## A SHOCKING TALE: TEM OBSERVATIONS OF DEFORMED OLIVINE IN ORDINARY CHONDRITES

A. Ruzicka¹,² and R. Hugo². ¹Cascadia Meteorite Laboratory and Portland State University, Portland, OR 97207, U.S.A. ²Department of Geology, Portland State University, Portland, OR 97207 U.S.A. E-mail: ruzickaa@pdx.edu.

**Introduction:** Previous transmission electron microscopy (TEM) work suggested that Portales Valley [H7 (S1)] and MIL 99301 [LL6 (S1)] experienced syn-metamorphic shock deformation at an early stage [1, 2]. To better interpret the microstructures of these and other chondrites, we used TEM to study two L6 chondrites with a nominally simple shock history, including Park (S1) and Leedey (S4, revised from a literature designation of S3 based on our optical microscopy data). Our expectation was that these L chondrites could have been shocked to various degrees from a cold starting temperature at a late stage, possibly even in the same, major collisional event that affected the L parent body approximately 0.5 Ga ago [e.g., 3].

**Results:** Olivine microstructures found for Leedey include (1) moderately high dislocation densities ($10^9-10^{10} \text{ cm}^{-1}$); (2) predominantly **b**=[001] dislocations with straight segments oriented along two to three distinct line vectors, with only occasional bowing of dislocations, suggesting low temperature deformation; (3) rare low angle grain boundaries indicating limited recovery; (4) small (10 s of nanometers) **b**=[001] dislocation loops.

Specific microstructures found for olivine in Park include (1) a moderate dislocation density ($10^8 \text{ cm}^{-1}$); (2) a prevalence of **b**=[001] dislocations, predominantly bowed, with some **b**=[100] dislocations, indicative of deformation at an elevated temperature; (3) occasional tangles formed by intersections of **b**=[001] dislocations with **b**=[100] dislocations; (4) no low angle grain boundaries (subgrain boundaries), indicating limited recovery; and (5) small (10 s of nanometers) **b**=[001] dislocation loops, suggestive of dislocation climb.

**Discussion:** Altogether, Leedey appears to be a good example of an S4 chondrite that was relatively strongly deformed from a cold starting temperature. This conforms to the hypothesis that this meteorite could have been affected by the relatively late shock event that affected the L chondrite parent body. Using the data for Leedey as a guide, we suggest that Portales Valley could have been deformed to an S3 shock stage with subsequent annealing to obtain S1 characteristics. In contrast to Leedey, Park appears to have been subjected to low-to-moderate shock pressures at elevated temperatures. There is no evidence that Park was significantly more deformed than it is now, so the low shock stage probably cannot be attributed to an annealing (recovery) process. Instead, we suggest that Park largely escaped the late, strong shock event that affected most L chondrites and instead records one or more, early, relatively weak shock events that occurred while the L chondrite parent body was being metamorphosed. This history of weak shock at elevated temperature is similar to that experienced by MIL 99301 [2]. We now have TEM evidence for early collisions affecting each of the H, L, and LL parent bodies.

**References:** [1] Hutson M. et al. 2007. Abstract #5072. 42nd Meteoritics and Planetary Science. [2] Hutson M. et al. 2009. Abstract #1081. 40th Lunar and Planetary Science Conference. [3] Bogard D. D. 1995. *Meteoritics* 30:244–268.

## MACRO-, TO MICRO- TO NANOSCALE ADVANCED SEM/EDS ANALYSIS USING STAGE CONTROL AND AN ANNULAR FOUR-CHANNEL SILICON DRIFT DETECTOR: APPLICATIONS IN IMPACT STUDIES

T. Salge¹, J. Berlin¹, R. Terborg¹, K. Howard², H. Newsom³ and A. T. Kearsley². ¹Bruker Nano GmbH, Schwarzschildstr. 12, 12489 Berlin, Germany. E-mail: tobias.salge@bruker-nano.de. ²Department of Mineralogy, Natural History Museum, London SW7 5BD, UK. ³Institute of Meteoritics, Department of Earth and Planetary Sciences, University of New Mexico, Albuquerque, NM 87131, USA.

**Introduction:** Imaging of ever smaller structures, in situ within large samples, requires low electron beam energy to enhance spatial resolution, and therefore also use of low energy X-ray lines for elemental analysis. To separate significant overlapping peaks e.g., N-K (392 eV) and Ti-L₁ (395 eV) [1], the incorporation of line deconvolution algorithms [2] in energy dispersive X-ray (EDX) software is of crucial importance. In this paper we describe applications of a new system to impact studies.

**Methods:** Without adequate X-ray count statistics, deconvolution is unlikely to be effective. We therefore used an annular Silicon Drift Detector (SDD), the Bruker XFlash 5060F. This has parallel processing of data from four channels, giving an output count rate of 1.1 Mcps. It is placed between the pole piece and sample, covering a very large solid angle of X-ray collection, rapidly acquiring sufficient data even on beam sensitive samples. High take-off angle and collection of X-rays from four different directions allow data collection across samples with substantial surface topography. Automated stage control and spectrum imaging (an entire EDS spectrum is collected at each pixel) allow large data sets to be acquired within a short time.

**Examples of Applications:** 1. Large area, high resolution mapping (with tiling or stitching of neighboring areas) is useful for understanding processes in the formation of tektites [3], revealing flow textures and layering, without destructive section preparation. Coalescence textures formed during the transition from melt to solid are seen, with surface pitting produced by microimpact collisions in the impact plume, and surface etching by chemical attack in the impact plume, or later weathering.

2. Large area, high resolution spectrum imaging of the matrix at impact melt breccia of the Chicxulub impact crater (Yax-1 borehole, Unit 5 861.72 m) [3] reveals the distribution of unusual, resorbed andradite garnets. Analysis at low accelerating voltage reveals secondary mineral formation, such as < 500 nm NaCl and < 150 nm Fe-Ti-oxides associated with garnet resorption, indicating a complex history of matrix genesis during the fluid evolution of high temperature seawater-derived brine.

3. Complex topographic surfaces, such as the interior of experimental Stardust foil craters, can be imaged very rapidly and in detail in order to locate projectile residues [4] without loss of data due to “shadowing” of any area at depth.

**References:** [1] Salge et al. 2010. Abstract #I6.4. 17th International Microscopy Congress. [2] Aßmann A. and Wendt M. 2003. *Spectrochimica Acta* 58:711–716. [3] Howard K. T. 2011. *Geological Society of London* 573–591. [3] Salge T. and Newsom H. 2010. Abstract #5337. 73rd Annual Meeting of the Meteoritical Society. [4] Kearsley et al. 2011. Submitted to 74th Annual Meeting of the Meteoritical Society.
TRACE ELEMENT GEOCHEMISTRY OF THE NAKHLITE NORTHWEST AFRICA 5790

M. E. Sanborn¹, M. Wadhwa¹, J. B. Balta², R. Mayne³ and H. Y. McSween Jr². ¹SESE, Arizona State University, Tempe, AZ, 85287-1404. E-mail: Matthew.Sanborn@asu.edu. ²Dept. of Earth and Planetary Sciences, University of Tennessee, Knoxville, TN. ³Dept. of Geology, Texas Christian University, Fort Worth, TX.

**Introduction:** Northwest Africa (NWA) 5790 is a recently recovered hot desert nakhlite that brings the total number of samples of this SNC subgroup to 13. Petrographically similar to the other known nakhlites [1], NWA 5790 contains phenocrysts of augite and olivine, embedded in a fine-grained mesostasis. The mesostasis is composed primarily of feldspar, phosphate, olivine, pyroxene, and oxides. An initial investigation of NWA 5790 noted that the whole-rock rare earth element (REE) abundances were elevated in comparison to the other known nakhlites [2]. Here, we report REE and selected trace and minor element abundances measured in situ in individual minerals of NWA 5790 with the goal of constraining its petrologic history and understanding its relationship to the other nakhlites.

**Results and Discussion:** The concentrations of REE and other selected trace elements were measured in augite, olivine, mesostasis, and phosphate (apatite) of NWA 5790 using the Cameca IMS-6f ion microprobe at Arizona State University. The augite has a concave-downward REE pattern that is LREE-depleted ([La/Sm]$_{CI}$ ~0.4), with a small negative Eu anomaly (Eu/Eu* ~0.9) and a slight decrease from the middle- to the HREE ([Gd/Lu]$_{CI}$ ~1.5). This mineral exhibits a moderate range of REE abundances (La ~1.5–3.2 × CI). The REE as well as Ti, Y, and Zr abundances in the augite cores of the NWA 5790 nakhlite are higher than those in the other nakhlites (e.g., Nakhla and Lafayette) with the exception of MIL 03346 augite cores (which exhibit similar abundances to those of NWA 5790) [3,4]. The olivine in NWA 5790 is HREE enriched ([Lu/Gd]$_{CI}$~12) with low REE abundances (Lu~1.2 × CI). The fine-grained mesostasis was analyzed with a beam spot large enough to obtain a “bulk” analysis. It has a LREE-enriched pattern (La~60-95xCI; [La/Lu]$_{CI}$~5-6) that parallels that of the NWA 5790 whole-rock. Apatite usually occurs as very small grains in the mesostasis; however, at least one apatite grain was large enough to obtain a clean ion microprobe analysis. The apatite in NWA 5790 is significantly LREE-enriched ([La/Lu]$_{CI}$~50), and has the highest REE abundances of all the analyzed phases (La~1100 × CI). This mineral also exhibits a significant negative Eu anomaly (Eu/Eu* ~0.29) similar to that in other nakhlite apatites (e.g., [3]).

**Conclusions:** The REE patterns of the various mineral of NWA 5790 are similar to those observed in the other known nakhlites indicating that NWA 5790 crystallized from a parent melt that originated from the same geochemical source reservoir as the other nakhlites. However, the elevated whole-rock REE concentrations [2] coupled with the relatively higher REE, Ti, Y, and Zr abundances observed in the augite cores suggest that the NWA 5790 parent melt was more evolved than those of most previously studied nakhlites (e.g., Nakhla [3]).

**References:** [1] Treiman A. 2005. *Chemie der Erde* 65:203–270. [2] Jambon A. et al. 2010. Abstract #1696. 41st Lunar and Planetary Science Conference. [3] Wadhwa M. and Crozaz G. 1995. *Geochimica et Cosmochimica Acta* 59:3629–3645. [4] Day J. et al. 2006. *Meteoritics & Planetary Science* 41:581–606.

CHONDRULES: CLUES TO EARLY PLANETESIMALS

I. S. Sanders¹ and E. R. D. Scott². ¹Trinity College Dublin, Ireland. E-mail: isanders@tcd.ie. ²University of Hawaii, USA.

**Introduction:** Most chondrules were evidently made in huge batches of closely spaced droplets [1] about 2 Myr after the solar system began. Modeling by [2] suggests that at that time enormous fan-shaped plumes of chondrule droplets, like ‘spray from a hose-pipe nozzle’, would have emerged downstream from the overshooting fraction of molten impactors following oblique low-velocity collisions and partial planetesimal mergers. If chondrules originated in this way, then what can they tell us about their planetesimal antecedents?

**Planetesimal Oxidation State:** Two main kinds of molten planetesimal probably existed – reduced and oxidized – to account for Type I (FeO-poor) and Type II (FeO-bearing) chondrules respectively. Reduced planetesimals perhaps accreted from partially condensed matter in the young, hot inner nebula, as proposed by [3], and their cores in some cases had evidently formed in the first 0.5 Myr [4]. The timing of disruption of one particularly large molten planetesimal, which left the IVA iron parent body in its wake, suitably coincides with the time of chondrule formation [5,6]. Oxidized planetesimals, on the other hand, perhaps accreted in a cold part of the disk, possibly later or further from the sun than the reduced ones, and incorporated grains of water ice. As the planetesimals became heated, hydrated and eventually melted, the H₂O plausibly oxidized the metal, and supplied the ¹⁶O-poor oxygen whose abundance in chondrules in E, H, L, LL and R chondrites correlates so well with FeO/(FeO + MgO). To make Type II chondrules in conventional ways is, incidentally, beset with problems [7]

**Planetesimal Anatomy:** A typical molten planetesimal perhaps had a coating of cold loose regolith passing downwards with increasing temperature into sintered rigid ‘crust’, through a zone of incipient melting into the molten silicate interior from which the metal core was separating. Hence the overshooting plume would have carried a diverse suite of materials. The spray would probably have sampled much more mantle than core, thus explaining iron depletion in all chondrite groups relative to CIs. Phenocrysts in the melt would explain chondrules with mm-sized single crystals. With olivine phenocrysts settling through time, later-formed chondrules would have become increasingly pyroxene-rich, as is observed [8]. Incipiently melted fragments of the crust would account for chondrules packed full of unmelted ‘relict’ grains [9]. Small solid objects, including fragments of sintered crust [10], perhaps became wetted and engulfed by droplets they encountered in flight, accounting for the presence of these objects within chondrules.

**Reservoirs of Ingredients:** Chondrules and other debris launched by collisions presumably accreted to new or existing planetesimals before having time to become homogenized in the disk by radial mixing and turbulence. Thus each chondrite parent body (chondrite group?) was dominated by ingredients supplied by recent impacts in its ‘feeding zone’ and so acquired its own distinctive petrographic and chemical characteristics [11].

**References:** [1] Alexander C. M. O’D. et al. 2008. *Science* 320:1617–1619. [2] Asphaug E. 2011. Abstract #1647. 42nd Lunar and Planetary Science Conference. [3] Bland P. A. and Ciesla F. J. 2010. Abstract #1871. 41st Lunar and Planetary Science Conference. [4] Burkhardt C. et al. 2008. *Geochimica et Cosmochimica Acta* 72:6177–6197. [5] Yang J. et al. 2007. *Nature* 446:888–891. [6] Blichert-Toft et al. 2010. *Earth and Planetary Science Letters* 296:469–480. [7] Fedkin A. V. and Grossman L. 2010. Abstract #1448. 41st Lunar and Planetary Science Conference. [8] Kita N. T. et al. 2005. Abstract #1750. 36th Lunar and Planetary Science Conference. [9] Ruzicka A. et al. 2010. Abstract #1956. 41st Lunar and Planetary Science Conference. [10] Libourel G. and Krot A. N. 2007. *Earth and Planetary Science Letters* 251:1–8. [11] Jones R. H. 2010. *Workshop on chondrules*, New York.
XANES ANALYSIS OF ORGANIC RESIDUES FROM THE IRRADIATION OF ASTROPHYSICAL ICE ANALOGS AND COMPARISON WITH Stardust SAMPLES

S. A. Sandford¹, M. Nuevo¹, S. N. Milam², G. D. Cody³, A. L. D. Kilcoyne⁴, B. T. De Gregorio⁵ and R. M. Stroud⁶. ¹NASA-Ames Research Center, Moffett Field, CA, USA. E-mail: Scott.A. Sandford@nasa.gov. ²NASA Goddard Space Flight Center, Greenbelt, MD, USA. ³Carnegie Institution of Washington, Washington, DC, USA. ⁴Advanced Light Source, Berkeley, CA, USA. ⁵NASA Johnson Space Center, Houston, TX, USA. ⁶Naval Research Laboratory, Washington, DC, USA.

**Introduction:** The NASA Stardust mission successfully collected authentic cometary grains from Comet 81P/Wild 2 [1], including organic-rich materials [2]. X-ray absorption near-edge structure (XANES) analysis of these samples indicates that these grains contain O-rich and N-rich organic materials having a broad variety of functional groups: carbonyls, C=C bonds, aliphatic chains, amines, amides, etc. [2, 3]. Stardust samples were also recently shown to contain the amino acid glycine [4]. Some of the organic materials found in Stardust samples contain very little aromatic carbon and resemble, at least superficially, the organic residues produced by the UV irradiation of interstellar/cometary ice analogs. Organic residues produced from the irradiation of astrophysical ice analogs are known to contain a large suite of prebiotic molecules including amino acids [5-7], and amphiphilic compounds (fatty acids) [8]. In the work reported here, residues were produced in the laboratory from the UV irradiation of mixtures of ices containing H₂O, CH₃OH, CO, and NH₃ in relative proportions 100:50:1:1 at 7 K. Additional residues were produced from mixtures with no NH₃, and mixtures containing alkanes or naphthalene (C₁₀H₈). C-, N-, and O-XANES spectra of these residues were measured using Beamline 5.3.2 at the Advanced Light Source in Berkely. These XANES spectra were used to assess the samples' organic functional group chemistry and overall atomic composition, as well as their N/C and O/C ratios. The first results of this study indicate the presence of a number of chemical bonds and functions, namely, carbonyls, C=C bonds, alcohols, amides, amines, and nitrile groups. The relative proportions of these functional groups can be compared with XANES measurements of Stardust samples [9]. The samples were also found to be modified by accumulated X-ray exposure, the principal change being the conversion of some aliphatic carbon into aromatic carbon.

**References:** [1] Brownlee D. E. et al. 2006. *Science* 314:1711. [2] Sandford S. A. et al. 2006. *Science* 314:1720. [3] Cody G. D. et al. 2008. *Meteoritics & Planetary Science* 43:353. [4] Elsila J. E. et al. 2009. *Meteoritics & Planetary Science* 44:1323. [5] Bernstein M. P. et al. 2002. *Nature* 416:401. [6] Muñoz Caro G. M. et al. 2002. *Nature* 416:403. [7] Nuevo M. et al. 2008. *Origins of Life and Evolution of the Biosphere* 38:37. [8] Dworkin J. P. et al. 2001. *Proceedings of the National Academy of Sciences* 98:815. [9] Nuevo M. et al. 2011. *Advances in Space Research* in press.

APATITE AS A PROBE OF VOLATILE COMPOSITION IN HED MAGMA

A. R. Sarafian, M. F. Roden and A. E. Patiño-Douce. University of Georgia. E-mail: povault@uga.edu.

**Introduction:** The HED parent body is a well-sampled planetary body, with nearly 1000 known meteorites. Although this parent body is thought to be volatile-depleted [1], apatite, a volatile-bearing (F, Cl, OH, S) mineral, is a minor constituent of eucrites, along with a volatile-free phosphate, merrillite. Representative basaltic eucrites (Stannern, Nuevo Laredo, Pasamonte, Juvinas, LEW 88010, LEW 88009, EET 87542, and GRA 98098) were selected for study on the basis of bulk rock TiO₂ and Mg# [2]. Four cumulate eucrites, Moore County, Serra de Magé, ALH 85001, and EET 87548 were also selected.

**Results:** Apatite compositions were measured using the JEOL 8600 electron microprobe at the University of Georgia; operating conditions and procedures followed those of Patiño-Douce et al. [3]. We assume X<sub>Cl</sub> + X<sub>F</sub> + X<sub>OH</sub> = 1 and calculate OH by difference. A typical apatite from basaltic eucrites has 40.7 P₂O₅, 55.7 CaO, 0.2 Cl, 3.6 F (all values in wt%), and no detectable SrO, Na₂O, Ce₂O₃, FeO, MgO, SO₃, Y₂O₃, AsO₄, or SiO₂. A typical apatite from cumulate eucrites has 41.8 P₂O₅, 56.2 CaO, 3.0 F (all values in wt%), and no detectable Cl, SrO, Na₂O, Ce₂O₃, FeO, MgO, SO₃, Y₂O₃, AsO₄, or SiO₂. The F:Cl:OH ratio for apatite in basaltic eucrites and cumulate eucrites are 10.5:0.53:1 and 11.8:0:1 respectively.

**Discussion:** The F-rich nature of eucrite apatites indicate either an intrinsically volatile-poor parent body or the eucritic magmas were devolatilized and lost H₂O and Cl. However, we found a measurable vacancy in the anion site (~15% in cumulate eucrites and up to 25% in basaltic eucrites), which we inferred to be occupied with OH. Chlorine concentration increases slightly (~0.2wt%) as Mg# decreases. Although partition coefficients are poorly constrained for the halogens between apatite and melt, a first approximation for melt composition can be obtained from partition coefficients for F and Cl between a fluid-saturated basaltic melt and apatite [4]; we estimate the Cl/F weight ratio in the melt to be ~0.25. Patiño-Douce et al. [3] described a thermodynamic model suggesting that a rock with both merrillite and apatite reflects relatively low H₂O content, high halogen content (in this case F), and high P content compared to typical terrestrial samples containing apatite only. The last stages of the volatile-depleted magma on the HED parent body must have concentrated enough F (and possibly H₂O) to crystallize apatite. The HED parent body differs from the volatile-rich parent body of GRA 06128/9 where the late stage fluid was Cl rich, and chlorapatite replaced merrillite [5].

**Acknowledgments:** We would like to thank Joe Boesenberg at the American Museum of Natural History and Kevin Righter at the Johnson Space Center for facilitating the loan of thin sections.

**References:** [1] Papike J. J. 2008. In *Reviews in Mineralogy*, vol. 36. *Planetary Materials*. Mineralogical Society of America: Washington, D.C. pp. 7-01-7-10. [2] *BVSP, Basaltic volcanism on the terrestrial planets* 1981. New York: Pergam on Press Inc. 1286 p. [3] Patiño-Douce A. E. et al. 2011. *Chemical Geology*, submitted. [4] Mathez E. A. and J. D. Webster. 2005. *Geochimica et Cosmochimica Acta*. 69:1275-1286. [5] Shearer C. et al. 2010. *Geochimica et Cosmochimica Acta*. 74:1172-1199.
HANDLING HEAVENLY JEWELS—35 YEARS OF ANTARCTIC METEORITE PROCESSING AT JOHNSON SPACE CENTER

C. E. Satterwhite¹, K. M. McBride¹, R. Harrington¹, C. M. Schwarz¹ and K. Righter². ¹ESCG Jacobs Technology, NASA Johnson Space Center, 2101 Nasa Pkwy, Houston, TX 77058, USA. E-mail: cecilia.e. satterwhite@nasa.gov. ²NASA-JSC, 2101 Nasa Pkwy, Houston, TX 77058, USA.

**Introduction:** The ANSMET program began in 1976, and since that time more than 18,000 meteorites have been processed in the Meteorite Processing Lab at Johnson Space Center in Houston, TX [1]. The meteorites are collected and returned to JSC on a freezer truck and remain frozen until they are initially processed.

**Initial Processing of Meteorites:** Initial processing involves drying the meteorites in a nitrogen glove box for 24-48 h, photographing, measuring, weighing and writing a description of the interior and exterior. The meteorite is broken and a representative sample is sent to the Smithsonian Institution for classification.

**Newsletter and Requests:** Once initial processing has been complete and the meteorites have been classified, the information is published in the Antarctic Meteorite Newsletter [2, 3]. The newsletter is published twice yearly and is sent electronically to researchers around the world and is also available on line. Researchers are asked to fill out a request form and submit it to the Meteorite Working Group secretary. All sample requests will be reviewed by either the meteorite curator or the Meteorite Working Group depending on the type of meteorite and the research being conducted.

**Processing for Sample Requests:** In the meteorite processing lab, meteorite samples are prepared several different ways. Most samples are prepared as chips obtained by use of stainless steel chisels in a chipping bowl or rock splitter. In special situations where a researcher needs a slab the meteorite samples can be bandsawed in a dry nitrogen glove box with a diamond blade, no liquids are ever introduced into the cabinet. The last type of sample preparation is thin/thick sections. The meteorite thin section lab at JSC can prepare standard 30-micron thin sections, thick sections of variable thickness (100–200 microns), or demountable sections using superglue.

**Information for Researchers:** It is important that researchers fill the sample request form completely, in order to make sure the meteorite is processed correctly [4]. Researchers should list any special requirements on the form, i.e., packaging of samples (poly vs. stainless), thick sections and thickness needed, superglue needed, interior chips, exterior chips, fusion crust, contamination issues, all concerns should be listed so processing can be done accurately and any concerns the researcher has can be addressed before the meteorites are broken.

**References:** [1] http://curator.jsc.nasa.gov/antmet/index.cfm. [2] http://curator.jsc.nasa.gov/antmet/classdb.cfm. [3] http://curator.jsc.nasa.gov/antmet/ann/ann.cfm. [4] http://curator.jsc.nasa.gov/antmet/request dates.cfm.

MEASUREMENT OF LOW-TEMPERATURE HEAT CAPACITY OF METEORITES

M. W. Schaefer¹, G. J. Consolmagno², D. Britt³ and B. E. Schaefer¹. ¹Louisiana State University. E-mail: mws@lsu.edu. ²Vatican Observatory. ³University of Central Florida.

**Introduction:** The heat capacity of a meteorite is a mass-weighted sum of the heat capacities of its component minerals, unlike thermal conductivity or porosity, which are functions of the fabric of the meteorite. As such it is an independent constraint on the mineral composition of a meteorite. It has been shown previously [1] that many meteorite groups can be categorized by their grain density and magnetic susceptibility; the measurement of heat capacity might provide yet another, independent, way of determining the bulk composition of a whole meteorite. One can look for systematic differences between various meteorite groups; systematic differences between falls and finds; and systematic differences between weathered and unweathered samples.

