FORMATION OF TiO₂ NANOPARTICLES IN A CAI-LIKE OBJECT
FROM AN AOA IN THE ALHA77307 CO3.0 CARBONACEOUS
CHONDRITE

Jangmi Han and Adrian J. Brearley. Department of Earth and Planetary Sciences, MSC03-2040, 1University of New Mexico, Albuquerque, NM 87131, USA. E-mail: jmhan@unm.edu.

**Introduction:** Amoeboid olivine aggregates (AOAs) consist of forsterite and lesser amounts of refractory, Ca-Al-rich phases such as Al,Ti-bearing pyroxene, ±anorthite, ±spinel, and rarely melilite. AOAs probably represent aggregates of nebular gas-solid condensates [1, 2]. The refractory component of AOAs shows evidence of gas-solid reactions that can constrain the origin and thermal history of the AOAs. Here, we report microstructural studies of an AOA from the CO3.0 chondrite, ALHA77307, aimed at understanding the origin of AOA and the relationship between the refractory component and forsterite.

**Methods:** SEM/BSE imaging, X-ray mapping, and FIB-TEM were used to study the AOA. TEM observations were made on two FIB sections from a CAI-like object within the AOA [3].

**Results:** The AOA (approximately 350 µm in size) contains CAI-like nodules with a spinel-rich core, a layer of intergrown spinel and diopside, and a diopside rim [3]. The boundary between the spinel core and the diopside layer is highly irregular and embayed indicating a reaction relationship. One FIB section (C) was cut from the spinel core to the diopside rim and a second (D) from a complex spinel-diopside intergrowth in the spinel core.

Both FIB sections are highly compact with no evidence of pores. The spinel core regions consist of a dense aggregate of submicron to microsized grains with an equilibrated grain boundary microstructure, i.e., 120 triple junctions. The spinel in both FIB sections exhibits a complex intergrowth texture with irregularly shaped Al,Ti-rich diopside grains (11-32 wt% Al₂O₃; 2-17 wt% TiO₂), often partially enclosed by spinel. Within the FIB D section, numerous nanoparticles, <100 nm in size, decorate the grain boundaries between spinel and diopside. The nanoparticles are elongated, faceted to subrounded and are usually crystallographically oriented relative to the spinel. They also exhibit a complex striated texture indicative of a disordered intergrowth of phases. EDS and EFTEM analyses show that the inclusions contain only Ti and O. FFT analysis of HRTEM images of several particles are consistent with the presence of two TiO₂ polymorphs, rutile, and anatase.

**Discussion:** To our knowledge, this is the first documented occurrence of nanoparticles of rutile and anatase within any CAI-like object. Ti oxides are not predicted equilibrium condensate phases; instead, Ti is incorporated into perovskite or Al,Ti-rich diopside. In this AOA, the formation of TiO₂ is clearly related to the reaction of spinel to form Al,Ti-rich diopside, a process that may be the result of disequilibrium back-reaction of spinel with the nebular gas [3]. Metastable formation of TiO₂ may have occurred during reaction between spinel, diopside, and gas under conditions where the gas was supersaturated in Ti, favoring the condensation of simple oxide phases onto the spinel, by coherent nucleation and growth. Alternatively, TiO₂ could have exsolved at high temperatures from a Ti-saturated, Al-rich diopside.

**References:** [1] Chizmadia L. J. et al. 2002. *Meteoritics & Planetary Science* 37:1781–1796. [2] Krot A. N. et al. 2004. *Chemie der Erde* 64:185–239. [3] Han J. and Brearley A. J. 2011. Abstract #1673. 42nd Lunar and Planetary Science Conference.

EVIDENCE FOR FORMATION OF Al–Ti PYROXENE BY
REACTION BETWEEN PEROVSKITE AND MELILITE IN A CAI-
LIKE OBJECT FROM AN AOA IN THE ALHA77307 CO3.0
CHONDRITE

J. Han and A. J. Brearley. Department of Earth and Planetary Sciences, MSC03-2040, 1University of New Mexico, Albuquerque, NM 87131, USA. E-mail: jmhan@unm.edu.

**Introduction:** Amoeboid olivine aggregates (AOAs), the most common type of refractory inclusion in carbonaceous chondrites, consist mainly of olivine and lesser amounts of refractory components such as spinel, Al,Ti-bearing diopside, and anorthite. Rarely, melilite and perovskite occur in AOAs [1, 2]. We have studied a fine-grained, perovskite-bearing CAI-like object in an AOA in the pristine CO3 chondrite ALHA77307 (CO3.0) in an effort to constrain the origins and thermal histories of refractory materials in AOAs. In addition, we hope to constrain the relationship between the refractory component and olivine in AOAs.

**Method:** The AOA was studied using SEM/BSE imaging and elemental X-ray mapping and then sectioned using FIB technique. The section was subsequently in detail by TEM techniques to characterize its microstructure and microchemistry.

**Results and Discussion:** The irregularly shaped AOA (approximately 300 µm in size) investigated in this study consists of approximately 35 vol% of a dense CAI-like refractory object enclosed by porous olivine grains. The CAI-like object contains numerous nodules consisting of spinel rimmed by a continuous layer of Al,Ti-bearing diopside. Minor perovskite grains, <3 µm in size, are present along grain boundaries between the spinel core and diopside rim.

The FIB section was prepared across a region of two neighboring CAI-like nodules where two approximately 2 µm perovskite grains occur between the spinel core and diopside rim. The FIB section consists of perovskite, diopside, spinel, and melilite with highly unequilibrated grain boundary microstructures. The perovskite grains are irregular in shape and completely surrounded by a layer 0.2–0.4 µm wide consisting of fine-grained diopside and minor spinel. The melilite is completely separated from the perovskite by this layer. Melilite has a high density of defects and dislocations and shows evidence of pores in localized areas.

Perovskite and spinel are nearly pure CaTiO₃ and MgAl₂O₄, whereas melilite is highly gehlenitic, but compositional homogeneous. In contrast, pyroxene shows large variations in chemical composition, with Al₂O₃ contents from 0 to 29 wt%, and TiO₂ contents from 0 to 16 wt%. Significantly, pyroxene grains in contact with perovskite have the highest TiO₂ contents, whilst those furthest away from perovskite contain <5 wt% TiO₂.

Unlike another CAI-like object that we have studied in ALHA77307 [3], this inclusion contains unequivocal compositional and textural evidence for a direct reaction between perovskite and melilite to form Al-Ti-rich pyroxene. The high degree of compositional and textural disequilibrium demonstrates that this reaction occurred under highly dynamic conditions, possibly during fast cooling. One possible scenario is that reaction of perovskite with the gas during melilite condensation stabilized Al–Ti pyroxene, until a reaction rim of sufficient thickness developed to isolate the perovskite from the gas phase. Melilite was then able to condense, as a stable phase.

**References:** [1] Krot A. N. et al. 2004. *Chemie der Erde* 64:185–239. [2] Chizmadia L. J. et al. 2002. *Meteoritics & Planetary Science* 37:1781–1796. [3] Han J. and Brearley A. J. 2011. Abstract #1673. 42nd Lunar and Planetary Science Conference.
5443

PROGRESSIVE AQUEOUS ALTERATION OF CR CHONDRITES

E. R. Harju¹, A. E. Rubin² and J. T. Wasson¹,². ¹Department of Earth & Space Science, UCLA, L.A., CA 90095-1567, USA. E-mail: harju@ucla.edu. ²IGPP, UCLA, L.A., CA 90095-1567, USA.

Although CR chondrites have experienced little or no thermal metamorphism, they have undergone varying degrees of parent-body aqueous alteration. O-isotopic measurements of CR chondrites show increasing δ¹⁷O and δ¹⁸O in more aqueously altered meteorites. However, these measurements have limitations because of terrestrial weathering and sample heterogeneity. What is needed is a petrologic scale for aqueous alteration in CR chondrites analogous to the one constructed for CM chondrites [1]. We examined 20 CR chondrites and used petrographic and compositional parameters to draft a preliminary CR alteration index.

The scale ranges from 2.0 (most altered, equivalent to petrologic type 1) to 3.0 (negligibly altered, roughly equivalent to that of LL3.0 Semarkona). With increasing CR chondrite alteration, metallic Fe-Ni grains develop rinds of magnetite, S-bearing oxides and hydroxides, and other OH-bearing phases. Chondrule mesostases have been altered to phyllosilicate in some CR chondrites. Mafic silicate phenocrysts in chondrules in Renazzo, Al Rais, and A-881595 show incipient alteration.

QUE 99177 and MET 00426 were classified as type 3.0 by [2]; most of the CR chondrites we studied appear to have similar degrees of aqueous alteration to these meteorites, clustering around type 2.9-3.0. These samples can be distinguished from one another by the presence in some chondrules of transparent isotropic glass (QUE 99177, MET 00426, EET 92062, LAP 02342, Gao-Guenie [b], GRV 021710 [2]) and by TEM studies [2] indicating the extent of matrix hydration.

Renazzo and A-881595 contain ≥50 µm-size metal grains that have been partially converted to magnetite; small metal grains have been largely to completely converted to magnetite. Chondrule phenocrysts show incipient alteration along grain surfaces.

In CR-an Al Rais most metal grains not embedded deep within chondrules have substantial alteration rinds of magnetite and a S-bearing oxide. Many of the chondrules contain polysynthetically twinned low-Ca pyroxene grains displaying alteration along twin boundaries. Al Rais is more altered than Renazzo or A-881595.

GRO 95577 is similar to CM2.0 chondrites. All of the metal is nearly or completely oxidized; mafic-silicate phenocrysts have all been converted to phyllosilicate [3]. We found that some of the former metal grains consist of at least nine distinct nested layers of phyllosilicate. Because similarly sized metal grains in CR LAP 02342 are compositionally uniform, this suggests that the layering in GRO 95577 represents episodic alteration of an initially homogeneous substrate.

We used thermogravimetric analysis to examine the differences in bulk indigenous water among relatively unaltered CR chondrites. Preliminary results indicate that some less-altered samples contain about 1 wt% less water than Renazzo (5.67 wt%, [4]). Correction for volatiles in weathering products is required.

Further development and application of this index will enhance our understanding of parent-body aqueous alteration process on the CR parent body and offer a guide to sample selection in future studies of CR chondrites.

**References:** [1] Rubin A. E. et al. 2007. *Geochimica et Cosmochimica Acta* 71:2361-2382. [2] Abreu N. M and Brearley A. J. 2010. *Geochimica et Cosmochimica Acta* 74:1146-1171. [3] Weisberg M. K. and Huber H. 2007. *Meteoritics & Planetary Science* 42:1495-1503. [4] Wiik H. B. 1956. *Geochimica et Cosmochimica Acta* 9:279-289.

5165

NANOCRYSTALLINE P-BEARING PENTLANDITE AND CHROMIUM NITRIDES FROM CM2 CHONDRITES Y-791198 and Y-793321

D. Harries¹ and F. Langenhorst². ¹Bayerisches Geoinstitut, Universität Bayreuth, Bayreuth, Germany. E-mail: dennis.harries@uni-bayreuth.de. ²Institut für Geowissenschaften, Friedrich-Schiller-Universität Jena, Jena, Germany.

**Introduction:** Phosphorous-bearing Fe,Ni sulfides are characteristic accessory phases in CM2 chondrites, but enigmatic in their mineralogy and origin. Extensive textural and microanalytical data has been provided by [1], and a TEM study by [2] has shown that the material is nanocrystalline, but its structure could not be identified. We have prepared several FIB sections of P-bearing sulfides from two selected CM2 chondrites: Yamato(Y)-791198 is an unrecciated rock with a rather low level of aqueous alteration and no significant secondary heating [3]. On the contrary, Y-793321 is a regolith breccia and has experienced an episode of thermal metamorphism at 250-500 °C [4]. This choice is intended to draw conclusions about the thermal stability and the origin of P-bearing sulfides and associated phases.

**Results:** TEM shows that grains in both meteorites are not homogeneous internally. Besides the nanocrystalline sulfide (<20 nm) we have found in certain grains myriads of <100 nm sized flakes of chromium nitrides. SAED derived d-values are consistent with CrN (carlsbergite) in Y-791198 and CrN + Cr₂N in Y-793321. The presence of chromium and nitrogen has been confirmed by EDX and EELS. This is the first reported observation of chromium nitrides in chondritic meteorites.

SAED patterns of the nanocrystalline sulfides in Y-791198 show two broad rings corresponding to d-values of 0.30 and 0.18 nm. In Y-793321 the principal rings occur at the same positions, but splitting occurs and additional, weaker rings join. The corresponding d-values of strong and weak rings in Y-793321 match to a pentlandite structure. HR-TEM imaging shows that the nanocrystallites in Y-793321 are larger (close to 20 nm) and better defined than in Y-791198. Hence, the difference in SAED patterns is due to better crystallinity in Y-793321, which we interpret as a result of thermal annealing. This is supported by the development of a lattice preferred orientation (LPO) of nanocrystallites in Y-793321, which is evident by circumferential intensity variations along the diffraction rings. The LPO is likely a transitional state during recrystallization toward a single pentlandite crystal. Chemical shift of P Kα determined by EMPA-WDS suggests a mix of phosphate and phosphide and EELS indicates abundant oxygen in the sulfide portion. While phosphide is likely incorporated into the pentlandite structure, phosphate groups probably reside on the large internal boundary surfaces. The same likely applies to lithophile trace elements measured by [1].

**Conclusions:** Comparison of Y-791198 and Y-793321 shows that the nanocrystalline sulfide is thermally not stable and undergoes recrystallization during processing at temperatures below 500 °C. However, the refractory nitrides must have formed at elevated temperatures in excess of 500 °C. This implies a complex, multistage formation history. Scenarios will be discussed.

**Acknowledgments:** NIPR is gratefully acknowledged for providing meteorite samples.

**References:** [1] Nazarov M. A. et al. 2009. *Petrology* 17:101-123. [2] Devouard B. and Buseck P. R. 1997. *Meteoritics & Planetary Science* 32:A34. [3] Nakamura T. 2005. *Journal of Mineralogical and Petrological Sciences* 100:260-272. [4] Nakamura T. 2006. *Earth and Planetary Science Letters* 242:26-38.
5166

## SULFIDES IN CM AND CM/CI-LIKE CHONDRITES AND THEIR RECORD OF THERMAL METAMORPHISM: A FIB-TEM STUDY

D. Harries¹ and F. Langenhorst². ¹Bayerisches Geoinstitut, Universität Bayreuth, Bayreuth Germany. E-mail: dennis.harries@uni-bayreuth.de.
²Institut für Geowissenschaften, Friedrich-Schiller-Universität Jena, Jena, Germany.

**Introduction:** Assemblages containing iron- and nickel-rich monosulfides such as troilite (FeS), pyrrhotite (Fe₁-xS) are interesting proxies for physicochemical conditions of formation, alteration, and metamorphism of planetary materials. Fe deficiency in pyrrhotite is directly related to f/S₂ and temperature [1] and can be linked indirectly to fO₂ if certain buffer assemblages are present, for example, magnetite plus metallic iron [2]. The results of Fe deficiency are various crystallographic superstructures [3]. To explore the effect of f/S₂ and temperature on sulfide mineralogy, we have sampled sulfide grains from two CM2 chondrites (Y-791198, Y-793321), two anomalous, metamorphic CM/CI-like chondrites (Y-86720, B-7904), and one anomalous CI chondrite (Y-980115) by FIB and studied them by analytical TEM.

**Results:** Yamato-791198 and Y-793321 contain troilite with abundant pentlandite and pyrrhotite exsolution. In both cases pyrrhotite occurs as strongly twinned 6C superstructure (approximately Fe₉.₉₂S). In Y-793321, which has experienced mild metamorphism [4], the textures of the sulfide assemblages are significantly coarser than in Y-791198.

The anomalous CM/CI-like chondrites Y-86720 and B-7904 show intimate associations of troilite with metallic (Fe,Ni) and only traces of 6C-pyrrhotite as narrow (< 100 nm) rims at grain margins. In Y-86720 we have found pseudomorphs of troilite and metal after euhedral, hexagonal crystals—likely former pyrrhotite. Both contain well crystallized anhydrite, but only B-7904 contains magnetite, which is absent in Y-86720.

In Y-980115 euhedral sulfide crystals similar to 4C-pyrrhotite (Fe₀.₈七S) in typical CI chondrites [5] are composed of polycrystalline troilite and pentlandite and are likely pseudomorphs as well. Also here, 6C-pyrrhotite occurs at grain margins and rims of internal voids. However, (Fe,Ni) metal is absent.

**Conclusions:** Compared to pristine Y-791198 the thermal overprint on Y-793321 had only minor effects on sulfide assemblages, mostly leading to coarsening through annealing at temperatures below 150 °C.

Y-86720 and B-7904 experienced more severe heating and low f/S₂ at the iron-troilite buffer. Preservation of magnetite in B-7904 points to peak temperatures below 570 °C and absence in Y-86720 suggest a higher metamorphic grade. The magnetite + troilite + metal assemblage suggests reducing conditions with log fO₂ below -25.8 in case of B-7904. 6C-pyrrhotite and anhydrite in both meteorites cannot be in equilibrium with this assemblage and likely formed during cooling.

Conversion of pyrrhotite to troilite in Y-980115 likely indicates similar low f/S₂ conditions as in B-7904. However, metal did not form, possibly due to shorter heating and kinetic inhibition.

**Acknowledgments:** NIPR is gratefully acknowledged for providing meteorite samples.

**References:** [1] Rau H. 1976. *Journal of Physics and Chemistry of Solids* 37:425-429. [2] Whitney J. A. 1984. *American Mineralogist* 69: 69-78. [3] Harries D. et al. 2011. *American Mineralogist* 96:716-731. [4] Nakamura T. 2006. *Earth and Planetary Science Letters* 242:26-38. [5] Berger E. L. et al. 2011. *Geochimica et Cosmochimica Acta* 75:3501-3513.

5182

## THE SIGNIFICANCE OF Li ISOTOPE VARIATIONS IN CO CHONDRITES: BULK-ROCK, INTER AND INTRACHONDRULE DIFFERENCES

J. Harvey¹,², M. A. Fehr¹, J. L. Sharrock³, I. J. Parkinson¹ and R. H. James⁴. ¹Department of Earth and Environmental Sciences, The Centre for Earth, Planetary, Space and Astronomical Research (CEPSAR), The Open University, Milton Keynes, UK. E-mail: feejh@leeds.ac.uk. ²School of Earth & Environment, University of Leeds, Leeds, UK. ³School of Earth Sciences, Grant Institute, University of Edinburgh, Edinburgh, UK. ⁴National Oceanography Centre, Southampton, UK.

The crystallization of chondrule phenocrysts has the potential to create distinctive ⁷Li/⁶Li profiles [1, 2] due to the more rapid diffusion of ⁶Li compared to ⁷Li in the presence of a compositional gradient. In well characterized samples it may be possible to extract cooling rates from the diffusion profiles. The mobility of Li during silicate weathering also results in significant fractionation of ⁶Li from ⁷Li, with ⁷Li preferentially entering the solution (e.g., [3]); this is a process that modifies the ⁷Li/⁶Li ratio of both terrestrial and meteorological material [3, 4]. The interpretation of lithium isotope diffusion profiles and fractionation patterns may therefore provide the means to quantify the effects of both primary and secondary processes in chondrites.

We will present data for intra- and interchondrule δ⁷Li variation (differences in ⁷Li/⁶Li relative to NIST L-SVEC), determined by ion microprobe and MC-ICP-MS respectively. This is complemented by bulk data for Ornans (CO3.3) and Lancé (CO3.4). The aims of this study are (1) to assess the extent to which Li isotope diffusion profiles in chondrule phenocrysts are preserved (2) to examine the extent of ⁷Li/⁶Li fractionation of bulk-rock and chondrules, in addition to intra-chondrule δ⁷Li variations in the context of aqueous alteration. High Mg# (>0.99) in chondrule cores suggest that primitive geochemical compositions may have been retained. In contrast, lower rim Mg# (≤0.80) suggest diffusive exchange with matrix during cooling or subsequent secondary alteration. However, the same Mg# gradients are also observed close to fractures in the interior of chondrule phenocrysts and thus are unlikely to be primary, suggesting that Li isotope fractionation during chondrule cooling may have been overprinted. Bulk-rock δ⁷Li values for Ornans (4.0 ± 0.4) and Lancé (2.9 ± 0.1) are marginally heavier than previously reported [5].

**References:** [1] Parkinson et al. 2007. *Earth and Planetary Science Letters* 257:609-621. [2] Beck et al. 2004. *Geochimica et Cosmochimica Acta* 68:2925-2933. [3] Kisakurek et al. 2004. *Chemical Geology* 212:27-44. [4] Sephton et al. 2006. *Meteoritics & Planetary Science* 41:1039-1043. [5] Seitz et al. 2007. *Earth and Planetary Science Letters* 260:582-596.
5510

Mg ISOTOPIC COMPOSITION OF THE SOLAR WIND BY SIMS
ANALYSIS OF GENESIS TARGETS

V. S. Heber¹, A. J. G. Jurewicz², P. Janney², M. Wadhwa², K. D. McKeegan¹ and D. S. Burnett³. ¹Department of Earth and Space Sciences, UCLA, Los Angeles, CA, USA. E-mail: heber@ess.ucla.edu. ²Arizona State University, Tempe, AZ, USA. ³CalTech, Pasadena, CA, USA.

**Introduction:** In order to deduce the isotopic compositions in the solar nebula of volatile elements, e.g., noble gases, O, and N, from analyses of the solar wind (SW), we must understand the magnitude of mass-dependent fractionation between the SW and the photosphere. With the exception of evaporation effects in CAI's, mass-dependent fractionation of Mg isotopes is small, as evidenced by Mg isotopic compositions of terrestrial igneous and meteoritic samples which agree within ~1‰ [1]. Thus, if we assume that the “terrestrial” Mg isotopic composition is also representative of the solar photosphere, we can use measurements of the SW captured by Genesis to test models of isotopic fractionation in formation of SW. For example, if the inefficient Coulomb-drag model [2] is correct, we would expect that the Mg isotopic composition in the SW is ~10‰ per amu [2] lighter than the terrestrial composition.

**Experimental and Results:** Fragments of a silicon target from the passive collector, a 25,26Mg implant (Si target) and a terrestrial magnetite (containing low abundance of Mg-bearing inclusions) were measured in depth-profiling mode with the UCLA Cameca ims 1270. 100 × 100 µm² craters were sputtered with a O₂⁺ beam in O₂ atmosphere (1.2E-5 torr). 7.5 keV impact energy and a field aperture of 40 × 40 µm² were applied to improve depth resolution. Mg isotopes and ²⁸Si were collected simultaneously on electron multipliers (Mg) and a Faraday cup (Si). A mass resolving power of 2100 sufficiently resolved ²⁴MgH from ²⁵Mg. Data are corrected for deadtime, blank and instrumental mass fractionation.

The 25,26Mg implant standard was homogeneously irradiated (25Mg/26Mg from three areas ~5 cm apart agree within 3‰). The 25Mg/26Mg ratio was calibrated by solution MC-ICPMS at ASU to be 0.9571 ± 0.0051 (2σ stdev). Uncertainties in the Genesis data are ~5‰ (1σ). Due to unresolved issues with deadtime corrections, we only report δ26Mg/²⁵Mg calibrated against the implant standard: 0.2 ± 4.1‰ (4 runs, 1σ stdev). This preliminary result agrees with [3, 4]. Thus, at present the isotopic fractionation in SW can neither be confirmed nor excluded. Future analyses that include more standards, a better control on the deadtime and more Genesis analyzes will reduce our uncertainties.

**References:** [1] Young et al. 2004. Reviews in mineralogy and geochemistry, vol. 55. pp. 197–230. [2] Bodmer et al. 2000. *Journal of Geophysical Research* 105:47–60. [3] Rieck et al. 2010. Abstract #2391. 41st Lunar and Planetary Science Conference. [4] Humayun et al. 2011. Abstract #1211. 42nd Lunar and Planetary Science Conference.

