Shock Metamorphism Research Articles

Iron-nickel metallic components bearing silicate-melts and coesite from Ramgarh impact structure, west-central India: Possible identification of the impactor

The Ramgarh structure (rim-to-rim diameter ~2.4 km) in the Vindhyan Supergroup of sedimentary rocks (including sandstone, shale and minor limestone) of the Mesoproterozoic age in the west-central India, is India’s third confirmed asteroid impact crater. This eroded structure is roughly rectangular in shape and resembles to the Barringer Crater, USA. The presence of central peak and its current crater diameter/depth ratio of ~12 well corroborate the range (10–20) of terrestrial complex asteroid impact craters. The mm-sized, iron-rich (FeO ~50 wt.% in average), spherule-like particles, recovered from the alluvium inside the Ramgarh structure, have internal morphology similar to those of the accretionary lapilli described in known impact craters. The in-situ LA-ICP-MS analyses also suggested high Co–Ni (up to 13,000 and 2500 ppm, respectively)-rich areas locally within these spherules/lapilli. A few non-in-situ, mm-sized particles, recovered from the rim of the structure show the presence of coesite, one of the diagnostic indicators of shock metamorphism. A few fragments of iron-rich, Ca–Al–silicate glasses recovered from the soil inside the structure and outside of the western crater rim include the presence of dendritic magnetite with occasional inclusions of relict native iron. Our microprobe analyses confirm that these metallic irons contain high proportions of Co (~350–3000 ppm), Ni (~200–4000 ppm) and Cu (~2200–7000 ppm) and possibly could be the relict component of a Cu-rich iron meteorite impactor. The field observation and relative enrichment of compatible and incompatible trace elements in the spherule-like substance (recovered from the alluvium inside the Ramgarh structure) as compared to target rocks suggests that hydrothermal activity played an important role in the evolution of the crater.

Shock metamorphism in plagioclase and selective amorphization.

Plagioclase feldspar is one of the most common rock‐forming minerals on the surfaces of the Earth and other terrestrial planetary bodies, where it has been exposed to the ubiquitous process of hypervelocity impact. However, the response of plagioclase to shock metamorphism remains poorly understood. In particular, constraining the initiation and progression of shock‐induced amorphization in plagioclase (i.e., conversion to diaplectic glass) would improve our knowledge of how shock progressively deforms plagioclase. In turn, this information would enable plagioclase to be used to evaluate the shock stage of meteorites and terrestrial impactites, whenever they lack traditionally used shock indicator minerals, such as olivine and quartz. Here, we report on an electron backscatter diffraction (EBSD) study of shocked plagioclase grains in a metagranite shatter cone from the central uplift of the Manicouagan impact structure, Canada. Our study suggests that, in plagioclase, shock amorphization is initially localized either within pre‐existing twins or along lamellae, with similar characteristics to planar deformation features (PDFs) but that resemble twins in their periodicity. These lamellae likely represent specific crystallographic planes that undergo preferential structural failure under shock conditions. The orientation of preexisting twin sets that are preferentially amorphized and that of amorphous lamellae is likely favorable with respect to scattering of the local shock wave and corresponds to the “weakest” orientation for a specific shock pressure value. This observation supports a universal formation mechanism for PDFs in silicate minerals.

Preferred orientation distribution of shock-induced planar microstructures in quartz and feldspar.

