Boesenberg, J.S.

Franchi, I.A.

Greenwood, R.C.

Gregory, J.D.

Humayun, M.

Mayne, R.G.

Silver, A.P.

English

Open Research Online (The Open University)

Open Research OnlineThe Open University’s repository of research publicationsand other research outputsChoteau makes three: A characterization of the thirdmember of the Vermillion subgroupConference or Workshop ItemHow to cite:Gregory, J.D.; Mayne, R.G.; Boesenberg, J.S.; Humayun, M.; Silver, A.P.; Greenwood, R.C. and Franchi, I.A.(2016). Choteau makes three: A characterization of the third member of the Vermillion subgroup. In: 47th Lunar andPlanetary Science Conference, 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.ukCHOTEAU MAKES THREE: A CHARACTERIZATION OF THE THIRD MEMBER OF THE VERMILLION SUBGROUP. J. D. Gregory1, R. G. Mayne1, J. S. Boesenberg2, M. Humayun3, A. P. Silver3, R. C. Greenwood4, and I. A. Franchi4 . 1School of Geology, Energy, and the Environment, Texas Christian University, TCU Box 298830, Fort Worth, TX. 76129, 2Dept of Earth, Environmental, and Planetary Sciences, Brown Universi-ty, 324 Brook Street, Providence, RI. 02912, 3Dept of Earth, Ocean, and Atmospheric Science, Florida State Univer-sity, P.O. Box 3064520, Tallahassee, FL 32306-4520. 4Dept of Physical Sciences, The Open University, Walton Hall, Milton Keynes, MK7 6AA, UK.   Introduction:  The Choteau pallasite was discov-ered in Montana in 2011 when it was purchased at an estate sale. Choteau was initially classified as an un-grouped pallasite based on the unusual oxygen isotope composition, which appeared to be similar to that of the acapulcoites and lodranites [1]. The purpose of the present study is to gain a better understanding of Choteau's petrogenesis and establish whether it can be linked to any previously classified pallasites or pallasite groupings. Methods:  A slab of Choteau was provided by the Oscar E. Monnig Meteorite Collection at TCU and three 1-inch round thin sections were produced from it. Back-scattered electron imaging was undertaken using the SEM in the Mineral Sciences Department at the Smithsonian's NMNH. Using these images, locations were selected for electron microprobe analysis, at the University of Oklahoma. Lastly, one of the thin sec-tions was sent to Florida State University for LA-ICP-MS to obtain rare earth elemental data for both the metal and silicate phases. New oxygen isotope data was also collected at the Open University following the methods of [2]. Results: The oxygen isotope composition of Choteau measured in this study (δ17O = 0.211±0.091‰; δ18O = 2.52±0.21‰; Δ17O‰ = -1.102±0.017‰; where Δ17O = δ17O-0.52δ18O) is very similar to pyroxene pallasites Vermillion and Yamato-8451 (Figure 1) [3].  Olivine compositions were found to be homoge-nous throughout, with an average value of Fe/Mn = 33.5±2.5 and Fo89.7±0.6 (Figure 2). Olivine grains ranged from highly angular to subhedral with lengths up to 1 cm.  Both orthopyroxene and clinopyroxene are present in Choteau. Orthopyroxene was found as individual grains or along the boundaries of the clinopyroxene, which was strictly observed as inclusions within the olivine. The orthopyroxene composition of En88.0±1.1Wo1.9±0.5 and the average clinopyroxene com-position of En56.4±3.9Wo38.7±4.8 (Figure 2) are very simi-lar to those found in Vermillion and Y-8451[3]. Merrillite was observed in highly anhedral shapes most often adjacent to an olivine crystal with Fe/Mg = 0.03±0.01. Chromite was common throughout each thin section, appearing as euhedral to subhedral crys-tals along the borders of olivine crystals or as anhedral inclusions within the olivine and pyroxene. The com-position was homogenous with Mg# (Mg/(Mg+Fe) x 100) = 38.9±9.5 and Cr# (Cr/(Cr+Al) x 100) = 86.9±4.0. Both of these values are very similar to the chromite compositions found in Vermillion (Mg# ≈ 40 and Cr# ≈ 94) [3].   