Evolution of the winonaite parent body: Clues from silicate mineral trace element distributions

Abstract— We have measured the trace element compositions of individual plagioclase, pyroxene, and olivine grains in 6 different winonaites that span the range of textures and mineralogies observed in these meteorites. Textural evidence in these meteorites, including the presence of a plagioclase/clinopyroxene‐rich lithology and coarse‐grained olivine lithologies, suggests that they may have experienced some silicate partial melting. However, trace element distributions in these lithologies do not show any clear signatures for such an event. Pyroxene trace element compositions do exhibit systematic trends, with abundances generally lowest in Pontlyfni and highest in Winona. The fact that the same trends are present for both incompatible and compatible trace elements suggests, however, that the systematics are more likely the result of equilibration of minerals with initially heterogeneous and distinct compositions, rather than partial melting of a compositionally homogeneous precursor. The winonaites have experienced brecciation and mixing of lithologies, followed by varying degrees of thermal metamorphism on their parent body. These factors probably account for the variable bulk rare earth element (REE) patterns noted for these meteorites and may have led to re‐equilibration of trace elements in different lithologies.     

Mineralogy of iron sulfides in CM1 and CI1 lithologies of the Kaidun breccia: Records of extreme to intense hydrothermal alteration

The polymict Kaidun microbreccia contains lithologies of C‐type chondrites with euhedral iron sulfide crystals of hydrothermal origin. Our FIB‐TEM study reveals that acicular sulfides in a CM1 lithology are composed of Fe‐rich pyrrhotite with nonintegral vacancy superstructures (NC‐pyrrhotite), troilite, and pentlandite, all showing distinct exsolution textures. Based on phase relations in the Fe‐Ni‐S system, we constrain the temperature of formation of the originally homogeneous monosulfide solid solution to the range of 100–300 °C. In some crystals the exsolution of pentlandite and the microtextural equilibration was incomplete, probably due to rapid cooling. We use thermodynamic modeling to constrain the physicochemical conditions of the extreme hydrothermal alteration in this lithology. Unless the CM1 lithology was sourced from a large depth in the parent body (internal pressure >85 bar) or the temperatures were in the lower range of the interval determined, the water was likely present as vapor. Previously described light δ³⁴S compositions of sulfides in Kaidun's CM1 lithology are likely due to the loss of ³⁴S‐enriched H₂S during boiling. Platy sulfide crystals in an adjacent, intensely altered CI1 lithology are composed of Fe‐poor, monoclinic 4C‐pyrrhotite and NC‐pyrrhotite and probably formed at lower temperatures and higher fS₂ relative to the CM1 lithology. However, a better understanding of the stability of Fe‐poor pyrrhotites at temperatures below 300 °C is required to better constrain these conditions.     

Mineralogy,petrology, chronology,and exposure history of the Chelyabinsk meteorite and parent body

Three masses of the Chelyabinsk meteorite have been studied with a wide range of analytical techniques to understand the mineralogical variation and thermal history of the Chelyabinsk parent body. The samples exhibit little to no postentry oxidation via Mössbauer and Raman spectroscopy indicating their fresh character, but despite the rapid collection and care of handling some low levels of terrestrial contamination did nonetheless result. Detailed studies show three distinct lithologies, indicative of a genomict breccia. A light‐colored lithology is LL5 material that has experienced thermal metamorphism and subsequent shock at levels near S4. The second lithology is a shock‐darkened LL5 material in which the darkening is caused by melt and metal‐troilite veins along grain boundaries. The third lithology is an impact melt breccia that formed at high temperatures (~1600 °C), and it experienced rapid cooling and degassing of S₂ gas. Portions of light and dark lithologies from Chel‐101, and the impact melt breccias (Chel‐102 and Chel‐103) were prepared and analyzed for Rb‐Sr, Sm‐Nd, and Ar‐Ar dating. When combined with results from other studies and chronometers, at least eight impact events (e.g., ~4.53 Ga, ~4.45 Ga, ~3.73 Ga, ~2.81 Ga, ~1.46 Ga, ~852 Ma, ~312 Ma, and ~27 Ma) are clearly identified for Chelyabinsk, indicating a complex history of impacts and heating events. Finally, noble gases yield young cosmic ray exposure ages, near 1 Ma. These young ages, together with the absence of measurable cosmogenic derived Sm and Cr, indicate that Chelyabinsk may have been derived from a recent breakup event on an NEO of LL chondrite composition.     

