MR54A-01 INVITED
The Effect of Material Properties on the Impact Cratering Process
The cratering process is strongly affected by the physical and chemical properties of target material, including porosity, volatile content and natural mixtures of diverse rocks. On Earth craters in water-saturated sediments are larger than their energy-equivalents in dry soils, which, in turn, are larger than their energy-equivalents in crystalline rocks. On Mars, the distinctly lobate outer boundary (and sometimes several overlapping sets of flow lobes) of impact crater ejecta blankets suggests mobilization of subsurface volatiles, in contrast to evidence from the much drier Moon and Mercury, where craters are surrounded by ballistically emplaced ejecta. A thorough understanding of the behavior and influence of material characteristics on the impact process is crucial for using impact cratering as a tool to better understand the physical, geological, and biogeochemical processes on a given planetary body. The presence of volatiles, namely water, in the target may affect shock propagation and consequently target melting and the final crater morphology. When the target is a mixture of materials with very different impedance, as is the case for rocks and water, the shock wave propagation may be affected by the interaction of the original shock wave with shocks reflected at material boundaries. Single- and multiple-shocked materials experiencing the same peak shock pressure will reach substantially different thermodynamic states, with the multiple-shocked material having lower shock temperature and shock entropy. As a result, for mixtures of materials with very different impedances the maximum shock pressure alone may not be the best criterion for estimating melting and vaporization. Water vaporization may also influence the ejection process by affecting the amount and dispersion of shocked ejecta. We carried out a series of numerical studies to investigate the effect of water and/or ice in the target on the cratering process. Initial results indicate that there is no single incipient and complete shock pressure for shock melting of ice in permafrost. These values depend on dry rock porosity and on the amount and shape of the water/ice inclusions. Neglecting to properly address target properties in numerical models may ultimately limit our understanding of the cratering process and result in predictions that are in disagreement with observations.
MR54A-02
Constraints on Thermodynamics of the Lower Mantle from New Shock-wave Experiments in the MgSiO3 and Mg2SiO4 systems
We present the results of recently published [1,2,3] and new shock-wave equation-of-state measurements in the Mg-Si-O system. Dynamic experiments have been performed in our laboratory at pressures up to 245 GPa using both low-pressure starting materials (Mg2SiO4 forsterite, MgSiO3 glass, and MgSiO3 enstatite) and high pressure polycrystalline aggregates (Mg2SiO4 wadsleyite and MgSiO3 perovskite), synthesized and recovered from high pressure using innovative techniques in the multi-anvil apparatus. All of these phases were characterized by low porosity and high purity with the exception of the perovskite, which contained 15-25% majorite. Differences in initial density and internal energy between these materials lead to distinct Hugoniots, each characterized by multiple phase regimes. By comparing the difference in energy at constant volume between two or more Hugoniots within a given regime, the Grueneisen parameter (gamma) of the phase can be determined. Furthermore, with the exception of perovskite, melting is inferred along all the Hugoniots at high pressures, which provides thermodynamic data for melts at P-T conditions difficult to achieve with other methods. In concert with molecular dynamics simulations [4], the results of our experiments have important implications for processes occurring in the lower mantle. At high pressures corresponding to those of the core-mantle boundary (CMB), significant density increases (which cannot be reconciled with the formation of post-perovskite) occur upon melting. This implies that melts with compositions over the entire Mg/Si range likely for the mantle would be negatively or neutrally buoyant at conditions close to the CMB, supporting the popular hypothesis calling for stagnant, partial melt to explain ultra-low velocity zones at the CMB. Furthermore, whereas gamma for solids normally decreases with increasing pressure, our results indicate the reverse behavior for melts. This unusual thermodynamic behavior substantially changes estimates of the adiabatic temperature gradient in a terrestrial magma ocean, which requires revision of existing models of crystallization behavior and attendant dynamics during cooling in this early period of Earth history. [1] Akins et al., 2004, GRL, 34, doi:10.1029/2004GL020237 [2] Luo et al., 2004, JGR, 109, B05205, doi:10.1029/2003JB002860 [3] Mosenfelder et al., 2007, JGR, 112, B06208, doi:10.1029/2006JB004364 [4] Stixrude and Karki, 2005, Science, 310, 297-299
MR54A-03
Temperature Measurements and Melting of Shock-Compressed Minerals
Laser-driven dynamic compression experiments permit exploration of material properties at pressures and temperatures spanning those encountered throughout the interiors of large rocky planets. Accurate experimental constraints on the melting of materials under these conditions have traditionally proven difficult to obtain, but are critically important to understanding interior dynamics. Here we report on absolute temperature measurements obtained with an optical pyrometer having high temporal resolution, <100ps, coupled to a kJ laser loading the sample with a 1-4 ns pulse at 527 nm wavelength. Temporally-resolved observations of the thermal emission from the shocked sample are collected with simultaneous pressure and optical reflectivity data to measure high- pressure melting. Results are discussed for a number of minerals relevant to the properties of the core-mantle boundary region, including α-quartz (SiO2) and fused silica, enstatite (MgSiO3), olivine ((Mg, Fe)2SiO4), and periclase (MgO). This work was performed under the auspices of the U.S. DOE by LLNL under Contract No. W-7405-ENG-48.