Shocked quartz and feldspar grains commonly exhibit planar microstructures, such as planar fractures, planar deformation features, and possibly microtwins, which are considered to have formed by shock metamorphism. Their orientation and frequency are typically reported to be randomly distributed across a sample. The goal of this study is to investigate whether such microstructures are completely random within a given sample, or whether their orientation might also retain information on the direction of the local shock wave propagation. For this work, we selected samples of shatter cones, which were cut normal to the striated surface and the striation direction, from three impact structures (Keurusselkä, Finland, and Charlevoix and Manicouagan, Canada). These samples show different stages of pre‐impact tectonic deformation. Additionally, we investigated several shocked granite samples, selected at different depths along the drill core recovered during the joint IODP‐ICDP Chicxulub Expedition 364 (Mexico). In this case, thin sections were cut along two orthogonal directions, one parallel and one normal to the drill core axis. All the results refer to optical microscopy and universal‐stage analyses performed on petrographic thin sections. Our results show that such shock‐related microstructures do have a preferred orientation, but also that relating their orientation with the possible shock wave propagation is quite challenging and potentially impossible. This is largely due to the lack of dedicated experiments to provide a key to interpret the observed preferred orientation and to the lack of information on postimpact orientation modifications, especially in the case of the drill core samples.

Volatiles in lunar felsite clasts: Impact-related delivery of hydrous material to an ancient dry lunar crust

In this detailed geochemical, petrological, and microstructural study of felsite clast materials contained in Apollo breccia samples 12013, 14321, and 15405, little evidence was found for relatively enriched reservoirs of endogenic lunar volatiles. NanoSIMS measurements have revealed very low volatile abundances (≤2–18 ppm hydrogen) in nominally anhydrous minerals (NAMS) plagioclase, potassic alkali feldspar, and SiO2 that make up a majority of these felsic lithologies. Yet these mineral assemblages and clast geochemistries on Earth would normally yield relatively high volatiles contents in their NAMS (∼20 to ≥80 ppm hydrogen). This difference is particularly notable in felsite 14321,1062 that exhibits extremely low volatile abundances (≤2 ppm hydrogen) and a relatively low amount of microstructural evidence for shock metamorphism given that it is a clast of the most evolved (∼74 wt.% SiO2) rock-type returned from the Moon. If taken at face value, ‘wet’ felsic magmas (∼1.2–1.7 wt.% water) are implied by the relatively high hydrogen contents of feldspar in felsite clasts in Apollo samples 12013 and 15405, but these results are likely misleading. These felsic clasts have microstructural features indicative of significantly higher shock stress than 14321,1062. These crustal lithologies likely obtained no more water from the lunar interior than the magma body producing 14321,1062. Rather, we suggest hydrogen was enriched in samples 12013 and 15405 by impact induced exchange, and/or partial assimilation of volatiles added to the surface of the Moon by a hydrated impactor (asteroid or comet) or the solar wind. Thus, the best estimate for magmatic water contents of felsic lunar magmas comes from 14321,1062 that leads to a calculated magmatic water content of ≤0.2 wt.%. This dry felsic magma has a slightly greater, but comparable water content to the ancient mafic magmas implied by the other lithologies that we have studied. Based on this and expanding evidence for a significantly dry ancient or early degassed Moon it is likely that some recent estimates (100's ppm) of the water abundances in the lunar parental magma ocean have been overestimated.

Extraterrestrial, shock-formed, cage-like nanostructured carbonaceous materials

AbstractShock caused by impacts can convert carbonaceous material to diamond. During this transition, new materials can form that depend on the structure of the starting carbonaceous materials and the shock conditions. Here we report the discovery of cage-like nanostructured carbonaceous materials, including carbon nano-onions and bucky-diamonds, formed through extraterrestrial impacts in the Gujba (CBa) meteorite. The nano-onions are fullerene-type materials and range from 5 to 20 nm; the majority shows a graphitic core-shell structure, and some are characterized by fully curved, onion-like graphitic shells. The core is either filled with carbonaceous material or empty. We show the first, natural, 4 nm sized bucky-diamond, which is a type of carbon nano-onion consisting of multilayer graphitic shells surrounding a diamond core. We propose that the nano-onions formed during shock metamorphism, either the shock or the release wave, of the pre-existing primitive carbonaceous material that included nanodiamonds, poorly ordered graphitic material, and amorphous carbonaceous nanospheres. Bucky-diamonds could have formed either through the high-pressure transformation of nano-onions, or as an intermediate material in the high-temperature transformation of nanodiamond to nano-onion. Impact processing of planetary materials was and is a common process in our solar system, and by extension, throughout extrasolar planetary bodies. Together with our previous discovery of interstratified graphite-diamond in Gujba, our new findings extend the range of nano-structured carbonaceous materials formed in nature. Shock-formed nano-onions and bucky-diamonds are fullerene-type structures, and as such they could contribute to the astronomical 217.5 nm absorption feature.