Petrogenesis and shock metamorphism of the enriched lherzolitic shergottite Northwest Africa 7755

Northwest Africa (NWA) 7755 is a newly found enriched lherzolitic shergottite. Here, we report its detailed petrography and mineralogy. NWA 7755 contains both poikilitic and non‐poikilitic lithologies. Olivine has different compositional ranges in the poikilitic and non‐poikilitic lithologies, Fa_30–39 and Fa_37–40, respectively. Pyroxene in the non‐poikilitic lithology is systematically Fe‐richer than that in the poikilitic lithology. The chromite grains in non‐poikilitic lithology are highly Ti‐richer than those in the poikilitic lithology. The chemical variations of olivine, pyroxene, and chromite between the poikilitic and non‐poikilitic lithologies support a two‐stage formation model of lherzolitic shergottites. Besides planar fractures and strong mosaicism in olivine and pyroxene, shock‐induced melt veins and pockets are observed in NWA 7755. Olivine grains within and adjacent to melt veins and/or pockets have either transformed to ringwoodite, amorphous phase, or dissociated to bridgmanite plus magnesiowüstite. Merrillite in melt veins has completely transformed to tuite; however, apatite only has partially transformed to tuite, indicating a relatively sluggish transformation rate. The partial transformation from apatite to tuite resulted in fractional devolatilization of Cl and F in apatite. The fine‐grained mineral assemblage in melt veins consists mainly of bridgmanite, minor magnesiowüstite, Fe‐sulfide, Fe‐phosphide, and Ca‐phosphate minerals. The coexistence of bridgmanite and magnesiowüstite in these veins indicates a shock pressure of >~24 GPa and a temperature of 1800–2000 °C. Coesite and seifertite are probably present in NWA 7755. The presence of these high‐pressure minerals indicates that NWA 7755 has experienced a more intense shock metamorphism than other enriched lherzolitic shergottites.     

The Chelyabinsk meteorite: New insights from a comprehensive electron microscopy and Raman spectroscopy study with evidence for graphite in olivine of ordinary chondrites

We present results of petrographic, mineralogical, and chemical investigations of three Chelyabinsk meteorite fragments. Three distinct lithologies were identified: light S3 LL5, dark S4–S5 LL5 material, and opaque fine-grained former impact melt. Olivine–spinel thermometry revealed an equilibration temperature of 703 ± 23 °C for the light lithology. All plagioclase seems to be secondary, showing neither shock-induced fractures nor sulfide-metal veinlets. Feldspathic glass can be observed showing features of extensive melting and, in the dark lithology, as maskelynite, lacking melt features and retaining grain boundaries of former plagioclase. Olivine of the dark lithology shows planar deformation features. Impact melt is dominated by Mg-rich olivine and resembles whole-rock melt. Melt veins (<2 mm) are connected to narrower veinlets. Melt vein textures are similar to pegmatite textures showing chilled margins, a zone of inward-grown elongated crystals and central vugs, suggesting crystallization from supercooled melt. Sulfide-metal droplets indicate liquid immiscibility of both silicate and sulfide as well as sulfide and metal melts. Impact melting may have been an important factor for differentiation of primitive planetary bodies. Graphite associated with micrometer-sized melt inclusions in primary olivine was detected by Raman mapping. Carbon isotopic studies of graphite could be applied to test a possible presolar origin.     

