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Petrologic and Isotopic Classifications of Ungrouped Achondrite Nwa 8186: Implications for a Ck/Cv Asteroidal Origin

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Petrologic and Isotopic Classifications of Ungrouped Achondrite Nwa 8186: Implications for a Ck/Cv Asteroidal Origin 46th Lunar and Planetary Science Conference (2015) 1472.pdf PETROLOGIC AND ISOTOPIC CLASSIFICATIONS OF UNGROUPED ACHONDRITE NWA 8186: IMPLICATIONS FOR A CK/CV ASTEROIDAL ORIGIN. P. Srinivasan1, F. M. McCubbin1, C. B. Agee1, K. Ziegler1, M. E. Sanborn2, Q.-Z. Yin2. 1Institute of Meteoritics and Dept. of Earth and Planetary Sciences, University of New Mexico, Albuquerque, NM 87131, USA. 2Dept. of Earth and Planetary Sciences, UC Davis, CA 95616, USA. Email: [email protected]. Introduction: The discovery of primitive achondrites and models of partially differentiated chondrite parent bodies have reintroduced the hypothesis of a common origin for chondrites and achondrites. This single-body hypothesis indicates that both groups could coexist on the same parent body in radial layers of increasing thermal grade towards the core [1]. Thermal evolution models on early accreting bodies predict the formation of an unmelted chondritic crust as a consequence of melting by radiogenic heating [2-7]. Therefore, achondritic components would form within the inner, hotter regions of the parent body, and chondritic components would remain on the outer, cooler regions. Figure 1. Plot of NWA 8186 and other achondrite and 17 54 CK and CV chondrites share a resemblance in chondrite meteorite groups in O- Cr space. mineralogy, oxygen and chromium isotopes [8-10], and Literature data are from [15,16] and references therein. recent discoveries of CV-like (e.g. NWA 3133 [11-13]) and CK-like [14] metachondrites and achondrites Results: Examination of the meteorite sample showed significant fracturing in all mineral phases. indicate a compellingly similar provenance that fits with the single-body hypothesis. NWA 8186 is an Many 120° triple junctions were also observed, ungrouped achondrite showing geochemical and typically occurring between olivine and plagioclase isotopic similarities to CK-like carbonaceous feldspar. Olivine is the dominant phase, comprising chondrites. This meteorite plots precisely on the CCAM >80% of the sample (Figure 2), and has a Mg# of line for oxygen isotopes, and in the region where CK, ~0.65, Fe/Mn = ~127, and NiO ranges from 0.5-1.5 wt.%. Nanometer-sized Ni-rich metal (or Ni-rich oxide) CV, and CO chondrites plot for Cr isotopes (Figure 1). Preliminary analyses showed NWA 8186 is dominated blebs were also found in the olivine. Plagioclase by olivine and plagioclase, with minor amounts oxides, feldspar, ~10% of the sample, is An51, and augite, <5% augite, and Cl-apatite. No traces of iron metal or troilite of the sample, is Fs11En39. Modal abundances indicate were identified [14]. We have further examined textures that this meteorite is a dunite, in accordance with IUGS and compositions in this meteorite in order to obtain a classification protocols [17-19]. refined petrologic classification of this sample, and to Four oxides phases were observed totaling about assess the likelihood that this achondrite is linked with ~5% of the meteorite. The dominant oxide phase is the CK/CV parent body. magnetite, and some of the magnetite grains display Methodology: For the analysis of NWA 8186, a thick exsolution lamellae of hercynitic spinel ~1 x 0.5 cm thin section was used that was carbon (FeAl2O4), thin ilmenite (FeTiO3) lamellae, and coated for quantitative electron-beam procedures. A micrometer-sized areas of titanomagnetite (Figure 3). FEI Quanta 3D FEG SEM was used to produce BSE Sulfides were also identified from SEM X-ray and EDS X-ray maps, and EPMA analyses were maps, which had previously not been recognized in completed on all observed phases using a JEOL 8200 NWA 8186 [14]. The modal abundance of sulfides is superprobe. Figure 2. BSE mosaic of NWA 8186 overlain by a sulfur- map (blue) and phosphorus-map (red). This sample is roughly 1x0.5 cm, primarily composed of olivine, and is highly fractured. Apatite (light red) and merrillite (dark red) are found clustered in the left region of the section. Sulfides tend to be larger in size towards the right region of the section. 