<figure><img src="image_12.png" /><figcaption>Fig. 1. Backside depth profile of SW Mg isotopes. This mode produces a complete profile, avoiding surface contamination and transient sputter effects otherwise encountered when sputtering from the front side. Isotopic ratios were calculated from the sum of the counting rates over the complete profile.</figcaption></figure>

5372

ATOM-PROBE TOMOGRAPHY OF METEORITIC AND
SYNTHETIC NANODIAMONDS

P. R. Heck¹,², M. J. Pellin²,³,⁴,⁵, A. M. Davis¹,²,³,⁵, D. Isheim⁶, D. N. Seidman⁶, J. Hiller⁴, A. Mane⁷, J. Elam⁷, M. R. Savina²,⁴, T. Stephan¹,²,³,⁴, F. J. Stadermann⁸,⁹, X. Zhao⁸,⁹, T. L. Daulton⁹,¹⁰ and C. Floss⁸,⁹. ¹Robert A. Pritzker Center for Meteoritics and Polar Studies, Department of Geology, The Field Museum, Chicago, IL, USA. E-mail: prheck@fieldmuseum.org. ²Chicago Center for Cosmochemistry. ³Department of the Geophysical Sciences, University of Chicago, Chicago, IL, USA. ⁴Materials Science Division, Argonne National Laboratory, Argonne, IL, USA. ⁵Enrico Fermi Institute, University of Chicago, Chicago, IL, USA. ⁶Northwestern University Center for Atom-Probe Tomography, Department of Materials Science & Engineering, Northwestern University, Evanston, IL, USA. ⁷Energy Systems Division, Argonne National Laboratory, Argonne, IL, USA. ⁸Laboratory for Space Sciences. ⁹Physics Department. ¹⁰Center for Materials Innovation, Wash. University, St. Louis, MO, USA.

**Introduction:** Because of their small size (approximately 3 nm [1]) meteoritic nanodiamonds [2] are the least understood component found in acid residue containing presolar grains. Their abundance is higher than for any other presolar grains [2], and the knowledge of their origins is therefore of great interest. Isotope anomalies in trace elements have been observed from bulk analyses of nanodiamond residue (e.g., [2]), but bulk C and N isotopic ratios are normal [3]. This and other observations have led to suggestions that only a small fraction of nanodiamonds is presolar (e.g., [4]). Another possibility is that the anomalies are carried by an associated abundant disordered non-diamond sp² C phase [5]. Carbon isotope analyses of individual nanodiamonds would test if ¹²C/¹³C ratios in any diamonds are anomalous. The atom probe tomograph (APT [6]) is currently the only instrument that can analyze the C isotopic compositions of individual nanodiamonds. The first APT results on Allende DM [7] nanodiamond residue (ADM) were recently presented [8, 9]. To correct for a potential instrumental bias, we have now prepared synthetic nanodiamond samples as analytical standards for the APT.

**Methods:** We used a dual-beam focused ion-beam (FIB) microscope to prepare APT samples of ADM [8]. In another approach, we deposited meteoritic or synthetic nanodiamonds directly on presharpened microtips for APT, and embedded them in a stable matrix using atomic-layer deposition [10] of W or Cu. APT analyses were performed with a Cameca LEAP4000XSi tomograph equipped with a picosecond UV laser [6].

**Results:** Direct deposition sample preparation eliminates the need for FIB lift-out work and improves sample stability. We succeeded in APT analyses of ADM and obtained well-defined C peaks in time-of-flight mass spectra. Our 3D-tomographic reconstructions exhibit isolated and clustered nanodiamonds. The C isotopic compositions of different APT tips of the same aliquot are identical within analytical uncertainties. At the meeting, we will present more data that will help to address the origin of the diamonds and also present APT data of synthetic nanodiamonds.

**References:** [1] Daulton T. L. et al. 1996. *Geochimica et Cosmochimica Acta* 60:4853–4872. [2] Lewis R. S. et al. 1987. *Nature* 326:160–162. [3] Russell S. S. et al. 1996. *Meteoritics & Planetary Science* 31:343. [4] Dai Z. R. et al. 2002. *Nature* 418:157–159. [5] Stroud R. M. et al. 2010. *Meteoritics & Planetary Science* 73:5352S. [6] Seidman D. N. and Stiller K. 2009. *MRS Bull.* 34:717–721. [7] Lewis R. S. et al. 1989. *Nature* 339:117–121. [8] Heck P. R. et al. 2011. Abstract #2112. 42nd Lunar and Planetary Science Conference. [9] Stadermann F. J. et al. 2011. Abstract #2134. 42nd Lunar and Planetary Science Conference. [10] Elam J. W. et al. 2006. *Applied Physics Letters* 89:053124.
MODELING THE THERMAL CONSTITUTION AND THERMAL EVOLUTION OF ASTEROIDS CONSIDERING SINTERING PROCESSES AND HEATING BY ²⁶Al AND ⁶⁰Fe

S. Henke¹, H. -P. Gail¹, M. Trieloff², W. Schwarz² and T. Kleine³. ¹Institut für theoretische Astrophysik ZAH Heidelberg. E-mail: shenke@ita.uni-heidelberg.de. ²Institut für Geowissenschaften Heidelberg. ³Institut für Planetologie, Westfälische Wilhelms- Universität Münster.

We present models for the internal constitution and thermal evolution of planetesimals of about 100 km radius, assuming spherical symmetry.

We solve the one dimensional heat conduction equation and the hydrostatic pressure equation. Heating in our model is due to ²⁶Al, ⁶⁰Fe and long living isotopes as well as the potential energy released during shrinking of the bodies during compaction. We chose the new measured ⁶⁰Fe-halflife (2.6 Ma) reported by [1].

We implemented compaction processes by pressure induced cold compaction based on [2], and by sintering due hot isostatic pressing based on the theory of [3] and [4]. A porosity dependent heat conductivity based on results of laboratory measurements [5, 6] is used.

We show results of model calculations for the first 200 Ma of evolution after body formation and compare this to data for the cooling history of H-chondrites. We fit the initial parameters (time of body formation after CAI formation, environmental temperature, heat conduction of the chondritic material) to data of H-chondrites.

The high-porosity layer that remains on the surface in our model causes much higher temperatures in the outer layers of the body than in the analytic model of Miyamoto [7], thus layers of certain petrologic type are much closer to the surface.

**References:** [1] Rugel F. et al. 2009. *Physical Review Letters* 103:07250. [2] Güttler C. et al. 2009. *The Astrophysics Journal* 701:130. [3] Kakar A. K. and Chaklader A. C. D. 1967. *Journal of Applied Physics* 38:3223. [4] Rao A. S. and Chaklader A. C. D. 1972. *Journal of the American Ceramic Society* 55:596. [5] Yomigida K. and Matsui T. 1983. *Journal of Geophysical Research* 88:9513. [6] Krause M. et al. 2011. *arXiv* 1101.2586. [7] Miyamoto M. et al. 1981. 12th Lunar and Planetary Science Conference p. 1145.

R-PROCESS ELEMENTS IN PRESOLAR SiC GRAINS

T. Henkel¹, D. Rost¹ and I. C. Lyon¹. ¹The University of Manchester, SEAES, Oxford Road, Manchester, M13 9PL, UK. E-mail: torsten.henkel@manchester.ac.uk.

**Introduction:** R-process elements are of special interest in presolar grains. It is still unclear how many sites have contributed and/or if the r-process is universal [1, 2] which would allow dating r-process material [3, 4]. Analyzing r-process material in presolar grains however is a major challenge as the abundances of these elements are very low (≤ppm) and the grains very small (few micrometers in size) resulting in less than 10⁶ atoms per element per grain.

Dating single grains would help establish the grains' lifetime in the interstellar medium (ISM). This is of special interest as the grains are bombarded by cosmic radiation during their travel through the ISM which limits their theoretical lifetime through sputtering [5] and might implant material [6]—although the latter could also happen during the formation of the grains or while incorporated into the forming solar system.

Traditionally secondary ion mass spectrometry (SIMS) has been used for in situ measurements of single grains but ionization efficiencies are too small to analyze isotope ratios of trace elements. This can be overcome by using laser post-ionization of secondary neutrals which improves the instrument sensitivity significantly.

**Experimental:** We used our time-of-flight SIMS instrument [7] which has an F₂-excimer laser (157 nm, 7.9 eV) for nonresonant ionization of secondary neutrals. Relative sensitivity factors for major elements have already been determined from silicate glass standards [7]. Sensitivity improvements from postionization by up to 2 orders of magnitudes have been achieved.

We have been able to detect heavy element impurities (Pd, Ag, Sn and Pt) in gold foil which are typically on the order of a few ppm only. The improved sensitivity allowed the determination of all the major isotope ratios of these elements demonstrating the instrument capabilities.

**Results:** So far one presolar SiC grain (1.5 µm in size) has been analyzed which was located with time-of-flight SIMS and confirmed with energy dispersive X-ray analysis. The abundances of heavy elements above iron in the grain were below the detection limits. The grain has a δ³⁰Si-value of 20 ± 50‰ with an Al abundance of approximately 1.5% relative to Si and other light elements well below 1%. The grain is therefore not of supernova origin and most likely a mainstream grain which might explain the very low or absent heavy elements.

**Outlook:** The postionization is now working reliably and has been proven capable of detecting heavy elements in low abundances. We are currently locating more presolar grains in our acid-residue and gently separated samples which we will analyze using laser postionization to search for r-process patterns. More results will be presented at the conference.

**References:** [1] Qian Y.-Z. and Wasserburg G. J. 2000. *Physics Reports* 333:77–108. [2] Otsuki K. et al. 2003. *New Astronomy* 8:767–776. [3] Cowan, J. J. and Sneden C. 2006. *Nature* 440:1151–1156. [4] Frebel A. et al. 2007. *The Astrophysical Journal* 660:L117–L120. [5] Jones A. P. et al. 1994. *The Astrophysical Journal* 433:797–810. [6] Lyon I. C. et al. 2007. *Meteoritics & Planetary Science* 42:373–385. [7] Henkel T. et al. 2007. *Review of Scientific Instruments* 78:055107.
5180

BADDELEYITE OCCURRENCES IN MARTIAN METEORITES AND
THE POTENTIAL FOR IN SITU GEOCHRONOLOGY OF ZAGAMI

C. D. K. Herd¹ and T. J. McCoy². ¹Department of Earth and Atmospheric Sciences, 1-26 Earth Sciences Building, University of Alberta, Edmonton, AB, Canada T6G 2E3. ²Department of Mineral Sciences, Smithsonian National Museum of Natural History, 10th & Constitution NW, Washington, DC 20560-0119, USA. E-mail: herd@ualberta.ca.

**Introduction:** Baddeleyite (ZrO₂) is of interest for its potential use in U-Pb geochronology of Martian meteorites [1]. Although it is more common in Martian meteorites than previously recognized, small grain sizes require in situ approaches for analysis, i.e., ion probe mass spectrometry. The primary step, therefore, is locating and documenting baddeleyite grains in thin section. Here we report on the results of this process applied to Zagami, especially the dark, mottled lithology (DML) [2]. Our observations have implications for the occurrence of baddeleyite in Martian meteorites, and the potential for application of in situ U-Pb geochronology methods to this meteorite.

**Methods:** Thin sections reflecting the range of Zagami lithologies were examined, including normal Zagami and DML (USNM 6545-2), DML (UH 218) and a late-stage melt pocket (LSMP; UH 233). The Particle Search method, which maps Zr hotspots by WDS (Cameca SX-100 electron microprobe; University of Alberta) is the most efficient method for locating baddeleyite grains. Grain locations were then verified, documented and measured using the FEI Nova NanoSEM (Smithsonian).

**Results:** A total of 481 grains were located in the three thin sections; 320 in DML (specimens UH 218 and USNM 6545-2), 95 in Normal Zagami (USNM 6545-2) and 66 in the LSMP (UH 233). These occurrences correspond to frequencies of 1 grain approximately every 1.3 mm² in DML, 1 every 0.6 mm² in Normal Zagami, and 1 every 0.5 mm² in the LSMP. In each case, baddeleyite was found in association with late-stage crystallization products. Most commonly it is found within fine-grained, silica-rich mesostasis pockets, within coarse mesostasis (specific to LSMP) or enclosed within titanomagnetite.

**Implications:** The primary factor influencing baddeleyite occurrence is buildup of Zr in late-stage melt [3]. Baddeleyite may be expected to occur in any Martian basalt that started with sufficient Zr content in the parental melt, and within which Zr remained incompatible in all other igneous minerals through the course of crystallization. Accordingly, baddeleyite is common in DML, an evolved, incompatible element-enriched lithology within an already Zr-rich rock [3]. That baddeleyite is relatively common in normal Zagami was unexpected, but its occurrence in association with late-stage crystallization phases is consistent with buildup of Zr as crystallization proceeds.

There is no lack of baddeleyite grains in Zagami. Based on our observations, the best places to find grains of sufficient size and abundances for in situ U-Pb geochronology are (1) within coarse mesostasis in LSMP (average grain size 3.6 ± 2.4 µm, *n* = 26); and (2) enclosed within titanomagnetite (4.5 ± 3.2 µm, *n* = 51). The largest grain found in LSMP coarse mesostasis is 14 × 9 µm. The largest grains found enclosed in titanomagnetite are 10 × 8 µm and 22 × 6 µm (where 6 µm is the maximum width).

**References:** [1] Herd C. D. K. et al. 2007. Abstract #1664. 38th Lunar and Planetary Science Conference. [2] McCoy T. J. et al. 1999. *Geochimica et Cosmochimica Acta* 63:1249–1262. [3] Herd C. D. K. and McCoy T. J. 2011. Abstract #1801. 42nd Lunar and Planetary Science Conference.

5291

NOMCOM REVIEW OF TYPE SPECIMEN REPOSITORIES

C. D. K. Herd¹, C. L. Smith², J. N. Grossman³, M. K. Weisberg⁴ and L. Welzenbach⁵. ¹Department of Earth and Atmospheric Sciences, University of Alberta, Edmonton, AB, Canada T6G 2E3. E-mail: herd@ualberta.ca. ²Department of Mineralogy, Natural History Museum, London, SW7 5BD, UK. ³NASA Headquarters, Mail Stop 3V71, 300 E Street SW, Washington, DC 20546, USA. ⁴Earth and Planetary Sciences, Kingsborough College, City University of New York, Brooklyn, NY 11235, USA. ⁵Department of Mineral Sciences, National Museum of Natural History, Smithsonian Institution, Washington, DC 20560, USA.

**The Issue:** The Nomenclature Committee (NomCom) Guidelines for Meteorite Nomenclature [1] require that “type specimens of all new meteorites are preserved in collections that make material available for research” and that “type specimens must be deposited in institutions that have well-curated meteorite collections and long-standing commitments to such curation.” Despite the fact that the terms “well-curated” and “long-standing” are vague, NomCom has, to date, approved nearly 90 different repositories to house type specimens, some of which, in retrospect, appear not to have been good choices. Because type specimens are in many cases the only materials, from a specific meteorite, available for research, NomCom has decided to re-evaluate its current policies. The goal is to assure that all type specimens will be available to future researchers and that they will be curated in an appropriate manner.

**The Plan:** To begin the evaluation process, NomCom has developed a simple questionnaire for curators of existing repositories to complete online. The questionnaire will serve several goals: to update the contact information for repositories, including individuals with curatorial responsibility and those with institutional oversight; to invite respondents to participate in the development of a “best practices” document; and to create an archive of any collections policies they may have. The information collected will be made available as a community resource. The ‘best practices’ document will eventually include a set of minimum guidelines that we expect any collection to meet if acting as a type specimen repository.

**Community Response:** Over the next few weeks, individuals with curatorial responsibility for type specimen repositories will receive a letter by email with a link to a page where they can provide the requested contact information, as well as share any policy or procedure documents. We ask that the community assist NomCom in this endeavor, and we welcome any feedback as the process continues.

**Reference:** [1] Guidelines for Meteorite Nomenclature, 2011. http://meteoricalsociety.org/bulletin/nc-guidelines.htm.
STUDIES ON PRISTINE SAMPLES OF BUZZARD COULEE

R. K. Herd¹,³, C. Samson¹, C. Fry¹, D. Melanson¹, P. J. A. McCausland² and R. Ernst¹. ¹Department of Earth Sciences, Carleton University, Ottawa, Ontario, Canada K1S 5B6. ²Department of Earth Sciences, University of Western Ontario, London, Ontario, Canada N6A 5B7. ³Earth Sciences Sector, Natural Resources Canada, Ottawa, Ontario, Canada K1A 0E8. E-mail: herd@nrcan.gc.ca.

**Introduction:** Eighteen individual samples of Buzzard Coulee, an H4 chondrite that fell south of Lloydminster, Saskatchewan, on November 20, 2008 were obtained by the National Meteorite Collection of Canada. Their masses range from 109.14 g to 8.80 g. Most are fully to almost-fully crusted and were collected before the winter of 2008–2009.

**Methodology:** To document this suite of samples in their “as received” condition, and to study varying masses of the same meteorite fall, they have first been characterized using independent non-destructive techniques, prior to any mineralogical and textural work. Each fragment, weighed and photographed upon acquisition, has been tracked with checks on weights and with photographs at all stages. Here we report the results from laboratory measurements of bulk volume and density (by silica beads, 3D laser imaging, and X-ray micro-CT scans), grain volume, grain density and porosity (by helium pycnometry), and magnetic susceptibility (in different laboratories and using different instruments).

**Results:** The silica bead method [1] gave sample volumes yielding an average density of $3.49 \pm 0.06 \text{ g cm}^{-3}$ for the sample suite. The volumes from the 3D laser imaging method gave a slightly more consistent average suite density of $3.46 \pm 0.03 \text{ g cm}^{-3}$. X-ray micro-CT scan data used to calculate the sample volumes yielded an average density of $3.41 \pm 0.04 \text{ g cm}^{-3}$. These values agree with other findings using the bead method (3.26–3.45 g cm$^{-3}$ for six fragments [2]) and 3D laser imaging (3.5 g cm$^{-3}$ for one 151.7 g fragment [3]), and are typical for H chondrites [4]. The fragments have an average porosity of $6.14 \pm 1.5\%$ and a grain density of $3.69 \pm 0.03 \text{ g cm}^{-3}$, similar to reported values from other H chondrites within a standard deviation of the mean [5]. Since the 3D laser density values were more consistent, they were divided into the volume magnetic susceptibility readings to give the mass magnetic susceptibility ($X$) of each sample. The average log of $X$ was $5.2 \pm 0.056 \times 10^{-9} \text{ m}^3 \text{ kg}^{-1}$, typical of an H chondrite [6–8]. The magnetic fabric defines an oblate ellipsoid. The laser based density measurements plotted vs. magnetic susceptibility, and vs. porosity, show that the data cluster within 2 standard deviations of the average, forming a single population, unlike the bimodal distribution reported previously [2].

**Summary:** These studies define a methodology for preliminary examination of new meteorite falls and finds [8], or of specimens from sample return missions to space. Digitized data is preserved on samples for archival and curatorial purposes, and can be used to select subsets of them for further research. Variations in their physical properties may be linked to internal variations such as concentrations of metallic minerals or zones of shock. The X-ray micro-CT scans in particular may be used to define metal concentrations and to calculate bulk metal contents. Portions of the meteorites where polished thin-sections would yield critical details of mineralogy and texture can be pre-selected.

**References:** [1] Macke R. J. et al. 2009. *Planetary and Space Science* doi: 10.1016/j.pss.2009.11.006. [2] Hildebrand A. R. et al. 2009. Abstract #1893. 40th Lunar and Planetary Science Conference. [3] Walton E. L. et al. 2009. Abstract #2072. 40th Lunar and Planetary Science Conference. [4] Consolmagno G. J. and Britt D. T. 1998. *Meteoritics & Planetary Science* 33:1231–1241. [5] Consolmagno G. J., Britt D. T. and Macke R. J. 2008. *Chemie der Erde* 68:1–29. [6] Rochette P. et al. 2003. *Meteoritics & Planetary Science* 38:251–268. [7] Rochette P. et al. 2009. *Elements* 5:223–228. [8] Smith D. L. et al. 2006. *Meteoritics & Planetary Science* 41:355–373.

TIME-RESOLVED STUDIES OF HYPERVELOCITY VERTICAL IMPACTS INTO POROUS PARTICULATE TARGETS: EFFECTS OF PROJECTILE DENSITY ON EARLY-TIME COUPLING AND CRATER GROWTH

B. Hermalyn and P. H. Schultz. Brown University, Providence, RI 02912-1846, USA. E-mail: Brendan_Hermalyn@brown.edu.

**Introduction:** The depth and duration of energy and momentum coupling in an impact shape the formation of the crater and distribution of ejected material. Metrics for crater growth (e.g., crater diameter and depth) and ejecta velocity are usually approached from late-stage parameters [1, 2]. Implicit in most of these studies is a reliance on the point-source approximation, which collapses the initial details of energy and momentum coupling into a single parameter [3]. This approximation breaks down at early times while the projectile is still transferring its energy and momentum to the target, however [4]. The flow-field inside the transient crater is reflected in the ejecta velocity distribution.

In this study, we present new experimental measurements of the ejecta distribution and crater growth from early- to main-stage flow to examine the role of impactor density on the depth and duration of coupling. The technique employed allows expression of the ejecta velocities as a function of both time and launch position for the first time, as well as direct comparison to and benchmarking of hydrocodes.

**Experimental Methodology:** A suite of hypervelocity vertical impact experiments into Ottowa quartz sand was performed at the NASA Ames Vertical Gun Range using a variety of different projectile materials ranging in density from 0.9g/cc to 7.8g/cc. A non-intrusive, high-speed (15,000 fps) imaging technique (particle tracking velocimetry or PTV) tracks the motion of individual sand grains during excavation and allows temporally and spatially resolved measurements of the ejecta velocities.

**Results and Analysis:** The earliest stages of excavation result in high-speed ejecta derived from the upper surface near the impact point. Dense projectiles penetrate deeper and couple later in the target, whereas underdense projectiles couple quickly relatively closer to the surface. Crater growth metrics for both the depth and diameter of the crater demonstrate non-proportional crater growth, where the crater first exhibits more relatively downward growth before transitioning to relatively outward expansion. This flow-field center evolution manifests in migrating ejection angles for much of crater formation. The temporal nature of the data also enables validation of computational parameters (e.g., EOS) using the CTH hydrocode.

**Conclusions:** The effect of projectile density on both the depth and duration of coupling has implications for planetary impacts, affecting models of ballistic ejecta emplacement and source depths of ejecta from primary craters (e.g., Apollo samples). Temporal (rather than spatial) description of the ejecta dynamics allows a physical understanding for ejecta produced by artificial impacts, such as the Deep Impact mission and the recent LCROSS impact. Investigation of the time-resolved growth of the crater is essential for comparison to the azimuthally varying ejecta velocities from oblique impact events.

**References:** [1] Housen et al. 1983. *Journal of Geophysical Research* 88:2485–2499. [2] Barnouin-Jha et al. 2007. *Icarus* 188:506–521. [3] Holsapple and Schmidt. 1987. *Journal of Geophysical Research* 92:6350–6376. [4] Hermalyn and Schultz 2010. *Icarus* 209:866–870.
5158

PHOTOPHORESIS ON CHONDRULES—NEW EXPERIMENTAL RESULTS

A. Hesse, J. Teiser and G. Wurm. Faculty of Physics, University Duisburg-Essen, Essen, Germany. E-mail: alexander.hesse@stud.uni-due.de.

**Introduction:** Photophoresis, as a fourth light-induced force alongside radiation pressure, Poynting-Robertson drag and the Yarkovski effect, exceeds these by orders of magnitude at conditions found in a protoplanetary disk [1, 2].

In contrast to the aforementioned forces, photophoresis is not based on a direct acceleration by photons: Gas molecules of the surrounding atmosphere stick to the surface of a unidirectionally illuminated particle. They take on the different temperatures along the illumination axis and therefore leave the surface at the corresponding thermal velocities. This results in a net momentum transfer that is directed away from the light source (e.g., the central star of the protoplanetary disk).

With its inherent pressure- and particle-size-dependence, it not only qualifies as an additional mechanism for particle transport, but also provides an effective way of sorting. These properties make photophoresis an important aspect in explaining the occurrence of size-sorted chondrules. In a first campaign of microgravity experiments, the efficiency of photophoresis acting on chondrules could clearly be shown. A follow-up project was initialized to shed light on more detailed aspects of photophoretic acceleration, including rotational properties.

**Experiment:** To quantify the effect, particularly with regard to chondrules, experiments were carried out at the ZARM drop tower in Bremen, providing approximately 9 s of microgravity with a residual acceleration of less than $10^{-6}$ g.