Spatial U-Pb age distribution in shock-recrystallized zircon – A case study from the Rochechouart impact structure, France

Age determination of impact structures via the zircon U-Pb system remains challenging and often ambiguous due to highly variable effects of shock metamorphism on U-Pb geochronology. It is, therefore, crucial to link the observed zircon microtextures, including their temperature and pressure conditions associated with their formation, directly to the U-Pb ages preserved. Here, we analyzed three recrystallized zircon grains and one plastically deformed zircon crystal from the medium-sized Rochechouart impact structure in the northwestern Massif Central of France. For the Rochechouart impact structure the impact age (206.92 ± 0.32 Ma [40Ar/39Ar]) as well as the tectono-themal history is well established making this study site ideal to test concepts about U-Pb systematics in shocked zircon and to differentiate between shock-driven age resetting and pre-impact crystallization and metamorphic overprinting. Zircon microstructures were studied using scanning electron imaging, cathodoluminescence imaging, and electron backscatter diffraction (EBSD) mapping. Further, we conducted U-Pb Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS) depth-profiling analysis, allowing us to interpret the resultant age data in discrete steps with increasing ablation time/depth. The U-Pb depth profiling data demonstrate that plastically strained grains are incompletely reset and preserve rim and interior age domains reflecting typical pre-impact (pre-Variscan and Variscan) regional tectonic ages. Our results also reveal that the granular crystals encountered contain microstructural evidence for “former reidite in granular neoblastic” (FRIGN) zircon, reflecting both high-pressure (⩾30 GPa) and high-temperature (⩾1200 °C) conditions. This signifies that FRIGN zircon is now known from an additional, medium sized, impact structure further supporting the hypothesis that this impact induced microstructure is commonly preserved. In addition, FRIGN zircon has a high potential to preserve completely impact-reset crystal domains (∼204 to 207 Ma) that can be identified by our combined analytical approach of U-Pb depth profiling and EBSD mapping, and thus are suitable for determining reliable impact ages.

К вопросу о методологии изучения и классификации месторождений полезных ископаемых

The modeling methodology is considered, and the static models that reflect the state of objects at the time of studying are analyzed. Based on these data, genetic retrospective models are constructed taking into account the modern data. They reflect the history of the processes that led to the formation of deposits. Models characterize the morphology, mineral and chemical composition of mineral bodies, conditions of occurrence, and features of surrounding rocks. Theoretical approaches and the content of modern genetic classification of mineral deposits are the core basis of knowledge about the geology of deposits. It is linked to the classification of igneous and metamorphic rocks described in the petrographic code, as well as to the modern lithology. Classification is necessary to systematize the entire variety of natural mineral objects, the origin of which is often ambiguous. It creates the basis for a unified approach to all natural objects that exist in the Earth's interior. The conditions for the formation of endogenous, exogenous and metamorphogenic deposits are considered. Endogenous objects are divided into three groups: magmatic, metasomatic, and hydrothermal. Among the exogenous, the sedimentogenetic, diagenetic, and catagenetic ranks are highlighted. Metamorphic deposits contain dynamothermal, regional-metasomatic, and migmatite classes, as well as thermal, dislocation, and impact metamorphism classes. Examples of typical deposits are given.

