Meteoritics & Planetary Science, Volume 41, Number 4 (2006)
http://hdl.handle.net/10150/641262
2024-03-28T14:36:10ZShock and thermal history of Martian meteorite Allan Hills 84001 from transmission electron microscopy
http://hdl.handle.net/10150/656128
Shock and thermal history of Martian meteorite Allan Hills 84001 from transmission electron microscopy
Barber, David J.; Scott, Edward R. D.
Microstructures in the Allan Hills 84001 meteorite were studied using optical and electron microscopy, putting emphasis on shock effects, which are widespread. Some orthopyroxene exhibits only (100) slip, but more typical grains suffered extensive slip, microfracturing, and frequently contain (100) clino-inversion lamellae. In fracture zones, shock deformation of orthopyroxene has produced all three effects in profusion, together with intergranular pockets of orthopyroxene glass and intragranular glass lamellae, which were apparently created by shearing on low index planes, usually (100) or {110}. Both types of plane are loci that pseudo-planar fractures tend to follow. Thus, the glass lamellae, which have not been observed in other meteorites, probably formed by frictional heating during the sliding of microscale corrugated surfaces, one over another, leading to local melting. We infer that the orthopyroxene glass and the fracture zones both formed from shear stresses created by strong shock. Ubiquitous undeformed micrometer and submicrometer euhedral chromites in orthopyroxene and plagioclase glasses and carbonate probably crystallized after shock heating and fracture zone formation. Nanocrystals of eskolaite (Cr2O3) coating silica glass grains are probably also a result of shock-induced thermal decomposition of chromite. Iron sulfides (pyrite and pyrrhotite were identified) tended to be associated with plagioclase glass. A carbonate disk showing no evidence for shock deformation had a substructure of elongated, slightly misoriented subcells in the exterior; interior regions had more eqiaxed subcells. Both microstructures probably formed during growth, but the conditions are undetermined. Chemical composition varied on a micron scale, but the rim of the disk was more ferroan; oxide precipitates and voids were widely distributed as in fracture-filling carbonates. If the fracture zones and opx glass are the result of strong shock, as we deduce, it is very unlikely that pores could have filled by carbonate long after the fracture zones formed. We infer that the carbonate, like the phosphate, olivine, pyrrhotite, eskolaite, and many euhedral, submicrometer chromites, crystallized during the final stages of the impact that created the fracture zones and glasses with compositions of plagioclase, silica, and orthopyroxene.
2006-01-01T00:00:00ZCarbonaceous materials in the acid residue from the Orgueil carbonaceous chondrite meteorite
http://hdl.handle.net/10150/656127
Carbonaceous materials in the acid residue from the Orgueil carbonaceous chondrite meteorite
Garvie, Laurence A. J.; Buseck, Peter R.
Insoluble organic matter (IOM) dominates the HF/HCl residue of the Orgueil (CI) carbonaceous chondrite meteorite. The IOM is composed primarily of two C-rich particle types. The first has a fluffy texture similar to crumpled tissue paper, and the second type occurs as solid or hollow nanospheres. High-resolution transmission electron microscope (HRTEM) images of the fluffy material show it is poorly ordered, with small, irregularly shaped regions having fringes with 0.34-0.38 nm spacings and locally 0.21 nm cross-fringes. Nanodiamonds occur in the fluffy material. The rounded C-rich particles are common in the residue and their HRTEM images show neither fringes nor nanodiamonds. Both types of carbonaceous materials have a high aromatic component, as revealed by electron energy-loss spectroscopy (EELS), with up to 10 at% substitution by S, N, and O. The average compositions of the fluffy material and nanospheres are C100S1.9N3.7O4.9 and C100S2.4N5.0O3.9, respectively. The structural and chemical heterogeneity of the carbonaceous materials may represent material from multiple sources.
2006-01-01T00:00:00ZThe rate of small impacts on Earth
http://hdl.handle.net/10150/656126
The rate of small impacts on Earth
Bland, Philip A.; Artemieva, Natalya A.
