In this study, we compare data for two separate Tafassasset stones and supply new oxygen isotope data for our sample. We include a discussion of the debate surrounding the classification of Tafassasset and offer a hypothesis for its origin based upon new information

Franchi, I. A.

Gardner-Vandy, K. G.

Greenwood, R. C.

Killgore, M.

Lauretta, D. S.

English

Open Research Online (The Open University)

Open Research OnlineThe Open University’s repository of research publicationsand other research outputsTafassasset: The Saga ContinuesConference or Workshop ItemHow to cite:Gardner-Vandy, K. G.; Lauretta, D. S.; Killgore, M.; Franchi, I. A. and Greenwood, R. C. (2008). Tafassasset: TheSaga Continues. In: 39th Lunar and Planetary Science Conference (Lunar and Planetary Science XXXIX), 10-14 Mar2008, League City, Texas, USA, p. 2307.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/lpsc2008/pdf/2307.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.ukTAFASSASSET: THE SAGA CONTINUES  K. G. Gardner-Vandy1, D. S. Lauretta1,2, M. Killgore2, I. A. Fran-chi3, and R. C. Greenwood3, 1Lunar and Planetary Laboratory, University of Arizona, Tucson, AZ 85721, kgard-ner@lpl.arizona.edu, 2Southwest Meteorite Center, Tucson, AZ 85712, 3Planetary and Space Sciences Research Institute, The Open University, Milton Keynes, MK7 6AA, United Kingdom.    Introduction: Tafassasset, a meteorite that consists of 26 stones found in the Tenere desert of Niger be-tween 2000 and 2001, has yet to be definitively classi-fied, and its dubious origin is likely due to consider-able textural and compositional heterogeneities be-tween individual stones.  It appears that at least two different lithologies of Tafassasset have been studied.  The first (Tafa-Paris) was classified by [1] as a meta-morphosed CR chondrite based upon an oxygen iso-tope composition in line with the CR chondrites (Fig-ure 1) and the presence of metal-bearing relict chon-drules similar to the chondrules found in the CR chon-drite Renazzo (Figure 2).  The authors assert that the differences between Tafassasset and Renazzo can be explained by various degrees of metamorphism.  Figure 1: Oxygen Isotope Plot of Primitive Achond.    Figure 2: Figure 2a from [1] showing a relict chon-drule similar to the chondrules in Renazzo  Soon after, [2] found that Tafa-Paris and another Tafassasset sample (Tafa-Freiburg) both have refrac-tory element ratios depleted relative to the CI chon-drites, with Al/Mg and Mn/Mg ratios more like those of other primitive achondrites.  In the end, they ex-clude a relationship between the CR chondrites and Tafassasset and confirm a pairing between Tafa-Paris and Tafa-Freiburg.  Likewise, [3] agrees that their sample of Tafassasset, similar in composition to Tafa-Freiburg and absent of relict chondrules, is not a metamorphosed CR and is more like Brachina, and they assert that there are two groups of brachinites in existence: one main group and another that includes Tafassasset and the anomalous LEW 88763.  Finally, [4] analyze a sample of Tafassasset from Marvin Kill-gore (Tafa-MK) which seems analogous to that studied by [2-3].  In their study, [4] assess a relationship be-tween Tafassasset and other FeO-rich primitive achon-drites, concluding that the major mineral composition of Tafassasset (Fa28.3 olivine and Fs23.5 low-Ca pyrox-ene) is much more similar to the brachinites and other FeO-rich ungrouped primitive achondrites such as RBT 04239 than the CR chondrites.   Perhaps the most vexing issue in the Tafassasset confusion is the existence of two competing sets of information: CR-like oxygen isotope data and FeO-rich silicate composition.  How can an FeO-rich, re-crystallized meteorite have an oxygen isotope compo-sition in line with the FeO-poor CR chondrites?  