MR12A-01 INVITED
Discovering New Minerals in the Early Solar System: a Nano-mineralogy Investigation
Nano-mineralogy is a study of earth and planetary materials at nano-scales, focused on characterizing nano- features (like inclusion, exsolution, coating, pore) in minerals and revealing nano-minerals/particles. With current high-resolution analytical electron microscopy, we are now capable to carry out nano-imaging, in-situ non-destructive chemical and structural analyses on geo-materials easier and faster. During an ongoing nano-mineralogy investigation of primitive meteorites, new minerals formed in the early solar system are being discovered. Each of the discoveries adds new information on the early solar evolution. Presented here are a few new minerals observed in refractory inclusions from Allende, including an ultra-refractory titania phase that likely formed among the first condensates, a new hibonite mineral by alteration, a Mo-rich oxide, and a Nb-rich oxide.
MR12A-02
Accretion and Preservation of Organic Matter in Carbonaceous Chondrites as Revealed by NanoSIMS Imaging.
Carbonaceous chondrites are the most primitive known meteorites. Their parent bodies accreted several discrete components of the early solar system: CAIs, other silicates, oxides, sulfides, ice, organics, and noble gases. Radioactive decay of short live radionucleides quickly heated these parent bodies and drove thermal metamorphism and aqueous alteration of their constituents. Despite this post-acretionary modification, at least some components of the organic matter in the carbaceous chondrites retained distinctive isotopic and molecular properties that may relate to their pre-acretionary origins in the protosolar nebula or in the molecular cloud that gave birth to it [1]. These processes that gave rise to early solar-system organic matter and the extent to which it was modified by parent body processes are still a matter of debate [2]. We have acquired NanoSIMS images of matrices of several CI, CM, CR and CV chondrites to document, in- situ, the distribution of organics and their textural and chemical relationships to co-existing inorganic components. Importantly, we performed these analyses on essentially unmodified fragments of matrix material pressed into indium, rather than on extracts, which have been the focus of most previous work on meteoritic organic matter. Specifically, we simultaneously collected H, D, 12C, 18O, 26CN, 28Si and 32S with a spatial resolution of 200 nm. Inorganic constituents of the imaged domains were determined by SEM imaging and EDS analysis. We identify two textural classes of organic constituents: diffuse organic matter and organic particles ~ 1 micron in diameter. The particles are common and do not exhibit any textural association with any inorganic matrix constituent. This distribution is consistent with previous observations by fluorescence optical microscopy [3]. These organic particles are likely primarily composed of insoluble organic matter (IOM) that grew prior to accretion as pure organic particules and was preserved in the matrix. In contrast to some observations of nm-scale HRTEM observations of chondritic matrices [4], organics do not seem to be associated with sulfides or sulfates. Instead, they are found intermixed with clay minerals within the matrix. We also found that a subset of organic particles in the matrices of CI, CM and CR chondrites are D rich (as previously reported by [5]). Profiles across these particles reveal that no significant isotopic exchange has occurred between these D-rich organic grains and the surrounding clays. This suggests that the isotopic composition of these grains remained unchanged during the parent body evolution, in contrast with conclusions from bulk measurements [2]. It has been previously suggested that relatively D-depleted water circulated through the parent bodies of the volatile-rich carbonaceous chondrites for 3 My. Known rates of water mobility through polymerized organic compounds and of D/H exchange between organic hydrogen and water lead one to predict that organic particles should have fully equilibrated with their surrounding phases in much less time than this. We speculate that this paradox might be evidence for exceptionally refractory character of H-C bonds in meteoritic IOM, or extreme D-exchange behavior of some organic moieties like radicals evidenced in IOM. [1] Pizzarello et al. (2006) in MESS II 625-651; [2] Alexander et al. (2007) GCA 71, 4380-4403 ; [3] Alpern and Benkheiri (1973) EPSL 19, 422-428; [4] Brearley and Abreu 32th LPSC; [5] Busemann et al (2006) Science 312, 727-730.
MR12A-03
The Role of Fe,Ni Metal and Fe,Ni Sulfide Nanoparticles in Catalytic Organic Synthesis in the Early Solar System: Evidence From Carbonaceous Chondrites.
