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An H Chondrite Melt Clast in an Ll Chondrite: Evidence for Mixing of Ordinary Chondrite Parent Bodies

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An H Chondrite Melt Clast in an Ll Chondrite: Evidence for Mixing of Ordinary Chondrite Parent Bodies 46th Lunar and Planetary Science Conference (2015) 2678.pdf AN H CHONDRITE MELT CLAST IN AN LL CHONDRITE: EVIDENCE FOR MIXING OF ORDINARY CHONDRITE PARENT BODIES. C. M. Corrigan1, N. G. Lunning2, and K. Ziegler3. 1Smithsonian Institution, National Museum of Natural History, MRC 119, 10th and Constitution Ave. NW, Washington DC, 20560, USA. 2University of Tennessee, Knoxville, TN 37996, USA. 3Institute of Meteoritics, University of New Mexico, Albu- querque, NM 17131, USA. E-mail: [email protected]. Introduction: In the course of studying impact yses were performed on 1-2 mg fragments of material melts in ordinary chondrites (OC) [1-4] we have iden- using the laser-fluorination method of [17] at the Uni- tified a number of impact melt and other secondary versity of New Mexico. Molecular O2 was extracted in clasts (i.e., not chondrules) within OC impact breccias. a BrF5-atmosphere, cryogenically and gas- This is part of an ongoing search for ancient melt clasts chromatographically purified, and the isotope ratios to gain insight into the lunar cataclysm [5-10] and measured on a gas source mass spectrometer (Delta small body scattering in the early Solar System (caus- PlusXL), with an analytical precision for Δ17O of 0.02 ing the late heavy bombardment [11-13]). ‰. Results are, respectively, δ17O 2.640, 2.449, and Potential melt clasts: Within the OC impact brec- 2.485; δ18O 3.691, 3.329, and 3.410; Δ17O 0.691, cias involved in this study, a range of types of inclu- 0.691, and 0.691 ‰ (reference TFL slope = sions have been identified. Some are microporphyritic 0.528; values are all linearized). Results from these impact melt clasts (such as those found in LEW 85397, analyses place the MET 01004 clast at the low end of EET 87595 and PCA 02071 [3]). Others fall along a the H chondrite field (Fig 4), in agreement with the continuum of igneous textures ranging between electron microprobe data. quenched glasses and fully crystalline clasts. Discussion: Given this clast’s almost complete MET 01004 melt clast: In the brecciated Antarctic lack of metal and sulfides, its igneous texture, and its LL5 ordinary chondrite MET 01004 we found an ex- shock features, we believe that this is an impact melt tremely light colored clast (Fig. 1). The clast is ap- clast that has been shock metamorphosed. Its oxygen proximately 3 x 4 cm on the exposed face of the mete- isotope composition, mineral chemistry, and bulk orite. Thin sections were made of this clast and ana- composition are consistent with those of H chondrites, lyzed using instrumentation at the Smithson- rather than with its host breccia (LL chondrite). ianBackscattered electron imagery and elemental map- The geologic history of this meteorite is an interest- ping were completed using the FEI NanoSEM at the ing one. Multiple possibilities for how an H chondrite Smithsonian, which is equipped with a THERMO- impact melt clast was emplaced onto the LL chondrite NORAN energy dispersive X-ray analytical system. parent body exist. One possibility is that impact melt- Electron microprobe analyses were conducted using ing of the H chondrite material occurred on its parent the Smithsonian’s JEOL 8900R Superprobe. body and this material was then ejected and incorpo- Results: Optical microscopy and SEM imag- rated into the LL body. This event or subsequent im- ing/mapping show that the clast is composed of oli- pact shock may have then shocked the clast. Another vine, pyroxene and plagioclase (Figs. 2, 3), some of possibility is that the H chondrite material melted as it which is poikilitically enclosing olivine. Very little struck the LL chondrite body (in which case it would metal (<1 vol.