Meteorite heat capacity is also an essential parameter in a number of modeling techniques of interest to planetary scientists, including thermal models of asteroids and icy bodies expected to have a significant rocky component similar to meteorites; models of the evolution of small body orbits as a function of thermal inertia via the Yarkovsky and YORP effects; the modeling of meteor brightness as a function of the meteor initial size and speed; and parameters used in the modeling of the response of planetary surfaces to impact cratering.

To perform such measurements on a suitably large, representative number of samples across meteorite types requires a quick and non-destructive way of measuring the heat capacity of whole rocks. Because heat capacity is itself a strong function of temperature, it is also desirable to measure it over a range of temperatures.

**Experimental Procedure:** We are developing a new, non-destructive procedure for measuring the heat capacity of minerals and rocks, based on the technique of [2]. An insulated dewar of liquid nitrogen is placed on a balance, and its decrease in mass from evaporation is measured over time. A small (several grams) sample is dropped into the dewar, and the increased amount of nitrogen evaporated by the heat input from the sample is determined. This mass of nitrogen is then used to calculate the average heat capacity over the temperature range between room temperature and the boiling temperature of liquid nitrogen. By having the sample at different starting temperatures, it will be possible to derive the temperature dependence of the heat capacity as well.

**Preliminary Results:** Since last year, we have experimented with a number of different combinations of containers and starting conditions. We have developed a mathematical model for the flow of heat from the system, in order to suggest ways to optimize the shape and size of the liquid nitrogen container. During this development phase, measurements of quartz are underway for comparison to published values. The average heat capacity over this temperature range derived for the samples measured to date is consistent with literature values. The technique appears to hold good promise of providing rapid, non-destructive measurements of this important thermal parameter.

**References:** [1] Consolmagno G. J. et al. 2008. *Chemie der Erde*— *Geochemistry* 68:1–29. [2] Koch A. et al. 2010. *Abstract #5163*. *Meteoritics & Planetary Science* 45.
DIFFERENTIATION OF Fe-Ni SPHERULES IN IMPACT GLASS

M. Schaffer, T. Kasama, R. E. Dunin-Borkowsky, A. N. MacDonald and C. Bender Koch. University of Copenhagen, Denmark

Metallic, micron sized Fe-Ni spherules are frequently reported from glasses formed during impact processes. They have a high preservation potential and are a valid indicator of impactor type.

However, the details of their formation remain obscure and their potential as specific indicator of impact details unexploited.

In this work on the impact glass from Wabar we report on of the relation between the simple spherules themselves and the hosting glass using SEM, and a detailed study of the spherules using a combination of FIB/SEM reconstruction and TEM.

Overall, we identified a late stage differential contraction of the spherules in the hosting glass, facilitating mechanical separation of the spherules from the host. The structure of the spherules is consistent with entry of a small nugget of the meteorite and some associated air into the molten glass. The spherical shape is acquired as the molten glass induces melting of the metal and its minor components. While the metal is molten a density related differentiation of the light components like iron sulfide and voids results in the accumulation of these at or close to the surface of the spheres.

With these principal processes being established the huge variation in the detailed sphere morphology can be rationalized.

<figure><img src="image_27.png" /><figcaption>Fig. 1. Composite Fe-Ni spherule from Wabar impactite glass.</figcaption></figure>

NALBACH (SAARLAND, GERMANY) AND WABAR (SAUDI ARABIA) GLASS—TWO OF A KIND?

M Schmieder¹, W. Müller² and E. Buchner¹,³. ¹Institut für Planetologie, Universität Stuttgart, Herdweg 51, 70174 Stuttgart, Germany. E-mail: martin.schmieder@geologie.uni-stuttgart.de. ²Diefflerstr. 217, 66809 Nalbach/Saar, Germany. ³Rieskrater-Museum Nördlingen, Eugene-Shoemaker-Platz 1, 86720 Nördlingen, Germany.

**Introduction:** The recent discovery of enigmatic glasses near Nalbach (Saarland, Germany) by one of the authors (W. M.) [1] poses the major question whether these glasses were produced by an impact event [2]. Although no shocked quartz grains have been detected so far, the Nalbach glass shows a number of features typical for terrestrial impact glasses, such as those found at, e.g., Monturaqui (Chile), Henbury (Australia), or the Libyan Desert Glass. In particular, the Nalbach glass exhibits a striking similarity to the Wabar (Saudi Arabia) impact glass [3, 4].

**Nalbach and Wabar Glass—Comparison and Interpretation:** The Nalbach and Wabar glasses both exhibit bluish to greenish varieties with a fluidal and schlieren-rich, vesicular, SiO₂-rich glassy groundmass (Fig. 1A, B). Both glass types contain abundant ballen α-cristobalite as a high-temperature (> 1000 °C) modification of silica [2]. Microscopic Fe-Ni spheres considered as droplets of iron meteoritic matter [2–4] are distributed in a very similar way in both glass types. The Nalbach and Wabar glasses show microstructural evidence for a dynamic liquid interaction between the silicic melt, incorporated Fe-Ni droplets, and gas (vesicles), which is expressed by the occurrence of flattened Fe-Ni spheres (Fig. 1C, D) and microspheres adherent to the inner surfaces of vesicles (Fig. 1E, F). Our preliminary observations are consistent with the formation of both glasses as an “emulsion” of an iron meteoritic melt in a hot, degassing target melt [4] and, like for Wabar, favor an impact origin for the Nalbach glass. Further analyses are ongoing.

**References:** [1] Müller W. 2011. Prims: A possible Holocene meteorite impact in the Saarland region, West Germany. Scribd document no. 51477759 (available online). [2] Buchner et al. 2011. This volume. [3] Hörz F. et al. 1989. Proceedings, 19th Lunar and Planetary Science Conference. pp. 697–709. [4] Schmieder M. and Buchner E. 2009. Abstract volume, 1st AICAC Meeting, Amman, Jordan, CD-ROM.

Fig. 1. Nalbach and Wabar glass. **A:** Uncut specimen, Nalbach (from [1]). **B:** Polished section, Wabar. **C:** Backscattered electron image (BEI) of vesicular glassy melt matrix (gray) and flattened Fe-Ni sphere (white) at the melt/vesicle interface, Nalbach. **D:** Similar to C but from Wabar (BEI). **E:** Fe-Ni microspheres adherent to the inner surface of a vesicle, Nalbach (BEI). **F:** Similar to E, with larger Fe-Ni sphere, Wabar (BEI).

<figure><img src="image_28.png" /></figure>
5060

IMPACTITES AND RELATED LITHOLOGIES IN
GERMANY—CURRENT STATE OF KNOWLEDGE

M Schmieder¹, W. Müller², L. Förster³ and E. Buchner¹,⁴. ¹Institut für Planetologie, Universität Stuttgart, Herdweg 51, 70174 Stuttgart, Germany. E-mail: martin.schmieder@geologie.uni-stuttgart.de. ²Diefflerstr. 217, 66809 Nalbach/Saar, Germany. ³Eichkamp 35, 23714 Bad Malente. ⁴Rieskratermuseum, Eugene-Shoemaker-Platz 1, 86720 Nördlingen, Germany.

**Background:** Germany, as the country of the Nördlinger Ries and pioneer impact studies [1], is of particular significance to impact research. The approximate 24 km and approximate 14.6 Ma Ries crater and the approximate 3.8 km Steinheim Basin in S Germany, both associated with a full suite of proximal impact ejecta (i.e., lithic impact breccias, suevites, and impact melt rocks), count among the best preserved impact structures on Earth [2, 3]. Distal Ries ejecta comprise the Central European tektite ("moldavite") strewn field, parts of which extend into Lusatia/Saxony [4], as well as the "Brock-horizont" layer in Bavaria and Baden-Württemberg [5]. Apart from the Ries and Steinheim craters, some enigmatic structures have been proposed to be of impact origin [6–10], however, all of these geologic features currently lack evidence for shock metamorphism and/or meteoritic matter as proof for impact.

**New Discoveries:** Fluidal, schlieren-rich, and vesicular silicatic glass containing abundant Fe–Ni spheres and ballen-textured α-cristobalite was recently discovered (W.M.) near Nalbach in the Saarland. The petrography and geochemistry of the glass revealed a striking resemblance to the Wabar impact glass found in Saudi Arabia and support a likely impact origin of the Nalbach glass [11, 12].

Erratic boulders and cobbles of Fennoscandian impact melt rocks were found (L. F.) in Pleistocene glacial till near Sieversdorf and Braak in Schleswig-Holstein, as well as from the Baltic Sea coast near Sassnitz on Rügen Island, Mecklenburg-Western Pomerania. Shock petrographic investigations suggest that these rocks were glacially transported over distances of up to approximately 800 km from the approximately 19 km Dellen and approximately 9 km Mien impact structures, Sweden, as their source craters [13, 14].

Magnetic Fe–Ni and silicatic spherules were reported from Neogene (Miocene) cave sediments of the Swabian Alb, Baden-Württemberg [15], which might suggest a link to the Ries-Steinheim event; similar magnetic spherules were found in Eocene-Oligocene clays in Hesse [16]. It will be worthwhile undertaking a first focused search for distal ejecta produced by large Paleozoic to Cenozoic terrestrial impacts in the German sedimentary record.

**References:** [1] Shoemaker E. M. and Chao E. C. T. 1961. *Journal of Geophysical Research* 66:3371–3378. [2] Buchner E. et al. 2010. *Meteoritics & Planetary Science* 45:662–674. [3] Buchner E. and Schmieder M. 2010. *Meteoritics & Planetary Science* 45:1093–1107. [4] Lange J. M. 1995. *Proceedings, 26th Lunar and Planetary Science Conference*. pp. 827–828. [5] Buchner E. et al. 2003. *Icarus* 191:360–370. [6] Rutte E. 1974. *Ohberrh. geol. Abh.* 23:66–105. [7] Ernstson K. et al. 2010. *Journal of Siberian Federal University Engineering and Technologies* 1:72–103. [8] Fehr K. T. et al. 2005. *Meteoritics & Planetary Science* 40:187–194. [9] Auth R. 2010. *Buchenblätter* 14:44–46. [10] Harting M. et al. 2003. Abstract #5201. 66th MetSoc. [11] Buchner E. et al. 2011. This conference. [12] Schmieder M. et al. 2011. This volume. [13] Förster L. et al. 2010. Abstract #5026. 73rd MetSoc. [14] Meyer K.-D. 1987. *Der Geschiebesammler* 20:125–146. [15] Schmieder M. et al. 2008. Abstract #55. Paneth, 15–16. [16] Schidlowski M. and Ritzkowski S. 1972. *Neues Jahrbuch für Mineralogie, Geologie und Paläontologie*. 3:170–182.

5316

RELICT STRUCTURE OF A HYDROUS MINERAL IDENTIFIED IN
WILD 2 DUST

S. Schmitz and F. E. Brenker. Geoscience Dept., Goethe University, Altenhoefer Allee 1, D-60438 Frankfurt / Main, Germany. E-mail: schmitz@em.uni-frankfurt.de.

**Introduction:** In January 2006, the Stardust spacecraft returned to Earth two unprecedented and independent types of extraterrestrial samples: bonafide samples of a comet, and the first grains of contemporary interstellar dust. Over a thousand dust particles from comet Wild 2 that have been collected by the Stardust spacecraft have been extensively investigated [1–5]. None of them ever yield compelling evidence for a hydrous phase. Here we present for the first time TEM data from a terminal particle (TP) (track 32) indicating the former existence of the phyllosilicate sepiolite or a Na-poor saponite.

**Technique:** A TEM grid from the terminal particle (TP) of Track 32 (C2027, 3, 32, 3, 8) was investigated in great detail using transmission electron microscopy. We performed high-resolution (HRTEM), bright field–(BF) imaging and selected area electron diffraction (SAED) to characterize the crystal structures. Additional energy dispersive X-ray (EDX) analyses were performed for determining the chemical composition. D-spacings were obtained from lattice fringe images and SAED patterns respectively.

**Results:** The complex TP of track 32 shows two clearly distinct mineral phases. The first, enstatite (Mg/Si 0.919; Fe/Si 0.026; Ca/Si 0.011; Al/Si 0.010; Cr/Si 0.011) shows high stacking disorder parallel (100) which includes alternating clinoenstatite (CLEN) and orthoenstatite (OREN) lamellae, (100) twins and pronounced stacking disorder. Attached and partly enclosed into the enstatite a structurally separated second phase (Mg/Si 0.694; Fe/Si 0.037; Ca/Si 0.026; Al/Si 0.032; Cr/Si 0.015; Mn/Si 0.015; Ti/Si 0.013) was discovered showing significantly different compositional and textural behavior. Chemical characterization is consistent with sepiolite or Na-poor saponite, both are phyllosilicates. Detailed TEM studies reveal sigmoidal fractures in this second phase which are typically observed as the result of dehydration in smectites or similar hydrous phases under vacuum. Microstructural data (HRTEM, SAED) are partly consistent with a high temperature triclinic clinoenstatite structure. The structural deviation is most likely due to the lower Mg/Si ratio of the new phase. Collected EELS spectra didn't show OH-related pre peaks indicating complete dehydration of the sample. Our data suggest that sepiolite or low Na saponite are the most likely candidates for the missing hydrous phase of comet Wild 2 samples, which transform to high temperature triclinic enstatite due to impact heating during capture into Aerogel.

**References:** [1] Brownlee et al. 2006. *Science* 314:1711–1716. [2] Flynn et al. 2006. *Science* 314:1731–1735. [3] Zolensky et al. 2006. *Science* 314:1735–1739. [4] Schmitz S. and Brenker F. E. 2008. *The Astrophysical Journal Letters* 681:L105–108. [5] Schmitz et al. 2009. *Geochimica et Cosmochimica Acta* 73:5483–5492.
RELATIONSHIP BETWEEN FEO CONTENT AND ÅÂ¹⁰O IN CHONDRULES FROM CR CHONDRITES: LINKING OXYGEN FUGACITY AND O-ISOTOPE EVOLUTION

D. L. Schrader¹, H. C. ConnollyJr.¹,²,³, D. S. Lauretta¹, K. Nagashima⁴ and G. R. Huss⁴. ¹LPL, University of Arizona, Tucson, AZ 85721, USA. ²Department of Physical Sciences, Kingsborough Community College of CUNY, Brooklyn, NY 11235 and Earth and Environmental Sciences, The Graduate Center of CUNY, New York, NY 10024, USA. ³Department of Earth and Planetary Sciences, AMNH, NY 110024, USA. ⁴HIGP, University of Hawai'i at Manoa, Honolulu, HI 96822, USA, E-mail: schrader@lpl.arizona.edu.

**Introduction:** In situ O-isotope measurements of chondrules provide clues about their solid precursors, melting history, and exchange with a gas reservoir [1, 2]. We present observations which constrain the formation environment of ferromagnesian chondrules from CR chondrites, part of an ongoing study to understand the formation conditions of type-II chondrules [2, 3].

**Methods:** We investigated chondrules from CR chondrites QUE 99177, GRA 95229, and GRA 06100. We analyzed element abundances of the olivines in 8 type-I and 12 type-II chondrules with the Cameca SX-50 electron microprobe at LPL. These same grains were analyzed with the UH Cameca ims 1280 ion microprobe for their O-isotope composition [2].

**Results:** O-isotope composition of olivine in type-II (> Fa₁₀) chondrules varies with the abundance of FeO-poor relict grains. Most type-II chondrules without relict grains have olivine phenocrysts with O compositions that cluster slightly above the TF line (Δ¹⁷O = 0.2 ± 1.0‰, 2σ). A second population (from MAC 87320 [1,2] and GRA 95229) of relict-free type-II chondrules has phenocrysts with compositions that plot below the TF line near the CCAM line (Δ¹⁷O = -1.6 ± 0.5‰). Relict grains within type-II chondrules have Δ¹⁷O = -4.4 ± 1.1‰, similar to olivine in type-I chondrules (Δ¹⁷O = -3.2 ± 0.9‰) [2]. Type-II chondrules with FeO-poor relict grains contain both ¹⁶O-rich (Δ¹⁷O = -2.5 ± 1.1‰) and ¹⁶O-poor (Δ¹⁷O = 0.2 ± 0.7‰) FeO-rich olivine.

**Discussion:** The FeO content of olivine and degree of melting experienced by chondrules are both related to ÅÂ¹⁷O. In general FeO-poor olivine has low Δ¹⁷O, and FeO-rich olivine has high Δ¹⁷O. Type-I chondrule olivine has the lowest Δ¹⁷O and FeO content. FeO-rich phenocrysts in type-II chondrules without relict grains have the highest Δ¹⁷O. Type-II chondrules with relicts contain olivine with both low Δ¹⁷O and high Δ¹⁷O.

These data suggest that FeO and Δ¹⁷O of olivine increased concurrently during type-II chondrule formation. FeO-poor, ¹⁶O-rich precursors contributed to the final isotopic composition of type-II chondrules [2]. The oxidation states and isotopic compositions of type-II chondrule olivines may have been established by: (1) the composition of their silicate precursors, a mixture of ¹⁶O-rich FeO-poor type-I chondrule fragments and ¹⁶O-poor FeO-rich material; (2) exchange between the chondrule melt and an ¹⁶O-poor, oxidizing gas reservoir; or (3) chondrule precursor materials such as H₂O ice or reduced carbon [2,4]. Components of these hypotheses are not mutually exclusive, and a combination of processes is plausible.

**Acknowledgments:** This work was supported by the DDF of the NAI (DSL, PI) and NASA Cosmochemistry (HCC, PI).

**References:** [1] Krot A. N. et al. 2006. *Geochimica et Cosmochimica Acta* 70:767–779. [2] Connolly H. C. Jr. and Huss G. R. 2010. *Geochimica et Cosmochimica Acta* 74:2473–2483. [3] Schrader D. L. 2008. *Geochimica et Cosmochimica Acta* 72:6124–6140. [4] Connolly H. C. Jr. et al. 1994. *Nature* 371:136–139.

FORMATION OF REFRACTORY METAL ALLOYS: THE FIRST CONDENSATES OF THE SOLAR SYSTEM?

D. Schwander¹,², T. Berg², H. Palme³, U. Ott¹ and G. Schönhense². ¹Max-Planck-Institut für Chemie, Joh.-Joachim-Becher-Weg 27, D-55128 Mainz, Germany. E-mail: d.schwander@mpic.de. ²Institut für Physik, Johannes Gutenberg-Universität, Staudingerweg 7, D-55128 Mainz, Germany. ³Forschungsinstitut und Naturmuseum Senckenberg, Senckenberganlage 25, D-60325 Frankfurt am Main, Germany.

Sub-micrometer sized refractory metal nuggets (RMN) separated from acid resistant residues of the Murchison meteorite have concentrations of the refractory metals Os, W, Ru, Mo, Ir that match calculated high temperature condensates having equilibrated with a solar composition gas at 1400–1600 K at 10⁻⁴ bar. The presence of the proper amounts of the easily oxidizable elements Mo and W in the alloy makes alteration in the meteoritic environment or during chemical processing very unlikely and thus supports a condensation origin [1]. A TEM study of a subset of these grains indicates a hexagonal close-packed crystal structure in all cases, supporting single phase condensation [2].

The most important objective in further studies is the in situ identification of the RMN: In polished sections of Allende and Murchison we have begun a systematic search for submicron refractory metal particles in order to see if the morphology and chemistry of these particles is identical to the particles recovered from acid resistant residues.

First results indicate that the RMN are very rare and are probably restricted to certain areas of the meteorites. It is presently unclear if the RMN are associated with CAI or occur as isolated grains in matrix, or both.

At the same time we have produced additional acid residues from Murchison. In one of these residues another population of RMN was identified. By the time of the conference we will present the compositions of these RMN. It is of interest to see if this new population of RMN covers exactly the same range in condensation temperatures as the particles of the population described by [1]. Furthermore we have started to produce acid residues from Allende to see if there are particles which could be similar or identical to those from Murchison.

**References:** [1] Berg T. et al. 2009. *The Astrophysical Journal* 702:L172–L176. [2] Harries D. et al. 2011. Abstract #1837. 42nd Lunar and Planetary Science Conference.
## ALHA77005: MONITOR FOR TERRESTRIAL INFLUENCE ON NOBLE GASES AND ANALOGUE FOR MARS

S. P. Schwenzer¹,², J. Schwanethal¹, U. Ott², A. G. Tindle¹, M. Anand¹ and S. P. Kelley¹. ¹The Open University, Walton Hall, Milton Keynes, MK7 6AA, UK. E-mail: s.p.schwenzer@open.ac.uk. ²Max-Planck Institut für Chemie, J.-J. Becher Weg, 55128 Mainz, Germany.

**Introduction:** The noble gases in Martian meteorites are derived from the Martian interior and atmosphere, and cosmogenic nuclides [1], but quantifying the components is complicated by terrestrial cold desert storage which can cause serious contamination, partially obscuring the extraterrestrial heavy noble gas signatures [2, and references therein] upon which key observations have been made (e.g., Martian atmosphere in EETA 79001, fractionated or ancient Martian atmosphere in ALHA77005 [see 1 for references]). This study investigates the rim and interior of ALHA77005 to characterize terrestrial air contamination, and derive potential correction procedures.

**Sample and Methods:** We obtained rim and interior samples of ALHA77005 (NASA, split ALH 77005, 225, 227, thin section 54). Each split was gently crushed, ~20 grains were handpicked, irradiated at McMaster reactor (Canada), and measured by melting grains individually by laser heating for Ar isotope analysis (MAP215-50). Microscopy, virtual microscope documentation, and electron microprobe (Cameca SX 100) were done at the OU.

**Results and Discussion:** Analysis of the rim of the sample revealed open fractures, which are lined by alteration minerals up to ~0.5 mm into the meteorite, but appear alteration free deeper into the sample.

Ar-Ar analysis reveals clear differences between rim and interior samples. Interior samples yield a range of ³⁶Ar/⁴⁰Ar of 0.001 to 0.003 but little correlation with ³⁹Ar/⁴⁰Ar. In contrast, exterior sample grains fall into two groups, one group displaying variable ³⁹Ar/⁴⁰Ar with a clear trend towards terrestrial atmospheric ³⁶Ar/⁴⁰Ar, but some analyses with low ³⁶Ar are associated with high ⁴⁰Ar/³⁶Ar. Average ⁴⁰Ar/³⁶Ar in these samples is lower than the values measured at Mainz (MZ) by the fusion of bulk-sample (50 mg) and by spot fusion measurement of the sample at OU [⁴⁰Ar/³⁶Ar 1229 (MZ) and 887 (OU)]. If relative probability plots of ⁴⁰Ar/³⁶Ar<sub>tr</sub> (cosm. corr. with (³⁸Ar/³⁶Ar)<sub>tr</sub> = 0.244, [3]) are investigated, both populations show two peaks with the exterior sample being considerably closer to terrestrial atmosphere (⁴⁰Ar/³⁶Ar<sub>tr</sub> of ~350 and 800 exterior; ~750 and 1400 interior). The second maximum of the interior sample is identical with ⁴⁰Ar/³⁶Ar<sub>tr</sub> measured in ol and pyx separates of ALHA77005 [4].

**Conclusions:** Our Ar-Ar study reveals significant air contamination even in the inner parts of the ALHA77005 sample. Future work will add heavy noble gas and trace element data in order to evaluate correction procedures for terrestrial contamination. The goal is to be able to see through terrestrial contamination even if only a small and weathered sample is available for study. Moreover, this will serve as an analogue to enhance our understanding of gas incorporation processes in cold deserts, which will help measuring Ar and noble gases on Mars [5,6].

**References:** [1] Swindle T. D. 2002. Martian noble gases. Reviews in Mineralogy and Geochemistry, vol. 47. pp. 171–190. [2] Schwenzer S. P. et al. 2009. *Polar Science*, doi:10.1016/j.polar.2009.06.00. [3] Wiens R. C. 1988. *Earth and Planetary Science Letters* 91:55–65. [4] Schwenzer S. P. et al. 2006. *Meteoritics & Planetary Science* 41:A161. [5] Bogard D. D. 2009. *Meteoritics & Planetary Science* 44:3–14. [6] Talboys D. L. et al. 2009. *Planetary and Space Science* 57:1237–1245.

## THE SECONDARY MINERAL-FORMING FLUID IN THE NAKHLITES

S. P. Schwenzer¹ and J. C. Bridges². ¹Dept. of Earth Sciences, CEPSAR, The Open University, Milton Keynes MK7 6AA, UK; s.p.schwenzer@open.ac.uk. ²Space Research Centre, Dept. of Physics & Astronomy, University of Leicester LE1 7RH, UK; E-mail: j.bridges@le.ac.uk.

