Anand, M.

Barnes, J.J.

Barrett, T.J.

Charlier, B.L.A.

Franchi, I.A.

Grady, M.M.

Greenwood, R.C.

English

Open Research Online (The Open University)

Open Research OnlineThe Open University’s repository of research publicationsand other research outputsThe isotopic composition of chlorine in apatite fromeucritesConference or Workshop ItemHow to cite:Barrett, T.J.; Barnes, J.J.; Anand, M.; Franchi, I.A.; Greenwood, R.C.; Charlier, B.L.A. and Grady, M.M.(2016). The isotopic composition of chlorine in apatite from eucrites. In: 47th Lunar and Planetary ScienceConference, 21-25 Mar 2016, 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.ukTHE ISOTOPIC COMPOSITION OF CHLORINE IN APATITE FROM EUCRITES.  T. J. Barrett1, J. J. Barnes1, M. Anand1,2, I. A. Franchi1, R. C. Greenwood1, B. L. A. Charlier1, and M. M. Grady1,2. 1Department of Physical Sciences, The Open University, Walton Hall, Milton Keynes, MK7 6AA, UK (Email: thom-as.barrett@open.ac.uk), 2Department of Earth Sciences, Natural History Museum, London, SW7 5BD, UK.   Introduction: Chlorine is an important element be-ing incompatible in nearly all silicates, extremely vola-tile and strongly hydrophilic [1]. The Cl isotope com-position (37Cl/35Cl) of meteorites, therefore, can pro-vide useful information regarding the isotopic reser-voirs that were present in the early Solar System and any secondary processes that may have occurred on different parent bodies [2, 3]. There is relatively a nar-row range in the Cl isotope compositions of terrestrial rocks, and martian and chondritic meteorites (δ37Cl ~ -4 to +1 ‰) [2-4], with an exception to this rule being the Moon. It has recently been demonstrated that lunar samples display a significant range in Cl isotopic com-position, especially lunar apatite [Ca5(PO4)3(F,Cl,OH)], which is a common accessory mineral that can contain abundant Cl, and which shows δ37Cl values ranging from typical terrestrial values of ~ -4 ± 2 ‰ to ex-tremely enriched values of +81 ± 2 ‰ [5-11].  The Howardite-Eucrite-Diogenite (HED) suite of meteorites represents the largest suite of crustal and sub-crustal rocks available from a differentiated basal-tic asteroid (most probably 4 Vesta) [12-13]. The eu-crites are also some of the oldest igneous rocks in the Solar System, typically with crystallization ages from 4547 Ma to 4559 Ma [14-15]. Therefore, characteriz-ing the volatile inventories of eucrites can help us un-derstand the processes operating on asteroid parent bodies early in the Solar System history. In order to investigate the history of volatiles in eucrites, we have measured Cl isotopic compositon of apatite from the cumulate eucrite Moore County and the basaltic eucrite Dar al Gani 945 (DaG 945). Apatite grains in these two samples were previously analyzed for their H isotopic composition [16].  Methods:  The Cl content and isotopic composi-tions were measured using the Cameca NanoSIMS 50L at The Open University in multi-collection mode based upon the protocol of Tartèse et al. [9] and Barnes et al. [10], acquiring negative secondary ions of 16O1H, 18O, 35Cl, 37Cl and 40Ca19F, simultaneously on electron mul-tipliers. A primary current of ~50 pA (in Cs+ mode) was used for the analyses. Typically a 3 × 3 µm area was analyzed, however for some larger grains this area was increased to 5 × 5 µm.  No electronic gating was used.  Results: A total of 12 measurements from nine in-dividual apatite grains were obtained. Apatite in both samples is typically located within mesostasis regions (Fig. 1).  Figure 1. Representative Back Scattered Electron (BSE) images of each eucrite. Pl = plagioclase, Px = pyroxene, Ap = apatite, ilm = ilmenite, Cr = chromite, Si = silica phase, Zr = zircon. Red boxes represent the approximate locations of the sputter pits correspond-ing to the analyses of Cl isotopes. White boxes repre-sent the approximate locations of H isotope measure-ments [16]. Apatite grains from Moore County display a large range in Cl content, from ~ 70 to 700 ppm Cl which is consistent with the results of previous studies [17]. Considerable intra-grain variability in Cl abundance is also observed in Moore County with a single grain having a range of ~ 360 ppm. Apatite crystals from DaG 945 display a more restricted variation (both inter as well as intra-grain) with values ranging from ~ 25 to 60 ppm Cl (Fig. 2). The δ37Cl values obtained for apatite from Moore County (~ +0.7 ± 2.7 ‰ to ~ +3.6 ± 1.8 ‰) are typi-cally within error of terrestrial values and chondrites, with the exception of two grains which are more en-riched (~ +6.6 ± 3.4 ‰ and +13.1 ± 3 ‰). DaG 945, however, shows a significant enrichment in δ37Cl with values ranging from ~ +27 ± 4.4 ‰ to ~ +35 ± 6.2 ‰. The results we have obtained form two distinct popula-tions one with a weighted average of +4.1 ± 3.4 ‰ (95 % confidence level, n = 7) (Moore County) and the other with an average of +31 ± 4.4 ‰ (95 % confi-dence level, n = 5) (DaG 945).  