Showing papers in "Meteoritics & Planetary Science in 2004"

Modeling damage and deformation in impact simulations

Abstract: Numerical modeling is a powerful tool for investigating the formation of large impact craters but is one that must be validated with observational evidence. Quantitative analysis of damage and deformation in the target surrounding an impact event provides a promising means of validation for numerical models of terrestrial impact craters, particularly in cases where the final pristine crater morphology is ambiguous or unknown. In this paper, we discuss the aspects of the behavior of brittle materials important for the accurate simulation of damage and deformation surrounding an impact event and the care required to interpret the results. We demonstrate this with an example simulation of an impact into a terrestrial, granite target that produces a 10 km-diameter transient crater. The results of the simulation are shown in terms of damage (a scalar quantity that reflects the totality of fragmentation) and plastic strain, both total plastic strain (the accumulated amount of permanent shear deformation, regardless of the sense of shear) and net plastic strain (the amount of permanent shear deformation where the sense of shear is accounted for). Damage and plastic strain are both greatest close to the impact site and decline with radial distance. However, the reversal in flow patterns from the downward and outward excavation flow to the inward and upward collapse flow implies that net plastic strains may be significantly lower than total plastic strains. Plastic strain in brittle rocks is very heterogeneous; however, continuum modeling requires that the deformation of the target during an impact event be described in terms of an average strain that applies over a large volume of rock (large compared to the spacing between individual zones of sliding). This paper demonstrates that model predictions of smooth average strain are entirely consistent with an actual strain concentrated along very narrow zones. Furthermore, we suggest that model predictions of total accumulated strain should correlate with observable variations in bulk density and seismic velocity.

Modal mineralogy of carbonaceous chondrites by X‐ray diffraction and Mössbauer spectroscopy

Abstract: Carbonaceous chondrites are among the most analyzed geological materials on Earth. However, despite this attention, and unlike most terrestrial rocks, little is known on the abundance of individual phases within them. Here, we show how a combination of several novel X-ray diffraction (XRD) techniques (including a high-brightness X-ray MicroSource®), and Mssbauer spectroscopy, allows a complete modal mineralogy to be ascertained from even the most highly unequilibrated, fine-grained chondrites for all minerals of abundance >1 wt%. Knowledge of the modal mineralogy of a sample also allows us to calculate grain density. We analyzed Allende, Murchison, Tagish Lake, and Orgueil. Based on our modal data, the grain density estimates for Allende, Murchison, and Orgueil are close to literature values. In the case of Tagish Lake, there is no published grain density, although a bulk density measurement does exist. Taking our estimate of grain density, and the measured bulk density, we calculate an exceptionally high porosity of 41% for this meteorite, similar to some chondritic IDPs and in line with a porosity calculated from an entry model for the Tagish Lake fireball. Although it is an oxidized CV, magnetite is present in Allende at a level of <0.5 wt% or <0.3 vol%, a result that is substantiated by several other instrumental studies. This may be an oxidized meteorite, but that oxidation is not manifested in abundant magnetite. In addition, we note appreciable fayalitic olivine in Orgueil, detected by both XRD and Mossbauer. We employed MicroSource® XRD to look at heterogeneity in mineral abundance in Orgueil and found substantial variation, with phyllosilicates varying inversely with olivine. The data suggest that Orgueil was initially composed primarily of anhydrous materials, which have been partially, but not completely, altered. Although the data are preliminary, comparison between our XRD modal assessment, bulk chemistry, grain density, and Mossbauer data, suggests that our estimates of mineral abundance are robust. The advent of MicroSource® XRD allows similar modal data to be acquired from samples as small as a few hundred micrograms.

Mn-Cr isotope systematics of the D'Orbigny angrite

Abstract: We have conducted a detailed study of the Mn-Cr systematics of the angrite D'Orbigny. Here, we report Cr isotopic abundances and Mn/Cr ratios in olivine, pyroxene, glass, chromite, and bulk rock samples from D'Orbigny. 53Cr excesses in these samples correlate well with their respective Mn/Cr ratios and define an isochron with a slope that corresponds to an initial 53Mn/55Mn ratio = (3.24 ± 0.04) x 10^(-6) and initial 53Cr/52Cr ratio of Ɛ(53) = 0.30 ± 0.03 at the time of isotopic closure. The 53Mn/55Mn ratio of the D'Orbigny bulk rock is more than two-fold the 53Mn/55Mn ratio of the angrites Lewis Cliff 86010 (LEW) and Angra dos Reis (ADOR) and implies an older Mn-Cr age of 4562.9 ± 0.6 Ma for D'Orbigny relative to a Pb-Pb age of 4557.8 ± 0.5 Ma for LEW and ADOR. One of the most unusual aspects of D'Orbigny is the presence of glass, a phase that has not been identified in any of the other angrites. The Mn-Cr data for glass and a pyroxene fraction found in druses indicate that they formed contemporaneously with the main phases of the meteorite. Since the Mn-Cr age of D'Orbigny is ~5 Ma years older than the angrites LEW and ADOR, D'Orbigny likely represents an earlier stage in the evolution of the angrite parent body.

Spectral reflectance-compositional properties of spinels and chromites: Implications for planetary remote sensing and geothermometry

Abstract: Reflectance spectra of spinels and chromites have been studied as a function of composition. These two groups of minerals are spectrally distinct, which relates largely to differences in the types of major cations present. Both exhibit a number of absorption features in the 0.326 μm region that show systematic variations with composition and can be used to quantify or constrain certain compositional parameters, such as cation abundances, and site occupancies. For spinels, the best correlations exist between Fe2+ content and wavelength positions of the 0.46, 0.93, 2.8, Restrahelen, 12.3, 16.2, and 17.5 μm absorption features, Al and Fe3+ content with the wavelength position of the 0.93 m absorption feature, and Cr content from the depth of the absorption band near 0.55 μm. For chromites, the best correlations exist between Cr content and wavelength positions of the 0.49, 0.59, 2, 17.5, and 23 μm absorption features, Fe2+ and Mg contents with the wavelength position of the 1.3 μm absorption feature, and Al content with the wavelength position of the 2 μm absorption feature. At shorter wavelengths, spinels and chromites are most readily distinguished by the wavelength position of the absorption band in the 2 m region ( 2.1 μm for chromite), while at longer wavelengths, spectral differences are more pronounced. The importance of being able to derive compositional information for spinels and chromites from spectral analysis stems from the relationship between composition and petrogenetic conditions (pressure, temperature, oxygen fugacity) and the widespread presence of spinels and chromites in the inner solar system. When coupled with the ability to derive compositional information for mafic silicates from spectral analysis, this opens up the possibility of deriving petrogenetic information for remote spinel- and chromite-bearing targets from analysis of their reflectance spectra.