Asteroids tens to hundreds of meters in diameter constitute the most immediate impact hazard to human populations, yet the rate at which they arrive at Earth's surface is poorly known. Astronomic observations are still incomplete in this size range; impactors are subjected to disruption in Earth's atmosphere, and unlike the Moon, small craters on Earth are rapidly eroded. In this paper, we first model the atmospheric behavior of iron and stony bodies over the mass range 1-10^12 kg (size range 6 cm-1 km) taking into account deceleration, ablation, and fragmentation. Previous models in meteoritics deal with rather small masses (<10^5-10^6 kg) with the aim of interpreting registered fireballs in atmosphere, or with substantially larger objects without taking into account asteroid disruption to model cratering processes. A few earlier attempts to model terrestrial crater strewn fields did not take into account possible cascade fragmentation. We have performed large numbers of simulations in a wide mass range, using both the earlier "pancake" models and also the separated fragments model to develop a statistical picture of atmosphere-bolide interaction for both iron and stony impactors with initial diameters up to ~1 km. Second, using a compilation of data for the flux at the upper atmosphere, we have derived a cumulative size-frequency distribution (SFD) for upper atmosphere impactors. This curve is a close fit to virtually all of the upper atmosphere data over 16 orders of magnitude. Third, we have applied our model results to scale the upper atmosphere curve to a flux at the Earth's surface, elucidating the impact rate of objects 1 km diameter on Earth. We find that iron meteorites >5 x 10^4 kg (2.5 m) arrive at the Earth's surface approximately once every 50 years. Iron bodies a few meters in diameter (105-106 kg), which form craters 100 m in diameter, will strike the Earth's land area every 500 years. Larger bodies will form craters 0.5 km in diameter every 20,000 years, and craters 1 km in diameter will be formed on the Earth's land area every 50,000 years. Tunguska events (low-level atmospheric disruption of stony bolides >108 kg) may occur every 500 years. Bodies capable of producing hazardous tsunami (~200 m diameter projectiles) should strike the Earth's surface every ~100,000 years. This data also allows us to assess the completeness of the terrestrial crater record for a given area over a given time interval.
2006-01-01T00:00:00ZPetrology and chemistry of MIL 03346 and its significance in understanding the petrogenesis of nakhlites on Mars
http://hdl.handle.net/10150/656125
Petrology and chemistry of MIL 03346 and its significance in understanding the petrogenesis of nakhlites on Mars
Day, James M. D.; Taylor, Lawrence A.; Floss, Christine; McSween, Harry Y.
Antarctic meteorite Miller Range (MIL) 03346 is a nakhlite composed of 79% clinopyroxene, ~1% olivine, and 20% vitrophyric intercumulus material. We have performed a petrological and geochemical study of MIL 03346, demonstrating a petrogenetic history similar to previously discovered nakhlites. Quantitative textural study of MIL 03346 indicates long (>1 x 10^1 yr) residence times for the cumulus augite, whereas the skeletal Fe-Ti oxide, fayalite, and sulfide in the vitrophyric intercumulus matrix suggest rapid cooling, probably as a lava flow. From the relatively high forsterite contents of olivine (up to Fo43) compared with other nakhlites and compositions of augite cores (Wo38-42En35-40Fs22-28) and their hedenbergite rims, we suggest that MIL 03346 is part of the same or a similar Martian cumulate-rich lava flow as other nakhlites. However, MIL 03346 has experienced less equilibration and faster cooling than other nakhlites discovered to date. Calculated trace element concentrations based upon modal abundances of MIL 03346 and its constituent minerals are identical to whole rock trace element abundances. Parental melts for augite have REE patterns that are approximately parallel with whole rock and intercumulus melt using experimentally defined partition coefficients. This parallelism reflects closed-system crystallization for MIL 03346, where the only significant petrogenetic process between formation of augite and eruption and emplacement of the nakhlite flow has been fractional crystallization. A model for the petrogenesis of MIL 03346 and the nakhlites (Nakhla, Governador Valadares, Lafayette, Yamato-000593, Northwest Africa (NWA) 817, NWA 998) would include: 1) partial melting and ascent of melt generated from a long-term LREE depleted mantle source, 2) crystallization of cumulus augite (olivine, magnetite) in a shallow-level Martian magma chamber, 3) eruption of the crystal-laden nakhlite magma onto the surface of Mars, 4) cooling, crystal settling, overgrowth, and partial equilibration to different extents within the flow, 5) secondary alteration through hydrothermal processes, possibly immediately succeeding or during emplacement of the flow. This model might apply to single - or multiple - flow models for the nakhlites. Ultimately, MIL 03346 and the other nakhlites preserve a record of magmatic processes in volcanic rocks on Mars with analogous petrogenetic histories to pyroxene-rich terrestrial lava flows and to komatiites.
2006-01-01T00:00:00Z