Discussion: In both samples apatite display con-sistent δD values (Fig. 3). There is no correlation be-tween δ37Cl and δD values (Fig. 3), in keeping with the conclusions drawn from lunar rocks [7]. The apatite in these meteorites are also characterized by low H2O contents (typically < 200 ppm), with DaG 945 showing more variability than Moore County in terms of H2O content [16].   2746.pdf47th Lunar and Planetary Science Conference (2016) Figure 2. Plot of Cl content (ppm) versus δ37Cl value (‰) for individual measurements of apatite. Uncer-tainties are reported at the 2σ level. The model put forward by Sharp et al. [5] suggests that δ37Cl enrichment in lunar basalts could be the product of isotopic fractionation during extensive out-gassing of Cl from a magma sufficiently poor in H, such that the magma degassed Cl in the form of metal chlorides. If the H2O content of apatite in the meteor-ites studied here reflect, relatively, the melt composi-tion then this mechanism [5] could provide a viable explanation. Furthermore, DaG 945 is a basalt whose parental magma may have erupted under similar degas-sing conditions as in the models proposed by Sharp et al. [5] and Ustunisik et al. [18], and could have pro-duced the heavy Cl isotopic compositions observed in the apatite from this sample. However, Moore County, which is a cumulate eucrite and likely to have formed at depth, is unlikely to have suffered significant mag-matic degassing. It is, therefore, possible that Moore County retained its original Cl isotopic composition (~ +4 ‰).  A different model, put forward by Boyce et al. [7], shows that in the case of lunar basalts the heavy δ37Cl values are likely to have been inherited from a residual magma ocean reservoir in the Moon (urKREEP) which had undergone Cl isotope fractionation early in the Moon’s history. Currently no urKREEP-like compo-nent has been proposed for Vesta although any body that has undergone differentiation via a magma ocean is likely to have a similarly incompatable element rich residuum [19]. In the eucrites studied here, however, the La/Yb ratios do not show a relationship consistent with the heavy Cl isotope signature being attributable to assimilation of an incompatible element enriched melt [20-21].  Conclusions: The isotopic composition of Cl rec-orded by apatite from the eucrites show a similar range in δ37Cl values to those reported for lunar apatite [e.g., [7, 10]. The results obtained here form two distinct populations with weighted averages of +4.1 ± 3.4 ‰ and +31 ± 4.4 ‰, for Moore County and DaG  945, respectively. The mechanism of 37Cl isotope enrich-ment on Vesta is likely to have been the degassing of Cl in the form of metal chlorides. In order to establish fully whether or not degassing of Cl played a role in the magmatic histories of the eucrites, however, the analyses of an expanded sample set is required.  Figure 3. Plot of δD value versus δ37Cl value for eu-critic apatite. Where possible apatite grains that have been analyzed for both δ37Cl and δD values are sepa-rated out. White symbols indicate in-situ δ37Cl meas-urements plotted with the mean δD value for that me-teorite (H isotope data are taken from  [16]). References: [1] Sharp Z. D. et al. (2010) GCA, 74, 264–273. [2] Sharp Z. D. et al. (2007) Nature, 446, 1062–1065. [3] Sharp Z. D. et al. (2013) GCA, 107, 189–204. [4] Williams J. T. et al (2015) LPSC XLVII, Abstract #2641. [5] Sharp Z. D. et al. (2010) Science, 329, 1050–1053. [6] Wang Y. et al. (2012) MetSoc LXXV, Abstract #5170. [7] Boyce J. W. et al. (2015) Sci. Adv., 1. [8] Trieman A. H. et al., (2014) Amer. Min., 99, 1860–1870. [9] Tartèse R. et al., (2014) MaPS, 49, 2266–2289. [10] Barnes J. J. et al., (2016) LPSC XLVII, Abstract #1439. [11] Robinson K. L. et al LPSC XLVII. Abstract #2199 [12] McCord T. B. et al. (1970) Sci-ence, 168, 1445–1447. [13] De Sanctis M. C. et al (2012) Science, 336, 697-700. [14] Misawa K. et al. (2005) GCA, 69, 5847–5861. [15] Zhou Q. et al. (2013) GCA, 110, 152–175. [16] Barrett T. J. et al. in review with MaPS. [17] Sarafian A. R. et al. (2013) MaPS, 48, 2135–2154. [18] Ustunisik G. et al. (2015) Amer. Min., 100, 1717–1727. [19] Mandler B. E. and Elkins-Tanton L. T. (2013) MaPS, 48, 2333-2349. [20] Yamaguchi A. et al. (2009) GCA, 73, 7162–7182. [21] Mittlefehldt D. W. and Lindstrom M. M. (2002) GCA, 67, 1911–1935.  Acknowledgements:  STFC are thanked for a stu-dentship to TJB. We thank the Smithsonian Institution for the loaning the polished section of Moore County (USNM 929-2). 2746.pdf47th Lunar and Planetary Science Conference (2016)

The isotopic composition of chlorine in apatite from eucrites

The isotopic composition of chlorine in apatite from eucrites

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