High-calcium pyroxene as an indicator of igneous differentiation in asteroids and meteorites

Abstract: Our analyses of high quality spectra of several S-type asteroids (17 Thetis, 847 Agnia, 808 Merxia, and members of the Agnia and Merxia families) reveal that they include both low- and highcalcium pyroxene with minor amounts of olivine ( ~0.4. High-calcium pyroxene is a spectrally detectable and petrologically important indicator of igneous history and may prove critical in future studies aimed at understanding the history of asteroidal bodies. The silicate mineralogy inferred for Thetis and the Merxia and Agnia family members requires that these asteroids experienced igneous differentiation, producing broadly basaltic surface lithologies. Together with 4 Vesta (and its smaller "Vestoid" family members) and the main-belt asteroid 1489 Magnya, these new asteroids provide strong evidence for igneous differentiation of at least five asteroid parent bodies. Based on this analysis of a small subset of the near-infrared asteroid spectra taken to date with SpeX at the NASA IRTF, we expect that the number of known differentiated asteroids will increase, consistent with the large number of parent bodies inferred from studies of iron meteorites.

Aerogel keystones: Extraction of complete hypervelocity impact events from aerogel collectors

Abstract: In January 2006, the Stardust mission will return the first samples from a solid solar system body beyond the Moon and the first samples of contemporary interstellar dust ever collected. Although sophisticated laboratory instruments exist for the analysis of Stardust samples, techniques for the recovery of particles and particle residues from aerogel collectors remain primitive. Here, we describe our recent progress in developing techniques for extracting small volumes of aerogel, which we have called "keystones," which completely contain particle impacts but minimize the damage to the surrounding aerogel collector. These keystones can be fixed to custom-designed micromachined silicon fixtures (so called "microforklifts"). In this configuration, the samples are self-supporting, which can be advantageous in situations where interference from a supporting substrate is undesirable. The keystones may also be extracted and placed onto a substrate without a fixture. We have also demonstrated the capability of homologously crushing these unmounted keystones for analysis techniques that demand flat samples.

The nature of the groundmass of surficial suevite from the Ries impact structure, Germany, and constraints on its origin

Abstract: Surficial suevites from the Ries impact structure have been investigated in the field and using optical and analytical scanning electron microscopy. The groundmass of these suevites comprises calcite, clay minerals, impact melt glass, crystallites (plagioclase, garnet, and pyroxene), francolite, and Ba-phillipsite. The latter zeolite is a secondary phase. Abundant textures have been observed: intricate flow textures between the various groundmass phases, globules of each phase in the other phases, spheroids of pyrrhotite in calcite, the "budding-off" of clay globules into silicate glass and/or calcite, euhedral overgrowths of francolite on apatite clasts, and quench-textured crystallites in the groundmass. Groundmass-forming calcite displays higher FeO, MnO, and SiO2 contents than limestone target material. The composition of suevite "clay minerals" is highly variable and not always consistent with montmorillonite. Three types of glasses are distinguished in the groundmass. Type 1 glasses are SiO2-rich and are clearly derived from sandstones in the sedimentary cover, while the protoliths of the other two glass types remains unclear. Analytical data and micro-textures indicate that the calcite, silicate glass, francolite, and clay minerals of the groundmass of the Ries suevites represent a series of impact-generated melts that were molten at the time of, and after, deposition. On cooling, plagioclase, pyroxene, and garnet crystallized from the groundmass. These results are at variance with the current, traditional descriptive definition of suevite. Given that Ries is the original type occurrence of "suevite," some modification to the traditional definition may be in order. As the results of this study are most consistent with the groundmass of Ries surficial suevites representing a mix of several types of impact-generated melts, we suggest that a possible origin for these suevites is as some form of impact melt flow(s) that emanated from different regions of the evolving crater.

Silica-rich igneous rims around magnesian chondrules in CR carbonaceous chondrites: Evidence for condensation origin from fractionated nebular gas

Abstract: The outer portions of many type I chondrules (Fa and Fs 800 K) ambient nebular temperatures and escaped remelting at lower ambient temperatures. We suggest that these rims formed either by gas-solid condensation of silica-normative materials onto chondrule surfaces and subsequent incomplete melting, or by direct SiO(gas) condensation into chondrule melts. In either case, the condensation occurred from a fractionated, nebular gas enriched in Si, Na, K, Mn, and Cr relative to Mg. The fractionation of these lithophile elements could be due to isolation (in the chondrules) of the higher temperature condensates from reaction with the nebular gas or to evaporation-recondensation of these elements during chondrule formation. These mechanisms and the observed increase in pyroxene/olivine ratio toward the peripheries of most type I chondrules in CR, CV, and ordinary chondrites may explain the origin of olivine-rich and pyroxene-rich chondrules in general.

Dynamical and compositional assessment of near‐Earth object mission targets

Abstract: Using an H-plot analysis, we identify 234 currently known near-Earth objects that are accessible for rendezvous with a "best case" delta-V of less than 7 km/s. We provide a preliminary compositional interpretation and assessment of these potential targets by summarizing the taxonomic properties for 44 objects. Results for one-half (22) of this sample are based on new spectroscopic measurements presented here. Our approach provides an easy-to-update method for giving guidelines to both observers and mission analysts for focusing on objects for which actual mission opportunities are most likely to be found. Observing prospects are presented for categorizing the taxonomic properties of the most accessible targets that are not yet measured.

Origin and emplacement of the impact formations at Chicxulub, Mexico, as revealed by the ICDP deep drilling at Yaxcopoil-1 and by numerical modeling

