DEGREE OF AQUEOUS ALTERATION IN SIX CM/CI CARBONACEOUS CHONDRITES

D. Takir and H. Y. McSween Jr. Department of Earth & Planetary Sciences and Planetary Geosciences Institute, University of Tennessee, Knoxville, TN 37996-1410. E-mail: dtakir@utk.edu.

**Introduction:** We have begun a project to quantify the degree of aqueous alteration in CM/CI carbonaceous chondrites, obtain spectra of these chondrites, and measure spectra of possibly related outer Main Belt asteroids in order to explore the nature of aqueous alteration on these asteroids [1, 2]. Here, we present petrographic and geochemical analyses for six CM/CI carbonaceous chondrites (LAP 02277, MIL 07700, QUE 99770, Cold Bokkeveld, Bells, and Ivuna). We applied and compared the alteration scales of Browning et al. [3] and Rubin et al. [4], which include detailed petrographic observations and electron microprobe analyses.

**Petrology and Geochemistry of CM/CI Chondrites:** [3] proposed a scale to track the degree of aqueous alteration of CM chondrites. This scale is based on the relative progress of coupled substitutions in the progressive alteration of cronstedtite to Mg-serpentine, which increases as alteration proceeds, the so-called mineralogic alteration index (MAI). [3] proposed two additional indicators of aqueous alteration, the abundance of isolated anhydrous mineral fragments (derived from disaggregation of chondrules) in the matrix and the relative percentage of chondrule alteration. Applying this scale to the chondrites analyzed thus far, we found a good correlation between the MAI and the volume percent of chondrule alteration. [4] proposed an alteration scale that ranges downward from moderately altered chondrites (petrologic subtype 2.6) to highly altered chondrites (petrologic subtype 2.0). [4] used petrologic properties that result from early and intermediate alteration processes, which include hydration of matrix and formation of phyllosilicates and production of large clumps of serpentine-tochilinite intergrowths (PCP). [4] also used petrologic properties that reflect progressive alteration processes and include oxidation of metallic Fe-Ni, alteration of mafic silicate phenocrysts in chondrules, changes in PCP composition (increase in the phyllosilicate/sulfide ratio), and changes in carbonate mineralogy. Applying this scale to our chondrites, we also found a good correlation between the petrologic type and the "FeO"/SiO₂ ratio.

**Implications:** The two scales correlate for the meteorites analyzed thus far except for Bells, which is thought to be extensively brecciated and altered [5]. Once we analyze additional meteorites, we will utilize the most quantifiable or significant features in both scales. High-quality reflectance spectra of the analyzed chondrites are being taken in order to correlate the 3-µm band spectral properties (e.g., shape, depth, width, band center) with the petrological/geochemical parameters. When coupled with telescopic spectra, this investigation will allow a better understanding of the nature of aqueous alteration of outer Main Belt asteroids spanning the 2.5 < a < 4.0 AU region, and will lead to crucial constraints on how, when, and where this alteration occurred.

**References:** [1] Takir D. et al. 2011. Abstract #1182. 42nd Lunar and Planetary Science Conference. [2] Takir D. and Emery J. P. 2010. Abstract #53.09. 42nd American Astronomical Society, Division for Planetary Science. [3] Browning L. B. et al. 1996. *Geochimica et Cosmochimica Acta* 60:2621-2633. [4] Rubin et al. 2007. *Geochimica et Cosmochimica Acta* 71:2361-2382. [5] Brearley A. J. 1995. *Geochimica et Cosmochimica Acta* 59:2291-2317.

ATMOSPHERIC OXYGEN EXCHANGE IN MICROMETEORITES

S. Taylor¹, Y. Guan² and G. Matrajt³. ¹CRREL, 72 Lyme Road, Hanover, NH 03755, USA. ²Geol. & Planet. Sci, Caltech Pasadena, CA 91125, USA. ³Department of Astronomy, University of Washington, Seattle WA 98105, USA.

**Introduction:** Micrometeorites (MM), 30 µm to 2 mm extraterrestrial particles, are heated to varying degrees entering the Earth's atmosphere. Heating, melting and volatilization can change their chemical and isotopic composition; mass-dependent isotopic fractionation is seen in severely heated MMs [1].

Single minerals of olivine and pyroxene partially melt or are affected by sub-solidus reactions [2] entering the Earth's atmosphere. We think this altered zone is demarcated by the presence of magnetite crystallites whose number and size decrease toward the center of the particle (Fig. 1a). The thickness of this zone is quite uniform for a given MM but varies among single mineral MMs probably due to the duration and temperature of heating and on the composition of the mineral.

**Sample Description and Analysis:** We measured ¹⁸O and ¹⁶O in the core and near the magnetite edge of four single mineral MMs of forsteritic composition to determine whether oxygen exchange during atmospheric entry altered their isotopic compositions. Analyses were made using the Cameca ims 7f-Geo at CalTech.

**Discussion:** Results show that all the forsterite MMs are isotopically light (Fig. 1b) similar to relict grains in carbonaceous chondrites [3]. Isotope analyses made near the rim (adjacent to the magnetite zone) and in the center of the MM show no gradient in isotopic composition (Fig. 1b). The one analyses made within the magnetite rim, Ol-7-8, plots in the same region as terrestrial olivines and is isotopically heavier than its core.

These data support the idea that oxygen exchange with the Earth's atmosphere produces a visible magnetite zone in iron containing minerals. The lack of an isotopic gradient in the homogeneous looking interior of these MMs suggests that their centers have not been altered during atmospheric heating and thus can be compared to values from other solar system materials.

**References:** [1] Alexander et al. 2002. *Geochimica et Cosmochimica Acta* 66:173-183. [2] Toppani et al. 2001. *Meteoritics & Planetary Science* 36:1377-1396. [3] Leshin et al. 1997. *Geochimica et Cosmochimica Acta* 61:835-845.

<figure><img src="image_30.png" /><figcaption>Fig. 1(a) Single mineral olivine MM Ol 5-A123; (b) a three-oxygen isotope plot showing analyses for the center and edge of four single mineral olivine MMs.</figcaption></figure>
EVOLUTION AND PROPERTIES OF DECIMETER BODIES IN
THE SOLAR NEBULA

J. Teiser, I. Engelhardt, and G. Wurm. Faculty of Physics, University
Duisburg-Essen. E-mail: jens.teiser@uni-due.de.

**Introduction:** The first step in planet formation is the coagulation of micron sized dust particles to macroscopic dust agglomerates at low collision velocities. With increasing particle sizes the collision velocities also increase. Centimeter sized dust agglomerates settle towards the midplane of protoplanetary disks and drift towards the central star. On their way, they can sweep up small (100 µm) particles at collision velocities between 1 m/s and 10 m/s, as small particles are well coupled to the gas [1, 2].

**Experimental Aspects:** Here, we present an experimental study on the evolution and the macroscopic mechanical properties of decimeter dust agglomerates in a self-consistent growth scenario. A beam of small dust agglomerates (approximately 100 µm) is generated using a sieve with a mesh width of 250 µm. The generated particles are accelerated by gravity and hit the targets at collision velocities between 1.5 and 7 m s⁻¹ depending on the drop height. The dust agglomerates consist of quartz dust with irregularly shaped grains in the size range between 0.1 and 10 µm and 80% of the mass between 1 and 5 µm, as considered relevant for protoplanetary disks. Different target sizes are used in the range between 3 mm and 3.5 cm in diameter.

**Results:** In the experiments targets grow by a combination of direct sticking and re-accretion of ejecta due to gravity. This is consistent with bodies in protoplanetary disks, as they move through the gas and impact ejecta are exposed to a head wind, which accelerates them back to the target surface. Under certain conditions, this headwind is of the same order as gravity in the laboratory experiments. However, in all experiments targets grow with a homogeneous structure, independent of the impact velocities and of the target sizes. The ratio between re-accretion and direct sticking becomes larger for increasing target sizes. As the target porosities and shapes do not depend on the target size, this leads to the conclusion, that the mechanisms leading to growth do not influence the properties of the resulting targets.

For the development of coagulation models the mechanical strength of dust aggregates is very important, which is mainly determined by the volume filling of the agglomerates. The volume filling of the forming targets increases with increasing impact velocity, as the dynamic pressure scales with $v^2$. However, for impact velocities larger than 5 m/s the volume filling reaches a saturation at a volume filling of 32% and does not increase further. This value is also consistent with the maximum volume filling feasible by local compression as already described by [3].

**Conclusions:** The experiments lead to the conclusion that the properties of growing dust aggregates do not change whether they grow by direct sticking or by re-accretion, but only on the impact velocities they are exposed to. However, once a volume filling of 32% is reached, further collisions do not lead to further compaction. For decimeter bodies only the maximum volume filling feasible by local compression has to be considered. The growth history is erased by numerous impacts and the resulting bodies have a homogeneous structure.

**References:** [1] Weidenschilling S. J. and Cuzzi J. N. 1993. *Protostars and Planets III* 1031ff. [2] Weidenschilling S. J. 1997. *Icarus* 127:290. [3] Wurm G. et al. 2005. *Icarus* 178:253.

$^{60}$Fe-$^{60}$Ni ANALYSIS OF PORPHYRITIC CHONDRULES FROM
PRIMITIVE ORDINARY CHONDRITES

M. Telus, G. R. Huss, K. Nagashima and R. C. Ogliore. University of
Hawai'i at Mānoa, Honolulu, HI 96822. E-mail: telus@higp.hawaii.edu.

**Introduction:** The $^{60}$Fe-$^{60}$Ni system ($t_{1/2}$ = 2.6 Myr) provides the resolution necessary to date a variety of early solar system processes. However, the initial solar system abundance of $^{60}$Fe is poorly constrained. Ion probe measurements provide some of the best estimates of the initial $^{60}$Fe abundance in the solar system. But the existing data have several problems: (1) data reduced using the mean of measured ratios is subject to a systematic bias that overestimates the $^{60}$Fe abundance [1,2]; (2) most detections of $^{60}$Fe come from radiating pyroxene chondrules, some of which may not have remained closed systems since formation [3]; (3) Fe/Ni ratios in large phenocrysts of porphyritic chondrules, which likely have remained isotopically closed, are lower than in radiating chondrules, making measurements difficult. Here we report data gathered with an improved multi-collection protocol on porphyritic chondrules from type 3.0-3.1 ordinary chondrites.

**Methods:** We have added some new chondrites to our study. QUE 97008 is classified as L3.05 [4]. Raman spectroscopic measurements made using the protocol of [5] give results consistent with this classification. Chondrules were selected for high Fe content and low Ni content using SEM-EDS and EMPA.

Fe and Ni isotopes were measured in multicollection mode at the University of Hawai'i using the Camecaims 1280 ion microprobe. Nickel-60, $^{61}$Ni, and $^{62}$Ni, were measured on electron multipliers (EMs) and $^{56}$Fe was measured on a Faraday cup. We used a 6-7 nA $^{16}$O⁻ primary beam. Each spot was presputtered using a 25 µm raster, which was reduced to 15µm for the measurements. A mass resolving power of ~5500 was achieved using multicollector exit slits of 250 µm. Major molecular interferences at the masses of the Ni isotopes were checked at the end of each measurement. EM gains were monitored by repeated measurements of standards and corrections for detector drift were applied to the data. Isotope ratios were calculated from time-interpolated total counts, following [1], and data were corrected for detector background, deadtime, and tails of interference peaks. Internal mass-fractionation corrections were performed for each point.

**Results:** We measured five porphyritic chondrules, two from QUE 97008 and three from Bishunpur (LL3.1). Four are porphyritic olivine or olivine-pyroxene chondrules and one is a barred olivine chondrule. The Fe/Ni ratios in these chondrules are factors of 5–10 lower than in radiating pyroxene chondrules, and we were not able to resolve clear evidence for $^{60}$Fe in any of them. However, the upper limits for the initial $^{60}$Fe/$^{56}$Fe ratios for these chondrules (1–2 × 10⁻⁷) are broadly consistent with previously reported detections [e.g., 3, 6]. We will continue our efforts to measure radiating pyroxene and porphyritic chondrules to refine estimates of the initial $^{60}$Fe/$^{56}$Fe ratio in the early solar system.

**References:** [1] Ogliore R. C. et al. 2011. *Nuclear Instruments and Methods in Physics Research Section B*, doi: 10.1016/j.nimb.2011.04.120. [2] Huss G. R. et al. 2011. Abstract #2608. 42nd Lunar and Planetary Science Conference. [3] Telus M. T. et al. 2011. Abstract #2559. 42nd Lunar and Planetary Science Conference. [4] Grossman J. N. and Brearley A. J. 2005. *Meteoritics & Planetary Science* 40:87–122. [5] Bonal L. et al. 2006. *Geochimica et Cosmochimica Acta* 70:1849–1863. [6] Tachibana et al. 2006. *The Astrophysical Journal* 639:L87–L90.
## OXYGEN ISOTOPE RATIOS IN CHONDRULES FROM PRIMITIVE CR CHONDRITE MET 00426: A CORRELATION WITH CHONDRULE Mg#

T. J. Tenner¹, D. Nakashima¹, T. Ushikubo¹, N. T. Kita¹, and M. K. Weisberg²,³. ¹University of Wisconsin-Madison, USA. E-mail: tenner@wisc.edu. ²Kingsborough Community College, CUNY, USA. ³American Museum Natural History, NY, USA.

**Introduction:** Meteorite Hills (MET) 00426 is classified as a CR 3.00 chondrite [1]. Its chondrules are prevalently large (300 µm to 2 mm dia.), with olivine and pyroxene Mg# > 95 (where Mg# = mole% [MgO]/[MgO + FeO]). A tiny volume percentage consists of smaller type II chondrules. Previous in situ O-isotope analyses of chondrules in CR chondrites [2–4] exhibit O-isotope ratios similar to those in carbonaceous [3–6] and ordinary [7] chondrites; i.e., $\Delta^{17}O: ?8.0 to +1\%$ (where $\Delta^{17}O = \delta^{17}O - 0.52 \times \delta^{18}O$). Recent studies of chondrules in Y81020 (CO3) [5] and Acfer 094 (ungrouped-C) [6] show correlations between O-isotope ratios and Mg#; i.e., Mg# > 95 and Mg# < 95 chondrules cluster at $\Delta^{17}O = -5\%$ and $-2\%$, respectively. However, the relationship between Mg# and O-isotope ratios in chondrules has yet to be investigated for CR chondrites. Here we present O-isotope ratios of 29 chondrules in MET 00426, measured with the WiscSIMS IMS-1280, using a 10 × 15 µm Cs+ primary beam [7], and discuss their relationship with corresponding Mg#.

**Results:** All individual spot data from chondrules in MET 00426 plot along a slope-1 line in the oxygen three-isotope diagram. $\Delta^{17}O$ values of multiple spots (olivine and/or pyroxene) within a given chondrule are indistinguishable ($\pm 0.5\%$ 2 SD). Type II (Mg# 53–62) chondrule fragments (n = 3) have $\Delta^{17}O$ values of 0.4 ± 0.2‰. Type I chondrules (n = 26) have $\Delta^{17}O$ values that increase from -6‰ to -1‰ as Mg# decreases from 99 to 94. Only one of 29 chondrules has a relict grain (relict olivine $\Delta^{17}O: -3.2\%$; host olivine/pyroxene $\Delta^{17}O: -1.6\%$, Mg#relict./host: 97.7).

**Discussion:** Our observations indicate that chondrules in MET 00426 formed in a variable environment where an increase in oxidation state (which lowers chondrule Mg# [8]) is correlated with increasing $\Delta^{17}O$ values. Two factors, (1) addition of H₂O, and (2) an increase in dust enrichment (relative to solar system) potentially contributed to more oxidizing conditions. A 5‰ increase in $\Delta^{17}O$ over a narrow range in chondrule Mg# (99–95) could be from oxidation of precursor material by addition of H₂O with positive $\Delta^{17}O$, due to cooling of the local disk, under relatively low dust enrichment ($< 200 \times$ solar system [8]). The inferred positive $\Delta^{17}O$ of the H₂O added to precursor material may have been similar to that of H₂O that accreted into the chondrite [9]. Increased dust enrichment ($> 1000 \times$ solar system [8]) would be necessary to form type II chondrules (Mg# approximately 55), along with a slightly higher H₂O contribution to precursor material (relative to type I chondrules) to yield their approximately 0.4‰ $\Delta^{17}O$ values. Such variability in the chondrule-forming environment could have occurred spatially and/or through time.

**References:** [1] Abreu N. M. and Brearley A. J. 2010. *Geochimica et Cosmochimica Acta* 74:1146–1171. [2] Connolly Jr. H. C. and Huss G. R. 2010. *Geochimica et Cosmochimica Acta* 74:2473–2483. [3] Chaussidon M. et al. 2008. *Geochimica et Cosmochimica Acta* 72:1924–1938. [4] Libourel G. and Chaussidon M. 2011. *Earth and Planetary Science Letters* 301:9–21. [5] Tenner T. J. et al. 2011. Abstract #1426. 42nd Lunar Prospector Spacecraft. [6] Ushikubo T. et al. 2011. Abstract #1183. 42nd Lunar Prospector Spacecraft. [7] Kita N. T. et al. 2010. *Geochimica et Cosmochimica Acta* 74:6610–6635. [8] Ebel D. S. and Grossman L. 2000. *Geochimica et Cosmochimica Acta* 64:339–366. [9] Clayton R. N. and Mayeda T. K. 1999. *Geochimica et Cosmochimica Acta* 63:2089–2104.

## SIDEROPHILE ELEMENTS IN THE ELGA (IIE) METAL

S. N. Teplyakova¹, M. Humayun². ¹Vernadsky Institute of Geochemistry and Analytical Chemistry, Russian Academy of Sciences, Moscow, Kosygina st.19, 119991 (E-mail: elga.meteorite@gmail.com) ²National High Magnetic Field Laboratory and Department of Earth, Ocean & Atmospheric Science, Florida State University, 1800 E. Paul Dirac Drive, Tallahassee, FL 32310, USA.

**Introduction:** The IIE irons belong to a non-magmatic group of iron meteorites because magmatic Ga-Ni, Ir-Ni trends are poorly defined [1]. It was suggested that IIE metal like IAB metal formed as a pool of impact-generated melt in a chondritic body [1]. Based on oxygen isotopes, silicate inclusions in IIE irons resemble H chondrites [2]. The IIE iron Elga meteorite is the finest octahedrite containing silicate inclusions (SIs) [3]. Based on concentrations of Ni, Ga, Ge in the metal the Elga meteorite was classified as IIE iron [4, 5]. Here, we report results on mineralogy and LA-ICP-MS siderophile element analyses of the metal in the Elga IIE and discuss the possible genetic relations between this meteorite and chondritic precursor material.

**Results and Discussion:** Elga consists of Fe-Ni metal with silicate (18 vol%) and troilite (less than 1 vol%) inclusions. The metal consists of taenite (taenite, plessite, martensite)—9.5 vol%, kamacite—85 vol%, and schreibersite—4.5 vol%.

A long laser track (1.2 cm) was analyzed in metal in octahedritic structure and aureole for 36 elements including 22 siderophiles. Bulk major and trace element abundances are (ppm) and RSD [%]: Fe 911825.1 ± 2, Ni 75682.0 ± 17, Co 4623 ± 7, P 4880 ± 168, S 2136 ± 97, Si 520 ± 231, Cr 17 ± 120, Cu 145 ± 17, Ga 11 ± 11, Ge 75.4 ± 11, As 11.4 ± 15, Mo 12.1 ± 11, Ru 8.3 ± 0.1, Rh 1.4 ± 11, Pd 3.7 ± 14, W 1.1 ± 13, Re 0.4 ± 16, Os 5.03 ± 12, Ir 4.8 ± 11, Pt 11.2 ± 11, Au 1.2 ± 18.

The elemental abundances of Co, Cu, Ge, Ga and Ir, are similar to those previously reported for Elga [1, 4], while the abundances of W are lower and As, Au are higher. The metal exhibits a flat refractory siderophile pattern relative to CI and Ni. The metal is enriched (1–1.7xCI) in siderophile elements (W, Re, Os, Ir, Ru, Pt, Mo), and depleted in Sb, Cu, Ga, Ge, and Sn relative to CI and Ni. Our model calculations of partial melting of H chondrite metal showed depletion in refractory siderophiles of partial melts and enrichment in solids that are in equilibrium with these liquids. Elga metal exhibits a pattern similar to solid metal that is in equilibrium with a 40% partial melt of bulk H chondrite metal (~6% partial melt of H chondrite), for most elements except Sb. Therefore siderophile pattern in Elga metal is consistent with fine-grained unfractionated metal particles of H-chondrite or solid metal crystallized from H-chondrite precursor.

**References:** [1] Wasson J. T. and Wang J. 1986. *Geochimica et Cosmochimica Acta* 50:725–732. [2] Clayton R. N. and Mayeda T. K. 1996. *Geochimica et Cosmochimica Acta* 60:1999–2017. [3] Plyashkevich L. N. 1962. *Meteoritika* 22:51–60. [4] Wasson J. T. 1970. *Geochimica et Cosmochimica Acta* 34:957–964. [5] Dyakonova M. I. 1979. *Nauka* 48. [6] Humayun M. et al. 2007. *Geochimica et Cosmochimica Acta* 71:4609–4627. [7] Kong P. et al. 1998. *Meteoritics & Planetary Science* 33:993–998.
### DATING CORE CRYSTALLIZATION OF IIAB AND IIIB IRON METEORITES USING THE PALLADIUM-SILVER DECAY SYSTEM

K. J. Theis¹, M. Schönbächler¹, G. K. Benedix²,³, M. Rehkämper³ and R. Andreasen³. ¹SEAES, University of Manchester, M13 9PL, UK, E-mail: karen.theis@postgrad.manchester.ac.uk. ²IARC/Department of Mineralogy, The Natural History Museum, Cromwell Road, London, UK SW7 5BD. ³IARC and Dept. of Earth Science and Engineering, Imperial College London, SW7 2AZ, UK.

**Introduction:** The IIAB and IIIAB iron meteorites exhibit distinct fractional crystallization trends, which indicate that they formed by the crystallization of a metallic liquid during the segregation of an iron core in an asteroidal parent body. Core segregation for the IIAB parent body is estimated to have occurred around 1.2 Ma after the formation of Allende CAIs based on the Hf-W decay system [1] and Pt-Re-Os dating of core crystallization yielded an age of 4546 ± 46 Ma [2]. The ¹⁰⁷Pd-¹⁰⁷Ag decay system can also be used to date these iron meteorites and it has the advantage that it yields a higher degree of precision due to the relatively short half-life of ¹⁰⁷Pd (6.5 Ma). Previous data show [3, 4] that IIAB iron meteorites may define two isochrons with ages of (1) 8.9 Ma after solar system formation for IIABs with high Ir contents and (2) 10.9 Ma for IIABs, which are inferred to have formed later in the crystallization sequence based on their low content of the compatible element Ir. Likewise, the IIAB iron meteorites also display fractional crystallization trends, but with relatively high scatter in the Ir versus Au diagram. The scatter is thought to be the result of parental melt that was trapped within the crystallizing metal [5]. As these conclusions are based on only a few data, this study aims to further constrain the Pd-Ag isochrons for both IIAB and IIIAB iron meteorites.

**Analytical Techniques:** Ion exchange chemistry [6] is used to separate Pd and Ag from four IIAB (Sikhote-Alin (x2), North Chile and Coahuila) and three IIIAB (Thunda, Boxhole and Henbury) iron meteorites. Silver isotopes are analyzed on the Nu Plasma MC-ICPMS at Manchester relative to the NIST SRM978a Ag standard. Palladium and Ag concentration are acquired by isotope dilution.

**Results and Discussion:** The results for North Chile are in good agreement with those of previous studies [3, 4] and fall on the same linear correlation in the Pd-Ag isochron diagram, corresponding to an age of 10.9 Ma after CAI formation [3]. Coahuila falls between the two IIAB isochrons [3] indicating that the sample analyzed here may be disturbed or formed slightly later (by ~1 Ma). Sikhote-Alin, which crystallizes late in the sequence according to elemental data [7], plots well below both isochrons. This suggests very late crystallization or later resetting. Our analyzed sample displays an unusually high Ag content (4.77 ppb) and a duplicate will be analyzed to confirm the data. The IIABs will also be analyzed and the results presented at the conference.

**References:** [1] Markowski A. et al. 2006. *Earth & Planetary Science Letters* 250:104-115. [2] Cook D. L. et al. 2004. *Geochimica et Cosmochimica Acta* 68:1413-1431. [3] Schönbächler M. et al. 2010 *Meteoritics & Planetary Science* 45:A183. [4] Chen J. H. et al. 2002. *Geochimica et Cosmochimica Acta* 66:3793-3810. [5] Wasson J. 1999. *Geochimica et Cosmochimica Acta* 63:2875-2889. [6] Schönbächler M. et al. 2007. *International Journal of Mass Spectrometry* 261:183-191. [7] Wasson J. T. et al. 2007. *Geochimica et Cosmochimica Acta* 71:760-781.