Testing the genetic relationship between fluid alteration and brecciation in CM chondrites

Boriskino is a poorly studied CM chondrite with numerous millimeter‐ to centimeter‐scale clasts exhibiting sharp boundaries. Clast textures and mineralogies attest to diverse geological histories with various degrees of aqueous alteration. We conducted a petrographic, chemical, and isotopic study on each clast type of the breccia to investigate if there exists a genetic link between brecciation and aqueous alteration, and to determine the controlling parameter of the extent of alteration. Boriskino is dominated by CM2 clasts for which no specific petrographic type could be assigned based on the chemical compositions and modal abundances of constituents. One clast stands out and is identified as a CM1 lithology, owing to its lack of anhydrous silicates and its overall abundance of dolomite‐like carbonates and acicular iron sulfides. We observe that alteration phases near clast boundaries exhibit foliation features, suggesting that brecciation postdated aqueous alteration. We measured the O‐isotopic composition of Ca‐carbonates and dolomite‐like carbonates to determine their precipitation temperatures following the methodology of Verdier‐Paoletti et al. (2017). Both types of carbonates yield similar ranges of precipitation temperatures independent of clast lithology, ranging from ?13.9 ± 22.4 (2σ) to 166.5 ± 47.3 °C, precluding that temperature alone accounts for the differences between the CM1 and CM2 lithologies. Instead, we suggest that initial water/rock ratios of 0.75 and 0.61 for the CM1 and CM2 clasts, respectively, might control the extent of aqueous alteration. Based on these estimates, we suggest that Boriskino clasts originated from a single parent body with heterogeneous distribution of water either due to local differences in the material permeability or in the initial content of ice available. These conditions would have produced microenvironments with differing geochemical conditions thus leading to a range of degrees of aqueous alteration.     

Newly identified “Tunnunik” impact structure,Prince Albert Peninsula,northwestern Victoria Island,Arctic Canada

Regional geological mapping of the glaciated surface of northwestern Victoria Island in the western Canadian Arctic revealed an anomalous structure in otherwise flat‐lying Neoproterozoic and lower Paleozoic carbonate rocks, located south of Richard Collinson Inlet. The feature is roughly circular in plan view, approximately 25 km in diameter, and characterized by quaquaversal dips of approximately 45°, decreasing laterally. The core of the feature also exhibits local vertical dips, low‐angle reverse faults, and drag folds. Although brecciation was not observed, shatter cones are pervasive in all lithologies in the central area, including 723 Ma old dikes that penetrate Neoproterozoic limestones. Their abundance decreases distally, and none was observed in surrounding, horizontally bedded strata. This circular structure is interpreted as a deeply eroded meteorite impact crater of the complex type, and the dipping strata as the remnants of the central uplift. The variation in orientation and shape of shatter cones point to variably oriented stresses with the passage of the shock wave, possibly related to the presence of pore water in the target strata as well as rock type and lithological heterogeneities, especially bed thickness. Timing of impact is poorly constrained. The youngest rocks affected are Late Ordovician (approximately 450 Ma) and the impact structure is mantled by undisturbed postglacial sediments. Regional, hydrothermal dolomitization of the Ordovician limestones, possibly in the Late Devonian (approximately 360 Ma), took place before the impact, and widespread WSW–ENE‐trending normal faults of probable Early Cretaceous age (approximately 130 Ma) apparently cross‐cut the impact structure.     

Ni/S/Cl systematics and the origin of impact‐melt glasses in Martian meteorite Elephant Moraine 79001

Martian meteorite Elephant Moraine A79001 (EET 79001) has received considerable attention for the unusual composition of its shock melt glass, particularly its enrichment in sulfur relative to the host shergottite. It has been hypothesized that Martian regolith was incorporated into the melt or, conversely, that the S‐enrichment stems from preferential melting of sulfide minerals in the host rock during shock. We present results from an electron microprobe study of EET 79001 including robust measurements of major and trace elements in the shock melt glass (S, Cl, Ni, Co, V, and Sc) and minerals in the host rock (Ni, Co, and V). We find that both S and major element abundances can be reconciled with previous hypotheses of regolith incorporation and/or excess sulfide melt. However, trace element characteristics of the shock melt glass, particularly Ni and Cl abundances relative to S, cannot be explained either by the incorporation of regolith or sulfide minerals. We therefore propose an alternative hypothesis whereby, prior to shock melting, portions of EET 79001 experienced acid‐sulfate leaching of the mesostasis, possibly groundmass feldspar, and olivine, producing Al‐sulfates that were later incorporated into the shock melt, which then quenched to glass. Such activity in the Martian near‐surface is supported by observations from the Mars Exploration Rovers and laboratory experiments. Our preimpact alteration model, accompanied by the preferential survival of olivine and excess melting of feldspar during impact, explains the measured trace element abundances better than either the regolith incorporation or excess sulfide melting hypothesis does.     