Abstract: We present and interpret results of petrographic, mineralogical, and chemical analyses of the 1511 m deep ICDP Yaxcopoil-1 (Yax-1) drill core, with special emphasis on the impactite units. Using numerical model calculations of the formation, excavation, and dynamic modification of the Chicxulub crater, constrained by laboratory data, a model of the origin and emplacement of the impact formations of Yax-1 and of the impact structure as a whole is derived. The lower part of Yax-1 is formed by displaced Cretaceous target rocks (610 m thick), while the upper part comprises six suevite-type allochthonous breccia units (100 m thick). From the texture and composition of these lithological units and from numerical model calculations, we were able to link the seven distinct impact-induced units of Yax-1 to the corresponding successive phases of the crater formation and modification, which are as follows: 1) transient cavity formation including displacement and deposition of Cretaceous megablocks; 2) ground surging and mixing of impact melt and lithic clasts at the base of the ejecta curtain and deposition of the lower suevite right after the formation of the transient cavity; 3) deposition of a thin veneer of melt on top of the lower suevite and lateral transport and brecciation of this melt toward the end of the collapse of the transient cavity (brecciated impact melt rock); 4) collapse of the ejecta plume and deposition of fall-back material from the lower part of the ejecta plume to form the middle suevite near the end of the dynamic crater modification; 5) continued collapse of the ejecta plume and deposition of the upper suevite; 6) late phase of the collapse and deposition of the lower sorted suevite after interaction with the inward flowing atmosphere; 7) final phase of fall-back from the highest part of the ejecta plume and settling of melt and solid particles through the reestablished atmosphere to form the upper sorted suevite; and 8) return of the ocean into the crater after some time and minor reworking of the uppermost suevite under aquatic conditions. Our results are compatible with: a) 180 km and 100 km for the diameters of the final crater and the transient cavity of Chicxulub, respectively, as previously proposed by several authors, and b) the interpretation of Chicxulub as a peak-ring impact basin that is at the transition to a multi-ring basin.

Fine‐grained, spinel‐rich inclusions from the reduced CV chondrites Efremovka and Leoville: I. Mineralogy, petrology, and bulk chemistry

Abstract: Fine-grained, spinel-rich inclusions in the reduced CV chondrites Efremovka and Leoville consist of spinel, melilite, anorthite, Al-diopside, and minor hibonite and perovskite; forsterite is very rare. Several CAIs are surrounded by forsterite-rich accretionary rims. In contrast to heavily altered fine-grained CAIs in the oxidized CV chondrite Allende, those in the reduced CVs experienced very little alteration (secondary nepheline and sodalite are rare). The Efremovka and Leoville fine-grained CAIs are 16O-enriched and, like their Allende counterparts, generally have volatility fractionated group II rare earth element patterns. Three out of 13 fine-grained CAIs we studied are structurally uniform and consist of small concentrically zoned nodules having spinel±hibonite±perovskite cores surrounded by layers of melilite and Al-diopside. Other fine-grained CAIs show an overall structural zonation defined by modal mineralogy differences between the inclusion cores and mantles. The cores are melilite-free and consist of tiny spinel±hibonite±perovskite grains surrounded by layers of anorthite and Al-diopside. The mantles are calcium-enriched, magnesium-depleted and coarsergrained relative to the cores; they generally contain abundant melilite but have less spinel and anorthite than the cores. The bulk compositions of fine-grained CAIs generally show significant fractionation of Al from Ca and Ti, with Ca and Ti being depleted relative to Al; they are similar to those of coarsegrained, type C igneous CAIs, and thus are reasonable candidate precursors for the latter. The finegrained CAIs originally formed as aggregates of spinel-perovskite-melilite ± hibonite gas-solid condensates from a reservoir that was 16O-enriched but depleted in the most refractory REEs. These aggregates later experienced low-temperature gas-solid nebular reactions with gaseous SiO and Mg to form Al-diopside and anorthite. The zoned structures of many of the fine-grained inclusions may be the result of subsequent reheating that resulted in the evaporative loss of SiO and Mg and the formation of melilite. The inferred multi-stage formation history of fine-grained inclusions in Efremovka and Leoville is consistent with a complex formation history of coarse-grained CAIs in CV chondrites.

Hydrothermal alteration in the core of the Yaxcopoil‐1 borehole, Chicxulub impact structure, Mexico

Abstract: Petrographic, electron microprobe, and Raman spectrometric analyses of Yaxcopoil-1 core samples from the Chicxulub crater indicate that the impact generated a hydrothermal system. Relative textural and vein crosscutting relations and systematic distribution of alteration products reveal a progression of the hydrothermal event in space and time and provide constraints on the nature of the fluids. The earliest calcite, halite, and gaylussite suggest that the impactite sequence was initially permeated by a low temperature saline brine. Subsequent development of a higher temperature hydrothermal regime is indicated by thermal metamorphic diopside-hedenbergite (Aeg3Fs(18-33)En32 11Wo(47-53)) after primary augite and widespread Na-K for Ca metasomatic alkali exchange in plagioclase. Hydrothermal sphene, apatite, magnetite ± (bornite), as well as early calcite (combined 3 to 8 vol%) were introduced with metasomatic feldspar. A lower temperature regime characterized by smectite after probable primary glass, secondary chlorite, and other pre-existing mafic minerals, as well as very abundant calcite veins and open-space fillings, extensively overprinted the early hydrothermal stage. The composition of early and late hydrothermal minerals show that the solution was chlorine-rich (Cl/F >10) and that its Fe/Mg ratio and oxidation state increased substantially (4 to 5 logO2 units) as temperature decreased through time. The most altered zone in the impactite sequence occurs 30 m above the impact melt.The lack of mineralogical zoning about the impact melt and convective modeling constraints suggest that this unit was too thin at Yaxcopoil-1 to provide the necessary heat to drive fluids and implies that the hydrothermal system resulted from the combined effects of a pre-existing saline brine and heat that traveled to the Yaxcopoil-1 site from adjacent areas where the melt sheet was thicker. Limonite after iron oxides is more common toward the top of the sequence and suggests that the impactite section was subjected to weathering before deposition of the Tertiary marine cover. In addition, scarce latest anatase stringers, chalcopyrite, and barite in vugs, francolite after apatite, and recrystallized halite are the likely products of limited post- ydrothermal ambient-temperature diagenesis, or ocean and/or meteoric water circulation.

Radar observations of asteroid 25143 Itokawa (1998 SF36)

Abstract: We observed 25143 Itokawa, the target of Japans Hayabusa (MUSES-C) sample-return mission, during its 2001 close approach at Arecibo on twelve dates during March 18-April 9 and at Goldstone on nine dates during March 20-April 2. We obtained delay-Doppler images with range resolutions of 100 ns (15 m) at Arecibo and 125 ns (19 m) at Goldstone. Itokawas average circular polarization ratio at 13 cm, 0.26 ± 0.04, is comparable to that of Eros, so its cm-to-m surface roughness probably is comparable to that on Eros. Itokawas radar reflectivity and polarization properties indicate a near-surface bulk density within 20% of 2.5 g cm^(-3). We present a preliminary estimate of Itokawas shape, reconstructed from images with rather limited rotation-phase coverage, using the method of Hudson (1993) and assuming the lightcurve-derived spin period (12.132 hr) and pole direction (ecliptic long., lat. = 355°, -84°) of Kaasalainen et al. (2003). The model can be described as a slightly asymmetrical, slightly flattened ellipsoid with extents along its principal axes of 548 x 312 x 276 m ± 10%. Itokawas topography is very subdued compared to that of other asteroids for which spacecraft images or radar reconstructions are available. Similarly, gravitational slopes on our Itokawa model average only 9° and everywhere are less than 27°. The radar-refined orbit allows accurate identification of Itokawas close planetary approaches through 2170. If radar ranging planned for Itokawas 2004 apparition succeeds, then tracking of Hayabusa during its 2005 rendezvous should reveal Yarkovsky perturbation of the asteroids orbit.

