Numerical modeling of impact‐induced hydrothermal activity at the Chicxulub crater

Abstract— Large impact events like the one that formed the Chicxulub crater deliver significant amounts of heat that subsequently drive hydrothermal activity. We report on numerical modeling of Chicxulub crater cooling with and without the presence of water. The model inputs are constrained by data from borehole samples and seismic, magnetic, and gravity surveys. Model results indicate that initial hydrothermal activity was concentrated beneath the annular trough as well as in the permeable breccias overlying the melt. As the system evolved, the melt gradually cooled and became permeable, shifting the bulk of the hydrothermal activity to the center of the crater. The temperatures and fluxes of fluid and vapor derived from the model are consistent with alteration patterns observed in the available borehole samples. The lifetime of the hydrothermal system ranges from 1.5 to 2.3 Myr depending on assumed permeability. The long lifetimes are due to conduction being the dominant mechanism of heat transport in most of the crater, and significant amounts of heat being delivered to the near‐surface by hydrothermal upwellings. The long duration of the hydrothermal system at Chicxulub should have provided ample time for colonization by thermophiles and/or hyperthermophiles. Because habitable conditions should have persisted for longer time in the central regions of the crater than on the periphery, a search for prospective biomarkers is most likely to be fruitful in samples from that region.     

Reconstruction of the Morasko meteoroid impact—Insight from numerical modeling

The Morasko strewn field located near Poznań, Poland comprises seven impact craters with diameters ranging from 20 to 90 m, all of which were formed in glacial sediments around 5000 yr ago. Numerous iron meteorites have been recovered in the area and their distribution suggests a projectile with the trajectory from NE to SW. Similar impact events producing crater strewn fields on average happen every 500 yr and pose a serious risk for modern civilization, which is why it is of utmost importance to study terrestrial strewn fields in detail. In this work, we investigate the Morasko meteoroid passage through the atmosphere, the distribution of its fragments on the ground, and the process of forming individual craters by means of numerical modeling. By combining atmospheric entry modeling, Pi‐group scaling of transient crater size and hydrocode simulations of impact processes, we constructed a comprehensive model of the Morasko strewn field formation. We determined the preatmospheric parameters of the Morasko meteoroid. The entry mass is between 600 and 1100 tons, the velocity range is between 16 and 18 km s^?1, and the trajectory angle is 30–40^°. Such entry velocities and trajectory angles do not deviate from typical values for near‐Earth asteroids, although the initial mass we determined can be considered as small. Our studies on velocities and masses of crater‐forming fragments showed that the biggest Morasko crater was formed by a projectile about 1.5 m in diameter with the impact velocity ~10 km s^?1. Environmental consequences of the Morasko impact event are very localized.     

Propagation of impact‐induced shock waves in porous sandstone using mesoscale modeling

Generation and propagation of shock waves by meteorite impact is significantly affected by material properties such as porosity, water content, and strength. The objective of this work was to quantify processes related to the shock‐induced compaction of pore space by numerical modeling, and compare the results with data obtained in the framework of the Multidisciplinary Experimental and Modeling Impact Research Network (MEMIN) impact experiments. We use mesoscale models resolving the collapse of individual pores to validate macroscopic (homogenized) approaches describing the bulk behavior of porous and water‐saturated materials in large‐scale models of crater formation, and to quantify localized shock amplification as a result of pore space crushing. We carried out a suite of numerical models of planar shock wave propagation through a well‐defined area (the “sample”) of porous and/or water‐saturated material. The porous sample is either represented by a homogeneous unit where porosity is treated as a state variable (macroscale model) and water content by an equation of state for mixed material (ANEOS) or by a defined number of individually resolved pores (mesoscale model). We varied porosity and water content and measured thermodynamic parameters such as shock wave velocity and particle velocity on meso‐ and macroscales in separate simulations. The mesoscale models provide additional data on the heterogeneous distribution of peak shock pressures as a consequence of the complex superposition of reflecting rarefaction waves and shock waves originating from the crushing of pores. We quantify the bulk effect of porosity, the reduction in shock pressure, in terms of Hugoniot data as a function of porosity, water content, and strength of a quartzite matrix. We find a good agreement between meso‐, macroscale models and Hugoniot data from shock experiments. We also propose a combination of a porosity compaction model (ε–α model) that was previously only used for porous materials and the ANEOS for water‐saturated quartzite (all pore space is filled with water) to describe the behavior of partially water‐saturated material during shock compression. Localized amplification of shock pressures results from pore collapse and can reach as much as four times the average shock pressure in the porous sample. This may explain the often observed localized high shock pressure phases next to more or less unshocked grains in impactites and meteorites.     

