We describe a new member of the CR chondrite clan and compare it to other members of this group

Franchi, I.A.

Greenwood, R.G

Killgore, M.

Lauretta, D.S.

Verchovsky, A.V.

English

Open Research Online (The Open University)

Open Research OnlineThe Open University’s repository of research publicationsand other research outputsThe Fountain Hills meteorite: A new CBa chondritefrom ArizonaConference or Workshop ItemHow to cite:Lauretta, D.S.; Killgore, M.; Greenwood, R.G; Verchovsky, A.V. and Franchi, I.A. (2004). The Fountain Hillsmeteorite: A new CBa chondrite from Arizona. In: 35th Lunar and Planetary Science Conference, 15-19 Mar 2004,Houston, Texas, USA.For guidance on citations see FAQs.c© [not recorded]Version: [not recorded]Link(s) to article on publisher’s website:http://www.lpi.usra.edu/meetings/lpsc2004/pdf/1255.pdfCopyright 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 FOUNTAIN HILLS METEORITE: A NEW CBa CHONDRITE FROM ARIZONA.  D. S. Lauretta1 M. Killgore2, R. C. Greenwood3, A. B. Verchovsky3, I. A. Franchi3  1Lunar and Planetary Laboratory, University of Arizona, Tucson, AZ 85721, USA, lauretta@lpl.arizona.edu. 2Southwest Meteorite Laboratory, PO Box 95, Payson, AZ 85547, USA. 3Planetary & Space Sciences Research Institute, Open University, Walton Hall, Milton Keynes MK7 6AA, United Kingdom.   Introduction:  The CR clan of meteorites has been established by [1] and a detailed review of these mete-orites is presented in [2]. There are several kinds of carbonaceous chondrites included in this clan: Carbo-naceous Renazzo-like (CR), Carbonaceous with High metal (CH), Carbonaceous Bencubbin-like (CB), and LEW 85332, a unique-metal rich meteorite. The CB chondrites are further divided into two subgroups: CBa and CBb. Here we describe a new member of the CR chondrite clan, Fountain Hills, a recent find with a total mass of ~60 g. Analytical Techniques: A polished slab of the Fountain Hills chondrite was surveyed with back-scattered electron imaging and X-ray mapping using the Cameca SX-50 electron microprobe at the U of AZ. Mineral compositions were determined by EMPA. Different analytical conditions were used to analyze silicates and metals.  Oxygen and nitrogen isotope ratios of silicates from Fountain Hills were analyzed at the Open Uni-versity, Milton Keynes, U.K. Details of the analytical techniques are given in [3] and [4]. Results:  The bulk oxygen isotopic composition of Fountain Hills was analyzed twice and averaged. The results are δ17O = -1.45 ‰ and δ18O = +1.28 ‰, rela-tive to VSMOW. These values are compared to those from other major chondrite groups in Figure 1. The O-isotopic composition of Fountain Hills is indistin-guishable from that of Bencubbin and Weatherford.  Fountain Hills contains about 1ppm of indigenous nitrogen with a moderate enrichment in 15N observed in a high temperature release. The peak δ15N value is +48 ‰ (Figure 2).  A back-scattered electron image of Fountain Hills is shown in Figure 3 and a Mg-Ca-Al combined X-ray map is presented in Figure 4. Observed chondrules in Fountain Hills range in size from 250 microns to 3500 microns in diameter. The average chondrule size has a diameter of ~1200 microns. Porphyritic and barred-olivine chondrules are the most abundant. All analyzed chondrule olivines are MgO-rich, with an average composition of Fo97 and Fe/Mn ratios of ~35. There is very little variation in bulk olivine compositions with olivines ranging from Fo96 to Fo98. These olivines con-tain 0.14 – 0.29 wt.% CaO, 0.04 – 0.17 wt.% Al2O3, 0.06 – 0.12 wt.% MnO, and 0.48 – 0.58 wt.% Cr2O3. Low-Ca pyroxene has a composition of En96 and an Fe/Mn ratio of ~23. It contains 1.7 – 1.9 wt.% CaO, 1.1 – 1.5 wt.% Al2O3, 0.09 – 0.11 wt.% MnO, and 0.9 – 1.0 wt.% Cr2O3. Plagioclase is extremely alkali-poor with a composition of An99 and contains ~1.0 wt.% MgO and ~0.4 wt.% FeO. High-Ca pyroxene appears to be augite with a composition of En59Wo35 and an Fe/Mn ratio of ~23. Metal in Fountain Hills occurs interstitial to chon-drules and silicate fragments. The composition of metal is fairly constant throughout the meteorite. We performed 76 analyses of metal at random locations. In all cases the metal is kamacite, which contains 6.3 – 6.8 wt.% Ni, 0.17 – 0.36 wt.% Co, 0.16 – 0.25 wt.% P, 0.05 – 0.19 wt.% Cr, 0 – 0.16 wt.% Cu, and 0 – 0.10 wt.% Si.  Discussion: The Fountain Hills meteorite is similar to the CB chondrites in bulk oxygen isotopes and metal and silicate compositions. The nitrogen isotopic measurement suggests the presence of trace amounts of nitrogen with a 15N enrichment typical of Bencub-bin-like meteorites. We have not located any CAIs in Fountain Hills. These inclusions are also rare in other CB chondrites. There are some intriguing differences between Fountain Hills and the other CB chondrites. Primarily, porphyritic chondrules are abundant in Fountain Hills and are extremely rare in the other CBs. However, Fountain Hills is most closely related to the CBa subgroup, making it the fourth member of this set.  Acknowledgments: We thank Ken Domanik for assistance with the microprobe. This work was sup-ported by NASA grant NAG5-11355 (PI: DSL). References: [1] Weisberg M. K.(1995) Proc. NIPR Symp. Ant. Met., 8, 11-32. [2] Krot A. N. et al. (2002) MAPS, 37, 11490. [3] Miller M.F. eal (1999) Rapid Commun. Mass. Spectrom. 451-t 13, 1211-1217.  [4] Verchovsky A. B. and Fisenko A. V. (1998) Science 281, 1165-1168.   Lunar and Planetary Science XXXV (2004) 1255.pdf Figure 3. Back-scattered electron image of the Fountain Hills chondrite. The meteorite contains ~25 vol.% kamacite metal, which serves as a ma-trix for refractory silicates, mostly forsterite and anorthite. Figure 1. Bulk oxygen-isotopic composition of the Fountain Hills meteorite compared to that of other major carbonaceous chondrite groups. Fig-ure modified from [2]. Figure 4. Combined X-ray elemental maps in Mg (red), Ca (green), and Al Kα (blue) of the CB chondrite Fountain Hills. The meteorite contains abundant forsteritic olivine (red). Anorthite is a combination of Ca and Al and appears light blue. Several anorthite-rich chondrules are evident.  Figure 2. Nitrogen-isotopic composition of the Fountain Hills meteorite as a function of release temperature. The N released at low temperature (< 800 oC) is likely terrestrial contamination.Lunar and Planetary Science XXXV (2004) 1255.pdf

The Fountain Hills meteorite: A new CBa chondrite from Arizona

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The Fountain Hills meteorite: A new CBa chondrite from Arizona

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