Chondrules originating from the L/LL4-class chondrite Bjurböle are exposed to an intense laser beam in the microgravity environment of the drop capsule at varying ambient pressures. Their motion is observed with two synchronized cameras aligned perpendicular to each other and the incident direction of the laser beam. In the recorded videostreams the three-dimensional motion of the chondrules can be obtained and rotational frequencies can be determined.

**Results:** The results for the linear motion are consistent with the outcome of the initial study [3] and the measured accelerations of approximately $10^{-4}$ m s$^{-2}$ are in accordance with the estimates of model forces [4]. Moreover for certain rotating chondrules transverse components in the acceleration could be detected, which hints at a process similar to the Yarkovski effect but with photophoretic momentum transfer. These results are the first experimental data on the interplay between particle rotation, photophoresis and material transport.

**References:** [1] Wurm G. and Krauss O. 2006. *Icarus* 180:487–495. [2] Wurm G. and Haack H. 2009. *Meteoritics & Planetary Science* 44:689–699. [3] Wurm G. et al. 2010. *Icarus* 208:482–491. [4] Rohatschek H. 1995. *Journal of Aerosol Science* 26:717–734.

5144

TYPE II CHONDRULE ORIGIN IN O AND C CHONDRITES

R. H. Hewins¹,² and B. Zanda¹,². ¹MNHN & CNRS, 75005 France.

²Department of Earth and Planet Sciences, Rutgers University, Piscataway, NJ, USA. E-mail: hewins@rci.rutgers.edu.

Relict grains are widespread in chondrules: many Type I chondrules contain olivine not crystallized from their associated Al-rich melts [1], including grains with very low $\Delta^{17}O$ [2, 3] and GOA with several specific O isotope compositions closer to TFL than solar [4]. This suggests chondrule formation with various dust/gas ratios and relatively inefficient recycling. Addition of SiO to the rims of many chondrules changes them from IA to IAB to IB, perhaps in regions of higher dust/gas ratio generating high $P_{SiO}$ during chondrule heating, with higher $\Delta^{17}O$ [5]. The Nancy open-system chondrule model [1, 3–5] has recently been extended to Type II chondrules formed from Type I material by adding Fe and O from the gas [6]. We use our recent extensive EMP data [7] for Type II chondrules in Paris and other CM chondrites to develop the Nancy-type II chondrule model.

Type II chondrules contain abundant Na, FeO, and S, consistent with formation in large dusty clumps yielding high $P_{Na}$, etc. The common survival of relict forsterite of Type I composition in Type II chondrules in both CC and OC strongly supports a Type I precursor. Oxidation of Fe from metal in Type I chondrules [6] would explain the jump in FeO from Type IA olivine composition to the most magnesian Type IIA chondrule olivines. However, Type II chondrules in both CC and OC have positively correlated FeO and MnO, consistent with condensation rather than oxidation [7]. Olivine in Type II chondrules in Paris and other CMs extends up to $Fa_{70}$, more ferroan than in OC. The OC chondrules experienced less condensation of FeO, possibly because of lower $P_{H_2O}$, and we find $f_{O_2}$ from QFI about ½ log unit lower.

Type II chondrule olivine in CMs has an Fe/Mn ratio like that in CO [8], micrometeorites and Wild 2 [9], but distinct from that in OC [7] and therefore comes from a different reservoir. The occurrence of IIA-IIB chondrules in both Paris and Semarkona indicates Fe- and Si-enrichment paths in both reservoirs. CR chondrites, on the other hand, contain Type IIs with intermediate Fe/Mn and O isotope characteristics split between CM-CO and OC reservoirs [8, 10], and thus indicate changing redox conditions related to variations in dust/gas ratios and/or ice contents.

OC contain clusters of chondrules, both Type I and Type II, with similar $\Delta^{17}O$, consistent with recycling side by side. This suggests turbulent mixing on a local scale before and during transient heating events. In CC, Type II chondrules are much less abundant and formed at higher $f_{O_2}$ than in OC while Type I chondrules have $\Delta^{17}O$ mostly closer to solar than those in OC. Thus Type IIs were moved to a domain containing abundant dust, including Type Is, themselves formed in several oxygen isotopic reservoirs. This assemblage was not recycled before accretion.

**References:** [1] Libourel G. et al. 2006. *Earth and Planetary Science Letters* 251:232–240. [2] Yurimoto H. and Wasson J. T. 2002. *Geochimica et Cosmochimica Acta* 66:4355–4363. [3] Jones R. H. 2004. *Geochimica et Cosmochimica Acta* 68:3423–3438. [4] Libourel G. and Chaussidon M. 2011. *Earth and Planetary Science Letters* 301:9–21. [5] Chaussidon M. et al. 2008. *Geochimica et Cosmochimica Acta* 72:1924–193. [6] Villeneuve J. 2010. Ph.D., CRPG Nancy. [7] Hewins R. H. et al. 2011. Abstract #1914. 42nd Lunar and Planetary Science Conference. [8] Berlin J. et al. 2009. *Meteoritics & Planetary Science* 46:513–533. [9] Dobricá E. 2010. Ph.D., Univ. Paris Sud. [10] Connolly H. C. Jr. and Huss G. R. 2010. *Geochimica et Cosmochimica Acta* 74:2473–2483.
5175

### 3D STRUCTURE OF CARBONACEOUS CHONDRITES

D. C. Hezel¹,², P. Elangovan¹,³, R. L. Abel¹,⁴, L. E. Howard¹ and R. Armstrong¹. ¹Natural History Museum, Department of Mineralogy, Cromwell Road, SW7 5BD, London, UK. ²Department of Geology and Mineralogy, University of Cologne, Zülpicher Str. 49b, 50674 Cologne, Germany. ³Department of Electronic Engineering, University of Surrey, Guildford, GU2 7XH, Surrey, UK. ⁴Department of Earth Science and Engineering, Imperial College, London, SW7 2AZ, UK. E-mail: dominik.hezel@uni-koeln.de.

**Introduction:** A number of important conditions of the solar nebula prevailing during the formation of chondrite components can only be learned from petrographic data. Component modal abundances for accurate mass balance calculations, chondrule size distributions to determine sorting processes, component spatial distributions for parent body agglomeration processes, distribution of metal and sulfide between chondrules and matrix to study their origin and formation, orientation of chondrules to study the place of impregnation of their paleomagnetic field and many more are examples of cosmochemical questions where petrographic data are mandatory.

So far petrographic studies were mostly restricted to 2D slices. Such measurements often provide inaccurate results, or are entierly unsuitable for a given problem. Micro computer-aided tomography (μ-CT) is an emerging technique that has the potential to solve these shortcomings and provide accurate 3D petrographic measurements. In recent years, much progress was made in scanning and quantifying rocks and meteorites [1–6]. First, the quality of the scans became much more accurate, with resolutions down to <1 µm. Second, powerful computer hard-and software now allows to reduce the tens of GB of data a single scan produces. We developed a plugin for ImageJ especially designed to quantify meteorite μ-CT data. We are continuously adding new tools to this plugin to make it a standard software package for reducing meteorite μ-CT data.

**Technique:** We used the Metris X-Tek HMX ST 225 CT at the Natural History Museum, London for scanning. Density is the prime information stored in the voxels (3D equivalent of pixels) of a tomographic scan. Segmenting chondrules from matrix is in particular difficult as both have similar densities and, hence, overlap greatly in gray values. ImageJ is a widely used, public domain image analyses tool. We developed a plugin—called *PhaseQuant*—that, among other features, employs statistical methods to automatically segment and quantify components whose gray values overlap, such as chondrules and matrix.

**Results:** We scanned a range of carbonaceous chondrites (Allende, Renazzo, Al Rais, etc.) and are producing a set of standard 3D images and videos of the samples. We currently apply *PhaseQuant* to quantify the data sets to determine component 3D modal abundances, size as well as their spatial distributions. In addition, sulfide and metal abundances between chondrules and matrix are studied. The results will be presented at the meeting.

**References:** [1] Ebel D. S. and Rivers M. L. 2007. *Meteoritics & Planetary Science* 42:1627–1646. [2] Ebel D. S. et al. 2008. *Meteoritics & Planetary Science* 43:1725–1740. [3] Friedrich J. M. et al. 2008a. *Planetary and Space Science* 56:895–900. [4] Friedrich J. M. 2008b. *Computers and Geosciences* 34:1926–1935. [5] Friedrich J. M. et al. 2008c. *Earth and Planetary Science Letters* 275:172–180. [6] Hezel D. C. et al. 2010. *Earth and Planetary Science Letters* 296:423–433.

5245

### XANES AND FERROUS-FERRIC RATIOS IN THE NAKHLITE SECONDARY PHASES

L. J. Hicks¹, J. C. Bridges¹ and S. J. Gurman². ¹Space Research Centre, University of Leicester, Leicester LE1 7RH, UK. E-mail: j.bridges@le.ac.uk. ²Department of Physics & Astronomy, University of Leicester, Leicester LE1 7RH, UK.

**Introduction:** X-ray absorption near edge structure (XANES) observes spectra created by the energies of the 3d orbitals in transition metals like iron [1]. Our aim is to report observations and analyses of the Fe K XANES in standards and nakhlite samples in order to gain a calibration between the XANES and the ferrous-ferric ratio of the measured material. This will allow us to constrain the oxidation state, and how it changes, within the nakhlite alteration minerals such as phyllosilicate [2].

**Method and Samples:** We have used the I18 Beamline for Microfocus Spectroscopy at the UK *Diamond Synchrotron*. This enabled a spatial resolution of 2 × 2.5 µm, measuring energy spectra ranging from 6900 to 7500 eV with a resolution of 0.25 eV, but concentrating on the XANES region from 7092 to 7122 eV with an energy resolution of 0.1 eV. The nakhlite samples have also been analyzed with Scanning Electron Microscopy at 15 kV. The nakhlite samples are NWA 817, Y000593, Y000749, Nakhla, NWA 5790, Governador Valadares, Lafayette, NWA 998, and Miller-Range 03346, along with standards of goethite, haematite, nontronite, magnetite, augite, pyrrhotite, San Carlos olivine, and Barwellolivine.

The pre absorption edge centroid position was calculated by measuring the average position of the normalized intensity summed over the XANES prepeak region subtracted from a curve-fitting baseline. Thus, the calculated position is greatly influenced by the structure of the L₂ and L₃ edges observed as prepeak features, with the L₃ edge typically consisting of two significant peaks which vary in intensity between the left peak (approximately 7112.2 eV) and the right peak (approximately 7113.7 eV) attributed to the presence of Fe²⁺ and Fe³⁺ respectively [3].

**Results and Conclusions:** Barwell L6 olivine (Fe³⁺/ΣFe = 0) has K-edge positions and calculated pre-edge centroid positions of 7118.8 and 7112.3 eV. With an increased oxidation state, as seen in hematite and goethite (Fe³⁺/ΣFe = 1), the edge positions and pre-edge centroid were 7122.2 and 7114.7 eV respectively. There is a clear shift seen in these and other standards, in the pre-edge features and edges associated with increasing ferrous-ferric ratios.

XANES measurements across veins, observed in the nakhlites, showed increases in energy for the K-edge and centroid position towards the central gel feature of the vein. Across one particular Lafayette vein, ten points each 2 µm apart were analyzed, the K-edge and centroid increased from 7118.6 and 7112.1 eV, in the surrounding olivine, to 7121.0 and 7114.8 eV in the central gel of the vein. Based on these observations, the central gel feature has K-edge and centroid positions between those of the magnetite and hematite standards, indicating an approximate Fe³⁺/ΣFe ratio between 0.67 and 1.0. Observing the XANES spectra throughout these measurements, there is also a distinct change in the pre-edge structure in the outer-vein material from that of the surrounding olivine. This outermost vein material is phyllosilicate, measuring a ratio Fe³⁺/ΣFe ≈ 0.4 and becomes more ferric with each measurement towards the central gel of the vein. Other vein minerals found in Lafayette include siderite, where XANES suggests a Fe³⁺/ΣFe ratio between 0 and 0.2.

By observing the approximate normalized intensities of the two pre-edge peaks, a left/right peak intensity ratio reveals a trend from a high left peak changing to an increasing right peak associated with an increased Fe³⁺/ΣFe ratio from 0 to 1, as measured in the standards. Such a trend is also observed in the veining features from the surrounding olivine into the vein and to the central gel, again indicating an increase in oxidation state towards the centre of the vein.

**References:** [1] Bunker G. 2010. In *Introduction to XAFS*. p. 137. [2] Changela H. G. and Bridges J. C. 2011. *Meteoritics & Planetary Science* 45:1847–1867. [3] Cressey G. et al. 1993. *Physics and Chemistry of Minerals* 20:111–119. [4] Marion G. M. et al. 2003. *Geochimica et Cosmochimica Acta* 67:4251–4266.
5154

NEUTRON CAPTURE RECORDS OF MESOSIDERITES

H. Hidaka¹ and S. Yoneda². ¹Department of Earth & Planetary Systems Science, Hiroshima University, Higashi-Hiroshima 739-8526, Japan. E-mail: hidaka@hiroshima-u.ac.jp. ²Department of Science and Engineering, National Museum of Nature and Science, Tokyo 169-0073, Japan.

**Introduction:** Mesosiderites are stony-iron meteorites composed of approximately equal fractions of metallic Fe-Ni and silicates. The silicates have mostly basaltic, gabbroic, and pyroxenitic components, and are petrologically similar to eucrites and howardites. The cosmic-ray exposure ages of mesosiderites are intermediate between those of stony and iron meteorites, and vary from <10 to 340 Ma [1], and they are long enough to study their exposure history from the accumulation of the cosmogenic products [2-4]. Our interest is to use the neutron irradiation records in mesosiderites (1) to determine the dependence of the thermal neutron production rate on the chemical composition of the target, and (2) to investigate a possibility of the initial irradiation record of mesosiderite silicates on the parent body before formation of the mesosiderites.

**Experiments:** Six mesosiderites, Dalgaranga, Estherville, Morristown, NWA 1242, NWA 2932, and Vaca Muerta, were used in this study. Silicate inclusions, each with a mass of 0.2-0.3 g, collected from each sample were digested individually using an HF-HClO₄ mixture. The sample was then evaporated to dryness and dissolved in 1 mL of 2 M HCl. The Sm and Gd fractions were chemically separated using a conventional ion exchange method [5]. The isotopic compositions of Sm and Gd were determined using a VG54-30 thermal ionization mass spectrometer equipped with seven Faraday cup collectors.

**Results and Discussion:** All six samples showed significant ¹⁵⁰Sm/¹⁴⁹Sm and ¹⁵⁸Gd/¹⁵⁷Gd isotopic shifts from neutron capture reactions corresponding to neutron fluences of (1.3-21.8) × 10¹⁵ n cm⁻². Among the six mesosiderites used in this study, the thermal neutron capture record of the Vaca Muerta silicates differed from those of the other mesosiderites, and showed a larger fluence of neutrons than expected based on its cosmic-ray exposure age. The thermal neutron fluence of the Vaca Muerta silicates was 0.67 × 10¹⁶ n cm⁻², i.e., larger than the expected value (1.51 × 10¹⁶ n cm⁻²). The parameter for the degree of neutron thermalization, ε<sub>Sm</sub>/ε<sub>Gd</sub>, also revealed a significant difference between the thermal neutron production conditions in Vaca Muerta (0.76) and the other mesosiderites (0.93-1.20). These results suggest a two-stage irradiation occurred for the silicates of Vaca Muerta on the surface of the parent body (> 50 Ma) and during its transit to earth (138 Ma). Our model is consistent with the results of the ⁸¹Kr-Kr CRE ages of the Vaca Muerta pebble, which were estimated to be > 60 Ma in the first CRE age before the second-stage irradiation [6].

**References:** [1] Terribilini D. et al. 2000. *Meteoritics & Planetary Science* 35:617-628. [2] Begemann F. et al. 1976. *Geochimica et Cosmochimica Acta* 40:353-368. [3] Albrecht A. et al. 2000. *Meteoritics & Planetary Science* 35:975-986. [4] Welten K. et al. 2001. *Meteoritics & Planetary Science* 36:939-946. [5] Hidaka H. and Yoneda S. 2007. *Geochimica et Cosmochimica Acta* 71:1074-1086. [6] Bajo K. and Nagao K. 2011. *Meteoritics & Planetary Science*, in press.

5511

DEEP IMPACT SITE AS IMAGED BY STARDUST NEXT ON
COMET 9P/TEMPEL 1 REVEALS A STRONG SURFACE AND
EXTENSIVE EJECTA BLANKET

A. R. Hildebrand¹, M. I. Ibrahim¹, R. D. Cardinal¹ and L. Maillet¹.
¹Department of Geosciences, University of Calgary, Calgary, AB, T2N 1N4, Canada. E-mail: ahildebr@ucalgary.ca.

**Introduction:** The Stardust NexT spacecraft imaged much of ~6 km-diameter comet Tempel 1's surface during its February 14, 2011, flyby. The spacecraft captured the crater created during the Deep Impact mission when a "smart" 370 kg projectile impacted its surface at 10.2 km s⁻¹ on July 4, 2005 [1]. One of the Deep Impact mission goals was to assess the surface physical characteristics of Tempel 1 through study of the size and shape of the resulting crater, however, the ejected gas and dust plumes were sufficiently opaque and long lasting to prevent the Deep Impact flyby spacecraft from imaging the impact site during its 800 s-long viewing window necessitating another imaging effort.

**Imaged Crater and Ejecta Blanket:** The impact location was projected using Deep Impact smart projectile imagery coupled with telemetered guidance data [1]. The impact site reported by the Stardust NexT team at the post-flyby briefing was a relatively subdued 100-150 m-diameter feature at the predicted impact position. However, the impact actually occurred ~400 m north (comet's reference frame) where a ~20 m diameter bright spot occurs with two associated bright rays. The impact location is confirmed by the position of the initial light captured by the Deep Impact flyby spacecraft; it is further confirmed by both the position of the shadow generated by the impact ejecta plume and the path of specular reflections from the comet's surface of the glowing projectile as it traversed a dust layer near the comet.

The impact feature is a relatively dramatic bright albedo feature on Tempel 1's surface, and is the greatest visible change in appearance of the comet's surface between the two flybys. The impact was oblique with an elevation angle to the surface of ~30° [1, 2]; at low impact angles both natural and artificial craters develop asymmetric ejecta blankets. The ~130° chevron-shaped ejecta blanket aligns to the impact direction with ejecta spanning a broad "V" open downrange. The two rays defining the edges of the chevron extend ~150 m; this ~7° ratio is typical of the crater diameter to ray lengths observed at other oblique impact sites.

**Implications for Tempel 1 Surface and Cratering Processes on Small Bodies:** The ~20 m crater size and ejecta blanket characteristics imply a surface strength of >~10 kPa near the strong end of previous estimates [2]. The ~20 m size also implies an ~10⁵ kg ejecta volume near the small end of proposed ejecta volumes [2]. The extensive ejecta blanket (that survived a perihelion passage before the second imaging attempt) indicates ejecta excavation at velocities of order 1 mm s⁻¹ for a large fraction of the crater's ejecta despite the 10.2 km s⁻¹ impactor speed and a relatively strong impacted surface at odds with current expectations; this also implies that regolith generation by hypervelocity impacts on small bodies is an efficient process. The ~1.5 km-thick, comet-shrouding dust layer inferred from reflections of the glowing projectile near impact explains the initial diffuse light signature recorded by the Deep Impact flyby spacecraft; light from the compact (subpixel size) ejecta cone was scattered to form a broad symmetric shape several pixels across.

**References:** [1] A'Hearn et al. 2005. *Science* 310:258-264. [2] Holsapple K. A. and Housen K. R. 2007. *Icarus* 187:345-356.
IMPACT CRATERING EXPERIMENTS ON POROUS SINTERED
TARGETS OF DIFFERENT STRENGTHS

K. Hiraoka¹, S. Hakura¹, A. M. Nakamura¹, A. Suzuki², and S. Hasegawa³.
¹Earth and Planetary Science, Kobe University, 1-1 Rokkodai-cho, Nada-ku, Kobe, 657-8501 Japan. E-mail: amnakamu@kobe-u.ac.jp.
²Center for Planetary Science (CPS), Kobe University, ³ISAS/JAXA, 3-1-1 Yoshinodai, Chuo-ku, Sagamihara, Kanagawa 252-5210, Japan.

**Introduction:** In order to derive from crater morphologies or the dimensions on the surface of asteroids some information on the physical properties of the surface or the impacting bodies, it is important to understand the impact cratering process and derive scaling laws. Extensive studies on cratering have been performed experimentally and theoretically in the past decades, however, there're still not enough laboratory data of cratering on porous targets. In this laboratory study, we changed the strength of the targets while we kept the porosity constant.

**Experiments:** The targets were sintered glass beads of 40% porosity [1]. The static tensile strength of the targets was measured by Brazilian disc test [2] and between 6.5 and 640 kPa. These values were similar to or lower than those of carbonaceous chondrite meteorites [3]. Low-velocity impact experiments (velocities of 240 and 344 m s⁻¹) were performed using a small He gas-gun at Kobe University. Projectiles were glass and nylon spheres of 1/8 inches and were shot vertically to the targets. We also performed high-velocity impact experiments (2.94–3.46 km s⁻¹) for the strongest targets using a two-stage light-gas gun at the Institute of Space and Astronautical Science (ISAS). Projectiles were 0.94 mm glass and 1.18 mm alumina spheres and were shot horizontally.

**Results:** The depth-diameter ratio of the craters was less than 0.2 for the craters on two weakest targets (strengths of 6.5 and 38 kPa). Others had the ratio larger than 0.3. The crater diameter increased with decreasing strength of the targets for the low-velocity shots. However, the diameter was less dependent on the strength for the weaker targets. The change of the dependence probably indicates the strength-gravity transition of the cratering process. We calculated the crater volume from the vertical cut profile of the craters and applied a crater scaling law that covers both strength and gravity regimes [4] to the low-velocity results to derive the scaling parameters. We found that thus derived scaling parameters reproduced the crater volumes of the high-velocity experiments.

**Summary:** We performed impact cratering experiments of constant porosity but different strength using sintered glass beads targets. It was shown that there was a change in dependence of the crater diameter on target strength. We derived the crater scaling parameters for this porous material that covers both strength and gravity regimes of cratering.

**References:** [1] Setoh M. et al. 2010. *Icarus* 205:702-711. [2] Nakamura A. M. et al. 2007. *Journal of Geophysical Research* 112:E02001. [3] Tsuchiyama A. et al. 2009. 72nd Annual Meeting of the Meteoritical Society. pp. 5189. [4] Holsapple K. 1993. *Annual Review of Earth Planetary Sciences* 21:333-73.

THE OMANI-SWISS METEORITE SEARCH PROJECT—RECENT
CAMPAIGNS AND OUTLOOK

B. A. Hofmann¹,³, E. Gnos², U. Eggenberger³, F. J. Zurfluh³, S. Boschetti⁴ and A. Al-Rajhi⁵.
¹Natural History Museum Bern, Bern, Switzerland. E-mail: beda.hofmann@geo.unibe.ch.
²Muséum d'histoire naturelle de la Ville de Genève, Genève, Switzerland.
³Institut für Geologie, Universität Bern, Bern, Switzerland.
⁴Institut für Geophysik, ETH Zurich, Zurich, Switzerland.
⁵Directorate General of Minerals, Ministry of Commerce and Industry, Muscat, Oman.

**Introduction:** A total of approximately 5500 meteorite samples recovered during ten field campaigns are attributable to approximately 690 fall events. We find that a significant part of the surface of Oman is suitable for meteorite search and that more than two-thirds of all meteorites found belong to large strewn fields containing several hundreds to thousands stones. The 40-day 2009/2010 field campaign was conducted mainly along a transsect from NW to SE, from the interior of Oman to the coast. The 23-day 2011 search campaign was conducted along a SW-NE axis in the central part of the Oman desert. Main aims were the continuation of systematic searches by foot in a variety of areas, test searches in unsampled areas and the conduction a geomagnetic survey in the JaH 091 strewn field.

**Systematic Searches by Foot:** During the 2009/2010 and 2011 field campaigns we completed systematic foot searches on a total of 21 quadrangles of 500 × 500 m each (a total of 5.25 km²), covering all major areas and soil types of interior Oman suited for meteorite search. We found at least 12 different meteorites (48 stones) on this surface, corresponding to an apparent fall density of 2.3 meteorites km⁻². This is significantly higher than estimated find densities from searches by car (1.0 km⁻²) where small meteorites (few g) are likely missed.