### SCALE-DEPENDENT METHODS FOR NITROGEN ELEMENTAL QUANTIFICATION IN INSOLUBLE ORGANIC MATTER BY IMAGING WITH THE NANOSIMS

A. Thomen and F. Robert. Laboratoire de Minéralogie et de Cosmochimie du Muséum. E-mail: athomen@mnhn.fr.

**Introduction:** Correlated microscopic variations between elemental and isotopic compositions of N in insoluble organic matter (IOM) of carbonaceous chondrites were tentatively search in recent studies [1, 2]. At a micrometer scale, i.e., the typical size of a hotspot in the IOM, this topic is addressed by quantitative imaging with a NanoSIMS. We proposed here two methods of N elemental quantification with “improper” samples, (i.e., rough sample surface) the surfaces of which often mimics those of natural samples (IPDs or IOM).

**Materials and Method:** The calibration of the N concentrations was done with: a charcoal, a type 3 kerogen, a type 1 kerogen and Murchison IOM, with increasing N/C from 2.2 × 10⁻³ to 3.27 × 10⁻², respectively. Several 10 × 10 µm² images of CN⁻, C⁻ and C₂⁻ of 128² pixels of the standards were acquired by rastering the Cs⁺ primary beam with ~2pA and 200 nm diameter. Ion images are sequentially reconstructed. Steady-state equilibration is achieved by implantation of a Cs⁺ dose of 2 × 10¹⁷cm⁻² prior to the analysis. Image files are processed with L'IMAGE software (L.Nittler copyright). The CN⁻/C⁻ and CN⁻/C₂⁻ ratios are extracted at two different spatial scales with the whole image defined by (i) one ROI and by (ii) ~300 ROIs (that is 6² pixels for each ROI). Each scale has its own drawbacks: (i) at the image scale, the artifacts of the chemical ion ratios linked to the topographic state of the samples, have a tendency to vanish, (ii) at a 6² pixels scale (0.2 µm²), our calibration accounts for topographic effects and the possible chemical heterogeneities of the standard. On ideal standards, i.e., polished and chemically homogeneous standards, the two approaches should yield identical results.

**Results:** Whatever the scale, the precision and detection limit for the N content in IOM is better determined by the CN⁻/C₂⁻ than by the CN⁻/C⁻ ratios. At the scale (i) inversion of the 95% confidence interval for the mean CN⁻/C⁻ ratios corresponds to a relative uncertainty of ±60% of the atomic N/C ratio. In the same way for CN⁻/C₂⁻, the relative uncertainty is ±22%. At the scale (ii) we observe for the CN⁻/C⁻ and CN⁻/C₂⁻ distributions of each standards, pronounced tails with clear departures from Gaussian distribution. These non-Gaussian distribution strongly affect the correlation coefficient of least-square regression and thus the precisions of the calibrations. When transformed in log-normal distributions, the variance of the residues is nearly constant, illustrating the improvement of the regression quality.

At the scale (ii) prediction interval [3] is used to quantify the uncertainties on the N content measured at a micrometer scale: inversion of 95% level gives a relative uncertainty of ±33% for the calibration based on the CN⁻/C₂⁻ but ±100% for CN⁻/C⁻ (that is covering the whole range of natural variations in kerogens or IOM!). We will show several applications of these methods for IOM of chondrites at the conference.

**References:** [1] Busemann H. et al. 2006. *Science* 312:728-730. [2] De Gregorio B. T. et al. 2011. *Geochimica et Cosmochimica Acta* 15:4454-4470. [3] Draper N. and Smith H. 2008. *Applied Regression Analysis*.
5463

BASAL BRECCIA AT MANICOUAGAN: FORMATION MECHANISMS, THE SUEVITE CONNECTION AND LUNAR-MARTIAN COMPARISONS

L. M. Thompson. Planetary and Space Science Centre, University of New Brunswick, 2 Bailey Drive, Fredericton, New Brunswick E3B 5A3 Canada. E-mail: lthompso@unb.ca.

**Introduction:** The term suevite has been used to describe impact-derived, polymict breccias containing a mixture of shocked and unshocked, lithic and melt fragments and generally considered to possess clastic matrices [1]. Recent re-interpretation of the type suevites of the Ries crater as clast-rich impact melts [2], has sparked a renewed interest in the nature of these impactite lithologies. This work concerns suevites associated with the ~80 km rim diameter, 214 Ma Manicouagan impact structure of Quebec, Canada.

**Suevite at Manicouagan:** Suevites are well exposed at the outer edge of the central island and occur as weakly consolidated, friable layers and lenses immediately underlying, and in contrast to, the more coherent, columnar jointed, typically vesiculated, basal, clast-laden impact melt (IM) sheet. They exhibit fine-grained, clastic matrices enclosing angular to rounded lithic and mineral fragments, and blebs, schlieren, stringers and larger masses of melt. Lithic fragments are polymict, range from <mm to tens of m in size and exhibit evidence of various degrees of deformation and shock prior to incorporation into the suevite. This indicates derivation from disparate sources and extensive mixing. Melt inclusions range in size from fine, cm-scale stringers and blebs to larger, m-size masses, which are typically clast laden and may be vesiculated, resembling the basal, clast laden IM that is exposed in the vicinity, indicating derivation from this source. Contacts of the larger melt masses and melt-rimmed clasts are typically irregular, the melt appearing to be disaggregated and strung out, grading into finer melt schlieren and stringers. Flow textures within IM immediately adjacent to suevite and fluidal, elongate melt blebs within the suevite indicate flow parallel to contacts and interaction of melt that was still relatively hot and plastic. Striated surfaces of some clasts attest to the dynamic and erosive nature of the suevite. In places, a gradation is observed from clastic- to melt-dominated suevite immediately adjacent to basal IM. As well, facies within the basal IM, which exhibit a high proportion of fine, clastic material, indicate a continuum between allochthonous, clastic breccias, through suevite to basal, clast laden IM.

**Conclusion:** Suevite exposed at the base of the IM sheet at Manicouagan is not a distinct impactite facies. It represents the dynamic interface between the basal IM and underlying clastic breccias and can be modeled by the Kelvin-Helmholtz phenomenon. The melt and clastic breccia represent two fluids moving at different velocities relative to one another, whereby perturbations are amplified and mixing occurs resulting in suevite. This work provides new insights into the formation mechanism of this little-studied variant of suevite and has implications for the formation of suevites in other settings, as well as possible links to lunar and Martian breccias, e.g., Martian megabreccia.

**References:** [1] von Engelhardt W. and Graup G. 1984. *Geologische Rundschau* 73:2:447–481. [2] Osinski G. R., Grieve R. A. F., and Spray J. G. 2004. *Meteoritics & Planetary Science* 39:10:1655–1684.

5279

EBSD STUDY OF LATTICE PREFERRED ORIENTATION (LPO) OF THE HARZBURGITE NWA 5480

B. J. Tkalcec and F. E. Brenker. Goethe University, Geoscience Institute, 60438 Frankfurt, Germany. E-mail: tkalcec@em.uni-frankfurt.de.

**Introduction:** Classified as belonging to the Howardite-Eucrite-Diogenite (HED) group of achondrite, Northwest Africa 5480, like other olivine-rich diogenites, is thought to represent an ultramafic cumulate formed within a magma chamber in the upper mantle of the differentiated asteroid 4 Vesta, or a Vesta-like body [1]. Dominated by olivine (57 vol%) and orthopyroxene (42 vol%) NWA 5480 has further been classified a harzburgite [2], whereby the distribution of olivine and orthopyroxene is very heterogeneous, with some areas displaying up to 90% of either of the two minerals. In this study, structural analysis was performed on the olivine grains of NWA 5480 using electron backscatter diffraction (EBSD), which allows us to measure and visualize the crystallographic orientation of the crystal axes to discover any lattice preferred orientation (LPO) [3].

**NWA 5480 Analysis:** The sample was categorized into two regions for targeted analysis: (a) Zone A, dominated by coarse-grained olivine, and (b) Zone B, dominated by orthopyroxene-olivine schlieren. A total of 1361 EBSD crystallographic orientation measurements of coarse-grained olivine were recorded from 58 sites (each site covering 1 mm²) within Zone A and a total of 148 measurements of finer-grained olivine were recorded from 20 sites within Zone B. Only EBSD measurements with a mean angular deviation (MAD) of <1 were accepted and recorded. The EBSD results of Zones A and B display pronounced yet distinctly differing LPOs, suggesting two separate deformation processes and/or events. A comparison with olivine LPO in terrestrial cumulates and deformed mantle peridotites illustrates the unlikeliness that the olivine LPO from Zone A of NWA 5480 was formed through cumulation or compaction processes. In contrast, a distinct similarity to the olivine LPO formed by pencil glide ((0kl) [100] glide system), typical of plastic deformation in the terrestrial mantle, causes us to consider alternative formation processes for NWA 5480, including the feasibility of convection within the Vestan mantle.

**References:** [1] McSween H. Y. Jr. et al. 2010. *Space Science Reviews* doi:10.1007/s11214-010-9637-z. [2] Beck A. W. and McSween H. Y. Jr. 2010. *Meteoritics & Planetary Science* 45:850–872. [3] Prior D. J. et al. 1999. *American Mineralogist* 84:1741–1759.
5115

A HYDRATED CLAST IN THE MOKOIA CV3 CARBONACEOUS CHONDRITE: EVIDENCE FOR INTENSIVE AQUEOUS ALTERATION IN THE CV PARENT BODY

K. Tomeoka¹ and I. Ohnishi¹,². ¹Department of Earth and Planetary Sciences, Faculty of Science, Kobe University, Nada, Kobe 657-8501, Japan. E-mail: tomeoka@kobe-u.ac.jp. ²JEOL Ltd., 3-1-2 Musashino, Akishima, Tokyo 196-8558, Japan.

We report the discovery of a highly hydrated clast in the Mokoia CV3 carbonaceous chondrite that contains eight chondrules and one amoeboid olivine inclusion (AOI), embedded in a matrix dominated by hydrous phyllosilicates. Anhydrous silicates in the chondrules and AOI were extensively replaced by phyllosilicates (mostly saponite with minor phlogopite). The mineralogy and texture indicate that the clast resulted from intensive aqueous alteration of a precursor equivalent to the host Mokoia meteorite, and it can be regarded as the CV2-type lithology.

This clast reveals abundant evidence of progressive and dynamic aqueous alteration processes. The chondrules were replaced inward from their margins, and the alteration products were disaggregated and mixed into the matrix; as a result, the chondrules increasingly became smaller in size and irregular in shape. Some of the chondrules were separated into pieces. The results indicate that a major proportion of the matrix materials were produced by the alteration of chondrules and inclusions and that there was a process in which the alteration products and the original matrix material were extensively and uniformly mixed together.

The matrix of the clast has a composition intermediate between saponite and serpentine and contains abundant framboidal magnetite, being similar to the matrix of the CI chondrites. The results suggest that more advancing alteration of the clast leads to a CI1-like lithology and raise the possibility that the CI1 chondrites could have resulted from aqueous alteration of a precursor that had a type 3 lithology (which consists of chondrules and inclusions set in a matrix) under conditions similar to those for the alteration of the Mokoia clast.

The results indicate that there was a highly wet region within the Mokoia parent body, where intensive aqueous alteration occurred, and that this clast escaped subsequent thermal metamorphism. The past studies of CV dark clasts [e.g., 1-3] and the present study of the Mokoia clast suggest that the CV parent body had a heterogeneous distribution of water and that various degrees of aqueous alteration, thermal metamorphism, and shock metamorphism took place in different regions. These inferences imply that the CV parent body was dynamic in terms of the physical and chemical processes.

The presence of the clast supports the previously proposed model [4], which suggests that the aqueous alteration in the chondrules/rims in Mokoia occurred in a wet region (or regions) different from where the host meteorite was finally lithified and that these aqueously altered chondrules/rims were afterward mixed with anhydrous matrix grains by brecciation on the parent body.

**References:** [1] Kojima T. and Tomeoka K. 1996. *Geochimica et Cosmochimica Acta* 60:2651-2666. [2] Krot A. N. et al. 1997. *Meteoritics & Planetary Science* 32:31-49. [3] Ohnishi I. and Tomeoka K. 2002. *Meteoritics & Planetary Science* 37:1843-1856. [4] Tomeoka K. and Ohnishi I. 2010. *Geochimica et Cosmochimica Acta* 74:4438-4453.

5464

"DOGGED" SEARCH OF FRESH NAKHLA SURFACES REVEALS NEW ALTERATION TEXTURES

T. Tomkinson¹ M. R. Lee² D. F. Mark¹ and F. Stuart¹. ¹SUERC, East Kilbride, Glasgow, U.K. ²GES, Glasgow University, UK. E-mail: tim.tomkinson@glasgow.ac.uk.

**Introduction:** Nakhla is the only nakhlite fall and hence is the best sample available for investigating products of aqueous alteration on Mars. Alteration products including clays, salts, carbonates and Fe-silicate gels have been previously observed filling fractures within this meteorite [1-3]. These materials have been interpreted as remnants of an ancient aqueous alteration event related to an impact-induced hydrothermal system or sustainable subsurface brine [3,4]. However, the poor crystallinity of the Fe-silicates suggests a short formation time scale on Mars. Given their microstructural, crystallographic and chemical complexity these alteration products are still not fully understood. Hitesh and Bridges [3] have examined the fine-scale structure of the Fe-silicate gels and clays by transmission electron microscope (TEM) study of foils cut from veins exposed in polished thin sections using the focused ion beam (FIB) technique. Here we have chosen to examine alteration product-coated grain surfaces in freshly broken samples of Nakhla, and we have also extracted layers of alteration products using a micromanipulator for high-resolution chemical and microstructural characterization.

**Methods:** The sample studied (BM1913, 25) was kindly loaned by the Natural History Museum (London). It is a fresh interior surface fragment (1.6 g) prepared by Dr. Caroline Smith and has not been carbon or gold coated or washed. The zones of weakness along which the sample has broken expose large areas of alteration products. These samples were characterized by field-emission SEM, low-voltage scanning transmission electron microscopy, and by TEM.

**Results:** We have observed a variety of alteration products that have not been previously described. The Fe-silicate gels are composed of multiple thin sheets, some of which are concentrically zoned with respect to Si, Cl, Ca, Mn and Fe and over scales of ~0.1 mm. The gels are also cross-cut by veins of a CI-rich material that have a nanoscale fibrous structure and are decorated by tens of nanometre sized euhedral calcium sulphate crystals. Additionally we have observed radial growth structures emanating from halite grains, and sites displaying stages of nucleation and growth of the gel.

The features that we have observed suggest that the Fe-silicate gel formed during fluctuations in fluid compositions, and the presence of CI- rich alteration products cross-cutting the gel indicates subsequent injection of a saline fluid. The sharp contacts of this later material with the gel suggest that this brine percolated through Nakhla at very low temperatures [5] thus not remobilizing and equilibrating with the surrounding material. One important message of this study is that caution is required when samples are being prepared. The nanoscale features that we have found would undoubtedly be lost in conventional thin section manufacture and may explain why they have not been observed previously.

**References:** [1] Bunch T. E. and Reid A. M. 1975. *Meteoritics* 10:303-324. [2] Gooding J. L. et al. 1991. *Meteoritics* 26:135-143. [3] Changela H. G. and Bridges J. C. 2011. *Meteoritics & Planetary Science* 45:1847-1867. [4] Rao M. N. et al. 2006. Abstract #1969. 37th Lunar and Planetary Science Conference. [5] Hausrath E. M. and Brantley S. L. 2010. *Journal of Geophysical Research* 115.
CRYSTALLOGRAPHY AND ORIGINS OF ETCH PITS IN NAKHLA OLIVINE

T. Tomkinson¹, M. R. Lee², D. F. Mark¹, F. Stuart¹ and P. Lindgren². ¹SUERC, East Kilbride, Glasgow, UK. ²GES, Glasgow University, UK. E-mail: tim.tomkinson@glasgow.ac.uk.

**Introduction:** Etch pitted olivine grains have been observed within the nakhlite meteorite MIL 03346 [1] but have rarely been described from Nakhla [2]. Velbel et al. in 2010 [1] concluded that the pits in MIL 03346 olivine were a result of terrestrial weathering in Antarctica as they occur only within the outermost few hundred microns of the meteorite. However, the pitted olivines that we have found in Nakhla are on freshly exposed pieces of the meteorite that were taken from the interior of this fall (~5 cm into the sample). As these etch pits occur only on some olivines and the majority are filled with alteration products of Martian origin, they must have formed on Mars.

**Results:** The Nakhla etch pits range from diamond to wedge to circular in shape, reflecting the crystallographic orientation of the grain surface on which they have formed. The largest of the two pitted regions found has an array of 80 by 100 round pits with a preferred orientation. The smaller region is a mix of small arrays and single lines of diamond shaped pits. The largest pit observed is ~30 µm and the smallest is sub-micron in diameter. Some of the pits are empty or filled with a hydrous Fe-silicate that has been interpreted by [3] to be a gel, while others contain a Cr-rich material that was exposed during sample preparation. If this Cr enriched material occurs beneath the amorphous gel it is likely to predate the gel formation.

**Implications:** These etch pits can be used to obtain information on the duration of exposure to aqueous solutions. For these calculations pit volumes are required, and they will be obtained by imaging their profiles using SEM and making digital elevation models of the largest pits (~30 µm diameter) using Alicaona MEX software, and by excavating cross sections through them using the focused ion beam (FIB) technique. These data will yield the minimum duration of exposure to the etching fluid [4]. As the pits are observed in only two of the 14 olivine grains so far examined, we will investigate the crystal orientation of the etched olivine grains in order to assess whether there is a preferred etching orientation. Alternatively the etching could predate the main phase of aqueous activity when the fluid was exposed to a limited surface area of the sample when it was less densely fractured.

**Acknowledgments:** The Nakhla sample (BM1913_25) used for this study was from the U.K. Natural History Museum and kindly allocated by Dr Caroline Smith.

**References:** [1] Velbel M. A. et al. 2010. Abstract #2223. 41st Lunar and Planetary Science Conference. [2] Velbel M. A. 2010. AGU Fall Meeting. Abstract #P52B-07. [3] Changela H. G. and Bridges J. C. 2011. Meteoritics & Planetary Science 45:1847–1867. [4] Olsen A. A. 2007. American Mineralogist 92:598–602.

STEPS TOWARD PRECISE Ar/Ar CHRONOLOGIES FOR FLUID-ROCK INTERACTION THROUGHOUT THE SOLAR SYSTEM

T. Tomkinson¹, D. F. Mark¹, M. R. Lee² and F. Stuart¹. ¹SUERC, East Kilbride, UK. ²GES, Glasgow University, UK. E-mail: t.tomkinson@glasgow.ac.uk.

**Introduction:** Determination of ages for pre-terrestrial fluid-rock interaction has only been carried out on two of the twelve known nakhlites (Lafayette and Yamato 200593). The only K-Ar ages for the K-bearing "iddingsite" range from 0-670 ± 91 Ma [1]. Within the nakhlites fine-grained iddingsite is present in discrete veins and as grain coatings. The low K-content and fine-grain size of the iddingsite hinder the Ar/Ar approach as: (1) only small amounts of material are available for isotope extraction, and (2) potential recoil of ³⁹Ar and ³⁷Ar from the fine-grained clays during irradiation. This contribution will focus on novel approaches for Ar/Ar dating of fine-grain K-bearing clay in meteorites using cutting edge mass spectrometer technology (high-sensitivity high-resolution HELIX Split Flight Tube noble gas mass spectrometer), a range of irradiation protocols (e.g., in vacuo encapsulation, D-D fusion neutrons and irradiation of thin sections), and a variety of Ar extraction tools (e.g., CO₂ and UV lasers, bulb furnace). Analog materials from the Isle of Arran (Scotland) have been collected to aid initial investigations. After testing of our approaches we will target K-bearing clay in both the nakhlites (e.g., Nakhla) and carbonaceous chondrites (e.g., Murchison).

**Arran Analogs:** Samples for a "proof of concept" study were sourced from the east coast of Arran, where a highly altered Carboniferous basaltic lava flow is exposed. The flow consists of grains of plagioclase and olivine that show similarities to olivine crystals from the nakhlites. Veins and patches of iddingsite are evident. These alteration products are been utilized to test and refine sample preparation methods, detection limits and achievable spatial resolution prior to analysis of the meteorites. This analogue material is especially useful as its age (c. 330 Ma) falls mid-way between the estimates for alteration of Lafayette (i.e., 0–670 Ma), its K-content is comparable to that of the nakhlite iddingsite, and veins of alteration products are 2–20 µm wide, also similar to the average size of veins in the nakhlites (4–16 µm [2]).

**Analytical Methodology:** SUERC has taken delivery of the first two Thermo Fisher HELIX SFT noble gas mass spectrometers. We have built a custom designed low volume extraction line, which is dedicated to Ar/Ar dating of extraterrestrial materials. Attached to this system is a New Wave UP-213 UV laser that enables in situ analysis of material with a spatial resolution (spot size) of 4 µm. The HELIX SFT is able resolve isobaric interferences at mass 36 allowing for the first time a true ³⁶Ar measurement and accurate atmospheric correction. Ion counting electron multipliers and high-sensitivity resistor Faraday will allow for precise measurement of small ion beams, facilitating determination of precise Ar/Ar ages.

**References:** [1] Swindle T. D. et al. 2000. Meteoritics & Planetary Science 35:107–115. [2] Changela H. G. and Bridges J. C. 2011. Meteoritics & Planetary Science 45:1847–1867.
MASON GULLY: THE SECOND METEORITE RECOVERED BY THE DESERT FIREBALL NETWORK

M. C. Towner¹, P. A. Bland¹, P. Spurný², G. K. Benedix³, K. Dyl¹, R. C. Greenwood⁴, J. Gibson⁴, I. A. Franchi⁴, L. SHRbený², A. W. R. Bevan⁵ and D. Vaughan⁶. ¹IARC, Dept Earth Sci. & Eng., Imperial College London, SW7 2AZ, UK. E-mail: m.towner@imperial.ac.uk. ²Astronomical Institute of the Academy of Sciences, Fričova 298, CZ-251 65 Ondřejov Observatory, Czech Republic. ³IARC, Department of Mineralogy, Natural History Museum, London SW7 5BD, UK. ⁴PSSRI, Open University, Walton Hall, Milton Keynes MK7 6AA, UK. ⁵Department of Earth and Planetary Sciences, Western Australian Museum, Locked Bag 49 Welshpool DC, WA 6986, Australia. ⁶PO BOX 187, Nedlands, Perth, WA 6909, Australia.

**Introduction:** Mason Gully (MG) is the second meteorite recovered using data obtained by the Desert Fireball Network (DFN) project [1, 2]. MG has been classified as an H5 ordinary chondrite, with minimal weathering, due to its short terrestrial exposure age in an area of low precipitation.

**Recovery:** Analysis of the camera images gave orbital information [3], and a prediction of the fall position to an area of 2–3 km². Searching was carried out on foot, by a team of six people, over 12 days in November 2010. The meteorite was recovered within 150 m of the predicted fall site. We have now undertaken 4 field campaigns, and recovered meteorites on 2 of them: a 50% success rate, validating the concept of a network based in the Australian outback.

**Physical Characteristics:** MG has a mass of 24.54 g. The stone is ~3 cm in long dimension, and 50% fusion crusted. Individual pyroxene, olivine, and feldspar crystals are visible on the broken surface, with no evidence of alteration of silicates. Some rust patches around metal grains on original broken surfaces. Although the meteorite appears to have high porosity (based on observations ofawn surfaces), metal in the interior shows no sign of alteration. Fusion crust is black and fresh. We compared alteration of MG crust, broken surfaces, and interior, to a recent H chondrite fall (recovered <3 weeks later with no rain between fall and recovery). In contrast, MG experienced 23 cm of rain between fall and recovery. MG was less weathered. The overall level of alteration was consistent with a 7 month residence time in the desert [4].

**Petrography:** The meteorite has a typical petrologic type 5 texture, with discernible, but not distinct chondrules. Chondrule types include barred olivine and radiating pyroxene. Minerals are heterogeneously distributed. Modal mineralogy (in vol%) is: olivine (33%); orthopyroxene (38%); clinopyroxene (5%); plagioclase (8%); metal (11%); sulfide (5%); minor components around 1%.

**Mineral Compositions and Geochemistry:** Mineral compositions as determined by EMP: olivine, Fa19.2 ± 0.6; low-Ca pyroxene, Fs16.4 ± 0.4Wo1.4 ± 0.3; chromite (Fe/Fe + Mg = 0.84; Cr/Cr + Al = 0.86). Oxygen isotopes: δ¹⁷O = 3.04; δ¹⁸O = 4.42; Δ¹⁷O = 0.74 (all in per mil).