Shock experiments on anhydrite and new constraints on the impact‐induced SOx release at the K‐Pg boundary

Abstract– We have performed six shock experiments at nominal peak‐shock pressures of 12.5, 20, 33, 46.5, 64, and 85 GPa using polycrystalline anhydrite discs embedded in ARMCO‐Fe sample containers and the shock reverberation technique. The recovered samples were analyzed using X‐ray powder diffraction and transmission electron microscopy (TEM). The X‐ray diffraction patterns recorded on all samples are compatible with the anhydrite structure; extra‐peaks have not been observed. Peak intensities decrease and peak broadening increases progressively in the pressure range from 0 to 46.5 GPa. At higher pressures, peak broadening diminishes and the X‐ray diffraction pattern of the 85 GPa sample resembles essentially that of unshocked, well‐crystallized anhydrite. Related structural changes at the nanoscale include in the pressure regime up to 20 GPa “cold” deformation phenomena such as cracks and deformation twins. Dislocation density increases up to 33 GPa and the strain increases up to 46.5 GPa. In the pressure range from 46.5 to 85 GPa, high postshock temperatures caused annealing of the deformation features. Increasing density and size of voids in the anhydrite samples shocked at 64 and 85 GPa indicate partial decomposition of anhydrite. Recalculation of the peak‐shock pressure in the experiments to a more realistic natural loading path indicates the onset of degassing of anhydrite in the pressure range of 30–41 GPa.     

Structural analysis of the collar of the Vredefort Dome,South Africa—Significance for impact‐related deformation and central uplift formation

Abstract— Landsat TM, aerial photograph image analysis, and field mapping of Witwatersrand supergroup meta‐sedimentary strata in the collar of the Vredefort Dome reveals a highly heterogeneous internal structure involving folds, faults, fractures, and melt breccias that are interpreted as the product of shock deformation and central uplift formation during the 2.02 Ga Vredefort impact event. Broadly radially oriented symmetric and asymmetric folds with wavelengths ranging from tens of meters to kilometers and conjugate radial to oblique faults with strike‐slip displacements of, typically, tens to hundreds of meters accommodated tangential shortening of the collar of the dome that decreased from ?17% at a radius from the dome center of 21 km to <5% at a radius of 29 km. Ubiquitous shear fractures containing pseudotachylitic breccia, particularly in the metapelitic units, display local slip senses consistent with either tangential shortening or tangential extension; however, it is uncertain whether they formed at the same time as the larger faults or earlier, during the shock pulse. In addition to shatter cones, quartzite units show two fracture types—a cmspaced rhomboidal to orthogonal type that may be the product of shock‐induced deformation and later joints accomplishing tangential and radial extension. The occurrence of pseudotachylitic breccia within some of these later joints, and the presence of radial and tangential dikes of impact melt rock, confirm the impact timing of these features and are suggestive of late‐stage collapse of the central uplift.     

Re‐evaluating the age of the Haughton impact event

Abstract— We have re‐evaluated the published age information for the Haughton impact structure, which was believed to have formed ?23 Ma ago during the Miocene age, and report new Ar/Ar laser probe data from shocked basement clasts. This reveals an Eocene age, which is at odds with the published Miocene stratigraphic, apatite fission track and Ar/Ar data; we discuss our new data within this context. We have found that the age of the Haughton impact structure is ?39 Ma, which has implications for both crater recolonization models and post‐impact hydrothermal activity. Future work on the relationship between flora and fauna within the crater, and others at high latitude, may resolve this paradox.     