Experimental ejection angles for oblique impacts: Implications for the subsurface flow‐field

Abstract: A simple analytical solution for subsurface particle motions during impact cratering is useful for tracking the evolution of the transient crater shape at late times. A specific example of such an analytical solution is Maxwells Z-Model, which is based on a point-source assumption. Here, the parameters for this model are constrained using measured ejection angles from both vertical and oblique experimental impacts at the NASA Ames Vertical Gun Range. Data from experiments reveal that impacts at angles as high as 45° to the targets surface generate subsurface flow-fields that are significantly different from those created by vertical impacts. The initial momentum of the projectile induces a subsurface momentum-driven flow-field that evolves in three dimensions of space and in time to an excavation flow-field during both vertical and oblique impacts. A single, stationary pointsource model (specifically Maxwells Z-Model), however, is found inadequate to explain this detailed evolution of the subsurface flow-field during oblique impacts. Because 45° is the most likely impact angle on planetary surfaces, a new analytical model based on a migrating point-source could prove quite useful. Such a model must address the effects of the subsurface flow-field evolution on crater excavation, ejecta deposition, and transient crater morphometry.

The Meteoritical Bulletin, No. 88, 2004 July

Abstract: The Meteoritical Bulletin No. 88 lists information for 1610 newly classified meteorites, comprising 753 from Antarctica, 302 from Africa, 505 from Asia (495 of which are from Oman), 40 from North America, 5 from South America, 4 from Europe, and 1 of unknown origin. Information is provided for 9 falls (Alby sur Cheran, Al Zarnkh, Devgaon, Kamioka, Kendrapara, Maromandia, New Orleans, Sivas, and Villalbeto de la Pena). Noteworthy specimens include a eucrite fall (Alby sur Cheran), 6 martian meteorites, 13 lunar meteorites, and 12 irons including one weighing 3 metric tons (Dronino).

Observations at terrestrial impact structures: Their utility in constraining crater formation

Abstract: Hypervelocity impact involves the near instantaneous transfer of considerable energy from the impactor to a spatially limited near-surface volume of the target body. Local geology of the target area tends to be of secondary importance, and the net result is that impacts of similar size on a given planetary body produce similar results. This is the essence of the utility of observations at impact craters, particularly terrestrial craters, in constraining impact processes. Unfortunately, there are few well-documented results from systematic contemporaneous campaigns to characterize specific terrestrial impact structures with the full spectrum of geoscientific tools available at the time. Nevertheless, observations of the terrestrial impact record have contributed substantially to fundamental properties of impact. There is a beginning of convergence and mutual testing of observations at terrestrial impact structures and the results of modeling, in particular from recent hydrocode models. The terrestrial impact record provides few constraints on models of ejecta processes beyond a confirmation of the involvement of the local substrate in ejecta lithologies and shows that Z-models are, at best, first order approximations. Observational evidence to date suggests that the formation of interior rings is an extension of the structural uplift process that occurs at smaller complex impact structures. There are, however, major observational gaps and cases, e.g., Vredefort, where current observations and hydrocode models are apparently inconsistent. It is, perhaps, time that the impact community as a whole considers documenting the existing observational and modeling knowledge gaps that are required to be filled to make the intellectual breakthroughs equivalent to those of the 1970s and 1980s, which were fueled by observations at terrestrial impact structures. Filling these knowledge gaps would likely be centered on the later stages of formation of complex and ring structures and on ejecta.

Tectonic influences on the morphometry of the Sudbury impact structure: Implications for terrestrial cratering and modeling

Abstract: Impact structures developed on active terrestrial planets (Earth and Venus) are susceptible to pre-impact tectonic influences on their formation. This means that we cannot expect them to conform to ideal cratering models, which are commonly based on the response of a homogeneous target devoid of pre-existing flaws. In the case of the 1.85 Ga Sudbury impact structure of Ontario, Canada, considerable influence has been exerted on modification stage processes by late Archean to early Proterozoic basement faults. Two trends are dominant: 1) the NNW-striking Onaping Fault System, which is parallel to the 2.47 Ga Matachewan dyke swarm, and 2) the ENE-striking Murray Fault System, which acted as a major Paleoproterozoic suture zone that contributed to the development of the Huronian sedimentary basin between 2.45-2.2 Ga. Sudbury has also been affected by syn- to post-impact regional deformation and metamorphism: the 1.9-1.8 Ga Penokean orogeny, which involved NNW-directed reverse faulting, uplift, and transpression at mainly greenschist facies grade, and the 1.16-0.99 Ga Grenville orogeny, which overprinted the SE sector of the impact structure to yield a polydeformed upper amphibolite facies terrain. The pre-, syn-, and post-impact tectonics of the region have rendered the Sudbury structure a complicated feature. Careful reconstruction is required before its original morphometry can be established. This is likely to be true for many impact structures developed on active terrestrial planets. Based on extensive field work, combined with remote sensing and geophysical data, four ring systems have been identified at Sudbury. The inner three rings broadly correlate with pseudotachylyte (friction melt) -rich fault systems. The first ring has a diameter of ~90 km and defines what is interpreted to be the remains of the central uplift. The second ring delimits the collapsed transient cavity diameter at ~130 km and broadly corresponds to the original melt sheet diameter. The third ring has a diameter of ~180 km. The fourth ring defines the suggested apparent crater diameter at ~260 km. This approximates the final rim diameter, given that erosion in the North Range is <6 km and the ring faults are steeply dipping. Impact damage beyond Ring 4 may occur, but has not yet been identified in the field. One or more rings within the central uplift (Ring 1) may also exist. This form and concentric structure indicates that Sudbury is a peak ring or, more probably, a multi-ring basin. These parameters provide the foundation for modeling the formation of thisrelatively large terrestrial impact structure.