Numerous studies have shown that carbonaceous chondrites contain a wide variety of both soluble and insoluble organic compounds. These compounds formed in a variety of different astrophysical environments including the interstellar medium, the solar nebula and on asteroidal parent bodies. The solid or insoluble organic material (IOM) in carbonaceous chondrites is likely the complex end product of synthesis and processing in all of these environments. Although the bulk chemistry and structure of IOM in carbonaceous chondrites is well understood, important questions remain as to the exact spatial occurrence and distribution of organic material within carbonaceous chondrites. Such information may provide important insights into the possible mechanisms of formation of organic material at the grain scale. We have examined the matrices of three CM carbonaceous chondrites, Y791198, Murchison and ALH81002 using a range of different TEM techniques. Mineralogically, the matrices of these meteorites consist of phyllosilicates and/or amorphous materials associated with sulfides, oxides and carbides. Using energy filtered TEM several distinct occurrences of organic material have been identified, notably associations with nanoparticles of sulfide and carbide. Sulfides have grain sizes that are commonly <100 nm with thin layers of poorly graphitized C (<1 nm) on their surfaces. This carbonaceous layer often contains nitrogen suggesting that it is organic in character. In addition, nanoparticles of Fe,Ni carbides that occur either singly or in clusters are often embedded in carbonaceous material that is also N-bearing. These carbides have experienced partial oxidation to magnetite around their rims. The ubiquitous spatial association between sulfide and carbide nanoparticles and carbonaceous material indicates a genetic relation between these phases. This association can be most readily explained by Fischer-Tropsch-type (FTT) catalysis reactions involving catalytic hydrogenation of CO to form higher molecular weight hydrocarbons. We infer that the Fe,Ni carbides were originally Fe metal grains that were carburized during catalysis. Carbide formation during FTT synthesis on Fe metal catalysts is a well-recognized phenomenon. Based on the presence of poorly- graphitized carbon on the surfaces of most nanoparticles, catalysis within the solar nebula appears to be most likely, because estimated temperatures of aqueous alteration for CM2 chondrites (<50° C) are too low to promote graphitization.
MR12A-04
Siderophile element distribution in metal-sulfide nodules of EH3 Sahara 97072: A relict condensation signature overprinted by transient melting events
Evidence of a condensation signature has been detected in the textures and compositions of metal-sulfide nodules (MSN) from the unequilibrated enstatite chondrite Sahara 97072 (EH3). There are two end-member models, both of which can reproduce the siderophile element relations and distribution among the kamacite (αFeNi), schreibersite (FeNiP3), and perryite [(FeNi)x(SiP)y] observed in the MSN. These minerals may have condensed as a single metal alloy that subsequently decomposed in transient melting events or they could have condensed as separate phases. Most likely the minerals were produced from some combination of both processes. Kamacite, schreibersite, and perryite compositions in MSN can reasonably be recombined to produce a metallic phase with Co/Ni and P/Ni ratios similar to those in primitive CI meteorites. The recombined metal has siderophile element (Ir, Ru, Co, Pd, Au, Ga,) ratios closer to CI values than kamacite alone, suggesting it could have been a condensing metal that remained in equilibrium with a cooling reduced gas of near solar composition until Au and Ga condensed. Alternatively, nearly CI chondritic Pd/Ru ratios in schreibersite suggest it could be an early condensate. Perryite may have initially formed as a condensate, as a product of metal sulfurization, or both, but, if so, it appears to also have re-crystallized from a liquid rich in Ni and Si during partial melting of the MSN. Fe, Ni, and P distributions in kamacite near perryite and schreibersite suggest they are primitive minerals in disequilibrium with the metal, indicating they formed in transient melting events as opposed to slow metamorphic heating. The siderophile element distribution in Sahara 97072 MSN is consistent with early condensing material that was later reprocessed by transient melting events.
MR12A-05
Shocking H2O Ice: The Role of Phase Changes during Impact Crater Formation
New experimental data and cratering calculations illustrate the complex response of H2O ice to shock
compression. We present peak and post-shock temperature measurements from shocked H2O ice. In
experiments with shock pressures between 8 and 14 GPa, initially ~150 K ice is compressed to a
supercritical state. In the time frame of the experiment, the supercritical H2O releases to the saturation
vapor curve and does not achieve full decompression. Further decompression requires a significant volume
expansion. In general, the time scale of expansion will depend on the internal energy and the surrounding
conditions (e.g., confined or unconfined). The temperature data validate a new 5-Phase hydrocode equation
of state model for H2O, which includes ice Ih, VI, VII, liquid, and vapor.