Shock effects in the Willamette ungrouped iron meteorite

A slab of the Willamette ungrouped iron contains elongated troilite nodules (up to ~2 × 10 cm) that were crushed and penetrated by wedges of crushed metal during a major impact event. What makes this sample unique is the contrast between the large amount of shock damage and the very small (~1%) amounts of shock melting in the large troilite nodules. The postshock temperature was low, probably ?960 °C. The Widmanstätten pattern has been largely obscured by an episode of postshock annealing that caused recrystallization of the kamacite. The shock and thermal history of Willamette includes (1) initial crystallization and formation of multicentimeter‐size troilite nodules from trapped melt, (2) impact‐induced melting of metal‐sulfide assemblages to form lobate taenite masses a few hundred micrometers in size, (3) impact‐crushing of the nodules and jamming of metal wedges into them, (4) simultaneous crushing of metal grains adjacent to sulfide throughout the meteorite, (5) postshock annealing causing minor recrystallization of metal and troilite, and (6) a late‐stage shock event (and additional annealing) producing Neumann lines in the kamacite.     

Impact melting of the largest known enstatite meteorite: Al Haggounia 001, a fossil EL chondrite

Al Haggounia 001 and paired specimens (including Northwest Africa [NWA] 2828 and 7401) are part of a vesicular, incompletely melted, EL chondrite impact melt rock with a mass of ~3 metric tons. The meteorite exhibits numerous shock effects including (1) development of undulose to weak mosaic extinction in low‐Ca pyroxene; (2) dispersion of metal‐sulfide blebs within silicates causing “darkening”; (3) incomplete impact melting wherein some relict chondrules survived; (4) vaporization of troilite, resulting in S₂ bubbles that infused the melt; (5) formation of immiscible silicate and metal‐sulfide melts; (6) shock‐induced transportation of the metal‐sulfide melt to distances >10 cm; (7) partial resorption of relict chondrules and coarse silicate grains by the surrounding silicate melt; (8) crystallization of enstatite in the matrix and as overgrowths on relict silicate grains and relict chondrules; (9) crystallization of plagioclase from the melt; and (10) quenching of the vesicular silicate melt. The vesicular samples lost almost all of their metal during the shock event and were less susceptible to terrestrial weathering; in contrast, the samples in which the metal melt accumulated became severely weathered. Literature data indicate the meteorite fell ~23,000 yr ago; numerous secondary phases formed during weathering. Both impact melting and weathering altered the meteorite's bulk chemical composition: e.g., impact melting and loss of a metal‐sulfide melt from NWA 2828 is responsible for bulk depletions in common siderophile elements and in Mn (from alabandite); weathering of oldhamite caused depletions in many rare earth elements; the growth of secondary phases caused enrichments in alkalis, Ga, As, Se, and Au.     

Experimental generation of shock‐induced pseudotachylites along lithological interfaces