46th Lunar and Planetary Science Conference (2015) 1472.pdf Discussion: In addition to NWA 8186 (this study and [14]), previous studies have noted the potential association of heavily metamorphosed chondritic samples (above petrologic grade 6) to CV and CK chondrites [11-13]. Thermal evolution models have been used to suggest that the CK/CV parent body could have radially evolved into a layered body with a metallic core, silicate magma ocean, and undifferentiated chondritic crust [2-7]. The crust of the CK/CV parent body would increase in metamorphic grade towards the mantle and might include reduced CVs in the outer crust, oxidized CVs in the mid-crust, and CK chondrites in the lower crust [2]. CK and oxidized-CV meteorites are some of the most oxidized carbonaceous chondrites known. Ilmenite-magnetite pairs in CKs indicate an fO2 of Figure 3. BSE image of an exsolved magnetite (Mag) grain QFM+3 – QFM+5 [20-22]. Exsolution of magnetite, with thick Fe-rich spinel (Sp) lamellae, thin ilmenite (I) spinel, and ilmenite phases, as well as the presence of lamellae, and blocky areas of titanomagnetite (TM). NiO-rich olivine and absence of Fe-rich metal indicates Ol=olivine, P=pyroxene, F= feldspar, Me = merrillite, Sf = this meteorite also formed at high fO2. sulfide. Two formation mechanisms are being postulated <1% of the sample, and they typically occur in close for CK-like achondrites. 1) NWA 8186 might have relation to the oxides. Figure 2 displays an S X-ray map formed as a residue from partial melting of CK overlain on a BSE image. These micrometer-sized chondrite, indicating proximity to the silicate magma sulfides are monosulfide solid solution, mss, (Fe,Ni)1- ocean, or 2) NWA 8186 might represent a cumulate that xS, with x values ranging from 0.02-0.11, and an formed as the crystallization product of a CK partial average composition of Fe=26.2%, Ni=29.6%, melt that erupted onto the surface of the CK/CV parent S=33.3%, and Co=0.4%. Pentlandite solid solution, body. Exploring the link between CK chondrites and Fe4.27Ni4.73S7.84, with composition of Fe=27.4%, Ni=32.0%, S=33.3%, and Co=0.377% was also found CK-like achondrites is currently being probed through coexisting with mss. high-pressure/high-temperature experimental studies. The P X-ray map overlain on Figure 2 indicates the The connection between these two groups will help phosphate distribution in NWA 8186. Cl-rich apatite constrain the thermal and magmatic evolution of the (Ca5(PO4)3Cl), composed of ~53.4% CaO, 41.5% P2O5, CK/CV parent body. 4.8% Cl, and 0.43% F, was seen surrounding olivine References: [1] Wood J.A. (1958) Smithson. Astrophys. Obs. grains. Figure 4 is a plot of X-site occupancy (mol.%) Tech. Report, No. 10, Cambridge, Mass. [2] Elkins-Tanton in apatite grains. Apatite in NWA 8186 mainly consists L.T. et al. (2011) EPSL, 305, 1-10. [3] Weiss B.P. et al. of chlorine in the X site with minor F and a missing (2013) Annu. Rev. Earth Planet. Sci., 41, 529-560. [4] Ghosh structural component that is likely OH. Merrillite A. et al. (1998) Icarus, 134, 187-206. [5] Hevey P. et al. (Ca18Na2Mg2(PO4)14) was also found associated with (2006) MAPS, 41, 95-106. [6] Sahijpal S. et al. (2011) JGR, 116, E06004. [7] Sramek O. et al. (2012) Icarus, 217, 339- apatite, and composed of ~46.5% CaO, 46.7% P2O5, 354. [8] Greenwood R.C. et al. (2010) GCA, 74, 1684-1705. 3.5% MgO, 2.78% Na2O. [9] Qin L. et al. (2010) GCA, 74, 1122-1245. [10] Trinquier Figure 4. A. et al. (2009) Science, 324, 374-376. [11] Schoenbeck T.W. Truncated et al. (2006) LPSC, 37, #1550. [12] Shukolyukov A. et al. ternary plot (2011) LPSC, 42, #1527. [13] Irving A.J. et al. (2004) AGU, of X-site 85, #P31C-02. [14] Agee C.B. et al. (2014) Met. Soc., 77, occupancy #5385. [15] Sanborn M.E. et al. (2014) LPS XLV, Abstract (mol.%) in #2032. [16] Jenniskens P. et al. (2014) MAPS, 49, 1388-1425. apatite [17] Le Maitre et al. (1989) Black. Sci. Pub. 193 pp. [18] Le grains. Bas et al. (1991) J. Geol. Soc. London 148, 825-833 [19] Le Apatite is Cl- Maitre et al. (2002) Cam. Uni. Press 252 pp. [20] Huber H. et rich, and al. (2006) GCA, 70, 4019-4037. [21] Righter K. et al. (2007) depleted in Polar Sci., 1, 25-44. [22] Geiger T. et al. (1995) PSS, 43, 485- OH(?) and F. 498. .
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