Impact lithologies and their emplacement in the Chicxulub impact crater: Initial results from the Chicxulub Scientific Drilling Project, Yaxcopoil, Mexico

Abstract: The Chicxulub Scientific Drilling Project (CSDP), Mexico, produced a continuous core of material from depths of 404 to 1511 m in the Yaxcopoil-1 (Yax-1) borehole, revealing (top to bottom) Tertiary marine sediments, polymict breccias, an impact melt unit, and one or more blocks of Cretaceous target sediments that are crosscut with impact-generated dikes, in a region that lies between the peak ring and final crater rim. The impact melt and breccias in the Yax-1 borehole are 100 m thick, which is approximately 1/5 the thickness of breccias and melts exposed in the Yucatan-6 exploration hole, which is also thought to be located between the peak ring and final rim of the Chicxulub crater. The sequence and composition of impact melts and breccias are grossly similar to those in the Yucatan-6 hole. Compared to breccias in other impact craters, the Chicxulub breccias are incredibly rich in silicate melt fragments (up to 84% versus 30 to 50%, for example, in the Ries). The melt in the Yax-1 hole was produced largely from the silicate basement lithologies that lie beneath a 3 km- thick carbonate platform in the target area. Small amounts of immiscible molten carbonate were ejected with the silicate melt, and clastic carbonate often forms the matrix of the polymict breccias. The melt unit appears to have been deposited while molten but brecciated after solidification. The melt fragments in the polymict breccias appear to have solidified in flight, before deposition, and fractured during transport and deposition.

A note on the snow line in protostellar accretion disks

Abstract: The temperature of ice grains in a protostellar disk is computed for a series of disk models. The region of stability against sublimation is calculated for small ice grains composed of either pure ice or "dirty" ice. We show that in the optically thin photosphere of the disk the gas temperature must be around 145 K for ice grains to be stable. This is much lower than the temperature of 170 K that is usually assumed.

The Chicxulub Scientific Drilling Project (CSDP)

Abstract: available online at http://meteoritics.org 787 © Meteoritical Society, 2004. Printed in USA. The Chicxulub Scientific Drilling Project (CSDP) Jaime URRUTIA-FUCUGAUCHI,1 Joanna MORGAN,2 Dieter STÖFFLER,3 and Philippe CLAEYS4 1Instituto de Geofisica, Universidad Nacional Autónoma de México, Coyoacán 04510, Mexico City, Mexico 2Earth Sciences and Engineering, Imperial College London, SW7 2AZ, United Kingdom 3Natural History Museum, Humboldt University, D-10099, Berlin, Germany 4Department of Geology, Vrije Universiteit Brussel, Pleinaan 2, B-1050, Brussels, Belgium In the June and July issues of Meteoritics & Planetary Science, we present the initial results of the Chicxulub Scientific Drilling Project (CSDP). The Chicxulub crater is a unique impact structure associated with one of the most dramatic geological events in the Phanerozoic. It is buried beneath carbonate sediments in the northern Yucatán Peninsula, southeastern Mexico. It has a diameter of 180–200 km and a complex multi-ring structure (Fig. 1). It represents the youngest and the best preserved of only three large multi-ring impact basins documented in terrestrial geological record. The impact that excavated the Chicxulub crater has been dated at 65 Ma and, therefore, is related to the event that marks the Cretaceous/Tertiary (K/T) boundary and the extinction of about 75% of species (50% of genera, including the dinosaurs) (Alvarez et al. 1980; Hildebrand et al. 1991). This crater presents a unique opportunity to discover new important information on the formation and characteristics of such large multi-ring impact craters, their environmental and climatic effects, and their implications for geological and biological evolution. The Chicxulub crater has been the focus of numerous studies, particularly in the last decade. Studies include offshore and onshore geophysical surveys, drilling/coring, laboratory analyses of impact breccias and melt, and computer modeling (e.g., Sharpton et al. 1992; Morgan et al. 1997, 2000; Collins et al. 2002; Ebbing et al. 2001). Drilling inside the structure and in its immediate vicinity has been carried out within the oil exploratory program by PEMEX with intermittent core recovery and, more recently, by the National University of Mexico (UNAM) Chicxulub drilling program resulting in continuous core recovery. The need for drilling/coring within the deeper part of the impact basin, where the crater floor is buried under several hundred meters of Tertiary carbonate sediments, was recognized already during the early studies of the crater because only the uppermost crater-related deposits were drilled (Urrutia-Fucugauchi et al. 1996; Rebolledo-Vieyra et al. 2000). With the initiation of the International Continental Scientific Drilling Program (ICDP), interest in drilling the crater increased and CSDP was developed as part of an international collaboration within the framework of ICDP. The drilling project was financed by ICDP and the National University of Mexico (UNAM) and was coordinated by UNAM. The CSDP borehole Yaxcopoil-1 was drilled from December 2001 through March 2002 in the southern sector of the crater at ~62 km radial distance from the approximate crater center at Chicxulub Puerto (Fig. 1). The Yaxcopoil-1 (Yax-1) borehole was planned to core continuously into the lower part of the post-impact carbonate sequence, the impact breccias, and the displaced Cretaceous rocks. The drill site at Hacienda Yaxcopoil (Figs. 2a and 2b) was selected based on integration of gravity, magnetics, magnetotelluric and offshore seismic surveys, pre-existing boreholes of PEMEX and UNAM programs, site conditions and access, land ownership, water availability, and an environmental impact assessment (Urrutia-Fucugauchi et al. 2001). An INDECO rotary drill rig from Perforaciones Industriales Termicas, S. A. (PITSA) and the coring device from the Drilling, Observation, and Sampling of the Earth’s Continental Crust (DOSECC) were used for the drilling/ coring operations. Rotary mode was used to drill from the surface to the depth of 404 m. After running wireline logs and casing the borehole, drilling was continued with wireline coring of the carbonate sequence and impact lithologies. Cores 63.5 mm in diameter were obtained to the depth of 993 m. At this depth, the HQ coring string became stuck and eventually was left in the hole as casing. Coring resumed with an NQ string (core diameter of 47.6 mm) to the final depth of 1511 m. Geophysical logging was conducted after completion of drilling through the first 400 m and reaching the final depth of 1511 m. Observations included hole deviation and azimuth (caliper, SP), magnetic susceptibility, radioactive element contents, gamma ray, electrical resisitivity, temperature, and conventional and waveform sonic. The Yax-1 borehole is open and available for experiments under a ten-year agreement between the Yaxcopoil Hacienda and UNAM. Several geophysical logging campaigns have been conducted after completion of the drilling project. A core laboratory and temporary repository was established at the University of Yucatán in Mérida. Facilities for digital photography of core boxes and core segments and 788 J. Urrutia-Fucugauchi et al. an automated digital core scanner were available for documentation. Cores were transported from the drill site daily, and information was made available on the CSDP web site through the ICDP information system. After completion of drilling operations, cores were packed and shipped to the UNAM Core Repository in Mexico City. Cores were further examined and then cut longitudinally in halves (one half is the project archive, while the other one is available for sampling by CSDP science team). Full cores and halves were digitally scanned for high resolution images, some of which are presented in the papers included in these two volumes of MAPS. The CSDP recovered core from the lower Tertiary carbonate sequence, impact breccias, and overlying Cretaceous carbonates, down to the depth of 1511 m. Core recovery was 98.5%. The Tertiary carbonates were cored between 404 m and 795 m. Impact breccias were cored between 795 m and 895 m, presenting an unexpectedly thin sequence of impactites, given the greater thickness of such breccias in wells located toward and outward the crater center relative to Yax-1. Beneath the impact breccias, a sequence of carbonate rocks (limestones, dolomites, and anhydrite) were recovered. The dip of the carbonates varies from being subhorizontal to up to 60 degrees, and these rocks contain thin dikes of breccia and melt. Impact breccias have been divided into six units which, from top to bottom, are (prelimary log names): redeposited suevites (794.63–808.02 m), suevites (808.02–822.86 m), chocolate-brown melt breccias (822.86– 845.80 m), glass-rich variegated suevites (845.80–861.06 m), green monomicitic-autogene melt breccias (861.06– 884.92 m), variegated polymicitic, allogenic clast melt breccias (884.92–894.94 m) (Dressler et al. 2003). A similar subdivision with a different classification of the lithologies Fig. 1. Location of the CSDP borehole Yax-1. Fig. 2a. Drilling operations at Hacienda Yaxcopoil. CSDP-Introduction 789 has been proposed in Dressler et al. (2003) and in other papers included in these volumes (e.g., Tuchscherer et al. 2004; Stöffler et al. 2004). The Cretaceous sedimentary rocks below about 895 m are cut with dikes of polymict breccias and display zones of monomict brecciation. They appear to represent “megablocks” displaced by the impact event. Anhydrite layers, the thickness of which varies from a few centimeters to up to 15 m, and make some 27% of the megablocks. Organic-rich layers and oil-bearing units are present at depths between 1410 and 1455 m (according to Kenkmann et al. [2004], this layer starts at 1263 m.). Scientific study of Yax-1 core samples and complementary studies are allowing researchers to: 1) evaluate the link between the Chicxulub crater and global K/ T boundary layer and mass extinction; 2) study large-scale cratering processes; 3) investigate post-impact crater modifications, environmental evolution, and faunal recovery; and 4) provide additional constraints on target compositions and deformation styles characteristic of the zone flanking the collapsed transient cavity. Initial studies have already provided important progress on these issues and, at the same time, left questions unanswered and opened new lines of inquiry. For instance: 1. The proportion of basement material within the impact breccias is higher than observed at other craters, opening exciting possibilities for investigations of excavation models and the nature of the Yucatán crust. 2. The impact breccia layers are thinner than predicted, with implications for crater structure, breccia emplacement, and crater formation. 3. Although there are layers of anhydrite in the target rocks, there is a surprising lack of evidence for anhydrite in the impact breccias. 4. Several different analyses document the importance of post-impact hydrothermal alteration within the core samples. 5. Recovered blocks of Cretaceous and impactites provide valuable material for laboratory analyses, despite the effects of subsequent alteration. 6. The origin of the thick carbonate sequence beneath the breccias (>600 m) has provoked an interesting debate and requires further study. 7. Initial studies of the impactite and early Paleocene stratigraphy have provided interesting age constraints and information on sequence completeness (for example the identification of the 29r/29n chron boundary just above the appearance of the first Danian fossils), provoking a debate about the age of the Chicxulub impact. In two consecutive volumes of MAPS, we present the initial results of the CSDP Science Team. These volumes include papers on the petrology, mineralogy, geochemistry, hydrothermal alteration, and degree of shock of the impactites, as well as interpretations of their depo