Figure 1: Oxygen isotope composition of Choteau compared with Vermillion and Y-8451.   Figure 2: Pyroxene and olivine compositions measured in Choteau.   The discovery of plagioclase makes Choteau unique in that it is only the second known pallasite to contain plagioclase, following NWA 10019 [4, 5]. However, the plagioclase found in Choteau (An9.0Ab85.6Or5.4) is nearly endmember albite, while NWA 10019 plagioclase is more calcic (An50 to An84) (Figure 3). The plagioclase occurs as highly irregular inclusions in both orthopyroxene and clinopyroxene 2393.pdf47th Lunar and Planetary Science Conference (2016)and small veins filling fractures were also observed in orthopyroxene (Figure 4).  Figure 3: Plagioclase compositions of Choteau as compared with those of NWA 10019 [5].   Figure 4: Plagioclase inclusions and veins within the orthopyroxene of Choteau pallasite.  LA-ICP-MS results showed a strong positive Eu anomaly in plagioclase and a strong negative Eu anom-aly in pyroxene. The olivine REE-contents were too low to be quantified. When compared with other pallasites, Choteau metal shows compositional similari-ties to Vermillion with Ni, Ga, Ir, Ge, and Au having remarkably similar values (Figure 5). Discussion: Choteau is currently classified as an ungrouped pallasite. The oxygen isotopic composition and mineral chemistry strongly suggest it is related to both Vermillion and Y-8451. LA-ICP-MS analyses of the metal within Choteau (Figure 5) confirm that it has a composition between that of Vermillion and Y-8451, lying closer to or overlapping Vermillion for every element examined so far.  Based on the similarities of chemistry, petrology, and oxygen isotopic data between Choteau and Vermil-lion and Y-8451, we suggest that this be formally rec-ognized as a new grouplet, and known as the Vermil-lion pallasites. However, Choteau does have an Δ17O value that lies >0.3‰ away from the other two grouplet members. This indicates that isotopic equilibrium was not achieved during their formation. which may indi-cate they were formed over short timescales. Perhaps most significant, however, is the presence of plagioclase in Choteau, which was not noted during its classification or in any other member of its grouplet. The only other plagioclase-bearing pallasite, NWA 10019, has a different oxygen isotope composition [5]. The identification of plagioclase in two pallasites hail-ing from different parent bodies may require a shift in our current thoughts on pallasite petrogenesis. References: [1] Ruzicka, A. (2014) The Meteoritical Bulletin, No 102. [2] Miller M. F. et al. (1999) Rapid comm. Mass Spectrm. 13, 1211-1217. [3] Boesenberg, J. S. et al. (2000) Meteoritics & Planet. Sci ,35, 757-769. [4] Agee, C. B. et al. (2015) 78th Annual Meteoritical Society Meeting, Abstract #5084. [5] Boesenberg, J. S. et al. This meeting. [6]Wasson J. T. (1999) GCA 63, 2875-2889. [7] Wasson J. T. and Choi B. G. (2003) GCA 67, 3079-3096. Acknowledgments: Special thanks to Dr. Tasha Dunn at Colby College, Dr. George Morgan of University of Oklahoma, and the Mineral Sciences staff at NMNH.   Figure 5: Comparison of Choteau's metal composi-tion with the IIIABs [6], the main-group pallasites (PMGs [7]), and the pyroxene-bearing pallasites Ver-million and Y-8451. A). Ni vs Ir. B). Ni vs Ga. 2393.pdf47th Lunar and Planetary Science Conference (2016)

Choteau makes three: A characterization of the third member of the Vermillion subgroup

Choteau makes three: A characterization of the third member of the Vermillion subgroup

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