In the following study, we obtain our own data of Tafa-Paris and compare it to previously reported data on Tafa-MK.  We also supply new oxygen isotope data for the Tafa-MK sample and offer a hypothesis for the origin of the Tafassasset stones as a whole.  Analytical Techniques: Two chips of bulk Tafa-MK were sent to Open University for oxygen isotopic analysis. Two polished thin sections of Tafa-Paris were obtained from the Muséum National d’Histoire Naturelle, Laboratoire de Minéralogie in Paris, France for analysis on the Cameca SX50 electron microprobe at LPL.  The conditions for the analyses on the elec-tron microprobe were 15 kV accelerating voltage, 20 nA beam current and 1-µm beam size for olivine and pyroxene; and 15 kV, 8 nA and 10-µm beam size for plagioclase.       Initial Results: The two chips of Tafa-MK ana-lysed here have nearly identical oxygen isotopic com-position, averaging δ17O = -0.41 ‰  and  δ18O = 2.25 Lunar and Planetary Science XXXIX (2008) 2307.pdf‰  with ∆17O = -1.58 (Figure 1).  This is different than the compositon found by [1] for Tafa-Paris: δ17O = 0.18 ‰  and  δ18O = 2.94 ‰  with ∆17O = -1.35.     Our observations show that Tafa-Paris is texturally and mineralogically indistinguishable from Tafa-MK; each contains subhedral olivine with pyroxene and interstitial plagioclase, with chromite, metal and troil-ite as minor phases.  Figures 3 and 4 show BSE images of typical regions in each section; note the identical relationships between the olivine and pyroxene (me-dium grays) to the metal and troilite (white) and inter-stitial plagioclase (dark gray).  The composition of the olivine, pyroxene and plagioclase is also nearly identi-cal.  As reported in [4], we found Tafa-MK to have Fa28.3 olivine, Fs23.5 low-Ca pyroxene, and An23.6-36.5 plagioclase.  Here, we find Tafa-Paris to have Fa29.1 olivine, Fs24.1 low-Ca pyroxene, and An23.3-45.1 plagio-clase.  We do not observe relict chondrules in either Tafa-Paris or Tafa-MK.      Despite minor olivine and pyroxene compositional differences, Tafa-MK and Tafa-Paris plot near each other on the XFs,opxn versus XFa diagram shown in Fig-ure 5.  Both Tafa-MK and Tafa-Paris plot within the LL ordinary chondrite field and along an equilibrium trend with other FeO-rich primitive achondrites and highly metamorphosed ordinary chondrites. This fig-ure emphasizes the compositional differences between Tafassasset and the CR chondrites as the latter plots much to the lower left near olivine Fa1-8. Discussion: In the quest to understand the origin of Tafassasset, consideration of our new oxygen isotope data is imperative.  As discussed here, all of the Tafas-sasset stones circulating through the scientific commu-nity can be paired, yet already two of the stones have distinct oxygen isotopic compositions.  To explain the oxygen isotope differences between the Tafassasset stones, we hypothesize that a parent body of FeO-rich material partially differentiated and cooled (without reaching isotopic homogeneity) in a region of the neb-ula with oxygen-isotopic composition similar to the CR chondrites.  Further study is necessary to access a second scenario in which a CR chondrite precursor had a high concentration of FeO-rich material that then partialy melted to form stones like Tafassasset.     References:  [1] Bourot-Denise M. et al. (2002) LPS XXXIII, Abstract #1611. [2] Zipfel J. et al. (2002) Meteoritics & Planet. Sci., 37, A155. [3] Nehru C. E. et al. (2003) LPS XXXIV, Abstract #1370. [4] Gardner K. G. et al. (2007) LPS XXXVIII, Abstract #2086.  Figure 3: BSE image of Tafa-MK (1.46 mm across)   Figure 4: BSE image of Tafa-Paris (1.46 mm across)    Figure 5: XFs,opxn vs. XFa for Primitive Achondrites and L and LL OCs Lunar and Planetary Science XXXIX (2008) 2307.pdf

Tafassasset: The Saga Continues

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