Abstract— To understand the mechanism of formation of shock‐induced pseudotachylites and particularly the role that rock heterogeneities and interfaces play in their formation, shock recovery experiments were carried out on samples consisting of two distinct lithologies (dunite and quartzite). It was possible to generate melt veins of 1–6 μm width along lithological interfaces at moderate shock pressures (6 to 34 GPa). The magnitudes of displacement along the interface, strain rate, and the kinetic heat production indicate that friction is an important heat source that largely contributes to the energy budget of the melt veins. The experimentally produced veins resemble natural S‐type pseudotachylites. The geometry of the veins depends on the orientation of the interface with respect to the shock front and includes strong variations in thickness, formation of melt pockets and injection veins, sudden changes in vein orientation, and sharp vein margins. Two types of melt were observed: vesicle‐free and vesicular melts. Dense vesicle‐free melt rock is likely to represent high‐pressure melts. Vesicular melts were also generated during shock compression, but they remained in a molten state during pressure release and continued shearing. Intermingling of comminuted olivine and melt suggests that ultracataclasis of olivine induced by a dynamic tensile failure is a precursor stage to frictional melting. Shock wave interferences at the lithological interface provide the necessary stress conditions to start dynamic failure of olivine. The composition of the frictional melts ranges from olivine‐normative to enstatite‐normative and is, thus, largely determined by olivine melting. The validity of the sequence of friction melting susceptibilities of rock‐forming minerals inferred from tectonically‐produced pseudotachylites is confirmed and can now be applied to ultra‐high strain rates during shock compression.     

40Ar‐39Ar chronology of lunar meteorites Northwest Africa 032 and 773

Abstract— The ⁴⁰Ar‐³⁹Ar dating technique has been applied to the lunar meteorites Northwest Africa 032 (NWA 032), an unbrecciated mare basalt, and Northwest Africa 773 (NWA 773), (composed of cumulate and breccia lithologies), to determine the crystallization age and timing of shock events these meteorites may have experienced. Stepped heating analyses of several different samples of NWA 032 gave complex age spectra but indistinguishable total ages with a mean of 2.779 ± 0.014 Gyr. Possible causes of the complex age spectra obtained from NWA 032 include recoil of ³⁹Ar, or the presence of pre‐shock ⁴⁰Ar incorporated into shock‐melt veins. The effects of shock veins were investigated by laser fusion of 20 small samples expected to contain varying proportions of the shock veins. The laser ages show a narrow age distribution between 2.61–2.86 Gyr and a mean of 2.73 ± 0.03 Gyr, identical to the total age of ?2.80 Gyr obtained for the bulk sample. Diffusion calculations based on the stepped heating data indicate that Ar release can be reconciled by release from feldspar (and possibly shock veins) at low temperatures followed by pyroxene at higher temperatures. The exposure age of NWA 032 is 212 ± 11 Myr, and it contains low trapped solar Ar. Stepped heating of cumulate and breccia portions of NWA 773 also give a relatively young age of 2.91 Gyr. The presence of trapped Ar in the breccia makes the age determination of this component less precise, but release of Ar appears to be from the same mineral phase, assumed to be plagioclase, in both lithologies. A marked difference in exposure age between the 2 lithologies also exists, with the breccia having spent 81 Myr longer at the lunar surface; this finding is consistent with the higher trapped Ar content of this lithology. Assuming that 2.80 Gyr and 2.91 Gyr are the crystallization ages of NWA 032 and NWA 773 respectively, these two meteorites are the youngest lunar mare basalts available for study.     

Shock experiments with the H6 chondrite Kernouvé: Pressure calibration of microscopic shock effects

Abstract— Shock‐recovery experiments were carried out on samples of the H6 chondrite Kernouvé at shock pressures of 10, 15, 20, 25, 30, 35, 45, and 60 GPa and preheating temperatures of 293 K (low‐temperature experiments) and 920 K (high‐temperature experiments). Using a calculated equation of state of Kernouvé, pressure‐pulse durations of 0.3 to 1.2 μs were estimated. The shocked samples were investigated by optical microscopy to calibrate the various shock effects in olivine, orthopyroxene, oligoclase, and troilite. The following pressure calibration is proposed for silicates: (1) undulatory extinction of olivine <GPa; (2) weak mosaicism of olivine from 10–15 GPa to 20–25 GPa; (3) onset of strong mosaicism of olivine at 20–25 GPa; (4) transformation of oligoclase to diaplectic glass completed at 25–30 GPa (low‐temperature experiments) and at 20–25 GPa (high‐temperature experiments); (5) onset of weak mosaicism in orthopyroxene at 30–35 GPa (low‐temperature experiments) and at 25–30 GPa (high‐temperature experiments); and (6) recrystallization or melting of olivine starting at 45–60 GPa (low‐temperature experiments) and at 35–45 GPa (high‐temperature experiments), and completed above 45–60 GPa in the high‐temperature experiments. Troilite displays distinct differences between the samples shocked at low and high temperatures. In the low‐temperature experiments, the following effects can be observed in troilite: (1) undulatory extinction up to 25 GPa, (2) twinning up to 45 GPa, (3) partial recrystallization from 30 to 60 GPa, and (4) complete recrystallization >35 GPa; whereas in the high‐temperature experiments, troilite shows (1) complete recrystallization from 10 up to 45 GPa and (2) melting and crystallization above 45 GPa. Localized shock‐induced melting is observed in samples shocked to pressures >15 GPa in the high‐temperature experiments and >30 GPa for the low‐temperature experiments in the form of FeNi metal and troilite melt injections and intergrowths and as pockets and veins of whole‐rock melt. Obviously, the onset and abundance of shock‐induced localized melting strongly depends on the initial temperature of the sample.     