Evaluating planetesimal bow shocks as sites for chondrule formation

Abstract: We investigate the possible formation of chondrules by planetesimal bow shocks. The formation of such shocks is modeled using a piecewise parabolic method (PPM) code under a variety of conditions. The results of this modeling are used as a guide to study chondrule formation in a one- dimensional, finite shock wave. This model considers a mixture of chondrule-sized particles and micron-sized dust and models the kinetic vaporization of the solids. We found that only planetesimals with a radius of ~1000 km and moving at least ~8 km/s with respect to the nebular gas can generate shocks that would allow chondrule-sized particles to have peak temperatures and cooling rates that are generally consistent with what has been inferred for chondrules. Planetesimals with smaller radii tend to produce lower peak temperatures and cooling rates that are too high. However, the peak temperatures of chondrules are only matched for low values of chondrule wavelength-averaged emissivity. Very slow cooling (<~100s of K/hr) can only be achieved if the nebular opacity is low, which may result after a significant amount of material has been accreted into objects that are chondrule-sized or larger, or if chondrules formed in regions of the nebula with small dust concentrations. Large shock waves of approximately the same scale as those formed by gravitational instabilities or tidal interactions between the nebula and a young Jupiter do not require this to match the inferred thermal histories of chondrules.

Concentration and variability of the AIB amino acid in polar micrometeorites: Implications for the exogenous delivery of amino acids to the primitive Earth

Abstract: — Micrometeorites (MMs) are extraterrestrial particles ranging in size from 25 μm to 2 mm that survive atmospheric entry and are collected on the Earth's surface. They represent the largest mass flux (MF) of extraterrestrial material (30,000 ± 20,000 t/yr) to the present-day Earth. Studies of large collections of MMs suggest that about 20% have not been heated to high temperatures and that they contain organic carbon. Since non-protein amino acids have been found in some carbonaceous meteorites, they might also be found in unmelted MMs. However, previous searches for amino acids in MMs were inconclusive. We combined a new extraction method for amino acids with a highly sensitive analytical method to detect and quantitate amino acids in MMs collected at the South Pole. We found the non-protein amino acid α-amino isobutyric acid (AIB) in one of our samples. The non-detection of this amino acid in the other samples analyzed suggests that there are amino acid-containing and amino acid-free MMs, with ˜14% of the MMs containing AIB. Since the MF of MMs is much higher than that of carbonaceous chondrites (CMs), amino acids in these small particles would represent an important source of exogenous delivery of organic molecules. Therefore, the results are discussed on the basis of their implications for astrobiology.