**References:** [1] Bland P. A. et al. 2009. Abstract #1664. 40th Lunar and Planetary Science Conference, www.desertfireballnetwork.org. [2] Spurný P. et al. 2009. Abstract #1498. 40th Lunar and Planetary Science Conference. [3] Spurný P. et al. 2011. *Meteoritics and Planetary Science* 46 (this conference). [4] Bland P. A., Berry F. J., and Pillinger C. T. 1998. *Meteoritics & Planetary Science* 33:127–129.

HOW BIG IS THE MOROKWENG IMPACT STRUCTURE, SOUTH AFRICA? A REAPPRAISAL OF ITS PRESENT SIZE USING REMOTE SENSING, GEOPHYSICS AND GIS

C. Townsend¹, M. A. G. Andreoli²,³, G. Cooper², S. Misra³ and I. Stengel⁴,⁵. ¹³0, Arnold Road, DL1 1JG, U.K. E-mail: skymapper639@impactcraters.info, skymapper639@impactcraters.info. ²School of Geoscience, University of the Witwatersrand, Johannesburg 2050, South Africa. ³SGS, University of KwaZulu-Natal, Durban-4000, South Africa. ⁴NamibGeoVista Geoconsult & Imaging, Windhoek, Namibia. ⁵Department of Geography, University of Pretoria, Pretoria, South Africa.

**Introduction:** The Morokweng impact structure in South Africa, formed close to the Jurassic-Cretaceous boundary (approximately 145 ± 2 Ma) by the impact of an LL-6 chondrite body [1 and references therein], could be one of the largest known terrestrial craters if the issue surrounding its final diameter can be resolved. Morokweng, presently buried under cover of Cenozoic Kalahari continental deposits, is described to have a diameter between approximately 70 km [2, 3] and 340 km [4]. The present integrated study, involving the use of Landsat 7 imagery, SRTM-DEM data, and the results of a magnetic and gravity survey, coupled with field observations and borehole data, suggest the minimum diameter of this structure is likely to be approximately 160 km.

**Techniques of Study:** The Landsat 7 imagery and DEM were investigated for arcuate and impact related topographical features; these datasets were merged with magnetic and gravity data and analysed spatially using a Geographical Information System (GIS).

**Observations:** The Morokweng structure is characterized by a large, elliptically shaped gravity low, approximately 155 × 125 km in diameter, centered on the melt sheet. When correlated with the SRTM-DEM and Landsat 7 imagery, it appears sharply truncated to the North, West and Southwest by the Kuruman Hills. Enhancement of the gravity data has revealed a smaller elliptical anomaly some 55 × 68 km in diameter, the center of which reveals a small low at 23.5° E 26.5° S, being the approximate center of the melt sheet.

**Conclusions:** We suggest that the Morokweng structure has a minimum diameter of approximately 160 km, with a probable final diameter of approximately 200 km [cf. 5, 6], making it one of the largest known terrestrial impact structures.

**References:** [1] Misra S. et al. 2011. Abstract #1102. 42nd Lunar and Planetary Science Conference. [2] Henkel H. et al. 2002. *Journal of Applied Geophysics* 49:129–147. [3] Reimold W. U. et al. 2002. *Earth and Planetary Science Letters* 201:221–232. [4] Corner B. et al. 1997. *Earth and Planetary Science Letters* 146:351–364. [5] Baker T. 2001. A Remote Sensing Investigation of the Morokweng Impact Structure, South Africa. University of Greenwich, UK. B.Sc. thesis (Unpublished). [6] Andreoli M. A. G. et al. 2008. Abstract #1236. 39th Lunar and Planetary Science Conference.
## ANGRA DOS REIS: RECONCILIATION OF COMPOSITION AND TEXTURE, INVOKING IMPACT

A. H. Treiman. Lunar and Planetary Institute, 3600 Bay Area Boulevard, Houston TX 77058 E-mail: treiman#lpi.usra.edu.

**Introduction:** The angrite meteorites are ancient basaltic rocks from a highly differentiated body [1-5]. Angra dos Reis itself (AdoR) is unique in being a pyroxenite, not a basalt. By mineralogy and chemistry, AdoR has been interpreted as an igneous cumulate [1, 6], but its texture (Fig. 1, [7]) suggests a more complex history.

**AdoR Texture:** AdoR consists of 93% fassaite pyroxene, with olivine, Mg-Fe spinel, merrillite, etc. [6]. The fassaite occurs in two textural patterns: poikilitic megacrysts up to 3 mm across, seen in thin section as disjoint grains in optical continuity; and equant 'groundmass' grains of ~100 µm across (Fig. [7]). Originally [6], groundmass grains were interpreted as cumulus and the megacrysts as intercumulus infill. Alternatively, [7] suggested that the megacrysts were porous phenocrysts, and that the groundmass represented devitrified magma. The former idea is physically implausible [7], and the latter chemically difficult [1].

**Yet Another Idea:** By analogy with the 'granular bands' in the ALH 84001 meteorite [8], the 'groundmass' fassaite grains in AdoR could be interpreted as annealed [9] remnants of shock-deformed (or melted) material. The poikilitic megacrysts would be unrecrystallized remnants of original cumulus grains. This explanation reconciles AdoR's bulk composition (pyroxene cumulate) with its texture, at the expense of requiring a multi-stage history. The relatively young age of AdoR, 4557.6 versus 4564.4 Ma for quenched angrites [4,10], would then represent shock metamorphic equilibration. This evidence for strong shock deformation and extended cooling duration is consistent with the concept of an angrite planetesimal or dwarf planet [5].

**References:** [1] Mittlefehldt D. W. and Lindstrom M. M. 1990. *Geochimica et Cosmochimica Acta* 54:3209-3218. [2] Mittlefehldt D. W. et al. 1998. Non-chondritic meteorites from asteroidal bodies. Ch. 4 in *Planetary materials* (ed. J. J. Papike), *Reviews in Mineralogy* 36. [3] Mittlefehldt D. W. et al. 2002. *Meteoritics & Planetary Science* 37:345-369. [4] Amelin Y. 2008. 1997. *Geochimica et Cosmochimica Acta* 72:221-232. [5] Weiss B. P. 2008. *Science* 322:713-716. [6] Prinz M. et al. 1977. *Earth and Planetary Science Letters* 35:317-330. [7] Treiman A. H. 1989. 19th Lunar and Planetary Science Conference. pp. 443-450. [8] Treiman A. H. 1998. *Meteoritics & Planetary Science* 33:753-764. [9] Fittipaldo M. A. et al. 2005. *Meteoritics & Planetary Science* 42:573-589. [10] Markowski A. et al. 2007. *Earth and Planetary Science Letters* 262:216-229.

<figure><img src="image_31.png" /><figcaption>Fig. 1. Angra dos Reis thin section, crossed polars, 2.1 mm across. All fassaite pyroxene. Megacryst of grains in optical continuity (brown), crossed by bands of smaller grains in random orientations.</figcaption></figure>

## THE ROLE OF SYNCHROTRON RADIATION-BASED X-RAY MICROTOMOGRAPHY IN EXAMINATION OF SMALL EXTRATERRESTRIAL SAMPLES RECOVERED BY SPACECRAFT IN THE Stardust AND HAYABUSA MISSIONS

A. Tsuchiyama¹, T. Nakamura², M. Uesugi³, K. Uesugi⁴, T. Nakano⁵, A. Takeuchi⁴, Y. Suzuki⁴, Y. Iida¹, R. Niimi¹, R. Noguchi¹, T. Matsumoto¹, J. Matsuno¹, T. Nagano¹, Y. Imai¹, T. Ogami², T. Noguchi⁶, M. E. Zolensky⁷, T. Mukai³, M. Abe³, T. Yada³ and A. Fujimura³.
¹Department of Earth & Space Science, Osaka University, Osaka, Japan. E-mail: akira@ess.sci.osaka-u.ac.jp.
²Department of Earth & Planetary Mat. Science, Tohoku University.
³JAXA.
⁴JASRI/SPring-8.
⁵GSJ/AIST.
⁶The Coll. of Sci. at Ibaraki University.
⁷NASA JSC.

**Introduction:** Coma dust particles of comet 81P/Wild 2 coma were returned to Earth by NASA's Stardust mission in January 2006 and have been examined in laboratories [1]. Regolith particles of asteroid 25143 Itokawa were returned by JAXA's Hayabusa mission in June 2010 and preliminary examination has started [2, 3]. Particle sizes of both the samples are small (<approximately 100 µm) and their total amounts are very small compared with Apollo lunar samples. Nondestructive microtomography gives unique 3-D data and also plays an important role in later destructive analyses in order to optimize the information obtained from these small, precious particles. Our research group has used a synchrotron radiation (SR) based microtomography system for these samples. An overview of these results will be presented here.

**SR-Based Microtomography:** Tomography using X-ray absorption is a conventional method to give spatial distribution of X-ray linear attenuation coefficients (LACs) as a digital image (CT image). SR X-ray beams with high flux density and high coherence give CT images of high spatial resolution (nano-scale resolution is possible by X-ray microscope optics using a FZP) and high S/N ratio. High flux X-ray beams are easily monochromated, and this gives quantitative LAC values in CT images, which can be used for identification of minerals and their rough chemical compositions. We used microtomography systems at BL20B2 and BL47XU of SPring-8 for the sample examination.

**Stardust Samples:** Comet coma dust was captured using silica aerogel at a relative velocity of 6.1 km s⁻¹ by the Stardust spacecraft. Dust particles were disrupted during the capture and impact tracks were formed. Relatively coarse crystalline grains, harvested from impact tracks, were examined by microtomography, which determined how to best cut the sample particles for later oxygen isotope analysis [4]. 3-D morphologies of impact tracks were also examined, and densities of impacted dust particles were estimated based on these data [5].

**Hayabusa Samples:** A quantitative 3-D mineral distribution was obtained for each particle by imaging at 7 and 8 keV. These 3-D mineral maps were essential for planning later destructive analysis in the preliminary examination analysis flow. External 3-D shape features of these particles were also examined to reveal details of regolith formation and evolution on Itokawa [3].

**References:** [1] Brownlee D. E. et al. 2006. *Science* 314:1711-1716. [2] Nakamura T. et al. 2011. This issue. [3] Tsuchiyama A. et al. 2011. This issue. [4] Nakamura T. et al. 2008. *Science* 321:1664-1667. [5] Iida Y. et al. *Meteoritics & Planetary Science* 45:1302-1319.
5243

ORIGIN AND EVOLUTION OF ITOKAWA REGOLITH PARTICLES
BASED ON THREE-DIMENSIONAL SHAPES AND SIZES OF
HAYABUSA SAMPLES

A. Tsuchiyama¹, M. Uesugi², T. Matsushima³, T. Michikami⁴, T. Kadono⁵, T. Nakamura⁶, K. Uesugi⁷, T. Nakano⁸, S. A. Sandford⁹, R. Noguchi¹, T. Matsumoto¹, J. Matsuno¹, T. Nagano¹, Y. Imai¹, A. Takeuchi⁷, Y. Suzuki⁷, T. Ogami⁶, J. Katagiri³, M. Ebihara¹⁰, T. R. Ireland¹¹, F. Kitajima¹², K. Nagao¹³, H. Naraoka¹², T. Noguchi¹⁴, R. Okazaki¹², H. Yurimoto¹⁵, M. E. Zolensky¹⁶, T. Mukai², M. Abe², T. Yada², A. Fujimura², M. Yoshikawa² and J. Kawaguchi². ¹Department Earth & Space Science, Osaka University, Osaka, Japan. E-mail: akira@ess.sci.osaka-u.ac.jp. ²JAXA. ³Department Engineering Mechanical & Energy, University of Tsukuba, Tsukuba, Japan. ⁴Fukushima National College of Technology. ⁵ILE, Osaka University, Osaka, Japan. ⁶Department Earth & Planetary Mat. Science, Tohoku University, Tohoku, Japan. ⁷JASRI/SPring-8. ⁸GSJ/AIST. ⁹NASA ARC. ¹⁰Graduate School of Science & Engineering, Tokyo Metropolitan University, Tokyo, Japan. ¹¹Research School of Earth Science, ANU. ¹²Department Earth & Planetary Science, Kyushu University, Kyushu, Japan. ¹³Geochemical Research Center, The University of Tokyo, Tokyo, Japan. ¹⁴The College of Science at Ibaraki University, Ibaraki, Japan. ¹⁵National History Science, Hokkaido University, Hokkaido, Japan. ¹⁶NASA JSC.

**Introduction:** Particles of S-type asteroid 25143 Itokawa were successfully recovered by the Hayabusa mission, and at least 1500 recovered particles have been identified as having an Itokawa origin [1, 2]. This is the first sample recovered from an asteroid and returned to Earth, and the second extraterrestrial regolith to have been sampled, the first being the Moon, which was sampled by the Apollo and Luna missions. We obtained three-dimensional (3-D) structures of Itokawa particles using X-ray microtomography in the Hayabusa sample preliminary examination in order to understand their textures in comparison with meteorites and the 3-D shape features in connection with the regolith formation and evolution on the asteroid [2].

**Experiments:** Forty particles of approximately 30–180 µm in size were imaged at BL47XU of SPring-8 with effective spatial resolutions of approximately 200 or 500 nm. Imaging at two X-ray energies of 7 and 8 keV made identification of minerals in CT images possible. A successive set of 3-D CT images, which shows quantitative 3-D mineral distribution, was obtained for each particle.

**Results and Discussion:** Modal mineral abundances of the whole sample and the bulk density (3.4 g cm⁻³) calculated form the modal abundance and average chemical compositions of the minerals [1] are similar to those of LL chondrites. 3-D structures of the particles having textural variations indicate a mixture of equilibrated and less-equilibrated chondrite materials. No particles showing melting were observed, indicating low impact velocities similar to typical relative impact velocities among asteroids (approximately 5 km s⁻¹). 3-D shape features of the particles (size and shape distributions together with presence of particles with rounded edges) suggest that they were formed by meteoroid impacts on the asteroid surface, transported to the smooth terrain, and eroded in situ there by granular processes, which include major resurfacing processes in small asteroids [3]. The above discussion indicates that the returned sample roughly represents the general surface materials on Itokawa.

**References:** [1] Nakamura T. et al. 2011. Abstract #1776. 42nd Lunar and Planetary Science Conference. [2] Tsuchiyama A. et al. 2011. Abstract #1777. 42nd Lunar and Planetary Science Conference. [3] Miyamoto H. et al. 2007. *Science* 316:1011–1014.

5103

EVIDENCE FOR EXTINCT ³⁶Cl FROM EXCESS ³⁶Ar IN ALLENDE SODALITE

G. Turner¹, R. Burgess¹, S. P. Kelley² and G. J. Wasserburg³. ¹SEAES, University of Manchester, UK, E-mail: Grenville.turner@manchester.ac.uk. ²Dept. of Earth and Environmental Science, Open University, UK. ³Division of Geological and Planetary Science, California Institute of Technology, USA.

**Introduction:** Excesses of ³⁶S have previously been reported in sodalite (Na₈Al₆Si₆O₂₄Cl₂) and wadalite (Ca₆Al₂Si₂O₁₆Cl₃) in CAI and chondrules from the Allende and Ningqiang meteorites and used to infer the presence of ³⁶Cl (half-life 3 × 10⁵ a) in the early solar system [1–3]. In spite of the fact that ³⁶Cl decays predominantly (98%) to ³⁶Ar, all attempts to locate the very large amounts of ³⁶Ar implied by these ³⁶S excesses have been negative. Here we present results of a new method to identify ³⁶Ar from ³⁶Cl decay, based on low fluence fast neutron activation.

**Method:** We have irradiated two batches of Allende sodalite (Pink Angel) with different fluencies of fast neutrons in the Cd-shielded CLICIT facility of the Oregon State TRIGA reactor. In the absence of trapped argon, the upper intercept of a plot of ³⁶Ar/³⁸Ar against ³⁷Ar/³⁸Ar defines a relationship between ³⁶Ar and ³⁸Ar from the action of cosmic ray-induced secondary neutrons and mono-isotopic ³⁶Ar from ³⁶Cl decay. By combining data from two irradiations, the two contributions can be unambiguously distinguished. Figure 1 shows allowed combinations of ³⁶Cl/³⁵Cl and the product of cosmogenic secondary neutron fluence and ³⁷Cl cross section for each series. The intersection corresponds to ³⁶Cl/³⁵Cl = (1.9 ± 0.5) × 10⁻⁸ and <σ₃₇.Φ> = (1.1 ± 0.8) × 10⁻¹⁰.

The latter is in accord with the neutron fluence observed in CAIs in Allende [4]. The ³⁶Cl/³⁵Cl ratio is 3 orders of magnitude lower than the highest value inferred from ³⁶S excesses. If interpreted chronologically, i.e., as a closure age, it would correspond to a period of 3Ma or so of activity involving the precipitation of sodalite, which is consistent with I-Xe data [5]. Alternatively, the low ratio may represent ³⁶Ar loss in line with the low activation energy for Ar diffusion in sodalite. The presence of excess ³⁶Ar together with ³⁶S supports the idea that live ³⁶Cl was present in the sodalite, as opposed to the ³⁶S being inherited. The mechanism for producing ³⁶Cl several million years after the formation of the solar system is still unknown.

**References:** [1] Lin Y. et al. 2005. *Proceedings of the National Academy of Sciences* 102:1306–1311. [2] Hsu W. et al. 2006. *The Astrophysical Journal* 640:525–529. [3] Jacobsen B. et al. 2009. Abstract #2553. 40th Lunar and Planetary Science Conference. [4] Wasserburg G. J. et al. 2011. *Geochimica et Cosmochimica Acta*, in press. [5] Turner G. et al. *Meteoritics & Planetary Science* 44:A5255.

<figure><img src="image_32.png" /></figure>
DECIPHERING THE HISTORY OF CARBONATES AND ASSOCIATED MINERALS DURING SECONDARY ALTERATION WITHIN CR1 GRO 95577

M. A. Tyra, C. Le Guillou and A. J. Brearley. Department of Earth & Planetary Sciences, MSC03-2040, University of New Mexico, Albuquerque, NM 87131, USA. E-mail: matyra@unm.edu.

**Introduction:** Secondary carbonates in CR chondrites are commonly pure CaCO₃ but can contain up to 7 wt% FeO, 14 wt% MgO, and 2 wt% MnO [1]. The highly altered CR1 chondrite GRO 95577, however, is an exception; Ca-carbonate, dolomite, and Fe-rich carbonate have all been identified and carbonate oxygen isotope compositions suggest that Ca-carbonate formation predated Fe-rich carbonate [2]. Carbonates occur in the matrix with other alteration phases. We have studied carbonate mineral relationships in GRO 95577 using SEM and TEM to understand their chemical heterogeneity and formation sequence.

**Results:** Carbonates often occur spatially correlated with pyrrhotite and some magnetite. Pyrrhotite ranges from euhedral-to-abraded, and magnetite, individually and within framboids, is euhedral. Some Fe-rich carbonate encrusts calcite and pyrrhotite. Calcite, Fe-rich carbonate, and pyrrhotite show dissolution features. Fibrous and porous phyllosilicate rims are systematically found rimming all other secondary phases.

We extracted a FIB section (27 µm across) that transects matrix, Fe-rich carbonate with associated fibrous phyllosilicate, pyrrhotite, heterogeneous carbonate, and a lamellar pentlandite-phyllosilicate intergrowth. Three distinct carbonate compositions occur (1) nearly-pure Ca-carbonate (some Mn), (2) an intermediate Ca-rich carbonate with highly variable Mg, Mn, and Fe, and (3) a carbonate high in Fe, Mg, and Mn, and almost free of Ca. The Fe-rich carbonate is located within 500 nm of the pyrrhotite, but is separated by a layer of Ca-carbonate and intermediate carbonate grains from the pyrrhotite. Intermediate carbonate contains four chemically-zoned areas within 1 µm with Ca/Mg ranging from 1 to 2. These zones are parallel to a pyrrhotite crystal face. Fine-grained matrix phyllosilicates and fibrous grains have compositions and *d*-spacings consistent with serpentine (15 < Mg# < 30 atom%).

**Discussion:** Pyrrhotite is almost always associated with Ca-carbonate, which may have formed early in the precipitation sequence together with magnetite. The zoned intermediate carbonate precipitated after pyrrhotite, as it grew parallel to a pyrrhotite crystal face. Intermediate carbonate is also richer in Fe next to the pyrrhotite, which may indicate that local chemical heterogeneity of the fluid could be controlled by pyrrhotite alteration. Fe-rich carbonate always surrounds both Ca-carbonate and pyrrhotite, and is a later stage phase. Lastly, Fe-rich carbonate dissolution and replacement by phyllosilicates is evident both on the SEM and TEM scale where 100 nm-wide serpentine fibers grow parallel to Fe-rich carbonate grain surfaces. These serpentine have a larger grain size than matrix, are also found intergrown with pentlandite, and may result from late stage precipitation in voids at grain boundaries. This interplay between sulfides, magnetite, carbonates, and phyllosilicates may result from heterogeneous and changing nano-scale chemical environments.

**Acknowledgments:** This work was supported by NASA grant NNG06GG37G (A. J. Brearley).

**References:** [1] Weisberg et al. 1993. *Geochimica et Cosmochimica Acta* 57:1567–1586. [2] Tyra M. A. et al. 2011. Abstract #1639. 42nd Lunar and Planetary Science Conference.

DEVELOPMENT OF A METHOD FOR QUANTITATIVE ANALYSIS BY SYNCHROTRON RADIATION X-RAY CT, TOWARD HAYABUSA2 MISSION

M. Uesugi¹ and K. Uesugi². ¹Osaka University, JAXA/JSPEC, 3-1-1 Yoshinodai Sagamihara, Kanagawa 252-5210, Japan. E-mail: uesugi@planeta.sci.isas.jaxa.jp. ²JASRI / SPring-8 1-1-1, Kouto, Sayo-cho, Sayo-gun, Hyogo 679-5198 Japan.

**Introduction:** Hayabusa2, subsequent mission of Hayabusa, targets 1999 JU3, a C-type asteroid which thought as a parent body of carbonaceous chondrite. Synchrotron radiation X-ray CT is one of the important tools of the initial analysis of those retuned samples, because it enables us not only to observe samples three-dimensionally but also to analyze materials inside the meteorites quantitatively by estimating X-ray absorption of materials [1-4]. By comparing calculated values of linear attenuation coefficient (LAC) and observed LAC values those expressed as contrasts of the slice images, we can determine materials inside the samples [2-4]. In this paper, we observed several carbonaceous chondrites by SR-X-Ray CT. Uesugi et al. [4] showed that histograms of the LAC are useful for rough determination of meteorite class. Using obtained histograms, we discuss the method for the quantitative analysis of Hayabusa2 samples.

**Results and Discussions:** Experiments were carried out at BL20B2 in SPring-8, Japan. X-ray CT data were obtained by optimized environment for the CT observation of meteorites [4]. Figure shows the histograms of carbonaceous chondrites obtained by 30 keV X-ray.

Corresponding materials of each peak in histograms are determined by thresholding the slice images around the peak. Peaks of chondrules appear at 3.1, 4.3 and 4.3 cm⁻¹ and peaks of matrix appear at 6.2, 8.0, and 9.2 cm⁻¹, for Murchison, Allende, and Y-81020, respectively. The peak values of chondrule are equivalent to Fo# 97, 91, and 91 of olivine, and En# 96, 87, and 87 of pyroxene, for Murchison, Allende, and Y-81020, respectively. These values depend on the chemical compositions of each component, and enable us to determine the meteorite class of the samples, though we should increase the number of data to increase accuracy.

**References:** [1] Tsuchiyama A. et al. 2005. *American Mineralogist* 90:132–142. [2] Nakamura T. et al. 2008. *Science* 321:1664–1667. [3] Tsuchiyama A. et al. 2011. Abstract #1777. 42nd Lunar and Planetary Science Conference. [4] Uesugi M. et al. 2010. *Earth and Planetary Science Letters* 299:359–367.

<figure><img src="image_33.png" /></figure>
5296

THE PARENT BODY OF THE CA. 480 KYR-OLD TUNGUSKA-LIKE IMPACT OVER ANTARCTICA

M. van Ginneken¹, C. Suavet²,³, C. Cordier¹, L. Folco¹, P. Rochette³ and C. Sonzogni³. ¹Museo Nazionale dell'Antartide, Università di Siena, Italy. E-mail: vanginneken@unisi.it. ²Massachusetts Institute of Technology, USA. ³CEREGE, Université Aix-Marseille III, France.

**Introduction:** Meteoritic ablation spheres (MAS) recently found on top of the Victoria Land Transantarctic Mountains (TAM) and in the EPICA Dome-C and Dome-Fuji ice core document the impact of a 10⁸ kg (or larger) cosmic body in the Antarctic region ca. 480 kyr ago [1-3]. The exact nature of the impactor is unknown, and whether the impactor struck the Antarctic ice sheet or exploded in the atmosphere is matter of debate. In order to address these questions, we discuss high-precision oxygen isotope compositions of two aggregates of MAS from the TAM.