Mineralogy and petrogenesis of lunar magnesian granulitic meteorite Northwest Africa 5744

Lunar meteorite Northwest Africa (NWA) 5744 is a granulitic breccia with an anorthositic troctolite composition that may represent a distinct crustal lithology not previously described. This meteorite is the namesake and first‐discovered stone of its pairing group. Bulk rock major element abundances show the greatest affinity to Mg‐suite rocks, yet trace element abundances are more consistent with those of ferroan anorthosites. The relatively low abundances of incompatible trace elements (including K, P, Th, U, and rare earth elements) in NWA 5744 could indicate derivation from a highlands crustal lithology or mixture of lithologies that are distinct from the Procellarum KREEP terrane on the lunar nearside. Impact‐related thermal and shock metamorphism of NWA 5744 was intense enough to recrystallize mafic minerals in the matrix, but not intense enough to chemically equilibrate the constituent minerals. Thus, we infer that NWA 5744 was likely metamorphosed near the lunar surface, either as a lithic component within an impact melt sheet or from impact‐induced shock.     

Elemental,isotopic, and structural changes in Tagish Lake insoluble organic matter produced by parent body processes

Here, we present the results of a multitechnique study of the bulk properties of insoluble organic material (IOM) from the Tagish Lake meteorite, including four lithologies that have undergone different degrees of aqueous alteration. The IOM C contents of all four lithologies are very uniform and comprise about half the bulk C and N contents of the lithologies. However, the bulk IOM elemental and isotopic compositions vary significantly. In particular, there is a correlated decrease in bulk IOM H/C ratios and δD values with increasing degree of alteration—the IOM in the least altered lithology is intermediate between CM and CR IOM, while that in the more altered lithologies resembles the very aromatic IOM in mildly metamorphosed CV and CO chondrites, and heated CMs. Nuclear magnetic resonance (NMR) spectroscopy, C X‐ray absorption near‐edge (XANES), and Fourier transform infrared (FTIR) spectroscopy confirm and quantitate this transformation from CR‐like, relatively aliphatic IOM functional group chemistry to a highly aromatic one. The transformation is almost certainly thermally driven, and probably occurred under hydrothermal conditions. The lack of a paramagnetic shift in ¹³C NMR spectra and 1s‐σ* exciton in the C‐XANES spectra, both typically seen in metamorphosed chondrites, shows that the temperatures were lower and/or the timescales were shorter than experienced by even the least metamorphosed type 3 chondrites. Two endmember models were considered to quantitatively account for the changes in IOM functional group chemistry, but the one in which the transformations involved quantitative conversion of aliphatic material to aromatic material was the more successful. It seems likely that similar processes were involved in producing the diversity of IOM compositions and functional group chemistries among CR, CM, and CI chondrites. If correct, CRs experienced the lowest temperatures, while CM and CI chondrites experienced similar more elevated temperatures. This ordering is inconsistent with alteration temperatures based on mineralogy and O isotopes.     