MR54A-04
Combining Observations of Shock-induced Minerals with Calculations to Constrain the Shock History of Meteorites.
All available evidence from shock Hugoniot and release adiabat measurements and from shock recovery experiments supports the hypothesis that the conditions for shock-induced phase transitions are similar to the conditions under which quasistatic phase transitions are observed. Transitions that require high temperatures under quasistatic pressures require high temperatures under shock pressures. The high-pressure phases found in shocked meteorites are almost invariably associated with shock melt veins. A shock melt vein is analogous to a pseudotachylite, a sheet of locally melted material that was quenched by conduction to surrounding cooler material. The mechanism by which shock melt veins form is not known; possible mechanisms include shock collisions, shock interactions with cracks and pores, and adiabatic shear. If one assumes that the phases within the vein crystallized in their stability fields, then available static high-pressure data constrain the shock pressure range over which the vein solidified. Since the veins have a sheet-like geometry, one may use one-dimensional heat flow calculations to constrain the cooling and crystallization history of the veins (Langenhorst and Poirier, 2000). Although the formation mechanism of a melt vein may involve transient pressure excursions, pressure equilibration of a mm-wide vein will be complete within about a microsecond, whereas thermal equilibration will require seconds. Some of our melt vein studies have indicated that the highly-shocked L chondrite meteorites were exposed to a narrow range of shock pressures, e.g., 18-25 GPa, over a minimum duration of the order of a second. We have used the Autodyn(TM) wave propagation code to calculate details of plausible impacts on the L-chondrite parent body for a variety of possible parent body stratigraphies. We infer that some meteorites probably represent material that was shocked at a depth of >10 km in their parent bodies.
MR54A-05
Natural occurrence of a new mineral with an olivine structure and pyroxene composition in the Tenham L6 chondrite
Here we report a new mineral with an olivine structure and a pyroxene composition, which occurs in shock- induced melt veins of the Tenham L6 chondrite. This new phase was identified with transmission electron microscopy (TEM) using SAED and EDS. It occurs in clusters of acicular crystals in a glassy matrix within shock melt veins. The crystals have a distinctive curvature and aspect ratios up to 25, with width ranging from 5 nm to 20 nm and length up to 500 nm. EDS analyses provide relative cation abundances that are consistent with a pyroxene stoichiometry: Na0.06Ca0.02Mg0.71Fe0.20Al0.11Si0.94O3. These compositions are similar to that of majorite garnet from the vein center and to that of the vitrified perovskite from the vein edge. Single-crystal and polycrystalline SAED patterns are consistent with the olivine structure and space group (Pbnm). The refined cell parameters for this orthorhombic structure are: a = 0.4782 nm, b = 1.0119 nm, and c = 0.5946 nm. The new olivine-structured phase crystallized either at the rapidly quenched margins of large veins or within thin, less than 0.03 mm melt veins. Our hypothesis is that the extremely rapid quench in these samples led to significant under-cooling of the melt to temperatures below the metastable melting curve of olivine. This allowed the rapid crystallization of the new phase with high-entropy compositional features and unusual morphologies. Our diffraction and EDS data pose a problem of how to accommodate a pyroxene-like composition in an olivine structure. If we write an olivine chemical formula based our EDS data from new phase (Na0.06Ca0.04Mg1.01Fe0.29Al0.10Si0.24)1.74()0.26Si1.00O 4, the olivine structure requires about 0.25 formula units of Si4+ in octahedral M1 sites, about 0.25 formula units of vacant M sites (()), and 0.10 formula units of Na+ and Ca2+ in M2 octahedral sites. Our observations demonstrate that the olivine structure can accommodate significant non-stoicheometry at high pressure and temperature. If non-stoichiometry and excess silica occur in equilibrated olivines, one might expect them to also occur in hot regions of the deep upper mantle. We suggest that it would be useful to investigate the stability of this phase via ab initio calculations and high-pressure experiments.