**Methods:** Two MAS aggregates, namely #20c.25 and #20c.351, from micrometeorite trap #20c, Miller Butte, Antarctica, were selected with a Scanning Electron Microscope (SEM) for oxygen isotope analysis. Two samples of host granite detritus, of which one was cleaned of weathering products, were also selected to study the effect of weathering and bedrock contamination. Oxygen isotope compositions were determined using the IR-laser fluorination coupled with mass spectrometry (IRMS) technique adapted for the study of extraterrestrial materials following [4].

**Results:** SEM observations show a larger amount of weathering products in #20c.351 compared to #20c.25. Oxygen isotope compositions are δ¹⁸O = 2.98 and Δ¹⁷O = -3.26 for #20c.25, δ¹⁸O = 6.50 and Δ¹⁷O = -0.54 for #20c.351, δ¹⁸O = 13.65 and Δ¹⁷O = 0.03 for cleaned host granite, and δ¹⁸O = 4.97 and Δ¹⁷O = 0.09 for host granite with weathering products (analytical uncertainties are ±0.27‰ for δ¹⁸O and ±0.07‰ for Δ¹⁷O).

**Discussion:** Comparison of oxygen isotope compositions of MAS aggregates and host granites indicate that alteration broadly shifts the MAS aggregate composition toward those of host granite, with maximum alteration on the Terrestrial fractionation line. Pristine bulk composition of the MAS, derived from modal composition of sample #20c.25 [1], is Δ¹⁷O ≈ -3.86‰, thus similar to those of CV, CO, and CK chondrites and of anhydrous silicates in short period comet 81P/Wild 2. Mixing models indicate that during the formation of MAS, interaction with oxygen from the Antarctic ice was likely minor, therefore ruling out direct impact on the ice sheet and favouring a Tunguska-like airburst in the low-atmosphere. This conclusion is consistent with the continental distribution of the paired MAS [1].

**Conclusion:** MAS from the TAM and the EPICA Dome-C and Dome-Fuji ice cores resulted from a Tunguska-like airburst over Antarctica ca. 480 kyr ago, implying an object several tens of meters in size or larger, either an asteroid of CV, CO or CK composition or a short-period comet similar to 81P/Wild 2.

**Acknowledgments:** This work was supported by the PNRA and by the EU-funded ORIGINS project.

**References:** [1] van Ginneken et al. 2010. *Earth and Planetary Science Letters* 293:104-113. [2] Narcisi et al. 2007. *Geophysical Research Letters* 34:L15502. [3] Misawa et al. 2010. *Earth and Planetary Science Letters* 289:287-297. [4] Suavet et al. 2010. *Earth and Planetary Science Letters* 293:313-320.

5330

MONT DIEU A IIE NON-MAGMATIC IRON METEORITE WITH CHONDRULES

N. Van Roosbroek¹,², S. Goderis¹, V. Debaille³, G. Hublet³, Ph. Claeys¹. ¹Earth System Sciences, Vrije Universiteit Brussels, B-1050 Brussels, E-mail: nadia.vanroosbroek@student.kuleuven.be. ²Geo-institute, Katholieke Universiteit Leuven, B-3001 Leuven. ³Département des Sciences de la Terre et de l'Environnement, Université Libre de Bruxelles, B-1050 Brussels, Belgium.

The ~450 kg fragment of the Mont Dieu iron meteorites at the Museum of the Royal Belgian Institute of Natural Sciences has further been characterized, with special focus on the silicate inclusions. As the original fragments in the collection of the Musée National d'Histoire Naturelle in Paris show much more rust damage [1, 2], the good state of preservation of this large fragment provides a unique way to study the Mont Dieu meteorites. Recently, Mont Dieu II was classified as a fine octahedrite IIE iron meteorite [3]. The metal phase shows a clear widmanstätten texture, composed essentially of kamacite, with fine lines of Ni-rich taenite, and locally troilite associated with schreibersite [3].

The presented study focuses on the abundant large, rounded, greenish silicate inclusions present in Mont Dieu. These inclusions are mainly composed of olivine (~SiO₂: 39.7 wt%, FeO: 15.8 wt%, MgO: 43.2 wt%, MnO: 0.4 wt%), pyroxene (~SiO₂: 60.0 wt%, FeO: 9.3 wt%, MgO: 31.2 wt%, MnO: 0.4 wt%, CaO: 0.9 wt%, Al₂O₃: 0.2 wt%, TiO₂: 0.1 wt%) and plagioclase (~SiO₂: 64.6 wt%, Al₂O₃: 21.1 wt%, Na₂O: 9.3 wt%, K₂O: 0.9 wt%, FeO: 0.8 wt%, CaO: 2.7 wt%), with chromite, troilite, schreibersite, (chlor)apatite and metal as minor mineral phases. The bulk composition has also been determined with ICP-OES and ICP-MS. Large chondrules (0.8–1.5 mm) are present in the examined inclusions, of which two are barred olivine chondrules. This feature appears unique in non-magmatic iron meteorites. The inner part of the chondrules consists mainly of olivine (~SiO₂:41.3 wt%, Fe₂O₃: 16.3 wt%, MgO: 40.7 wt%, MnO: 0.26 wt%), pyroxene (~SiO₂:58.3 wt%, Fe₂O₃: 9.7 wt%, MgO: 30.6 wt%, MnO: 0.21 wt%, CaO: 0.79 wt%, Al₂O₃: 0.04 wt%, TiO₂: 0.03 wt%), plagioclase (~SiO₂: 65.9 wt%, Al₂O₃: 20.7 wt%, Na₂O: 8.2 wt%, K₂O: 0.88 wt%, Fe₂O₃: 1.6 wt%, CaO: 2.4 wt%) and troilite, crossed with metal veins. The chondrules are surrounded by troilite, (chlor) apatite, metal, chromite and schreibersite. The fayalite and ferrosilite molar contents of the seven chondrules are similar to those observed in H-type ordinary chondrite that have been linked to IIE NMI based on their oxygen isotopic compositions [4]. Due to the presence of obvious chondrules in the silicate inclusions, Mont Dieu can be considered as part of the Netschaëvo class within the IIE group. Based on these characteristics, the Mont Dieu II meteorite promises to provide more insight in the further understanding of the IIE NMI formation process.

**References:** [1] Grossman. 1997. *Meteoritics & Planetary Science* 31 (Suppl.): A159–166. [2] Desrousseaux et al. 1996. *Meteoritics & Planetary Science* 31:A36. [3] Van Den Borre et al. 2007. *Meteoritics & Planetary Science* 42:A153. [4] Olsen et al. 1994. *Meteoritics* 200–213.
5153

CHONDRULES IN THE TIESCHIT CHONDRITE: A CHEMICAL-
PETROGRAPHIC APPROACH

M. E. Varela¹, E. Zinner² and G. Kurat³. ¹ICATE/CONICET, San
Juan, Argentina. E-mail: evarela@icate-conicet.gob.ar. ²Physics
Department, Washington University, St. Louis, MO, USA. ³Department
Lithospheric Science, University of Vienna, Vienna, Austria (deceased).

**Introduction:** Although large amounts of petrological and geochemical data on chondrules exist, there is still no consensus, neither on the different processes that were active during and after chondrule formation, nor on the nature of the processes that give rise to liquid droplets [e.g., 1]. However, most of the scientists working on chondrules today believe that crystals in chondrules are precipitation products of the liquid (e.g., melt) and that mesostasis represents residual liquid of the bulk melt. Here we compare the texture of three chondrules from the UOC Tieschitz L/H3.6 (PTS, Tieschitz L 3441–3443, NHMV) with the chemical composition of their clear glassy mesostasis (residual melt?) in order to contribute to our knowledge of the chondrule forming mechanism.

**Results:** Tieschitz 3-(2) is a PO chondrule (glass: 13%, euhedral Ol [Fo98]: 87%), 2.3 mm in apparent diameter and containing several metal and sulfide belts and diffusion controlled aggregation at the surface (lobate surface). Tieschitz L(A) is a POP chondrule (diameter: 680 µm) with a rim of low-Ca pyroxene (glass: 3.6%, Ol [Fo98]: 51%, Px [En97.6, Fs1.4]: 45%). Tieschitz XII is a glass-rich (glass: 64%) chondrule that contains olivine plates (16.5%), Ca-rich pyroxene (2.5%), sulfide dendrites, and a large spinel that is surrounded by a cryptocrystalline matrix (16.5%) [2]. Normalized REE abundance patterns of the glassy mesostasis are shown in the figure.

The studied objects have almost identical elemental abundance
patterns but different mineralogy and texture. Trace element abundances
and patterns appear not to correlate with the textures and/or the
mineralogy of the objects, as was previously observed in some chondrules
from UOCs [3]. Glasses show no sign of geochemical fractionation but
instead a clear sign of vapor fractionation. Unraveling the role of liquids
in crystal growth is crucial for our understanding of chondrule formation
processes.

**References:** [1] Zanda 2004. *Earth and Planetary Science Letters* 224:1–17. [2] Kurat et al. 2010. Abstract #5315. Lunar and Planetary Science Conference. [3] Engler et al. 2007. *Icarus* 192:286.

<figure><img src="image_34.png" /></figure>

5044

AEROGEPHYSICAL ANOMALIES OVER SERRA DA CANGALHA
(SdC) IMPACT STRUCTURE, BRAZIL

M. A. R. Vasconcelos and A. P. Crósta. Institute of Geosciences, University of Campinas, Campinas, SP, Brazil. E-mail: vasconcelos@ige.unicamp.br; alvaro@ige.unicamp.br.

**Introduction:** SdC is a complex impact structure ~13 km in diameter with a central uplift of ~5.8 km in diameter [1]. It was formed in undisturbed Phanerozoic sedimentary rocks of the Parnaíba Basin, northeastern Brazil. Inside SdC there are four stratigraphic units, namely the Longá, Piauí, Poti and Pedra de Fogo formations, from the lowermost to the uppermost unit. The Longá Fm. comprises dark shales of the Devonian/Lower Carboniferous representing the oldest rocks exposed in the central uplift. The other three units comprise basically by sandstones.

**Data and Results:** We used relatively high-resolution aeromagnetic and aerogammaspectrometric data, acquired using a flight line spacing of 500 m and flight height of 100 m. This portion of the Parnaíba Basin exhibits a conspicuous NE-SW magnetic anomaly related to crystalline basement. The regional magnetic field the same directional trend and it was partially removed by filtering the data, revealing a 15 nT residual magnetic anomaly over the crater still oriented towards NE-SW. We then applied the analytic signal method in order to position the anomaly over the source. The result revealed an anomaly coinciding with the area of occurrence of the the Longá Fm. in the middle of the central uplift, together with other anomalies that can be related to the NW boundary of the central uplift. The Euler deconvolution method [2] was applied to the analytical signal results and showed 1000–1500 m deep sources under the central uplift possibly related to basement rocks. Gammaspectrometric data provided interesting results related to lithologic/mineralogic characteristics of exposed rocks. The K channel marked anomalies with high values (0.5%) related to all vertical ridges within the central uplift, as well as the boundary and center of the central uplift, whereas the Th channel marked the topographic contrast that defines the annular basin (4 ppm) and the rim (20 ppm). The U channel shows very well defined boundaries related to the central uplift and rim (3 ppm). A ternary map is presented in the Fig. 1.

**Conclusions:** An uncharacteristic high concentration of radioelements is observed mainly into the central uplift. This high concentration might be explained by the absorption of the radioelements by clay minerals and/or their combination with iron oxides, increasing the concentration on the surface. Indeed, high amounts of iron oxides are observed in thin-sections of rocks from the collar and in breccias from the central depression. These conspicuous anomalies suggest that the radioelements were remobilized and combined with oxides and clays, possibly as a result of the circulation of hydrothermal fluids in later stages of the cratering process.

**References:** [1] Kenkmann T. et al. 2010. Abstract #1237. 41st Lunar and Planetary Science Conference. [2] Thompson D. T. 1982. *Geophysics* 47:31–37.

<figure><img src="image_35.png" /><figcaption>Fig. 1. Ternary aerogammaspectrometric map (RGB = K, Th, U channels, respectively).</figcaption></figure>
## ACCRETION DISK ORIGIN OF EARTH'S WATER: LABORATORY EXPERIMENTS

L. Vattuone¹, M. Smerieri¹, L. Savio², K. Muralidharan³, M. J. Drake⁴ and M. Rocca¹. ¹Dipartimento di Fisica dell'Università di Genova and IMEM-CNR Unità operativa di Genova, Via Dodecaneso 33, 16146 Genova, Italy. ²IMEM-CNR Unità operativa di Genova, Via Dodecaneso 33, 16146 Genova, Italy. ³Department of Materials Sciences and Engineering, University of Arizona, Tucson, AZ 85721, USA. ⁴Lunar and Planetary Laboratory, University of Arizona, Tucson, AZ 85721, USA.

The presence of water on the Earth is an enigma. It is generally agreed that it was too hot at 1 AU for hydrous minerals to be stable in the accretion disk. Thus, Earth's water is conventionally believed to be delivered by comets or wet asteroids after Earth formed. However, wet asteroids and comets have elemental and isotopic properties that are inconsistent with those of Earth [1] limiting the amount of water derived from comets and wet asteroids. Drake [1] proposed that water was introduced during planet growth in the accretion disk in a form stable under high temperature conditions. This hypothesis is supported by the presence of water in the disks around young stars [2] and by numerical simulations of water adsorption on silicate grains under conditions corresponding to those in the accretion disk [3–5] which showed that molecular chemisorption of water on forsterite (a main constituent of the dust grains) might account for the formation of several Earth oceans [4]. Here we show by laboratory experiments (see Figure) that water adsorbs dissociatively on the olivine {100} surface at the temperature (approximately 500–1500 K) and water pressure (approximately 10⁻⁸ bar) expected for the accretion disk, leaving an OH adlayer that is stable at least up to 900 K. This high temperature stability may result in the accretion of many Earth oceans to Earth and other terrestrial planets, provided that a viable mechanism to produce water from hydroxyl exists.

**References:** [1] Drake M. J. 2005. *Meteoritics & Planetary Science* 40:519–527. [2] Bethell T. and Bergin E. 2009. *Science* 326:1675–1677. [3] Stimpfl M., Walker A. M., Drake M. J., de Leeuw N. H., and Deymier P. 2006. *Journal of Crystal Growth* 294:83–95. [4] Muralidharan K., Deymier P., Stimpfl M., de Leeuw N. H., and Drake M. J. 2008. *Icarus* 198:400–407. [5] Muralidharan K. and Drake M. J., 2011. *Meteoritics & Planetary Science*. This conference.

<figure><img src="image_36.png" /><figcaption>Fig. 1. Schematic drawing of the ultra high vacuum chamber (UHV) and quadrupole mass spectrometer (QMS). Blue filled circles represent water molecules. Green is olivine crystal.</figcaption></figure>

## Q NOBLE GASES IN THE ORGUEIL Hf/HCl RESIDUE: A HIGH PRESSURE EXPERIMENT

A. B. Verchovsky¹, W. Montgomery² and M. A. Sephton². ¹Open University, Milton Keynes, UK. ²Earth Science and Engineering, Imperial College London, South Kensington, SW7 2AZ, UK.

**Introduction:** The origin of the planetary noble gases (PNG) carrier (Q) in meteorites remains enigmatic. All attempts to separate Q have been unsuccessful in spite of a number of characteristic distinctive features. It is carbonaceous and accounts for a small (approximately 5%) fraction of the total carbonl in meteorites [1]. Solvation experiments with pyridine [2] suggest that it has an organic structure that can be partially destroyed by the reagent, resulting in a partial loss of PNG. Q is destroyed by oxidizing reagents such as HNO₃, H₂O₂, HClO₄ and oxidized in pure oxygen atmosphere at low temperatures (400–600 °C). It is more readily transformed into diamond by shock metamorphism than the other organics [3]. On the other hand it is less susceptible to parent body metamorphism than most of the macromolecular material [4]. In the present study we decided to examine how Q reacts to high static pressures. For this purpose a HF/HCl residue from Orgueil has been taken to high pressure and modest temperature in a diamond-anvil cell (DAC). The treated and original samples then were analyzed for C, N, Ar and Xe and by micro-Raman.

**Experimental:** A small aliquot (approximately 10 ng) of the HF/HCl residue was loaded into a membrane-type DAC containing type II diamonds with 500 µm culets. A rhenium gasket with a 200 µm diameter sample chamber and a preindented thickness of approximately 50 µm depth was placed between the diamonds to contain the sample. The sample was compressed to 9.7 GPa and heated at 250–300 °C for 7 h, then allowed to cool. Pressure was monitored continuously using the ruby florescence method. [5]

C, N, and noble gases have been analyzed using mass spectrometric multi-element analyzer Finesse at the Open University. The gases were extracted in three combustion steps—300, 700, and 1000 °C. In the first step most of surface contamination has been removed; at 700 °C most of the gases from Q are released, and the last step is required just to confirm that no more gases can be released. Raman spectra have been obtained using the mapping method for areas approximately 20 × 20 µm with resolution of 1 µm.

**Results and Discussion:** The Raman spectra of the original and treated sample are similar. They show usual broad D and G bands characteristics of macromolecular material. No diamond feature is seen in the treated sample, which does not necessarily mean that diamond has not been formed under high pressure, but rather that it is small in size and has low abundance.

The noble gas results indicate that ³⁶Ar and ¹³²Xe have been lost during the high pressure expose: 70% of Ar and 40% of Xe. Since at 300 °C no PNG losses are expected from Q the result suggests that it is the pressure that resulted in the noble gas release.

**Conclusions:** Static high pressure affects the Q resulting in its structural transformation and loss of noble gases. We plan to treat the samples with variable pressure and more detailed investigations of the pressure-treated material in order to understand the observed effects.

**References:** [1] Lewis R. S. et al. 1975. *Science* 190:1251–1262. [2] Marrocchi Y. et al. 2005. *Earth and Planetary Science Letters* 236:569–578. [3] Gobel R. et al. 1978. *Journal of Geophysical Research* 83:855–867. [4] Verchovsky A. B. et al. 2002. *Earth and Planetary Science Letters* 199:243–255. [5] Mao H. K., Xu J., and Bell P. M. 1986. *Journal of Geophysical Research Solid Earth Planets* 91:4673–4676.
CONCORDANT EARLY SOLAR SYSTEM TIME SCALES FROM
Pb-Pb AND EXTINCT CHRONOMETERS

M. Wadhwa, A. Bouvier and G. A. Brennecka. ¹Arizona State
University, Tempe, AZ 85287, USA. E-mail: wadhwa@asu.edu.

In recent years, high-resolution chronometers based on long- and short-lived radionuclides have been applied extensively to toward determining the time scales, with sub-Ma precision, of events that occurred in the early solar system (ESS). However, these chronometers have often yielded discordant ages (e.g., [1, 2]). Our recent work focuses on high-precision Pb-Pb and Al-Mg systematics of several meteorites and their components (e.g., [1-4]). We also showed that $238\text{U}/235\text{U}$ in meteoritic materials can be variable [5, 6]. We recently presented a Pb-Pb age of 4568.2$^{+0.2}_{-0.4}$ Ma for a CAIs from the NWA 2364 CV3 meteorite [3]; although its U isotopic composition was not measured, the uncertainty in its reported Pb-Pb age incorporates possible deviation of its $238\text{U}/235\text{U}$ ratio from the previously assumed value (=$137.88$). We find that the Al-Mg ages using the NWA 2364 CAIs as the anchor agree with those obtained using D'Orbigny as the anchor (Fig. 1), implying that the relatively old age for this CAIs is robust and $26$Al was uniformly distributed in the ESS. These Al-Mg ages also agree with the Mn-Cr and Pb-Pb ages (Fig. 1). One exception is Asuka-881394, the Pb-Pb age of which is approximately 1 Ma older than its Al-Mg and Mn-Cr ages. Its Pb-Pb age was calculated assuming that its $238\text{U}/235\text{U}$ has the same value as that which we measured in other achondrites [4, 7], which may not be a valid assumption. Nevertheless, Fig. 1 shows that our recent high-precision chronological investigations yield consistent time scales for ESS events such as planetesimal differentiation.

**References:** [1] Wadhwa M. et al. 2007. Workshop on Chronology of Meteorites and the ESS. Abstract #4053. [2] Wadhwa M. et al. 2009. *Geochimica et Cosmochimica Acta* 73:5189-5201. [3] Bouvier A. and Wadhwa M. 2010. *Nature Geoscience* 3:637-641. [4] Bouvier A. et al. 2011. *Geochimica et Cosmochimica Acta* (submitted). [5] Bouvier A. et al. 2011. Goldschmidt Conference (submitted). [6] Brennecka G. et al. 2010. *Science* 327:449-451. [7] Brennecka G. and Wadhwa M. 2011. Goldschmidt Conference (submitted). [8] Sugiura N. and Yamaguchi A. 2007. Abstract #1431. 38th Lunar and Planetary Science Conference. [9] Shukolyukov A. and Lugmair G. 2008. Abstract #2094. 39th Lunar and Planetary Science Conference. [10] Shukolyukov A. et al. 2009. Abstract #1381. 40th Lunar and Planetary Science Conference. [11] McKibbin S. et al. 2010. Abstract #5172. 73rd Meteoritical Society Meeting.

<figure><img src="image_37.png" /><figcaption>Fig. 1. Pb-Pb ages (calculated using measured $238\text{U}/235\text{U}$ ratios for NWA 2976 and the angrites [4, 7], and inferred values for NWA 2364 CAIs and Asuka-881394) compared to Al-Mg and Mn-Cr ages. Pb-Pb and Al-Mg data from [2–7] and references therein; Mn-Cr data from [2, 8–11].</figcaption></figure>

A LASER PROBE $^{40}$Ar/$^{39}$Ar INVESTIGATION OF SHERGOTTITE NORTHWEST AFRICA 4797: IMPLICATIONS FOR SHOCK METAMORPHISM PRIOR TO EJECTION

E. L. Walton¹,², S. P. Kelley³, C. D. K. Herd², A. J. Irving⁴ and T. E. Bunch⁵. ¹Dept. Phys. Sci., Grant MacEwan University, Canada ²Dept. Earth & Atm. Sci., University of Alberta, Canada. ³Dept. Earth Sci., The Open University, UK. ⁴Dept. of Earth & Space Sci., University of Washington, USA. ⁵Dept. Geol., Northern Arizona University, USA. E-mail: ewalton@ualberta.ca.

**Introduction:** We report laser probe argon isotope analyses of "lherzolitic" shergottite Northwest Africa (NWA) 4797. The results are discussed in the context of deciphering shock history. NWA 4797 is texturally complex, with strong shock overprinting primary igneous features [1]. Shock effects in olivine and pyroxene include strong mosaicism, grain boundary melting, reduced birefringence, shock blackening, and pervasive fracturing. Maskelynite has not been observed in NWA 4797; rather, precursor igneous plagioclase has been completely melted, vesiculated and recrystallized by shock to form plagioclase microlites in glass. A vein (approximately 1 mm; 10.8 vol%) of quenched melt transects the sample, from which skeletal olivine (Fo$_{69-79}$), pyroxene (En$_{44-51}$) and chromite have crystallized in alkali-rich glass containing spheres of iron sulfide.

**Samples, Methods, and Results:** A total of 37 extractions were obtained on recrystallized plagioclase, the shock melt vein and igneous groundmass using a New Wave 193 nm eximer laser ablation system (approximately 70 µm diameter melt pits). This system prevents heating outside laser pits, precisely targeting plagioclase and the vein. Corrections for mass spectrometer discrimination, irradiation interferences and cosmogenic $^{36}$Ar concentrations have been applied to all argon data following extraction.

Groundmass. Extractions yielded a wide range of ages from 690 ± 30 Ma to several apparent ages >4.5 Ga. All groundmass analyses yielded ages older than the conventional range for shergottites (approximately 200–600), indicating the presence of excess argon.

Recrystallized Plagioclase. Several young ages were obtained, ranging from 50 ± 20 Ma to 150 ± 15 Ma. However, many of the plagioclase analyses were contaminated by groundmass, yielding ages up to 2760 ± 40 Ma. These older ages are correlated with the Ca/K ratio of the analysis.

Shock melt vein. A transect of laser pits crossing the melt vein showed on average lower $^{40}$Ar/$^{36}$Ar ratios within the vein in contrast to other shergottites. This is reflected in the mean age of the vein (800 Ma) compared with the groundmass (2600 Ma).

Cosmic Ray Exposure (CRE) ages. A CRE age of 2.2 ± 0.4 Ma was measured using spallogenic $^{36}$Ar and $^{38}$Ar production.

**Discussion:** The CRE age of 2.2 ± 0.4 Ma approximates the time of impact ejection. We interpret young ages from recrystallized plagioclase to indicate the timing of argon loss during an impact event on Mars. While the youngest age is significantly older than the CRE age, this may reflect partial loss during the impact event, but clearly the ages are younger than conventional crystallization ages for shergottites. The combined lack of a Martian atmospheric signature and significantly younger ages in recrystallized plagioclase is attributed to longer cooling times for this melt. Argon extractions from the shock melt vein show lower $^{40}$Ar/$^{36}$Ar ratios compared to shock melts in other shergottites. We favor a formation mechanism by injection rather than *in situ* pore collapse for the shock melt vein. These observations make NWA 4797 a unique shergottite worthy of further study.