**Introduction:** The nakhlite secondary minerals are mainly a siderite/ smectite and serpentine/silicate gel/salt assemblage which varies in composition between the nakhlites [1, 2]. This increasingly accurate understanding of the secondary mineralogy now allows us to model the associated fluid. Fluid modeling also helps test our hypothesis that the alteration occurred in the aftermath of an impact into ice-bearing Martian crust [2]. We utilize experience on thermochemical modeling of Martian impact hydrothermal systems [3], with the presence of CO₂ [4] to derive likely formation conditions of the nakhlite alteration assemblage.

**Methods:** We use CHILLER [5] to model the fluid. Lafayette bulk chemistry [6], olivine [6], mesostasis compositions, and combinations of them are used as dissolving host rock chemistry. The initial fluid is a diluted brine with Fe, Ca and Mg in the 10⁻² mol L⁻¹ range, charge balanced by Cl⁻. We modeled H₂CO₃ concentrations between 0.1 × 10⁻⁴ mol L⁻¹ H₂CO₃ and 5 mol L⁻¹. Each analysis has data for water to rock ratios (W/R) between 1 and 100,000. W/R is defined as the amount of rock that completely dissolved in the given amount of initial fluid. Model temperatures (at 10 bar) are 10 and 100 °C.

**Results and Conclusions:** At both temperatures the nature of the mineral assemblages can be grouped e.g. high W/R, where Fe and Al mainly form minerals such as hematite (at 100 °C) or goethite (at 10 °C), but silicates—such as are seen in the nakhlites—are not formed. At intermediate W/R phyllosilicates like some of those observed [2] are predicted: nontronite, chlorite, talc, serpentine, depending on T and composition of the host rock, occur together with hematite/goethite. At low W/R smectite (nontronite) is not formed from most host rock chemistries, but the assemblages include serpentine, chlorite and other minerals. In the presence of CO₂, carbonates precipitate replacing any oxide and forming Mg-(Ca)-siderite at low W/R. At 10 °C and W/R of 1.5–8 the model carbonate contains ~45% sd, 32% cc, 23% mg, which matches some of the compositions in Nakhla [1]. From our observation of Lafayette with carbonate being precipitated first followed by smectite and serpentine [2], we assume that CO₂ was exhausted by carbonate precipitation and was followed by phyllosilicate. During initial carbonate formation the pH is buffered by the presence of dissolved H₂CO₃. Smectite (nontronite 43%) with serpentine (antigorite 36%) and traces of chlorite plus goethite, apatite, pyrite precipitated in our model from Lafayette bulk rock, W/R ~380 and pH 8. Our results are consistent with the nakhlite mineral assemblages and W/R ratios decreasing from Lafayette to Nakhla and low fluid temperatures

**References:** [1] Bridges J. C. et al. *Space Science Reviews* 2001. 96:365–392. [2] Changela H. G. and Bridges J. C. 2010. *Meteoritics & Planetary Science* 45:1847–1867. [3] Schwenzer S. P. and Kring D. A. 2009. *Geology* 37:1091–1094. [4] Schwenzer S. P. and Kring D. A. 2009. *Meteoritics & Planetary Science* 44:A188. [5] Reed M. H. and Spycher N. F. 2006. Users Guide for CHILLER: A Program for Computing Water-Rock Reactions, Boiling, Mixing and Other Reaction Processes in Aqueous-Mineral-Gas systems and Minplot Guide.U. of Oregon. [6] Treiman A. H. 2005. *Chemie der Erde* 65:203–270.
## THERMAL AND IMPACT HISTORY OF H CHONDRITES: WAS THE ONION SHELL STRUCTURE PUNCTURED BY IMPACTS DURING METAMORPHISM?

Edward R. D. Scott¹, Tatiana V. Krot¹, Joseph I. Goldstein² and G. Jeffrey Taylor¹. ¹HIGP, University of Hawaii, Honolulu, HI, USA. E-mail: escott@hawaii.edu. ²Dept. of Mechanical and Industrial Engineering, University of Massachusetts, Amherst, MA, USA.

**Introduction:** The thermal and impact histories of ordinary chondrites provide important clues to the formation and impact histories of asteroids. Bodies that were heated by ²⁶Al and were undisturbed by impacts developed an onion-shell structure with the least metamorphosed type 3 material on the outside and types 4-6 arranged in concentric shells in an onion-shell structure. Trieloff et al. [1] argued from their Ar-Ar and Pb-Pb ages and Pu fission track data for 9 H4-6 chondrites that the onion-shell structure was undisturbed during cooling and that the metallographic cooling rates, which showed little or no correlation between type and cooling rate [2], were flawed or compromised by later impact reheating. Harrison and Grimm [3] discarded fast metallographic cooling rates and inferred that only a very small fraction of the age and cooling rate data for H chondrites were inconsistent with an onion-shell model.

**Methods:** We have studied taenite grains and shock levels in 13 H4-6 chondrites which are largely shock stage S1/S2. Metallographic cooling rates were determined from central Ni contents of taenite grains using the Wood technique to constrain thermal histories during kamacite growth at 500-600 °C. Cloudy taenite particles, which formed below 350 °C, were measured using a FEI field emission SEM with a resolution of 1-2 nm.

**Results:** Zones of cloudy taenite are clearly visible in all chondrites except for three fast cooled H4 chondrites. The 0.1 µm scale of these intergrowths precludes any diffusion during reheating that could modify the Ni zoning in taenite on scales of micrometers. Thus the metallographic cooling rates could not have been modified by impact reheating. Cloudy zone dimensions are inversely correlated with metallographic cooling rate and are consistent with iron meteorite data [4]. Our results and published data [2] for 11 H4 chondrites give cooling rates of 5-100 °C Myr⁻¹ except for three that cooled at > 5000 °C Myr⁻¹ and which anchor the H4-6 sequence of Trieloff et al. [1]. Our cooling rates for these H4s are consistent with the Trieloff ages but are far too high for chondrites that were heated by ²⁶Al. These H4s could not have cooled undisturbed near the surface and were probably excavated ~3 Myr after the H body accreted so that they quickly cooled near the surface. Early impacts like this one could have shock melted sulfides in low-shock type 5 and 6 material [5, 6]. We predict that most H4s will have Ar-Ar ages that are younger than the Trieloff samples. Similarly, H5 and H6 chondrites both show wide ranges of cooling rates that are incompatible with undisturbed cooling. Butsura, for example, cooled about five times faster than four other H6 chondrites including Estacado and Guarena. We infer that several impacts punctured the H chondrite body to type 6 depths while it was cooling causing disturbances in the thermal histories of many H chondrites.

**References:** [1] Trieloff M. et al. 2003. *Nature* 422:502-506. [2] Taylor G. J. et al. 1987. *Icarus* 69:1-13. [3] Harrison K. P. and Grimm R. E. 2010. *Geochimica et Cosmochimica Acta* 74:5410-5423. [4] Yang J. et al. 2010. *Geochimica et Cosmochimica Acta* 74:4471-4492. [5] Rubin A. E. 2004. *Geochimica et Cosmochimica Acta* 68:673-689. [6] Scott E. R. D. et al. 2010. *Meteoritics & Planetary Science* 45:A184.

## TWO TYPES OF GRAPHITE CRYSTALS WITHIN THE KRYMKA CHONDRITE (LL3.1)

V. P. Semenenko and A. L. Girich. Institute of Environmental Geochemistry, NAS of Ukraine, Palladina 34a, Kyiv-142, 03180, Ukraine. E-mail: virasem@i.com.ua.

Previous studies [4, 5] showed that carbonaceous xenoliths in Krymka contain uniformly dispersed regular crystals of graphite, which were probably formed as a result of metamorphism from primary organic compounds. Recently, we found similar crystals of graphite (≤4 x 0.2 µm in size) within a primitive, fine-grained rim around a polyzone, olivine-pyroxenes macrochondrule (6 x 4.3 mm in size). The rim contains rare grains of Ca, Al-rich minerals (hibonite, anorthite, diopside and spinel), and its SiO₂/MgO and FeO/(FeO + MgO) ratios are higher (3.11 and 0.86, respectively) than those of the opaque matrix [1].

The macrochondrule consists of several concentric zones: the silicate core is surrounded by two metal-troilite-rich zones, which are separated from each other by a silicate zone. This whole assemblage is enclosed by the fine-grained rim. The metal-troilite mantles contain rounded or irregular graphite grains with a fine, polycrystalline structure. The grains are larger (up to 100 µm in size) than metal-associated carbon inclusions in primitive chondrites [3]. By these mineralogical features this unique macrochondrule differs from other known chondrules and macrochondrules [2].

The following speculations about the nature of the two types of graphites found within the studied object can be made. Their presence points to an enrichment in carbon of the precursor material of the macrochondrule and its primitive rim. The existence of two metal-troilite mantles in the macrochondrule attests to its multistage formation. A fine-grained, silicate precursor of the macrochondrule was probably mantled by a metal-troilite dust in a mineralogically heterogeneous dusty environment at least twice, like those of accretionary, fine-grained spherules within the Krymka xenolith BK 13 [6]. The following flash melting of the macrochondrule precursor, probably with partial evaporation of its surface led to the separation of immiscible metal-troilite-silicate liquids and the preservation of the melted metal-troilite mantles. Crystallization of graphite during the formation of macrochondrule must be connected with the presence of C-bearing material, for example C-containing metal or organic compounds in the precursor. The primitive, fine-grained rim of the macrochondrule was formed due to deposition of dusty particles onto its surface from a dusty environment before or during accretion of the Krymka parent body. Regular graphite crystals within the fine-grained rim were probably formed from primary organic compounds due to shock metamorphism of the Krymka parent body. Thus, the two types of graphite crystals, which differ in size, shape and structure, have different nature, i.e. metamorphic within the rim and "igneous" within the macrochondrule, due to distinct processing of the primary C-containing material.

**Acknowledgments:** We are sincerely grateful to Claude Perron for helpful discussion and correction of our English.

**References:** [1] Huss G. R. et al. 1981. *Geochimica et Cosmochimica Acta* 45:33-51. [2] Hutchison R. and Bridges J. C. 1995. *Meteoritics* 30:523-524. [3] Mostefaoui et al. 2000. *Geochimica et Cosmochimica Acta* 64:1945-1964. [4] Semenenko V. P. et al. 2004. *Geochimica et Cosmochimica Acta* 68:455-475. [5] Semenenko V. P. et al. 2005. *Geochimica et Cosmochimica Acta* 69:2165-2182. [6] Semenenko V. P. et al. 2001. *Meteoritics & Planetary Science* 36:1067-1085.
THE END TO END MARS SAMPLE RETURN CAMPAIGN

M. A. Sephton¹, C. D. K. Herd² and the MEPAG E2E-iSAG team. ¹Earth Science and Engineering, Imperial College London, South Kensington, United Kingdom, SW7 2AZ. E-mail: m.a.septon@imperial.ac.uk ²Earth and Atmospheric Sciences, University of Alberta, Edmonton, Canada, T6G 2E3.

**Introduction:** Mars Sample Return (MSR) is a top priority for the next decade of space exploration [1]. Returned samples would allow their detailed investigation by the most advanced instruments on Earth providing new insights into the history of Mars. The success of MSR would require multiple flight missions and a subsequent non-flight project, a scenario now termed the “MSR Campaign.” The campaign objectives need to be understood in detail at an early stage in order to plan the role of each component mission. The End to End International Science Analysis Group (E2E-iSAG) was chartered by the Mars Exploration Program Analysis Group (MEPAG) to undertake this task by building on efforts of previous MEPAG-chartered groups. Since August 2010 the E2E-iSAG has been (1) defining potential objectives, (2) defining candidate criteria for sampling and landing sites definition (3) defining reference landing sites (4) developing a first draft of a returned sample measurement plan, and (5) initial inputs to technology planning for the overall sample return campaign (from the first mission to the analysis of samples on Earth), and not just one segment of this program.

**Results:** The scientific objectives proposed by E2E-iSAG are, in priority order (1) Critically assess any evidence for past life or its chemical precursors, and place detailed constraints on the past habitability and the potential for preservation of the signs of life, (2) Quantitatively constrain the age, context and processes of accretion, early differentiation and magmatic and magnetic history of Mars, (3) Reconstruct the history of surface and near-surface processes involving water, (4) Constrain the magnitude, nature, timing, and origin of past planet-wide climate change, (5) Assess potential environmental hazards to future human exploration, (6) Assess the history and significance of surface modifying processes, including, but not limited to: impact, photochemical, volcanic, and Aeolian, (7) Constrain the origin and evolution of the Martian atmosphere, accounting for its elemental and isotopic composition with all inert species, (8) Evaluate potential critical resources for future human explorers. With an additional obvious but unprioritised objective—determine if the surface and near-surface materials contain evidence of extant life.

The findings of the E2E-iSAG most relevant to the meteorological community are those involving sample selection, sample sizing and sample distribution once back on Earth. Sample types of highest priority to the objectives listed above are: (1) Lacustrine or hydrothermal deposits, (2) Hydrothermally altered rocks, (3) Low-T fluid-altered rocks, (4) Unaltered Igneous rocks, (5) Regolith, (6) Airfall dust, and (7) Atmosphere, rocks with trapped atmosphere. Subsurface rocks made accessible by either drilling or natural excavation would be desirable. Sample suites would be preferable for some sample types. The total proposed returned sample mass would be about 500 g with a proposed subdivision of this amount between sample numbers and types detailed in the E2E-iSAG report. With decades of experience involving curation and analysis of extraterrestrial samples, the meteorological community may have a new opportunity to apply their skills to Mars samples returned by a campaign of dedicated space missions.

**Reference:** [1] Squyres S. W. et al. 2011. Washington D.C.: The National Academies Press.

CATALYTIC HYDROGENATION OF THE MACROMOLECULAR MATERIAL IN METEORITES

M. A. Sephton¹, W. Meredith², A. Verchovsky³, J. S. Watson³ and C. E. Snape². ¹Earth Science and Engineering, Imperial College London, South Kensington, SW7 2AZ, UK, E-mail: m.a.septon@imperial.ac.uk. ²School of Chemical, Environmental and Mining Engineering, University of Nottingham, Nottingham, NG7 2RD, UK. ³Planetary and Space Sciences Research Institute, Open University, Milton Keynes, MK7 6AA, UK.

**Introduction:** By far the major organic component in meteorites is a large and relatively complex organic network or macromolecule [1]. The macromolecule is insoluble in common organic solvents and is generally assumed to be completely indigenous owing to its high molecular weight and immobility. However, it is this attribute which creates analytical difficulties and repulses straightforward investigations.

In recent years non-degradative techniques such as nuclear magnetic resonance spectroscopy [2, 3] and degradative methods such as hydrous pyrolysis [4, 5] have provided insights into the make up the macromolecular material. One additional and particularly effective method involves the catalytic addition of hydrogen to the macromolecule to liberate its constituent units [6]. The products of catalytic hydrogenation are highly amenable to analysis by mass spectrometric techniques. Catalytic hydrogenation is highly efficient and has released more bound carbon from the macromolecule than any other thermal degradation technique. This approach has also produced the largest ever aromatic units from the thermal degradation of meteorites and has partly reconciled the disharmony between observations of large interstellar molecules and their smaller meteoritic counterparts [6].

To date, this method has only been applied to the type II Murchison meteorite. New experiments have extended this work to other meteorites of differing petrologic type (types I and III), revealing the potential influence of differing organic source materials and subsequent effects of parent body alteration. Moreover, the previous off-line method has been supplemented by an on-line approach that could provide the opportunity for the analysis of a wider range of meteorites only available in small amounts.

Preliminary data suggest the presence of occluded units within the macromolecular material that may reveal which organic compounds formed first during macromolecule construction before being swamped and immobilized by the forming organic network. Contaminant organic compounds are also occasionally present reflecting the propensity of the macromolecule to retain terrestrial compounds despite previous extraction with organic solvents.

**References:** [1] Sephton. 2002. *Natural Product Reports* 19:292–311. [2] Gardinier et al. 2000. *Earth and Planetary Science Letters* 184:9–21. [3] Cody G. D. et al. 2002. *Geochimica et Cosmochimica Acta* 66:1851–1865. [4] Sephton M. A. et al. 1998. *Geochimica et Cosmochimica Acta* 62:1821–1828. [5] Sephton M. A. et al. 2000. *Geochimica et Cosmochimica Acta* 64:321–328. [6] Sephton M. A. et al. 2004. *Geochimica et Cosmochimica Acta* 68:1385–1393.
## EXPERIMENTAL TEST OF THE CARBON MONOXIDE SELF-SHIELDING MODEL FOR THE EARLY SOLAR SYSTEM'S OXYGEN ISOTOPE EVOLUTION

X. Y. Shi¹, Q.-Z. Yin², Y.-C. Chang¹, Z. Luo¹, H. Huang¹, C.-Y. Ng¹. ¹Department of Chemistry, University of California, Davis, CA 95616, USA. E-mail: xyshi@ucdavis.edu. ²Department of Geology, University of California, Davis, CA 95616, USA. E-mail: qyin@ucdavis.edu.

We designed a laboratory experiment to test the carbon monoxide self-shielding (COSS) model advanced to explain the oxygen isotope distribution in the early Solar System materials [1–3]. Our unique windowless vacuum ultraviolet (VUV) ultra high resolution laser photodissociation and photoionization mass spectrometry is designed to mimic the solar nebular photochemistry environment and experimentally verify if the CO photodissociation at VUV wavelengths (90–110 nm) would produce the expected mass independent oxygen isotope fractionation as predicted in the recently revived self-shielding model [1–3]. This model has been invoked to explain the peculiar oxygen isotope distribution observed in early solar system materials and has a specific prediction for the Sun’s oxygen isotope composition [4], a top science priority of NASA’s GENESIS mission.

However, the first experimental test of COSS model by [5] using the Advanced Light Source (ALS) at Lawrence Berkeley National Laboratory (LBNL), poses a serious question on the COSS model, although the validity of [5] were immediately challenged by [6–8]. The significance of COSS model for the early solar system warrants a second opinion with closer experimental scrutiny. If the COSS model stands the test of experimental verification, it has major implications for the origin and transport of water in the Solar System that governs the ultimate habitability of planets.

Our VUV laser system can generate four to five orders of magnitude higher optical resolution, with brightness more than 6 orders of magnitude greater than the broadband synchrontron light source at ALS at LBNL. This will ensure higher signal-to-noise ratios when studying weak transitions or transitions of less abundant isotopologues, such as ¹³C¹⁷O. Our initial experimental results support a “slope-1” ¹⁶O depletion line as the COSS model postulates and observed in the early solar system material and now observed in the Sun [9].

**References:** [1] Clayton R. N. 2002. *Nature* 415:860. [2] Yurimoto and Kuramoto 2004. *Science* 305:1763. [3] Lyons and Young 2005. *Nature* 435:317. [4] Yin Q.-Z. 2004. *Science* 305:1729. [5] Chakraborty et al. 2008. *Science* 321:1328. [6] Lyons et al. 2009. *Science* 324:1516a. [7] Federmann and Young 2009. *Science* 324:1516b. [8] Yin Q.-Z. et al. 2009. *Science* 324:1516c. [9] McKeegan et al. 2011. *Science*, in press.

## FINE MORPHOLOGICAL FEATURES OF MINERAL GRAINS IN THE NORTON COUNTY AUBRITE

K.O. Shkurenko, V.P. Semenenko, T.M. Gorovenko and V. M. Slyvinsky. Institute of Environmental Geochemistry, NAS of Ukraine, Palladina 34a, Kyiv-142, 03180, Ukraine. E-mail: koshkurenko@gmail.com.

Previous scanning electron microscopy (SEM) study of chondrites [1, 4] and pallasites [2, 5] indicates the presence of a primary and secondary sculptures on the surface of mineral grains. Most of the sculptures are thought to be produced during thermal and shock effects connected with a complex, multistage formation history of the meteorites. Here we present the data on a SEM and energy dispersive spectroscopy (EDS) examination of sculptural features and chemical composition of a surface of grains separated from a fine-grained fraction of the Norton County achondrite [6]. The following minerals and their intergrowths are detected as separate grains: enstatite, forsterite, troilite, kamacite, brezinaite, ferroalabandite, daubreelite, and oldhamite. Iron hydroxides, regular crystals of calcite and gypsum, and fine-grained films of manganite are detected as weathering products. As we know, this is the first time the brezinaite is found in the Norton County achondrite. It was revealed as a friable grain of green-grayish colour with perfect-developed cleavage [6]. The EDS data indicate to the following composition (wt%): 48.2 Cr, 41.2 S, 6.78 Ca, 1.72 Mg, and admixture of Mn, Fe, Ti, Ni, Mg, and Si. The presence of Ca, Ni, Mg, Mn, and Si is probably due to analytical contamination by surrounding minerals. The presence of refractory elements such as Ti, V, and Cr is the main chemical feature of the sulfides that indicates to a high temperature formation of the aubrite. For example, troilite contains (wt%) up to 2.91 Ti, 1.04 Cr and 0.27 V, daubreelite does up to 2.04 Ti and 0.24 V, and brezinaite—up to 0.79 Ti.

The surface of the examined mineral grains and their fractures is characterized by the following sculptures: (1) elongated voids (rivulet-like voids of [3]) which are parallel to the axis c of enstatite crystals; (2) inclusions of troilite on enstatite, Cu-Fe-Ti-alloy on olivine, and some unidentified minerals which result in their preferable crystallization on a mineral phase boundary during slow cooling; (3) submicron pores on enstatite and olivine grains that could indicate possibly some gas inclusions; (4) well-developed cleavage along (110) for enstatite, and lamellar deformations on enstatite, olivine, troilite and kamacite formed as a result of shock; (5) sticked drops, emulsions and streams of melt on enstatite, olivine and troilite grains produced by shock heating; (6) thin growth steps on a face of a troilite crystal that testify to their slightly unequilibrated formation probably as result of a post-shock heating.

Thus, the SEM and EDS-study of a small portion of the material allows to get additional data about a mineralogical composition and a fine structure of the achondrite. The morphological and chemical features support the previous petrographic results [3] on important role of multistage shock metamorphism in the cosmic history of the Norton County parent body.

**References:** [1] Cristophe Michel-Levy M. 1981. *Earth and Planetary Science Letters* 54:67–80. [2] Kvasnytsya V. and Semenenko V. P. 2003. *Meteoritics & Planetary Science* 7:A10. [3] Okada A. et al. 1988. *Meteoritics* 23:59–74. [4] Semenenko V. P. and Tertichnaya B. V. 1994. 25th Lunar and Planetary Science Conference. pp. 1261–1264. [5] Semenenko V. P. et al. 2008. *Proceedings of Ukranian Minerologic Society* 5:68–74 (in Ukrainian). [6] Semenenko V. P. et al. 2010. *Proceedings of Ukranian Minerologic Society* 7:28–35 (in Ukrainian).
A STUDY OF EVAPORATION OF MURCHISON CHONDRITE AND CAI INCLUSIONS IN EFREMOVKA USING THE KNUDSEN EFFUSION CELL

S. I. Shornikov and O. I. Yakovlev. RAS Vernadsky Institute of Geochemistry & Analytical Chemistry, Kosygin st., 19, Moscow, 119991, Russia. E-mail: sergey.shornikov@gmail.com.

**Introduction:** Vaporization (condensation) is among the key physicochemical processes that form the meteorite substance composition. These processes played an especially important role at formation of the substance of white inclusions in carbonaceous chondrites (CAI). The problems of the origin and physicochemical conditions for CAI formation remain the subject of continuous discussions [1]. A weak side of various genetic models of CAI is their construction on a purely theoretical basis without due reliance on experimental data. Therefore, of special interest are the results of the study on evaporation of CAI substance from the Efremovka meteorite [2].

**Experiment:** The experiments were performed by the mass spectrometric method at evaporation from a Knudsen effusion tungsten cell of CAI inclusions in Efremovka and Murchison sample. To reduce the interaction of cell material the samples were placed in a rhenium boat on the bottom of a tungsten cell. It did not fully rule out the reducing action of tungsten. The temperature dependence of changes in the studied melts was determined at temperatures up to 2900 K. Experiments were spent in the conditions of step temperature lifting with step of 50 K at the constant temperature about 15 min. The chosen conditions of step heating allowed to keep a molecular mode of the expiration of steam at the given weight of the sample and optimum to reduce the experiment duration.

**Results and Discussion:** We identified the gas phase composition (monoatomic forms: K, Na, Fe, Mg, Ca, Al; oxide forms: FeO, SiO, TiO, TiO₂, CaO, AlO, Al₂O₃) and found the partial pressure values of vapor species.