Zircon Microstructures Record Deformation History of Shock- and Tectonically-generated Pseudotachylites: a Case Study from the Vredefort Impact Structure, South Africa

AbstractHigh-strain rate deformation can cause in situ melting of rocks, resulting in the formation of dark, micro- to nanocrystalline pseudotachylite veins. On Earth, pseudotachylite veins form during meteorite impacts, large landslides, and earthquakes. Within the Vredefort impact structure, both impact-generated and (pre-impact) tectonically-generated pseudotachylite veins have been described, but are challenging to distinguish. Here, we demonstrate a genetic distinction between two pseudotachylite veins from Vredefort by studying their petrography, degree of recrystallization and deformation, cross-cutting relationships and the deformation microstructures in associated zircon. We conclude that Vein 1 is pre-impact and tectonically-generated, and Vein 2 is impact-generated. In agreement, zircon microstructures in Vein 1 contain planar deformation bands (PDBs), attributed to tectonic deformation, whereas zircon microstructures in Vein 2 reveal microtwin lamellae, indisputable evidence of shock metamorphism. Thus, deformation microstructures in zircon may provide a new criterion for distinguishing the genetic origin of pseudotachylite veins. Zircons that have been removed from their context (i.e., alluvial or detrital zircon, zircon from Lunar breccia) should be interpreted with caution in terms of their deformation history. For example, zircon with PDBs cannot reliably be used as a marker for shock deformation, because this feature has been shown to form in purely tectonic settings.

Conference report: Large Meteorite Impacts and Planetary Evolution VI

Conference report: Large Meteorite Impacts and Planetary Evolution VI

Mineral chemistry and in situ U[sbnd]Pb geochronology of the mare basalt Northwest Africa 10597: Implications for low-Ti mare volcanism around 3.0 Ga

Unbrecciated mare basalts are rare in the lunar meteorite collection. Found in 2015, Northwest Africa (NWA) 10597 is a medium-grained low-Ti mare basalt with a subophitic texture. The meteorite consists mostly of mm-sized pyroxene and plagioclase, with minor olivine, spinel, ilmenite, phosphates, silica, and trace Zr-rich minerals, such as baddeleyite, zirconolite and tranquillityite. A portion of plagioclase and silica has been transformed to their high-pressure polymorphs due to shock metamorphism. NWA 10597 has a low TiO2 content (2.9 wt%) but is relatively enriched in rare earth elements (REE) (Laaverage = 65 × CI) with an overall unfractionated pattern except for a negative Eu anomaly. Calculated REE concentrations of parent melts in equilibrium with Mg-rich pyroxene and Ca-rich plagioclase suggest no significant assimilation of REE-rich melts after the onset of pyroxene crystallization. In situ UPb analyses of baddeleyite and apatite reveal a mutually consistent age of ~3.0 Ga, which is also in excellent agreement with that of low-Ti mare basalts NWA 4734 and LaPaz Icefield (LAP) 02205 dated with other independent techniques. The concordance suggests no significant thermal disturbance in the UPb isotopic system of NWA 10597 although it was heavily shocked. NWA 10597 closely resembles NWA 4734 in terms of petrographic texture, mineral chemistry and geochronology, indicating a pairing relationship.

Special issue of MAPS in honor of Wolf Uwe Reimold on occasion of his 65th birthday

Special issue of <i>MAPS</i> in honor of Wolf Uwe Reimold on occasion of his 65th birthday

Petrogenesis and Shock Metamorphism of Basaltic Lunar Meteorites Northwest Africa 4734 and 10597