**Reference:** [1] Walton E. L. et al. 2009. Abstract #1464. 40th Lunar and Planetary Science Conference.
STABLE ISOTOPES: A NONCARBONACEOUS PEDIGREE FOR UREILITES, IN COMMON WITH ALMOST ALL DIFFERENTIATED PLANETARY MATERIALS

Paul H. Warren. Institute of Geophysics, UCLA, Los Angeles, CA 90095-1567, USA; E-mail: pwarren@ucla.edu.

The abundant, diverse ureilite meteorites are peridotitic asteroidal mantle restites that have remarkably high bulk carbon contents (average 3 wt%) and have long been linked to the so-called carbonaceous chondrites. This term is “somewhat of a misnomer” [1], because the high petrologic type carbonaceous chondrites are, if anything, C-poor compared to ordinary chondrites. But it has long been customary to describe as “carbonaceous” a distinctive set of chondrites that show kinship in various ways without necessarily being carbon-rich. Here, the word carbonaceous is used in that strict, meteoritics-customary sense.

Ureilite oxygen isotopic compositions, i.e., diversely negative $\Delta^{17}O$, viewed in isolation, have long been viewed as confirming the carbonaceous-chondritic pedigree hypothesis. However, a very different picture emerges through analysis of a compilation of recently published high-precision isotopic data for Cr [2–8]; especially 8], Ti [9–10] and Ni [11–13] for ureilites and various other planetary materials (earlier publications by same first authors not cited). Ureilites have lower $\varepsilon^{62}\text{Ni}$ and far lower $\varepsilon^{50}\text{Ti}$ and $\varepsilon^{54}\text{Cr}$ than any known variety of carbonaceous chondrite. On a plot of $\varepsilon^{50}\text{Ti}$ versus $\varepsilon^{54}\text{Cr}$, and similarly $\Delta^{17}O$ versus $\varepsilon^{54}\text{Cr}$, ureilite compositions cluster far from and in a direction approximately orthogonal to a trend internal to the carbonaceous chondrites, and the carbonaceous chondrites are separated by a wide margin from all other planetary materials. Thus, notwithstanding the impressive resemblance to carbonaceous chondrites in terms of diversely negative $\Delta^{17}O$, the ureilite precursors accreted from preponderantly noncarbonaceous (sensu stricto) materials.

Despite total depletion of basaltic matter, ureilites retain moderate py (≡ pyroxene/[olivine + pyroxene]) ratios, averaging 30 vol%; a result expected from simple partial melting of moderate-SiO$_2$/(FeO + MgO) noncarbonaceous chondritic matter, but implying an additional process of major reduction of FeO if the precursor material were carbonaceous-chondritic. The ureilites' diversity in py, silicate-core mg and $\Delta^{17}O$ implies the parent asteroid accreted a diverse mélange of noncarbonaceous materials.

The striking bimodality of planetary materials on the $\varepsilon^{50}\text{Ti}$ versus $\varepsilon^{54}\text{Cr}$ and $\Delta^{17}O$ versus $\varepsilon^{54}\text{Cr}$ diagrams may be an extreme manifestation of the effects of episodic accretion of early solids in the protoplanetary nebula. However, an alternative, admittedly speculative, explanation is that the bimodality corresponds to a division between materials that originally accreted in the outer solar system (carbonaceous) and materials that accreted in the inner solar system (noncarbonaceous, including the ureilites).

**References:** [1] Krot A. N. et al. 2004. In *Meteorities, Comets, and Plants*, edited by Davis A. M. Treatise on Geochemistry, vol. 1. pp. 83–128. [2] Yamashita K. et al. 2005. *NIPR Symposium Antarctica Meteorite* 100. [3] Shukolyukov A. et al. 2011. Abstract #1527. 42nd Lunar and Planetary Science Conference. [4] Ueda T. et al. 2006. *NIPR Symposium Antarctica Meteorite* 117. [5] Trinquier A. et al. 2008. *Geochimica et Cosmochimica Acta* 72:5146. [6] de Leuw S. et al. 2010. Abstract #2703. 41st Lunar and Planetary Science Conference. [7] Qin L. et al. 2010. *Meteoritics & Planetary Science* 45:1771. [8] Yamakawa A. et al. 2010. *The Astrophysical Journal* 720:150. [9] Leya I. et al. 2008. *Earth and Planetary Science Letters* 266:233. [10] Trinquier A. et al. 2009. *Science* 324:374. [11] Regelous M. et al. 2008. *Earth and Planetary Science Letters* 272:330. [12] Dauphas N. et al. 2008. *The Astrophysical Journal* 686:560. [13] Quitte G. et al. 2010. *The Astrophysical Journal* 720:1215.

NORTHWEST AFRICA 6693, A NEAR-ULTRAMAFIC ACHONDRITE WITH LOW (0.6 × CHONDRITIC) Mg/Si, Na-FELDSPAR, Ni-RICH Fo50 OLIVINE AND LOW $\Delta^{17}O$

P. H. Warren¹, A. E. Rubin¹, J. Isa¹, E. Thompson², I. Ahn³ and B-G. Choi⁴. ¹Institute of Geophysics, UCLA, Los Angeles, CA, USA; E-mail: pwarren@ucla.edu ²Lake Oswego, OR, USA. ³Korea Polar Research Institute, Korea, Japan. ⁴Earth Science Education, Seoul National University, Korea, Japan.

The NWA 6693 meteorite was found as a single 5.1 kg stone. Much of its surface is weathered, but the many scattered metal grains in the interior are rust-free. This may be partly a consequence of the metal composition, which clusters tightly at (by weight) Ni$_{81}$Fe$_{19}$Co$_{0.7}$. A mode shows (in vol%) pyroxene 70, olivine 16, feldspar 13, Cr-spinel 0.6, metal 0.4, with traces of Ni-rich sulfides and merrillite. Pyroxene composition clusters tightly at En$_{57}$Wo$_{3.3}$ and FeO/MnO = 90; with scattered En$_{40}$Wo$_{40}$ lamellae rarely (where blebby) large enough for EPMA. Olivine clusters tightly at Fo$_{50}$, with 0.77 wt% NiO (approximately 10% of the bulk rock's Ni occurs as NiO in olivine). The feldspar is Ab$_{92}$Or$_{2.5}$, except for rare grains of Ab$_{57}$Or$_{42}$. The spinel composition (in wt%) is 57.8 Cr$_{2}$O$_{3}$, 32.0 FeO, approximately 2.1 Fe$_{2}$O$_{3}$, 2.8 TiO$_{2}$, 1.7 MgO.

The igneous (no relict chondrules) texture is hard to categorize. The dominant pyroxene includes an oikocryst 5 mm across, enclosing chadacrysts of both olivine and feldspar. However, most pyroxenes are nonpoikilitic and <2 mm. Most olivines are blocky and 0.5–1 mm; some grains have embayed shapes and a few grains exhibit a mantle of pyroxene. The feldspar appears interstitial, yet separated outcrops exhibit optical continuity over distances as great as 2.5 mm. In places the feldspar wraps around small mafic silicates; cf. “pincer” textures observed [1] in impact-melted but essentially undifferentiated chondrites.

The bulk oxygen isotopic composition, determined by replicate laser-fluorination at KOPRI, averages $\delta^{18}\text{O} = 4.32\text{‰}$, $\delta^{17}\text{O} = 1.19\text{‰}$ and $\Delta^{17}\text{O} = -1.08\text{‰}$. These data are in principle consistent with the acapulcoite-lodranite clan [2], but in those meteorites the mafic silicates are highly magnesian, and feldspars have lower Na/Ca. The ungrouped LEW 88763, with $\Delta^{17}\text{O} = -1.2\text{‰}$, [2, 3], is a closer precedent to NWA 6693, but LEW 88763 consists mainly of olivine, its mafic silicates have higher mg (e.g., Fo$_{65}$) and its feldspars range to Ab$_{55}$An$_{44}$. Several CH/CB chondrites are also similar in O-isotopes to NWA 6693 [e.g., 4].

The bulk composition is in most respects close to chondritic. Two approximately 360 mg chips were analyzed by INAA and FB-EPMA, representing (A) fresh interior and (B) material including some rusty brown alteration. Highly siderophile elements (Os, Ir and Au) and Ni are essentially chondritic in (A); in (B) they are 0.3 × chondritic, except Ni, 0.5×. The REE pattern of (A) is sloped from 0.8 × CI at La-Sm, to 0.4 × CI at Lu, with a (+) Eu anomaly (1.5 × CI). Ga/Al (wt) = 0.00016. The numerically greatest divergence from typical chondritic composition is shown by highly volatile elements; e.g., Zn in (A) is <0.02 × CI. The most significant divergence, however, is that the mafic silicates are so ferroan, bulk Mg/Si (0.48, by wt.) is only about 0.6 × chondritic; and no juggling of the olivine/pyroxene ratio (to model vagaries of crystal accumulation) can shift Mg/Si to >> 0.8 × chondritic. Thus, in contrast with “primitive achondrites,” Mg/Si indicates NWA 6693 is no restite. The origin of this unique material is enigmatic.

**References:** [1] Rubin A. E. and Jones R. H. 2003. *Meteoritics & Planetary Science* 38:1507–1520. [2] Greenwood R. C. et al. 2007. Abstract #2163. 38th Lunar Prospector Spacecraft. [3] Swindle T. D. et al. 1998. *Meteoritics & Planetary Science* 33:31–48. [4] Ivanova M. A. et al. 2008. *Meteoritics & Planetary Science* 43:915–940.
A TALE OF TWO CRATERS: (U-Th)/He DATING AND MODELING OF AGE RESETTING OF TWO SMALL/LOW ENERGY IMPACT STRUCTURES

J-A. Wartho¹, M. C. van Soest¹, I. Ukstins Peate², D. T. King Jr.³, L. W. Petruny³ and K. V. Hodges¹. ¹School of Earth and Space Exploration, Arizona State University, PO Box 871404, Tempe, AZ 85287, USA. ²Department of Geoscience, University of Iowa, Iowa City, IA 52242, USA. ³Department of Geology and Geography, Auburn University, Box 870338, Auburn, AL 36849, USA.

**Introduction:** (U-Th)/He single apatite and zircon crystal dating has been successfully undertaken on two small/low-energy impact structures: Wetumpka (a ~5 km diameter shallow-marine structure [1]) and Monturaqui (a 350 m diameter structure [2]). A range of ages was obtained, but the youngest cluster of apatite/zircon ages yielded impact ages of 84.4 ± 1.4 (2σ) for Wetumpka [3] and 663 ± 90 Ka (95% confidence) for Monturaqui [2]. These (U-Th)/He ages are in agreement with previous stratigraphic [1] and cosmogenic/palaeomagnetic ages [4-5].

However, the target rocks of these two small size/low energy craters experienced heterogeneous and relatively low shock metamorphism conditions of 5-15 GPa (Wetumpka [6]) and <10-65 GPa (Monturaqui [7]) that relate to temperatures of 100-150 °C and 100-1500 °C, respectively [8]. Therefore, how did the radiogenic ⁴He in ~10% of the apatite and zircon crystals we have analyzed become reset during these short-lived impact events?

Mathematical modeling of He diffusion in apatite and zircon has been undertaken to better understand how these grains can become reset during impact events, and how the (U-Th)/He technique can be successfully applied to the dating of small impact structures.

**Results:** Mathematical modeling of the % of He diffusion loss from apatite and zircon crystals, has shown that (a) short, but high temperature fireball [e.g., 9], (b) very localized short high shock metamorphism events [e.g., 10], (c) longer duration but lower temperature hydrothermal events [e.g., 11] or (d) a combination of all three processes (a-c) are all capable of resetting (U-Th)/He ages in apatite and zircon grains during the formation of low energy or small impact structures.

**Conclusions:** As the He diffusion modeling shows, the unique fast diffusion properties of He in U-bearing minerals (e.g., apatite and zircon) make the low-temperature (U-Th)/He geochromometer a powerful tool for dating impact structures, especially low energy and small impact structures.

**References:** [1] King D. T. Jr. et al. 2006. *Meteoritics & Planetary Science* 41:1625-1631. [2] Ukstins Peate I. et al. 2010. Abstract #2161. 41st Lunar and Planetary Science Conference. [3] Wartho J.-A. et al. 2011. Abstract #1524. 42nd Lunar and Planetary Science Conference. [4] Valenzuela M. 2008. *Jaresbericht Annual Report* 27. [5] Valenzuela M. 2008. Abstract #5185. 72nd Meteoritical Society. [6] Neathery T. L. et al. 1976. *Geological Society of America Bulletin* 87:567-573. [7] Bunch T. E. and Cassidy W. A. 1972. *Contributions to Mineralogy & Petrology* 36:95-112. [8] French, B. M. 1998. *Traces of Catastrophe*. Lunar and Planetary Institute. 120 p. [9] Collins G. S. et al. 2005. *Meteoritics & Planetary Science* 40:817-840. [10] Biren M. B. et al. 2010. Abstract #2314. 41st Lunar and Planetary Science Conference. [11] Jöeleht A. et al. 2005. *Meteoritics & Planetary Science* 40:21-33.

COOLING RATES OF IVA AND IIIB METEORITES

J. T. Wasson and P. Hoppe. ¹University of California, Los Angeles, CA, USA. E-mail: jtwasson@ucla.edu. ²Max-Planck Institut f. Chemie, Mainz, Germany.

**Introduction:** A major puzzle in iron meteorite research is why metallographic cooling rates (MCR) show wide ranges (factor of 60 in group IVA [1], factor of 6 in group IIIB [2]) within fractionally crystallized (magmatic) iron-meteorite groups, and why MCR correlate with compositions. Possible answers are that essentially all the insulating silicates had been stripped from the metallic cores [3] or that there are effects associated with composition or impact histories that are not properly treated in the conversion of phase-composition data to MCR [4]. In an attempt to better assess relative cooling rates within these groups we developed a new method based on Co/Ni ratios in kamacite and taenite; our preliminary results indicate comparatively little variation in cooling rates within IVA and IIIB.

**Method:** Our approach is based on the variation with temperature of the double ratio (Co/Ni)α/(Co/Ni)γ [5]. We call this ratio Rαγ; it ranges from approximately 3 at 1000 K to approximately 30 at 720 K. Thus, the lower the cooling rate, the higher the value of Rαγ.

**Experimental:** We used the MPIC NanoSIMS to determine Co in kamacite and taenite; the spatial resolution was about 0.5 µm and the sampling depth <0.1 µm. By measuring the ratio near the interface we focused on the low-temperature record and minimized the effects of factors that may have affected the systems at high temperatures in the α + γ field. Our pilot study consisted of IVA irons Bishop Canyon and Duchesne with MCR differing by a factor of 25 and IIIB irons Haig and Cumpas with MCR differing by a factor of 4.5.

The IVA irons have similar Rαγ values of 22-23 implying that they cooled at rates that differ by less than a factor of 3 (2σ). The Rαγ in IIIB Cumpas was similar (approximately 21), but that in Haig was approximately 29 indicating that it cooled several times slower, in contrast to the Haig MCR which is 4.5 × higher than that for Cumpas [2].

**Discussion:** To help assess the reason for the contrasting IIIB cooling rates we reexamined the MCR results [2]; to help eliminate possible high-temperature effects we limited the comparison to narrow taenite lamellae (halfwidths between 4 and 10 µm). We discovered that the Haig data distribution is anomalous and Bella Roca partly anomalous and that the central Ni contents in the other 11 irons were unresolvable; irons differing in MCR by a factor of 6 showed the same central Ni contents implying identical cooling rates at low temperatures. We infer that the anomalous behavior of Haig in both studies reflects shock effects.

**Summary:** Our preliminary study using a new cooling rate method indicates no difference in cooling rates between two IVA irons with MCR differing by a factor of 25. Our results imply that the Haig IIIB iron cooled more slowly than Cumpas, in contrast to MCR data showing Haig to have cooled 4x faster. We suggest that this reflects shock damage in Haig that created tiny fractures in the taenite. These results cast doubt on published cooling rates showing large variations in groups IVA and IIIB.

**References:** [1] Yang J. et al. 2008. *Geochimica et Cosmochimica Acta* 72:3043. [2] Yang J. and Goldstein J. 2006. *Geochimica et Cosmochimica Acta* 70:3197. [3] Yang J. et al. 2007. *Nature* 446:888. [4] Wasson J., Matsunami Y., and Rubin A. 2006. *Geochimica et Cosmochimica Acta* 70:3149. [5] Widge S. and Goldstein J. 1977. *Metallurgical and Materials Transactions A* 8:309.
## ANALYSES OF TWO MARTIAN METEORITES FOR THE RAMAN LASER SPECTROMETER ONBOARD EXO MARS

I. Weber¹, U. Böttger², E.K. Jessberger¹ and H. W. Hübers²,³. ¹Institute of Planetology, Muenster, Germany. E-mail: sonderm@uni-muenster.de.
²Department of Experimental Planetary Physics, Institute of Planetary Research, German Aerospace Center, Berlin, Germany. ³Institut für Optik und Atomare Physik, Technische Universität Berlin, Berlin, Germany.

**Introduction:** Exo Mars is the first mission in ESA's Aurora Exploration Programme, which will start 2018. The Raman Laser Spectrometer (RLS) onboard Exo Mars will perform measurements on Mars to identify mineral products and organic compounds. In preparation for the data from Mars we investigate Martian meteorites as well as analogue materials with a Raman instrument working under "real" conditions. For a first complete mineralogical identification the material is investigated by light microscopy and SEM. Afterwards Raman analyses at room temperature and in Martian-like conditions were made. Here we present the results of these measurements on two Martian meteorites.

**Measurements and Experimental Setup:** The Raman measurements are performed with a confocal Raman microscope Witec alpha300 R system at room temperature under air at ambient pressure. The Raman laser excitation wavelength is 532 nm. The spectral resolution of the spectrometer is 4–5 cm⁻¹. A Nikon 10 × objective is used. The spot size on the sample in focus is about 1.5 µm. The laser power of 1 mW on the sample is chosen to represent the RLS instrument on Exo Mars.

**Samples:** The two Martian meteorites DaG 670 [1] and SAU 051 [2] are basaltic shergottites consisting of pyroxenes, maskelynite and relict plagioclase as well as of olivines. They were chosen for the investigations because the basaltic shergottites are the most likely candidate rocks to be found on the surface on Mars.

**Results:** Sections of both meteorites were scanned with light microscopy and eight regions of interest each are selected for detailed SEM and Raman investigations. The areas are marked with an 100 × 100 µm "L" using laser ablation. All regions were mapped with a JEOL JSM6610LV SEM working in low-vacuum to avoid artefacts in the Raman spectra from carbon coating. The Raman measurements are in progress and the results will be presented.

**References:** [1] Wadhwa M. et al. 2001. *Meteoritics & Planetary Science* 36:195–208. [2] Gnos E. et al. 2002. *Meteoritics & Planetary Science* 37:835–854.

## MAJOR-ELEMENT ABUNDANCES OF SURFACE MATERIALS ON MERCURY: FIRST RESULTS FROM THE MESSENGER X-RAY SPECTROMETER

S. Z. Weider¹, L. R. Nittler¹, R. D. Starr², W. V. Boynton³, L. G. Evans⁴, J. O. Goldsten⁵, T. J. McCoy⁶, R. L. McNutt Jr.⁵, C. E. Schlemm II⁵, S. C. Solomon¹ and A. L. Sprague³. ¹Carnegie Institution of Washington, Washington, DC 20015, USA. E-mail: sweider@ciw.edu.
²Catholic University, Washington, DC 20064, USA. ³University of Arizona, Tucson, AZ 85721, USA. ⁴Computer Sciences Corporation, Lanham-Seabrook, MD 20706, USA. ⁵Johns Hopkins University, Applied Physics Laboratory, Laurel, MD 20723, USA. ⁶Smithsonian Institution, Washington, DC 20013, USA.

**Introduction:** MESSENGER became the first spacecraft to orbit Mercury on 18 March 2011. One of the mission's major scientific objectives is to characterize the chemical composition of Mercury's surface in order to address key questions on the formation and geologic history of the least-explored, innermost planet [1]. The X-Ray Spectrometer (XRS) on MESSENGER [2] employs the technique of planetary X-ray fluorescence (XRF) to estimate quantitatively the abundances of major rock-forming elements (e.g., Mg, Al, Si, S, Ca, Ti, and Fe) on Mercury's surface. The instrument measures characteristic X-rays that are excited by the incident (and variable) flux of solar X-rays from within the top approximately 100 µm of the regolith.

**Preliminary Results:** Preliminary analysis of XRS data acquired during solar flare events indicates that Mercury's crust is substantially richer in Mg and poorer in Al and Ca than both lunar crustal rocks and terrestrial basalts. The derived Mg/Si, Al/Si, and Ca/Si ratios lie within the range of terrestrial ultramafic rocks. Analysis of "quiet Sun" data gives Mg, Al, and Si abundances consistent with the flare data. The flare spectra also indicate low total concentrations of Ti (<1 wt %) and Fe (<approximately 5 wt%) and a surprisingly high S abundance (S/Si approximately 0.1) in surface materials on Mercury.

**Discussion:** In order to explain Mercury's high density and metal/silicate ratio [3], it has been suggested that a primary crust and much of the original mantle may have been stripped away, e.g., by a giant impact event [4, 5]. The Mg/Si, Al/Si, and Ca/Si data reported here indicate a relatively low abundance of plagioclase feldspar, perhaps consistent with this model. However, models [e.g., 6] whereby Mercury originally formed from highly reduced (e.g., enstatite chondrite) material may also explain our elemental data. Melting of such precursory material produces basaltic and metal-rich (Fe, Ni-FeS) melts, leaving an ultramafic (mainly enstatite) residuum. Melting experiments [6] on material from the EH-chondrite Indarch have been used to suggest a "synthetic Mercurian basalt" composition whose Mg/Si, Al/Si, Ca/Si, and S/Si ratios lie within the range seen by MESSENGER, although even the relatively small amount of Fe seen is higher than predicted by the melting experiments. The relatively low apparent Fe and Ti abundances reported here indicate that these elements are not responsible for the thermal neutron absorption observed during MESSENGER's Mercury flybys [7].

**References:** [1] Solomon S. C. et al. 2001. *Planetary and Space Science* 49:1445–1465. [2] Schlemm C. E. et al. 2007. *Space Science Reviews* 131:393–415. [3] Lewis J. S. 1972. *Earth and Planetary Science Letters* 15:286–290. [4] Wetherill G. W. 1988. In: *Mercury*, University Arizona Press. pp. 670–691. [5] Benz W. et al. 1988. *Icarus* 74:516–528. [6] Burbine T. H. et al. 2002. *Meteoritics & Planetary Science* 37:1233–1244. [7] Lawrence, D. J. et al. 2010. *Icarus* 209:195–209.
## COSMOGENIC RADIONUCLIDES IN ANTARCTIC HOWARDITES: TERRESTRIAL AGES AND PAIRING

K. C. Welten¹, K. Nishiizumi¹ and M. W. Caffee². ¹Space Sciences Laboratory, University of California, Berkeley, CA 94720, USA. E-mail: kwelten@ssl.berkeley.edu. ²Department of Physics, Purdue University, West Lafayette, IN 47907, USA.

**Introduction:** In anticipation of the upcoming visit of the Dawn spacecraft to asteroid 4 Vesta, there is renewed interest in howardites, which are believed to dominate the surface of 4 Vesta. The Antarctic meteorite collection contains more than 100 howardites, including several showers, such as the Pecora Escarpment (PCA) 02009 pairing group [1, 2]. The identification of paired howardites is important to understand their heterogeneity on a cm to m scale. We have previously shown that cosmogenic radionuclides can help to identify paired HED meteorites, even when the individual samples show a wide range in composition [2]. In this work we present concentrations of cosmogenic ¹⁰Be (half-life = 1.36 Myr), ²⁶Al (0.705 Myr), and ³⁶Cl (0.301 Myr) in 24 howardites and 3 brecciated eucrites from the Antarctic meteorite collection.

**Experimental Procedures:** We dissolved howardite samples of 50–150 mg in HF/HNO₃. After dissolution, small aliquots of the dissolved samples were taken for chemical analysis. We separated Be, Al, and Cl and measured the radionuclide concentrations by accelerator mass spectrometry at Purdue’s PRIME Lab.

**Results and Discussion:** Chemical analysis of the samples shows a wide range in bulk compositions with Al ranging from 1 to 7 wt%, Ca from 2 to 8 wt%, and Mg from 4 to 15 wt%. The bulk composition of Mt. Pratt (PRA) 04401 is consistent with a large contribution of CM-type material [3,4].

