isotopically lighter than CM2 meteorites is difficult to reconcile with these isotopic compositions having been inherited during alteration of CM2 and CM1 on a single parent body. This would require heterogeneous fluid compositions from the onset of alteration, or evolution of fluid com-positions towards lighter values during late-stage al-teration without this later fluid affecting CM2 samples.  Models suggest variations in water:rock ratio should be reflected by variations in ∆17O [3,4] but these values show no systematic differences between CM2 and CM1 samples, therefore differences in water:rock ratio on the parent body also cannot explain these data.  In the absence of a mechanism to create heterogeneous fluids on the CM parent body, or other independent non-isotopic evidence for separate parent bodies, an alternative explanation is required.  Exposure of meteorites to fluid during Antarctic weathering is well known to move O-isotope composi-tions toward lighter values [14]. This would be the simplest explanation for the lighter composition of CM1 samples relative to CM2 falls and most Antarctic CM2 finds. However, when Antarctic CM1 samples are compared to pristine CM2 falls in plots of δ18O vs. ∆17O, there is little evidence for a significant isotopic shift in ∆17O towards the TFL in the expected direction of Antarctic weathering [14].  We are currently undertaking additional O-isotope analyses to evaluate the extent of terrestrial contamina-tion in CM1 meteorites. If CM1 isotope compositions indicate significant interaction with Antarctic precipi-tation, this raises the possibility that CM1 finds may actually be CM2 falls that were further hydrated on Earth.       References: [1]Howard K.T. et al. (2009) GCA, 73, 4576-4589. [2]Howard K.T. et al. (2009) MAPS, 44, A94. [3]Clayton R.N. and Mayeda T.K. (1984) EPSL, 67,151-161. [4]Clayton R.N. and Mayeda T.K. (1999) GCA, 63, 2089-2104.[5]Browning L. et al. (1996) GCA, 60, 2621-2633.[6] Cressey G. and Schofield P.F. (1996) Powder Diffraction, 11, 35-39. [7]Miller M.F. et al. (1999) Rapid Commun. Mass Spectrom., 13, 1211-1277 [8] Weisberg M.K. et al. (2009) MAPS, 44, 1355-1397. [9] Bland P.A. et al. (2004) MAPS, 39, 3-16. [10] Tomeoka  K. and Buseck P.R. (1988) GCA, 52, 1627-1640. [11] Lauretta D.S. et al. (2000) GCA, 64, 3263-3273.  [12] Barber D.J. (1985) Clay Minerals 20, 415-454. [13] Benedix G.K. et al. (2003) GCA, 67, 1577-1588.  [14] Bland P.A. et al. (2000) MAPS, 35, 1387-1395. Figure 1.  δ18O vs. δ17O.   Green = CM2 falls;  Orange = CM1 finds;   Blue square = QUE97990;  Grey diamonds = CM2 finds. 1595.pdf41st Lunar and Planetary Science Conference (2010)

Correlated modal mineralogy, aqueous alteration and oxygen isotope composition of CM Chondrites

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Correlated modal mineralogy, aqueous alteration and oxygen isotope composition of CM Chondrites

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