Results of experiments on the Murchison chondrites have shown that the temperature sequence of evaporation components melt to a gas phase is almost identical to the known data on evaporation of the lunar basalt [3]. The maximum concentration of silicon dioxide in melt is observed to temperature 2000 K. At high temperature its concentration sharply falls at increasing of concentrations CaO and Al₂O₃ (to 60–65 wt% Al₂O₃).

The CAI inclusions in Efremovka evaporation characterized of silicon monoxide presence in the gas phase with the high partial pressure (10⁻⁶ atm) at the temperature range 2400–2700 K. The partial pressure values of *p*(SiO) are closed to the same of *p*(Ca) and *p*(Al). It is obviously the such unusual values of *p*(SiO) are due to the high values of *p*(O₂) in comparison to typical for evaporation of Murchison chondrite and lunar basalt. For example, at temperature 2350 K the value of *p*(O₂) = 1.0 × 10⁻⁶ atm for CAI inclusions in Efremovka, and the value of *p*(O₂) = 2.0 × 10⁻⁷ atm for Murchison chondrite.

The obtained result has a genetic importance which showed of the special conditions of formation of CAI inclusions in Efremovka. The CAI inclusions in Efremovka feature is in their formation in the regions with high oxygen concentration.

**References:** [1] Shornikov S. I. and Yakovlev O. I. 2010. Abstract #1408. 41st Lunar and Planetary Science Conference. [2] Yakovlev O. I. et al. 1984. 15th Lunar and Planetary Science Conference. pp. 945–946. [3] Markova O. M. et al. 1986. *Geokhimiya* 11: 1559–1569 (in Russian).

SYNCHROTRON X-RAY IRRADIATION OF Stardust INTERSTELLAR CANDIDATES: FROM “NO” TO “LOW” DAMAGE EFFECTS

A. Simionovici¹, C. Allen, S. Bajt, R. Bastien, H. Bechtel, J. Borg, F. E. Brenker, J. C. Bridges, D. E. Brownlee, M. J. Burchell, M. Burghammer, A. Butterworth, P. Cloetens, A. M. Davis, C. Floss, G. Flynn, D. Frank, Z. Gainsforth, E. Grün, P. R. Heck, J. Hillier, P. Hoppe, L. Howard, G. R. Huss, J. Huth, A. T. Kearsley, A. J. King, B. Lai, J. Leitner, L. Lemelle, H. Leroux, R. Lettieri, W. Marchant, L. Nittler, R. Ogliore, F. Postberg, S. Sandford, J.A. Sans Tresseras, T. Schoonjans, S. Schmitz, G. Silversmit, V.A. Solé, R. Srama, T. Stephan, J. Stodolna, R. M. Stroud, S. Sutton, M. Trieloff, P. Tsou, A. Tsuchiyama, T. Tyliczs-zak, B. Vekemans, L. Vincze, A. J. Westphal, D. Zevin, M. E. Zolensky and > 29,000 Stardust@home dusters². ¹ISTerre, Observatory of Sciences of the Universe of Grenoble, BP 53, 38041 Grenoble, FRANCE. E-mail: alexandre.simionovici@ujf-grenoble.fr. ²ISPE team affiliations are listed at http://www.ssl.berkeley.edu/~westphal/ISPE.

**Introduction:** Although synchrotron radiation X-Ray fluorescence (SR-XRF) is among the least destructive analysis methods applied to rare extraterrestrial grains, we have observed radiation damage effects following high flux synchrotron analyses. Track 30 of the IS collector of the Stardust mission [1], containing 2 candidates dubbed Orion and Sirius [2] was analyzed at ESRF, France, on beamlines ID13 and ID22NI by nano-XRF/XRD scanning methods. Beam damage effects were noticed on both samples and a quantitative analysis of their irradiation history was established [3], allowing us to propose new experimental protocols as well as fluence limits, minimizing such effects in the future. The purpose of this study is to present these facts, analyze potential damage mechanisms and offer alternatives.

**Cosmic Irradiation Limit:** The cosmic X-rays background in the 3–300 keV energy range, most likely produced by active galactic nuclei components impacting our IS samples was fitted by a simple analytical expression by Gruber et al. [4] and a limit of cosmic irradiation of 3·10¹⁹ ph cm⁻², dubbed astrophysical limit (AL) was established. X-ray scanning of IS samples at fluences (flux · time) exceeding the AL are potentially damaging, except in specific cases, known in biochemistry as “recovery effects” appear, where multiple shorter exposures are preferable to long ones [7].

**Damage Mechanisms:** Main particle impact damage include electron or proton/alpha irradiation-induced heating, ion displacement and charging [6, 7] of microscopic samples, particularly material in an insulator matrix.

**References:** [1] Brownlee et al. 2006. *Science* 314:1711–1715. [2] Westphal et al. 2011. Abstract #2083. 40th Lunar and Planetary Science Conference. [3] Simionovici et al. 2011. Abstract #2812. 42nd Lunar and Planetary Science Conference. [4] Gruber D. E. et al. 1999. *The Astrophysical Journal* 520:124–129. [5] Egerton R. F. et al. 2004. *Micron* 35:399–409. [6] Lemelle L. et al. 2003. *Geochimica et Cosmochimica Acta* 67:1901–1910. [7] Oger P. et al. 2008. *Spectrochimica Acta Part B* 63:512–517.
5474

WHERE ARE ALL THE TERRESTRIAL METEORITES?

Michael J. Simms. National Museums Northern Ireland, Cultra, BT18 0EU, Northern Ireland. E-mail: michael.simms@nmni.com.

Meteorites clearly can reach Earth from the Moon and from Mars, launched by larger impacts. This raises the inevitable question: can solid debris be ejected from Earth by large impacts, to return subsequently, survive re-entry, and be recognized as a meteorite?

**Recognition:** Meteorites launched from Earth will not be recognizable from lithology or geochemistry. The only diagnostic feature would be a fusion crust formed during re-entry, but this will not develop on many sedimentary rocks [1]. The Antarctic collection programmes are the best prospect for recognizing a fusion-crusted terrestrial meteorite but no candidate has yet been reported. Might this absence be significant and not merely chance?

**Residence Times in Space and on Earth:** CRE ages for Lunar meteorites [2] are typically << 1 Ma, which suggests that ejected terrestrial debris will have comparably short space residence times. In geological terms return may be almost synchronous with the impact launch event. Maximum terrestrial residence times of achondrites are < 1 Ma [3]. Combined space and terrestrial residence times of a terrestrial meteorite, probably < 1 Ma, require a launching impact within the last few hundred ka for there to be any prospect of such a meteorite being recognized at the surface.

**Launch Conditions:** Minimum crater diameters necessary to achieve significant dispersal of Lunar and Martian material into space are 0.4 km and 3km respectively. Scaling-up to Earth-based parameters suggests a minimum crater diameter of ~25 km, even before the effects of atmospheric drag on Earth-launched projectiles is considered. Spallation may launch solid material at high velocities from a narrow and very shallow zone around the impact site [4], with vaporization of volatiles in the target rock perhaps providing further acceleration. To escape Earth's gravity requires launch velocities perhaps 75% of impact velocity, which exceeds most published estimates [5] and questions the potential existence of terrestrial meteorites. For the high shock pressures associated with a suitably large Earth impact, any potential meteoroids will be small, probably <20 cm [6], with obvious implications for survival during re-entry. The shallow location of the spallation zone has significant implications for recognizing 'terrestrial' meteorites. The target lithologies for many large impacts were sedimentary (e.g., limestone for the Chicxulub, Manicouagan, and Ries sites). Fusion crusts would not form on such lithologies and hence it would be effectively impossible to identify them as having re-entered the atmosphere as a meteorite.

**Conclusions:** A fusion crust is the only feature by which a terrestrially derived meteorite might be recognized. None have yet been reported. Minimum crater diameter of ~25 km and combined space and terrestrial residence times of < 1 Ma for potential Earth-derived meteorites precludes the existence of any at the surface today; a crater of such size and recency is unlikely to have been overlooked by impact hunters scouring Google Earth. Ancient terrestrial meteorites might be sought in strata that slightly post-date major impacts in the geologic past.

**References:** [1] Brandstätter F. et al. 2008. *Planetary and Space Science* 56:976-984. [2] Eugster O. et al. 2006. *Meteorites and the Early Solar System* II. pp. 829-851. [3] Jull A. J. T. 2006. *Meteorites and the Early Solar System* II. pp. 889-905. [4] Melosh H. J. 1984. *Icarus* 59:234-260. [5] Wells L. E. et al. 2003. *Icarus* 162:38-46. [6] Nyquist L. E. 2001. *Space Science Reviews* 96:105-164.

5151

A STUDY OF GLASSES IN HOWARDITES: IMPACT MELT CLASTS VERSUS PYROCLASTS

S. A. Singerling and H. Y. McSween. Department of Earth & Planetary Sciences, University of Tennessee, Knoxville, TN 37996, USA. E-mail: ssingerl@utk.edu.

**Introduction:** Theoretical calculations predict that explosive eruptions ought to have occurred on volatile-bearing asteroids of sufficient size [1, 2]. The resulting deposits, pyroclastics, should then be preserved in samples derived from these asteroids. The large asteroid 4 Vesta is one candidate that might have experienced explosive volcanism, but pyroclastic material has not yet been definitively identified in meteorite samples (HEDs) from this body [3]. Preliminary work by David Mittlefehldt has identified what may be pyroclasts in several howardite samples (personal communication). The purpose of this research is to identify, describe, and analyze glasses in howardites and distinguish their origin: impact melt-derived or pyroclastic.

The following table outlines expected differences between the two glass types: impact melt clasts and pyroclasts.

**Significance:** Impact melt clasts act as representative samples of surface lithologies, the study of which will further constrain the range of compositions that occur on Vesta. Pyroclasts, if recognized as such in HEDs samples, could yield information on eruptive conditions operating in/on Vesta and could indicate a new role for volatile elements on Vesta.

**Expected Results:** This work will focus on compositional results obtained from electron microprobe analysis. Variation diagrams will help distinguish the two compositionally (i.e., CaO versus MgO, FeO/MgO versus Al₂O₃) and in regard to volatile elements (i.e., Na₂O/CaO versus Fe# and K₂O/CaO versus Fe#). For an example of such diagrams relating to HEDs, see [4]. Impact melt clasts should plot near a howardite composition on such diagrams since they represent pre-existing material on the surface that was impacted. Pyroclasts should plot near a primitive basaltic eucrite composition since they represent the initial melt. Any xenocrysts in the impact melts clasts should have evolved compositions. Phenocrysts in pyroclasts should have primitive compositions. We expect to present chemical results at the meeting.

**References:** [1] Wilson L. and Keil K. 1997. *Meteoritics & Planetary Science* 32:813-823. [2] Wilson L. et al. 2010. *Meteoritics & Planetary Science* 45:1284-301. [3] Keil K. 2002. In *Asteroids III* pp. 573-85. [4] Mittlefehldt D. W. et al. 1998. In *Planetary materials*. pp. 4-122.

<table><thead><tr><th></th><th>Impact melt clasts</th><th>Pyroclasts</th></tr></thead><tbody><tr><td>Composition</td><td>Howardite-like and mineral-like<br/>Evolved melt<br/>Heterogeneous</td><td>Eucritic-like<br/>Initial melt<br/>Homogeneous</td></tr><tr><td>Volatile-element content</td><td>Low</td><td>High</td></tr><tr><td>Incompatible-element content</td><td>Low</td><td>High</td></tr><tr><td>Texture</td><td>Vitric<br/>Vitrophyric<br/>Dendritic<br/>Partly resorbed clasts<br/>Schlerin</td><td>Vitric<br/>Vitrophyric<br/>Dendritic</td></tr></tbody></table>
## EFFECTS OF SHOCK METAMORPHISM ON ZIRCONS FROM THE HAUGHTON IMPACT STRUCTURE

C. Singleton¹, G. R. Osinski¹,², and D. Moser¹. ¹Department of Earth Sciences, University of Western Ontario, ON N6A 5B7, Canada E-mail: asingle2@uwo.ca. ²Department of Physics and Astronomy, University of Western Ontario, London, ON N6A 5B7, Canada.

**Introduction:** The extreme pressure and thermal conditions attained during shock metamorphism at sometimes very limited spatial and temporal scales [1] result in unique microscopic features that remain to be fully explored in some common rock-forming minerals, including zircon. The type of shock effect(s) created in a given sample depends upon the pressures and temperatures involved as well as the composition, density and material's location in the target. This has prompted our analysis of zircon grains in variably shocked and exceptionally preserved crystalline basement assemblages of the Haughton impact structure, Canada. The purpose of this study is to identify and describe various effects in zircons grains caused by the impact event.

**Results:** Our results provide valuable new information regarding the effects of shock in zircon.

*Shock Effects in Quartz and Shock Levels.* In this sample suite it was found that quartz shows the full range of shock features including fracturing, PDFs, toasting, decrease in birefringence, diaplectic melting, full melting with loss of grain boundaries and finally flow banding. Based on these and other shock features shock levels from 1 to 7 were assigned to 52 samples of shocked crystalline material from the Haughton impact structure.

*Shock Effects in Zircon.* A number of shock effects were identified in the zircon grains. The first of these are a variety of fractures, which range from non-linear to linear. The non-linear fractures are also present in some un-shocked samples so these features do not always indicate shock. Other effects include less defined zoning and micro-faulting. Zircons that have been highly shocked show a distinct granular texture that is proposed to be due to recrystallization of the zircon.

*Raman Spectroscopy.* Raman spectroscopy investigations were carried out on eight zircons from various shock levels. Level 0 shows peaks at approximately 225, 292, 354, 413, 435, 819, 973 and 1003. Level 2 shows peaks at approximately 204, 223, 355, 438, 813, 973 and 999. Level 3 shows peaks at approximately 201, 222, 351, 434, 818, 973 and 1000. Level 5 shows peaks at approximately 201, 223, 355, 439, 973 and 1007. Level 6 shows peaks at approximately 201, 222, 353, 438, 972 and 1005. These results indicate mainly zircon type structures with some evidence for the scheelite-type phase and transition to reidite [2].

**Summary:** The shock effects identified in these samples range from shock level 1–7 indicating pressures ranging from 2 to 80 GPa which agrees with previously published estimates for Haughton samples [3]. The progression of shock effects seen in quartz is in keeping with other studies. A number of shock effects have been identified in zircon grains of various shock levels and raman spectroscopy investigations suggest evidence of zircon and scheelite-type phase and possible transition to reidite. [add some more text here to fill in the space]

**References:** [1] Langenhorst F. 2002. *Bulletin of the Czech Geological Survey* 77:265–282. [2] Gucsik A. 2007. Workshop on Impact Cratering II. [3] Stöffler D. 1966. *Contributions to Mineralogy and Petrology* 12:15–24.

## DISTRIBUTION AND ORIGIN OF MAFIC IMPACT EJECTA FROM THE LOCKNE CRATER, SWEDEN

A. S. L. Sjöqvist¹, P. Lindgren², E. F. F. Sturkell¹, and M. R. Lee². ¹Department of Earth Sciences, University of Gothenburg.

²School of Geographical and Earth Sciences, University of Glasgow, E-mail: axel@student.gu.se.

**Introduction:** The Lockne crater formed approximately 455 Myr ago in a target composed of a crystalline Proterozoic basement, overlain by early Palaeozoic sedimentary rocks, and seawater. The inner crater has a diameter of approximately 7 km, with an up to 2.5 km wide brim of ejected, crushed crystalline basement spread around it [1]. Large ejecta blocks within this rim range from a few meters to approximately 100 m in diameter are found as far 15 km from the edge of the crater. The largest ejecta blocks have created smaller, secondary craters around Lockne with diameters of 100–200 m (i.e., Målingen, Karsåtjärnen, and Kloxåsen) [2]. The field distribution of the Kloxåsen ejecta blocks is unknown but determining their relation to the rest of the impact structure is important for a fuller understanding of the formation of Lockne and other similarly sized craters in mixed (crystalline and sedimentary) targets. Here, we have mapped the distribution of ejecta around Kloxåsen. As the chemical and mineralogical composition of the Kloxåsen ejecta blocks is unknown we have also carried out a mineralogical and chemical analysis of this material to establish the source rock of the mafic ejecta blocks, to characterize the extent of hydrothermal alteration, and to determine the levels of shock in the ejecta.

**Results and Discussion:** Mapping of the Kloxåsen area showed that the mafic ejecta breccia is much more widely distributed than previously thought, although the granitic breccia dominates. Optical petrography, SEM-EDS, and Raman spectroscopy revealed extensive and curious hydrothermal alteration in the mafic clasts. Olivine has been completely replaced by serpentine and chlorite that in turn are wholly or partly replaced by calcite (and translucent sphalerite). This strongly depletes the clasts in Fe and Mg, and enriches them in Ca. Titanaugite has been replaced by quartz, chlorite, calcite, and anatase, although unaffected titanaugite grains still occur. Some biotites appear “split open” and are replaced by quartz and anatase, but no kinkbanding has been observed. Titanaugite affected by mechanical twinning suggests shock pressure of at least 5 GPa [3]. Fluid-inclusion analysis in calcite revealed that the hydrothermal system in Kloxåsen is the same as in the rest of the crater [2]. Sector-zoned titanaugite oikocrysts, which are only known to occur in CSDG (Central Scandinavian Dolerite Group) rocks [4], and whole-rock geochemistry conducted in this study, strongly suggest the Åsby dolerite as a protolith for the observed mafic impact ejecta at Kloxåsen.

**Acknowledgments:** The Royal Astronomical Society is thanked for funding this project.

**References:** [1] Lindström M. et al. 2005. *Impact tectonics*, Berlin: Springer. pp. 357–388. [2] Sturkell E. F. F. 1998. *Geologische Rundschau* 87:253–267. [3] Melosh H. J. 1989. *Impact cratering: A geologic process*. New York, NY: Oxford University Press. [4] Claeson D. T. et al. 2007. *Journal of Petrology* 48:711–728.
USING PETROLOGY AND MINERALOGY TO UNDERSTAND THE SURFACE OF VESTA: A COLLECTION OF FINE-GRAINED EUCRITES

S. E. Smith¹, R. G. Mayne¹ and C. M. Corrigan². ¹Monnig Meteorite Collection, School of Geology, Energy, and the Environment, TCU Box 298830, Texas Christian University, Fort Worth, TX 76109, USA. E-mail: samantha.e.smith@tcu.edu. ²Smithsonian Institution, National Museum of Natural History, 10th & Constitution NW, Washington, D.C. 20560-0119, USA.

**Introduction:** Recent work has suggested that fine-grained eucrites could be key to understanding the petrogenesis of Vesta [1]. These eucrites represent original surface or near-surface material that may give us information about Vesta's geologic past. It is unlikely that such units will be visible with the Dawn spacecraft, however, because of the brecciation on the surface of Vesta.

Previous studies, such as [1-3] indicate that individual fine-grained eucrites are not common; however, howardites are the most abundant member of the HEDs family and can contain unique eucrite clasts not found in eucrite-only samples. Therefore, the textural variety of eucrite clast types within howardites is far greater.

By studying these fine-grained eucrite clasts, we can better understand the different units found on the surface of Vesta, and what processes might have produced them.

**Methodology:** In this study, we have examined all howardites in the National Meteorite Collection at the Smithsonian for the presence of fine-grained eucrite clasts. These clasts were mapped using the FEI NovaSEM 600 scanning electron microscope in the Mineral Sciences Department at the Smithsonian. The resulting X-ray maps and BSE images have been used to analyze minerals present, modal abundances, and textures. The mineral chemistries of pyroxene and plagioclase within each of the clasts have been analyzed using the JEOL 9800R/5 electron microprobe, also at the Smithsonian. So far eight fine-grained eucrite clasts have been identified in five howardites.

**Results and Discussion:** After obtaining SEM and microprobe data we determined that there is a variety of textural and compositional features within the fine-grained eucrite clasts. Texurally, we found several clasts that exhibited ophitic textures and granular pyroxene and plagioclase. Some clasts were found to have a quenched, glassy texture and others contained skeletal grains. Compositionally, the fine-grained eucrites encompass both equilibrated and unequilibrated pyroxene compositions suggesting that they experienced a range of metamorphic conditions. Minor elements in pyroxene suggest that they formed from both early primitive and late stage, more evolved melts. There are also a large range of plagioclase compositions such as silica-enriched plagioclase composition glass to crystalline plagioclase with a compositional range of An₆₂-95. Most samples have unequilibrated plagioclase of An~10.

SEM and electron microprobe data show a large variety of textures and compositions within the data set. Such diversity has given us insight into the diverse geologic setting in which these eucrites were formed on Vesta.

**References:** [1] Mayne R. G., Sunshine J. M., McSween H. Y., McCoy T. J., Corrigan C. M. and Gale A. 2010. *Meteoritics & Planetary Science*, in press. [2] Stolper E. 1977. *Geochimica Cosmochimica Acta* 41:587-611. [3] Russell C. T. et al. 2004. *Planetary and Space Science* 52:465-489.

OBSERVATIONS OF ¹²CO-¹³CO PARTITIONING TOWARD SOLAR-TYPE PROTOSTARS AS PROXIES FOR SOLAR SYSTEM CHEMICAL EVOLUTION

R. L. Smith¹, K. M. Pontoppidan², E. D. Young¹,³ and M. R. Morris⁴. ¹Department of Earth and Space Sciences, University of California Los Angeles (UCLA), Los Angeles, CA, USA. E-mail: rsmith@ess.ucla.edu. ²Space Telescope Science Institute, Baltimore, MD, USA. ³Institute of Geophysics and Planetary Physics, UCLA, Los Angeles, CA, USA. ⁴Division of Astronomy and Astrophysics, Department of Physics and Astronomy, UCLA, Los Angeles, CA, USA.

**Introduction:** The solar system [¹²C]/[¹³C] ratio is approximately 86-89 [1-4], values which are peculiar as compared to local interstellar clouds [approximately 62-69; 4-6]. Explanations for this discrepancy in carbon isotopes have invoked Galactic chemical evolution (GCE), and assume that the solar system isotopically represents the local interstellar medium approximately 4.6 Gyr ago, and that presently-evolving protostars should be chemically similar to their parent clouds [7]. This study utilizes high-resolution near-infrared observations of CO toward local young stellar objects and molecular clouds as a probe of protostellar chemical evolution.

**Observations:** Near-infrared spectra of the CO rovibrational bands toward 12 local star-forming regions were observed using the CRIRES instrument at very high resolution (R = 95,000). These data trace several foreground clouds and protostellar envelopes, and two circumstellar disks, thereby representing varying stages of protostellar evolution. Precise CO isotopologue ratios were derived from the observed line profiles; a subset of these observations has been reported [8-10].

**Results:** Derived [¹²CO]/[¹³CO] ratios reveal a large spread (approximately 84–158) as compared to the solar system and local interstellar medium, indicating that protostars may vary significantly in [¹²C]/[¹³C] with respect to their parent clouds. Further, comparisons with the CO ice column densities derived from published optical depths [11] and upper limits for these objects reveal a general trend which suggests that ¹²CO-¹³CO partitioning between the ice and gas may influence the carbon isotopic composition in protostellar reservoirs. It is also apparent that complex chemistry, possibly including isotopic exchange with graphitic, hydrocarbon and/or other carbon reservoirs, likely contributes to the chemical evolution of these systems, and simple ice-gas fractionation is insufficient for explaining the observations to date.

**Conclusions:** A robust set of observations of [¹²CO]/[¹³CO] toward local star-forming regions suggests that protostars are not necessarily similar to their parent clouds in carbon isotopes, violating GCE assumptions for describing early solar system [¹²C]/[¹³C] evolution. Ice-gas partitioning in ¹²CO-¹³CO may play a role in determining the isotopic evolution of protostars, and may have significant implications for understanding the carbon isotopic budget in the early solar system.

**References:** [1] Scott P. C. et al. 2006. *Astronomy & Astrophysics* 456:675-688. [2] Coplen T. B. et al. 2002. *Pure and Applied Chemistry* 74:1987-2017. [3] Clayton D. D. and Nittler L. R. 2004. *Annual Review of Astronomy and Astrophysics* 42:39-78. [4] Wilson T. L. 1999. *Reports on Progress in Physics* 62:143-185. [5] Langer W. D. and Penzias A. A. 1993. *The Astrophysical Journal* 408:539-547. [6] Milam S. N. et al. 2005. *The Astrophysical Journal* 634:1126-1132. [7] Prantzos N. et al. 1996. *Astronomy & Astrophysics* 309:760-774. [8] Smith R. L. et al. 2009. *The Astrophysical Journal* 701:163-175. [9] Smith R. L. et al. 2010. *Meteoritics & Planetary Science* 45:5381. [10] Smith R. L. et al. 2011. Abstract #1281. 42nd Lunar and Planetary Science Conference. [11] Pontoppidan K. M. et al. 2011. *Astronomy & Astrophysics* 408:981-1007.
## OXYGEN ISOTOPE MEASUREMENTS OF SIMULATED WILD 2 IMPACT CRATER RESIDUES

C. J. Snead¹, K.D. McKeegan¹, A.T. Kearsley² and M.J. Burchell³. ¹Department of Earth and Space Sciences, University of California, Los Angeles, Los Angeles, CA 90095-1567, USA. E-mail: star dust2006@ucla.edu. ²Dept. of Mineralogy, The Natural History Museum, London SW7 5BD, UK. ³School of Physical Science, University of Kent, Canterbury, Kent CT2 7NH UK.