Concentrations of cosmogenic ¹⁰Be (15–25 dpm kg⁻¹) and ²⁶Al (52–109 dpm kg⁻¹) indicate that all howardites were exposed in space long enough (> 5 Myr) to reach > 90% of ¹⁰Be saturation level. The variations in ¹⁰Be and ²⁶Al are mainly due to differences in shielding, although the high ²⁶Al concentration (109 dpm kg⁻¹) in MET 96500 may be due to production of ²⁶Al by solar cosmic rays. Concentrations of cosmogenic ³⁶Cl range from 4 to 12 dpm kg⁻¹, with the exception of PRA 04401, with shows a value of 24 dpm kg⁻¹. The high ³⁶Cl concentration in PRA indicates a large contribution from neutron-capture ³⁶Cl, most likely in the Cl-containing CM clasts. The remaining variations in ³⁶Cl are due to a combination of differences in (i) the concentration of Ca, a major target element for the production of ³⁶Cl, (ii) shielding effects as a function of size and depth, and (iii) the terrestrial age. Using the relationship between the ³⁶Cl production rate from Ca and the ²⁶Al production rate [5], we derive terrestrial ages up to ~250 kyr for the Antarctic howardites. Based on the radionuclide results, we identified one pairing group at Miller Range (MIL) and one at Queen Alexandra Range (QUE), the latter including both howardites and diogenites. The radionuclide results can also be used to constrain the CRE histories of howardites when combined with recent cosmogenic noble gas results [6].

**Acknowledgments:** We thank the Meteorite Working Group for providing the meteorite samples.

**References:** [1] Beck A. et al. 2007. *Meteoritics & Planetary Science* 42:A17. [2] Welten K. et al. 2009. *Meteoritics & Planetary Science* 44:A216. [3] Mittlefehldt D. et al. 2011. Abstract #2569. 42nd Lunar and Planetary Science Conference. [4] Herrin J. et al. 2011. Abstract #2806. 42nd Lunar and Planetary Science Conference. [5] Leya I. and Masarik J. 2009. *Meteoritics & Planetary Science* 44:1061–1086. [6] Cartwright J. et al. 2011. Abstract #2655. 42nd Lunar and Planetary Science Conference.

## REVERSIBLE SAMPLE PREPARATION OF SMALL PARTICLES IN POTTED BUTTS FOR SIMS ANALYSES

A. J. Westphal¹, A. Butterworth¹, Z. Gainsforth¹, G. R. Huss², K. Nakashima², R. C. Ogliore² and J. Stodolna¹. ¹Space Sciences Laboratory, U. C. Berkeley, Berkeley, CA 94720, USA. E-mail: westphal@ssl.berkeley.edu. ²Hawai‘i Institute of Geophysics and Planetology, University of Hawaii at Mānoa, 1680 East-West Road, POST 504, Honolulu, HI 96822, USA.

**Introduction:** With the return of Stardust samples the cosmochemistry community faced the novel challenge of conducting coordinated analyses on a suite of extraterrestrial samples that are simultaneously extremely tiny (nanogram to picogram), extremely limited, and extremely complex and fine-grained. This has required the development of novel techniques [1]. Current techniques generally make efficient use of the samples [2], but isotopic analyses have required a sample preparation technique that requires contact with the sample and modification of the potted butt. Here we describe an easily reversible sample preparation technique for SIMS measurements that facilitates multiple rounds of TEM, STXM and isotopic analyses, thus making full use of rare particles. We anticipate that this technique will be appropriate for Hayabusa samples from 25143 Itokawa, which share the same challenging properties as Stardust samples.

**Silicon Nitride Window Mounting Protocols for SIMS:** The most common sample-preparation protocol for Stardust samples is to embed particles in epoxy and to use an ultramicrotome to produce multiple 100 nm-thick sections for TEM or STXM, and a potted butt that usually contains the remaining 70–90% of the original particle. Further rounds of ultramicrotomy can be done on the potted butt if it is preserved as a cylindrical “bullet.” To achieve sufficiently high precision to place samples on the solar-system three-isotope oxygen plot, measurements must be made in the bulk grain in the potted butt. We developed a reversible SIMS sample preparation technique as part of our coordinated analysis of the particle Iris [3–5]. X-ray fluorescence microprobe analysis showed that Iris is a complex particle with several phases not uncovered in the first rounds of ultramicrotomy. This technique involves no physical contact with the sample or modification of the potted butt. The mount consists of two components: a Cameca ims 1280-compatible Al round which holds the potted butt at a fixed altitude, and a “buckler,” which simultaneously surrounds the particle with a flat conductive plane, matching the altitude of the particle, and integrates a ring of 12 standards that closely match the known phases in the particle and that surround the particle. The buckler consists of a thin Al disk with embedded standards, polished to a mirror finish, with a central 3mm hole. We mounted a Au-coated, 500 nm-thick Si₃N₄ membrane in which we had previously drilled a 300 µm diameter hole using an ion mill, over the hole in Al disk. We used a motorized, encoded micromanipulator with 200 nm resolution (Sutter MP-285) to integrate the complete assembly, since ~1-µm precision is required in the three-dimensional alignment of the Si₃N₄ window with the particle. Using San Carlos olivine standards in potted butts, we have shown that O-isotopes are measured with ~2‰ precision (2σ) in Δ¹⁷O using this technique [5].

**References:** [1] Westphal A. J. et al. 2004. *Meteoritics & Planetary Science* 39:1375. [2] Zolensky M. et al. 2000. *Meteoritics & Planetary Science* 35:9. [3] Gainsforth Z. et al. 2011. *Meteoritics & Planetary Science* 46. This conference. [4] Ogliore R. et al. 2011. *Meteoritics & Planetary Science* 46. This conference. [5] Huss G. R. et al. 2011 *Meteoritics & Planetary Science* 46. This conference.
5434

## ANALYSIS OF TITANIUM ISOTOPE RATIOS IN REFRACTORY INCLUSIONS BY LA-MC-ICP-MS

C. D. Williams¹, M. Wadhwa¹, P. E. Janney¹, R. R. Hines¹, E. S. Bullock² and G. J. MacPherson². ¹School of Earth and Space Exploration, Arizona State University, Tempe, AZ, USA. E-mail: Curtis.Williams@asu.edu. ²US National Museum of Natural History, Smithsonian Institution, Washington, DC, USA.

**Introduction:** Calcium–aluminum-rich inclusions (CAIs) with fractionation and unknown nuclear (FUN) effects preserve a record of large isotopic heterogeneities at small spatial scales in the early solar system (ESS). Constraining the origin of these isotopic heterogeneities and their temporal and spatial distribution is critical for understanding stellar inputs into the solar nebula and the conditions that prevailed during the earliest epoch of Solar System formation. Thus far, no absolute dates have been determined for any FUN CAIs and there is little to no material remaining from previously studied ones for chronological investigations. As such, it is important to locate new FUN CAIs so that their distinctive isotopic signatures can be placed in temporal context in the ESS.

The FUN CAIs preserve large anomalies in several neutron-rich isotopes including $^{50}$Ti (e.g., [1, 2]). Therefore, the titanium isotope system is well suited for the identification of these objects. Furthermore, laser ablation multicollector inductively coupled plasma mass spectrometry (LA-MC-ICP-MS) allows precise, yet rapid analysis of Ti isotopic compositions, making this technique suitable for analyzing a large number of CAIs to search for ones with FUN effects. Here we report preliminary results of titanium isotope ratio measurements of two coarse-grained Allende CAIs by LA-MC-ICP-MS.

**Analytical Technique:** Titanium isotope ratios were measured with the Thermo Finnigan Neptune MC-ICP-MS at ASU. The isotope ratios were measured in two peak jumping steps: $^{44}$Ca, $^{46}$Ti, $^{47}$Ti, $^{48}$Ti, $^{49}$Ti, $^{50}$Ti (step 1) and $^{49}$Ti, $^{51}$V, $^{53}$Cr (step 2). All analyses were made in medium-resolution with count times of 8.4 s in step 1 and 4.2 s in step 2. Each run consisted of 20 integrations, resulting in a approximately 5 min analysis time. Gas blanks were measured at least four times per analytical session. In situ analyses of minerals in polished sections were made using a Photon Machines Analyte 193 excimer laser ablation system connected to the MC-ICP-MS using He as the carrier gas. Typical laser operating conditions included a 150 µm spot size and 4 Hz repeat rate at full power (7 mJ per pulse). Instrumental mass bias was corrected using the sample-standard bracketing technique (with a San Carlos augite megacryst as the bracketing standard) and internal normalization to a $^{49}$Ti/$^{47}$Ti ratio of 0.749766 [3]. Typical $^{48}$Ti intensity measured on the San Carlos augite (approximately 1.5 wt% TiO₂) was $1.6 \times 10^{-11}$ A.

**Results:** Several runs (10–15) were made on titanium-rich phases (predominantly fassaite) in each of two coarse-grained Allende CAIs (19B65 and CPM2-1). The average values for the Ti isotopic ratios and 2SD uncertainties for each CAIs were as follows: For 19B65, $\varepsilon^{46}\text{Ti}/^{47}\text{Ti} = 4.38 \pm 6.68$, $\varepsilon^{48}\text{Ti}/^{47}\text{Ti} = 0.98 \pm 1.96$ and $\varepsilon^{50}\text{Ti}/^{47}\text{Ti} = 10.50 \pm 4.35$; For CPM2-1, $\varepsilon^{46}\text{Ti}/^{47}\text{Ti} = 6.22 \pm 4.52$, $\varepsilon^{48}\text{Ti}/^{47}\text{Ti} = 2.87 \pm 2.58$ and $\varepsilon^{50}\text{Ti}/^{47}\text{Ti} = 0.67 \pm 7.74$. These values are within the reported range for normal, non-FUN CAIs (e.g., [1–3]). Analyses of additional CAIs are currently in progress.

**References:** [1] Niederer F. R. et al. 1980. *The Astrophysical Journal* 240:L73–L77. [2] Niemeyer S. and Lugmair G. W. 1981. *Earth and Planetary Science Letters* 53:211–225. [3] Niederer F. R. et al. 1985. *Geochimica et Cosmochimica Acta* 49:835–851.

5253

## POTENTIAL RELATIONSHIP BETWEEN TITANIUM AND ZIRCONIUM ISOTOPE HETEROGENEITIES WITHIN SOLAR SYSTEM MATERIALS

N. H. Williams, W. M. Akram and M. Schönbächler. SEAES, The University of Manchester, M13 9PL, UK. E-mail: Niel.Williams-2@postgrad.manchester.ac.uk.

**Introduction:** Isotopic heterogeneities in the neutron-rich isotopes $^{96}$Zr and $^{50}$Ti between bulk carbonaceous chondrites and the Earth–Moon system are now well established [1–3]. Previous work indicated that $^{96}$Zr and $^{50}$Ti anomalies in calcium–aluminum rich inclusions (CAI) might be correlated [4, 5]. This raises the question, whether Ti and Zr anomalies in bulk solar system materials are linked to the same carrier phase (i.e., CAI), which was heterogeneously distributed within the solar system. A potential $^{96}$Zr–$^{50}$Ti correlation may help to identify (i) a common nucleosynthetic site, in which these anomalies were generated and/or (ii) the processes experienced by the carrier phase(s) that allowed the specific nucleosynthetic signature to be preserved. A recent study revealed a linear relationship between $^{46}$Ti and $^{50}$Ti isotope anomalies within CAIs, bulk meteorites and the Earth, when normalizing to a fixed $^{49}$Ti/$^{47}$Ti ratio [3]. The isotopes $^{46}$Ti and $^{50}$Ti have a different nucleosynthetic production site [6], which indicates that most likely processes within the solar system lead to the observed $^{46}$Ti–$^{50}$Ti correlation.

In order to better constrain the potential relationship of Zr and Ti isotope anomalies in our solar system, we will obtain high precision Ti isotope analyses on a wide variety of solar system materials, which were previously analyzed for Zr isotopes [1].

**Analytical Techniques:** Titanium is separated via a two-step ion exchange procedure previously described in [7, 8]. The Zr data is reported elsewhere [1]. Samples are further purified using an additional separation step involving AG50-X8 cation resin. The resin is cleaned with 4 M HNO₃, then preconditioned and the samples are loaded onto the column in 0.25 M H₂SO₄ – 1% H₂O₂. Titanium is separated from Ca, Cr and V and other matrix elements in 4 M HNO₃.

All five Ti isotopes are analyzed simultaneously on a HR Nu plasma MC-ICPMS at high resolution. The samples are bracketed by an Alfa Aesar Ti standard solution. Instrumental mass fractionation is corrected using the exponential law relative to $^{49}$Ti/$^{47}$Ti. Calcium, V and Cr corrections are applied to account for isobaric interferences on $^{48}$Ti (Ca) and $^{50}$Ti (V and Cr). Samples are introduced in 0.5 M HNO₃ – 0.005 M HF mixtures by means of desolvating nebulizer.

The samples to be analyzed include solar system material from various heliocentric distances: the geostandards BHVO2 and Cody Shale, four carbonaceous chondrites (Allende, Murchison, Colony, and Orgueil), two ordinary chondrites (Richardton and Forest Vale), two eucrites (Pasamonte and Sioux County), CAIs from Allende and lunar basalts.

**References:** [1] Akram W. M. et al. 2011. Abstract #1908. 42nd Lunar and Planetary Science Conference. [2] Leya I. et al. 2008. *Earth and Planetary Science Letters* 266:233–244. [3] Trinquier A. et al. 2009. *Science* 324:374–376. [4] Leya I. et al. 2009. *The Astrophysical Journal* 702:1118–1126. [5] Harper C. L. et al. 1991. Abstract #1257. 22nd Lunar and Planetary Science Conference. [6] Clayton D. 2003. *Handbook of Isotopes in the Cosmos: Hydrogen to Gallium*. Cambridge University Press. [7] Schönbächler M. et al. 2004. *Analyst* 129:32–37. [8] Leya I. et al. 2007. *International Journal of Mass Spectrometry* 262:247–255.
5187

THE SPACE ACADEMY: A SUCCESSFUL STEM PARTNERSHIP

R. C. Wilson¹, J. J. Remedios¹, M. Barstow², S. Roberts³, T. Moore³, A. Ojha⁴ and S. Bull⁵. ¹EOS, Department of Physics and Astronomy, University of Leicester, Leicester, UK. E-mail: rcw15@le.ac.uk. ²College of Science and Engineering, University of Leicester, Leicester, UK. ³IESSG, Nottingham Geospatial Building, University of Nottingham, Nottingham, UK. ⁴Space Academy, National Space Center, Leicester, UK. ⁵Science Learning Center East Midlands, School of Education, University of Leicester, Leicester, UK.

**Introduction:** Launched in October 2008, the Space Academy is a collaboration between the University of Leicester, the University of Nottingham, the National Space Center, the Science Learning Center East Midlands, STEMNET and the East Midlands Development Agency. The project has an Earth/planetary research scientist at each university, dedicated full-time staff and outstanding teachers seconded as lead educators to the project. The Space Academy offers curriculum focused full-day programmes in biology, chemistry, physics, astronomy, earth sciences and applied sciences for students from Key Stages 3 to 5, tailor-made to the curriculum focus of the participating school. In addition, the Space Academy hosts an annual teacher CPD conference and INSET sessions for science educators.

**Methodology:** A suite of materials using space sciences and climate change as the context for learning and engagement in KS3-5 curriculum themes have been developed in collaboration with internationally recognized experts in space science, education and industry. Researchers and teachers work together to ensure that masterclass content is curriculum-focussed and include current discoveries and scientific information. The program is delivered by both research scientists experienced in public engagement and teachers (seconded from their schools) that are recognized as outstanding educators. An example Space Academy program includes an A Level/Cambridge Pre-U masterclass that uses "an away day to Mars" as the context for learning topics such as Newton's law and forces, gravitational potential and field strength, EM spectrum and spectroscopy through the investigation of physical properties/environment of Mars and its habitability. Similarly, a GCSE/A Level chemistry masterclass uses "Comets: harbingers of doom or agents of life?" as the context for learning low-temperature physics, simulating a comet nucleus, kinetics and mass spectrometry.

**Evaluation:** The Space Academy methodology has received international recognition by the science education community, enabling the project to contribute on an international-scale including (1) hosting the largest session at the NASA Space Exploration Educators Conference, (2) leading a session at the ESA human space-flight teachers workshop and (3) leading a session at the European Space Camp at the Andoya Rocket Range in Northern Norway. The Space Academy masterclasses and teacher CPD sessions have been externally evaluated by both the University of Leicester and an OFSTED inspector in terms of lasting student understanding and teacher effectiveness. There is evidence that the Space Academy program increases attainment in national examinations.

**Acknowledgments:** We acknowledge funding from the East Midlands Development Agency for this project 2008–2011.

5222

NORTHWEST AFRICA 5131: ANOTHER TAFASSASSET-LIKE METACHONDRITE RELATED TO THE CR CHONDRITE PARENT BODY

J. H. Wittke¹, T. E. Bunch¹,⁵, A. J. Irving²,⁵, D. Rumble III³ and P. P. Sipiera⁴,⁵. ¹Geology Program, SESES, Northern Arizona University, Flagstaff, AZ 86011, USA, E-mail: james.wittke@nau.edu. ²Earth & Space Sciences, University of Washington, Seattle, WA, USA. ³Geophysical Laboratory, Carnegie Institution, Washington, DC, USA. ⁴Field Museum of Natural History, Chicago, IL, USA. ⁵Planetary Studies Foundation, Galena, IL, USA.

**Petrography:** NWA 5131 is a 533 g achondrite previously thought to have affinities with LL chondrites (based largely on its olivine composition); however, new oxygen isotopic data clearly indicate that it is instead related to CR chondrites, and is in many ways quite similar to Tafassasset. The specimen is an inequigranular metamorphic aggregate of olivine (62 vol%, Fa₂₉.₃, FeO/MnO = 68), orthopyroxene (28 vol%, Fs₂₄.₇Wo₂.₁), and metal (5 vol%) with accessory clinopyroxene, intermediate plagioclase, chromite (cr# 82) and troilite. Finer grained portions consist of polygonal grains meeting at 120° junctions, but there also are regions with poikiloblastic texture. Orthopyroxene poikiloblasts up to 4 mm across enclose smaller chadacysts of olivine and metal. No chondrules can be distinguished, but the poikiloblastic texture may represent former chondrule regions that have been highly recrystallized.

**Oxygen Isotopes:** Results obtained by laser fluorination of acid-washed material (δ¹⁸O = 2.12, 2.10; δ¹⁷O = 0.42, -0.45; Δ¹⁷O = -1.536, -1.555‰, respectively) plot well below the TFL along the trend for CR chondrites.

**Discussion:** NWA 5131 provides additional supporting evidence for the existence of at least some regions of extensive thermal metamorphism within the CR chondrite parent body. Although the vast majority of CR chondrites are hydrated and of petrologic type 2, there are two known highly equilibrated CR6 chondrites (NWA 2994, NWA 3100) [1]. Tafassasset (like NWA 5131) is best termed a CR metachondrite, since it is completely lacking in chondrules. As we have already argued [1, 2], such thermally metamorphosed specimens, as well as the paired, very ancient igneous-textured achondrites NWA 011/2400/2976/4587/ 4901, with very similar oxygen isotopic compositions [3], imply much more complexity within the CR parent body than originally envisioned. Like the CV-CK chondrite parent body, it may well have a metallic core surrounded by a metamorphic mantle (perhaps containing plutonic igneous bodies) and a chondrule-rich regolithic crust. It remains to be seen whether NWA 5131 also exhibits the fractionated, nonchondritic compositional characteristics found in Tafassasset, NWA 2994 and NWA 3100 [1]. Hypotheses about the size and/or thermal history of the CR chondrite parent body may need to be reconsidered in light of discoveries of highly equilibrated specimens like NWA 5131.

**References:** [1] Bunch T. et al. 2008. Abstract #1991. 39th Lunar and Planetary Science Conference. [2] Bunch T. et al. 2005. Abstract #2308. 36th Lunar and Planetary Science Conference. [3] Yamaguchi A. et al. 2002. *Science* 296:334–336. [4] Floss C. et al. 2005. *Meteoritics & Planetary Science* 40:343–360. [5] Spivak-Birndorf L. et al. 2009. *72nd Met. Soc Mtg.*, #5390; Bouvier A. and Wadhwa M. 2010. Abstract #489. 41st Lunar and Planetary Science Conference.
A NOMENCLATURE SYSTEM FOR DIOGENITES CONSISTENT
WITH THE IUGS SYSTEM FOR NAMING TERRESTRIAL
ULTRAMAFIC ROCKS

J. H. Wittke¹, A. J. Irving², T. E. Bunch¹ and S. M. Kuehner². ¹Geology Program, SESES, Northern Arizona University, Flagstaff, AZ 86011, USA. E-mail: james.wittke@nau.edu. ²Earth & Space Sciences, University of Washington Seattle, WA 98195, USA.

**Background:** Traditionally diogenites have been considered to be extraterrestrial ultramafic rocks dominated by orthopyroxene and having igneous (presumed to be cumulate) textures. Many diogenites have been shocked, and exhibit cataclastic (crushed) textures, although few are true breccias (requiring rotation of clasts). With the discovery of many more specimens in both hot and cold deserts, the range of orthopyroxene compositions found in diogenites has expanded considerably [e.g., 1], and now spans a range from Fs₁₃ to Fs₃₄. Furthermore the modal mineralogy of diogenitic rocks now includes not only chromite (and minor Ni-poor metal) but variable amounts of olivine and calcic plagioclase (yet very little clinopyroxene).

Recently it has been suggested [2] that the name diogenite be expanded to include rocks containing > 40 vol% olivine (previously termed olivine diogenites by us [3]), and even to related rocks composed of more than 90vol% olivine (of which there are at least five known examples [4]).

**A Logical Nomenclature System:** The new terms *harzburgitic diogenite* and *dunitic diogenite* are very reasonable descriptors for those examples with 40-90vol% olivine and ≥90vol% olivine, respectively. However, in keeping with the established IUGS nomenclature for terrestrial ultramafic rocks [5], we suggest that the additional terms *olivine-orthopyroxenitic diogenite* and *noritic diogenite* be used to describe the few specimens with 10-40vol% olivine and with > 10 vol% plagioclase, respectively. The diagram below is modified from Figure 2.6 of [5], and on it are plotted the modes for 21 Northwest African and Arabian Peninsula diogenitic specimens that we have studied in detail. Note that diogenites can contain up to 10vol% plagioclase before becoming noritic, and such examples could be termed plagioclase-bearing diogenites (like their terrestrial peridotite counterparts).

**References:** [1] Bunch T. et al. 2007. *70th Meteorite Society Mtg., #5133*. [2] Beck A. and McSween H. 2010. *Meteoritics & Planetary Science* 45:850-872. [3] Irving A. et al. 2005. *Lunar and Planetary Science Abstract #2188*. [4] Bunch T. et al. 2010. *73rd Meteorite Society Mtg., #5315*. [5] LeMaitre et al. 2002. *Igneous Rocks: A Classification and glossary of terms*, 2nd ed. Cambridge University Press.

<figure><img src="image_38.png" /></figure>

ORGUEIL LOW-DENSITY PRESOLAR CARBON AIN'T GRAPHITE
BUT GLASSY CARBON

B. Wopenka¹, E. Groopman² and E. Zinner². ¹Earth and Planetary Sciences and ²Physics Department, Washington University, St. Louis, MO 63130, USA. E-mail: bwopenka@levee.wustl.edu.

Presolar carbonaceous grains from Orgueil traditionally have been called “graphite” [1]. Graphite is a crystalline solid with a well defined 3-D structure. A recent Raman study [2] of grains from the Orgueil high-density fraction OR1f (2.02–2.04 g cm⁻¹) confirmed that the majority of them (86%) were indeed graphitic. However, this is not the case for grains from Orgueil’s low-density separate OR1d (1.75–1.92 g cm⁻³). The majority of low-density carbon grains (11 of 20) that were analyzed by Raman spectroscopy followed by NanoSIMS analysis [3] are not graphite, but rather glassy carbon, an amorphous solid. Only 3 of 20 grains were graphitic, and the remainder (6 of 20) had a “kerogen-type” Raman signature, i.e., they also were non-crystalline sp²-bonded carbon. The physical properties of glassy carbon are well known to materials scientists. It has a unique structure with a random combination of small-sized (nanocrystalline) basal planes and edge planes with many large void spaces in between. The latter explains its low density (listed as 1.42 g cm⁻³) compared to the one for graphite (listed as 2.09–2.23 g cm⁻³).