Resolution of impact‐related microstructures in lunar zircon: A shock‐deformation mechanism map

Abstract– The microstructures of lunar zircon grains from breccia samples 72215, 73215, 73235, and 76295 collected during the Apollo 17 mission have been characterized via optical microscopy, cathodoluminescence imaging, and electron backscatter diffraction mapping. These zircon grains preserve deformation microstructures that show a wide range in style and complexity. Planar deformation features (PDFs) are documented in lunar zircon for the first time, and occur along {001}, {110}, and {112}, typically with 0.1–25 μm spacing. The widest PDFs associated with {112} contain microtwin lamellae with 65°/<110> misorientation relationships. Deformation bands parallel to {100} planes and irregular low‐angle (<10°) boundaries most commonly have <001> misorientation axes. This geometry is consistent with a dislocation glide system with <100>{010} during dislocation creep. Nonplanar fractures, recrystallized domains with sharp, irregular interfaces, and localized annealing textures along fractures are also observed. No occurrences of reidite were detected. Shock‐deformation microstructures in zircon are explained in terms of elastic anisotropy of zircon. PDFs form along a limited number of specific {hkl} planes that are perpendicular to directions of high Young’s modulus, suggesting that PDFs are likely to be planes of longitudinal lattice damage. Twinned {112} PDFs also contain directions of high shear modulus. A conceptual model is proposed for the development of different deformation microstructures during an impact event. This “shock‐deformation mechanism map” is used to explain the relative timing, conditions, and complexity relationships between impact‐related deformation microstructures in zircon.     

Time evolution and temperatures of hypervelocity impact‐generated tracks in aerogel

Abstract— Aerogel collectors have been used to capture cometary, interplanetary, and interstellar dust grains by NASA's Stardust mission, highlighting their importance as a scientific instrument. Due to the fragile and heterogeneous nature of cometary dust grains, their fragments are found along the walls of tracks that are formed during the capture process. These fragments appear to experience a wide range of thermal alteration and the causes of this variation are not well understood at a theoretical level as physical models of track formation are not well developed. Here, a general model of track formation that allows for the existence of partially and completely vaporized aerogel material in tracks is developed. It is shown that under certain conditions, this general track model reduces to the kinetic “snowplow” model that has previously been proposed. It is also shown, based on energetic considerations, that track formation is dominated by an expansion that is snowplow‐like in the later stages of track formation. The equation of motion for this snowplow‐like stage can be solved analytically, thus placing constraints on the amount of heating experienced by cometary dust fragments embedded in track walls. It is found that the heating of these fragments, for a given impact velocity, is expected to be greater for those embedded in larger tracks. Given the expected future use of aerogels for sample return missions, the results presented here imply that the choice of aerogel compositions can have a significant effect on the modification of samples captured and retrieved by these collectors.     

Formation of the Treysa quintet and the main‐group pallasites by impact‐generated processes in the IIIAB asteroid

Treysa and Delegate have compositions closely similar to those of IIIAB irons but plot above the IIIAB field on Ir‐Au diagrams; for this reason they are designated anomalous members of IIIAB. All refractory siderophiles share this anomaly. Wasson ( 1999 ) interpreted the large spread on IIIAB Ir‐Au diagrams to result from melt‐trapping and generated solid and liquid fractional crystallization tracks; almost all IIIAB irons fall between the tracks. In contrast, Treysa, Delegate, and three other irons (the Treysa quintet) plot beyond the liquid track. Ideal fractional crystallization cannot account for compositions that plot outside the region between the tracks. Possible explanations for the anomalous compositions of the Treysa quintet are that (1) these meteorites did not form in the IIIAB magma or (2) they formed by the mixing of early crystallized solids with a late liquid. The weight of the evidence including cosmic‐ray ages favor the second explanation. Although this explanation can account for positions plotting above the liquid track, it requires special circumstances. The infalling blocks must be assimilated and the resulting melt must crystallize quickly into pockets small enough (<1 m) to allow igneous gradients to be leveled by subsequent diffusion. The Treysa quintet shares the region beyond the liquid track with most main‐group pallasites (PMG), which may have also originated in the IIIAB body. It appears that Treysa, its relatives, and the PMG were formed in one or more impact events that mixed olivine and solid metal formed near the core‐mantle boundary with nearby magma. It is then necessary to cool the melt rapidly; the best way to achieve rapid cooling is by heat exchange with cooler solids. That the Treysa quintet and the PMG can be explained by the same processes operating on late IIIAB magma supports the conclusion that PMG formed on the IIIAB parent asteroid.     