%) or sulfide are present in the clast have had to have cooled long before it was admixed (significantly less than the ~3-6% typical for LL chon- into the LL parent body brecciated material). drites [14,15]). Those metal grains that are present are The clast found in MET 01004 shows several simi- coarse, as opposed to being disseminated throughout larities to the melt clast found in the Peace River L6 the clast. The clast is moderately shocked, with planar chondrite [18]. The overall texture and mineralogy are fractures and undulose extinction in olivine, pyroxene similar, though, the Peace River clast contains less and plagioclase (which is not maskelynitized). The feldspar than that in the MET 01004 clast. Both clasts overall texture of the clast is achondritic, lacking any are depleted in metal relative to bulk ordinary chon- remnant chondrule features. drites. The Peace River clast formed from LL chon- Mineral chemistry/Bulk Composition: Olivines and drite material, which was later, incorporated into an L pyroxenes were analyzed using the electron micro- chondrite (Peace River). probe. Olivines have values of Fa19, and pyroxenes are Previously reported lithic clasts (potentially impact Fs15. Compositions are within the H chondrite range melts) may be additional evidence of mixing between [15], far from the LL chondrite host rock (Fa27.5-30 and ordinary chondrite groups. An igneous textured clast in Fs23.2-25.7). When compared with bulk LL chondrites Chantonnay (L6) has bulk composition consistent with [14], this clast is depleted in Fe and S and enriched in H chondrites [19]. Lithic clasts in the L6 chondrites Y- Si. The H chondrite silicate bulk composition [16] is 75097 and Y-793241 have H chondrite oxygen isotope similar to this clast (Table 1). compositions, but the major element chemistry of these Oxygen isotope composition: Oxygen isotope anal- clasts has thermally equilibriated with the host L6 ma- 46th Lunar and Planetary Science Conference (2015) 2678.pdf terial [20]. Figure 3: BSE image of olivine chadacrysts poikilitically In any case, the impact melt clasts in MET 01004 enclosed by pyroxene and plagioclase. Field of view is 5 mm and Peace River provide evidence for impact melting (horizontally). and mixing between ordinary chondrite groups. Age dating: Impact melt clasts from OCs are dis- tinctly under-represented in terms of solar system ages [15], but if successfully obtained, these dates will lead to a better understanding of how early solar system bombardment affected the asteroid belt. We certainly recognize that this is a significant challenge. Age da- ting of the H chondrite clast and other clasts in MET 01004 will help us further understand mixing between ordinary chondrite parent bodies. References: [1] Corrigan & Lunning 2013 MAPS 48, Figure 4: Oxygen isotope composition of igneous textured #5256 [2] Corrigan et al., 2012, LPSC 43 #1577 [3] Corrigan melt clast from MET 01004. Filled (red) circles correspond and Lunning 2012, MAPS 47, #5320 [4] Corrigan & Lun- ning, 2013, LPSC 44 #2615 [5] Turner et al. 1973, 4th LPSC, to the melt clast discussed in this abstract. 1889 [6] Tera et al. 1974, EPSL 1, 19 [7] Kring & Cohen 2001 JGR 107, doi: 10.1029/2001JE001529. [8] Cohen et al. 2005, MAPS 40, 755 [9] Ryder et al. 2002, JGR 107, 6-1 [10] Hartmann 2003, MAPS 38, 579 [11] Tsiganis et al. 2005, Nature 435, 459 [12] Morbidelli et al. 2005, Nature 435, 462 [13] Gomes et al. 2005, Nature 435, 466 [14] Jarosewich, 1990, Mets 2, 323 [15] Gomes & Keil, 1980, Brazilian Stone Meteorites, U of NM, ABQ [16] Mason, B. (1965) Amer Mus Novitates 2223, 1-38 [17] Sharp (1995) Am J Sci 295, 1058- 1076. [18] Herd et al. 2013 Canad J Earth Sci 50, 14 [19] Yolcubal et al. (1997) JGR 102, 589 [20] Nakamura et al. (1994) Proc. NIPR Ant. Met. 7, 125 Figure 1: Photograph of igneous textured melt clast in LL breccia MET 01004. The large, light-colored clast (~3 x 4 cm) is the subject of this abstract. Table 1: Bulk composition of the igneous clast from MET 01004 compared to ordinary chondrites. MET 01004 Bulk LL H Chondrite Wt. % Igneous Chondrite Silicates Ratios Clast [14] [16] Fe/Si 0.54 0.94 0.44 Fe/Mg 0.64 1.2 0.52 Fe/Mn 34 58 38.6 S/Mg 0.01 0.12 N/A Figure 2: XPL image of olivine chadacrysts poikilitically enclosed by pyroxene and plagioclase. Si/Mg 1.2 0.92 1.2 .
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