Based on their first and second-order Raman spectra, the sp²-bonded carbon materials in Orgueil grains can be grouped into three different types [see also 2]. (1) Graphitic grains with more or less structural disorder have very narrow 1st- order peaks (D and G) and very strong second-order peaks; their D/G intensity ratios can be used to infer the graphitic in-plane crystallite size; (2) Allende-type grains have wider first-order peaks than graphitic grains, but with a distinctive feature: their D band is more intense than their G band; in addition they have a very weak (or no) second-order spectrum; and (3) kerogen-type grains have extremely wide first-order and no second-order peaks. From the Raman spectroscopic perspective, only grains of type 1 deserve to be called “graphite,” whereas grains of types 2 and 3 are amorphous. The very characteristic and unusual Raman signature of Allende-type grains is typical for glassy carbon and is not known to occur in any other form of sp²-bonded carbon. Thus, we propose that the carbon in both Allende and the majority of the low-density presolar carbon grains in Orgueil are a form of glassy carbon, rather than crystalline graphite. This finding not only explains the low density of this so-called “graphite” separate, but it is also consistent with the hypothesis, based on isotopic data [4], that grains from the low- and high-density fractions come from different stellar sources. The presolar graphite probably comes from AGB stars (and dominates the high-density separate), whereas the presolar glassy carbon comes from supernovae (and dominates the low-density separate). Indeed, all grains with Allende-type (i.e., glassy carbon) spectra of this study have isotopic signatures (such as ¹⁴N, ¹⁸O, ²⁸Si excesses and high ²⁶Al/²⁷Al ratios) indicative of a supernovae origin [3]. One possible explanation for the presence of glass (rather than crystals) in carbon material from supernovae could be the relatively rapid cooling of their ejecta.

**References:** [1] Jadhav M. et al. 2006. *New Astronomical Review* 50:591–595. [2] Wopenka B. et al. 2011. Abstract #1162. 42nd Lunar and Planetary Science Conference. [3] Groopman E. et al. This conference. [4] Zinner E. et al. 2006. *Proceeding of Science* (NIC-IX) 019.
## INVESTIGATING CARBONATE SURVIVAL IN STardust ALUMINUM FOILS

P. J. Wozniakiewicz¹, H. A. Ishii¹, A. T. Kearsley², M. J. Burchell³, J. P. Bradley¹, N. Teslich¹ and M. J. Cole³. ¹Institute of Geophysics and Planetary Physics, Lawrence Livermore National Laboratory (LLNL), 7000 East Avenue, Livermore, CA 94550, USA (wozniakiewicz1@llnl.gov). ²Impacts & Astromaterials Research Centre (IARC), Department of Mineralogy, Natural History Museum (NHM), London SW7 5BD, UK. ³Centre for Astrophysics and Planetary Science, School of Physical Sciences, University of Kent, Canterbury, Kent CT2 7NH, UK.

**Introduction:** Hydrous meteorites and interplanetary dust particles commonly contain carbonates believed to result from parent body aqueous alteration. Such alteration requires the prolonged presence of liquid water, and comets, generally believed to have remained frozen since their accretion, were not expected to contain these minerals. Intriguingly, the presence of carbonates in cometary nuclei was inferred from VEGA 1 and 2 mass spectrometry data obtained from comet 1P/Halley [1] and Spitzer space telescope spectra obtained from comet 9P/Temple 1 [2]. Recently, carbonates were positively identified in samples of comet 81P/Wild 2 collected in Stardust aerogel [e.g., 3, 4]. However, the authors recognized concerns over aerogel premission purity and postmission contamination. Confirmation of cometary carbonates may be provided by analyses of Stardust Al foils. However, collection by impact at 6 km s⁻¹ likely caused alteration requiring impact analog investigations to assess the effects.

**Method:** Powdered calcite was fired in light gas gun shots at flight spare Al foils under Stardust encounter conditions. Scanning electron microscopy with energy dispersive X-ray analysis (SEM EDX) was used to locate regions of residue. Focused ion beam (FIB) microscopy was used to prepare sections from preimpact projectile and postimpact crater residues for analysis and comparison by transmission electron microscopy (TEM).

**Results and Discussion:** TEM analyses show calcite residues in craters > 50 µm are a mixture of crystalline and amorphous material. Crystalline materials exhibit close to preimpact projectile compositions and polycrystalline structure. This material is interpreted as surviving impactor. Amorphous material is vesiculated and contains Al suggesting a melt origin and loss of volatiles. Although calcite decomposes releasing CO₂ at high pressures [e.g., 5], the amorphous residue chemistry is depleted in C but not O relative to the original projectile, possibly as a result of incorporating and oxidizing Al as observed in previous studies of phyllosilicate residues [6]. We conclude that where surviving material is not found, carbonates may be identified by their amorphous vesiculated residues which retain original cation to O ratios but have incorporated Al. The Al foils therefore represent a valuable sample set for investigation of cometary carbonates.

**Acknowledgments:** We thank NASA for providing Al foils, and STFC for support of the LGG. Parts of this work were performed under the auspices of U.S. DOE by LLNL under Contract DE-AC52-07NA27344. This work was supported by grants: NASA NNH07AG46I to HAI & LDRD 09-ERI-004 to JPB.

**References:** [1] M. N. Fomenkov et al. 1992. *Science* 258:266. [2] C. M. Lisse et al. 2006. *Science* 313:635. [3] S. Wirick et al. 2007. *Abstract* #1534. 38th Lunar Prospector Spacecraft. [4] G. J. Flynn et al. 2008. *Abstract* #1979. 39th Lunar Prospector Spacecraft. [5] Lange M. A. and Ahrens T. J. 1983. 14th Lunar Prospector Spacecraft p. 419. [6] Wozniakiewicz et al. *Abstract* #2357. 39th Lunar Prospector Spacecraft.

## KWAJALEIN ATOLL: A NEW COLLECTION SITE FOR MICROMETEORITES

P. J. Wozniakiewicz¹, J. P. Bradley¹, M. E. Zolensky², D. E. Brownlee³ and H. A. Ishii¹. ¹Institute of Geophysics & Planetary Physics, Lawrence Livermore National Laboratory (LLNL), 7000 East Avenue, Livermore, CA 94550, USA (wozniakiewic1@llnl.gov). ²NASA Johnson Space Center, Houston, TX 77058, USA. ³Department of Astronomy, University of Washington, Seattle, WA 98195, USA.

**Introduction:** Although micrometeorites are constantly arriving at the Earth's surface, they are quickly hidden amongst vast amounts of natural and anthropogenic terrestrial dust. Using magnets, metal-bearing micrometeorites have been successfully separated from deepsea sediments [e.g., 1, 2] and dissolved prehistoric (preindustry) limestones [e.g., 3] and salts [e.g., 4]. However, these samples do not fully represent the incident population of micrometeorites. More complete collections are obtained where the terrestrial dust flux is so low that micrometeorites represent the major dust component and additional separation techniques are not required, for example, from the upper atmosphere by high altitude planes [e.g., 5] or from polar ice, snow and well water [e.g., 6-8]. These samples are proving invaluable to understanding interplanetary dust. However, atmospheric collections are captured in silicone oil requiring removal by solvents, which may be problematic to some analyses (e.g., organics), and polar samples have been exposed to water ice [9].

We are evaluating collection of micrometeorites on Kwajalein atoll in the Pacific Ocean, more than 1000 miles from the nearest continent. From mid-December to mid-May, trade winds blow continuously from the northeast at 15-20 knots providing a continuous stream of ocean air to sample. The main sources of nonextraterrestrial dust are sea spray and coral, both of which are easily distinguishable from micrometeorites. We report the findings of our preliminary background samples.

**Method:** A high volume air sampler was installed at Kwajalein High School and fitted with a polycarbonate membrane filter with 5 µm perforations. The flow rate was set to sample approximately 100 m³ of air per hour. Filters were returned to LLNL for survey by optical microscopy. A selection of micrometeorite candidates were picked and analyzed by scanning electron microscopy.

**Findings and Future Work:** The sampler was intended to run for 7 days, but was stopped after only three due to clogging of the filters by salt. Despite the short exposure the filter contains "particles of interest" in addition to background aerosol and analysis results will be presented. NASA funding has been awarded to purchase, modify and install a new air sampler at Kwajalein airport and to design and implement several alternative collectors. These plans will also be presented.

**Acknowledgments:** We thank S. Yakuma, B. Barnett, D. Dearborn and T. Jones (LLNL) for use of their samplers and help throughout the project and Paula Fluhrer and her students at Kwajalein High School for running the experiment. Parts of this work were performed under the auspices of U.S. DOE by LLNL under Contract DE-AC52-07NA27344.

**References:** [1] Fredriksson K. 1956. *Nature* 177:32. [2] Pettersson H. and Fredriksson K. 1958. *Pacific Science* 12:71. [3] Dredge et al. 2010. *Scottish Journal of Geology* 46:7. [4] Davidson J. et al. 2007. *Abstract* #1545. 38th Lunar Prospector Spacecraft. [5] Brownlee et al. 1977. Proceedings, 8th Lunar Science Conference. p. 149. [6] Maurette et al. 1991. *Nature* 351:44. [7] Duprat et al. 2007. *Advances in Space Research* 39:605. [8] Taylor et al. 1998. *Nature* 392:899. [9] Bradley et al. 2011. *Abstract* #1320. 42nd Lunar Prospector Spacecraft.
TAKING PLANETARY SCIENCE AND ASTRONOMY TO
STUDENTS IN THE MIDDLE OF THE PACIFIC OCEAN

P. J. Wozniakiewicz¹, J. P. Bradley¹, M. E. Zolensky², D. E. Brownlee³,
H. A. Ishii¹. ¹Institute of Geophysics and Planetary Physics, Lawrence
Livermore National Laboratory (LLNL), 7000 East Avenue, Livermore,
CA 94550, USA (wozniakiewic1@llnl.gov). ²NASA Johnson Space
Center, Houston, TX 77058, USA. ³Dept. of Astronomy, University of
Washington, Seattle, WA 98195, USA.

**Introduction:** Kwajalein Island is the southernmost island in the Kwajalein atoll, one of the Micronesian nation of atolls and islands in the middle of the Pacific Ocean. The remote location means that approximately 300 children living on the island generally miss out on some learning opportunities in science taken for granted at most mainland schools; e.g., take your children to work days, talks and presentations by scientific visitors, and opportunities to get involved in current research. Such experiences can be the inspiration behind successful scientific and medical careers and without them potential scientists may not be aware of all career paths available to them. Working with Kwajalein Junior/Senior High School science teacher, Paula Fluhrer, we have initiated an outreach project that aims to provide the students at Kwajalein High School with inclass and extracurricular opportunities for hands-on participation in astronomy and planetary science.

**The Outreach Project:** For this outreach project we have identified two key tasks. First, we are introducing students to the field of planetary science and astronomy through a series of presentations and after school activities. The presentations are written to compliment their curriculum-based classes in Chemistry, Physics and Earth Sciences and are to be given by visiting scientists from LLNL and JSC in November 2011. The private donation of a Celestron Ultima 2000 telescope has allowed us to initiate an after school astronomy club to inspire potential astronomers and planetary scientists of tomorrow. Second, we are giving students the opportunity to gain some hands-on research experience through participation in the research collaboration between LLNL, JSC and Univ. of Washington that was recently awarded NASA funding to evaluate the collection of micrometeorites on Kwajalein [1]. Kwajalein is an ideal location to test micrometeorite collection because it is >1000 miles from the nearest continent, and the primary sources of nonextraterrestrial dust are sea salt and coral, both of which are easily distinguished from micrometeorites. Also, Kwajalein micrometeorites will not have been subjected to ice, snow, well water or silicone oil and solvents like other collections. Students will participate by running the air sampler to collect dust and by performing some searches by optical microscope (to be donated) to identify micrometeorite candidates. Follow-up analyses will be carried out at LLNL, and data collected on particles identified by students and its relevance will be presented to the students to keep them involved and excited about their role in the unfolding science.

**Acknowledgments:** We thank S. Yakuma, B. Barnett, D. Dearborn, and T. Jones (LLNL) for use of their samplers and help throughout the project. We thank Paula Fluhrer, a dedicated teacher at Kwajalein High School, for her enthusiastic collaboration in this project, and we thank her students for running the experiment. Parts of this work were performed under the auspices of U.S. DOE by LLNL under Contract DE-AC52-07NA27344.

**Reference:** [1] Wozniakiewicz P. J. et al. Abstract #5206. This conference.

ASTEROIDAL ATMOSPHERES: A NEW SUBJECT AREA

I. P. Wright, S. J. Barber, S. Sheridan, G. H. Morgan, D. J. Andrews, C. T. Pillinger and A. D. Morse. Planetary & Space Sciences, Open University, UK.

**Introduction:** A few years ago the Moon was widely thought to be a dry, barren place. Today, with results from both spacecraft observations [e.g., 1] and laboratory analyses [e.g., 2], the regolith of the Moon is considered to contain a number of different forms of water at various levels of concentrations (up to % levels). Indeed, it is now commonplace to talk about a lunar hydrosphere and its associated water cycle. One component of this arises from a substance that is seen to come and go [3], known as “space dew” [4], which is most likely to be the result of an interaction between the solar wind (H) and the lunar surface (O). It is a short step to postulate that an equivalent effect would also be associated with the surfaces of asteroidal bodies. And, by extension, these would also have atmospheres (perhaps more accurately considered as exospheres, although if cycling of dew at the surface of the Moon is a reality then perhaps “exosphere” is semantically incorrect). This would be on top of the large volumes of water that are already considered to exist within the upper layers of large bodies like Ceres, and as prominent surface deposits on asteroids such as 24 Themis.

**Lutetia:** On July 10, 2010, the Rosetta spacecraft (ESA's mission to rendezvous with, and land on, a cometary nucleus) flew past the asteroid 21 Lutetia. This is by far the largest such body thus far visited by a space mission and some spectacular images were obtained [5]. Amongst the investigations carried out during the flyby was a campaign mounted by various gas analysis instruments to search for evidence of an atmosphere around the asteroid. Included in this was the Ptolemy instrument, which is located on the Lander (Philae) element of the mission [6].

**Discussion:** Although there is not space here to describe the results from all of the Rosetta instruments it is clear that at closest approach (ca. 15,000 km, at the sub-solar point) the gas analysis instruments detected increases in signals from volatiles such as H₂O and CO₂. The question is: were these the result of the spacecraft traversing an asteroidal atmosphere, or were they somehow a result of spacecraft outgassing during the various manoeuvers that were necessary for tracking purposes during the flyby? At face value the results from Ptolemy would suggest asteroidal outgassing rates of about 5 × 10³ kg s⁻¹ (i.e., considering the entire surface of the body). In principle this is the sort of level that would be expected from an active cometary nucleus and clearly seems unlikely. However, if Rosetta had flown through a collimated outgassing feature, the globally normalized rates would appear to be much lower. Now, it could be argued that such a coincidence is unlikely, but we should point out that some asteroids, like 596 Sheila, are known to have tails (whilst certain comets have “tails” that point towards the Sun). Furthermore, recent results regarding the presence of water on the Moon should act to remind us of how quickly and completely a paradigm can change.

**References:** [1] Colaprete A. et al. 2010. *Science* 330:463–468. [2] Greenwood J. P. et al. 2011. *Nature Geoscience* 4:79–82. [3] Pieters C. M. et al. 2009. *Science* 326:568–572. [4] McCord T. B. et al. 2011. *OH/H₂O* in the Lunar Soil discovered by M3: Possible Space Dew? *Journal of Geophysical Research*, in review. [5] See, for instance http://www.esa.int/esaMI/Rosetta/ [6] Andrews D. J. et al. 2011. Ptolemy operations and results during the Lutetia flyby, *Planetary and Space Science*, submitted.
## SHOCK METAMORPHISM OF FRESH AND ALTERED BASALT PROTOLITHS FROM LONAR CRATER, INDIA: WHY ARE SNCS ALWAYS “FRESH” AND CLASS 2?

S.P. Wright. Institute of Meteoritics, University of New Mexico, Albuquerque, NM 87131, USA. E-mail: spwright@unm.edu.

**Introduction:** The study of shock metamorphism on basaltic minerals [1–3] is of much interest given that our only ~55 samples of Mars are shocked basalts. Identifying the source craters/regions of SNC meteorites [4] is also of interest as we have no sample return and hence no geologic context to compare to the increasing amount of remote data being returned from orbiters. This is a quandary as all ~40 shergottites have been shocked ~25–45 GPa, yet all remote data is of pixels of primarily unshocked basalt. Further, the finding of alteration minerals on Mars [5–6] has implications for water in the geologic history of Mars, and thus upcoming missions such as MSL [7]. Deccan basalt has been called an excellent analog for Surface Type 1 (basalt) found in the equatorial regions of Mars [8]. Comparisons to and implications for shergottites and data from Mars orbiters and Rovers are noted. For this study, from a previous classification [9], Class 1 (<20 PGa), Class 2 (20–40 GPa), Class 3 (40–60 GPa), Class 4 (60–80 GPa), and Class 5 (>80 GPa) shocked basalt, or “impact melts” of both fresh and altered basalt, along with one soil protolith, have been found, and petrographic and BSE images will be shown.

**Deccan and Lonar Background:** Prior to impact, Deccan basalts primarily composed of labradorite and augite erupted ~65 Ma. Aqueous alteration that we attribute to groundwater in the lower 3 of 6 flows resulted in chlorite, serpentine, celadonite, hematite, zeolite, and silica being deposited in the basalts. Impact ~570 ka [10] produced impactites from a wide range of shock pressures [9] of both fresh and altered protoliths.

**Petrography and BSE Images:** Class 2 shocked basalt show intense shattering and fracturing of clinopyroxene grains, and labradorite has been converted to maskelynite [2, 9]. These data are comparable to the petrography of shergottites [e.g., 11]. Petrographic images of impact melts exhibit schlieren and flow features similar to lechatelierite from Meteor Crater and other terrestrial impact melts. Classes 3 and 4 show flowing and vesiculated plagioclase glass, respectively (NOT maskelynite). New finds related to Mars alteration include 1.) melt glass “encasing” the alteration minerals listed above to preserve their pre-impact (groundwater) alteration now accessible as breccia clasts in the ejecta, and 2.) only Class 2 rocks with altered protoliths acquire “decompression cracks”, as there was no melting to accommodate the change in volume as seen in higher classes.

**Implications:** We receive a biased sampling as only fresh, Class 2 shocked basalts are ejected off Mars. The melting of higher shock classes and/or the “decompression cracking” of altered basalts must decrease the particle velocity term in the Huguenot equations, and hence the Martian escape velocity of 5.0 km s⁻¹ is not reached. These materials likely remain on Mars to be found by future rovers.

**References:** [1] Stöffler. 1971. *Journal Of Geophysical Research* 76:5541–5551. [2] Stöffler. 1972. *Fortschritte der Mineralogie* 49:50–113. [3] Stöffler. 1974. *Fortschritte der Mineralogie* 51:256–289. [4] Hamilton et al. 2003. *Meteoritics & Planetary Science* 38:871–885. [5] Ehlmann et al. 2010. *Geophysical Research Letters* [6] Glotch et al. 2010. *Geophysical Research Letters*. [7] Mahaffey. 2008 *ISSI* 25:255–268. [8] Bandfield et al. 2000. *Science* 287:1626–1630. [9] Kieffer. et al. 1976. 7th Lunar and Planetary Science Conference. 1391–1412. [10] Jourdan et al. 2011. *Geology*, in press. [11] McCoy et al. 1999. *Geochemistry Geophysics Geosystems* 6:1249–1262.

## INDICATORS FOR AN IMPACT DIRECTION IN THE CENTRAL PIT OF AN UNNAMED MARTIAN CRATER

G. Wulf, M. H. Poelchau and T. Kenkmann. ¹Institute of Geosciences – Geology, Albert-Ludwigs-University Freiburg, Germany. E-mail: gerwin.wulf@geologie.uni-freiburg.de.

**Introduction:** The ejecta blanket is the most distinctive indicator for the impact direction in oblique impacts, showing the formation of “forbidden” zones and “butterfly” patterns [1, 2]. The position of the central uplift relative to the crater center was proposed as another diagnostic feature [3], but could not be verified statistically [4] and might be caused by heterogeneous target structures [5]. A further promising indicator for obliquity is provided by the internal structure of central uplifts. Some terrestrial craters show a preferred stacking of layered bedrock in the central uplift [6, 7] with bedding strike perpendicular to the long axis of the crater ellipse [7]. Further analysis of Martian craters [8, 9] provide evidence that preferred strike orientation in the central uplift could be indicative for an impact direction. Here we present new results of the mapping of an unnamed central pit crater that confirm preferred strike orientation within central structures as a possible indicator for an impact direction.

**Methods:** An unnamed central pit crater south of Valles Marineris (15.8° S 63.7° W, 54.5 km crater rim diameter, 13 km pit diameter) was mapped with emphasis on structural trends of the exposed layered units in the central pit. We used high-resolution image data (HRSC, CTX, HiRISE) to determine the apparent strike values and additionally used an available HiRISE DTM of the western part of the central pit to get true strike and dip values using the Laytools extension for ArcGIS [10].

**Results:** The central pit crater can be characterized as an oblique impact crater using THEMIS infrared images showing a “forbidden” zone of the ejecta blanket which indicates an impact direction from 290–300°. Apparent and true strike orientation shows a very distinct preferred trend of 30–40° over the majority of the central pit. Strata predominantly dip WNW.

**Discussion:** An oblique impact and the impact direction from 290–300° could clearly be determined based on the ejecta pattern. The consistent 30–40° strike and westward dip of the analyzed stack of layers surrounding the central pit indicate material motion and shortening along 300–310° and a shear component top-to-the-SSE. This direction shows only a minute deviation from the impact direction inferred from the ejecta blanket. Shortening and stacking in the central crater floor is explained by the remnant horizontal momentum transferred from the impacting projectile to the target during the oblique impact [7]. The yet unsolved processes of central pit formation will also be considered and current models of pit formation will be cross-checked with the constraints given by the structural data gathered in this work. For instance, the orientation of central pit strata appear to be unaffected by the pit formation.

**References:** [1] Gault D. E. and Wedekind J. A. 1978. Proceedings, 9th Lunar and Planetary Science Conference. pp. 3843–3875. [2] Anderson J. L. B. et al. 2004. *Meteoritics & Planetary Science* 39:303–320. [3] Schultz P. H. and Anderson R. R. 1996. *Geological Society of American Special Paper* 302:397–417. [4] Ekholm G. A. and Melosh H. J. 2001. *Geophysical Research Letters* 28:623–626. [5] Collins G. S. et al. 2008. *Earth and Planetary Science Letters* 270:221–230. [6] Scherler D. et al. 2006. *Earth and Planetary Science Letters* 248:43–53. [7] Kenkmann T. and Poelchau M. H. 2009. *Geology* 37:459–462. [8] Wulf G. et al. 2011. Abstract #1440. 41st Lunar and Planetary Science Conference. [9] Poelchau M. H. et al. 2009. Abstract #1796. 40th Lunar and Planetary Science Conference. [10] Kneissl T. et al. 2010. Abstract #1640. 41st Lunar and Planetary Science Conference.
5007

SHOCK-PRODUCED COESITE IN THE XIUYAN CRATER, CHINA

W Xiao, M. Chen and X. Xie. Guangzhou Inst. Geochem., Chinese Academy of Sciences, Guangzhou 510640, China. E-mail: wsxiao@gig.ac.cn.

**Introduction:** The Xiuyan crater is located in the Liaodong Peninsula of northeastern China. It is a simple bowl-shaped crater with a diameter of 1.8 km. The crystalline basement around the crater is made up of Proterozoic metamorphic rock complex composed of granulite, hornblendite, gneisses tremolite marble, and marble [1]. A drilling positioning at the center of the crater reveals a crater-fill breccia unit of 188 m in thickness lain under lacustrine sediments [2]. Below the breccia layers are fractured bedrock of tremolite marble or still the breccia units at the crater bottom. A layer of impact breccia composed of clasts of granulite, gneiss, hornblendite and fragments of glass as well as elastic matrix, occurs at the lower part of the breccia units in the depth interval from 260 to 295 m. We have examined abundant coesite from the impact breccia.

**Results and Discussion:** Clasts of gneiss in the impact breccia have been investigated in petrology and mineralogy. The gneiss is composed of quartz, feldspar and small amount of amphibole. Feldspar was completely molten and transform to vesicular glass. Quartz in gneiss was transformed into silica glass. Amphibole was mostly transformed to iron-magnesium oxides, which commonly occur as irregular patches surrounding silica glasses. Coesite occurs within silica glass and commonly appears as polycrystalline aggregates of stringers and irregular patches, as well as granular, needle-like and dendritic crystals. The composition of coesite is identical to the silica glass. The occurrence of coesite is indicative of crystallization from a silica liquid [3]. The silica glass contains coesite up to 80% by volume. Raman spectrum of coesite displays the strongest peak at 521 cm⁻¹ and other weak peaks at 1146, 1038, 815, 785, 468, 427, 356, 326, 269 and 205 cm⁻¹, and all these peaks can be attributed to coesite [4]. The coesite associating with silica glass is considered to be a diagnostic feature for shock metamorphism of target rocks [5]. The occurrence of coesite provides a unambiguous evidence for an impact origin of the Xiuyan crater.