Hornblende alteration and fluid inclusions in Kärdla impact crater,Estonia: Evidence for impact‐induced hydrothermal activity

Abstract— The well‐preserved Kärdla impact crater, on Hiiumaa Island, Estonia, is a 4 km diameter structure formed in a shallow Ordovician sea ?455 Ma ago into a target composed of thin (?150 m) unconsolidated sedimentary layer above a crystalline basement composed of migmatite granites, amphibolites and gneisses. The fractured and crushed amphibolites in the crater area are strongly altered and replaced with secondary chloritic minerals. The most intensive chloritization is found in permeable breccias and heavily shattered basement around and above the central uplift. Alteration is believed to have resulted from convective flow of hydrothermal fluids through the central areas of the crater. Chloritic mineral associations suggest formation temperatures of 100–300 °C, in agreement with the most frequent quartz fluid inclusion homogenization temperatures of 150–300 °C in allochthonous breccia. The rather low salinity of fluids in Kärdla crater (<13 wt% NaCl_eq) suggests that the hydrothermal system was recharged either by infiltration of meteoric waters from the crater rim walls raised above sea level after the impact, or by invasion of sea water through the disturbed sedimentary cover and fractured crystalline basement. The well‐developed hydrothermal system in Kärdla crater shows that the thermal history of the shock‐heated and uplifted rocks in the central crater area, rather than cooling of impact melt or suevite sheets, controlled the distribution and intensity of the impact‐induced hydrothermal processes.     

Explicating the behavior of Mn‐bearing phases during shock melting and crystallization of the Abee EH‐chondrite impact‐melt breccia

Abstract— –Literature data show that, among EH chondrites, the Abee impact‐melt breccia exhibits unusual mineralogical characteristics. These include very low MnO in enstatite (<0.04 wt%), higher Mn in troilite (0.24 wt%) and oldhamite (0.36 wt%) than in EH4 Indarch and EH3 Kota‐Kota (which are not impact‐melt breccias), low Mn in keilite (3.6–4.3 wt%), high modal abundances of keilite (11.2 wt%) and silica (～7 wt%, but ranging up to 16 wt% in some regions), low modal abundances of total silicates (58.8 wt%) and troilite (5.8 wt%), and the presence of acicular grains of the amphibole, fluor‐richterite. These features result from Abee's complex history of shock melting and crystallization. Impact heating was responsible for the loss of MnO from enstatite and the concomitant sulfidation of Mn. Troilite and oldhamite grains that crystallized from the impact melt acquired relatively high Mn contents. Abundant keilite and silica also crystallized from the melt; these phases (along with metallic Fe) were produced at the expense of enstatite, niningerite and troilite. Melting of the latter two phases produced a S‐rich liquid with higher Fe/Mg and Fe/Mn ratios than in the original niningerite, allowing the crystallization of keilite. Prior to impact melting, F was distributed throughout Abee, perhaps in part adsorbed onto grain surfaces; after impact melting, most of the F that was not volatilized was incorporated into crystallizing grains of fluor‐richterite. Other EH‐chondrite impact‐melt breccias and impact‐melt rocks exhibit some of these mineralogical features and must have experienced broadly similar thermal histories.     

Evidence for an impact‐induced magnetic fabric in Allende,and exogenous alternatives to the core dynamo theory for Allende magnetization

We conducted a paleomagnetic study of the matrix of Allende CV3 chondritic meteorite, isolating the matrix's primary remanent magnetization, measuring its magnetic fabric and estimating the ancient magnetic field intensity. A strong planar magnetic fabric was identified; the remanent magnetization of the matrix was aligned within this plane, suggesting a mechanism relating the magnetic fabric and remanence. The intensity of the matrix's remanent magnetization was found to be consistent and low (~6 μT). The primary magnetic mineral was found to be pyrrhotite. Given the thermal history of Allende, we conclude that the remanent magnetization was formed during or after an impact event. Recent mesoscale impact modeling, where chondrules and matrix are resolved, has shown that low‐velocity collisions can generate significant matrix temperatures, as pore‐space compaction attenuates shock energy and dramatically increases the amount of heating. Nonporous chondrules are unaffected, and act as heat‐sinks, so matrix temperature excursions are brief. We extend this work to model Allende, and show that a 1 km/s planar impact generates bulk porosity, matrix porosity, and fabric in our target that match the observed values. Bimodal mixtures of a highly porous matrix and nominally zero‐porosity chondrules make chondrites uniquely capable of recording transient or unstable fields. Targets that have uniform porosity, e.g., terrestrial impact craters, will not record transient or unstable fields. Rather than a core dynamo, it is therefore possible that the origin of the magnetic field in Allende was the impact itself, or a nebula field recorded during transient impact heating.     