**Introduction:** Oxygen isotope measurements of samples from Comet 81P/Wild 2 collected by NASA's Stardust mission have provided surprising similarities to minerals in carbonaceous chondrites which have been interpreted as evidence for large-scale radial migration of dust components from the inner solar nebula to the accretion regions of Jupiter-family comets [1, 2]. If this interpretation is correct, what the studies have so far not achieved is a measurement of the average oxygen isotope composition of the dust component of a known comet parent body. The reason is that the analyses have so far been restricted to large terminal particles because the fine-grained component of impacting particles suffered from intimate mixing with oxygen-rich aerogel. However, the aluminum foil substrates on the Stardust collector provide an opportunity for a low background alternative to the aerogel collectors for measuring the O-isotopic composition of both the coarse and the fine-grained components of impacting dust particles. We have been developing the experimental approaches necessary to assess potential fractionation effects of capture by measuring impact craters produced by projectiles of known composition. Here we present results of oxygen isotope measurements of San Carlos olivine residues shot into aluminum foil targets at Stardust encounter velocities.

**Results:** Simulated impacts were produced by firing San Carlos Olivine into foil targets at approximately 6 km s⁻¹ using a light gas gun at the University of Kent in Canterbury [3]. Craters ranging from 20–50 microns in diameter were cut from the foil target, flattened and pressed into Indium. The samples were then mounted with a San Carlos Olivine standard for ion probe analysis by the UCLA IMS-1270. Measurements were made using a 10 keV, 1–2 nA Cs primary beam; spot size was approximately 20 µm. The detectors were configured for 2-oxygen isotope composition measurements; a Faraday cup was used to measure the ¹⁶O signal, while the ¹⁸O signal was measured using an electron multiplier. Four measured craters contained enough residue to sustain a ¹⁶O count rate >10⁶ counts s⁻¹; δ¹⁸O values for these craters, normalized to the San Carlos Olivine standard were 3.1 ± 0.2 ‰, 2.6 ± 0.4 ‰, 2.0 ± 0.8 ‰, 0.9 ± 1.2 ‰, and 4.2 ± 0.7 ‰. Quoted uncertainties are 1 SD.

**Discussion:** Crater measurements with ¹⁶O count rates > 10⁶ counts s⁻¹ reproduced the standard value of San Carlos olivine to within 4‰. The most accurate measurements are correlated with higher ¹⁶O count rates. Sample preparation of impact craters has a strong effect on the level of precision that can be achieved; craters must be flattened in order avoid distortion of the accelerating electric field and inaccuracies in the instrumental mass fractionation correction.

**References:** [1] McKeegan K. D. et al. 2006. *Science* 314:1724-1728. [2] Nakamura T. et al. 2008. *Science* 321:1664-1667. [3] Kearsley A. T. et al. 2006. *Meteoritics & Planetary Science* 41:167-180.

## ARAGONITE IN THE CM CARBONACEOUS CHONDRITES: A PROXY FOR THE MAGNITUDE OF AQUEOUS ALTERATION

M. R. Sofe, M. R. Lee and P. Lindgren. School of Geographical and Earth Sciences, University of Glasgow, Glasgow G128QQ, UK. E-mail: msofe@glasgow.ac.uk.

**Introduction:** The degree of aqueous alteration of CM carbonaceous chondrites can be tracked by changes in properties including in the volume of fine-grained matrix, the volume and chemical composition of its constituent phyllosilicates, and the extent of alteration of chondrules [e.g., 1-3]. The carbonate minerals aragonite, calcite and dolomite were also formed during alteration and whereas calcite is seen in all CMs, dolomite has been recorded only from the more highly altered meteorites [e.g., 4] and little is known about the occurrence of aragonite. Here we have sought to understand better the significance of the correspondence between carbonate mineralogy and degree of alteration by undertaking a systematic study of intermeteorite differences in the abundance and properties of aragonite.

**Methods:** Six CM2s were studied, namely Pollen, Murray, Mighei, LON 94101, Nogoya, and QUE 93005 (listed in the approximate order of increasing degree of alteration), together with the CM1 SCO 06043. Dolomite was identified by SEM imaging and X-ray microanalysis, and aragonite was distinguished from calcite using electron backscatter diffraction and Raman spectroscopy.

**Results:** Aragonite occurs in the matrices of five of the meteorites studied and is absent from the most highly altered samples (i.e., QUE 93005 and SCO 06043). Only Nogoya contains both aragonite and dolomite. The approximate abundance of aragonite, expressed as grains mm⁻², is: Pollen (0.20), Murray (0.30), Mighei (0.23), LON 94101 (0.10), Nogoya (0.17). The mean sizes of these aragonite grains are also similar: Pollen (21 µm), Murray (22 µm), Mighei (33 µm), LON 94101 (25 µm), Nogoya (14 µm). The largest grain (101 µm) was found in Mighei. Unlike calcite, the aragonite grains usually lack serpentine-tochilinite rims, although many of them have been partially replaced by matrix phyllosilicates. Most of the crystals occur in loose clusters and their c axes have a common alignment.

**Discussion:** As aragonite crystals are embayed by phyllosilicates they formed prior to the end of aqueous activity, and their common alignment suggests crystallization under uniaxial lithostatic pressure at depth within the parent body. Given that aragonite occurs only in the less altered meteorites and dolomite in the more highly processed CMs, with the crossover at Nogoya (classified as CM2.2 by [2]), these minerals track the degree of aqueous alteration by their presence or absence. We suggest that crystallization of aragonite rather than calcite is favored in solutions with Mg/Ca > approximately 1, and significantly higher Mg/Ca ratios will yield dolomite. If aragonite and dolomite crystallized after calcite, then the Mg/Ca of bicarbonate rich solutions must have increased with time, and reached significantly greater values in those meteorites that were more highly altered overall.

**Acknowledgments:** We thank the Natural History Museum (London) and NASA-JSC for loan of the samples studied.

**References:** [1] McSween H. Y. 1979. *Geochimica et Cosmochimica Acta* 43:1761-1770. [2] Rubin A. E. et al. 2007. *Geochimica et Cosmochimica Acta* 71:2361-2382. [3] Howard K. T. et al. 2009. *Geochimica et Cosmochimica Acta* 73:4576-4589. [4] Johnson C. A. and Prinz M. 1993. *Geochimica et Cosmochimica Acta* 57:2843-2852.
CL ZONING OF CALCITE IN CM CARBONACEOUS CHONDRITES AND ITS RELATIONSHIP TO DEGREE OF AQUEOUS ALTERATION

M. Sofe¹, M. R. Lee¹, P. Lindgren¹ and C. L. Smith². ¹School of Geographical & Earth Sciences, University of Glasgow, Glasgow G12 8QQ, UK. ²Department of Mineralogy, Natural History Museum, London, SW7 5BD, U.K. E-mail: Mahmood.Sofe@ges.gla.ac.uk.

**Introduction:** Carbonates occur in CM2 carbonaceous chondrites in a variety of phases including: Ca-carbonate (aragonite and calcite), dolomite and breunnerite [e.g., 1, 2]. Petrographic work has revealed multiple generations of Ca-carbonates including aragonite, calcite replacing olivine/pyroxene, calcite rimmed by tochilinite and partially or completely replaced by phyllosilicates, and calcite grains that are free of inclusions and rims. In this study we have sought to understand the conditions of Ca-carbonate crystallization within the CM parent body and how they may have varied between meteorites that show different magnitudes of aqueous alteration.

**Methods:** This study used the following CM2 falls: Murchison, Murray, Pollen, Mighei, and Nogoya. Over 300 grains were located and their textures and microstructures characterized using backscattered electron (BSE) and cathodoluminescence (CL) microscopy. Work by laser Raman spectroscopy and electron probe microanalysis (EPMA) was also undertaken for mineral identification and chemical analysis respectively.

**Results and Discussion:** Based on the aqueous alteration index of CM chondrites of [3], we related CL intensity and patterns of zoning in the Ca-carbonates into degree of alteration. The least altered meteorites studied (Murchison, Murray, Pollen and Mighei) contain 1.3–2.0 vol.% of Ca-carbonate (both aragonite and calcite) that have a variety of CL characteristics. All calcite grains that have replaced olivine/pyroxene and many grains of aragonite and non-rimmed calcite have a low CL intensity and are unzoned, and this can be related to the stability of fluid composition during crystal growth. Some aragonite and non-rimmed calcite grains, and all of calcites rimmed with tochilinite, display one of three styles of CL emission. (1) Concentric zoning with a triangle-shaped seed crystal in one part, subsequent zoning of hexagonal habit, and other bands of zoning to fill the pore space. Growth of this type was controlled by the change in fluid composition and delivery of ions to the growing crystal face. (2) Grains that have only a high CL intensity throughout as a consequence of compositional stability of the fluid during the crystal growth; however this fluid was different in chemical composition to that which crystallized grains with a low CL intensity. (3) Some grains have patchy low and high CL intensity which possibly reflects a complex growth history as a result of a changeable fluid chemical compositions. Ca-carbonate grains in Nogoya, the more highly aqueously altered CM2, show no CL variation and a low emission intensity; this implies that they crystallized from fluids that were stable in chemical composition. This correspondence between CL properties and degree of alteration agrees very well with the finding by [4] that concentrations of manganese and iron in the CM2 carbonates decrease with increasing alteration of their host rock.

**References:** [1] Lee M. et al. 2010. *Meteoritics & Planetary Science* 45:A115. [2] Brearley A. J. 1998. Abstract #1246. 29th Lunar and Planetary Science Conference. [3] Rubin E. et al. 2007. *Geochimica et Cosmochimica Acta* 71:2361–2382. [4] De Leuw S. et al. 2010. *Meteoritics & Planetary Science* 45:513–530.

$^{60}$Fe-$^{60}$Ni SYSTEMATICS IN THE ANGRITES

L. J. Spivak-Birndorf, M. Wadhwa and P. E. Janney. School of Earth and Space Exploration, Arizona State University, Tempe, AZ 85287-1404, USA. E-mail: Lev.Spivak-Birndorf@asu.edu.

**Introduction:** The abundance of $^{60}$Fe ($t_{1/2}$ = 2.6 Ma) in the early Solar System is not well constrained. Previously reported estimates of the initial $^{60}$Fe/$^{56}$Fe ratio vary considerably depending on the samples and techniques used (e.g., [1–3]). We have begun an investigation of the $^{60}$Fe-$^{60}$Ni systematics in angrites to constrain the abundance and distribution of $^{60}$Fe in the early Solar System, preliminary results of which were presented earlier [4]. Here we report Fe-Ni data for additional angrite samples.

**Materials and Methods:** Nickel isotopes and Fe/Ni ratios were measured by ICP-MS using methods described by [4]. The study includes two bulk samples and two mineral separates (olivine and pyroxene) from the quenched angrite D'Orbigny, as well as bulk samples of the plutonic angrites NWA 4590, NWA 4801, and NWA 6291 (paired with NWA 2999).

**Results:** The three plutonic angrite samples span a wide range of $^{56}$Fe/$^{58}$Ni ratios (approximately 70–10,000), and all have Ni isotopic compositions identical to the terrestrial value within analytical errors. In contrast, while bulk samples and mineral separates of D'Orbigny also span a wide range of $^{56}$Fe/$^{58}$Ni ratios (approximately 4600–11,000), all have excesses in $^{60}$Ni that correlate with their Fe/Ni ratios. The D'Orbigny samples define an internal $^{60}$Fe-$^{60}$Ni isochron corresponding to a $^{60}$Fe/$^{56}$Fe = (3.4 ± 1.5) × 10⁻⁹ and $^{60}$Ni₀ = -0.18 ± 0.29 at the time of the last equilibration of Ni isotopes in this meteorite.

**Discussion:** A recent investigation of $^{60}$Fe-$^{60}$Ni systematics in angrites showed that $\epsilon^{60}$Ni* is correlated with Fe/Ni ratios in bulk samples of D'Orbigny, Sahara 99555 (SAH 99555), and NWA 2999 [3]. The authors of that study interpreted this relationship as a whole-rock (WR) isochron corresponding to the timing of widespread Fe/Ni fractionation during differentiation of the angrite parent body. They further argue that the source reservoirs of the various angrites formed contemporaneously and have remained closed systems with respect to their $^{60}$Fe-$^{60}$Ni systematics. However, D'Orbigny and SAH 99555 crystallized at the same time [5, 6] and the $^{60}$Fe-$^{60}$Ni systematics of NWA 2999 are very similar to that of chondrites, making it possible that the angrite WR isochron reported by [3] actually corresponds to the time of formation of D'Orbigny and SAH 99555 from a chondritic source. Bulk samples of plutonic angrites NWA 4590 and NWA 4801 have $^{56}$Fe/$^{58}$Ni ratios as high as approximately 10,000 and should have excess $^{60}$Ni if their sources formed contemporaneously with those of the quenched angrites, as suggested by [3]. However, we find no $^{60}$Ni excesses in these samples indicating that $^{60}$Fe was extinct by the time the source reservoirs of these meteorites formed. Assuming that NWA 4590 and NWA 4801 formed at the same time, we determine a $^{60}$Fe/$^{56}$Fe ≤ 9 × 10⁻¹⁰ when this event occurred. This corresponds to an age of ≤4558.3 Ma relative to D'Orbigny, which is consistent with Pb-Pb ages reported for the plutonic angrites [5]. Finally, the $^{60}$Fe-$^{60}$Ni systematics of D'Orbigny and the age difference between CAIs and this angrite [7] allows us to estimate an initial Solar System $^{60}$Fe/$^{56}$Fe of (1.2 ± 0.6) × 10⁻⁸.

**References:** [1] Chen J. H. et al. 2009. *Geochimica et Cosmochimica Acta* 73:1461–1471. [2] Tachibana S. et al. 2006. *The Astrophysical Journal* 639:L87–L90. [3] Quitté G. et al. 2010. *The Astrophysical Journal* 720:1215–1224. [4] Spivak-Birndorf L. J. et al. 2011. Abstract #2281. 42nd Lunar and Planetary Science Conference. [5] Amelin Y. 2008. *Geochimica et Cosmochimica Acta* 72:221–232. [6] Amelin Y. 2008. *Geochimica et Cosmochimica Acta* 72:4874–4885. [7] Bouvier A. and Wadhwa M. 2010. *Nature Geoscience* 3:637–641.
## SHOCK-INDUCED MELT POCKET FORMATION IN NWA 4468: EVIDENCE FOR PHASE TRANSFORATION INDUCED IMPLOSION

J. G. Spray¹ and S. Boonsue¹. ¹Planetary and Space Science Centre, University of New Brunswick, Fredericton, NB E3B 5A3, Canada. E-mail: jgs@unb.ca.

Northwest Africa (NWA) 4468 is 675 g Martian meteorite recovered from the Western Sahara in 2006 [1]. It is an olivine basaltic shergottite, comprising clinopyroxene, olivine, maskelynite, chromite, ilmenite, Ca-phosphates, troilite and pyrrhotite. There have been several studies of NWA 4468, including its geochronology, petrology and bulk composition, but aspects of its shock history are less well constrained. The meteorite exhibits both bulk (maskelynite formation) and localized shock effects, the latter in the form of shock veins and melt pockets that contain various high pressure and/or high temperature polymorphs, glasses and neocrystallites. This work presents new results following extensive melt pocket characterization based on analytical field emission scanning electron microscopy and micro-Raman spectroscopy. The melt pockets are typically 2–5 mm in diameter and subspherical. Three components are identified in the melt pockets (1) fragmented/fractured mineral phases inherited from the bulk sample, (2) inherited mineral phases that have undergone solid-state phase transformation (e.g., olivine to ringwoodite), and (3) glasses and neocrystalline phases that originated via fusion. Metal phases (mainly Fe-Ti-Cr oxides) have undergone melting to yield disseminated clouds of immiscible spheres set within a silica-rich glass matrix. Three Ca-phosphate polymorphs have been recognized [2, 3]: α-, β- and γ- apatite (chlorapatite, merrillite and tuite, respectively), with their structures confirmed by Raman spectroscopy. Ca-hollandite and stishovite are also present. Evidence for a possible new high-pressure polymorph of low-Ca clinopyroxene is also presented and discussed. Ringwoodite is developed at the rim of the pockets, which, overall, suggest an increasing pressure gradient from periphery to core. Most if not all melt pockets are characterized by the presence of Ca-phosphate phases in their core regions. We propose that the relatively low pressure transformation of apatite to merrillite, which took place during the bulk shock event, may have provided the necessary energy for the thermal-pressure spike as a result of concomitant volume reduction and associated implosion.

**References:** [1] Irving A. J. et al. 2007. Abstract #1526. 38th Lunar Prospector Spacecraft. [2] Beck P. et al. 2007. *Geophysical Research Letters* 34:L01203, doi:10.1029/2006GL028141. [3] Xie X. et al. 2002. *Geochimica et Cosmochimica Acta* 66:2439–2444.

## CARBON RAMAN SPECTROSCOPY: NATURAL AND EXPERIMENTALLY-INDUCED VARIATIONS IN IDPS

N. H. Spring and H. Busemann. SEAES, The University of Manchester, UK. E-mail: nicole.spring-2@postgrad.manchester.ac.uk.

**Introduction:** Carbon Raman spectroscopy has been shown to complement other techniques in identifying levels of thermal alteration in meteorites. As it has high spatial resolution and is relatively non-destructive, it can also be applied to micron-sized IDPs. Many IDPs are thought to originate from comets rather than asteroids [1], and so are expected to show lesser degrees of parent body thermal alteration than meteorites. Indeed, the majority of IDPs contain organic matter (OM) that is similar to or more disordered than the most primitive insoluble organic matter (IOM) separated from meteorites [2, 3]. In particular, some of the IDPs collected to coincide with the passing though comet Grigg-Skjellerup's dust stream (GSC) show Raman characteristics which are among the most primitive to date, complementing NanoSIMS isotope data on H and N which conclude the same [4]. However, average parameters for GSC-IDPs, normal IDPs and the most unaltered IOM are similar, suggesting asteroids and comets acquired their OM from a common reservoir.

We have applied this technique to 27 IDPs to determine the degree of order of their OM, and identify the most primitive and C-rich particles for a detailed NanoSIMS study. Fragments of these IDPs have also been analyzed with FTIR spectroscopy [5]. Ultimately they will be examined in RELAX, a time of flight mass analyzer for Xe [6]. The samples are cluster particles of varying textures and compositions from eight different collections.

**Method:** We used a Renishaw inVia Raman microscope with a 514 nm laser, based at Manchester Metropolitan University to determine D and G band widths Γ and positions ω. Meteoritic IOM separates from several meteorite types were used as standards. The IDPs were unpressed and positioned on gold foil covered SEM stubs. The laser was focused using x50 magnification. The spot size was ~ 2 µm and the laser power was kept < 30 µW.

**Results:** Of the 20 IDPs yielding Raman parameters with low errors, all but three plot alongside the most primitive IOM (EET 92042, Semarkona, Murchison), agreeing with other studies that show IDPs have the most disordered C, and therefore least thermal alteration, adding to the likelihood that they are cometary material. The OM of the GSC particle is the most amorphous from the G band data, possibly indicative of irradiation in space.

Our data are comparable with literature trends (taken with 532 nm excitation [2]) if shifts in D and to a much lesser extent G band are accounted for. These shifts can be explained by (i) different laser excitation, whereby D and G ω decrease with increasing excitation wavelength [7]. (ii) Shifts due to laser power, whereby we observe differences of up to 100 cm⁻¹ in D band width between 25 and 125 µW for most disordered IOM. Given the smaller beam in the previous study [2], the power density they used was significantly greater than 125 µW. Adjusted accordingly, trends for IDPs and IOM agree with previous studies.

**Acknowledgments:** We thank V. Vishnyakov at the MMU DRIAM facility funded by EPSRC grant EP/F056117/1.

**References:** [1] Nesvorný D. et al. 2010. *The Astrophysical Journal* 713:816–836. [2] Busemann H. et al. 2011. Abstract #4086. *Corals II*. [3] Starkey N. A. and Franchi I. A. 2011. Abstract #1938. 42nd Lunar and Planetary Science Conference. [4] Busemann H. et al. 2009. *Earth and Planetary Science Letters* 288:44–57. [5] Busemann H. et al. 2011. *Meteoritics & Planetary Science* 46. This conference. [6] Busemann H. et al. 2010. Abstract #1947. 41st Lunar and Planetary Science Conference. [7] Ferrari A. C. and Robertson J. 2001. *Physical Review B* 64:075414.
## THE SUSCEPTIBILITY OF PHASE Q TO PYRIDINE: ARE CI CHONDRITES UNIQUE?

N. Spring¹, H. Busemann¹, N. Vogel², L. Huber², R. Wieler², C. Maden² and C.M.O'D. Alexander³. ¹SEAES, University of Manchester, UK. E-mail: nicole.spring-2@postgrad.manchester.ac.uk. ²ETH Zurich, Switzerland. ³Carnegie Institution of Washington, USA.

**Introduction:** The origin of the primordially trapped noble gases in the elusive phase Q is an unresolved question in meteoritics [1]. Acid-resistant residues, enriched in insoluble organic matter (IOM) in all meteorite classes contain Q-gases of remarkably similar composition, that are strongly fractionated relative to solar gases [2, 3]. Q-gas signatures in achondrites and comet Wild 2 dust [4, 5] show that phase Q was ubiquitously incorporated during accretion in the inner and outer solar system. Pyridine applied to the IOM of CI Orgueil released large amounts of Q-Xe due to swelling [6, 7]. In contrast, pyridine did not affect phase Q in LL, CV, CR and CM chondrites [7-9]. This may suggest that different types of phase Q exist. Either phase Q was modified due to particular conditions on the CI parent body (e.g., severe aqueous alteration) or processes on all other parent bodies (e.g., mild thermal alteration) modified a pyridine-susceptible fraction. Here we present noble gas data on IOM from pristine meteorites. In particular, we show that phase Q in CIs Ivuna and Orgueil are not affected by pyridine.

**Experimental:** We used an IR laser to extract noble gases in IOM (<1-5 mg) of CIs Orgueil and Ivuna, CV3 Allende, L3.05 QUE 97008, C2 Tagish Lake. IOM aliquots before and after treatment with pyridine were compared [see 8 for details]. Nb tubes were used to avoid sample loss during heating. As quantitative gas release has not been reached for all samples yet, we compare isotope and element ratios, which can readily distinguish the release of Q-gases [8, 9], whilst avoiding ambiguity due to sample loss during preparation and extraction.

**Results and Discussion:** All residues contain large amounts of Q noble gases, in agreement with their primitive character. All isotope and elemental ratios are in the range expected for IOM [2, 3]. As demonstrated earlier [8], the reported 60% loss of phase Q [6] would be visible in the elemental ratios (favoring the lighter element) and also Xe isotopes (Xe-HL would be more dominant). However, even the most suitable Xe isotopic ratios are typically identical (within 3%) in treated and untreated IOM. Similarly, the element ratios vary by typically 10-30%, however, without a systematic trend, hence not supporting the idea of phase Q loss due to pyridine treatment. To the contrary, more ratios point to a depletion of presolar phases relative to phase Q, which hence may simply reflect sample heterogeneity. Most importantly, treated and untreated CI Ivuna IOM shows similar ratios and we do not observe a significant loss of Q-gases in Orgueil. The CI parent body acquired and still carries the same phase Q and its noble gases as all other groups, despite different alteration processes.

**References:** [1] Wieler R. et al. 2006. In *Meteorites and the early solar system II*: 499-521. [2] Busemann H. et al. 2000. *Meteoritics & Planetary Science* 35:949-973. [3] Huss G. R. et al. 1996. *Geochemistry, Geophysics, Geosystems* 6:3311-3340. [4] Busemann H. and Eugster O. 2002. *Meteoritics & Planetary Science* 37:1865-1891. [5] Marty B. et al. 2008. *Science* 319:75-78. [6] Marrocchi Y. et al. 2005. *Earth and Planetary Science Letters* 236: 569-578. [7] Matsuda J.-I. et al. 2010. *Meteoritics & Planetary Science* 45:1191-1205. [8] Spring N. et al. 2010. Abstract #2640. 41st Lunar and Planetary Science Conference. [9] Busemann H. et al. 2008. Abstract #1777. 39th Lunar and Planetary Science Conference.