AbstractWe present comprehensive compositional and mineralogical results on two basaltic lunar meteorites Northwest Africa (NWA) 4734 and NWA 10597 to constrain their igneous mineralogy and metamorphic characteristics, and examine the potential pairing relationship among them and other meteorites (e.g., basaltic lunar meteorites collected from LaPaz Icefield, Antarctica (LAPs)). NWA 4734 and NWA 10597 are low‐Ti (3.2–3.5 wt.% TiO2), low‐Al (10–12 wt.% Al2O3), low‐K (880–1,300 ppm K) mare basalts derived from evolved parental magma (Mg# (molar Mg/[Mg + Fe] × 100) = 33.6–38.3) and are mostly composed of pyroxene (52.7–55.5 vol.%), plagioclase/maskelynite (27.5–29.3 vol.%), olivine (6.7–7.6 vol.%), and late‐formed components (i.e., mesostasis). Pyroxene and olivine in these two meteorites exhibit a multimodal compositional distribution, indicating multiple generations of these mafic minerals, which correspond to different evolution phases (e.g., magma chamber, ascending, and eruption) during the solidification of the basaltic parental magma. Immiscibility played an important role in the evolution of the late‐stage melts, inducing fractionation involving Fe‐rich and Si, K‐rich melts within mesostasis. Considering the extensive partial transformation of plagioclase to maskelynite across the sections, the average shock pressure endured by NWA 4734 and NWA 10597 is probably 23–29 GPa, which is consistent with the pressure condition (below 29.8 GPa) recorded by quartz. Similar textures, bulk composition, modal mineral proportions, and mineral compositions indicate that NWA 4734, NWA 10597, and the LAPs most likely originated from the same region of the Moon and both experienced intensive shock events.

Oxidation state of iron in fulgurites and Trinitite: Implications for redox changes during abrupt high-temperature and pressure events

Understanding the geochemical effects of shock metamorphism that occur on planetary surfaces is critical when using potentially shocked planetary samples. Fulgurites, formed by lightning strikes on Earth’s surface, could provide an analog for understanding shock metamorphism, because the intense heat and pressure of impact events are similar to those experienced during lightning strikes. The oxidation state generated during impact is the result of temperature (above 2000 K) and pressure (above 10 GPa) conditions, though the composition and physical nature of the country rock are also important. Atomic explosions such as the Trinity nuclear test and the resulting melted surface material Trinitite can also serve as proxies of shock metamorphism. This study compares samples of fulgurite and Trinitite glasses with the associated country rock using petrography, backscattered electron images, and Mössbauer spectroscopy, with a focus on evaluating redox changes associated with lightning-induced and atomic explosion metamorphism as a function of target material. Two fulgurite melts were also subjected to additional analysis using X-ray absorption spectroscopy to assess potential spatial variations in redox state via in-situ microscale measurements. Results indicate lightning-induced redox variations are heterogeneous at micron scales, although the net effect is overall target reduction with an average reduction of Fe3+ by 66%. Moreover, the data show that the physical state of the country rock (i.e., particulate vs. solid rock) has an effect on the magnitude of how the lightning-induced metamorphism effects are observed.

Frontiers in natural and un-natural glasses: An interdisciplinary dialogue and review

The purpose of this review is to compare and contrast silicate natural glasses with so-called un-natural glasses, bringing together literature and common issues in geology and technical glass science and engineering. The review is organized into three proposed ‘frontiers’ of glass science of interdisciplinary interest: 1) extreme conditions for glass formation, 2) crystallization in complex glasses, and 3) long-term glass alteration. In each frontier, examples of natural (geologic) as well as un-natural (accidental anthropogenic) glasses are presented, and their characteristics are compared and contrasted. Since crystallization is an important consideration, not only fully amorphous glasses but also partially crystallized glasses are considered. Natural glasses discussed include primarily impactites, obsidian, and basaltic glass, though glasses formed by other means are also considered. Un-natural glasses discussed include those created in atomic blasts, vitrified Iron Age hillforts, and slags from metallurgical processes. Conditions for extreme glass formation in different types of glasses formed by shock metamorphism are compared to modern time-resolved shock and high-pressure experiments for observing polyamorphism and densification changes. Silicate magmas producing obsidian and basaltic glasses are presented and the differences in crystallization pathways are discussed as they relate to changes in flow properties and eruptions. Vitrified hillfort materials and combustion natural glasses are then compared with respect to their felsic-mafic liquid phase separation and crystal assemblage. Finally, an appeal is made for the use of natural and un-natural glasses for understanding long-term glass corrosion as a function of composition. Glass corrosion models are considered as applied to both types of glasses, particularly relating to the neo-formed crystals, biologically-induced glass corrosion, and nuclear waste management. It is hoped that this unorthodox presentation will foster dialogue between earth scientists and technical glass scientists and further research into these and other frontiers of natural glass science.