Portales Valley: Petrology of a metallic‐melt meteorite breccia

Abstract— Portales Valley (PV) is an unusual metal‐veined meteorite that has been classified as an H6 chondrite. It has been regarded either as an annealed impact melt breccia, as a primitive achondrite, or as a meteorite with affinities to silicated iron meteorites. We studied the petrology of PV using a variety of geochemical‐mineralogical techniques. Our results suggest that PV is the first well‐documented metallic‐melt meteorite breccia. Mineral‐chemical and other data suggest that the protolith to PV was an H chondrite. The composition of FeNi metal in PV is somewhat fractionated compared to H chondrites and varies between coarse vein and silicate‐rich portions. It is best modeled as having formed by partial melting at temperatures of ?940–1150 °C, with incomplete separation of solid from liquid metal. Solid metal concentrated in the coarse vein areas and S‐bearing liquid metal concentrated in the silicate‐rich areas, possibly as a result of a surface energy effect. Both carbon and phosphorus must have been scavenged from large volumes and concentrated in metallic liquid. Graphite nodules formed by crystallization from this liquid, whereas phosphate formed by reaction between P‐bearing metal and clinopyroxene components, depleting clinopyroxene throughout much of the meteorite and growing coarse phosphate at metal‐silicate interfaces. Some phosphate probably crystallized from P‐bearing liquids, but most probably formed by solid‐state reaction at ?975‐725 °C. Phosphate‐forming and FeO‐reduction reactions were widespread in PV and entailed a change in the mineralogy of the stony portion on a large scale. Portales Valley experienced protracted annealing from supersolidus to subsolidus temperatures, probably by cooling at depth within its parent body, but the main differences between PV and H chondrites arose because maximum temperatures were higher in PV. A combination of a relatively weak shock event and elevated pre‐shock temperatures probably produced the vein‐and‐breccia texture, with endogenic heating being the main heat source for melting, and with stress waves from an impact event being an essential trigger for mobilizing metal. Portales Valley is best classified as an H7 metallic‐melt breccia of shock stage S1. The meteorite is transitional between more primitive (chondritic) and evolved (achondrite, iron) meteorite types and offers clues as to how differentiation could have occurred in some asteroidal bodies.     

Characterization and significance of shocked quartz from the Woodleigh mpact structure,Western Australia

Abstract— We re‐examined the buried Woodleigh structure in Western Australia, which has been inferred to be a multi‐ringed, 120 km diameter impact crater, because the proposed size and possible synchronicity with one of the pre‐Mesozoic mass extinction events has attracted controversy. We undertook a detailed study of the petrology and mineralogy of a number of samples of core from the Woodleigh‐1 borehole that was drilled into the central uplift of the structure. Crystalline Proterozoic basement rocks comprising granites and gneisses in the Woodleigh‐1 core contain minor brecciation in discrete veins and reveal clear evidence of shock metamorphism over the full extent of the core. Imaging of laboratory‐etched quartz showed that a large number of grains contain shock deformation lamellae. Microstructural and crystallographic analysis of these lamellae by TEM showed that they are planar deformation features (PDFs) that have subsequently undergone annealing and water assisted recrystallization. The available geological, petrographic, and mineralogical evidence suggest that Woodleigh is an eroded impact crater that is nearer to 60 km than 120 km in diameter. Future drilling projects should better constrain the level of erosion, and may reveal any preserved impact lithologies.     