**JaH 091 Strewn Field:** In the large (51.2 × 7.2 km) JaH 091 strewn field [1] we performed a geomagnetic survey of the largest impact sites, after recovery of all visible fragments. Only at the largest impact site a significant unrecognized mass (several 100 kg of fragments) was discovered. Future work on the data will constrain the buried mass, allowing a determination of the total mass of the JaH 091 meteorite fall on the ground (estimated at approximately 3000 kg).

**Outlook:** Future field work will aim to continue the quantification of meteorite densities in different areas of the Omani desert.

**Reference:** [1] Russell S. S., Folco L., Grady M., Zolensky M. E., Jones R., Righter K., Zipfel J., and Grossman J. N. 2004. *Meteoritics & Planetary Science* 39:A215–A272.
MAGNETIC SIGNATURE OF NWA 5029: A BASALTIC SHERGOTTITE RELATED TO NWA 480/1460

V. H. Hoffmann¹,², T. Mikouchi³, M. Torii⁴, M. Funaki⁵ and R. Hochleitner⁶. ¹Department of Geosciences, University of Tuebingen, Sigwartstr. 10, 72076 Tuebingen, Germany. ²Department of Geography-Environmental Sciences, University of Munich, Munich, Germany. ³Department of Earth and Planetary Sciences, University of Tokyo, Tokyo, Japan. ⁴Department Geosph.-Biosph.-Syst. Sci., Okayama University of Science, Okayama, Japan. ⁵NIPR, Tokyo, Japan. ⁶Mineralogical State Collection, Munich, Germany. E-mail: hoffmann.viktor@gmx.net.

**Introduction:** Northwest Africa (NWA) 5029 is a small, unpaired 14.67 g partly fusion-crusted stone which was found in Morocco in 2003 and classified as a basaltic shergottite [1] (diabasic-mafic-intermediate shergottite [2]). Here we report its magnetic properties and compare the data with NWA 480/1460, a diabasic/basaltic shergottite with similar mineralogy and texture [3, 4]. The opaque minerals have been identified and investigated by optical microscopy and EMPA.

**Samples and Investigations:** We used several chips and fragments of the type mass (approximately 1.4 g) of NWA 5029 for the magnetic experiments. For comparison, chips of NWA 480 (plus fragments) and NWA 1460 (both paired) were used. First magnetic data of NWA 480/1460 have been published by [5], whereby NWA 1460 corresponds to the sample of this study.

**Results:** The natural remanent magnetization (NRM) of NWA 5029 could be attributed to impact shock related processes on Mars, eventually resulting from the last and ejecting event, representing an SRM (shock remanent magnetization). The NWA 480/1460 NRM is most likely of terrestrial origin, an IRM which might be due to remagnetization by the use of hand magnets already in field by the nomads. The magnetic susceptibility (MS) values of the three stones are in the range which was found by [6] for Martian meteorites, and typical for diabasic/basaltic shergottites. The degree of anisotropy of magnetic susceptibility (AMS) is quite low in all cases (P between 1.071 and 1.090) despite the high degree of shock which should be reflected by the AMS. In the low-temperature range all three samples show essentially identical behavior: two magnetic transitions are detected in the range of 40-60 K (ilmenite and/or chromite) and at 120 K (magnetite) in both FC and ZFC IRM curves.

**Discussion and Conclusion:** The magnetic remanences (NRM) in these shergottites are carried by pure magnetite (Fe₃O₄) with a Curie-temperature (T_c) of about 575 °C. In agreement with other data, the magnetite is apparently exsolved (magnetite with ilmenite lamellae), that is, high-temperature oxidized titano-magnetite. Minor effects are related to substituted magnetite with a T_c of about 525 °C. Fe-sulfides such as monoclinic pyrrhotite (Fe₇S₈) could not be detected. The magnetic signature and texture of all three stones is very similar which leads to our hypothesis that the three rocks are very likely launch paired. We can conclude that the high-temperature oxidation occurred on the surface of Mars either during the primary formation processes or as a result of shock (at least 35 GPa) from the event which ejected the meteorites from the Mars surface into space.

**References:** [1] Mikouchi T. and Barrat J. A. 2009. Abstract #5344. 72nd Annual Meteoritical Society Meeting. [2] Irving A. 2011. http://www.imca.cc/mars/martian-meteorites.htm. [3] Barrat J. A. et al. 2002. *Meteoritics & Planetary Science* 37:487-499. [4] Irving A. and Kuehner S. 2003. Abstract #1503. 34th Lunar and Planetary Science Conference. [5] Rochette P. et al. 2005. *Meteoritics & Planetary Science* 40:4:529-540. [6] Rochette P. et al. 2009. *Meteoritics & Planetary Science* 44:405-427.

INCIPIENT PHYLLOSILICATE STRUCTURES IN SYNTHETIC Mg-SILICATES HYDRATED IN A HUMID CHAMBER

E. J. Hoffman¹, L. A. Veblen¹ and D. R. Veblen². ¹Physics Department, Morgan State University, Baltimore, MD 21251, USA. E-mail: eugene.hoffman@morgan.edu. ²Department of Earth and Planetary Sciences, Johns Hopkins University.

**Introduction:** In reactions with water synthetic Mg-silicate smokes from the Goddard Space Center Astrochemistry Lab [1] undergo a color change and produce proto-phylosilicates. These products appear to be the same whether hydration occurs during immersion in liquid water [2] or by exposure to a humid atmosphere [3]. The products have been detected by X-ray diffraction [3] and by electron microscopy [2, 3], although the protocrystalline structures disintegrate very easily under the electron beam.

We report here continuing efforts to characterize humid-chamber reaction products under the electron microscope.

**Experimental:** Synthetic Mg-silicate smoke was obtained, stored, pre-treated with flowing dry N₂, and incubated over deoxygenated deionized water as described [3]. Incubation temperatures were 17-20 °C.

A Philips EM420 electron microscope was used with several sample preparation methods, including suspension in ethanol and dropping onto a holey carbon grid [3]. Imaging and electron diffraction conditions were also subject to much experimentation. Experience with similarly beam-labile material [4] served as a guide.

**Results and Discussion:** Images and diffraction patterns were obtained more reliably although further work is warranted to examine the incipient structures more thoroughly before they fall apart.

**Acknowledgment:** This work was supported in part by an NSF HBCU-UP grant to Morgan State University and an NSF GeoEd grant to EJH.

**References:** [1] Nuth III, J. A. et al. 2000. *Journal of Geophysical Research* 105:10,387-10,396. [2] Chizmadia, L. J. et al. 2006. Abstract #2187. 37th Lunar and Planetary Science Conference. [3] Hoffman E. J. et al. 2011. Abstract #2526. 42nd Lunar and Planetary Science Conference. [4] Stubbs J. C. et al. 2009. *Geochimica et Cosmochimica Acta* 73:1563-1576.
## SHOCK BAROMETRY OF THE SILJAN IMPACT STRUCTURE, SWEDEN

S. Holm¹, C. Alwmark¹, W. Alvarez² and B. Schmitz¹. ¹Department of Geology, Lund University, Sölvegatan 12, 223 62 Lund, Sweden, E-mail: sanna.holm@geol.lu.se. ²Department of Earth and Planetary Science, University of California, 307 McCone Hall, Berkeley, CA 94720-4767, USA.

**Introduction:** The Siljan structure in south-central Sweden is the largest verified impact structure in Europe with an estimated diameter of ~50 km [1]. The structure is dated to $377 \pm 2$ Ma [2] and strongly eroded, exposing a surface that was originally located beneath the crater floor [1]. Despite its size, Siljan is poorly studied, and basic facts about the original morphology of the structure remain elusive. Information about the distribution of shocked quartz and shock barometry are lacking. Previous studies of microscopic shock metamorphic features from the structure are limited both with respect to the number of localities and the amount of data presented (e.g., [3, 4]). Here we present the first detailed characterization of the shock level distribution across the Siljan structure by extensive studies of shock metamorphic features in quartz.

**Materials and Methods:** Three hundred thin sections from seventy three localities across predominantly the central part of the Siljan structure were searched for shock metamorphic features in quartz with optical and u-stage microscopy (see [5] and [6] for details on technique). Measured optic axes and poles perpendicular to planar deformation feature (PDF) planes in individual quartz grains were determined and indexed using a stereographic projection template.

**Results and Discussion:** Out of 73 localities, 21 display microscopic shock metamorphic features (planar features; PFs, and PDFs). The PDF orientation and frequency pattern reveals that the samples have been subjected to different shock pressures. The most strongly shocked samples record pressures of 16–20 GPa, although locally in the samples higher pressures causing melting were attained, and occur at the center of the structure. Outwards from the geographical center pressures decrease in circular envelopes to ~5 GPa at 10 km distance. Observations of shatter cones outside of the zone with shocked quartz extend the total shocked area to approximately 30 km in diameter.

The deep erosion level of the structure prevents any assumptions about the original size of Siljan based on present morphological features. However, with the distribution of shocked quartz across the structure defined, we can estimate the original dimensions based on calculations in [7] (and references therein). Using the relationships summarized in [7], we conclude that the Siljan impact structure can have been as large as 90 km in diameter before erosion.

**References:** [1] Grieve R. A. F. 1988. In *Deep drilling in crystalline bedrock*. The Deep Gas Drilling in the Siljan Impact Structure, Sweden and Astroblemes vol.1. Springer Verlag, pp. 328–348. [2] Reimold W. U. et al. 2005. *Meteoritics & Planetary Science* 40:591–607. [3] Svensson N.-B. 1971. *Nature Physical Science* 229:90–92. [4] Tamminen J. and Wickman F. E. 1980. *GFF* 102:275–278. [5] Engelhardt W. von, and Bertsch W. 1969. *Contributions to Mineralogy and Petrology* 20:203–234. [6] Ferrière L. et al. 2009. *Meteoritics & Planetary Science* 44:925–940. [7] Therriault A. M. et al. 1997. *Meteoritics & Planetary Science* 32:71–77.

## PILOT DIGITALIZATION PROJECT FOR THE METEORITE COLLECTION AT THE ROBERT A. PRITZKER CENTER FOR METEORITICS AND POLAR STUDIES

J. L. Holstein¹, E. A. Pelker¹ and P. R. Heck¹,². ¹Robert A. Pritzker Center for Meteoritics and Polar Studies, Department of Geology, The Field Museum, 1400 S. Lake Shore Drive, Chicago, IL 60605, USA. E-mail: jholstein@fieldmuseum.org. ²Chicago Center for Cosmochemistry, The University of Chicago, Chicago, IL, USA.

**Introduction:** Digitalization of a research and educational meteorite collection is an important part of collection management. It will allow specimens to be examined remotely by researchers before a loan request is submitted. Through our website it will also serve to educate the public about the classification and components of meteorites. It also has the potential of evolving into an interactive exhibit and applications for handheld electronic devices. Other benefits include the reduced handling of fragile specimens, preserving a record of the appearance of a meteorite before cutting, and the potential to calculate the bulk density of individual meteorite specimens. We are undertaking a pilot project to digitalize selected specimens from the Robert A. Pritzker Center for Meteoritics and Polar Studies’ meteorite collection using three techniques: basic 2D digital photography, rotational (3D) digital photography, and 3D laser scans. The resulting macroscopic visual representations will also increase the visibility of our collection to a global audience of scientists, students, and the general public. The goal of this pilot project is to test three different digitalization techniques on selected specimens of different meteorite groups. This will form the basis for a proposal to digitize a larger fraction of our collection.

**Materials and Methods:** With over 8,000 meteorite specimens in the collection, we selected representative examples of the most important meteorite groups for this pilot study. Chosen were examples of ordinary chondrites (H, L, LL, R, E), carbonaceous chondrites, primitive and differentiated achondrites including Martian and lunar meteorites, pallasites, mesosiderites, and irons. Digital photographs of new specimens will be obtained before accession into our collection. This basic form of digitalization will also be used for all meteorites that we selected from our collection. A subgroup of cut specimens that show internal texture are placed on a turntable and are photographed at regular rotational increments. The set of images are combined into an animation that will be displayed on a webpage. For a third subgroup of specimens we will use a desktop 3-D laser scanner. This results in geometric models of the specimens that will later be placed on a webpage allowing users to visually study the specimens remotely. The models will be calibrated and allow us to calculate the bulk density of a specimen. This data can then be used to study the physical properties of meteorites, e.g., [1, 2].

**Acknowledgments:** The Robert A. Pritzker Center for Meteoritics and Polar Studies is supported by the Tawani Foundation.

**References:** [1] Consolmagno G. J. et al. 2006. *Meteoritics & Planetary Science* 41:331–342. [2] Macke R. J. et al. 2011. *Meteoritics & Planetary Science* 46:311–326.
5395

THE PLANETESIMAL SHOCK MODEL FOR CHONDRULE FORMATION: A MORE QUANTITATIVE ASSESSMENT

L. L. Hood¹ and S. J. Weidenschilling². ¹Lunar and Planetary Lab, University of Arizona, Tucson, Arizona 85721, USA. E-mail: lon@lpl.arizona.edu. ²Planetary Science Institute, 1700 E. Ft. Lowell, Tucson, Arizona 85719, USA.

**Introduction:** Gas dynamic shock waves in a low-temperature nebula are currently considered to be a plausible mechanism for providing the repetitive transient heating events that were apparently responsible for chondrule formation (e.g., [1]). Possible meteoritic constraints on the sources of chondrule-forming shocks include (1) isotopic evidence that chondrule formation most probably began 1–1.5 Myr after the formation of CAIs and continued for several Myr; (2) coexistence of chondrules in chondrites with products of later parent-body processes (e.g., igneous rock fragments); and (3) inferences that chondrule formation regions were large (more than several hundred km across) but also relatively localized to explain observed differences in properties of chondrules from different chondrite groups.

One class of nebular shocks that can potentially satisfy the above constraints is shocks generated by planetesimals passing through Jovian resonances, assuming that Jupiter formed approximately 1 Myr after CAIs [2–4]. Included are both planetesimal bow shocks produced by bodies ejected into eccentric orbits and impact vapor plume shocks produced by high-velocity collisions between planetesimals. Here, we employ an improved planetesimal accretion and orbital evolution code that includes the damping effects of collisions and mutual gravitational scattering between embryos. The goal is to provide more accurate estimates of potential chondrule formation efficiency by both planetesimal shock mechanisms.

**Planetesimal Evolution Code and Results:** A population of several hundred discrete bodies with diameters between approximately 750 and 2000 km and a total mass of approximately 1 Earth mass is initially distributed in 20 radial zones from 2 to 4 AU. In addition, a background population of smaller bodies is represented in the model by a statistical continuum. A symplectic N-body integrator is applied to calculate the orbital evolution of the large bodies. The accretion rate of discrete bodies is tracked and is used to estimate the collision rate with smaller bodies. After integration times of 250 kyr or more, only approximately 1% of the discrete bodies have $e > 0.3$ (corresponding to midplane crossing velocities $v_m > 5$ km s⁻¹). A few bodies rarely reach $e > 0.4$ ($v_m > 7.5$ km s⁻¹). The total cumulative area of the midplane that would be traversed by these bodies and their associated bow shocks over a period of several Myr is <1% of the total area. The code output shows that bodies with $e > 0.2$ and diameters of 1000–3000 km typically gained mass at rates of $10^{16}–10^{17}$ g year⁻¹ during the interval from 200 to 250 kyr. The corresponding impact interval for 1 km sized bodies (mass approximately $10^{15}$ g) is days to weeks. Implications of these results for chondrule formation efficiency in planetesimal-generated shocks will be presented at the conference.

**References:** [1] Desch S. J. et al. 2005. In *Chondrites and the Protoplanetary Disk*, Krot A. N. et al. eds., Astronomical Society Pacific Conference Series, vol. 341. pp. 849–872. [2] Hood L. L. 1998. *Meteoritics & Planetary Science* 33:97–107. [3] Weidenschilling S. J. et al. 1998. *Science* 279:681–684. [4] Hood L. L. et al. 2009. *Meteoritics & Planetary Science* 44:327–342.

5241

I-Xe AND Ar-Ar DATING OF ENSTATITE METEORITES

J. Hopp¹, M. Trieloff¹, U. Ott² and O. V. Pravdivtseva³. ¹Institute of Geosciences, University Heidelberg, INF 234-236, D-69120 Heidelberg, Germany. E-mail: jens.hopp@geow.uni-heidelberg.de. ²MPI for Chemistry Mainz, Joh.-J.-Becher-Weg 27, D-55128 Mainz, Germany. ³Laboratory for Space Sciences, Physics Department, Washington University, 1 Brookings Drive, St. Louis, MO 63130-4899, USA

**Introduction:** The decay of short-lived <sup>129</sup>I into <sup>129</sup>Xe ($t_{1/2}$ = 15.7 Ma) allows resolution of small timescales during early formation history of meteorite parent bodies. Few available data suggest enstatite meteorites are promising candidates to apply this chronometer, thus improving our understanding of the chronology of enstatite meteorite parent body formation and metamorphism. We report first results of a still ongoing I-Xe and Ar-Ar dating study of a suite of aubrites and enstatite chondrites (EH and EL groups, different petrological types).

**Results:** We focussed on meteorites with low weathering stages. In addition we measured some meteorites for which data already exist, because these are the first I-Xe results obtained at Institute of Geosciences, Heidelberg. So far we analyzed whole rock samples (80–250 µm) of the aubrites ALH 84024, Bishopville, Cumberland Falls, Mayo Belwa, Norton County, and Peña Blanca Spring. Because of low xenon and iodine concentrations, observed low <sup>129</sup>Xe/<sup>132</sup>Xe excess from <sup>129</sup>I-decay and problems with <sup>128</sup>Xe-memory precision was lowered and hence, most aubrites revealed no significant age information except for Peña Blanca Spring. For this meteorite we yielded a relatively well-defined isochron (1040–1300 °C) in <sup>129</sup>Xe/<sup>132</sup>Xe versus <sup>128</sup>Xe/<sup>132</sup>Xe space of about 4.2 ± 4.5 (2σ) Ma younger than the enstatite achondrite Shallowater which was used as age reference (4562.3 ± 0.4 (1σ) Ma, [2]). This age is similar though less precise to a previously reported (recalculated) I-Xe age of Peña Blanca Spring [1]. Analyses of enstatite separates from some of the aubrites with a higher sensitivity mass spectrometer at Washington University, St. Louis, are in progress.

Because of their much higher xenon and iodine concentrations measurements of chondrites are more significant. Up to now we analyzed mm-sized chips of the enstatite chondrites Abee (EH4) and EET 96135 (EH4/5) and for both obtained well-defined isochrons. Abee is calculated to be 0.7 ± 0.9 (2σ) Ma younger than EET 96135 and 0.2 ± 0.7 (2σ) Ma younger than Shallowater. The latter age difference again agrees with previous data [1]. The observed range in <sup>129</sup>Xe/<sup>132</sup>Xe ratios of extractions defining the isochrons of both meteorites are very different (2.7–7.8, Abee; 1.37–1.86, EET 96135). EET 96135 shows a much higher concentration of trapped <sup>132</sup>Xe (2.4E-9 cm³ STP g⁻¹) than Abee (8.5E-10 cm³ STP g⁻¹) explaining its lower <sup>129</sup>Xe excess.

Before measurement Xe is routinely separated from Ar at a cold trap (about 10% Ar remaining in Xe fraction). For both chondrites this argon fraction was used for simultaneous Ar-Ar dating. Absolute age evaluation still await standard measurements, but results indicate an approx. 0.3 Ga lower partial “plateau” age for Abee compared with EET 96135.

**References:** [1] Podosek F. A. 1970. *Geochimica et Cosmochimica Acta* 34:341–365. [2] Gilmour J. D. et al. 2009. *Meteoritics & Planetary Science* 44:573–580.
## MOLECULE CHEMISTRY IN SUPERNOVA EJECTA: INSIGHTS FROM PRESOLAR SiC GRAINS

P. Hoppe¹ and E. Zinner². ¹Max Planck Institute for Chemistry, Particle Chemistry Department, 55128 Mainz, Germany. E-mail: peter.hoppe@mpic.de. ²Laboratory for Space Sciences and Physics Department, Washington University, St. Louis, MO 63130, USA.

**Introduction:** Primitive solar system materials contain small quantities of presolar grains that formed in the winds of evolved stars or in the ejecta of stellar explosions [1]. Of particular importance are SiC grains from type II supernovae (SNeII), the X grains, characterized by isotopically light Si [2], and the “unusual” and C (subsequently designated U/C) grains, characterized by heavy Si [3–5]. Many of the isotopic signatures of these grains can be satisfactorily reproduced by ad hoc SN mixing models. In this approach, matter from different layers in a SNII [e.g., 6] is mixed in variable proportions. With ad hoc SN mixing, however, it is not possible to explain the isotope data of Fe in X grains [7] and of S in U/C grains [3–5]. While it cannot be completely ruled out that predictions for certain isotope abundances in current SN models [6] are in error, it is more likely that the problems with Fe and S point toward element fractionation processes during or prior to SiC formation. Here we will explore the possible role of molecule chemistry in SN ejecta in this respect.

**S and Fe in Supernova Grains:** The U/C grains have isotopically light S [3–5]. Because Si- and S-isotopic signatures (i.e., heavy or light Si and S) correlate very well in the interior of SNeII [6], ad hoc mixing models fail to account for heavy Si together with light S. Most X grains have higher than solar $^{57}$Fe/$^{56}$Fe but solar $^{54}$Fe/$^{56}$Fe [7]. The light Si of X grains requires significant contributions from the Si/S zone, which should lead to higher than solar $^{54}$Fe/$^{56}$Fe in X grains, contrary to observation.

**Discussion:** Models based on chemical kinetics predict formation of various molecules in the ejecta of massive SNe [8]. Molecules such as CO and SiO have been observed in the ejecta of SN1987A [9]. According to the models by [8], 13–34% of the ejected matter is in the form of molecules. The molecule inventory depends strongly on the degree of mixing. If H from the outer SN layers mixes with matter from interior layers, then H₂, H₂O, OH, N₂, O₂, CO, CO₂, SO, and SiO dominate. If no mixing occurs, O₂, SiS, and CO dominate. In this latter case about 25% of all molecules are SiS that formed in the Si/S zone. Ad hoc SN mixing shows that it is possible to produce heavy Si even if matter from the Si/S zone is admixed. If SiS molecules from the Si/S zone are incorporated efficiently by the growing SiC grains, while elemental S or other S molecules are not, it would be possible to condense SiC grains with light S and heavy Si. The Fe-isotopic signature of X grains requires preferential trapping of Fe from the He/C and He/N zones. According to the models of molecule chemistry in SN ejecta by [8], some (FeS)₂ dust precursors form in the Si/S zone which may prevent Fe from the Si/S zone to be trapped by the growing SiC X grains. Clearly, more detailed work is needed to follow up on these ideas.

**References:** [1] Zinner E. 2007. In *Meteorites, comets, and planets*, edited by Davis A. M. Treatise on Geochemistry, vol. 1.02. [2] Lin J. et al. 2010. *The Astrophysical Journal* 709:1157–1176. [3] Hoppe P. et al. 2010. *The Astrophysical Journal* 719:1370–1384. [4] Gyngard F. et al. 2010. *Meteoritics & Planetary Science* 45:A72. [5] Hoppe P. and Fujiya W. 2011. Abstract #1059. 42nd Lunar and Planetary Science Conference. [6] Rauscher T. et al. 2002. *The Astrophysical Journal* 576:323–348. [7] Marhas K. et al. 2008. *The Astrophysical Journal* 689:622–645. [8] Cherchneff I. and Lilly S. 2008. *The Astrophysical Journal* 683:L123–L126. [9] Wooden D. et al. 1993. *The Astrophysical Journal* 88:477–507.

## SIDEROPHILE ELEMENT PATTERNS OF SULFIDE-METAL ASSEMBLAGES FROM THE ALMAHATA SITTA POLYMICT BRECCIA

M. Horstmann¹, M. Humayun² and A. Bischoff¹. ¹Institut für Planetologie, WWU Münster, 48149 Münster, Germany. E-mail: marianhorstmann@uni-muenster.de. ²NHMFL & Deptartment of EOAS, Florida State University, Tallahassee, FL 32310, USA.