The orbit, atmospheric dynamics, and initial mass of the Park Forest meteorite

Abstract: The fireball accompanying the Park Forest meteorite fall (L5) was recorded by ground-based videographers, satellite systems, infrasound, seismic, and acoustic instruments. This meteorite shower produced at least 18 kg of recovered fragments on the ground (Simon et al. 2004). By combining the satellite trajectory solution with precise ground-based video recording from a single site, we have measured the original entry velocity for the meteoroid to be 19.5 ± 0.3 km/s. The earliest video recording of the fireball was made near the altitude of 82 km. The slope of the trajectory was 29 from the vertical, with a radiant azimuth (astronomical) of 21 and a terminal height measured by infrared satellite systems of 18 km. The meteoroid's orbit has a relatively large semi-major axis of 2.53 ± 0.19 AU, large aphelion of 4.26 ± 0.38 AU, and low inclination. The fireball reached a peak absolute visual magnitude of -22, with three major framentation episodes at the altitudes of 37, 29, and 22 km. Acoustic recordings of the fireball airwave suggest that fragmentation was a dominant process in production of sound and that some major fragments from the fireball remained supersonic to heights as low as ~10 km. Seismic and acoustic recordings show evidence of fragmentation at 42, 36, 29, and 17 km. Examination of implied energies/initial masses from all techniques (satellite optical, infrasound, seismic, modeling) leads us to conclude that the most probable initial mass was (11 ± 3) 10^3 kg, corresponding to an original energy of ~0.5 kt TNT (2.1 x 10^12 J) and a diameter of 1.8 m. These values correspond to an integral bolometric efficiency of 7 ± 2%. Early fragmentation ram pressures of 30 seconds.

Ejecta formation and crater development of the Mjølnir impact

Abstract: Crater-ejecta correlation is an important element in the analysis of crater formation and its influence on the geological evolution. In this study, both the ejecta distribution and the internal crater development of the Jurassic/Cretaceous Mjolnir crater (40 km in diameter; located in the Barents Sea) are investigated through numerical simulations. The simulations show a highly asymmetrical ejecta distribution, and underscore the importance of a layer of surface water in ejecta distribution. As expected, the ejecta asymmetry increases as the angle of impact decreases. The simulation also displays an uneven aerial distribution of ejecta. The generation of the central high is a crucial part of crater formation. In this study, peak generation is shown to have a skewed development, from approximately 50-90 sec after impact, when the peak reaches its maximum height of 1-1.5 km. During this stage, the peak crest is moved about 5 km from an uprange to a downrange position, ending with a final central position which has a symmetrical appearance that contrasts with its asymmetrical development.

Petrology and origin of amoeboid olivine aggregates in CR chondrites

Abstract: Amoeboid olivine aggregates (AOAs) are irregularly shaped, fine-grained aggregates of olivine and Ca, Al-rich minerals and are important primitive components of CR chondrites. The AOAs in CR chondrites contain FeNi metal, and some AOAs contain Mn-rich forsterite with up to 0.7 MnO and Mn:Fe ratios greater than one. Additionally, AOAs in the CR chondrites do not contain secondary phases (nepheline and fayalitic olivine) that are found in AOAs in some CV chondrites. The AOAs in CR chondrites record a complex petrogenetic history that included nebular gas-solid condensation, reaction of minerals with the nebular gas, small degrees of melting, and sintering of the assemblage. A condensation origin for the Mn-rich forsterite is proposed. The Mn-rich forsterite found in IDPs, unequilibrated ordinary chondrite matrix, and AOAs in CR chondrites may have had a similar origin. A type A calcium, aluminum-rich inclusion (CAI) with an AOA attached to its Wark-Lovering rim is also described. This discovery reveals a temporal relationship between AOAs and type A inclusions. Additionally, a thin layer of forsterite is present as part of the Wark-Lovering rim, revealing the crystallization of olivine at the end stages of the Wark-Lovering rim formation. The Ca, Al-rich nodules in the AOAs may be petrogenetically related to the Ca, Al-rich minerals in Wark- Lovering rims on type A CAIs. AOAs are chondrite components that condensed during the final stage of Wark-Lovering rim formation but, in general, were temporally, spatially, or kinetically isolated from reacting with the nebula vapor during condensation of the lower temperature minerals that were commonly present as chondrule precursors.

More evidence that the Chicxulub impact predates the K/T mass extinction

Abstract: Yaxcopoil-1 (Yax-1), drilled within the Chicxulub crater, was expected to yield the final proof that this impact occurred precisely 65 Myr ago and caused the mass extinction at the Cretaceous-Tertiary (K/T) boundary. Instead, contrary evidence was discovered based on five independent proxies (sedimentologic, biostratigraphic, magnetostratigraphic, stable isotopic, and iridium) that revealed that the Chicxulub impact predates the K/T boundary by about 300,000 years and could not have caused the mass extinction. This is demonstrated by the presence of five bioturbated glauconite layers and planktic foraminiferal assemblages of the latest Maastrichtian zone CF1 and is corroborated by magnetostratigraphic chron 29r and characteristic late Maastrichtian stable isotope signals. These results were first presented in Keller et al. (2004). In this study, we present more detailed evidence of the presence of late Maastrichtian planktic foraminifera, sedimentologic, and mineralogic analyses that demonstrate that the Chicxulub impact breccia predates the K/T boundary and that the sediments between the breccia and the K/T boundary were deposited in a normal marine environment during the last 300,000 years of the Cretaceous.

Shocked rocks and impact glasses from the El'gygytgyn impact structure, Russia

Abstract: The Elgygytgyn impact structure is about 18 km in diameter and is located in the central part of Chukotka, arctic Russia. The crater was formed in volcanic rock strata of Cretaceous age, which include lava and tuffs of rhyolites, dacites, and andesites. A mid-Pliocene age of the crater was previously determined by fission track (3.45 ± 0.15 Ma) and 40Ar/39Ar dating (3.58 ± 0.04 Ma). The ejecta layer around the crater is completely eroded. Shock-metamorphosed volcanic rocks, impact melt rocks, and bomb-shaped impact glasses occur in lacustrine terraces but have been redeposited after the impact event. Clasts of volcanic rocks, which range in composition from rhyolite to dacite, represent all stages of shock metamorphism, including selective melting and formation of homogeneous impact melt. Four stages of shocked volcanic rocks were identified: stage I (≤35 GPa; lava and tuff contain weakly to strongly shocked quartz and feldspar clasts with abundant PFs and PDFs; coesite and stishovite occur as well), stage II (35-45 GPa; quartz and feldspar are converted to diaplectic glass; coesite but no stishovite), stage III (45-55 GPa; partly melted volcanic rocks; common diaplectic quartz glass; feldspar is melted), and stage IV (>55 GPa; melt rocks and glasses). Two main types of impact melt rocks occur in the crater: 1) impact melt rocks and impact melt breccias (containing abundant fragments of shocked volcanic rocks) that were probably derived from (now eroded) impact melt flows on the crater walls, and 2) aerodynamically shaped impact melt glass "bombs" composed of homogeneous glass. The composition of the glasses is almost identical to that of rhyolites from the uppermost part of the target. Cobalt, Ni, and Ir abundances in the impact glasses and melt rocks are not or only slightly enriched compared to the volcanic target rocks; only the Cr abundances show a distinct enrichment, which points toward an achondritic projectile. However, the present data do not allow one to unambiguously identify a meteoritic component in the Elgygytgyn impact melt rocks.