The fall,recovery, and classification of the Park Forest meteorite

Abstract— On the night of March 26, 2003, a large meteorite broke up and fell upon the south suburbs of Chicago. The name Park Forest, for the village that is at the center of the strewnfield, has been approved by the nomenclature committee of the Meteoritical Society. Satellite data indicate that the bolide traveled from the southwest toward the northeast. The strewnfield has a southeast‐northwest trend; however, this is probably due to the effects of strong westerly winds at high altitudes. Its very low ⁵⁶Co and very high ⁶⁰Co activities indicate that Park Forest had a preatmospheric mass that was at least ～900 kg and could have been as large as ～7 times 10³ kg, of which only ～30 kg have been recovered. The average compositions of olivine and low‐Ca pyroxene, Fa_{24.7 ± 1.1} and Fs_{20.8 ± 0.7}, respectively, and its bulk oxygen isotopic composition, δ¹⁸O = +4.68%o, δ¹⁷O = +3.44%o, show that Park Forest is an L chondrite. The ferromagnesian minerals are well equilibrated, chondrules are easily recognized, and maskelynite is mostly ≤50 μm across. Based on these observations, we classify Park Forest as type 5. The meteorite has been strongly shocked, and based on the presence of maskelynite, mosaicism and planar deformation features in olivine, undulatory extinction in pyroxene, and glassy veins, the shock stage is S5. The meteorite is a monomict breccia, consisting of light‐colored, angular to rounded clasts in a very dark host. The light and dark lithologies have essentially identical mineral and oxygen isotopic compositions. Their striking difference in appearance is due to the presence of a fine, pervasive network of sulfide veins in the dark lithology, resulting in very short optical path lengths. The dark lithology probably formed from the light lithology in an impact that formed a sulfide‐rich melt and injected it into cracks.     

The effect of target lithology on the products of impact melting

Abstract— Impact cratering is an important geological process on the terrestrial planets and rocky and icy moons of the outer solar system. Impact events generate pressures and temperatures that can melt a substantial volume of the target; however, there remains considerable discussion as to the effect of target lithology on the generation of impact melts. Early studies showed that for impacts into crystalline targets, coherent impact melt rocks or “sheets” are formed with these rocks often displaying classic igneous structures (e.g., columnar jointing) and textures. For impact structures containing some amount of sedimentary rocks in the target sequence, a wide range of impact‐generated lithologies have been described, although it has generally been suggested that impact melt is either lacking or is volumetrically minor. This is surprising given theoretical constraints, which show that as much melt should be produced during impacts into sedimentary targets. The question then arises: where has all the melt gone? The goal of this synthesis is to explore the effect of target lithology on the products of impact melting. A comparative study of the similarly sized Haughton, Mistastin, and Ries impact structures, suggests that the fundamental processes of impact melting are basically the same in sedimentary and crystalline targets, regardless of target properties. Furthermore, using advanced microbeam analytical techniques, it is apparent that, for the structures under consideration here, a large proportion of the melt is retained within the crater (as crater‐fill impactites) for impacts into sedimentary‐bearing target rocks. Thus, it is suggested that the basic products are genetically equivalent but they just appear different. That is, it is the textural, chemical and physical properties of the products that vary.     

Impact melt dikes in the Sudbury multi-ring basin (Canada): Implications from uranium-lead geochronology on the Foy Offset Dike

Abstract To investigate the origin of Offset Dikes and their age relationships to major impact generated lithologies in the Sudbury multi-ring impact structure, such as the Main Mass of the Sudbury “Igneous” Complex, zircon and baddeleyite were dated by the U-Pb chronometer. The rocks analysed are one diorite and two quartz diorites from inside the Foy Offset, one quartz diorite from the contact zone, and two country rock samples collected at 10 and 30 m distances from the contact within the Levack Gneiss Complex. The 21 analysed zircon and baddeleyite fractions yield a crystallization age of 1852 +4/-3 (2σ) Ma for the accessory minerals in the Foy Offset Dike and an age of 2635 ± 5 Ma for the shocked Levack country rock, in which zircons show significant shock effects (multiple sets of planar fractures), in contrast to the totally unshocked zircons of the Offset Dike. Within given errors, the new age of 1852 Ma is identical to the pooled 1850 ± 1 Ma U-Pb age determined by Krogh et al. (1984) as the crystallization age of accessory phases in different lithologies of the Sudbury “Igneous” Complex, which has been interpreted to represent the coherent impact melt sheet of the Sudbury Structure. This excellent agreement of the ages substantiates that emplacement of the Offset Dikes occurred coevally with the formation of the impact melt sheet. Total absence of inherited zircons in the central part of the Foy Offset indicates melting of the precursor material at temperatures well above 1700 °C, which emphasizes the origin of the dike lithologies by impact melting.