MR54A-06
PartialLy Shock-Transformed Olivine in Shocked Chondrites: Mechanisms of Solid-State Transformation
High-pressure minerals, produced by shock meta-morphism, are common in and around melt veins in highly shocked chondrites. These minerals either crys-tallized from silicate melt in the shock-vein or formed by solid- state transformation of host-rock fragments entrained in the melt or along shock-vein margins. Olivine- ringwoodite transformation kinetics can be used to constrain shock duration if one knows P-T conditions and transformation mechanisms. Here we examine the solid-state transformation of olivine to ringwoodite and the formation of ringwoodite lamellae in Tenham. Partially transformed olivines show a variety of ringwoodite textures. Some have granular textures whereas others have straight or curved ringwoodite lamellae, made up of distinct (1 to 2 ?m) crystals. Many of these polycrystalline ringwoodite lamellae occur in pairs. Where these paired lamellae cross the are offset, suggesting that the lamellae are associated with shearing. Electron diffraction reveals that the ringwoodites in the polycrystalline lamellae, occur in roughly the same crystallographic orientation, defining a lattice-preferred orientation. TEM also shows that the remnant olivine is highly deformed, with high densities of complex dislocations. This olivine has a poorly organized sub-grain structure that grades into polycrystalline olivine. The nearby untransformed olivine is also highly de-formed, but less than the partially transformed olivine. TEM images of complex dislocation and sub-grain microstructures suggests that the transformation of olivine to ringwoodite involves extensive deformation. High densities of dislocations provide potential sites for heterogeneous nucleation of ringwoodite and may enhance Fe-Mg inter-diffusion. The differential stress at the initial stage of the shock results in high strains and local heating. The paired ringwoodite lamellae in olivine appear to result from shearing and possibly shear heating, where nucleation occurs on both sides of a shear band.
MR54A-07
Experimental constraints on P-T-t conditions for plagioclase amorphization and crystallization of high-pressure phases in shocked meteorites
Presence of amorphous plagioclase and high-pressure phases of plagioclase in shocked meteorites can be important constraints on the history of shock events. Here we report new experimental results on amorphization and crystalization kinetics of plagioclase (albite and labradorite) examined by in-situ X-ray observations at high pressure and temperature using Kawai-type high-pressure apparatus and synchrotron radtion. We have found that amorphization pressures become lower at higher temperatures. For example, we have observed that albite and labradorite become amorphous at 23 GPa and 1173 K, and 13 GPa and 973 K, respectively. These amorphization pressures are much lower than those reported in previous shock experiments and DAC studies at room T. We have also found that crystallization kinetics from amorphous plagioclases, which was examined by time-resolved X-ray diffraction measurements, is rather different among minerals. Jadeite first appears from amorphous plagioclases of both chemical compositions. Nucleation of other minerals such as stishovite, grossular, and Ca ferrite-type NaAlSiO4 is significantly delayed. Preliminary analysis of kinetic data based on Avrami rate equation indicates that the n-value for the formation of Jadeite is around 0.5, whereas the n-values for other phases are relatively large 3-4. Based on kinetic data obtained in albite, we have estimated P-T-t kinetic boundaries for the presence of amorphous albite, jadeite (no stishovite), and jadeite+stishovite. In the time scale of 1 sec, only jadeite is present between 1473-1973 K at around 20 GPa. Stishovite appears at higher T, and jadeite is not formed at lower T. It has been reported that jadeite is present without silica phase in shocked meteorites, which is possibly due to the difference in crystallization kinetics as demonstrated in the present study. These findings and further quantitative analysis of kinetic data make it possible to constrain P-T-t conditions of shocked meteorites based on the plagioclase breakdown.
MR54A-08
Formation of wadsleyite in a shock experiment - implications for the duration of shock events in meteorite parent bodies
We report the first observation of the high-pressure silicate phase wadsleyite in the recovery products of a shock experiment. Wadsleyite was detected by micro-X ray diffraction and EBSD. Wadsleyite grew from melt which formed by chemical reaction of periclase and silica during shock. Our findings show that the growth rate of high pressure silicate phases in shock-generated melts can be of the order of m/s and is probably not diffusion controlled. Our finding has important implications for the time scale of shock events recorded by meteorites and indicates that the presence of high pressure silicates found in shocked meteorites does not necessarily imply large impactor sizes. This work was supported by the NNSA Cooperative Agreement DOE-FC88-01NV14049 and NASA/Goddard grants under awards NNG04GP57G and NNG04GI07G. Use of the HPCAT facility was supported by DOE-BES, DOE-NNSA, NSF, DOD -TACOM, and the W.M. Keck Foundation. APS is supported by DOE-BES under Contract No. W-31-109-Eng-38.