Composition of impact melt particles and the effects of post-impact alteration in suevitic rocks at the Yaxcopoil-1 drill core, Chicxulub crater, Mexico

Abstract: Petrographical and chemical analysis of melt particles and alteration minerals of the about 100 m-thick suevitic sequence at the Chicxulub Yax-1 drill core was performed. The aim of this study is to determine the composition of the impact melt, the variation between different types of melt particles, and the effects of post-impact hydrothermal alteration. We demonstrate that the compositional variation between melt particles of the suevitic rocks is the result of both incomplete homogenization of the target lithologies during impact and subsequent post-impact hydrothermal alteration. Most melt particles are andesitic in composition. Clinopyroxene-rich melt particles possess lower SiO2 and higher CaO contents. These are interpreted by mixing of melts from the silicate basement with overlying carbonate rocks. Multi-stage post-impact hydrothermal alteration involved significant mass transfer of most major elements and caused further compositional heterogeneity between melt particles. Following backwash of seawater into the crater, palagonitization of glassy melt particles likely caused depletion of SiO2, Al2O3, CaO, Na2O, and enrichment of K2O and FeOtot during an early alteration stage. Since glass is very susceptible to fluid-rock interaction, the state of primary crystallization of the melt particles had a significant influence on the intensity of the postimpact hydrothermal mass transfer and was more pronounced in glassy melt particles than in wellcrystallized particles. In contrast to other occurrences of Chicxulub impactites, the Yax-1 suevitic rocks show strong potassium metasomatism with hydrothermal K-feldspar formation and whole rock K20 enrichment, especially in the lower unit of the suevitic sequence. A late stage of hydrothermal alteration is characterized by precipitation of silica, analcime, and Na-bearing Mg-rich smectite, among other minerals. This indicates a general evolution from a silica-undersaturated fluid at relatively high potassium activities at an early stage toward a silica-oversaturated fluid at relatively high sodium activities at later stages in the course of fluid rock interaction.

Alteration and metamorphism of CO3 chondrites: Evidence from oxygen and carbon isotopes

Abstract: Carbonaceous chondrites of the Ornans-type (CO3) form a well-documented metamorphic series. To investigate the conditions under which metamorphism took place, whole rock oxygen and carbon isotope analysis has been carried out on 10 CO3 chondrites (ALH A77307 [3.0], Colony [3.0], Kainsaz* [3.1], Felix* [3.2], Ornans* [3.3], ALH 82101 [3.3], Lance* [3.4], ALH A77003 [3.5], Warrenton* [3.6], and Isna [3.7] [*denotes a fall]). Whole rock oxygen isotope analysis was carried out by laser-assisted fluorination, whole rock carbon isotope analysis by continuous flow mass spectrometry. The results of this study indicate that the oxygen and carbon isotopes in CO3 finds have been significantly disturbed by terrestrial weathering processes. Conclusions based on the isotopic composition of such weathered finds may be significantly flawed. In particular, the Antarctic meteorite ALH A77307 (3.0), suggested as being close in composition to CO-CM chondrite precursor material, has experienced significant terrestrial contamination. Oxygen isotope data for CO3 falls indicates that there is a subtle increase in ?17O values with increasing metamorphic grade for sub-types 3.1 to 3.4. This increase does not persist to higher sub-types, i.e., Warrenton (3.6). These relationships are explicable in terms of the progressive formation of phyllosilicates, coupled with loss of primary phases such as melilite, and suggest that an aqueous fluid phase was present during metamorphism. Carbon abundance and δ13C values of CO3 falls decrease with increasing metamorphic grade. These trends reflect progressive changes in the nature of the organic macromolecular component during metamorphic heating and lend additional support to the evidence that CO3 chondrites are part of a metamorphic series. The most likely setting for metamorphism was on the CO3 parent body. The "Ornans paradox," whereby Ornans (3.3) should belong to a higher sub-type based on chemical compared to petrographic evidence, may result from local-scale redox differences on the CO3 parent body. A wide variety of classification schemes have been proposed for CO3 chondrites. In view of its simplicity and applicability, the scheme of Scott and Jones (1990) is regarded as the most useful in assigning sub-types to new CO3 samples.

Los Angeles: A tale of two stones

Abstract: We compare and contrast the mineralogy and petrology of the 2 stones of the extremely ferroan and Cr-poor martian meteorite, Los Angeles. The 2 stones are similar in many characteristics, strongly suggesting that they originated from a single flow or shallow intrusion. However, stone 2 is more ferroan and enriched in late-stage materials than its larger, and more widely studied, sibling. Stone 2 has a far higher abundance (~25 vol%) than stone 1 (10 vol%) of combined opaques, meaning not only conventional opaque minerals but also, and more abundantly, fine-grained symplectitic intergrowths of fayalite + ferroan augite + silica (interpreted as pyroxferroite breakdown material, PBM). The bulk composition of the PBM is close to that of stoichiometric pyroxferroite, with roughly 45 wt% FeO. Extensive zonation within the pyroxenes of both stones is consistent with origin by closed-system fractional crystallization of the parent basaltic melt(s). However, the compositional and modal disparity between the two stones suggests that they formed in an environment where at least mild multi-cm-scale differentiation occurred. Probably, in both stones, crystallization began from similar melts with mg ~27-28 mol%, but during crystallization, significant migration of the melt component occurred, perhaps by crystal settling and/or filter pressing. Stone 2 acquired an enhanced proportion of residual melt and, thus, higher proportions of late-stage materials such as PBM, oxides, and phosphates. Within the PBM, clinopyroxene poikiloblastically encloses fayalite and silica. At least some of the PBM had already formed by decomposition of pyroxferroite before the major shock that caused the very scarce brecciation within Los Angeles. However, the low abundance of fractures within PBM, in comparison to pyroxene and some other minerals, may be an indication that the textures of PBM regions typically did not assume their final detailed configuration until after the last major shock. The steep slope of a pyroxene mg-Cr correlation suggests that igneous crystallization occurred at higher fO2 in Los Angeles than in otherwise similar shergottites such as QUE 94201, Shergotty, and Zagami.