**Acknowledgments:** This research was supported by CAS (Kzcx2-yw-140), NNSF of China (40872041) and President Foundation of CAS.

**References:** [1] Chen M. 2008. *Chinese Science Bulletin* 53:392-395. [2] Chen M. et al. 2010. *Chinese Science Bulletin* 55:1777-1781. [3] Chen M. et al. 2010. *Earth and Planetary Science Letters* 297:306-314. [4] Boyer H. et al. 1985. *Physics and Chemistry of Minerals*. 12:45-48. [5] Chao E. C. T. et al. 1960. *Science* 132:220-222.

<figure><img src="image_39.png" /></figure>

5387

LA-ICP-MS STUDY OF METALLIC Fe-Ni FROM THREE MESOSIDERITES NEWLY FOUND IN GROVE MOUNTAINS, ANTARCTICA

L. Xu<sup>1,2</sup>, Y. Lin<sup>2</sup>, Y. Zhang<sup>2</sup>, C. Li<sup>1</sup>. <sup>1</sup>National Astronomical Observatories, Chinese Academy of Sciences, Beijing, China. E-mail: xul@nao.cas.cn. <sup>2</sup>Key Laboratory of the Earth's Deep Interior, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing, China.

**Introduction:** Mesosiderites consist of subequal amounts of metal and basaltic-pyroxenitic silicates. Previous analyses by neutron activation show possible genetic links of the metal to IIIBA iron meteorites, but with distinct differences in detail [1, 2]. In this study, we analysed in situ highly siderophile elements (HSE) and other trace elements in metallic Fe-Ni from three mesosiderites found in Grove Mountains, Antarctica, in order to clarify genetic relationship of the metallic Fe-Ni with other groups of meteorites.

**Results and Discussion:** Of the three mesosiderites, GRV 055364 is mild weathered. The silicate portion shows coarse-grained textures, and olivine clasts have coronas decorated by fine-grained chromite. GRV 050212 and GRV 021553 are significant altered, and highly brecciated. Their silicate portions are very heterogeneous and commonly zoned. Exsolution of low-Ca and augite are well developed, especially in GRV 050212.

HSE (Re, Os, Ir, Ru, Pt, Rh, Pd and Au), Ni, Co and other trace elements (W, Mo, As, Cu, Sb, Ga, Ge) of metal clasts from these meteorites were analyzed in situ by LA-ICP-MS. Metal grains from GRV 055364 show a wide range of compositions. There seems a correlation between the concentrations and abundances patterns of HSE. As the concentrations of HSE decrease, their CI-, Ni-normalized abundance patterns vary from refractory-rich (Re/Ni: 3.7 × CI) to flat patterns, and to slightly refractory-depleted (Re/Ni: 0.28 × CI). All of them have negative Pd-anomalies, and are depleted in the moderately volatile elements, including As, Cu, Sb, Ga and Ge. Analyses of GRV 021553 exhibit a relatively narrow range of compositions, with the HSE patterns similar to the refractory grains from GRV 055364. Those from GRV 050212 are the most HSE-enriched, and six out of nine analyses have Ni-normalized abundances of Re of 3.9-5.0 × CI. The most HSE-enriched grains show a steep slope of the abundance patterns toward Re.

Except for the HSE-depleted grains from GRV 055364, the HSE patterns of all other analyses from the three meteorites are very similar to those of metallic Fe-Ni from equilibrated H-group chondrites [3, 4]. Our analyses confirm a close genetic relationship of metallic Fe-Ni between mesosiderites and H-group chondrites. In contrast, HSE-rich IIIBA irons show no negative Pd-anomaly with lower Re concentrations relative to other HSE [5], different from those of metal from the mesosiderites. The wide variation of HSE of metal Fe-Ni from GRV 055364 is probably due to fractional crystallization of molten metal.

**Acknowledgments:** This work was supported by the Knowledge Innovation Program of Chinese Academy of Sciences (KZCX2-YW-Q08) and NSF of China (41073053).

**References:** [1] Hassanzadeh J. et al. 1990. *Geochimica et Cosmochimica Acta* 54:3197-3208. [2] Wasson J. T. et al. 1974. *Geochimica et Cosmochimica Acta* 38:135-149. [3] Horan M. F. et al. 2003. *Chemical Geology* 196:27-42. [4] Fischer-Goedde M. et al. 2010. *Geochimica et Cosmochimica Acta* 74:356-379. [5] Mullane E. et al. 2004. *Chemical Geology* 208:5-28.
## HAYABUSA SAMPLE CURATION AT PLANETARY MATERIAL SAMPLE CURATION FACILITY IN JAXA

T. Yada¹,², A. Fujimura¹,², M. Abe²,¹, T. Nakamura⁴, T. Noguchi⁵, R. Okazaki⁶, T. Okada²,¹, Y. Ishibashi¹, K. Shirai², M.E. Zolensky⁷, S. Sandford⁸, M. Uesugi¹, Y. Karouji¹, M. Ueno²,¹, T. Mukai³, M. Yoshikawa²,¹, and J. Kawaguchi¹,². ¹JAXA's Space Exploration Center. ²Institute Space Astronautical Science. ³Japan Aerospace Exploration Agency, Sagamihara, Kanagawa 252-5210, JAPAN, E-mail: yada@planeta.sci.isas.jaxa.jp. ⁴Department Earth Science, Graduate School Science, Tohoku University, Sendai, Miyagi 980-8578 JAPAN. ⁵College Sci., Ibaraki University, Mito, Ibaraki 310-8512, JAPAN. ⁶Department Earth Planet. Science, Graduate School Science, Kyushu University, Fukuoka 812-8581, JAPAN. ⁷NASA Johnson Space Center, Mail Code KT, Houston, TX 77058, USA. ⁸NASA Ames Research Center, M/S 245-6, Moffett Field, CA 94035-1000, USA.

**Introduction:** A Japanese asteroid explorer Hayabusa has successfully returned its reentry capsule on June 13th, 2010. As detailed previously [1], a series of processes have been held at the Planetary Material Sample Curation Facility in JAXA. The sample container was extracted from the reentry capsule and then introduced into clean chambers to be opened in there. After opening, the sample catcher, which consists mainly of two rooms (Room-A and -B) and a rotational cylinder, was extracted from the container. Then the particles inside the catcher were recovered with three different sampling methods by (1) direct pickup from the catcher, (2) Teflon spatula scooping and (3) compulsory fall onto a silica glass disk, followed by analyses by FESEM-EDS. We eventually concluded that the rocky particles must have originated from S-type Asteroid 25143 Itokawa based on their mineralogy and elemental ratios relative to remote sensing results of the asteroid surface [2].

**Sample Curation for Initial Analyses:** There are more than a thousand particles of >10 µm in size recognised on the silica glass disk which gathered those fallen compulsory from the catcher Room-A. Among them, more than a hundred particles have been removed and more than fifty were distributed for the initial analyses thus far. Their preliminary results are presented in [2–10].

**Future Schedule for Further Sample Distribution:** The initial analyses are still in progress, and we will distribute some of particles recovered from the catcher Room-B. Then some part of the particles will be distributed to NASA, based on the Memorandum of Understanding (MOU) between Japan and USA for the Hayabusa mission. Finally, in the near future an international Announcement of Opportunity (AO) for sample analyses will be open to any interested party.

**References:** [1] Fujimura A. et al. 2011. Abstract #1829. 42nd Lunar and Planetary Science Conference. [2] Nakamura T. et al. 2011. Abstract #1766. 42nd Lunar and Planetary Science Conference. [3] Tsuchiyama A. et al. 2011. Abstract #1766. 42nd Lunar and Planetary Science Conference. [4] Ebihara M. et al. 2011. Abstract #1902. 42nd Lunar and Planetary Science Conference. [5] Yurimoto H. et al. 2011. Abstract #1755. 42nd Lunar and Planetary Science Conference. [6] Noguchi T. et al. 2011. Abstract #1596. 42nd Lunar and Planetary Science Conference. [7] Okazaki R. et al. 2011. Abstract #1653. 42nd Lunar and Planetary Science Conference. [8] Nagao K. et al. 2011. Abstract #2119. 42nd Lunar and Planetary Science Conference. [9] Kitajima F. et al. 2011. Abstract #1855. 42nd Lunar and Planetary Science Conference. [10] Naraoka H. et al. 2011. Abstract #1672. 42nd Lunar and Planetary Science Conference.

## AB INITIO CALCULATION ON PHOTODISSOCIATION OF CO MOLECULES TO TEST THE SELF-SHIELDING MODEL

A. Yamada¹, S. Nanbu² and M. Ozima¹. ¹Department of Earth and Planetary Science, University of Tokyo, Tokyo 113-0033, Japan. E-mail: yamada@eps.s.u-tokyo.ac.jp. ²Sophia University.

The mass-independently fractionated oxygen isotope in CAI of primitive meteorites is commonly attributed to self-shielding effect of CO [e.g., 1]. Besides its unusual implication that CAI oxygen must be the same as the solar oxygen, the self-shielding hypothesis is based on several dubious premises including no isotopic effect of photo-dissociation of CO. Here, we show from a quantum mechanical calculation that photo-dissociation of CO seems to give rise to yield substantial isotopic effect on the photo-dissociation.

Ab initio reaction dynamics simulations were performed to compute the photolysis rate for the $E^1 \angle ? \times \chi^1\Sigma^+$ electronic transition with non-adiabatic transition to $k^3\Pi$ and dissociative channel. This transition is considered one of the important transition for a CO self-shielding model. We used the Born-Oppenheimer approximation; in the first step the time independent Schrödinger equation was solved only for the electromotion, and in the second step we performed the time dependent wavepacket dynamics for the nuclei-motion in the potential energy curves. The theoretical photo-dissociation cross section as a function of wavelength of excitation light was estimated by the Fourier transform of the autocorrelation function, $\langle\phi(0)\phi(t)\rangle$ [2]. Assuming the Boltzmann distribution for vibrational and rotational state, we summed up to get the total cross section. Using photo-dissociation cross sections calculated for respective isotopes ($i = 16, 17, 18$), we will discuss photo-dissociation rate $J_i = \int I(z, \lambda)\phi_i(\lambda)\sigma_i(\lambda)d\lambda$, where $i$ is for an isotope, $I$ is light intensity, $\phi$ is the quantum yield assumed unity, $\sigma$ is the cross section.

**References:** [1] Lyons et al. 2009. *Geochimica et Cosmochimica Acta* 73:4998–5017. [2] Heller 1978. *Journal of Chemical Physics* 68:2066–2075.
T. Yokoyama¹, K. Misawa¹,², and O. Okano³. ¹Graduate University for Advanced Studies, Tachikawa, Tokyo 190-8518, Japan. E-mail: yokoyama.tatsunori@nipr.ac.jp. ²Antarctic Meteorite Research Center, NIPR, Tachikawa, Tokyo 190-8518, Japan. ³Faculty of Science, Okayama University, Okayama 700-8530, Japan.

**Introduction:** Alkali-rich igneous fragments were identified in the brecciated LL chondrites, Krähenberg (LL5) [1], Bhola (LL3-6) [2], and Yamato (Y)-74442 (LL4) [3–5], and show characteristic fractionation patterns of alkaline elements (e.g., Na approximately 0.5 × CI, K approximately 12 × CI, Rb approximately 45 × CI, and Cs approximately 70 × CI [6]). The alkali-rich fragments in Krähenberg, Bhola, and Y-74442 are very similar in mineralogy, petrography, and mineral chemistry, implying that they could have formed from related precursor materials [5]. In order to understand origin of these alkali-rich fragments as well as to constrain timing of their formation, we have undertaken Rb-Sr isotopic studies on alkali-rich fragments in Bhola and Y-74442.

**Results and Discussion:** Abundances of rubidium in the whole-rock samples of Bhola and Y-74442 (50–100 mg in weight) are 2–10 times of those of ordinary chondrites, indicating a contribution of alkaline-rich fragments. Alkali-rich fragments separated from Bhola and Y-74442 show extreme enrichments of rubidium (10–70 × chondrites). Model ages of Bhola and Y-74442 whole-rock samples are calculated to be 4.67 and 4.63 Ga, respectively, when LL-chondrite ⁸⁷Sr/⁸⁶Sr initial ratio of 0.69900 [7] (using $\lambda_{87Rb} = 1.402 \times 10^{-11} a^{-1}$; adjusted NBS-987 ⁸⁷Sr/⁸⁶Sr to 0.710250) is used. Bhola and Y-74442 (including three data points of [8]) yield a whole-rock Rb-Sr age of 4596 ± 96 Ma with an initial ⁸⁷Sr/⁸⁶Sr of 0.7000 ± 0.0046. Relatively young ³⁹Ar-⁴⁰Ar ages of approximately 4200 Ma were reported for alkali-rich fragments in Krähenberg and Bhola [9], and the whole-rock sample of Y-74442 [10], which suggests that they suffered impact event(s) with the partial degassing of Ar.

Potassium, rubidium, and cesium are also classified as large ion lithophiles and are partitioned into residual phases during crystallization. The alkali-rich fragments in Krähenberg and Bhola possess flat REE patterns, which is different from the alkali-rich early planetary objects, Graves Nunatak (GRA) 06128/06129, showing LREE/HREE fractionations [11]. Geochemistry (i.e., solid/liquid fractionation process) could not be responsible for the enrichments of heavier alkalis in the Krähenberg, Bhola, and Y-74442 fragments. Taking into account for the lack of detectable potassium isotopic fractionation in the Krähenberg fragment [12] along with the old formation ages of approximately 4.56 Ga for the Krähenberg and Y-74442 fragments [1, 13], an alkali-rich component could have formed during an early stage of solar system evolution and incorporated into precursor materials of fragments prior to a final melting event.

**References:** [1] Kempe W. and Müller O. 1969. *Meteorite Research*, pp. 418. [2] Noonan A. F. et al. 1978. Geological Survey Open File Report 78-701, 311. [3] Yanai K. et al. 1978. Memoirs of the National Institute Polar Research Special Issue 8:110. [4] Ikeda Y. and Takeda H. 1979. Memoirs of the National Institute Polar Research Special Issue 15:123. [5] Yokoyama T. et al. 2011. Abstract #1941. 42nd Lunar Prospector Spacecraft. [6] Wlotzka F. et al. 1983. *Geochimica et Cosmochimica Acta* 47:743. [7] Minster J. F. and Allègre C. J. 1981. *Earth Planetary Sciences* 56:89. [8] Okano O. et al. 1990. *Geochimica et Cosmochimica Acta* 54:3509. [9] Trieloff M. et al. 1994. *Meteoritics* 29:541. [10] Kaneoka I. and Nagao K. 1993. *Proceedings of NIPR Symposium Antarctica Meteorites* 6:88. [11] Day J. M. D. et al. 2009. *Nature* 457:179. [12] Humayun M. and Clayton R. N. 1995. *Geochimica et Cosmochimica Acta* 59:2131. [13] Nishiya N. et al. 1995. Okayama University of Earth Science Report 2:91.

Z. Yoldi-Martinez¹, P. Beck¹, G. Montes-Hernandez², R. Chiriac³, E. Quirico¹, L. Bonal¹, B. Schmitt¹ and F. Moynier⁴. ¹UJF-Grenoble 1/CNRS-INSU, Institut de Planetologie et d'Astrophysique de Grenoble (IPAG), France, E-mail: pierre.beck@obs.ujf-grenoble.fr. ²Institut des Sciences de la Terre (IsTERRE), UJF-Grenoble 1, France. ³Université de Lyon, Université Lyon 1, Laboratoire des Multimatériaux et Interfaces UMR CNRS 5615.

**Introduction:** Carbonaceous chondrites are considered as amongst the most primitive Solar System samples available. One of their primitive characteristics is their enrichment in volatile elements. This includes hydrogen, which is present in hydrated and hydroxylated minerals. More precisely, the mineralogy is expected to be dominated by phyllosilicates in the case of CM chondrites, and by Montmorillonite type clays in the case of CI. Here, in order to characterize and quantify the abundance of low-temperature minerals in carbonaceous chondrites, we performed thermogravimetric analysis of matrix fragments of Tagish Lake, Murchison and Orgueil.

**Methods:** Chunks of the matrix of Murchison, Tagish Lake, and Orgueil were extracted for TGA analysis following [1]. TGA experiments were performed with TGA/SDTA 851e Mettler Toledo under the following conditions: sample mass of about 10 mg, platinum crucible of 150 µL with a pinhole, heating rate of 10 °C min⁻¹, and inert N₂ atmosphere of 50 mL min⁻¹. Sample mass loss and associated thermal effects were obtained by TGA/SDTA. In order to integrate the different mass loss steps, the TGA first derivative (mass loss rate) was used. TGA apparatus was calibrated in terms of mass and temperature. A total of about 15 reference minerals were analyzed for comparison.

**Exchangeable Water:** The three samples studied show significant endothermic mass loss below 200 °C. Two distinct peaks are present on the first derivative curve in the case of Murchison and Orgueil, corresponding to adsorbed H₂O (peak centered at 70 °C) and water molecules trapped in mesopores (peak centered at 130 °C). Tagish Lake shows the presence of adsorbed H₂O only. The total amount of this “exchangeable” water is 2.8, 6 and 7.8 wt% for Murchison, Tagish Lake and Orgueil respectively.

**-OH bearing mineral phases:** In the case of Orgueil, a significant amount of mass loss is centered at 250 °C, likely related to the decomposition of ferrihydrite, confirming previous studies [2]. In addition, mass releases are observed at 570 °C and 730 °C, which can be attributed to serpentine and Mg-rich phyllosilicate. In the case of Murchison, the occurrence of a Mg-rich serpentine is suggested by the presence of a mass loss at 730 °C, together with cronstedtite (mass loss at 380 °C and 545 °C). Tagish Lake shows a thermal behavior distinct from that of Murchison and Orgueil since (1) Mg-serpentine appear to be absent (2) the first derative curve shows that the mass loss occurs at 570 °C with a shoulder at 520 °C.

**Conclusions:** This study shows that TGA enables to quantify the amount of “exchangeable water” for a given meteorite sample. This water might play a dynamic role on the early water cycle of hydrous asteroids. In addition, TGA are of great help for estimating the H budget in various chondrite groups and understanding the radial transfer of volatiles in the Solar System.

**References:** [1] Montes-Hernandez et al. submitted. [2] Tomeoka K and Buseck P.R. 1988. *Geochimica et Cosmochimica Acta* 52:1627–1640.
5470

EARLY FORMATION OF DIFFERENTIATED PARENT BODIES AND LATE FORMATION OF CHONDRITE PARENT BODIES BEYOND THE ASTEROID BELT

E. D. Young. Department of Earth & Space Sciences, University of California Los Angeles, 595 Charles E. Young Drive East, Los Angeles, CA 90095. E-mail: eyoung@ess.ucla.edu.

**Introduction:** Our understanding of the chronology of planetesimal formation has changed significantly in the past several years. High-precision chronometers (e.g., $^{182}$Hf/$^{182}$W, $^{26}$Al/$^{26}$Mg) reveal that differentiated bodies formed very rapidly, often with metal-silicate separation ages nearly indistinguishable from CAI formation (i.e., time zero). These same constraints require that chondrite parent bodies generally formed several million years after CAIs. In retrospect, thermal models for planetesimal formation and evolution could have been used to reach these same conclusions. Although thermal models are often employed to simulate the evolution of individual bodies for detailed comparisons with meteorological data [1], the spectrum of possible evolution histories provides robust constraints on feasible combinations of accretion times and peak temperatures (e.g., differentiated vs. undifferentiated; [2]).

**Method:** A spherically symmetric finite difference code that simulates the thermal evolution of planetesimals, taking into account enthalpies of both water and rock phase transitions and convective enhancement of heat flow, was constructed. The base state assumes 30% by volume water. The heat sources in these calculations are the short-lived nuclides. The code was applied to a wide range of initial conditions that map out possible and impossible combinations of time and peak temperature.

**Results:** Mapping results in terms of accretion time (post-CAI) and radii of the bodies shows clearly that silicate-metal segregation dated by the Hf/W system *requires* accretion of these parent bodies prior to 1.5 Myr post-CAI. The result is robust and is consistent with recent dating of igneous meteorites [3]. A minimum radius of ~30 km applies for formation of bodies destined for metal-silicate segregation at 1.5 Myr, with smaller radii possible at <1.5 Myr. Chondrite parent bodies in which temperatures did not exceed ca. 1,000 K *must* have formed >1.5 Myr post-CAI for all radii >15 km, consistent with the emerging new chronological constraints for chondrules and their host rocks. For bodies that never suffered temperatures greater than 500 K (e.g., some carbonaceous chondrites), accretion times are limited to >2.2 Myr for radii >15 km. Thermal models require early (<1.5 Myr) formation of differentiated bodies and late (>2 Myr) formation of carbonaceous chondrite parent bodies.

**Discussion:** Models for the dynamics of the early solar system linked to giant planet migration raise significant questions about the provenance of bodies populating the asteroid belt. The constraints on accretion times provided by thermal models, in apparent agreement with emerging chronological constraints, suggest that carbonaceous chondrites may represent objects accreted at heliocentric distances well beyond the asteroid belt (e.g., if accretion times for bodies tens of kilometers in diameter varied with distance R from the nascent Sun according to $\tau$(yrs) ~ 2000 ($R$/1AU)$^{3/2}$ [4]).

**References:** [1] Young E. D. et al. 1999. *Science* 286:1331–1335. [2] Ghosh A. et al. 2006. *Meteorites and the early solar system II*, 555–556. [3] Kleine T. et al. 2009. *Geochimica et Cosmochimica Acta* 73:5150–5188. [4] Weidenschilling S. J. 2000. *Space Science Reviews* 92:295–310.

5453

EVOLUTION TO WEAK NUCLEAR STATISTICAL EQUILIBRIUM AND THE NEUTRON-RICH IRON GROUP ISOTOPES

T. Yu and B. S. Meyer. Department of Physics and Astronomy, Clemson University, Clemson, SC 29634-0978, USA.

**Introduction:** FUN CAIs and hibonite grains show roughly correlated excesses and deficits in the neutron-rich iron-group isotopes such as $^{48}$Ca and $^{50}$Ti (e.g., [1] and references therein). Such effects seem to call for correlation of these isotopes in the precursor dust of the Solar System. From a nucleosynthesis point of view, this makes sense because the neutron-rich iron-group isotopes are copiously co-produced in low-entropy expansions of neutron-rich matter [2], which presumably occur in rare Type Ia (thermonuclear) supernovae. The production of these isotopes in such expansions, however, depends on how neutron-rich the material becomes, which in turn depends on the complicated nuclear dynamics of hot, dense, neutron-rich matter.

**Weak Interaction Rates:** To explore the production of the neutron-rich iron-group nuclei, we have developed tools [3] to follow the evolution of nuclear reaction networks into weak nuclear statistical equilibrium (WSE). Weak interaction rate compilations have been calculated before (e.g., [4, 5]). However these compilations do not guarantee a network evolves to a weak nuclear statistical equilibrium (WSE) that is consistent with the other nuclear data in the problem because the compilations treat forward and reverse rates separately. We have developed tools to compute the reverse weak rates from detailed balance. This enables us to compare network and equilibrium calculations consistently.

**Results:** With our tools, we calculated the nucleosynthesis in matter at fixed high temperature (6 × 10$^9$ K) and density (9 × 10$^9$ g cc$^{-1}$) and found that $Y_e$ (number of net electrons per nucleon) evolves to 0.397, the WSE value. If we instead do not insist on weak detailed balance, $Y_e$ evolves to 0.419 with rates from reference [3]. This value is inconsistent with WSE. The resulting $^{48}$Ca abundance differs significantly in the two calculations. Another interesting result is that the system evolves into WSE via a quasi-equilibrium, not a nuclear statistical equilibrium (NSE). NSE is a state in which all nuclei are in equilibrium under strong and electromagnetic, but not necessarily weak, reactions. The reason for this is that, as dense, hot matter is undergoing the electron captures to reduce $Y_e$, the number of heavy nuclei $Y_h$ must also change. The tight balance between forward and reverse strong and electromagnetic reactions causes the effective timescale for $Y_h$ to evolve to become comparable to that for $Y_e$ to change.

**Conclusions:** A proper treatment of forward and reverse weak-interaction rates is important for calculations of the yields of neutron-rich isotopes in low-entropy freeze-outs. We also have found that in such low-entropy environments, the nuclear populations can evolve to WSE through a quasi-equilibrium instead of a regular NSE. This result will be important for those computing effective neutronization rates in dense Type Ia supernovae.

**References:** [1] Meyer B. S. and Zinner E. In *Meteorites and the early solar system II* (Tucson: University of Arizona Press), pp. 69–108. [2] Meyer B. S. et al. 1996. *The Astrophysical Journal* 462:825–838. [3] See https://sourceforge.net/projects/nucnettools/ [4] Fuller G. M. et al. 1985. *The Astrophysical Journal* 293:1–16. [5] Langanke K. and Martinez-Pinedo G. 2000. *Nuclear Physics A* 673:481–508.