Using the 5-Phase EOS, we model impact cratering onto icy satellites. After passage of the impact-generated
shock wave, material beneath the growing transient crater has a layered composition: vapor, liquid, high-
pressure phases (ices VII and VI), and ice Ih. The high pressure phases cannot fully decompress without a
large volume increase. Thus, these phases initially unload to the pressure along the phase boundary; this
pressurized region affects the excavation flow field. The changes in crater excavation lead to differences in
crater size and amount of ejecta compared to excavation in a homogeneous target. The differences are
significant for large craters (e.g., complex craters on Ganymede and Callisto).
The modified excavation flow field also concentrates highly shocked material in the crater floor. In cases
where a large, hot plug is buried during crater collapse, explosions occur as the material cools by
transforming to vapor, producing features similar to central pits observed on Ganymede, Callisto, and Mars.
The behavior of shocked H2O ice during decompression should lead to a variety of features that depend
on the ambient conditions specific to each icy planetary body.
http://www.shock.eps.harvard.edu/preprints/
MR12A-06
Where are all the Strongly Shocked Meteorites?
Recent studies of high pressure minerals in melt veins have shown that the S6 shock stage, designating the most strongly shocked chondriticmeteorites, corresponds to a peak shock pressure of about 25 GPa and an effective shock pressure duration of the order of one second. (Sharp and De Carli 2006) Calculations of the probability distribution of asteroid-asteroid impact velocities in the early solar system indicate a broad peak over the range of 2-7 km/s with a mean of 5.29 km/s. (Bottke et al 1994) The peak pressure at the point of impact between two bodies of chondritic composition would be about 25 GPa for a 2 km/s impact. For a 7 km/s impact, the peak pressure would be about 100 GPa. We would therefore expect to find a substantial population of meteorites showing evidence of having been shocked to pressures over the range between 25 and 100 GPa. In fact, there are very few chondrites that appear to have been shocked to pressures above 25 GPa. Here we present the results of Autodyn (TM) hydrocode calculations of asteroid-asteroid impacts over the velocity range of 2-7 km/s to provide a measure of the range of expected shock effects in meteorites
MR12A-07
New evidence for an impact origin of Taihu lake, China: Possible trigger of the extinction of LiangChu Culture 4500 years ago
Abstract Here we report new evidence of an impact crater in south-east of China, Taihu Lake in Jiangsu Province. An impact origin was originally proposed on the basis of fractured quartz, wavy extinction quartz, and shatter cones in the sandstone of Devonian Wutong formation in the islands of Taihu lake (Wang, et al., 1992, 1993, 2000). In the absence of additional evidence, the impact origin hypothesis has fallen into disfavour. Here we report studies of sedimentary samples, which could be ejecta from Taihu, found in a small lake in the vicinity of Taihu lake. The samples consist of irregularly-shaped quartz-rich concretions found in lake sediments. Preliminary studies indicate that these samples contain angular fragments of shocked quartz. The stratigraphic age of the lake sediments is similar to that of the 65 km diameter Taihu lake. If the impact origin of Taihu lake can be conclusively established, it is of the correct age to explain the mysterious disappearance of the LiangChu culture about 4500 years ago.
MR12A-08
Structure and Stability of Jarosite at High Temperature and Pressure
Jarosite, KFe3(SO4)2(OH)6, and its related sulfates commonly occur in acid drainage environments as the weathering products of sulfide ore deposits. They can also precipitate from aqueous sulfates due to oxidation of H2S in epithermal environments and hot springs associated with volcanic activities. In 2004, jarosite was detected by the Mars Exploration Rover Mössbauer spectrometer, which has been interpreted as a strong evidence for the existence of water (and thus life) on Mars. In this work, we have investigated the crystal structure and thermodynamic stability of jarosite at temperatures up to 650 K and/or pressures up to 8 GPa using in situ neutron and synchrotron X-ray diffraction. To avoid the large incoherent scattering of neutrons by hydrogen, a deuterated sample was synthesized and characterized. Rietveld analysis of the obtained diffraction data allowed determination of unit-cell parameters, atomic positions and atomic displacement parameters as a function of temperature and pressure. In addition, the coefficients of thermal expansion, bulk moduli and pressure-temperature stability regions of jarosite were determined.