A case study of impact‐induced hydrothermal activity: The Haughton impact structure,Devon Island,Canadian High Arctic

Abstract— The well‐preserved state and excellent exposure at the 39 Ma Haughton impact structure, 23 km in diameter, allows a clearer picture to be made of the nature and distribution of hydrothermal deposits within mid‐size complex impact craters. A moderate‐ to low‐temperature hydrothermal system was generated at Haughton by the interaction of groundwaters with the hot impact melt breccias that filled the interior of the crater. Four distinct settings and styles of hydrothermal mineralization are recognized at Haughton: a) vugs and veins within the impact melt breccias, with an increase in intensity of alteration towards the base; b) cementation of brecciated lithologies in the interior of the central uplift; c) intense veining around the heavily faulted and fractured outer margin of the central uplift; and d) hydrothermal pipe structures or gossans and mineralization along fault surfaces around the faulted crater rim. Each setting is associated with a different suite of hydrothermal minerals that were deposited at different stages in the development of the hydrothermal system. Minor, early quartz precipitation in the impact melt breccias was followed by the deposition of calcite and marcasite within cavities and fractures, plus minor celestite, barite, and fluorite. This occurred at temperatures of at least 200 °C and down to ?100–120 °C. Hydrothermal circulation through the faulted crater rim with the deposition of calcite, quartz, marcasite, and pyrite, occurred at similar temperatures. Quartz mineralization within breccias of the interior of the central uplift occurred in two distinct episodes (?250 down to ?90 °C, and <60 °C). With continued cooling (<90 °C), calcite and quartz were precipitated in vugs and veins within the impact melt breccias. Calcite veining around the outer margin of the central uplift occurred at temperatures of ?150 °C down to <60 °C. Mobilization of hydrocarbons from the country rocks occurred during formation of the higher temperature calcite veins (>80 °C). Appreciation of the structural features of impact craters has proven to be key to understanding the distribution of hydrothermal deposits at Haughton.     

The Fountain Hills impact‐modified CB chondrite and thermal history of the CB asteroid

Abstract— Fountain Hills is a metal‐rich chondrite with mineral and whole chondrite oxygen isotope compositions that suggest it is a CB chondrite. However, its petrologic characteristics suggest that it has been modified by shock and recrystallization to a greater degree than other CB chondrites. It differs texturally from the CB chondrites in that its metal is interstitial to the silicate and does not occur as discrete clasts as in the other CB chondrites. Portions of Fountain Hills appear to be recrystallized and it contains large (mm‐size) olivine rimmed by pyroxene. A characteristic of the CB chondrites is the presence of small sulfide blebs in large metal clasts and anomalously heavy (¹⁵N‐enriched) nitrogen often associated with metal surrounding the sulfide blebs, but Fountain Hills lacks sulfide and its nitrogen is relatively light. The differences between Fountain Hills and the other CB chondrites can be attributed to a secondary process, most likely impact‐melting and recrystallization, that overprinted its primary features and it is inferred that Fountain Hills is an impact‐modified CB chondrite.     

Northwest Africa 482: A crystalline impact‐melt breccia from the lunar highlands

Abstract— Northwest Africa 482 (NWA 482) is a crystalline impact‐melt breccia from the Moon with highlands affinities. The recrystallized matrix and the clast population are both highly anorthositic. Clasts are all related to the ferroan anorthosite suite, and include isolated plagioclase crystals and lithic anorthosites, troctolites, and spinel troctolites. Potassium‐, rare‐earth‐element‐, and phosphorus‐bearing (KREEP) and mare lithologies are both absent, constraining the source area of this meteorite to a highland terrain with little to no KREEP component, most likely on the far side of the Moon. Glass is present in shock veins cutting through the sample and in several large melt pockets, indicating a second impact event. There are two separate events recorded in the ⁴⁰Ar‐³⁹Ar system: one at ～3750 Ma, which completely reset the K‐Ar system, and one at ?2400 Ma, which caused only partial degassing. These events could represent, respectively, crystallization of the impact‐melt breccia and later formation of the glass, or the formation of the glass and a later thermal event. The terrestrial age of the meteorite is 8.6 ± 1.3 ka. This age corresponds well with the modest amount of weathering in the rock, in the form of secondary phyllosilicates and carbonates. Based on terrestrial age and location, lithology, and chemistry, NWA 482 is unique among known lunar meteorites.     