**Introduction:** Among the meteorites recovered from the strewn field of asteroid 2008 TC₃ in the Nubian Desert, two unique rocks were discovered dominated by sulfide-metal intergrowths with ureilitic material attached [1]. The samples MS-158 and MS-166 were studied by optical and electron optical microscopy (SEM, EPMA). Siderophile element abundances were determined in situ on polished thin sections applying LA-ICP-MS at Florida State University following the procedures of [2].

**Results:** MS-158 is dominated by large sulfide domains, often sprinkled with small (<15 µm) metal inclusions, rounded to irregularly shaped zoned metal grains (cores ~7–10 wt% Ni) with sharp Ni-rich rims (up to ~35 wt% Ni) and reduced ureilitic material attached or enclosed, suggesting a genetic relationship. MS-166 contains similar areas but is additionally characterized by metal grains with minor, irregular zoning (~7–23 wt% Ni) surrounded by small, Ni-rich metal grains (~30 wt% Ni) embedded in an intergrowth of FeS, a kind of Fe-oxide and minor metal. Siderophile element patterns in MS-158 metal cores are enriched in refractory siderophile elements (W to Rh; 2–10 × CI, Ni-norm.) and rims are depleted showing nearly complementary patterns (0.02–0.6 × CI, Ni-norm.) for these elements. Ga, Ge, and Sn show lower abundances in the rims with respect to the cores. Pd and Sb are enriched in the rims. Au and As abundances are similar in both regions. Metal in MS-166 is in some cases more strongly enriched in refractory siderophiles (~2–40 × CI, Ni-norm.). Ni-zoning for siderophile element patterns was not reflected as prominently in grains embedded in the Fe oxide-bearing assemblage. Only one measurement exhibited similar depleted abundances as found in MS-158. The range of measured Ir concentrations spans almost three orders of magnitude in the metal grains.

**Discussion:** Siderophile element patterns indicate that the metal grains in the assemblages formed via in situ fractional crystallization. The cores of the metals crystallized early in this process as indicated by the high abundances of refractory siderophile elements. The sharp Ni-rich rims depleted in the latter elements crystallized from a more evolved liquid with higher S content. The texture on thin section level supports in situ fractional crystallization. Using the determined core compositions, values for solid metal–liquid metal partition coefficients (D) were estimated [3, 4] with the assumption that the original liquid had a chondritic relative pattern for refractory siderophiles. From the estimated D values, the range of original S content of the melts is estimated as 15–25% S. However, the range of volatile element abundances in calculated liquid compositions is too large to be matched with any specific class of bulk carbonaceous chondrite material. The observed range for Ir is similar to IIIBAB iron meteorites [5].

**References:** [1] Bischoff A. et al. 2010. *Meteoritics & Planetary Science* 45:1638–1656. [2] Humayun M. et al. 2007. *Geochimica et Cosmochimica Acta* 71:4609–4627. [3] Chabot N. L. and Jones J. H. 2003. *Meteoritics & Planetary Science* 38:1425–1436. [4] Chabot N. L. et al. 2009. *Meteoritics & Planetary Science* 44:505–519. [5] Wasson J. T. 1999. *Geochimica et Cosmochimica Acta* 63:2875–2889.
RARE EARTH ELEMENT (REE) ABUNDANCES OF SULFIDES
FROM E CHONDRITE LITHOLOGIES OF THE ALMAHATA
SITTA POLYMICT BRECCIA

M. Horstmann¹, M. Humayun² and A. Bischoff¹. ¹Institut für Planetologie, WWU Münster, 48149 Münster, Germany. E-mail: marianhorstmann@uni-muenster.de. ²NHMFL & Department of EOAS, Florida State University, Tallahassee, FL 32310, USA.

**Introduction:** A large variety of different E chondrite types was found among the Almahata Sitta samples recovered from the Nubian Desert of northern Sudan [1]. We report REE abundances in oldhamite and keilite from selected samples determined by laser ablation-ICP-MS at FSU with a UP193FX excimer laser system using USGS and NIST standards for calibration [2].

**Samples and Methods:** In situ measurements (10 or 20 µm spots) of REE abundances of oldhamite and keilite were conducted on polished thin sections of samples previously described [1]: oldhamite: MS-14 (EH3, n = 1), MS-17 (EL3/4, n = 2), MS-52 (EL 6, n = 5); keilite: MS-159 (EL IMR, n = 3), MS-D (EL 6, n = 8).

**Results:** Oldhamite: MS-14 oldhamite exhibits a pattern with flat HREE (~30 × CI), LREE decreasing from La to Nd (~35–60 × CI), and positive Eu (~60 × CI) and Yb (~70 × CI) anomalies. In MS-17, oldhamite grains both show similar patterns with flat HREE (~85–140 × CI), slightly bowl-shaped LREE (La/Ce ~1.1, La/Sm ~0.9; ~90–120 × CI), and negative Eu anomalies (~50 × CI). All oldhamite grains in MS-52 are characterized by bow-shaped patterns (LREE ~85–200, La/Sm ~0.5; HREE ~220–120 × CI, Gd/Lu ~1.7) with pronounced negative Eu anomalies (~25–30 × CI). Different grains within the same section show very homogenous REE compositions. In comparison with MS-17, Eu anomalies are more pronounced in MS-52 (Eu/Eu* = 0.15 vs. 0.45, calculated as EuN/(√SmN × GdN); normalized to CI). Keilite: REE patterns of keilite in MS-159 and MS-D are similar between different grains and characterized by LREE depletions (≤ ~4 × CI) relative to HREE (around ~10–20 × CI) and negative Eu anomalies (≤ ~1.5 × CI).

**Discussion:** REE patterns gained for oldhamite in the different samples are very similar to patterns reported in the literature for E chondrite and aubrite oldhamite [e.g., 3, 4]. The oldhamite in MS-14 indicates that its main REE signature originates from nebular processes because magmatic processes fail to generate positive anomalies in Eu and Yb [e.g., 3–5]. Patterns found for MS-17, although of type EL3/4, and MS-52 are similar to patterns found in EL6 (equilibrated) chondrites [4] and may result from prolonged thermal metamorphism and re-equilibration of oldhamite with surrounding phases [3–5]. Keilite REE patterns share some similarity with alabandite REE patterns from the literature [e.g., 6]. Both minerals have been previously interpreted to form by igneous processes [6,7].

**References:** [1] Bischoff A. et al. 2010. *Meteoritics & Planetary Science* 45:1638–1656. [2] Gaboardi M. and Humayun M. 2009. *Journal of Analytical Atomic Spectrometry* 24:1188–1197. [3] Crozaz G. and Lundberg L. L. 1995. *Geochimica et Cosmochimica Acta* 59:3817–3831. [4] Floss C. and Crozaz G. 1993. *Geochimica et Cosmochimica Acta* 57:4039–4057. [5] Lodders K. and Fegley B. Jr. 1993. *Earth and Planetary Science Letters* 117:125–145. [6] Wheelock M. M. et al. 1994. *Geochimica et Cosmochimica Acta* 58:449–458. [7] Keil K. 2007. *Chemie der Erde*–*Geochemistry* 67:37–54.

MODAL MINERALOGY OF CR CHONDRITES BY PSD-XRD: ABUNDANCE OF AMORPHOUS Fe-SILICATE

K. T. Howard¹, G. K. Benedix¹, P. A. Bland¹,²,³ and D. L. Schrader⁴. ¹Mineralogy Dept., Impacts and Astromaterials Research Centre (IARC), The Natural History Museum, London, UK. ²IARC, Dept. of Earth Sci. & Eng., Imperial College, London, UK. ³Dept. of Applied Geology, Curtin University of Technology, Perth, Australia. ⁴Univ. of Arizona, Lunar & Planetary Laboratory, Tucson, USA. E-mail: kieren.howard@nhm.ac.uk.

**Introduction:** Position sensitive detector X-ray diffraction (PSD-XRD) allows us to quantify the mineralogy of bulk CR chondrite powders down to the nano-scale for all phases >1wt%.

**Samples and Method:** We studied 7 CRs. Weathering complicates interpretations in 3 finds, so we report on: GRO 03116; GRA 06100; MET 00426 and Al Rais (fall). Data were collected by PSD-XRD and quantified using a pattern stripping technique [2]. Here each mode is for an 80 mg sample, < in [2]. Cu radiation induces Fe- fluorescence that is used to quantify the Fe-rich amorphous component [2]. XRD data were crosschecked using Co radiation. Average values are reported in volume %.

**Results and Discussion:** Olivine (35%) and pyroxene (40%) exist in nearly equal volumes in all samples. In samples like MET 00426, these phases show no evidence for peak broadening in XRD patterns: a common feature of even minor aqueous alteration. Our total anhydrous silicate (ol+pyx) abundance (75%) is less than reported in petrographic studies (85–95%) [3,4]. XRD shows CRs typically contain 3–4% metal, 1–2% magnetite and rare carbonate (<1%). Al Rais has a distinct mineralogy reflecting a greater volume of dark inclusions [3] and more advanced aqueous alteration, it contains abundant magnetite (17%) and only rare metal. XRD patterns for Al Rais show obvious diffraction peaks from crystalline serpentine (20%). All other samples show no evidence for diffraction from phyllosilicate, indicating it is not present in well crystalline form. Transmission electron microscopy (TEM) also shows only rare nano-crystalline phyllosilicate in MET 00426 [1]; if this contributed significantly (>1%) to the bulk composition, XRD would resolve it in broad reflections.

**Meteorite Dark Matter:** Subtraction of crystalline phases in pattern fitting reveals large residual X-ray counts in all samples but Al Rais. These residual counts are fluorescence from Fe-rich X-ray amorphous material, typically: FeNiS, FeₓOₓ, FeOOH(±ₓH₂O) and potentially also Fe, Mg phyllosilicates. Modeling the residual component in XRD patterns, using background intensities [2] of these phases, fails to account for the required counts or requires unrealistic abundances ruled out by mass balance calculations. TEM shows that amorphous Fe-silicate is common in the matrices of MET 00426 and QUE 99177 (CR) [1]. Modeling the XRD residuals with large volumes (≤15%) of this amorphous Fe-silicate [1] and less FeNiS (5–10%) and FeₓOₓ/FeOOH(±ₓH₂O) (≤10%) yields good agreements with bulk compositions. Petrographic studies counting amorphous material as silicate also explains discrepancies with XRD estimates of total silicate volume.

**Conclusion:** Amorphous Fe-silicate can be a significant component of CRs. It appears to be destroyed by the progression of aqueous alteration, hence its absence in Al Rais. The question remains: is it a pristine product of the solar nebular [1], or a proto-alteration product of early stage parent body processing?

**References:** [1] Abreu N. M and Brearely A. J. 2010. *Geochimica et Cosmochimica Acta* 74:1146–1171. [2] Howard K. T. et al. 2010. *Geochimica et Cosmochimica Acta* 74:5084–5097. [3] Weisberg M. K. et al. 1993. *Geochimica et Cosmochimica Acta* 57:1567–1586. [4] Schrader D. L. et al. 2011. *Geochimica et Cosmochimica Acta* 75:308–325.
5062

REE MICRODISTRIBUTIONS IN NWA 4898: A HIGH-Al MARE BASALT

W. Hsu¹,², Y. Guan³, S. Li² and Y. Wang¹. ¹Purple Mountain Observatory, Nanjing, China. E-mail: wbxu@pmo.ac.cn. ²China University of Geosciences, Wuhan, China. ³GPS, Caltech, Pasadena, CA 91125, USA.

**Introduction:** Lunar meteorites extend diversity of lunar materials and yield complementary information on the formation and geological evolution of the Moon. Among them, NWA 4898 is a unique high-Al, low-Ti mare basalt [1]. It is much younger (3.6 Ga) than Apollo 14 high-Al basalts (3.95-4.33 Ga) and was derived from an extremely LREE-depleted mantle source [2]. Here we report ion microprobe analyses of REEs in NWA 4898 and provide additional constraints on its petrogenesis.

**Results:** NWA 4898 exhibits an intergranular texture, composed of fine-grained, chemically zoned pyroxenes and anorthite (maskelynite). Olivine occurs as megacrysts and often contains chromite and melt inclusions. Several shock-induced melt veins are visible in the sections. Pronounced normal REE zoning was observed in olivine, pyroxenes, and maskelynite, indicating shock metamorphism did not modify REE microdistributions in NWA 4898. Using the average REEs of maskelynite cores and appropriate partition coefficients for calcic plagioclase [3], we estimate the REE contents of the initial melt that anorthite crystallized from. The calculated melt has slightly lower REEs (20%) than the bulk sample analyzed by [1]. Melt inclusions in olivine are 40% higher than the bulk.

**Discussion:** REE abundances of the calculated initial melt are generally compatible with the bulk composition within analytical uncertainties (relatively large due to low REEs in maskelynite, especially HREEs). This suggests that NWA 4898 evolved in a close system and largely reflects geochemical characteristics of its source region. However, melt inclusions trapped in olivine (Fo₇₀) megacrysts do not represent the initial melt. They are enriched in CaO (13–16 wt%) and Al₂O₃ (13–15%) but depleted in MgO (1.3–4.5%), significantly more evolved [Mg/(Mg + Fe) = 0.27] relative to the bulk sample (0.46).

**Acknowledgment:** This work was supported by Chinese National Natural Science Foundation (grant nos. 40773046, 10621303) and by the Minor Planet Foundation of China.

**References:** [1] Greshake A. et al. 2008. Abstract #1631. 29th Lunar and Planetary Science Conference. [2] Gaffney A. et al. 2008. Abstract #1877. 29th Lunar and Planetary Science Conference. [3] Snyder G. A. et al. 1992. *Geochimica et Cosmochimica Acta* 56:3809–3823.

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5288

NONDESTRUCTIVE THREE-DIMENSIONAL IMAGING OF AN INTERPLANETARY DUST PARTICLE AT THE NANOSCALE

Z. W. Hu¹ and R. Winarski². ¹XNano Sciences Inc., P. O. Box 12852, Huntsville, AL 35812, USA. E-mail: zwhu@xnano.org. ²Center for Nanoscale Materials, Argonne National Laboratory, Argonne, IL 60439, USA.

Interplanetary dust particles (IDPs) collected from the stratosphere [1], perhaps dominated by comet fragments as implied by the latest dynamical model of zodiacal cloud [2], represent the most primitive extraterrestrial materials currently available for laboratory study [3–5]. Chondritic porous (CP) IDPs and cluster IDPs in general exhibit properties consistent with a cometary origin, and they are fragile, porous aggregates of mostly submicron-sized surviving presolar grains and nebular condensates. Much knowledge about early solar system processes has been gained from the laboratory analysis of the mineralogy and compositions of microtomed thin IDP sections (~≤100 nm thick). Yet, subtle, fine structures indigenous to interstellar grains and parent bodies preserved in IDPs may be altered by destructive processing. A question arises then: Is it possible to peer into delicately-bound-aggregate IDPs noninvasively to map their unaltered structures in three-dimensional (3D) nanoscale detail? Such information is crucial to our understanding of presolar and early solar nebular processes and the origin and evolution of comets and the solar system at a fundamental level. We have been tackling this issue using a nondestructive 3D nanoimaging approach that exploits the high penetrating power of hard X-rays and the high sensitivity of phase contrast imaging. Results from the initial experiment will be presented, along with the discussion of implications for understanding the structure and formation of interstellar grains, ice and dust agglomeration in the early solar nebula, and the origin and evolution of comets. New opportunities will also be discussed to investigate both presolar grains in situ in host IDPs and cometary structure and evolution at the fundamental level.

**Acknowledgment:** ZWH wishes to thank G. J. Flynn, D. E. Brownlee, M. E. Zolensky, L. R. Nittler, B. Lai, and J. Masor for valuable discussions, V. Rose for technical assistance, and NASA for allocating IDPs for this research. Use of the Center for Nanoscale Materials and the Advanced Photon Source at Argonne National Laboratory was supported by DOE, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC02-06CH11357

**References:** [1] Brownlee D. E. 1985. *Annual Review of Earth and Planetary Sciences* 13:147–173. [2] Nesvorný D. 2010. *The Astrophysical Journal* 713:816–836. [3] Ishii H. A. et al. 2008. *Science* 319:447–450. [4] Busemann H. 2009. *Earth and Planetary Science Letters* 288:44–57. [5] Messenger S. 2000. *Nature* 404:968–971.
THE IMPORTANCE OF COSMIC RAY EFFECTS FOR PALLADIUM-SILVER CHRONOMETRY IN IRON METEORITES: AN EVALUATION USING NOBLE GASES

L. Huber¹, M. Schönbächler², I. Leya³, R. Wieler¹, K. J. Theis², M. Rehkämper⁴ and G. K. Benedix⁵, ¹Institute for Geochemistry and Petrology ETH Zürich, CH-8092 Zürich, Switzerland. E-mail: liliane.huber@erdw.ethz.ch. ²SEAES, The University of Manchester, M13 9PL, UK. ³Space Research and Planetary Sciences, University of Bern, Switzerland. ⁴IARC and Dept. of Earth Science and Engineering, Imperial College London, SW7 2AZ, UK. ⁵IARC and Department of Mineralogy, The Natural History Museum, Cromwell Road, London, SW7 5BD, UK.

**Introduction:** The short-lived ¹⁰⁷Pd-¹⁰⁷Ag decay system (half-life = 6.5 Myr) is a powerful tool to date core crystallization of asteroids [1]. During the travel as meteoroids from the asteroid parent body to the earth galactic cosmic-ray (GCR) particles can produce cosmogenic Ag isotopes through interactions with Pd (e.g., ¹⁰⁶Pd (n,γ)) [2]. This production of Ag can potentially compromise the Pd-Ag dating system. It is therefore essential to determine the concentration of cosmogenic Ag in these meteorites. Reedy [2] modeled the cosmogenic production of Ag isotopes and obtained an upper limit of the ¹⁰⁷Ag/cos/¹⁰⁸Pd ratio of 1.4 × 10⁻⁷ for an exposure age of 1 Ga at a position of maximum GCR particle flux within meteoroids. This would be negligible for Pd-Ag age determinations, because the measured ratios are in the range of approximately 1 × 10⁻⁵ [1, 2].

This study aims to reassess these early calculations and utilizes noble gas concentrations as a measure for the fluence of cosmic-rays. This in turn can be used to estimate the magnitude of the GCR effects that may influence the Pd-Ag dating system.

One way to exclude that the Pd-Ag dating system is compromised by cosmogenic Ag is to compare deviations from Pd-Ag isochrones (which are defined by several iron meteorites [3]) with the amount of produced noble gases. If samples with very low noble gas concentrations fall with the same probability on the isochrone as those with high contents the influence of the GCR induced effects can be neglected.

**Samples and Methods:** The noble gases will be measured in samples adjacent to those which were used for Pd-Ag analyses [4]. New cosmic-ray exposure ages will be determined for Coahuila (IIAB), Boxhole (IIIA), and Goose Lake (IAB), which have not been analyzed for light noble gases previously. Further analyzed will be Toluca, Thunda, Canyon Diablo, Odessa, Henbury, Sikhote-Alin, and North Chile and for these samples the reported exposure ages will be verified based on the newly obtained noble gas data and recent physical model calculations [4].

For the noble gas analyses, the samples were cut into 80–150 mg pieces. Following cleaning with ethanol they are loaded into the sample storage arms of a high-T furnace, where they are heated in vacuum for two days at 80 °C to desorb atmospheric noble gases. The gas extraction occurs at 1700 °C using a RF-heating system. We will present the light noble gas data (He, Ne, Ar) and the corresponding cosmic ray exposure ages of the meteorites.

**References:** [1] Schönbächler M. et al. 2008. *Geochimica et Cosmochimica Acta* 72:5330–5341. [2] Reedy R. C. 1980. 11th Lunar and Planetary Science Conference. pp. 1169–1178. [3] Theis et al. 2011 *Meteoritics & Planetary Science*, this volume. [4] Ammon K. et al. 2009. *Meteoritics & Planetary Science* 44:485–503.

COMPONENTS OF DISTAL SUBDURY EJECTA

M. S. Huber¹ and C. Koeber¹,². ¹Department of Lithospheric Research, University of Vienna, Althanstrasse 14, A-1090 Vienna, Austria. E-mail: matthew.huber@univie.ac.at. ²Natural History Museum, Burgring 7, A-1090 Vienna, Austria.

**Introduction:** The 1850 Ma [1] Sudbury impact is one of the largest meteorite impact events recorded in Earth history. The economic mineral deposits within the Sudbury crater that are related to the vast differentiated melt sheet are well studied, but the ejecta have not been thoroughly characterized. Impact ejecta have been found from approximately 500 km to 950 km from the crater center [2, 3, 4]. The most distal ejecta are located in the “Colerain” region of Minnesota, about 950 km from the crater center. Near the US-Canada border in Ontario, a drill core through the ejecta layer was taken near the Pine River (“PR”) at a distance of approximately 650 km from the crater center. The nearest ejecta deposits are found south of Lake Superior in the Upper Peninsula of Michigan at the “Connors Creek” location, about 500 km from the crater center.

Different locations have varying thicknesses and compositions of ejecta. The deposits are located in slightly differing geological settings, but are generally similar near-shore continental shelf deposits with mostly thin bedded deposits both stratigraphically above and below the ejecta deposits [3]. Few surface exposures of the ejecta deposits have been found, but the Connors Creek location provides a small outcrop. Although part of the upper portions of the ejected material has been truncated by erosion, about two meters of ejecta are preserved, with glasses and accretionary lapilli as the main impact derived phases. The Colerain and PR samples preserve some of the fine textures lost to alteration and erosion at the Connors Creek location.

Samples have been characterized by petrographic and geochemical analyses. Samples from all sites have similar mineral content, including carbonates, shocked and unshocked quartz, various micas that appear to represent devitrified glass, pyrite, chalcopyrite, sphalerite, and some spherules of organic carbon (possibly locally derived, or possibly of similar origin to organic carbon observed in [5]). Samples are primarily silicates, with up to 23 wt.% Al₂O₃, and 3–4 wt.% MgO and Fe₂O₃. In addition, the trace elements V, Cr, Rb, and Zr are enriched.

From near to far, the main differences between the various deposits are (1) Accretionary lapilli are not present at the most distal locations; (2) Evolution of glass particles from angular toward rounded (spheroidal) shapes; (3) Decrease in the abundance of channel deposits; and (4) Decrease in locally derived detritus mixed with glasses (from approximately 20 vol% to < 5 vol%). These features are consistent with decreasing energy at greater distance from the crater. Thicknesses of deposits are not consistent with the theoretical calculations of [6], likely a result of reworking and wave action following initial deposition. There is a relationship between underlying strata and local expressions of the ejecta layer.

**Acknowledgments:** Special thanks to Klaus Schulz, Bill Addison, Greg Brumpton, Bill Cannon, and Mark Severson.

**References:** [1] Krogh et al. 2004. Ontario Geological Survey Special Volume 1:431–446. [2] Addison W. A. et al. 2005. *Geology* 33:193–196. [3] Cannon W. F. et al. 2010. *GSA Bulletin* 122:50–75. [4] Addison W. A. et al. 2010. *GSA Special Paper* 465:245–268. [5] Wright A. J. et al. 2010. *Precambrian Research* 177:23–38. [6] Melosh H. J. 1989. *Oxford Monographs on Geology and Geophysics* 11:87–111.
## PONTLYFNI, THE INSIDE STORY: DISTRIBUTION OF METAL AND SULFIDE REVEALED BY MICRO-COMPUTED TOMOGRAPHY

A. C. Hunt¹, G. K. Benedix¹,² and L. Howard³. ¹Impacts and Astromaterials Research Centre (IARC), The Natural History Museum, Department of Mineralogy, Cromwell Road, London, SW7 5BD, UK, E-mail: alison.hunt@nhm.ac.uk. ²Department of Earth Science and Engineering, IARC, Imperial College London, South Kensington Campus, London SW7 2AZ, UK. ³Department of Mineralogy, The Natural History Museum, Cromwell Road, London, SW7 5BD, UK.