## THE MASON GULLY METEORITE FALL IN SW AUSTRALIA: FIREBALL TRAJECTORY AND ORBIT FROM PHOTOGRAPHIC RECORDS

P. Spurný¹, P. A. Bland²,³, L. Shrbený¹, M. C. Towner², J. Borovička¹, A. W. R. Bevan⁴ and D. Vaughan⁵. ¹Astronomical Institute of the Academy of Sciences, Fričova 298, CZ-251 65 Ondřejov Observatory, Czech Republic. E-mail: spurny@asu.cas.cz. ²IARC, Department of Earth Sci. & Eng., Imperial College London, SW7 2AZ, UK. ³Dept. Applied Geol., Curtin University of Technology, GPO Box U1987, Perth WA 6845, Australia. ⁴Dept. Earth and Planet. Sci., Western Australian Museum, Locked Bag 49 Welshpool DC, WA 6986, Australia. ⁵PO BOX 187, Nedlands, Perth, WA 6909, Australia.

**Introduction:** We report a new instrumentally recorded meteorite fall, named Mason Gully, which was recorded by the Desert Fireball Network (DFN), an ambitious project dedicated to mapping of fireballs over the remote area of the Nullarbor Region of SW Australia, a very suitable place for meteorite recoveries [1]. This is the second case of such successful recovery after Bunburra Rockhole, the major milestone of this project [2].

**Results and Discussion:** The reported meteorite fall occurred over SW Australia on April 13th, 2010 at $10^h36^m12.7^s \pm 0.1^s$ UT (maximum brightness). The fireball designated DN130410 was recorded photographically and photoelectrically by two eastern stations of the DFN. Its luminous trajectory started at an altitude of 83.46 km and after a 73.8 km long flight terminated at an altitude of 23.84 km. The angle of the atmospheric trajectory to the Earth's surface was $53.9^\circ$. The object, with initial mass of about 40 kg, entered the atmosphere with a very low speed of $14.53 \text{ km s}^{-1}$, and during a 6.0-s long flight decelerated to the terminal speed of $4.1 \text{ km s}^{-1}$, when the fireball brightness decreased below the sensitivity limit of both DFOs. The maximum absolute (100-km distance) brightness of $-9.4$ magnitude was reached in a short flare at an altitude of 35.8 km. The initial meteoroid before its collision with Earth orbited the Sun on a very low inclined orbit defined by the following parameters: semimajor axis (AU): $2.470 \pm 0.004$; eccentricity: $0.6023 \pm 0.0007$; perihelion distance (AU): $0.98240 \pm 0.00007$; aphelion distance (AU): $3.958 \pm 0.008$; argument of perihelion (°): $18.95 \pm 0.03$; longitude of ascending node (°): $203.2112$; inclination (°): $0.832 \pm 0.013$; period (years): $3.882 \pm 0.009$. All angular elements are given in J2000 equinox. This is one of the most precise orbits ever determined for a meteorite. The computation of the darkflight, and determination of impact position was complicated by very strong lateral stratospheric winds reaching $45 \text{ m s}^{-1}$, shifting the resulting impact position for the meteorite of the recovered size (25 g) by about 6 km aside! In spite of this unfavorable fact the meteorite was found up to 150 m from the predicted site (for more details see [3]). This case along with the Bunburra Rockhole meteorite clearly show that this project in the remote and hostile environment of the Australian desert is meaningful, and that these observations can bring data of fundamental value to solar system research. We also proved the correctness of our observational and computational methods and models.

**References:** [1] Bland P. A. et al. 2006. *Meteoritics & Planetary Science* 41:5197. [2] Bland P. A. et al. 2009. *Science* 325:1525. [3] Towner M. C. et al. 2011. *Meteoritics & Planetary Science* 46, This conference.
NANOSIMS OXYGEN ISOTOPE ANALYSES ON GRIGGS-KJELLERUP COLLECTION IDPS

N. A. Starkey¹, I. A. Franchi¹ and J. Davidson². ¹PSSRI, Open University, Milton Keynes, MK7 6AA, UK. E-mail: n.starkey@open.ac.uk. ²Lunar & Planetary Laboratory, University of Arizona, Tucson, AZ 85721-0092, USA.

**Introduction:** The oxygen isotope composition of meteorites has revealed much about the origin and history of inner solar system materials and processes operating across a large volume of the protoplanetary disk. Our knowledge of oxygen isotope signatures beyond the asteroid belt is limited to a few analyses of particles from comet Wild 2 [1-4] and anhydrous CP-IDPs [4-7]. Almost all such analyses studied large, ferromagnesian silicate crystals with the predominant fine-grained material largely overlooked. While the fine grain size of this material prohibits isotopic measurements on a grain by grain basis, the signature and variability of the bulk isotopic composition of this material offers insight into early solar system oxygen isotopic reservoirs. Here we present NanoSIMS 50L high precision three-oxygen isotope spot analyses on a set of Grigg-Skjellerup collection IDPs from a larger sample set previously characterized by SEM, Raman and NanoSIMS (C,N,O isotope mapping) [8]. All the IDPs are anhydrous fine-grained particles containing presolar grains emphasizing the primitive nature of the samples [8].

**Method:** Particles were pressed into gold foil and a ≈ 50pA probe was rastered over 5 × 5µm² areas for analyses with ¹⁶O measured on a Faraday cup and ¹⁷O, ¹⁸O and ²⁴Mg¹⁶O on electron multipliers. The mass resolution was set to >10,000 (Cameca-definition). Isotope ratios (δ¹⁷O and δ¹⁸O) were normalized to SMOW using analyses of San Carlos olivine. The mixed mineralogy of the fine-grained samples precludes any matrix effect correction, with variations between the olivine standard and the main phases in the IDPs thought to be negligible. Combined 2σ precision from sets of five analyses on Eagle Station yield ±0.8‰ for δ¹⁸O and ±1.2‰ for δ¹⁷O, and mean values within 2σ of true.

**Results:** The IDPs display a wide range in δ¹⁸O values from approximately 0‰ to < -30‰, plotting close to, or above the CCAM line. The offset from the CCAM line appears to be related to ¹⁶OH interference revealed by a strong linear relationship with ¹⁶OH abundances. The reason ¹⁶OH is impacting the IDP results remains unclear as no such effect is observed for the olivine standard or Acfer094 matrix pressed into gold (despite similarly high ¹⁶OH). δ¹⁸O are far less susceptible to interference and therefore reflect the true values of the IDPs, most likely falling along the CCAM line. With two samples having δ¹⁸O values of -25 and -31‰, these results extend the limited range of oxygen isotopes previously reported for IDPs. While the mineralogical composition of each sputtered area is unknown, it may contain many thousands of small grains and it is therefore unlikely that such large variations are the result of random sampling of refractory ¹⁶O-rich material. These results indicate a much larger range in bulk compositions of cometary materials compared to that from asteroids, recording more primitive signatures of reservoirs in the early solar system.

**References:** [1] McKeegan K. D. et al. 2006. *Science* 314:1724-1728. [2] Nakamura T. et al. 2008. *Science* 321:1664-1667. [3] Joswiak D. J. et al. 2010. *Abstract* #2119. 41st Lunar and Planetary Science Conference. [4] Nakashima D. et al. 2011. *Abstract* #1240. 42nd Lunar and Planetary Science Conference. [5] Messenger et al. 2003. *Science* 300:105-108. [6] Aleon J. et al. 2009. *Geochimica et Cosmochimica Acta* 73:4558-4575. [7] Floss C. et al. 2011. *Meteoritics & Planetary Science* 45:1889-1905. [8] Davidson J. 2009. Ph.D. thesis, Open University.

OXYGEN ISOTOPE ANALYSIS OF A CHONDRULE-LIKE WILD 2 TERMINAL PARTICLE USING NANOSIMS

N. A. Starkey¹, I. A. Franchi¹, J. C. Bridges², H. G. Changela²,³ and L. J. Hicks². ¹PSSRI, Open University, Milton Keynes MK7 6AA, UK. E-mail: n.starkey@open.ac.uk. ²SRC, University of Leicester, Leicester LE1 7RH, UK. ³NRL, 4555 Overlook Ave. SW, Washington, D.C. 20375, USA.

**Introduction:** Oxygen isotopes provide a key tool in determining origins and links of diverse solar system materials. SIMS analyses of terminal particles returned from comet Wild 2 with sufficient analytical precision for useful comparison to meteorites are very limited [1-3]. Generally, analyses of grains have used potted butts or single grains pressed into gold in order to provide sufficient sample thickness to analyse with high current probes. At most, only one potted butt can exist for any characterized terminal particle and in many cases it is required for other analytical techniques or no longer contains cometary material. Here we utilize the more readily available microtomed sections as a means of extracting useful oxygen isotopic information. NanoSIMS 50L isotope imaging mode allows the material available for analysis from these very thin sections (≈70 nm) to be maximized, although analytical uncertainty reported by others using this technique is large [4].

**Sample:** The microtomed TEM section is from Stardust terminal particle #2, Track 154 (C2063,1,154,1,15). This approximately 4 × 1.5 µm² particle has previously been characterized by TEM [5] and is composed of Al-rich diopside and pigeonite with minor enstatite and forsterite, and is thought to be a fragment of an Al-rich chondrule rather than a CAIs [5].

**Analyses:** The back of the TEM grid containing the sections was reinforced with a 200 nm gold coat and a further 20 nm gold coat was deposited on the top of the sections to facilitate charge dissipation. A 2pA probe was rastered over an 8 × 8 µm² area for analysis. The Cameca-defined mass resolution was >10,000 with ¹⁶,¹⁷,¹⁸O, ²⁸Si and ²⁴Mg¹⁶O measured on electron multipliers. δ¹⁷O and δ¹⁸O were normalized to SMOW from San Carlos olivine measurements. Results were processed using L'image software and corrected for position drift, QSA effects and detector aging. The difference in matrix effect between the low-FeO pyroxene and the olivine standard are thought to be negligible compared to the precision and a correction is not applied. Reproducibility determined from analyses on comparable areas of San Carlos olivine was 0.9‰ for δ¹⁸O and 4‰ for δ¹⁷O and analyses of Eagle Station gave correct values within error of true value.

**Results:** The particle was surrounded by aerogel with the sample area being defined on the basis of ²⁴Mg¹⁶O and ¹⁶O counts. The isotopic composition of the particle was determined as: δ¹⁸O = -8.2 ± 2.3‰ and δ¹⁷O = -5.0 ± 5.4‰ (1σ errors). On a three-isotope diagram the sample plots just above (but well within 2σ) of the CCAM line. The isotopic composition is indistinguishable from that of Al-rich chondrules found in carbonaceous chondrites, in agreement with its mineralogy [5], and clearly distinct from unaltered CAIs-like materials. The chondrule-like compositions are similar to those reported previously [1-3] and support a common origin for chondrules in asteroids and comets.

**References:** [1] McKeegan K. D. et al. 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] Joswiak D. J. et al. 2010. *Abstract* #2119. 41st Lunar and Planetary Science Conference. [5] Bridges J. C. and Changela H. G. 2010. *Abstract* #2058. 41st Lunar and Planetary Science Conference.
RAMAN VARIATION OF ORGANIC MATTER IN LARGE
CLUSTER IDPs AND METEORITIC IOM

N. A. Starkey¹, I. A. Franchi¹ and C. M. O'D. Alexander². ¹PSSRI, Open University, Milton Keynes MK7 6AA, UK. E-mail: n.starkey@open.ac.uk. ²DTM, Carnegie Institution of Washington, 5241 Broad Branch Road, Washington, D.C. 20015, USA.

**Introduction:** CP-IDPs display a range of primitive features indicative of a cometary origin. The organic material (OM) present in IDPs may be the result of formation processes that occurred across a large volume of the protoplanetary disk to small-scale on the cometary body itself. Raman microscopy offers a nondestructive approach for determining general characteristics of the OM in IDPs. Compared to the large number of potential parent bodies or the huge volume of the protoplanetary disk that may have contributed to the formation of comets, the number of IDPs analyzed to date remains relatively small. Raman measurements were performed on nine IDPs (from five large cluster particles; collectors L2005 and L2006) offering the opportunity to investigate the variability of the OM on a number of scales. Previous Raman analyses of IDPs and IOM from meteorites have used a number of different wavelengths, predominantly green. While it has been shown that D band position (ωD) is influenced by excitation λ [1], it is rarely quantified. Interestingly, the wavelength dependent shift in the ωG is known to vary with degree of ordering of OM. Multiwavelength analyses made here offer an independent measure of organic maturity.

**Results:** Meteoritic IOM (samples as in [2]) and IDPs mapped using 514 nm and 473 nm lasers showed a reproducible downshift of 10–13 cm⁻¹ in ωD. For ωG, the IDPs displayed a larger downshift (approximately 4 cm⁻¹) than the IOM (negligible). The range of ωD and ωG in any one IDP cluster is limited, with values comparable to meteoritic IOM. For particles within individual clusters, ωD varies by <6 cm⁻¹ and ωG by <3 cm⁻¹. Each cluster IDP displays distinctive internal peak parameter covariation.

**Discussion:** The large ΓD and ΓG, high ωD, low ωG and ID/IG ratios of 0.9–1.3 indicate that the IDPs are all primitive. The samples plot near to the CI, CM and CR field. The dispersion in ωG with excitation λ is related to the degree of disordering of the carbon [1], and therefore the greater ωG dispersion observed for the IDPs suggests that the OM in the IDPs may be more disordered than meteoritic IOM, in contrast to their similar peak parameters. The IDPs selected for this study do not appear to be as primitive as Grigg-Skjellerup Collection (GSC) particles [3], but are comparable to other non-GSC IDPs [4].

While great isotopic and structural variability exists within individual particles at the sub-µm scale (e.g., [3]), these results indicate that the OM within IDPs is reasonably homogeneous at the few to tens of µm-scale, and that there is greater variability between particles than within. The variability between particles most likely reflects accretion from different reservoirs, potentially on the scale of individual comets given the distinctive signatures from GSC-IDPs [3]. Secondary processes on the comet, in interplanetary space or during atmospheric entry may also play a role despite low peak metamorphic temperatures (<200 °C (after [5]).

**References:** [1] Ferrari A. C. and Robertson J. 2001. *Physical Review B* 63:075414. [2] Alexander C. M. O'D. et al. 2007. *Geochimica et Cosmochimica Acta* 71:4380–4403. [3] Busemann H. et al. 2009. *Earth and Planetary Science Letters* 288:44–57. [4] Davidson J. 2009. Ph.D. thesis, Open University. [5] Busemann H. et al. 2007. *Meteoritics & Planetary Science* 42:1387–1416.

CHILI—APPROACHING THE FINAL FRONTIERS IN LATERAL
RESOLUTION AND SENSITIVITY FOR ISOTOPIC AND
CHEMICAL ANALYSIS

T. Stephan¹,²,³, A. M. Davis¹,²,⁴, M. J. Pellin¹,²,³,⁴, M. R. Savina¹,³, I. V. Veryovkin¹,³, A. J. King¹,², N. Liu¹,²,³, R. Trappitsch¹,²,³ and R. Yokochi¹,². ¹Chicago Center for Cosmochemistry. E-mail: tstephan@uchicago.edu. ²Department of the Geophysical Sciences, University of Chicago, Chicago, IL 60637, USA. ³Materials Science Division, Argonne National Laboratory, Argonne, IL 60439, USA. ⁴Enrico Fermi Inst., University of Chicago, Chicago, IL 60637, USA.

**Introduction:** Cometary, asteroidal, and interstellar dust collected by recent sample return missions Stardust and Hayabusa, by high-flying aircraft in the stratosphere, or extracted from meteorites all require analytical techniques with high lateral resolution and high sensitivity. In secondary ion mass spectrometry (SIMS), lateral resolution has reached approximately 50 nm and the useful yield—the ratio of detected to consumed atoms from a sample—only rarely exceeds 1%. The final frontiers, however, to determine the isotopic nature and the location of every single atom in a given sample seem to be no longer beyond imagination. The final limiting factors can be the number of atoms in an analyzed volume and how fast they can be detected.

CHILI (Chicago Instrument for Laser Ionization), a new resonance ionization mass spectrometry (RIMS) instrument, which is about to be completed at the University of Chicago, is a major step toward these frontiers [1]. CHILI is designed for isotopic and elemental analysis at a few nanometers spatial resolution and a useful yield of 40–50%.

**Major Components:** CHILI is equipped with a COBRA-FIB high-resolution liquid metal ion gun, which can be focused to 2.5 nm, and an e CLIPSE Plus field emission electron gun (both from Orsay Physics) with a beam diameter of <4 nm. The electron gun supports the built-in diffraction-limited (approximately 0.5 µm) optical microscope for sample imaging at high resolution. A piezoelectric driven stage for nanometer-scale motions and a sample holder that accepts a wide variety of samples are operational.

The flight tube for the time-of-flight mass spectrometer will be mounted vertically above the sample chamber, which resides in the center of an H-shaped laser table supported by active vibration cancelation devices.

For resonance ionization, six tunable Ti:sapphire solid state lasers pumped with three 40W Nd:YLF lasers will be used. This will allow at least three elements to be analyzed simultaneously.

Counting statistics limit the dynamic range and the precision of isotopic ratios in existing RIMS instruments [2, 3] due to dead time effects [4] of the single anode microchannel plate detectors. CHILI will initially be equipped with such a detector, but a new detector that will allow much higher count rates is presently under development, in collaboration with the Large-Area Picosecond Photo-Detectors Project (http://psec.uchicago.edu).

**Conclusions:** CHILI reflects many recent developments in instrument design, approaching their physical limits. CHILI will be applied to a multitude of cosmochemical problems where high lateral resolution and high sensitivity are indispensable.

**References:** [1] Stephan T. et al. 2011. Abstract #1995. 42nd Lunar and Planetary Science Conference. [2] Savina M. R. et al. 2003. *Geochimica et Cosmochimica Acta* 67:3215–3225. [3] Veryovkin I. V. et al. 2004. *Nuclear Instruments and Methods in Physics Research B* 219–220:473–479. [4] Stephan T. et al. 1994. *Journal of Vacuum Science & Technology A* 12:405–410.
## EXTRACTING PYROXENE MINERAL SPECTRA FROM MARTIAN METEORITES

N. R. Stephen¹,², G. K. Benedix¹,² and P. A. Bland¹,²,³. ¹Dept. of Mineralogy, The Natural History Museum, Cromwell Road, London, SW7 5BD, UK. E-mail: n.stephen@nhm.ac.uk. ²Impacts & Astromaterials Research Centre, (IARC) and Dept. of Earth Science and Engineering, Imperial College London, South Kensington Campus, London, SW7 2AZ, UK. ³Dept. Applied Geol., Curtin University of Technology, GPO Box U1987, Perth WA 6845, Australia.

**Introduction:** Without Martian sample return, Martian meteorites provide the best in situ opportunity to study Martian material [e.g. 1] on Earth and to compare to data collected via remote sensing techniques. Thermal emission spectroscopy is one method that has been used to produce several maps of the surface geology and mineralogy of Mars [2-4]. This method uses a spectral library of mostly terrestrial minerals to deconvolve whole rock spectra of the surface of Mars into its constituent minerals. The terrestrial minerals in the database cover a wide range of compositions. However, minerals in Martian meteorites have compositions still not represented in the spectral library and getting mineral separates from hand samples is difficult. One method that can be used is micro-FTIR [5-9]. In this study we are expanding ours and others previous studies [8-10] to measure spectra for the range of pyroxene compositions found in Martian meteorites and compare these to bulk rock spectra.

**Samples and Analytical Techniques:** Spectra of pyroxenes were measured in thin sections of Los Angeles (BM2000, M12) and Zagami (BM1966, 54 and MfN) using a Perkin Elmer SpectrumOne FTIR with an AutoIMAGE microscope.

**Results and Discussion:** Compositional differences of pyroxenes in Martian meteorites are reflected in their spectra. Pyroxene (augite and pigeonite) in the Martian meteorites have a much larger range of Mg and Fe in pigeonite and lower Ca contents in augite as compared to pyroxenes in the spectral library. There are two ways to extract pyroxene spectra using the microscope FTIR system (1) Individually measure each mineral grain or (2) create a full spectrum map of an entire thin section. The first method can be hampered by grain orientation, which is known to effect IR spectra. To test the usefulness of the microscope-derived spectra on deconvolutions, we will create a theoretical ‘bulk’ rock by combining individual mineral spectra together according to modal mineralogy analysis of thin sections and bulk powders. This ‘bulk’ will also be compared to the measured spectra of the bulk thin section.

**References:** [1] McSween H. Y. 2003. In *Meteorites, Comets, and Planets* (ed. A. M. Davis), Vol 1 *Treatise on Geochemistry* (eds H. D. Holland and K. K. Turekian). [2] Christensen P. et al. 2001. *Journal of Geophysical Research* 106:23823-23871. [3] Christensen P. et al. 2004. *Space Science Review* 110:85-130. [4] Christensen P. et al. 2004. *Science* 306:1733-1739. [5] Palomba et al. 2006. *Icarus* 182:68-79. [6] Klima R. and Pieters C. 2006. *Journal of Geophysical Research* 111:E01005. [7] Morlok A. et al. 2006. *Meteoritics & Planetary Science* 41:773-784. [8] Benedix G and Hamilton V. 2007. Abstract #1805. 38th Lunar and Planetary Science Conference. [9] Benedix G. K. and Hamilton V. E. 2009. Abstract #5082. 72nd Annual Meteoritical Society Meeting. [10] Hamilton V. E. 2000. *Journal of Geophysical Research* 105:9701-9716.

## MODELING INTERSTELLAR DUST DYNAMICS IN THE SOLAR SYSTEM: APPLICATION TO Stardust

V. J. Sterken¹,², N. Altobelli³, S. Kempf²,⁴, F. Postberg¹,⁵, G. Schwehm³, R. Srama¹,⁶, and E. Grün¹,⁴. ¹MPIK, Heidelberg, Germany E-mail: Veerle.Sterken@mpi-hd.mpg.de. ²IGEP, TU Braunschweig, Germany. ³ESA, Madrid, Spain. ⁴LASP, University of Colorado, Boulder, CO, USA. ⁵Institut für Geowissenschaften, Universität Heidelberg, Germany. ⁶IRS, TU Stuttgart, Germany.

**Introduction:** Interstellar dust (ISD) has been detected in the solar system with space missions like Helios [1], Ulysses [2-5], Galileo [3, 6], and Cassini [7], using onboard dust detectors [8]. In addition, in 2000 and 2002, the Stardust mission has exposed its aerogel collectors to the interstellar dust stream for sample return of interstellar dust. Up to now, a few ISD candidates are found in the aerogel, and efforts are on-going in the frame of the Inter Stellar Preliminary Examination (ISPE) [9]. For better understanding the results of the Stardust ISD capture, we calculated the relative densities and fluxes of ISD during the mission collection phase. Also an estimate of the absolute number of particles to be expected in the aerogel is made.

**Method:** Simulations of interstellar dust trajectories through the solar system are made, taking into account gravity, solar radiation pressure and Lorentz forces due to the interaction of the charged dust with the interplanetary magnetic field. The latter leads to time-variable fluxes. From the simulations, densities and fluxes are derived at the location of Stardust with respect to the undisturbed flow of ISD. An estimate is made for the total amount of particles hitting the collector, by starting with the MRN distribution of dust [10] derived from astronomical observations, and applying the particle filtering at the Heliospheric boundary [11] and in the inner solar system [4]. The method is tested against Ulysses data and modeling.

**Results:** The proposed method of estimating total interstellar dust numbers is evaluated and future work is defined.

**References:** [1] Altobelli N. et al. 2006. *Astronomy & Astrophysics* 448:243. [2] Grün E. et al. 1993. *Nature* 362:428. [3] Baguhl M. et al. 1996. *Space Science Reviews* 78:165-172. [4] Landgraf M. et al. 2000. *Journal of Geophysical Research* 105:10303. [5] Krüger H. et al. 2007. *Space Science Reviews* 130:401. [6] Altobelli N. et al. 2005. *Journal of Geophysical Research* 110:7102. [7] Altobelli N. et al. 2003. *Journal of Geophysical Research* 108:8032. [8] Srama R. et al. 2004. *Space Science Reviews* 114:465. [9] Westphal A. J. et al. 2011. Abstract #2083. 42nd Lunar and Planetary Science Conference. [10] Mathis J. S. 1977. *The Astrophysical Journal* Part 1 217:425-433. [11] Slavin J. D. et al. 2010. 12th International Solar Wind Conference 1216:497-501.
5482

INVESTIGATING PRESSURE MAGNITUDES AT DEPTH FOR
OBLIQUE IMPACTS INTO LAYERED TARGETS: APPLICATIONS
TO IMPACTS INTO SEDIMENTARY TARGETS

A. M. Stickle and P. H. Schultz. Brown University Geological Sciences, Providence RI, USA, 02912. E-mail: angela_stickle@brown.edu.