Confirmation of a meteoritic component in impact‐melt rocks of the Chesapeake Bay impact structure,Virginia, USA—Evidence from osmium isotopic and PGE systematics

Abstract— The osmium isotope ratios and platinum‐group element (PGE) concentrations of impact‐melt rocks in the Chesapeake Bay impact structure were determined. The impact‐melt rocks come from the cored part of a lower‐crater section of suevitic crystalline‐clast breccia in an 823 m scientific test hole over the central uplift at Cape Charles, Virginia. The ¹⁸⁷Os/¹⁸⁸Os ratios of impact‐melt rocks range from 0.151 to 0.518. The rhenium and platinum‐group element (PGE) concentrations of these rocks are 30–270x higher than concentrations in basement gneiss, and together with the osmium isotopes indicate a substantial meteoritic component in some impact‐melt rocks. Because the PGE abundances in the impact‐melt rocks are dominated by the target materials, interelemental ratios of the impact‐melt rocks are highly variable and nonchondritic. The chemical nature of the projectile for the Chesapeake Bay impact structure cannot be constrained at this time. Model mixing calculations between chondritic and crustal components suggest that most impact‐melt rocks include a bulk meteoritic component of 0.01–0.1% by mass. Several impact‐melt rocks with lowest initial ¹⁸⁷Os/¹⁸⁸Os ratios and the highest osmium concentrations could have been produced by additions of 0.1%–0.2% of a meteoritic component. In these samples, as much as 70% of the total Os may be of meteoritic origin. At the calculated proportions of a meteoritic component (0.01–0.1% by mass), no mixtures of the investigated target rocks and sediments can reproduce the observed PGE abundances of the impact‐melt rocks, suggesting that other PGE enrichment processes operated along with the meteoritic contamination. Possible explanations are 1) participation of unsampled target materials with high PGE abundances in the impact‐melt rocks, and 2) variable fractionations of PGE during syn‐ to post‐impact events.     

Rietveld analysis of X‐ray powder diffraction patterns as a potential tool for the identification of impact‐deformed carbonate rocks

Abstract— Previous X‐ray powder diffraction (XRD) studies revealed that shock deformed carbonates and quartz have broader XRD patterns than those of unshocked samples. Entire XRD patterns, single peak profiles and Rietveld refined parameters of carbonate samples from the Sierra Madera impact crater, west Texas, unshocked equivalent samples from 95 miles north of the crater and the Mission Canyon Formation of southwest Montana and western Wyoming were used to evaluate the use of X‐ray powder diffraction as a potential tool for distinguishing impact deformed rocks from unshocked and tectonically deformed rocks. At Sierra Madera dolostone and limestone samples were collected from the crater rim (lower shock intensity) and the central uplift (higher shock intensity). Unshocked equivalent dolostone samples were collected from well cores drilled outside of the impact crater. Carbonate rocks of the Mission Canyon Formation were sampled along a transect across the tectonic front of the Sevier and Laramide orogenic belts. Whereas calcite subjected to significant shock intensities at the Sierra Madera impact crater can be differentiated from tectonically deformed calcite from the Mission Canyon Formation using Rietveld refined peak profiles, weakly shocked calcite from the crater rim appears to be indistinguishable from the tectonically deformed calcite. In contrast, Rietveld analysis readily distinguishes shocked Sierra Madera dolomite from unshocked equivalent dolostone samples from outside the crater and tectonically deformed Mission Canyon Formation dolomite.     

Re‐examining the role of chondrules in producing the elemental fractionations in chondrites