**Introduction:** Winonaites are primitive achondrites with roughly chondritic mineralogy [1]. They are related to silicate inclusions in IAB iron meteorites via their oxygen isotope compositions [2]. Pontlyfni is the only fall among the winonaites. Previous studies have shown that Pontlyfni is probably the most reduced and finest-grained of the winonaites, with textures and mineralogy that vary widely [1, 3]. In particular, Fe-Ni metal and troilite are heterogeneously distributed, with metal occurring as veins up to 6 mm in length and as particles up to 1 mm in dimension [1]. Here, we aim to elucidate the structures and abundances of the metal- and troilite-rich regions using micro-computed tomography (CT). Micro-CT is particularly suited to studying meteorites such as the winonaites, as they contain minerals with significantly different x-ray attenuation coefficients, giving rise to clearly defined variations in the greyscale values of the resultant x-ray images, and allowing easy separation of mineralogical phases.

**Method:** We scanned an ~18 g piece of Pontlyfni (BM 1975, M6), which measures 3 cm by 2 cm by 2 cm. Micro-CT scanning was carried out at the NHM using a Metris X-Tek HMXST 225 CT scanner. The specimen was scanned using a W target, at 225 kV and 175 µA, with a 1.5 mm copper filter and 500 ms exposure at a voxel size of ~17 µm. Data were reconstructed with a cone beam algorithm using CT-PRO. Analysis of the resulting data was carried out using VG Studio Max 2.1.

**Discussion:** In hand specimen and thin section, Pontlyfni comprises silicate material with variably weathered metal and troilite, typical of the winonaites. Our new data reveal the highly brecciated nature of this meteorite. Angular and rounded silicate clasts measuring up to 4.5 mm in dimension are distinguished from a metal- and sulfide-rich matrix. Veins of metal and sulfide also cross-cut these clasts. The CT images reveal that metal and sulfide are more abundant in Pontlyfni than previously determined. Mineral modes calculated from combined elemental maps of Pontlyfni thin sections indicate an abundance of Fe-Ni metal + troilite of 30-39%, similar to published abundances of 30-37% [1, 3]. By comparison, new micro-CT data show a much higher volume of metal + sulfide (51.7%) and a lower volume of silicate phases (48.3%). Angular, chondritic silicate inclusions have been described in some IAB irons, with an abundance of up to 40% [4]. The combination of silicate-rich clasts set in a metal- and sulfide-rich matrix indicates that Pontlyfni more closely resembles a IAB iron meteorite, similar to some parts of Campo del Cielo or Caddo County, than a typical winonaite.

**References:** [1] Benedix G. K. et al. 1998 *Geochimica et Cosmochimica Acta* 62:2535-2553. [2] Clayton R. N. and. Mayeda T. K. 1996. *Geochimica et Cosmochimica Acta* 60:1999-2017. [3] Floss C. et al. 2008. *Meteoritics & Planetary Science* 43:657-674. [4] Benedix G. K. et al. 2000. *Meteoritics & Planetary Science* 35:1127-1141.

## OXYGEN ISOTOPIC COMPOSITION OF A STARDUST CHONDRULE FRAGMENT NAMED IRIS

G. R. Huss¹, R. C. Ogliore¹, K. Nagashima¹, A. L. Butterworth², Z. Gainsforth², J. Stodolna² and A. J. Westphal². ¹Univ. of Hawaii at Mānoa, USA. E-mail: ghuss@higp.hawaii.edu. ²University of California, Berkeley, USA.

**Introduction:** Oxygen isotopic composition is key to understanding the origins of and relationships among solar system materials. Measuring the O isotopic composition of Stardust samples is challenging because of the small grain sizes. The first O measurements were done on a CAI fragment [1]. A few measurements of relatively large fragments of less-refractory material have also been made [1-4]. Here we report data for Iris, a complex fragment with characteristics of a type II chondrule [5, 6].

**Methods:** Iris is a 23 × 15 × 5 µm terminal particle from Stardust track C2052.2.74. We mounted the Iris potted butt, without modification, in a specially designed holder [7]. The sample is in the middle of a one-inch Al disk behind a 500-nm-thick Si₃N₄ window with a 300-µm central hole. Appropriate polished standards are mounted with epoxy in a ring of holes on a cover plate that holds the Si₃N₄ window [7]. The sample was documented by SEM, and analysis spots were marked by putting a focused electron beam on the sample for several minutes to produce a spot visible as a depleted O signal in scanning ¹⁶O⁻ ion image. Oxygen-isotope data were collected using the UH irs 1280 ion microprobe in multicollection mode. A 25-30 pA Cs⁺ primary ion beam focused to ~2 µm was used. The mass resolving power for ¹⁷O⁻ was ~5500 (M/ΔM). Oxygen-16 was measured on the L1 Faraday cup, ¹⁷O on the monocollector electron multiplier (EM), and ¹⁸O on the H1 EM. The potential contribution from interfering ¹⁶OH⁻ signal to ¹⁷O⁻ was evaluated by checking ¹⁶OH⁻ after each measurement. Data were corrected for Faraday cup background, EM deadtime, detector yield, and ¹⁶OH⁻ contribution to ¹⁷O (typically <0.2‰). There is a systematic offset of ~2.7‰ in δ¹⁸O between samples behind the Si₃N₄ window and those mounted on the cover plate. We quantified this offset using San Carlos olivine. Uncertainties in δ¹⁸O include the measurement uncertainty and the uncertainty in the calibration of the offset. Those for Δ¹⁷O include the measurement uncertainty and the standard deviation of Δ¹⁷O from a group of standards.

**Results and Discussion:** We measured the O isotopic compositions of olivine, mesostasis, and chromite in Iris. The results are in close agreement and indicate that Iris is isotopically homogeneous. Mean Δ¹⁷O of Iris is ~0.3‰ ± ~2‰ (2σ), indistinguishable from terrestrial O. Iris thus appears to show relatively high Δ¹⁷O compared to other chondrule-like material from comet Wild 2 [2-4]. Mean δ¹⁸O for Iris is ~+8‰ ± ~2‰ (2σ) and plots near the intersection of the terrestrial fractionation line and carbonaceous chondrite anhydrous mineral line on an O three-isotope plot. Iris thus appears to have a higher δ¹⁸O than most anhydrous chondritic materials [cf. 8].

**References:** [1] McKeegan K. D. 2006. *Science* 314:1724-1728. [2] Nakamura T. et al. 2008. *Science* 321:1664-1667. [3] Nakashima D. et al. 2011. Abstract #1240. 42nd Lunar and Planetary Science Conference. [4] Nakamura T. et al. 2009. *Meteoritics and Planetary Science* 44:A153. [5] Gainsforth Z. et al. 2010. *Meteoritics & Planetary Science* 46:A60. [6] Gainsforth Z. et al. 2011. *Meteoritics & Planetary Science* 46: this issue. [7] Westphal A. J. et al. 2011. *Meteoritics & Planetary Science* 46: this issue. [8] Yurimoto H. et al. 2008. Reviews in Mineralogy and Geochemistry vol. 68. pp. 41-186.
5091

NEW DENSE COLLECTION AREA IN THE ATACAMA DESERT, CHILE: PRELIMINARY RESULTS

A. Hutzler¹, J. Gattacceca¹, P. Rochette¹, T. Jull², M. Valenzuela³, C. Cournede¹, M. Uehara¹, N. Laridhi-Ouazaa⁴, M. Gounelle⁵, R. Braucher¹. ¹CEREGE, UMR6635 CNRS; Aix-Marseille Université, Aix-en-Provence, France. ²NSF Arizona Accelerator Mass Spectrometer Facility, The University of Arizona, Tucson, AZ 85721, USA. ³Dpto. Geologia, Universidad de Chile, Chile Plaza Ercilla 803, Santiago, Chile. ⁴University of Tunis El Manar, Faculty of Sciences, Department of Geology, Crystalline Petrology and Sedimentary Unit, Campus 2092 Manar-II, Tunis, TUNISIE ⁵LMCM, Museum national d'Histoire naturelle, CP52, 61 rue Buffon, 75005 Paris, France.

The Atacama Desert (Chile) is the driest and one of the oldest deserts on Earth [1]. It thus offers good possibilities for meteorite recovery [2][3]. We describe here the main characteristics of a new dense collection area (DCA) located in the Central Depression of the Atacama Desert. We propose to name it Paposo, after the LL6 ordinary chondrite Paposo, found close by [4]. During a joint CEREGE-University of Chile expedition in 2010, we recovered 62 meteorites, all ordinary chondrites except for one CO3. Meteorites were collected searching on foot on a hilly deflation surface, and recognized because of their chocolate-brown color that contrasts with the darker local magmatic rock. Petrographic observations, electron microprobe analyses, and magnetic susceptibility were used for pairing. The 62 meteorites represent at least 44 different falls. The Paposo area thus qualifies as a new DCA.

Surface ages were determined using in situ produced cosmogenic ¹⁰Be concentrations measured in quartz pebbles collected on the surface. Assuming no denudation, Paposo area exposure ages are of several Myr. Regarding weathering, 75% of the meteorites have a weathering grade of W2 or W3. They are thus more weathered than in the other close-by chilean DCA of San Juan [3], but still less weathered compared to other hot desert DCAs in the world. Terrestrial ages are being determined with in-situ 14C and will be discussed [5].

Meteorites masses are between 1 g and 582 g, with 95% under 280 g. The collection shows an overabundance of small meteorites, and a lack of large ones, compared to other hot desert DCAs. The abundance of small stones indicates that only searching on foot allows efficient recovery of small meteorites. This mass distribution might also reflect a fragmentation of meteorites during weathering, although this not strongly supported by the low pairing rate.

H and L chondrites abundances are in agreement with Antarctic and hot deserts collections, with the same abundance of L and H. This tendency has to be confirmed with future search that will allow increasing meteorite density.

**References:** [1] Dunai et al. 2005. *Geology* 33:321–324. [2] Muñoz C., Guerra N., Martinez-Fris J., Lunar C., and Cerda J. 2007. *Journal of Arid Environments* 71:188–200. [3] Gattacceca et al. Description of the densest meteorite collection area in hot deserts: The San Juan meteorite field (Atacama Desert, Chile). *Meteoritics & Planetary Science*, in revision. [4] Meteoritical Bulletin, no. 89, 2005. *Meteoritics & Planetary Science* 40:A201–A263. [5] Jull A. J. T., Donahue D. J., and Linick T. W. 1989. *Geochimica et Cosmochimica Acta* 53:2095–2100.

5522

ASTEROID ITOKAWA (LL CHONDRITE) REGOLITH SIZE FREQUENCY CHARACTERISTICS AND COMPARISON TO DISAGGREGATED TAGISH LAKE (C2 UNGROUPED) METEORITE FRAGMENTS

M. I. Ibrahim and A. R. Hildebrand. University of Calgary. E-mail: emiibrah@ucalgary.ca.

**Introduction:** Asteroid compositional diversity is indicated by spectral and albedo variation and the lithologic range of recovered meteorites. Asteroid regoliths have proven to be unexpectedly ubiquitous and have been imaged with varying resolution on asteroids Gaspra, Ida, Mathilde, Eros, Itokawa, Stein, and Lutetia, and the analogous Martian moons, Deimos and Phobos. The characteristics of asteroid regolith, and the processes that produce it, are of increasing interest as asteroid sampling missions have begun and asteroid dynamic responses to orbit diversion techniques are considered. In this study, we begin by comparing Itokawa [1] regolith fragments' size-frequency and shape distributions between three different terranes. We then determine the size-frequency and shape distributions of fragments of disaggregating Tagish Lake meteorite [2] to compare fragmentation in the much weaker dark asteroid lithologies.

**Data Analysis and Results:** Image "st_2532629277_v.fits" from the Descent and touchdown sequence of Hayabusa [3] was used to calculate the size frequency distribution of fragments on four equal areas. The image resolution of 10 cm pixel⁻¹ allowed four areas of ~170 m² to be picked on the typical rough terrane and two smooth areas (within Komaba Crater and Muses-C). The cumulative size frequency distributions of fragments on the rough terrains yielded a slope of -1.63 and -1.32 while Komaba Crater and Muses-C had a slope of -3.33 and -3.07, respectively. The mean b/a axial ratios of the fragments were found to be 0.73 ± 0.03 and 0.75 ± 0.03 within Komaba Crater and the Muses-C region, respectively.

A total of 195 fragments of Tagish Lake meteorite were measured with a digital caliber and produced a cumulative size distribution slope of -5.7 and their calculated axial ratios b/a and c/a had mean values of 0.76 ± 0.01 and 0.53 ± 0.01, respectively. Note that working with fragments allows measuring of three axes; imagery data allows measurement of only two and an assumption of flat-lying fragments is required to conclude that the measured ratio corresponds to b/a.

**Discussion:** The size frequency within Komaba, a 30 m-diameter crater, had a steeper slope than those of the rough terrane (and even steeper than that of the Muses-C region where abundant fine material collected), indicating that impact-induced fracturing and intra-cratering flows are very effective in eliminating larger fragments and resulting in a more uniform size distribution consistent with observed terrestrial erosion processes.

Tagish Lake meteorite fragment samples show a significantly steeper slope for the cumulative size distribution compared to Itokawa terranes. The steeper slope of the Tagish Lake lithologies is interpreted to reflect the lower strength of these hydrated phyllosilicate lithologies (typical of other similar meteorite lithologies such as CI and CM). This implies that the easier fracturing of Tagish Lake-like lithologies will result in enhanced regolith generation on their parent bodies. However, the b/a fragment axial ratio remain the same for the lithologies of contrasting strengths.

**References:** [1] Fujiwara et al. 2006. *Science* 312:1330–1334. [2] Hildebrand A. R. et al. 2006. *Meteoritics & Planetary Science* 41:407–431. [3] http://darts.isas.jaxa.jp/planet/project/hayabusa/amica.
THREE-DIMENSIONAL MICRO-STRUCTURES OF DUST AGGREGATE BY RANDOM BALLISTIC DEPOSITION USING MICRO-TOMOGRAPHY

Y. Imai¹, A. Tsuchiyama¹, J. Blum², K. Uesugi³, A. Takeuchi³, T. Nakano⁴ and M. Uesugi⁵. ¹Department Earth & Space Sci., Osaka Univ. E-mail: imai@astroboy.ess.sci.osaka-u.ac.jp. ²TU Braunschweig. ³JASRI/SPring-8. ⁴GSJ/AIST. ⁵JAXA.

**Introduction:** In a formation scenario of planetary system, it is widely accepted that planetesimals form due to collisions and aggregation of sub-micron solid particles (dust). In recent years, several studies have been made on the dust aggregation process. Blum et al. (2006) [1] describe the dust aggregation by annalistic theoretical approach. Wada et al. (2008) [2] determined an equation of state of dust aggregates by three dimensional numerical simulations of dust aggregate collisions. In terms of experimental studies, Blum and Wurm (2008) [3] reviewed a series of their studies of experiments about dust aggregation, and clarified macroscopic picture of dust aggregation. However, three dimensional (3D) structure of the dust aggregate has not been determined experimentally yet. In order to reveal dust aggregation in protoplanetary disk, it is necessary to clarify 3D structure of the dust aggregates experimentally.

**Experiments:** Dust aggregate analogs were made in a laboratory as simulation of random ballistic deposition process in the early solar system. Single silica sphere grains of 1.5 microns in diameter were dropped down by cogwheel [3] with a feasible low-velocity in a vacuum chamber evacuated at 100 Pa. During the falling, the grains were aggregated with each other, and finally deposited on a glass fiber of 40 microns in diameter. 3-D structures of the deposited aggregates were obtained by nano-tomography using phase contrast at beamline BL47XU in SPring-8 with monochromatic X-ray beams of 8 keV. Successional slice images with the voxel size of 48 × 48 × 48 nm were reconstructed.

**Results and Discussion:** We succeed to recognize individual gains of the aggregates, and found that the aggregates are mainly composed of chains of grains with some branching. Positions of the center of gravity of individual grains were obtained using image analysis after binarization of the grains. The coordination numbers of the grains vary mainly from 2 to 3, and the fractal dimension of the aggregates ranges from 1.8 to 2.4. The numerical simulation of dust aggregation [2] showed that the fractal dimension of the aggregates does not exceed 2.5 in ballistic cluster-cluster aggregation process. The result of the numerical simulation is consistent with the result of the present study. However, some aggregates of the experiment have higher coordination numbers than those of the numerical simulation, although the fractal dimension of the experimental and numerical simulation aggregates are almost the same. We found that angles between neighboring grains in the experimental aggregates concentrate in 60 degree. In contrast, the aggregates of the numerical simulation do not have such feature. This suggests that there is difference of 3-D structure between the aggregates of experimental and numerical simulations, and the aggregates may have different physical properties such as strength and collisional sticking probability.

**References:** [1] Blum J. et al. 2006. *The Astrophysical Journal* 652:1768. [2] Wada et al. 2008. *The Astrophysical Journal* 677:1296–1308. [3] Blum J. and Wurm G. 2008. *Annual Review of Astronomy and Astrophysics* 46:21–56.

SULFUR ISOTOPE COMPOSITIONS OF TROILITE INCLUSIONS FROM THE MURCHISON CARBONACEOUS CHONDRITE

T. R. Ireland, P. Holden. Research School of Earth Sciences, The Australian National University, Canberra, ACT0200, Australia. E-mail: trevor.ireland@anu.edu.au.

**Introduction:** As one of the first projects undertaken on the new SHRIMP SI, we have developed a technique for rapid measurement of the four isotopes of sulfur. In this work we have analyzed troilite inclusions separated from the Murchison carbonaceous chondrite. The inclusions occur typically as rounded objects and are up to several hundred micrometers across. Most of the grains appear to be recovered from isolated sulfide grains in the matrix, although some fragments with adhering silicate suggest a chondrule origin. The grains were mounted in epoxy along with a pyrite reference.

**Methods:** In situ isotopic analysis was performed using sensitive high-mass resolution ion microprobe—stable isotope (SHRIMP-SI). A 6 nA primary Cs⁺ ion beam (13 keV) was focused to approximately 20 microns with S⁻ secondary ions extracted at 10 keV. No electron gun was required for charge compensation. The sulfur isotopes were collected in multiple collection mode: ³²S with 400 µm collector slit and 10¹⁰Ω current mode resistor, ³³S with 150 µm c.s. and 10¹²Ω, ³⁴S 200 µm c.s. and 10¹¹Ω, and ³⁶S with 200 µm c.s. and 10¹²Ω. In this mode of acquisition, SHRIMP SI is capable of measuring δ³⁴S to around 0.2 ‰ (2 SE) Δ³³S to around 0.3 ‰ (2 SE) and Δ³⁶S to 0.5 ‰ (2 SE).

**Results:** The Murchison troilite inclusions are uniform within each inclusion but show differences between inclusions of up to several permil in terms of mass dependent fractionation. No nonlinear effects have been detected at this stage in either Δ³³S or Δ³⁶S.

**Discussion:** SHRIMP analysis allows analysis of four-isotope S compositions with only a few nanograms of material. The external precision obtained for Δ³⁶S is similar to that attained by conventional gas source mass spectrometry. Mass dependent fractionation in the Murchison inclusions is consistent with evaporation/condensation in the solar nebula.
5231

NORTHWEST AFRICA 6704: A UNIQUE CUMULATE PERMAFIC ACHONDRITE CONTAINING SODIC FELDSPAR, AWARUITE AND "FLUID" INCLUSIONS, WITH AN OXYGEN ISOTOPIC COMPOSITION IN THE ACAPULCOITE-LODRANITE FIELD

A. J. Irving¹, R. Tanaka², A. Steele³, S. M. Kuehner¹, T. E. Bunch⁴, J. H. Wittke⁴ and G. M. Hupé. ¹Department of Earth & Space Sciences, University of Washington, Seattle, WA 98195, USA. E-mail: irving@ess.washington.edu ²Institute for Study of Earth's Interior, Okayama University, Misasa, Japan. ³Geophysical Laboratory, Carnegie Institution, Washington, DC, USA. ⁴Geology Program, SESES, Northern Arizona University, Flagstaff, AZ, USA.

**Petrography:** A very fresh, dense, yellowish-green stone with rounded, shiny exterior surfaces and small patches of remnant black fusion crust found in Algeria is a unique, medium grained (gs 0.3–1.5 mm), unshocked achondrite with an igneous cumulate texture. Smaller grains of olivine (Fa₅₁.₆-₅₃.₂; FeO/MnO = 120–140; NiO = 0.9–1.0 wt%) and chromite [Cr/(Cr + Al) = 89.0) are enclosed within large (up to 1.5 mm) orthopyroxene oikocrysts (Fs₄₁.₆-₄₂.₄Wo₂.₈-₃.₆, FeO/MnO = 81–106), which are surrounded by intercumulus indistinctly-twinned, very sodic plagioclase (Ab₉₂An₄Or₄) and tiny grains of awaruite (Ni = 75 wt%). Orthopyroxene grains contain curvilinear trains of tiny rounded to irregularly-shaped, clear to pinkish inclusions (< 2–30 µm), which appear on polished surfaces to be empty bubbles with smooth rounded walls. Confocal Raman spectroscopy of inclusions below thin section surfaces did not reveal any fluid species.

**Oxygen Isotopes:** Replicate analyses of acid-washed bulk material by laser fluorination gave: δ¹⁷O 1.015, 0.880; δ¹⁸O 3.922, 3.613; δ¹⁷O -1.048, -1.020 per mil, respectively. These results plot in the field for acapulcoites and lodranites [1, 2].

**Discussion:** This specimen is unlike any other known achondrite in its combined mineralogical features: mafic silicate minerals with elevated FeO/MnO ratios, extremely sodic plagioclase, trapped bubble trains and very Ni-rich metal. Despite its oxygen isotopic composition, NWA 6704 bears no mineralogical resemblance to members of the acapulcoite-lodranite clan. It appears to be a cumulate igneous rock related to an unknown "planetary" parent body, which evidently was capable of interior melting and crystal accumulation. The relatively ferroan silicates, very sodic plagioclase and now empty (?) fluid inclusions (see below) suggest a somewhat "Earth-like," differentiated parent body.

Bubble trains and cleavages in orthopyroxene; WOF 0.6 mm

**References:** [1] Clayton R. and Mayeda T. 1996. *Geochimica et Cosmochimica Acta* 60:1999–2017; [2] Irving A. et al. 2007. Abstract #1338. 38th Lunar and Planetary Science Conference.

<figure><img src="image_14.png" /></figure>

5232

PAIRED FINE GRAINED, PERMAFIC OLIVINE-PHYRIC SHERGOTTITES NORTHWEST AFRICA 2990/5960/6234/ 6710: TRACE ELEMENT EVIDENCE FOR A NEW TYPE OF MARTIAN MANTLE SOURCE OR COMPLEX LITHOSPHERIC ASSIMILATION PROCESSES

A. J. Irving¹, C. D. K. Herd², M. Gellissen³, S. M. Kuehner¹ and T. E. Bunch⁴. ¹Earth & Space Sciences, University of Washington, Seattle, WA 98195, USA, E-mail: irving@ess.washington.edu. ²Earth & Atmos. Sciences, University of Alberta, Edmonton, Canada. ³Institute of Geosciences, Universität zu Kiel, Germany. ⁴Geology, SESES, Northern Arizona University, Flagstaff, AZ 86011, USA.

**Background:** NWA 2990 was the first of at least five fresh crusted stones allegedly found in Mali representing an unusual type of Martian igneous lithology. Our initial studies [1] established that NWA 2990 is an olivine-phyric shergottite with unusually fine grainsize, and based upon its REE abundances we suggested that it had affinities to "enriched" shergottites. Bulk major and trace element compositions were measured on a very small fragment of the stone; however, subsequent analyses on more representative powders of paired stones now lead us to present revised compositional data, which are much more consistent with the observed mineralogy and have significant implications.

**Paired Specimens:** NWA 5960, NWA 6234 and NWA 6710 all share with NWA 2990 the same microporphyritic texture, mineralogy and mineral compositions. Bulk XRF analyses of NWA 5960 and NWA 6234 in the same laboratory establish that these are all permafic shergottites [2] with CaO = 6.5 wt%, MgO = 16.6 wt% and Mg/(Mg + Fe) = 0.572. The major element composition reported previously [1] evidently was on powder that undersampled the olivine and chromite microphenocrysts, yet bulk abundances of incompatible elements (residing in groundmass phases) were accurate. REE data for NWA 6234 by ICPMS show good agreement for common elements with INAA data for NWA 2990, but new data for Pr and Nd require that the affinity of these specimens be revised from "enriched" to intermediate.

**Discussion:** The more complete REE pattern implies either that NWA 2990 and pairings are related to a Martian mantle source distinct from the depleted, intermediate and "enriched" sources already known [3], or else that assimilative interaction of an "enriched" Martian magma with lithospheric materials (possibly hydrothermally altered?) changed its Sm/Nd ratio.