**Introduction:** Experimental and numerical models reveal the effects of low-impedance layers on top of competent substrates, including reduced shock pressure at depth in the substrate. This becomes increasingly important for the terrestrial bodies where low-impedance layers typically cover the surface (i.e., ice, sediments or mega-regolith). The earth, specifically, is 75-80% covered in sediments; therefore, understanding the effects of layered targets is important for our understanding of resultant crater structures seen on the surface and the shock effects recorded in the rock record. One useful case study is the Rock Elm impact structure in southwest Wisconsin, USA, which is a deeply eroded crater found in a region of otherwise virtually undeformed sedimentary rocks lying on top of a Precambrian crystalline basement. Field studies [i.e., 1] identified shock features within quartz grains within the structure; however, these features were formed at very low shock pressures (5-10 GPa) and other diagnostic shock effects such as shatter cones were absent.

**Approach:** A suite of experiments were performed at the NASA Ames Vertical Gun Range at the NASA Ames Research Center into planar PMMA targets with surface layers of varying materials and thicknesses. Direct comparisons were made using 3D CTH calculations to interpret the processes observed in high-speed imaging. Large-scale simulations, then, allow application to planetary impacts. For large-scale impacts into sedimentary targets with thicknesses on the order of one- to two-projectile diameters, pressure magnitudes at depth are determined and compared to field observations at the Rock Elm impact structure.

**Results:** Experiments show that damage structures and crater size in the competent substrate are significantly reduced or non-existent for impacts into a layered target. 3D CTH simulations reveal that this is a manifestation of the impact energy being absorbed/scattered in the surface layer and the layer acting as a wave guide for very oblique impacts (15-30°). For laboratory scale impacts, the peak pressure in the competent substrate drops by ~80% for a thin (1 projectile diameter) surficial sedimentary layer. In contrast, for surface ice layers, the peak pressure drops by only ~35% because of the lower impedance contrast. At larger scales, a 90% lower impedance (sedimentary layer) results in a 70% drop in peak pressure near the surface of the underlying bedrock [2].

The Rock Elm impact structure provides an application of these results. This crater formed in 500-600 m thick sediments covering a crystalline basement rock [1]. The finding that quartz in the central uplift is only weakly shocked may be a result of the impact into a soft, overlying sedimentary layer where the upper layers of sediments absorb much of the impact energy. Large-scale CTH models, calibrated by experiments, can be used to constrain pressure magnitudes at depth within such a target lithology.

**References:** [1] French B. M., Cordua W. S., and Plescia J. B. 2004. *Geological Society of America Bulletin* 116:188-200. [2] Stickle A. M. and Schultz P. H. 2010. Abstract U51B-0043. American Geophysical Union Fall Meeting.

5015

SHOCK CLASSIFICATIONS OF PLANETARY SILICATE ROCKS
REVISITED: UPDATE OF THE 1991 GCA AND 2007 IUGS
PROPOSALS

D. Stöffler. Museum für Naturkunde—Leibniz Institute at the Humboldt University Berlin, D-10115 Berlin, Germany. E-mail: dieter.stoeffler@mfn-berlin.de.

**Introduction:** A wide variety of rocks from all planetary bodies explored so far display shock effects attributed to hyper-velocity impacts acting on these bodies since their formation. The present record of shocked rocks includes terrestrial, lunar, and Martian crustal rocks and fragments from different asteroidal bodies (stony and iron meteorites). Since the first shock recovery experiments became available for calibration of shock effects around 1960 various classification systems have been proposed until a unified system was developed in the scope of the IUGS Systematics of Metamorphic Rocks [1].

**Current Classification Systems:** Shock classifications are based on the principle of "progressive shock metamorphism" defined first in 1966 [2, 3]. Subsequently, classification systems were proposed focusing on the rocks' parent bodies, i.e. Earth, Moon, Mars, and asteroids [4-7]. However, the systematics of shocked rocks is largely independent of the parent bodies but rather depends on the type of rock, its physical properties (e.g., constituent minerals and their shock impedance, porosity), and the ambient temperature during shock. Moreover, shock classifi-cations of different rocks were published in different, sometimes hardly accessible journals and books [1, 7], and a coherent summarizing review of the current state focusing on all major rock types independent of their parent body is missing as the IUGS systematics deals primarily with the classification of impactites in general [1].

**Recommended Shock Classifications of Planetary Rocks:** A set of shock classifications is proposed for different "basic" rock types based on pressure calibrated "equilibrium" shock effects of the main constituent minerals (quartz, plagioclase, olivine, pyro-xene), and on "disequilibrium" effects such as high pressure polymorphs and intergranular melt veins and pockets. The shock classified rock types comprise: (1) igneous and metamorphic crystalline rocks such as felsic (quartzo-feldspathic) rocks, mafic rocks, and ultramafic rocks, (2) consolidated sedimentary rocks such as sandstone, and (3) unconsolidated sediments such as sand and (planetary) regolith. These subgroups include shock classifi-cations for the following specific rocks: (1) granitoids and gneisses, quartzite; basalt, gabbro, norite, troctolite, anorthosite, basaltic achondrites; dunite, peridotite, pyroxenite, chondrites, chassignite, ureilite, aubrite, diogenite, (2) sandstone, and (3) sand, lunar regolith, asteroidal regolith. For these rocks 5-7 stages of shock metamorphism are defined on the basis of shock effects calibrated by shock recovery experiments on the major minerals (see references in [1, 7]).

**Outlook:** More shock recovery experiments on plagioclase, olivine, and pyroxene at low ambient temperatures (~100-250 K) are desirable for improving classification of extraterrestrial rocks.

**References:** [1] Fettes D. and Desmons J. 2007. *Meta-morphic rocks*. Cambridge University Press. [2] Stöffler D. 1966. *Contributions to Mineralogy and Petrology* 12:15-24. [3] Chao E. C. T. 1967. *Science* 156:192-202. [4] Dodd R. T. and Jarosewich E. J. 1979. *Earth and Planetary Science Letters* 44:335-340. [5] Van Schmus W. R. and Ribbe P. H. *Geochimica et Cosmochimica Acta* 31:747-765. [6] Stöffler D. et al. 1991. *Geochimica et Cosmochimica Acta* 55:3845-3887. [7] Joliff B. L. et al. 2006. *Reviews in Mineralogy & Geochemistry*, vol. 60. pp. 1-81 and 519-596.
COMBINED NANOSIMS AND TEM IN SITU ANALYSIS OF PRISTINE MATRIX MATERIAL OF ALHA77307 AND ACFER 094

A. N. Stojic¹, F. E. Brenker¹ and P. Hoppe². ¹Goethe Universitaet, Institut fuer Geowissenschaften, 60438 Frankfurt/Main, Germany. ²Max-Planck Institute for Chemistry, P.O. Box 3060, 55020 Mainz, Germany E-mail: stojic@em.uni-frankfurt.de.

Only a few structural studies on presolar silicates and oxides exist [1]. Commonly the NanoSIMS mapping technique is used to identify presolar grains in the meteoritic matrix from a polished thin section. To extract these presolar grains for further TEM analysis, a complex and risky lift out procedure by FIB is needed [1], reducing the availability of such grains for structural studies. In contrast, argon ion slicing (ArIS), a recently introduced TEM sample preparation technique in geosciences, yields super large electron transparent areas of up to 40,000 µm² [2]. The availability of extremely large thin foils makes a substantial change in the sequence of procedure steps possible, i.e., preparation of a large thin foil first, followed by the NanoSIMS scan on the previously obtained thin foil (Fig. 1). No additional sample preparation for TEM is required once the grains are identified by NanoSIMS.

Pieces of Acer 094 and ALHA77307 were ArIS treated until large electron transparent areas were obtained. These TEM thin foils were scanned for oxygen isotope ratios with a Cameca NanoSIMS 50. Based on large O-isotopic anomalies we detected two presolar grains (both silicates) out of eight ion images (10 × 10 µm²) in ALHA77307, and 7 presolar grains (six silicates and one oxide) out of 22 ion images (same size) from Acer 094.

**References:** [1] Vollmer et al. 2009. *The Astrophysical Journal* 700:774–782. [2] Stojic and Brenker. 2010. *European Journal of Mineralogy* 22:17–21.

<figure><img src="image_29.png" /><figcaption>Fig. 1. Thin foil section of ALHA77307 with NanoSIMS scan windows indicated.</figcaption></figure>

A COMPARISON OF POROSITY USING DIFFERENT MEASUREMENT TECHNIQUES

M. M. Strait. Alma College, Alma, MI 48801, USA. E-mail: straitm@alma.edu.

**Introduction:** Porosity is one of several physical properties of meteorites that are being explored by the planetary science community. Knowledge of porosity will affect the modeling of a variety of physical phenomenon, such as impact disruption. Porosity is being measured in a variety of ways by different laboratories, including a combination of helium pycnometry and glass beads to measure whole rock porosity [1], SEM/BSE imaging and computer processing to measure thin sections [2] and X-ray tomography on whole rock samples [3]. There are advantages and disadvantages to each of these techniques, but they each provide important information about porosity.

A related three-dimensional X-ray technique using Micro CT scanning has been reported [4]. This technique was applied by this laboratory to a suite of rocks that have also been measured using pycnometry and thin section imaging. The first report on this comparison looked at its use as a visualization technique [5]. Here we report on a comparison of porosity as measured by the three methods.

**Results and Discussion:** A set of five terrestrial samples that had previously been measured using pycnometry and imaging [6] were measured using the GE eXplore specCZT scanner at the University of Western Ontario. The set of samples chosen were low in porosity, and, as a result, the size of the pores are on the same size scale as the resolution of the Micro CT, limiting the usefulness of the technique to measure porosity. Other problems encountered were the variations in the color range from slice to slice in the three-dimensional image stack and the overlap of the color range in the center of the sample with the pore space further out into the sample. More work is being done to resolve color overlap problems, but the resolution problem cannot be overcome.

In general, we find that samples with low porosity (<5%) do not agree well between the three techniques, but the nature of the porosity is not well understood in these samples. There is better agreement with more highly porous samples (>10%), but again, there is some variation between the methods used.

Overall, we conclude that CT imaging is best used to get a basic view of the appearance of the porosity in a hand sample. The technique is easy to use, fast, and there is no sample preparation needed. Pycnometry and point counting produce better numerical measures for porosity, especially at smaller values. Pycnometry is the best technique for quick, reliable whole rock measurements, although it gives no visualization of the porosity in the sample. In conclusion, porosity needs to continue to be evaluated using multiple methods to get a better feel for both the numerical range of porosity values as well as the physical appearance of the pore space.

**References:** [1] Britt D. T. and Consolmagno G. J. 2003. *Meteoritics & Planetary Science* 38:1161–1180. [2] Strait M. M. and Consolmagno G. J. 2002. *Meteoritics & Planetary Science* 37:A137. [3] Friedrich J. M. et al. 2008. *Planetary and Space Science* 56:895–900. [4] McCausland P. J. A. et al. 2010. Abstract #2584. 41st Lunar and Planetary Science Conference. [5] Strait M. M. et al. 2011. Abstract #2052. 42nd Lunar and Planetary Science Conference. [6] Strait M. M. et al. 2007. *Meteoritics & Planetary Science* 42:A146.
5350

ULTRASENSITIVE DETERMINATION OF ⁸¹Kr-Kr COSMIC RAY EXPOSURE AGES OF EUCRITES AND BUNBURRA ROCKHOLE

I. Strashnov¹, P. A. Bland², P. Spurný³, M. C. Towner² and J. D. Gilmour¹. ¹SEAES, University of Manchester, Manchester, E-mail: Ilya.Strashnov@manchester.ac.uk. ²IARC, Department of Earth Science and Engineering, Imperial College London, SW7 2AZ, UK. ³Astronomical Institute of the Academy of Sciences, Fričova 298, CZ-251 65 Ondřejov Observatory, Czech Republic.

**Introduction:** Cosmic ray exposure ages (CRE ages) measure the length of time a meteoroid was exposed to cosmic rays as a small body between separation from its parent asteroid or planet and arrival at the Earth. They can be determined by comparing the equilibrium concentration of radioactive ⁸¹Kr to the concentrations of stable cosmogenic Kr isotopes. Among the various types of methods ⁸¹Kr-Kr CRE ages appear the most reliable for eucrites because no adjustment is needed for shielding and sample composition [1]. However, low ⁸¹Kr concentrations (few thousand atoms mg⁻¹ [2]) and low ⁸¹Kr/Kr ratios have been constraints on the use of the method since bulk (up to 1 g) samples have been required. Here we report ⁸¹Kr-Kr ages acquired by RIMSKI—Resonance Ionization Mass Spectrometer for Krypton Isotopes [3, 4].

**Results and Discussions:** Typical samples were grains of 1–3 mg, each containing a few thousand atoms of ⁸¹Kr and ~10⁵–10⁶ atoms of stable krypton isotopes. Between 3-6 replicate analyses have been made for each meteorite in this study. Gas was extracted by laser heating. Isotope ratios were corrected for mass discrimination using interspersed analyses of krypton sourced from the terrestrial atmosphere. For 7 eucrites our data are consistent with previously reported literature data [4] once account is taken of the recent revision of the ⁸¹Kr half-life to 2.29 × 10⁵ yr [5].

We have also analyzed two samples for which ⁸¹Kr-Kr ages have not been previously reported. Our preliminary age for the eucrite Lakangaon is 24.59 ± 0.87 Myr, while that for the anomalous eucrite Bunburra Rockhole is 26.71 ± 1.05 Myr.

Eugster and Michel [1] assign known eucrites to five CRE age clusters corresponding to presumed impacts on the parent body. Our data suggest that Lakangaon belongs to a cluster with Millbillillie, for which we obtain an ⁸¹Kr-Kr age of 23.57 ± 1.87 Myr (compare to the literature age of 22.38 ± 0.73 Myr [2]) and Bereba 26.29 ± 1.65 Myr (27.17 ± 2.29 Myr [2]).

Bunburra Rockhole [6] was recovered by The Desert Fireball Network in Western and has a calculated pre-atmospheric orbit suggesting its parent body is a member of the Aten group of earth-crossing asteroids. While our data suggest an ⁸¹Kr-Kr age similar to that of Millbillillie, Bereba and Lakangaon, oxygen isotope and other data imply that it did not originate on the eucrite parent body [6]. Our ⁸¹Kr age is distinctly different from those of the other anomalous eucrite falls, Pasamonte (7.7 ± 0.52 Myr [2]) and Ibitira (12.5 ± 0.52 Myr [2]), suggesting they sample three impacts in contrast to the five impacts on the Eucrite parent body that have produced 26 falls with measured ⁸¹Kr-Kr ages to date. Eucrite samples were provided by Natural History Museum. This work was funded by the STFC, grant number ST/G003068/1.

**References:** [1] Eugster O. and Michel Th. 1995. *Geochimica et Cosmochimica Acta* 59:177–199. [2] Shukolyukov A. and Begeman F. 1996. *Meteoritics & Planetary Science* 31:60–72. [3] Strashnov I. et al. 2011. *Journal of Analytical Atomic Spectrometry*, doi:10.1039/C1JA10051C, in press. [4] Strashnov et al. This conference. [5] Baglin C. M. 2008. *Nuclear Data Sheets* 109:2257–2437. [6] Bland P. A. et al. 2009. *Science* 325:1525–1527.

5273

RIMSKI—A RESONANCE IONIZATION MASS SPECTROMETER FOR DETERMINATION OF KRYPTON ISOTOPE RATIOS OF EXTRATERRESTRIAL SAMPLES

Strashnov I. and J. D. Gilmour. SEAES, University of Manchester, Manchester. E-mail: Ilya.Strashnov@manchester.ac.uk.

RIMSKI [1]—A Resonance Ionization Mass Spectrometer for Krypton Isotopes—is a newly developed ultrasensitive instrument with a cryogenic sample concentrator and a laser ion source designed for isotopic analysis of extraterrestrial material. Laser heating is used for extraction of krypton from samples into the mass spectrometer volume. Atoms continuously condense on a cold spot in the ion source, and are repeatedly released by laser heating. They are ionized in the evaporation plume using three tunable dye lasers and one frequency tripled Q-switched Nd:YAG laser. Two resonant transitions are employed (at 116.5 nm, produced by four-wave frequency mixing in Xe, and 558.1 nm) and are followed by single photon ionization at 1064 nm.

Multiple analyses of air aliquots and < 5 mg samples of meteorites, both containing ~10⁶ total krypton atoms, show precision ~1% for major isotope ratios. They are reproducible to this level through a day's analyses, allowing calibration for linear and non-linear mass discrimination by sample-standard bracketing. The mass spectrometer blank corresponds to ~5800 krypton atoms (~3300 atoms of ⁸⁴Kr). The lifetime against detection of krypton atoms is 7 min.

A further test of the instrument is provided by Kr isotope ratio determinations of eucrites with known isotope compositions. Cosmogenic ⁸¹Kr can be reliably detected at the few thousand atom level, corresponding to determination of cosmic ray exposure ages—the length of time a meteoroid has spent as a small body between separation from its parent asteroid or planet and arrival at the Earth—from samples of a few milligrams. Determined ages and literature values (in brackets [2]) of—Bereba: 26.29 ± 1.65 My (27.17 ± 2.29 My), Bouvante: 6.93 ± 0.33 My (6.99 ± 0.40 My), Pasamonte: 7.71 ± 0.49 My (7.70 ± 0.52 My), Padvarnikai: 14.32 ± 0.79 My (15.2 ± 2.08 My), Juvinas: 11.14 ± 0.45 My (11.04 ± 0.42 My), Millbillillie: 23.57 ± 1.87 My (22.38 ± 0.73 My), Stannern: 36.37 ± 2.08 My (37.10 ± 0.94 My) — agrees within experimental error once account is taken of the recent revision of the ⁸¹Kr half-life to 2.29 × 10⁵ years [3].

Further instrument development includes design of a new ion source and implementation of the Xe resonance ionization scheme similar to RELAX [4] that would allow simultaneous Xe/Kr analysis. Among planned measurements are primitive meteorites, presolar SiC grains as well as material from sample return messions e.g., Stardust or Genesis. Determination of elemental Kr/Xe ratios will allow aqueous and igneous processes to be traced. method applicability forcing use of bulk (up to 1 g) samples.

**Acknowledgment:** Eucrite samples were provided by Natural History Museum. This work was funded by the STFC, grant number ST/G00306811.

**References:** [1] Strashnov I. et al. 2011. *Journal of Analytical Atomic Spectrometry*, doi:10.1039/C1JA10051C, in press. [2] Shukolyukov A. and Begeman F. 1996. *Meteoritics & Planetary Science* 31:60–72. [3] Baglin C. M. 2008. *Nuclear Data Sheets* 109:2257–2437. [4] Crowther S. A. et al. 2008. *Journal of Analytical Atomic Spectrometry* 23:938–947.
5118

ELEMENTAL ANALYSIS OF IMPACT RESIDUES IN CRATERS
ON THE Stardust INTERSTELLAR FOILS

R. M. Stroud, C. Allen, A. Ansari, S. Bajt, N. Bassim, H. A. Bechtel, J. Borg, F. Brenker, J. Bridges, D. E. Brownlee, M. Burchell, M. Burghammer, A. L. Butterworth, P. Cloetens, A. M. Davis, R. Doll, C. Floss, G. J. Flynn, D. Frank, Z. Gainsforth, E. Grün, P. R. Heck, J. K. Hillier, P. Hoppe, L. Howard, G. R. Huss, J. Huth, B. Hvide, A. Kearsley, A. J. King, P. G. Kotula, B. Lai, J. Leitner, L. Lemelle, A. Leonard, H. Leroux, L. R. Nittler, R. C. Ogliore, W. J. Ong, F. Postberg, M. C. Price, S. A. Sandford, J. A. Sans Tresseras, S. Schmitz, T. Schoonjans, K. Schreiber, G. Silversmit, A. Simionovici, R. Srama, T. Stephan, J. Stodolna, S. R. Sutton, R. Toucoulou, M. Trieloff, P. Tsou, A. Tsuchiyama, T. Tyliczszak, B. Vekemans, L. Vincze, A. J. Westphal, M. E. Zolensky and >29,000 Stardust@home dusters. ISPE author affiliations are at http://www.ssl.berkeley.edu/~westphal/ISPE/.

**Introduction:** Preliminary imaging studies show the abundance of craters on the interstellar foils is 50 to 100 times the expected value for interstellar particle impacts [1]. Thus, the majority of the craters are likely due to other sources, such as secondary impacts from interplanetary dust particles that impacted the solar panels, or direct impact of high velocity nanoparticles.

In order to distinguish amongst the possible origins for the craters, elemental analyses of any retained impact residue, and/or crater morphology measurements, are necessary. Solar cell cover glass is primarily silica, but contains wt. % levels of Ce and Zn, which can be used as an indicator of spacecraft origin. The specific features of craters from the impact of high velocity nanoparticles, i.e., 10–50 nm particles traveling at 100–400 km s⁻¹, are unknown, however, lack of detectable residue and/or high depth-to-diameter and narrow-rimmed craters are expected to be diagnostic. Results from analog shot studies suggest that interstellar particle impact craters are likely to contain residue detectable by Auger spectroscopy, and have crater morphologies consistent with impact velocities of 5–25 km s⁻¹.

**Experimental:** Elemental analysis of six craters was performed, including those with symmetric and asymmetric geometries, and both larger and smaller than 1 µm. Auger measurements were done with the PHI 700 Auger Nanoprobe at Washington University [2]. EDX measurements were performed at the MPI for Chemistry in Mainz using a FEG SEM with a conventional EDX system, and at Sandia National Lab, NM, using a FEG SEM with a custom Bruker on-axis silicon drift-detector EDX system. FIB-TEM studies were carried out at the Naval Research Lab, with a FEI Nova 600 FIB-SEM and JEOL 2200FS TEM.

**Results and Discussion:** All six craters contained residues with levels of Si and O detectable by Auger or EDX, but not Ce, or Zn. In addition, Fe was detected in four craters, Mg in two, and Na in two. Subsequent TEM analysis of FIB sections from two craters first analyzed by EDX revealed residues ~10 to 50-nm thick, with major silica and minor levels of Ti, Ce and Zn. These measurements are consistent with analysis of tracks in aerogel from the IS collector, which revealed ~80% of tracks originate from secondary impacts. Additional elemental analysis of craters is planned to identify bona fide interstellar particle impacts.

**References:** [1] Stroud R. M. et al. 2011. Abstract #1753. 42nd Lunar and Planetary Science Conference. [2] Floss C. et al. 2011. This conference.

5194

ORDINARY CHONDRITE-RELATED GIANT COSMIC SPHERULES

C. Suavet¹,², C. Cordier¹, P. Rochette³, L. Folco¹, J. Gattacceca³, C. Sonzogni³, D. Damphoffer³. ¹MNA, Siena, Italy. ²MIT, Cambridge, MA, USA. E-mail: csuavet@mit.edu. ³CEREGE, Aix-en-Provence, France.

Micrometeorites are extraterrestrial particles smaller than approximately 2 mm collected on the Earth's surface [1]. Most of them melt during atmospheric entry; they are called cosmic spherules. A previous study [2] found that all three spherules with porphyritic olivine (PO) and cryptocrystalline (CC) textures were related to ordinary chondrites based on their oxygen isotopic signature. Here we report the oxygen isotope compositions of 15 giant (>800 µm) cosmic spherules from the Transantarctic Mountains, Antarctica [3], including six PO, eight CC and one barred olivine (BO) textural types [4].

Eleven spherules (approximately 70% of them: 4/6 PO and 6/8 CC, and the BO spherule) are related to ordinary chondrites based on oxygen isotopic compositions. In ordinary chondrite-related PO spherules olivines have Fa₈.₅⁻¹₁.₈ composition, they are Ni-poor to Ni-rich (0.04–1.12 wt%), and CaO-rich (0.10–0.17 wt%). Ordinary chondrite-related spherules also have high magnetite contents (approximately 2–12 wt%). One PO and one CC spherules are related to previously identified ¹⁷O-enriched cosmic spherules for which the parent body is unknown [2]. One CC spherule has an oxygen isotopic signature relating it with CM/CR carbonaceous chondrites.

These results show that the texture of cosmic spherules is not only controlled by atmospheric entry heating conditions but also depends on the parent body, whether it through orbital parameters (entry angle and velocity), or chemistry, mineralogy, or grain size of the precursor.