Abstract— The matrices of all primitive chondrites contain presolar materials (circumstellar grains and interstellar organics) in roughly CI abundances, suggesting that all chondrites accreted matrix that is dominated by a CI‐like component. The matrix‐normalized abundances of the more volatile elements (condensation temperatures <750–800 K) in carbonaceous and ordinary chondrites are also at or slightly above CI levels. The modest excesses may be due to low levels of these elements in chondrules and associated metal. Subtraction of a CI‐like matrix component from a bulk ordinary chondrite composition closely matches the average composition of chondrules determined by instrumental neutron activation analysis (INAA) if some Fe‐metal is added to the chondrule composition. Measured matrix compositions are not CI‐like. Sampling bias and secondary redistribution of elements may have played a role, but the best explanation is that ?10–30% of refractory‐rich, volatile depleted material was added to matrix. If most of the more volatile elements are in a CI‐dominated matrix, the major and volatile element fractionations must be largely carried by chondrules. There is both direct and indirect evidence for evaporation during chondrule formation. Type IIA and type B chondrules could have formed from a mixture of CI material and material evaporated from type IA chondrules. The Mg‐Si‐Fe fractionations in the ordinary chondrites can be reproduced with the loss of type IA chondrule material and associated metal. The loss of evaporated material from the chondrules could explain the volatile element fractionations. Mechanisms for how these fractionations occurred are necessarily speculative, but two possibilities are briefly explored.     

Fluid inclusion evidence for impact‐related hydrothermal fluid and hydrocarbon migration in Creataceous sediments of the ICDP‐Chicxulub drill core Yax‐1

Fluid inclusions studies in quartz and calcite in samples from the ICDP‐Chicxulub drill core Yaxcopoil‐1 (Yax‐1) have revealed compelling evidence for impact‐induced hydrothermal alteration. Fluid circulation through the melt breccia and the underlying sedimentary rocks was not homogeneous in time and space. The formation of euhedral quartz crystals in vugs hosted by Cretaceous limestones is related to the migration of hot (>200 °C), highly saline, metal‐rich, hydrocarbon‐bearing brines. Hydrocarbons present in some inclusions in quartz are assumed to derive from cracking of pre‐impact organic matter. The center of the crater is assumed to be the source of the hot quartz‐forming brines. Fluid inclusions in abundant newly‐formed calcite indicate lower cyrstallization temperatures (75–100 °C). Calcite crystallization is likely related to a later stage of hydrothermal alteration. Calcite precipitated from saline fluids, most probably from formation water. Carbon and oxygen isotope compositions and REE distributions in calcites and carbonate host rocks suggest that the calcite‐forming fluids have achieved close equilibrium conditions with the Cretaceous limestones. The precipitation of calcite may be related to the convection of local pore fluids, possibly triggered by impact‐induced conductive heating of the sediments.     

The formation of Ca‐, Fe‐rich silicates in reduced and oxidized CV chondrites: The roles of impact‐modified porosity and permeability,and heterogeneous distribution of water ices

CV (Vigarano type) carbonaceous chondrites, comprising Allende‐like (CV_oxA) and Bali‐like (CV_oxB) oxidized and reduced (CV_red) subgroups, experienced differing degrees of fluid‐assisted thermal and shock metamorphism. The abundance and speciation of secondary minerals produced during asteroidal alteration differ among the subgroups: (1) ferroan olivine and diopside–hedenbergite solid solution pyroxenes are common in all CVs; (2) nepheline and sodalite are abundant in CV_oxA, rare in CV_red, and absent in CV_oxB; (3) phyllosilicates and nearly pure fayalite are common in CV_oxB, rare in CV_red, and virtually absent in CV_oxA; (4) andradite, magnetite, and Fe‐Ni‐sulfides are common in oxidized CVs, but rare in reduced CVs; the latter contain kirschsteinite instead. Thus, a previously unrecognized correlation exists between meteorite bulk permeabilities and porosities with the speciation of the Ca‐, Fe‐rich silicates (pyroxenes, andradite, kirschsteinite) among the CVox and CVred meteorites. The extent of secondary mineralization was controlled by the distribution of water ices, permeability, and porosity, which in turn were controlled by impacts on the asteroidal parent body. More intense shock metamorphism in the region where the reduced CVs originated decreased their porosity and permeability while simultaneously expelling intergranular ices and fluids. The mineralogy, petrography, and bulk chemical compositions of both the reduced and oxidized CV chondrites indicate that mobile elements were redistributed between Ca,Al‐rich inclusions, dark inclusions, chondrules, and matrices only locally; there is no evidence for large‐scale (>several cm) fluid transport. Published ⁵³Mn‐⁵³Cr ages of secondary fayalite in CV, CO, and unequilibrated ordinary chondrites, and carbonates in CI, CM, and CR chondrites are consistent with aqueous alteration initiated by heating of water ice‐bearing asteroids by decay of ²⁶Al, not shock metamorphism.