**References:** [1] Bunch T. et al. 2009. Abstract #2274. 40th Lunar and Planetary Science Conference. [2] Irving A. et al. 2011. Abstract #1612. 42nd Lunar and Planetary Science Conference. [3] Borg L. and Draper D. 2003. *Meteoritics & Planetary Science* 38:1713–1731. [4] Lapen T. et al. 2010. Abstract #2448. 41st Lunar and Planetary Science Conference.

<figure><img src="image_15.png" /></figure>
## OXIDATION IN R CHONDRITES

Junko Isa, Alan E. Rubin, and John T. Wasson. Institute of Geophysics, University of California, Los Angeles, CA 90095-1567, USA. E-mail: jisa@ucla.edu.

R chondrites are among the most oxidized chondrite groups, characterized by the rarity or absence of metallic Fe-Ni [1-5], high Ni contents in the remaining metal (mainly awaruite) [2-5], abundant NiO-bearing olivine (Fa35-42) [5], the presence of Fe³⁺-bearing oxides and, in one sample (R5 LAP 04840), major hornblende and accessory biotite [6]. These mineralogical features reflect variations in local conditions on the R chondrite parent body. To understand such parent-body processes, we studied the bulk chemistry and petrology of 14 R chondrites. Our INAA data show only minor chemical variations in R chondrites [5, 7, 8]. Nevertheless, we observed diverse compositions among oxide phases in equilibrated R chondrites [7]. The amount of Fe³⁺ in magnetite-chromite solid solutions increases with the amount of NiO in olivine. In addition, the sample with the most Fe³⁺-enriched oxides (A-881988 and LAP 031156) are associated with the lowest olivine Fa (35 mole%), implying that these rocks are the most oxidized samples. These trends are not a function of petrologic type; this indicates that the degree of oxidation is independent of the temperature history and instead, probably reflects local conditions. Variations in redox state caused variations in the distribution of Fe and Ni among sulfides and oxides. For example, Fe occurs in metal (Fe⁰), is bivalent in silicates, sulfides and partly in oxides, and is trivalent in some oxides.

We used the optical microscope and point-counter to determine modal abundances and petrologic type; we also used EPMA for phase analysis. The respective abundances of silicate, sulfide and oxide are: R6 Y-980702, Fa38 (94 vol%, 5 vol%, 0.6 vol%), R4 A-881988, Fa35 (92 vol%, 5 vol%, 3 vol%), R4 LAP 031156, Fa35 (89 vol%, 7 vol%, 4 vol%), R4 NWA 2198, Fa39 (91 vol%, 8 vol%, 1 vol%), R4-clast in Bencubbin, Fa38 (94 vol%, 6 vol%, 0.7 vol%). The sulfide modal abundances (5-8 vol%) are not correlated with the phase compositions and have been affected by terrestrial weathering to different extents [9]. On the other hand, the oxide/silicate volume-ratios (0.007-0.04) are correlated with the olivine and magnetite compositions. The modal abundance of oxides increases with the amount of Fe³⁺ in oxide, and decreases with the olivine Fa value. The observed trends (the chief oxidant) probably result from the inhomogeneous distribution of water in the R chondrite parent body.

**References:** [1] Rubin A. E. and Kallemeyn G. W. 1989. *Geochimica et Cosmochimica Acta* 53:3035-3044. [2] Schulze H. et al. 1994. *Meteoritics* 29:275-286. [3] Rubin A. E. and Kallemeyn G. W. 1994. *Meteoritics* 29:255-264. [4] Bischoff A. et al. 1994. *Meteoritics* 29:264-274. [5] Kallemeyn G. W. et al. 1996. *Geochimica et Cosmochimica Acta* 60:2243-2256. [6] McCanta M. C. et al. 2008. *Geochimica et Cosmochimica Acta* 72:5757-5780. [7] Isa et al. 2010. Abstract #5403. *Meteoritics & Planetary Science*. [8] Greenwood et al. 2000. *Geochimica et Cosmochimica Acta* 64:3897-3911. [9] Rubin A. E. and Huber H. 2005. *Meteoritics & Planetary Science* 40:1123-1130.

## QUE 97186 AND RBT 04143: TWO EXTREME VARIETIES OF CV3 CARBONACEOUS CHONDRITES

H. Ishida, T. Nakamura, H. Miura, and Y. Kakazu. Department of Earth and Planetary Material Sciences, Tohoku University, Miyagi 980-8578 Japan. E-mail: h-ishida@s.tohoku.ac.jp.

CV3 meteorites of reduced type are known to retain the most primitive mineralogical characteristics of CV3 chondrites, because it has undergone the least degrees of aqueous alteration and thermal metamorphism [e.g., 1]. At the beginning of the present study, we have carried out petrologic study on six CV3 samples, RBT 04143, QUE 97186, GRA 06101, LAP 04843, LAP 02206, and LAP 02228 in order to identify reduced type CV3s. The results showed that RBT 04143 and QUE 97186 are classified to reduced CVs and further mineralogical and isotope study was carried out on the two meteorites to uncover their formation and evolution process.

RBT 04143 has a very porous matrix and rounded undeformed chondrules. Submicron to several-micron size silicates of mostly olivine and minor amounts of low- and high-Ca pyroxene are major phases in matrix. Individual matrix olivine were analyzed by using FE-EPMA and showed a wide range of Fe/(Mg + Fe) ratios from Fa0 to 80. The wide Fa variation indicates that olivine dust in the solar nebula had an extreme wide range of Fe/Mg ratios and RBT 04143 has undergone the least degrees of aqueous alteration and thermal metamorphism after accretion of parent body.

On the other hand, the matrix of QUE 97186 is highly compacted with porosity much lower than RBT 04143 and chondrules are flattened to high aspect ratios and show a preferred orientation. The texture strongly suggests that the meteorite has experienced shock impact on the meteorite parent body. Olivine and pyroxene phenocrysts in QUE 97186 chondrules show undulatory extinctions and planar deformation fractures. This result indicates that the meteorite experienced shock pressure around 20 GPa according to the results of previous shock recovery experiments [2] and shock-stage classification scheme [3].

Shock-induced heating resulted in homogenizing matrix olivine composition: QUE 97186 matrix olivine showed Fa range from Fa45 to 60, which is much narrower than that in RBT 04143. Low melting temperature material is also subject to heating: troilite grains in the matrix are totally or partially melted, suggesting that peak shock temperature exceeded Fe-FeS eutectic melting temperature of 1000 °C. So as to estimate the cooling rate from peak residual temperature (800 approximately 1300 °C based on internal energy increase derived from hugoniot calculation [2] and mineralogical observation), we simulated homogeneization of Fa with time by considering size distribution of matrix olivine and Fe-Mg diffusion length by both grain-boundary and lattice diffusion [4-6]. The results of simulation indicate that the cooling rates are in the range from $10^{-5}$-$10^{-1}$ K s⁻¹. This corresponds to the heating region smaller than 10 m on the QUE 97186 parent body.

**References:** [1] McSween. 1977. *Geochimica et Cosmochimica Acta* 41:1777-1790. [2] Nakamura et al. 2000. *Icarus* 146:289-300. [3] Stoffler et al. 1991 *Geochimica et Cosmochimica Acta* 55:3845-3867. [4] Yurimoto and Wasson. 2002 *Geochimica et Cosmochimica Acta* 66:1777-1790. [5] Mishin and Herzig. 1999 *Material Science and Engineering* 260:55-71. [6] Dohmen and Chakraborty. 2007. *Physics and Chemistry of Minerals* 34:409-430.
5213

THE QUESTION OF GEMS IN COMET 81P/WILD 2: STARDUST ANALOG IMPACTS OF FINE-GRAINED MINERAL AGGREGATES.

H. A. Ishii¹, P. J. Wozniakiewicz¹, A. T. Kearsley², M. J. Burchell³, J. P. Bradley¹, N. Teslich¹, M. C. Price³ and M. J. Cole³. ¹Institute of Geophysics and Planetary Physics, Lawrence Livermore National Laboratory, 7000 East Avenue, Livermore, CA 94550, USA E-mail: hope.ishii@llnl.gov. ²Impacts & Astromaterials Research Centre, Department of Mineralogy, Natural History Museum, London SW7 5BD, UK. ³Centre for Astrophysics and Planetary Science, School of Physical Sciences, University of Kent, Canterbury, Kent CT2 7NH, UK.

**Introduction:** During the Preliminary Examination phase for comet 81P/Wild two samples returned by NASA's Stardust mission, GEMS-like objects similar in texture to submicron amorphous silicate glass embedded with metal and sulfides (GEMS) found in cometary chondritic porous interplanetary dust particles [1] were found in hypervelocity impact tracks generated by capture of comet dust in silica aerogel at approximately 6.1 km s⁻¹. We showed that sulfide particles fired into aerogel resulted in the same texture as Stardust GEMS-like objects (sulfide-rimmed metal beads embedded in glass) [2]. We now explore results of hypervelocity impact of fine-grained mineral aggregate particles like those inferred to have impacted Stardust Al foil collectors [3, 4]. We ask if such aggregates can explain the heterogeneous glass chemistry seen in GEMS-like objects in impact tracks in Stardust aerogel as well as the mixtures of quenched melt and residual crystals seen in impact residues in Stardust foils.

**Materials and Methods:** Fine-grained artificial mineral aggregates were generated from <5 µm, relatively monodisperse, size-separated powders of olivine, diopside and pyrrhotite cemented together with an acrylate adhesive and fired by light gas gun (LGG) at flight spare Al foil and silica aerogel at Stardust encounter speed. SEM with EDX was used to locate impact craters with multi-mineral residue in foils for focused ion beam (FIB) thin section preparation. Impact tracks in aerogel were extracted by ultrasonic blades [5], compressed and embedded in epoxy for ultramicromed thin section preparation. Thin sections were analyzed using LLNL's Titan aberration-corrected (scanning) transmission electron microscope (S)TEM).

**Results and Discussion:** In foil, impacts of relatively-monodisperse aggregates result in complete melting, loss of S, incorporation of Al and segregation of Fe-rich, Al-bearing crystalline beads. The glass composition is intermediate in Mg and Ca and depleted in Fe relative to olivine and diopside precursors. In aerogel, aggregate impacts result in bulbous tracks. Results of aggregate impacts in the aerogel will also be presented as a function of track depth and compared with prior single component projectile shots. We will discuss implications of aggregate particles on localized heating and chemical inhomogeneity in impacts.

**Acknowledgments:** We thank NASA for Al foils and STFC for LGG support. This work was supported by NNH07AG46I and NNH11AQ79I to HAI and LDRD 09-ERI-004 to JPB. Portions of this work were performed under the auspices of US DOE by LLNL under Contract DE-AC52-07NA27344.

**References:** [1] Keller L. P. et al. 2006. *Science* 314:1728–1731. [2] Ishii H. A. et al. 2008. *Science* 319:447–450. [3] Price M. C. et al. 2010. *Meteoritics & Planetary Science* 45:1409–1428. [4] Leroux H. et al. 2008. *Meteoritics & Planetary Science* 43:143–160. [5] Ishii H. A. and Bradley J. P. 2006. *Meteoritics & Planetary Science* 41:233–236.

5072

FORMATION OF REFRACTORY INCLUSIONS FROM ¹⁶O-RICH AND ¹⁶O-POOR SOLIDS: EVIDENCE FROM COMPLEX CAIS IN CV CHONDRITES

M. A. Ivanova¹,², A. N. Krot³, K. Nagashima³ and G. J. MacPherson². ¹Vernadsky Institute of Geochemistry, Moscow 119991, Russia. E-mail: meteorite2000@mail.ru. ²Smithsonian Institution, Washington, DC. ³University of Hawai'i at Mānoa, Honolulu, HI 96822, USA.

**Introduction:** CAIs enriched in Zr, Sc and Y provide important records of the highest temperature processes in the early solar nebula [1–4]. We recently reported mineralogy, petrography, and O-isotope compositions of two complex refractory inclusions consisting of normal (Type B, CTA, or FTA) CAIs and Zr-rich CAIs (which probably are ultrarefractory) composed of Zr,Sc-rich pyroxenes, Zr-Y-Sc oxides, and Y-rich perovskites, 3N-24 from NWA 3118 and 33E-1 from Efremovka [5]. In both Zr-rich CAIs, the Zr, Sc-rich pyroxenes, the Y-rich perovskites and the Zr-Y-Sc-oxides are depleted in ¹⁶O (Δ¹⁷O ~−3 ± 2%), whereas the co-occurring Al,Ti-diopside and spinel are ¹⁶O-rich (Δ¹⁷O ~−23 ± 2%) [5]. In this regard 3N-24 and 33E-1 differ from an ultrarefractory CAI “Romulus” from the CM chondrite Murchison [4] that is uniformly ¹⁶O-rich (Δ¹⁷O ~−20%). In order to better understand this enigmatic dichotomy of O-isotope compositions in 3N-24 and 33E-1, we performed detailed spot analyses of the Zr, Sc-rich pyroxenes in 3N-24 along a profile outward from the Zr-Y-Sc-oxides into the enclosing Al, Ti-diopside to explore potential correlations between pyroxene isotopic and mineral chemical compositions.

**Results and Discussion:** In 3N-24, the Zr, Sc-rich pyroxenes form the halo around the Zr-Y-Sc-rich oxides intergrown with Y-rich perovskite. The pyroxenes have positive Zr-Sc, Zr-Y, and Zr-Hf correlations, and show a systematic change in Δ¹⁷O value from ¹⁶O-poor to ¹⁶O-rich and decreasing Zr, Sc, and Y contents with increasing distance from the Zr-Y-Sc oxides. However, chemical and O-isotope compositions of the pyroxenes appear to be decoupled. There is a sharp (~5 µm) boundary between the ¹⁶O-poor Zr, Sc-rich pyroxenes and the surrounding ¹⁶O-rich Al, Ti-diopside. Both types of pyroxenes enclose small (1–3 µm) spinel grains which are similarly ¹⁶O-rich (Δ¹⁷O ~−22 ± 3%).

In 33E-1, pyroxene in the core is highly enriched in Sc, Zr, Y and Hf, but correlations between these elements are not as clear as in 3N-24. Al-diopside from the rim of 33E-1 is more enriched in Zr, Sc, Y and Hf than that of 3N-24. As in 3N-24, we observed no correlation between O-isotope compositions of the Zr, Sc-rich pyroxene and their chemical compositions, although again the Zr, Sc-pyroxene in the CAI core is ¹⁶O-poor and the Al-diopside in the rim is ¹⁶O-rich.

We infer that the complex CAIs from Efremovka and NWA 3118 recorded the multistage formation of refractory inclusions and the coexistence of ¹⁶O-poor (Zr-rich oxides, perovskite and Zr, Sc-rich pyroxenes) and ¹⁶O-rich (Al-diopside, spinel) solids in the early solar system [6]. The Zr, Y, and Sc-rich ¹⁶O-poor mineral assemblages in 3N-24 and 33E-1 appear to be relict; they were partially melted and mixed with material formed in an ¹⁶O-rich environment. Oxygen-isotope exchange between the ¹⁶O-rich and ¹⁶O-poor materials during this melting was very limited.

**References:** [1] Davis A. M. 1991. *Journal of Geophysical Research* 90:1151–1154. [2] Simon S. B. et al. 1996. *Meteoritics & Planetary Science* 31:106–115. [3] El Goresy A. et al. 2002. *Geochimica et Cosmochimica Acta* 66:1459–1491. [4] Ushikubo T. et al. 2004. *Meteoritics & Planetary Science* 39:#5111. [5] Ivanova M. A. et al. 2011. Abstract #1728. 42nd Lunar and Planetary Science Conference. [6] Krot A. N. et al. 2010. *The Astrophysical Journal* 713:1159–1166.
THE TIMING OF AQUEOUS ALTERATION IN UNEQUILIBRATED ORDINARY CHONDRITES

B. Jacobsen¹, J. Matzel¹, A. N. Krot², I. D. Hutcheon¹, M. Telus² and K. Nagashima². ¹Lawrence Livermore National Laboratory. ²University of Hawai'i at Mānoa, USA.

**Introduction:** The time scale of secondary mineral formation in chondrites is important for distinguishing between nebular and asteroidal alteration processes. Although most chondrites are affected by varying degrees and types of alteration, the time frame for different episodes of fluid-assisted thermal metamorphism is poorly constrained. Aqueous activity occurred nearly simultaneously on the parent bodies of at least three classes of unequilibrated carbonaceous chondrites—CI, CM and CV—suggesting water played an important role in the chemical evolution of primitive solar system bodies [1]. Whereas the timing of aqueous alteration in CV chondrites is well constrained, the timing of formation of secondary minerals in unequilibrated ordinary chondrites (UOCs) is not. In order to constrain the timing of aqueous alteration on the UOC parent body, we studied the ⁵³Mn-⁵³Cr systematics of fayalitic olivine in the UOCs, Ngawi (LL3.0–3.6) and EET 90161 (L3.05).

**Results and Discussion:** Fayalitic olivine (Fa₅₀–₁₀₀) is a major constituent in the matrices of many UOCs and carbonaceous (CV, CK, CO) chondrites and both nebular and asteroidal models have been proposed to explain its origin [2]. Most recent models, however, favor fayalite formation during fluid-assisted thermal metamorphism on the chondrite parent bodies [3, 4]. In this regard, the ⁵³Mn-⁵³Cr systematics of fayalite can be used to constrain the timing of aqueous alteration on chondrite parent bodies. The Cr isotopic composition of fayalite in two UOCs, Ngawi and EET 90161, was measured with the NanoSIMS 50 at the Lawrence Livermore National Laboratory using a ¹⁶O⁺ primary beam focused to ~300 nm spatial resolution and a mass resolving power of ~3500. Fayalite in Ngawi, with ⁵⁵Mn/⁵²Cr ratios ranging between 1100 and 1950, shows no excess ⁵³Cr beyond analytical uncertainty. Fayalite grains in EET 90161 exhibit a much wider range in ⁵⁵Mn/⁵²Cr ratios of 430–20,000 and two fayalites with ⁵³Mn/⁵²Cr of ~19,000 show large ⁵³Cr excesses (δ⁵³Cr ~375‰) resolved at the 2σ level. A regression through the Mn-Cr data for EET 90161 fayalite defines a best fit line with a slope corresponding to (⁵³Mn/⁵⁵Mn)₀ = (2.6 ± 2.2)×10⁻⁶. The inferred initial ⁵³Mn/⁵⁵Mn ratio cannot be resolved yet from that in ordinary chondrite chondrules reported by [5], (5.1 ± 1.6)×10⁻⁶.

These data provide the first evidence for radiogenic ⁵³Cr in secondary phases in UOCs and demonstrate that the formation of fayalitic olivine during aqueous alteration occurred simultaneously, about 6 Ma after CAI formation, on at least three carbonaceous and one ordinary chondrite parent body. The time scale is consistent with heating of asteroids by ²⁶Al, and suggests that water played an important role in the chemical evolution of primitive solar system bodies.

**References:** [1] Hutcheon I. D. et al. 1998. *Science* 282:1865–1867. [2] Lauretta D. et al. 2003. *Geochimica et Cosmochimica Acta* 65:1337–1353. [3] Krot A. et al. 2004. Proceeding of National Institute of Polar Research 17:154–172. [4] Zolotov M. Yu. et al. 2006. *Meteoritics & Planetary Science* 41:1775–1796. [5] Yin Q.-Z. et al. 2007. *The Astrophysical Journal* 662:L43–L46.

TRANSPORT OF CHONDRITE COMPONENTS IN THE PROTOPLANETARY DISK: INTERPRETING THE CARBONACEOUS/ NON-CARBONACEOUS CHONDRITE DICHOTOMY

E. Jacquet¹, S. Fromang² and M. Gounelle¹. ¹LMCM, UMR 7202, CNRS & MNHN, CP 52, 57 rue Cuvier, 75005 Paris, France. E-mail: ejacquet@mnhn.fr. ²CEA, Irfu, SAp, Centre de Saclay F-91191 Gif-sur-Yvette. ³UMR AIM, CEA-CNRS-Univ. Paris VII, Centre de Saclay, F-91191, Gif-sur-Yvette, France.

**Introduction:** A fundamental observation in cosmochemistry is the first-order distinction between carbonaceous (“CCs”) and non-carbonaceous chondrites (Enstatite, Ordinary, Rumuruti chondrites, “EORs”). How this dichotomy arose in the protoplanetary disc and how it is to be understood chronologically and/or spatially [1] is inseparable from the long-standing debates on the chemical, isotopic and petrographic fractionations in the different chondrite groups (e.g. [2],[3]). In particular, it is important to investigate fractionations of petrographic components of chondrites during their several Ma-long [4] journey as free-floating objects in the disc until their final agglomeration.

**Approach:** We have revisited via simplified analytic calculations the radial dynamics of mm- and sub-mm-sized solids [5]. In order to draw conclusions independently of the details of the disk structure, we have based our reasonings on as generic a disk model as possible and compared our inferences to calculations for specific disc models published in the literature (e.g. [7–9]).

**Results and Discussion:** We find that the dynamics of solids are essentially controlled by a gas-solid “decoupling” parameter called S first introduced by [6]: If S < 1, solids are essentially tied to the gas and no (whole-rock) fractionation other than exchange with the gas phase occurs. If S > 1, solids tend to settle toward the disk midplane and drift toward the Sun faster than the gas. In many disk models, S should be an increasing function of time and heliocentric distance.

Based on (1) abundance of refractory inclusions (2) proportion of matrix (3) lithophile element abundances and (4) oxygen isotopic composition of chondrules, we find that the properties of CCs are most consistent with S < 1 when they accreted while those of EORs suggest S > 1. For example, complementarity in CCs ([10–11]) would have been maintained until accretion because differential drift between chondrules and dust grains was negligible. Given how S varies in disk models, this suggests that CC components agglomerated (but not necessarily formed) earlier on average than those of EORs (as suggested by [12]). Thus, refractory inclusions formed in the early phases of disk history [13], and found in considerable abundance in CCs, would have been depleted in the disk by the time EORs accreted because of drift toward the Sun.

If our first-order conjecture regarding the CC/EOR duality were to prove correct, this would open the way to considerable progress in the understanding of chondrite diversity in the accretion disk context.

**References:** [1] Wood J. A. 2005. *Ch. and the p. p. d.*, 953–971. [2] Wasson J. T. and Chou C.-L. 1974. *Meteoritics* 9:69–84. [3] Anders E. 1964. *Space Science Review* 3:583–714. [4] Villeneuve J. et al. 2009. *Science* 325:985–988. [5] Jacquet E. et al. (to be submitted to *Icarus*). [6] Cuzzi J. N. et al. 1996, in *Ch. and the p. p. d.*, 35–43. [7] Clarke C. J. and Pringle J. E. 1988. *MNRAS* 235:365–373. [8] Ciesla F. J. 2009. *Icarus* 200:655–671. [9] Hughes A. L. H. and Armitage P. J. 2010. *The Astrophysical Journal* 719:1633–1653. [10] Bland P. et al. 2005. Proceedings of the National Academy of Sciences 102:13755–13760. [11] Hezel D. C. and Palme H. 2010. *Earth and Planetary Science Letters* 294:85–93. [12] Chambers J. 2006. in *MESS2* 487–497. [13] Jacquet E. et al. *Astronomy & Astrophysics* 526:L8.
CARBON, NITROGEN, AND SILICON ISOTOPIC COMPOSITIONS
OF PRESOLAR SILICON CARBIDE GRAINS FROM THE LL3.1
CHONDRITE BISHUNPUR

M. Jadhav¹, K. Nagashima¹ and G. R. Huss¹. ¹Hawaiʻi Institute of Geophysics and Planetology, University of Hawaiʻi at Mānoa, Honolulu, HI 96822. E-mail: manavi@higp.hawaii.edu.

**Introduction:** Several studies have indicated that the N isotopic compositions of presolar SiC grains vary between different meteorites and among different size ranges in single meteorites [1-5]. The N compositions do not correlate with the cosmic ray exposure ages of the host meteorites, and terrestrial contamination is unlikely in most cases [3, 4]. We know that abundances and other characteristics of presolar grains correlate with the bulk compositions of the host meteorites [6, 7]. This suggests the possibility that N isotopes carry a record of the pre-accretionary radiation history of the material making up each chondrite. To test this hypothesis, we have begun an investigation of presolar SiC grains from various chondrites. Here, we report preliminary results of C, N, and Si isotopic measurements on presolar SiC grains from the Bishunpur (LL3.1) chondrite.