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

**References:** [1] Rubin A. E. and Grossmann J. N. 2010. *Meteoritics & Planetary Science* 45:114–122. [2] Suavet C., Alexandre A., Franchi I. A., Gattacceca J., and Sonzogni C. et al. 2010. *Earth and Planetary Science Letters* 293:313–320. [3] Rochette P. et al. 2008. *Proceedings of the National Academy of Sciences* 105:18206–18211. [4] Genge et al. 2008. *Meteoritics & Planetary Science* 43:497–515.
5105

CORRELATION BETWEEN <sup>54</sup>Cr ANOMALIES AND ACCRETION AGES OF METEORITE PARENT BODIES: IMPLICATIONS FOR THE EVOLUTION OF THE SOLAR NEBULA

N. Sugiura and W. Fujiya. Department of Earth and Planetary Science, University of Tokyo, Tokyo, Japan. E-mail: Sugiura@eps.s.u-tokyo.ac.jp.

**Introduction:** A fair correlation between <sup>54</sup>Cr anomalies and accretion ages is observed among meteorites, including iron meteorites, a pallasite, mesosiderites, an aubrite, HED meteorites, an angrite, NWA 011, ureilites and chondrites (including E, O, R, CK, CO, CV, CH-CB, CR, CM, and CI). The <sup>54</sup>Cr data are taken from [1-8], whereas the accretion ages were estimated in [9]. Data on the Eagle Station pallasite are not included because spallogenic Cr may contribute significantly to the <sup>54</sup>Cr anomaly [10].

The <sup>54</sup>Cr anomalies range from -0.9e in differentiated meteorites to 1.6e in primitive chondrites. The accretion ages range from <0.6 Ma after CAI for differentiated meteorites to 3.5 Ma after CAI for primitive chondrites. NWA 011 is an outlier of the trend that is defined by the other meteorites. It is an achondrite that formed early [11] but the ε<sup>54</sup>Cr is as high as those of C chondrites [1]. An explanation could be devised for the high ε<sup>54</sup>Cr. Hence, the correlation between <sup>54</sup>Cr anomalies and accretion ages is considered significant in spite of the presence of an outlier. The carriers of the <sup>54</sup>Cr anomalies were recently identified as tiny presolar oxide grains [12, 13]. Hence, we could elucidate the evolution of the solar nebula by investigating how the presolar grains were mixed and distributed in the solar nebula.

**Results:** We made one-dimensional numerical simulations to find out whether the correlation between <sup>54</sup>Cr anomalies and accretion ages can be explained by advection-diffusion of presolar grains injected into the solar nebula. The nebula is assumed to be in a steady state, moving inward at a constant velocity. The injection is assumed to have happened at the time of CAI formation. Adjustable parameters are the turbulence diffusion parameter (α) and the location of the injection. Accretion locations of parent bodies are assumed to shift gradually with time from 1 AU (differentiated meteorites) to 4 AU (primitive C chondrites). Results of the numerical simulations suggest that the correlation observed among meteorites can be reproduced if the presolar grains were injected at an appropriate distance that is proportional to α. For α = 10<sup>-3</sup> that is often assumed for the solar nebula, the injection location is ~100 AU.

**References:** [1] Bogdanovski O. and Lugmair G.W. 2004. Abstract #1715. 30th Lunar and Planetary Science Conference. [2] Shukolyukov A. and Lugmair G.W. 2006. Abstract #1478. 37th Lunar and Planetary Science Conference. [3] Shukolyukov A. and Lugmair G.W. 2006. *Earth and Planetary Science Letters* 250:200. [4] Trinquier A. et al. 2007. *The Astrophysical Journal* 655:1179. [5] Qin L. et al. 2010. *Meteoritics & Planetary Science* 45:1771. [6] Qin L. et al. 2010. *Geochimica et Cosmochimica Acta* 74:1122. [7] Yamakawa A. et al. 2010. *The Astrophysical Journal* 720:150. [8] Yamashita K. et al. 2010. *The Astrophysical Journal* 723:20. [9] Sugiura N. and Fujiya W. 2011. Meteoritical Society Meeting Abstract. This conference. [10] Papanastassiou D. A. and Chen J. H., 2011. Abstract #2195. 42nd Lunar and Planetary Science Conference. [11] Sugiura N. and Yamaguchi A. 2007. Abstract #1431. 38th Lunar and Planetary Science Conference. [12] Dauphas N. et al. 2010. *The Astrophysical Journal* 720:1577. [13] Qin L. et al. 2011. *Geochimica et Cosmochimica Acta* 75:629.

5106

ACCRETION AGES OF METEORITE PARENT BODIES

N. Sugiura and W. Fujiya. Department of Earth and Planetary Science, University of Tokyo, Tokyo, Japan. E-mail: Sugiura@eps.s.u-tokyo.ac.jp.

**Introduction:** Accretion of parent bodies is the first stage of the solar system evolution. Mainly based on W isotopic information [1] and heat source consideration [2], it is now generally accepted that iron meteorite and achondrite parent bodies accreted earlier than parent bodies of chondrites. However, details of the accretion ages are not well established for many meteorite parent bodies yet. Here we compile accretion ages (Tacc calculated from CAI formation) if available and if not available, we estimate Tacc based on chronological data, heat source consideration and thermal histories. Magmatic irons, HED achondrites, angrites, aubrites, NWA 2976, pallasites, mesosiderites, ureilites and chondrites (E, O, R, CK, CO, CV, CH-CB, CR, CM and CI) are included in this study. Instantaneous accretion of parent bodies is assumed for simplicity.

**Methods and Results:** Tacc of magmatic irons is estimated to be <0.6 Ma based on the W isotope [2]. Tacc of HED achondrites is estimated to be ~1.3 Ma, based on the core formation age at ~2.5 Ma after CAI [3]. Tacc of angrites is estimated to be 1.6 Ma, based on the model core formation age of ~4 Ma after CAI [4]. Tacc of aubrites is not constrained well and is estimated to be <1.8 Ma, according to the requirement of melting of chondritic materials due to <sup>26</sup>Al heating. Tacc of NWA 2976 (paired with NWA 011) is estimated to be ~1.5 Ma, based on the model Al-Mg age of 3.5 Ma after CAI [5]. W isotopic compositions of metal in pallasites are similar to those in magmatic irons [6]. Hence Tacc of pallasite is estimated to be <0.6 Ma. Although W isotopes in many mesosiderites appear to be affected by slow cooling, the least radiogenic composition is similar to those in magmatic irons [6]. Hence Tacc of mesosiderites is estimated to be <0.6 Ma. Estimate of Tacc of ureilites is complicated because of the possible loss of <sup>26</sup>Al by explosive volcanism. We favor Tacc~1.1 Ma though we cannot explain the details here.

Tacc of chondrites (E, O, R, CK, CO, CV, CH-CB and CR) are estimated based on the abundances of Al and the peak metamorphic temperatures. They are 1.8, 2.1, 2.4, 2.6, 2.8, 3.0, 3.3 and 3.5 Ma. The ambient temperatures for the C chondrites (except for CR) were assumed to be ~50 C. In the case of CR, presence of water ice and heat of hydration were taken into consideration and the ambient temperature is assumed to be ~100 C. The accretion ages of O, CO and CR reported here are similar to those reported by [1].

Tacc of CM and CI are estimated to be ~3.5 Ma, based on the Mn-Cr ages of carbonates [7, 8] and the thermal histories of the parent bodies. Here again, presence of water ice, heat of hydration and ambient temperature of ~100 C are taken into account.

**References:** [1] Kleine T. et al. 2009. *Geochimica et Cosmochimica Acta* 73:5150. [2] Hevey P. J. and Sanders I. S. 2006. *Meteoritics & Planetary Science* 41:95. [3] Touboul M. et al. 2008. Abstract #2336. 39th Lunar and Planetary Science Conference. [4] Markowski A. 2007. *Earth and Planetary Science Letters* 262:214. [5] SchillerM. et al. 2010. *Geochimica et Cosmochimica Acta* 74:4844. [6] Quitte G. et al. 2005. *Geochimica et Cosmochimica Acta* 69:1321. [7] Fujiya W. et al. 2010. Abstract #1644. 41st Lunar and Planetary Science Conference. [8] Fujiya W. et al., 2011. Abstract #1397. 42nd Lunar and Planetary Science Conference.
5128

METEORITES IN SLOVAKIA—SUMMARY

J. Svoren¹, V. Porubcan¹,², M. Husarik¹. ¹Astronomical Institute of SAS, SK-05960 Tatranska Lomnica, Slovak Republic, E-mail: astrsven@ta3.sk. ²Department of Astronomy, Comenius University, Mlynska dolina, SK-84248 Bratislava, Slovak Republic.

In the contribution an overview of Slovak meteorites from the viewpoint of the fall/find and location of the fragments at present, is presented. In the territory of the Slovak Republic (an area of 49 035 km²) three falls and three meteorite findings over two centuries have been registered. Additional details are in [1].

The first known meteorite listed in Hungarian transcript as Lenarto is a find of an iron meteorite found in October 1814 in the vicinity of the eastern-Slovak village of Lenartov, west of Bardejov. The meteorite is classified as a medium octahedrite, a transitional member of group IIIA-IIIB (2). Contains large amounts of occlusions of gases, mainly hydrogen, 90.4% of iron and 8.8% of nickel. The main mass (73.6 kg) is deposited in the museum of Budapest. Additional larger fragments are in Tubingen (3.3 kg), Vienna (3.2 kg) and Budapest (2.9 kg).

The second meteorite known as Magura is also a finding of an iron meteorite in around 1835 in the vicinity of the northern-Slovak village of Slanica near the town of Namestovo. A large number of boulders weighing from 1 to 46 kg were found. Approximately 1600 kg of the meteoric material was melted. There has never been found a crater. Magura is a heavily shocked, coarse octahedrite with 92.6% of iron and 6.7% of nickel. Iron from Magura is located in a number of museums around the world, with the largest pieces in Tubingen (45.5 kg), Vienna (30.2 kg), Budapest (18.5 kg), Berlin (10.1 kg) and London (9.0 kg) [2].

The first meteorite fall recorded in Slovakia is dated on July 24, 1837. The stone meteorite listed in Hungarian transcript as Gross-Divina fell at noon in the vicinity of village Divina near the town of Zilina. Classified as a chondrite. It is very rare in museum collections. The main mass (9.9 kg) is deposited in the Budapest museum and smaller fragments are in Prague (0.26 kg) and Wien (0.06 kg).

The second fall is a stone listed in Hungarian transcript as Nagy-Borove which fell in the vicinity of the village of Velke Borove near the town of Liptovsky Mikulas on May 9, 1985. Classified as a chondrite with well-defined glassy chondrules. It is very rare in museum collections. The largest piece (5.9 kg) is exposed in the museum in Budapest and additional smaller fragments are in Chicago (0.18 kg) and London (0.05 kg).

The last find is from August 1994, where a stone meteorite of the weight of 4.3 kg was found, during harvest in the vicinity of the village of Rumanova near the town of Sered. Classified as an H5 chondrite with 21.2% of iron, 18.2% of silicon and 13.2% of magnesium. Two largest fragments are deposited in the Slovak national museum in Bratislava (2.5 kg).

The last one is the fall of an H5 chondrite near the town of Kosice on February 28, 2010 [3].

**Acknowledgment:** This work has been supported by the APVV project No. 0516-10.

**References:** [1] Tucek K. 1981. *Meteorites and their occurrences in Czechoslovakia (in Czech)*. Prague: Academia. [2] Buchwald V. F. 1975. *Handbook of iron meteorites*. Berkeley: University California Press. [3] Toth J. and Svoren J. 2010. Meteorit Košice (in Slovak). In *Meteor news of the slovak astronomical society* 31:65-69.

5497

$^{40}$Ar-$^{39}$Ar DATING OF LARKMAN NUNATAK 06299: COMPARISON TO PAIRED SAMPLE LAR 06298 AND TO OTHER LL CHONDRITES

T. D. Swindle¹,², J. R. Weirich¹, C. E. Isachsen², A. Wittmann³ and D. A. Kring³. ¹Lunar and Planetary Laboratory, University of Arizona, Tucson, AZ, USA. E-mail: tswindle@email.arizona.edu. ²Department of Geosciences, University of Arizona, Tucson, AZ, USA. ³Lunar and Planetary Institute, Houston, TX, USA.

Larkman Nunatak (LAR) 06298 and 06299 are paired LL chondrite impact melt breccias. We previously analyzed four splits of LAR 06298 using $^{40}$Ar-$^{39}$Ar dating [1], and determined that all showed signs of partial degassing at ~1000 Ma or later, in some cases ~500 Ma or later. We were unable to obtain a more precise age because no two of the four splits had anything resembling concordant plateaus. Since LAR 06299 is paired with LAR 06298, we analyzed three splits of LAR 06299 to shed more light on the situation. Two of the splits look much like two of the LAR 06298 splits, with minimum ages of ~1000 Ma, but no good plateaus. The third split, however, yielded several extractions giving much younger ages, of ~400 Ma or less. Since minimum apparent ages are likely to be maximum ages of partial degassing, we interpret this as indicating a partial resetting event no more than 400 Ma ago. This split has a significant amount of $^{36}$Ar, indicating the presence of trapped Ar, and does not yield an isochron, so various assumptions about trapped Ar yield ages anywhere from 400 Ma to near zero. Since none of the seven splits has yielded a good plateau at an age that could represent a resetting event, further Ar-Ar analysis of LAR 06298 or LAR 06299 is unlikely to be worthwhile, but even an upper limit of ~400 Ma is significant.

First, this is the youngest degassing event recorded in an LL chondrite. In fact, of >20 LL chondrites for which shock ages have been determined, only one other (Oberlin, 420±30 Ma [2]) is <500 Ma. The impact melt textures of these two chondrites are macroscopically similar to that of Cat Mountain [3], which implies this degassing occurred during a major impact event (perhaps in the presence of a significant gravitational field [4]). Alternatively, the impact melt textures could have been established during an earlier event, and a later event, not yet detected petrographically, could have caused the degassing.

Second, with the return of samples of LL chondrite composition from asteroid 25143 Itokawa by Hayabusa [5], the history of shocked LL chondrites can potentially be linked to specific events and objects in the asteroid belt. For example, the ≤400 Ma degassing event may be related to the impact disruption of an LL-chondrite asteroid and reassembly of Itokawa, consistent with the calculated 75–1000 Ma age of the surface of Itokawa [6]. Unfortunately, we were unable to determine a precise degassing age for LAR 06298/06299, and its cosmic ray exposure age (20–25 Ma) is less than the calculated surface age of Itokawa, so we would need to find an LL chondrite with a better-preserved thermal history to fully make the connection. However, the LAR 06298/06299 results suggest that such a task may be possible.

**References:** [1] Weirich J. R. et al. 2009. *Meteoritics & Planetary Science* 44:A215. [2] Bogard D. D. et al. 1976. *Journal of Geological Research* 81:5664-5678. [3] Kring D. A. et al. 1996. *Journal of Geological Research* 101:29,329-29,371. [4] Rubin A. E. and Moore W. B. 2011. *Meteoritics & Planetary Science* 46: 737-747. [5] Nakamura T. et al. 2011. Abstract #1766. 42nd Lunar and Planetary Science Conference. [6] Michel P. et al. 2009. *Icarus* 200:503-513.
5074

THERMAL CONDUCTIVITY OF METEORITES

M. Szurgot. Center of Mathematics and Physics, Technical University of
Łódź, Al. Politechniki 11, 90 924 Łódź, Poland. E-mail: mszurgot@p.lodz.pl.

Thermal conductivity of meteorites has been studied by a number of
researchers [1-4]. The aim of the paper was to determine and analyze
thermal conductivity of various meteorites at room temperature.

Thermal conductivity *K* of meteorites was determined using the well-
known relationship

$$
K = D d C_p, \tag{1}
$$

where *D* is thermal diffusivity, *d* is bulk density and *Cp* is specific heat capacity.

According to our data, thermal conductivity of chondrites is in the
range of (1.5-4.5) W m⁻¹ K⁻¹, stony-irons (2.5-17) W m⁻¹ K⁻¹, and
octahedrites (23-67) W m⁻¹ K⁻¹. Exemplary values of *K* are: NWA 4039
(eucrite, 1.1 W m⁻¹ K⁻¹), HaH 286 (eucrite, 1.9 W m⁻¹ K⁻¹), Allende
(CV3, 1.4 W m⁻¹ K⁻¹), DaG 610 (H4, 2.5 W m⁻¹ K⁻¹), Gold Basin (L4,
3.7 W m⁻¹ K⁻¹), El Hammami (H5, 4.5 W m⁻¹ K⁻¹), Vaca Muerta
(mesosiderite-A1, 2.5 W m⁻¹ K⁻¹), Brahin (pallasite PMG, (5.4-
16.5) W m⁻¹ K⁻¹), Morasko (IAB-MG, (25-67) W m⁻¹ K⁻¹), Odessa
(IAB-MG, 45 W m⁻¹ K⁻¹), Campo del Cielo (IAB-MG, 52 W m⁻¹ K⁻¹),
and Gibeon (IVA, (23-25) W m⁻¹ K⁻¹).

The relationship between thermal conductivity and bulk density of meteorites has been established. Thermal conductivity can be expressed as a function of bulk density by various empirical equations. Power, linear, and quadratic fit are satisfactory.

According to the power function fit, we have

$$
K = K_0(d/d_0)^n, \quad (2)
$$

where *K* is thermal conductivity (W m⁻¹ K⁻¹), *d* is bulk density (kg m⁻³), and *d₀* = 1000 kg m⁻³. Coefficients *K₀* and *n* are constant for a given temperature (*K₀* = (0.044 ± 0.050) W m⁻¹ K⁻¹, and *n* = 3.4 ± 0.6, R² = 0.90, at 300 K).

Linear fit is given by the formula

$$
K = A \cdot d + B, \tag{3}
$$

where *K* is thermal conductivity (W m⁻¹ K⁻¹), *d* (kg m⁻³) is bulk density of meteorites, and coefficients *A* and *B* are constant for a given temperature (*A* = (8.81 ± 0.61)•10⁻³ W m² kg⁻¹, *B* = -26.7 ± 2.8W m⁻¹ K⁻¹ at 298 K, R² = 0.89).

*K* depend on *d* since *D*, *Cp*, and thermal capacity *Cp*•*d* depend on density of meteorites [5,6]. Literature data [2,4] confirm validity of equations (2) and (3). Using values of grain density *d*<sub>g</sub> and *K* measured by Opeil and coworkers [2] enable one to determine *A* and *B* values for various temperatures:

$$
A = 2.99 \cdot 10^{-3} \text{ W m}^2 \text{ kg}^{-1}, B = -5.84 \text{ W m}^{-1} \text{ K}^{-1} \text{ at } 100 \text{ K,}
$$

$$
A = 3.92 \cdot 10^{-3} \text{ W m}^2 \text{ kg}^{-1}, B = -8.50 \text{ W m}^{-1} \text{ K}^{-1} \text{ at } 200 \text{ K,}
$$

$$
A = 4.84 \cdot 10^{-3} \text{ W m}^2 \text{ kg}^{-1}, B = -10.9 \text{ W m}^{-1} \text{ K}^{-1} \text{ at } 300 \text{ K, } R^2 = 0.97.
$$

Conclusions: Relationship between thermal conductivity and density of meteorites has been established that can be applied for evaluation of thermal conductivities of extraterrestrial matter at various temperatures.

**References:** [1] Szurgot M. et al. 2008. *Crystal Research and Technology* 43:921-930. [2] Opeil C. P. et al. 2010. *Icarus* 208:449-454. [3] Beach M. et al. 2009. *Planetary and Space Science* 57:764-770. [4] Yomogida K. and Matsui T. 1983. *Journal of Geophysical Research* 88:9513-9533. [5] Szurgot M. and Wojtatowicz T. W. 2011. Abstract #5036. 74th Annual Meteoritical Society Meeting. [6] Szurgot M. 2011. Abstract #1150. 42nd Lunar and Planetary Science Conference.

5036

THERMAL DIFFUSIVITY OF METEORITES

M. Szurgot¹ and T. W. Wojtatowicz². ¹Center of Mathematics and Physics, Technical University of Łódź, Al. Politechniki 11, 90 924 Łódź. E-mail: mszurgot@p.lodz.pl. ²Institute of Physics, Technical University of Łódź, Wolczanska 219, 93 005 Łódź, Poland.

**Introduction:** Determination of thermal properties of meteorites is important to describe the evolution of asteroids and planets. Thermal diffusivity of selected meteorites has been studied by a number of researchers [1-6]. The aim of the paper was to determine and analyze thermal diffusivity of various meteorites at room temperature.

**Methods:** Thermal diffusivity *D* of meteorites was measured by the laser flash method at ambient conditions, at 298 K in one atmosphere air. Bulk density of the samples was determined by the Archimedean method. Relative errors of measurements of diffusivity are about 1-6%, and density about 1%.

**Results and Discussion:** Thermal diffusivity of achondrites is in the range of (0.4-2.4) × 10⁻⁶ m² s⁻¹, chondrites (0.5-2.0) × 10⁻⁶ m² s⁻¹, stony-irons (1.3-7.0) × 10⁻⁶ m² s⁻¹, and octahedrites (7-19) × 10⁻⁶ m² s⁻¹. Exemplary values of *D* are: NWA 4039 (eucrite, 0.41 × 10⁻⁶ m² s⁻¹), HaH 286 (eucrite, 0.93 × 10⁻⁶ m² s⁻¹), Zaklodzie (enstatite achond., 1.04 × 10⁻⁶ m² s⁻¹), Allende (CV3, 0.56 × 10⁻⁶ m² s⁻¹), NWA 4967 (CO3.2, 1.00 × 10⁻⁶ m² s⁻¹), NWA 1609 (H4-6, 1.08 × 10⁻⁶ m² s⁻¹), El Hammami (H5, 1.57 × 10⁻⁶ m² s⁻¹), DaG 610 (H4, 1.64 × 10⁻⁶ m² s⁻¹), Thuathe (H4/5, 2.01 × 10⁻⁶ m² s⁻¹), NWA869 (L4-6, 1.03 × 10⁻⁶ m² s⁻¹), Gold Basin (L4, 1.32 × 10⁻⁶ m² s⁻¹), Vaca Muerta (mesosiderite-A1, 1.34 × 10⁻⁶ m² s⁻¹), Brahin (pallasite PMG, (2.1-6.9) × 10⁻⁶ m² s⁻¹), Campo del Cielo (IAB-MG, 15.8 × 10⁻⁶ m² s⁻¹), Odessa (IAB-MG, 12.4 × 10⁻⁶ m² s⁻¹), Morasko (IAB-MG, (7-19) × 10⁻⁶ m² s⁻¹), Henbury (IIHAB, 10.3 × 10⁻⁶ m² s⁻¹), and Gibeon (IVA, 7.2 × 10⁻⁶ m² s⁻¹).

Measurements of 20 meteorites revealed relationship between thermal diffusivity and bulk density of meteorites. Thermal diffusivity is a linear function of bulk density which is expressed by the empirical equation

$$
D = A \cdot d + B, \tag{1}
$$

where *D* is thermal diffusivity (10⁻⁶ m² s⁻¹), *d* (kg m⁻³) is bulk density of meteorites, and coefficients *A* and *B* are constant for a given temperature (*A* = 2.49 × 10⁻⁹ m⁵ kg⁻¹ s⁻¹, *B* = -7.11 × 10⁻⁶ m² s⁻¹ at 298 K). Not only our but also literature data confirm validity of eq. (1). Coefficients *A* and *B* depend on temperature and express effect of temperature on thermal diffusivity. Using values of *D* and *d* measured for five meteorites (four chondrites and one octahedrite) by Osako [6] enabled us to determine *A* and *B* values for various temperatures:

$$
A = 2.37 \times 10^{-9} m^5 kg^{-1} s^{-1}, B = -6.19 \times 10^{-6} m^2 s^{-1} \text{ at } 100 \text{ K,}
$$

$$
A = 2.11 \times 10^{-9} m^5 kg^{-1} s^{-1}, B = -6.24 \times 10^{-6} m^2 s^{-1} \text{ at } 200 \text{ K,}
$$

$$
A = 2.10 \times 10^{-9} m^5 kg^{-1} s^{-1}, B = -6.52 \times 10^{-6} m^2 s^{-1} \text{ at } 300 \text{ K.}
$$

Conclusions: Relationship between thermal diffusivity and bulk
density of meteorites has been established that can be applied for
evaluation of thermal diffusivities of extraterrestrial matter at various
temperatures.

**References:** [1] Szurgot M. et al. 2008. *Crystal Research and Technology* 43:921-930. [2] Opeil C. P. et al. 2010. *Icarus* 208:449-454. [3] Beach M. et al. 2009. *Planetary and Space Science* 57:764-770. [4] Yomogida K. and Matsui T. 1983. *Journal of Geophysical Research* 88:9513-9533. [5] Matsui T. and Osako M. 1979. *Memories of the National Institute of Polar Research* 15:243-252 (Special Issue). [6] Osako M. 1981. *Bulletin of the National Science Museum Series E*, 4:1-8.