**Method:** A portion of the SiC-spinel separate prepared by [6] was deposited on gold foil mounts. Thirty-five SiC grains were identified by mapping the mounts for C, Si, Al, Mg, and Cr using wavelength dispersive spectrometry on the UH JXA-8500F field-emission electron microprobe. Most grains were < 1.5 µm in size. Isotopic analysis of C, N, and Si were performed using the UH ims-1280 ion microprobe. A 3-5 pA Cs⁺ primary beam focused to ~0.5 µm was rastered over 5 × 5 µm² regions and scanning isotope images were collected. Carbon and Si isotopes were measured first, followed by C and N (measured as CN''). Isotopic ratios were calculated from regions of interest using L'image software.

**Results and Discussion:** Thirty-three of the grains are mainstream grains with ¹²C/¹³C ratios ranging from 20 to 100 and δ²⁰Si and δ³⁰Si ranging from 0‰ to +200‰ relative to solar silicon. Two grains have ¹²C/¹³C ratios < 10 and can be classified as A+B grains. Nitrogen in most of the Bishunpur grains is highly enriched in ¹⁴N, with ¹⁴N/¹⁵N up to 12 times the solar value. Some grains appear to be less enriched in ¹⁴N than similar-sized SiC grains from Murchison [1], Orgueil [3], Colony [4], and ALHA77307 [5]. We are cautious to interpret this result because the less ¹⁴N-rich N compositions could be due to contamination by terrestrial N. While measuring C and Si isotopes, the C/Si ratio of a majority of the grains decreased over the measurement period. The ion images showed a large C signal around the grains that sputtered away during the course of the measurement. This indicates that the grains were covered by a C-rich contaminant. It is possible that N from what was left of this C-rich material affected the N compositions. We intend to check these measurements by preparing new grain mounts, taking special precautions to remove the C-rich material, as described in [3], and measuring more presolar SiC grains from Bishunpur.

**Acknowledgments:** Supported by NASA grant NNX08AB58G (GRH).

**References:** [1] Hoppe P. et al. 1994. *The Astrophysical Journal* 430:870-890. [2] Hoppe P. et al. 1996. *Geochimica et Cosmochimica Acta* 60:883-907. [3] Huss G. R. et al. 1997. *Geochimica et Cosmochimica Acta* 23:5117-5148. [4] Smith J. B. and Huss G. R. 2003. *Meteoritics & Planetary Science* 38:A118. [5] Smith J. B. et al. 2004. Abstract #2006. 35th Lunar and Planetary Science Conference. [6] Huss G. R. and Lewis R. S. 1995. *Geochimica et Cosmochimica Acta* 59:115-160. [7] Huss G. R. et al. 2003. *Geochimica et Cosmochimica Acta* 67:4823-4848.

RAMAN SPECTROSCOPY INVESTIGATIONS OF TAGISH LAKE NANODIAMONDS

T. Jakubowski¹, M. Kozanecki², U. Ott³ and A. Karczemska⁴. ¹Balonowa 5/2 Wroclaw, Poland. E-mail: illaenus@gmail.com. ²Department of Molecular Physics, Technical University of Łódź, Łódź, Poland. ³Department of Geochemistry, Max Planck Institute for Chemistry, Mainz, Germany. ⁴Institute of Turbomachinery, Technical University of Łódź, Łódź, Poland.

**Introduction:** Tagish Lake, a carbonaceous chondrite C2, fell on January 18, 2000, in Canada. According to [1], the meteorite contains about 3650-4330 ppm of nanodiamonds, while inferred from measurements of xenon [2] the abundance is 1800-2500 ppm. This is the highest nanodiamond abundance reported in chondrites. Raman spectroscopic signatures of separated nanodiamonds have been studied in the work presented here.

**Samples and Experiments:** Nanodiamonds were separated following established procedures [3] Raman spectra have been recorded using the confocal Raman micro-spectrometer T-64000 (Jobin-Yvon) with argon line λ = 514.5 nm.

**Results:** The spectra show a diamond peak at 1329 cm⁻¹ (FWHM of about 50 cm⁻¹) and a broad peak around 1600 cm⁻¹ (Fig. 1) [4]. Previous investigations of Allende meteorite presolar nanodiamonds showed peaks at 1326 and 1590 cm⁻¹ [5] what is in agreement of laboratory manufactured nanodiamonds Raman investigations [6].

Shifts of diamond peak positions (monocrystalline cubic diamond has Raman peak at 1332 cm⁻¹) occur because of the nanometer crystal sizes [6] or because of different polytypes [7] of diamond occurring in the sample.

**References:** [1] Grady M. M. et al. 2002. *Meteoritics & Planetary Science* 37:713-735. [2] Jakubowski T. et al. 2011. Abstract #2944. Goldschmidt 2011 Conference. [3] Amari S. et al. 1994. *Geochimica et Cosmochimica Acta* 58:459-470. [4] Jakubowski T. 2011. Ph.D. thesis, Technical University of Lodz. [5] Gucsik A. et al. 2008. Proceedings of the IAU Symposium 251 "Organic Matter in Space":335-339. [6] Osswald S. et al. 2009. *Physical Review B* 80:075419. [7] Phelps A. W. 1999. Abstract #1749. Lunar and Planetary Science Conference.

<figure><img src="image_16.png" /><figcaption>Fig.1. Raman spectra of nanodiamonds from Tagish Lake carbonaceous chondrite. The diamond peak is detected at the position 1329 cm⁻¹.</figcaption></figure>
5167

EVIDENCE FOR RHÖNITE IN ANGRITES D'ORBIGNY AND
SAHARA 99555

A. Jambon and O. Boudouma. CNRS UMR 7193, UPMC-PARIS 6, ISTEP 4 place Jussieu, 75005 Paris, France. E-mail: albert.jambon@upmc.fr.

**Introduction:** Angrites are rare achondrites with a specific and odd mineralogy: anorthite, calcic olivine and Al-Ti-diopside-hedenbergite are the major phases [1]. Magmatic angrites: D'Orbigny, SAH 99555 and NWA 1296 share textural and mineralogical similarities, with a decreasing grain size indicating an increasing cooling rate. An enigmatic Al-Ti-silicate was described in D'Orbigny [1]. It was later suggested that it could be rhönite [2]. More recently this mineral was discovered in another angrite, NWA 4590 of the plutonic type [3].

**Analytical Techniques:** We investigated three polished sections, two of D'Orbigny (NHM Wien; 1176-1179) and one of SAH 99555. We performed SEM imaging (Zeiss Ultra 55VP), EDS identification and EMP analyses at 15 kV, 10 nA (Cameca SX100). Raman spectra were obtained for D'Orbigny with the SEM built-in spectrometer

**Results:** Rhönite is present in both samples in close relationship with final crystallization phases. Its size is typically 20–100 µm. The structural formula is (20 O and 14 cations): $(Si_{3.95}Al_{1.73}Ti_{0.32})Fe_5(Fe_{0.17}, Ti_{0.75})Ca_{2.03}O_{20}$. Notice that the tetrahedral site is significantly deficient in Al. Since $Fe^{3+}$ is absent, excess Ti was assigned to the tetrahedral site. Rhönite is Mg-free like all other mafic phases during the final stages of crystallization of D'Orbigny and SAH 99555. Comparisons with NWA 4590 rhönite indicates a slightly different chemistry: in particular the absence of $Fe^{3+}$ and $Cr^{3+}$, and the Si, Al deficiency in the tetrahedral site.

**Interpretation and Conclusions:** Rhönite is a rare mineral found in terrestrial alkalic rocks and CAIs. It was also found in a Luna 4 sample [4]. It is characteristic of a low silica activity in Ca and Ti-rich melts. It may be present in other angrites as well even though not described until now and appears another oddity of angrites. As already suggested, it points to some similarity between angrites and terrestrial alkalic rocks [2].

**References:** [1] Mittlefehldt et al. 2002. *Meteoritics & Planetary Science* 37:345–369. [2] Jambon A. et al. 2005. *Meteoritics & Planetary Science* 40:361–375. [3] Kuehner S. M. and Irving A. J. 2007. *EOS Transactions of the American Geophysical Union* 88: Abstract P41A-0219. [4] Treiman A. 2008. *American Mineralogist* 93:488–491.

5309

CHLORINE IN THE PARIS CM: A PRELIMINARY STUDY

A. Jambon¹, O. Boudouma¹, B. Zanda², and F. Couffignal³. ¹UPMC-PARIS 6, ISTEP 4 place Jussieu, CNRS UMR 7193, 75005 Paris, France. E-mail: albert.jambon@upmc.fr. ²LMCM-DHT-CP52, Museum National d'Histoire Naturelle and CNRS UMR 7202, 61 rue Buffon, 75005 Paris, France. ³CAMPARIS 4 place Jussieu, 75005 Paris, France.

**Introduction:** Paris is a recently discovered CM chondrite with moderate aqueous alteration compared to other CM2 [1]. This specimen is presently studied by a consortium and we were allocated section 2010-6 by MNHN Paris. Here we focus on the abundance and distribution of chlorine in this sample compared to Mighei (MNHN section 2008-42) and Cold Bokkeveld (MNHN section 2010-16). The abundance of chlorine in CM chondrites is highly variable and poorly understood. This work is part of a more comprehensive survey of halogens in CMs.

**Analytical Techniques:** In this first approach we performed SEM imaging and EMP analyses. In addition, chemical maps by EDS and WDS allowed characterizing the chlorine distribution.

**Results:** EMP analyses in raster mode over areas of 100 × 100 µms yields the following average abundances: 1160 (± 240), 1620 (± 230) and 830 (± 180) ppm (± variability for 10 points) for Paris, Mighei and Cold Bokkeveld respectively. EDS chemical mapping is consistent with EMP results. For Paris, it indicates that olivine, troilite and kamacite are chlorine free, while there is a low chlorine background of about 1200 (± 600 = 2 sigma) ppm in the fine matrix and a few hotspots with several wt% Cl are. These hot spots correspond to tochilinite patches surrounded by cronstedtite.

This distribution contrasts with that observed in both Mighei and Cold Bokkeveld in which a chlorine background is present at the level of 1600 ppm Cl and 800 ppm Cl, respectively. In Mighei hotspots are absent but in Cold Bokkeveld, rare halite crystals of a few micrometers can be found. We did not find the Cl-tochilinite which appears to be specific of Paris.

Spot analyses of the Paris tochilinite indicate a variable Cl concentration from 1.0 to 3.7 wt%. When tochilinite is more closely associated with cronstedtite, the Cl content falls to an average of 5000 ppm and can be as low as 1000 ppm.

**Conclusions:** In situ analyses yield high chlorine values compared to all previous values obtained from bulk chemical analyses. For instance mean CM composition is given at 160 ppm Cl [2]. The presence of significant amounts of chlorine in tochilinite is, as far as we know of, specific of Paris and may explain its high chlorine abundance. The low level from bulk analysis may be due to high chlorine mobility in aqueous fluids.

**References:** [1] *The Meteoritical Bulletin* #97. [2] Wasson J. T. and Kallmeyn G. W. 1988. *Philosophical Transactions of the Royal Society of London A* 325:535.
NEW INSIGHTS ON HYDROUS PHASES IN R CHONDRITES NWA 6491 AND 6492

N. S. Jamsja¹, A. M. Ruzicka¹, M. Fries². ¹Cascadia Meteorite Laboratory, Portland State University, 17 Cramer Hall, 1721 SW Broadway, Portland OR 97207, USA. E-mail: niinaj@pdx.edu. ²Planetary Science Institute, 1700 E. Fort Lowell Road, Suite 106, Tucson, AZ 85719, USA.

**Introduction:** R chondrites are among the most oxidized meteorites and show evidence of hydrous alteration represented by phases such as biotite and hornblende [1-3]. We recently reported two hydrous phases (HP1 and HP2) that were believed to be related to laihunite and cronstedtite in the NWA 6491 and 6492 R chondrites, genomit breccias containing type 3-5 and 3-6 material [4]. In this abstract we summarize new results from both meteorites that provide additional information about these hydrous and other phases.

**Results:** Additional study of electron microprobe (EMPA) and SEM data from both meteorites, and Raman spectroscopy data from NWA 6492, show that the HP2 phase, believed to be related to laihunite [4], is indeed an intermixture of laihunite and olivine. Two-component mixing models based on EMPA data imply olivine with approximately Fa₆₀-₇₀, whereas preliminary Raman data suggest olivine with approximately Fa₈₈. Both Fa values are significantly higher than typical for equilibrated R chondrites (Fa₃₇-₄₁) [5]. EMPA data also imply that the olivine and laihunite intermixture may contain water.

Additional analysis of EMPA data suggests that the HP1 phase, earlier suggested to be cronstedtite [4], is in fact goethite that is intermixed with one or more other phases. Raman microscopy conducted on an atypical goethite-bearing assemblage in NWA 6492 suggests a complex intermixture of jarosite and anhydrite in the core, and goethite and hematite in a rim and in veins that cross-cut the core.

**Conclusions:** We infer that laihunite, goethite, anhydrite and jarosite are of pre-terrestrial origin. We base our conclusion on: (1) the overall low weathering grade of NWA 6491 and 6492; (2) the lack of terrestrial cross-cutting weathering veins; (3) the systematic change of goethite texture and grain size from petrographic type 3 to type 5 and 6 clasts, which implies that it was present at the time of metamorphism in the R chondrite parent body; (4) the apparent broken edges of certain laihunite grains in mineral and lithic clasts which implies that laihunite was present prior to brecciation; and (5) veining structures, which imply formation of sulfate prior to goethite.

Laihunite, goethite, and jarosite probably formed by aqueous alteration under hydrous and oxidizing conditions during open system processes. Laihunite likely formed by the breakdown of ferrous olivine.

**References:** [1] McCanta M. C. et al. 2008. *Geochimica et Cosmochimica Acta* 72:5757-5780. [2] McCanta M. C. et al. 2007. Abstract #2149. 38th Lunar and Planetary Science Conference. [3] Mikouchi T. et al. 2007. Abstract #1928. 38th Lunar and Planetary Science Conference. [4] Jamsja N. and Ruzicka A. 2011. Abstract #2324. 42nd Lunar and Planetary Science Conference. [5] Hutchison R. 2004. Meteorites: A petrological, chemical and isotopic synthesis.

GERZEH, A PREHISTORIC EGYPTIAN METEORITE

D. Johnson¹, M. M. Grady¹,², and J. Tyldesley³. ¹PSSRI, Open University, Walton Hall, Milton Keynes, MK76AA, UK, E-mail: D.Johnson@open.ac.uk. ²Natural History Museum, Cromwell Road, London, UK. ³KNH Centre for Biomedical Egyptology, Manchester University, Oxford Road, Manchester, M139PT, UK.

Iron from meteorites has been utilized in artefacts by peoples from different cultures, e.g., the Inuit of Greenland, who fashioned knives and weapons from the Cape York meteorite [1]. One of the most ancient examples of worked iron meteorites comes from a prehistoric Egyptian cemetery. The site of El Gerzeh, 40 miles south of Cairo, was excavated in 1911-1912, and 301 graves dating from around 3300 BCE were discovered [2]. A few of the graves contained rare and precious materials such as gold and carnelian. Two graves, Tombs 67 and 133, were also found to contain iron beads; at the time of excavation these examples of Egyptian pre-dynastic culture were considered to be the earliest specimens of worked iron. Subsequent analysis revealed the iron to contain significant levels of nickel, leading to identification of their meteoritic origin [3]. Iron meteorites were also used to make other items later in Egyptian history, including a dagger blade from Tutankhamen's tomb (18th dynasty, 1340-1320 BCE), but no artifacts have been found to be made from meteorite iron after this date.

The most recently published analysis of the Gerzeh iron beads was in 1929 [3]. Our study is intended as an illustration of the non-destructive analysis of an intact artifact, in order to allow a better understanding of this historically rare and precious sample. Optical imaging, electron microscopy and EDS were used to both define and analyze the bead. A thick weathering layer of oxidized iron was easily seen; this incorporates rounded quartz grains that originated in the grave, which was filled with sand. Patches of the oxidized areas have degraded and partially fallen away, probably post-excavation, allowing us to examine underlying regions in which small areas of metal were observed. The metal has an average composition of 9 wt% Ni. The tubular bead structure interior was also found to be filled with ferric oxides and sand, at one end of the bead plant fibres of the strand used to string the beads together were also observed.

This study explores one of the earliest examples of the use of meteoritic iron in artifacts. It is impossible to estimate the original mass of the meteorite: nine beads were discovered in the two tombs, but because of the rarity of iron in early Egypt, it is assumed all beads were produced from the same meteorite. The meteorite must have fallen prior to approximately 3300 BCE, as this is the accepted date of the tombs. It is also possible that it fell beyond Egyptian borders as some of the other materials found in the tombs are believed to have arrived in Egypt through foreign trade routes.

**Acknowledgments:** We would like to thank Dr Karen Exell, Curator of Egypt and the Sudan, at the Manchester Museum, for the loan of a Gerzeh bead, accession number: 5303.

**References:** [1] Buchwald V. F. 1975. *Handbook of iron meteorites*, University of California Press, Berkeley, pp. 1418. [2] Wainwright G. A. 1912. *Revue Archeologique* 19:255-259. [3] Desch C. H. 1929. Reports on the metallurgical examination of specimens for the Sumerian Committee of the British Association. Reports of the British Association for the Advancement of Science.
5404

## METEORITES, ORGANICS AND FISCHER-TROPSCH TYPE REACTIONS: PRODUCTION AND DESTRUCTION

N. M. Johnson¹, A. S. Burton¹,² and J. A. Nuth III³. ¹NASA Goddard Space Flight Center, Astrochemistry Laboratory, Code 691, Greenbelt, MD 20771, USA. ²NASA Postdoctoral Program, CA, USA. ³NASA GSFC, solar systems Exploration Division, Code 690, Greenbelt, MD 20771, USA. E-mail: natasha.m.johnson@nasa.gov.

**Background:** There has been an ongoing debate about the relative importance about the various chemical reactions that formed organics in the early solar system. One proposed method that has long been recognized as a potential source of organics is Fischer-Tropsch type (FTT) synthesis [e.g., 1, 2]. This process is commonly used in industry to produce fuels (i.e., complex hydrocarbons) by catalytic hydrogenation of carbon monoxide [3]. Hill and Nuth [4] were the first to publish results of FTT experiments that also included Haber-Bosch (HB) processes (hydrogenation of nitrogen. Their findings included the production of nitrile-bearing compounds as well as trace amounts of methyl amine. Previous experience with these reactions revealed that the organic coating deposited on the grains is also an efficient catalyst [5] and that the coating is composed of insoluble organic matter (IOM) and could be reminiscent of the organic matrix found in some meteorites [6]. This current set of FTT-styled experiments tracks the evolution of a set of organics, amino acids, in detail.

**Experimental Procedure:** The FTT/HB reactions are conducted in a closed gas circulating system that monitors the evolved gases in real time using Fourier transform infrared spectroscopy. The experiment reported here was heated to 550 °C and used a catalyst of iron-silicate amorphous grains. The gas mixture combines 75 torr of CO, 75 torr of N₂ and 550 torr of H₂. Individual reactions proceed until approximately 90% of the CO is depleted. The reactants are then pumped away, the bulb is refilled with the same gas mixture and another reaction using the same catalyst is carried out. Fifteen such runs were done at the specified temperature and results in coated iron silicate grains approximately 10% by mass carbon and 0.2% nitrogen. A complete description of the experiment methodology can be found in previous publications [4, 5]. The coated catalyst/grain sample was analyzed at different time steps to follow the evolution of the amino acids. The amino acids were analyzed using derivatization and liquid chromatography-fluorescence detection/time-of-flight mass spectrometry.

**Results:** We will present a time sequence for a select set of amino acids and their abundance over the course of the reaction. The data show that the amino acids are produced early in the reaction cycle and then are either destroyed or are incorporated into the IOM on the catalyst/grain surface. We will also report on the generation of a rare, nonbiogenic amino acid.

**References:** [1] Kress M. E. and Tielens A. G. G. M. 2001. *Meteoritics & Planetary Science* 36:75-91. [2] Hayatsu R. and Anders E. 1981. In *Cosmo and geochemistry*, edited by Bosche. pp. 1-39. [3] Hindermann J. P. et al. 1993. *Catalysis Reviews-Science and Engineering* 35:1-127. [4] Hill and Nuth J. A. 2003. *Astrobiology* 3:291-304. [5] Nuth J. A. et al. 2008. *The Astrophysical Journal* 673:L225-L228. [6] Johnson N. M. et al. 2004. Abstract #1876. 35th Lunar and Planetary Science Conference.

5475

## EXSOLUTION IN FELDSPAR IN THE TUCTUAC (LL5) CHONDRITE: A NEW PERSPECTIVE ON COOLING RATES FOR METAMORPHOSED CHONDRITES

R. H. Jones and A. J. Brearley. Department of Earth and Planetary Sciences, University of New Mexico, NM, USA. E-mail: rjones@unm.edu.

**Introduction:** Petrologic type 4-6 ordinary chondrites have undergone metamorphism on their parent bodies. There are currently few means to obtain quantitative estimates of peak temperatures and cooling rates for this process. We recently observed possible exsolution of K-feldspar in albitic plagioclase in the LL5 chondrite Tuxtuacs [1], and in L5 Roy (1933) [2]. Since exsolution microstructures can potentially be used as cooling rate indicators, we have studied this feature further using transmission electron microscopy (TEM).

**Methods:** We obtained quantitative WDS analyses of feldspar using EPMA. A TEM sample was extracted from a feldspar grain in Tuxtuac using focused ion beam (FIB) extraction techniques, and was characterized on a JEOL 2010 200 kV TEM.

**Results:** Exsolution in albitic feldspar occurs in several relict chondrules in a thin section of Tuxtuac [1]. Textures vary from grains that are predominantly albitic with very fine-scale (sub-µm) K-rich (sub)parallel lamellae, to regions of grains that contain approximately equal amounts of coarser (up to 10 µm) intergrown albite and K-feldspar. EPMA analyses give compositions of An₉Ab₈₉Or₂ and An₃Ab₁₈Or₇₉ for the two phases, measured in coarse-grained areas with a 5 µm electron beam [1].

We selected a grain with fine-scale (sub-µm) K-rich lamellae, and cut a FIB section perpendicular to the length of the lamellae. The section shows an exsolution texture of K-feldspar lenses in host albite: albite has albite twinning with individual twin widths of 100-160 nm. The section intersects several K-feldspar lenses, which have maximum widths of 90 nm up to ~0.5 µm. K-feldspar lamellae also contain albite twins. In addition, some local regions within the albite have very fine-scale K-feldspar lamellae that are just a few nanometers in thickness and occur on a wavelength of 60-90 nm. The interface between K-feldspar and albite exhibits a zigzag structure and significant strain at the interface. This zigzag interface is caused by the evolution of ordering and development of albite twins in the K-feldspar.

**Discussion:** Our study shows conclusively that albitic plagioclase in Tuxtuac has an exsolution microstructure. The microstructure is very comparable to that seen in plagioclase of similar compositions in a terrestrial syenite intrusion, that probably cooled through the exsolution interval of around 600 to 500 °C over a time scale of 10⁴ years [3,4]. This cooling rate is significantly faster than metallographic cooling rates calculated for LL5 and LL6 chondrites of tens of degrees per million years, or less [5,6], over a similar temperature range. There is no obvious explanation for this large difference in cooling rates. To date, we have not observed exsolution in any type 6 chondrites, which would be predicted to cool more slowly than type 5, according to an onion-shell model.

**References:** [1] Kovach H. A. and Jones R. H. 2010. *Meteoritics & Planetary Science* 45:246-264. [2] Gallegos J. and Jones R. H. 2011. This conference. [3] Brown W. L. and Parsons I. 1988. *Contributions to Mineralogy and Petrology* 98:444-454. [4] Brown W. L. et al. 1983. *Contributions to Mineralogy and Petrology* 82:13-25. [5] Taylor G. J. et al. 1987. *Icarus* 69:1-13. [6] Willis J. and Goldstein J. I. 1981. *Proceedings of Lunar and Planetary Science* 12B:1135-1143.