Chondrite Shock Metamorphism History Assessed by Non-Destructive Analyses on Ca-Phosphates and Feldspars in the Cangas de Onís Regolith Breccia

The Cangas de Onís regolith breccia is an H5-H6 ordinary chondrite that fell to Earth in 1866. It is constituted by 60% H6-H5 clasts within an H5 clastic matrix. All clasts are affected by intense fissuration, with an intricate pattern filled by kamacite and troilite, shock metamorphism of plagioclase into maskelynite. The chondrite is composed of low-Ca pyroxene, olivine and plagioclase as the main silicate phases, with two types of phosphates, taenite-kamacite blebs and troilite. The specimen was studied by micro-Raman spectroscopy, Fourier-transform infrared spectrophotometry (FTIR), spectral cathodoluminescence and computer tomography. The ease with which the specimens can be prepared for analysis using these techniques, and the speed with which relevant information can be obtained, make them excellent tools for the study of non-replaceable materials. Moreover, the Raman and FTIR results offer enough resolution to reveal heterogeneities in the shocked metamorphism throughout the specimen. The obtained results showed that the extent of the metamorphic conditions within the studied sample is heterogeneous, which leads us to believe that the last accretionary event that took place in this regolithic breccia was not significant enough to allow for overall homogenization.

A light, chondritic xenolith in the Murchison (CM) chondrite – Formation by fluid-assisted percolation during metasomatism?

The main mineralogical characteristics of a large light-colored clast within the Murchison CM breccia are discussed in detail including data on the mineralogy, bulk chemistry, organics, and oxygen isotopes. Petrographic study shows that the white clast consists of two areas with different granoblastic textures: (1) a coarse-grained (average grain size: ∼200 μm) and (2) a fine-grained lithology (average grain-size: ∼20 μm). The Fa-content of olivine in the clast is the same as Fa within olivine from Rumuruti (R) chondrites (Fa: ∼38 mol%); however, the concentrations of the elements Ni and Ca in olivine are significantly different. The fragment also contains Ca-rich pyroxene, ∼An30-38-plagioclase/maskelynite, Cr-rich spinel, several sulfide phases, a nepheline-normative glass, and traces of merrillite and metal. The occurrence of maskelynite and nepheline-normative amorphous phase in restricted areas of the well-recrystallized rock may indicate remarkable P-T-excursions during shock metamorphism. The O-isotope composition of the clast falls below the terrestrial fractionation line (TFL), lying in the field of CM chondrites and is significantly different from data for bulk R chondrites. The study of the soluble organic matter revealed a highly-oxidized carbon chemistry and organomagnesium compounds reflecting high temperature and pressure processes.

(U‐Th)/He zircon dating of Chesapeake Bay distal impact ejecta from ODP site 1073

AbstractSingle crystal (U‐Th)/He dating has been undertaken on 21 detrital zircon grains extracted from a core sample from Ocean Drilling Project (ODP) site 1073, which is located ~390 km northeast of the center of the Chesapeake Bay impact structure. Optical and electron imaging in combination with energy dispersive X‐ray microanalysis (EDS) of zircon grains from this late Eocene sediment shows clear evidence of shock metamorphism in some zircon grains, which suggests that these shocked zircon crystals are distal ejecta from the formation of the ~40 km diameter Chesapeake Bay impact structure. (U‐Th/He) dates for zircon crystals from this sediment range from 33.49 ± 0.94 to 305.1 ± 8.6 Ma (2σ), implying crystal‐to‐crystal variability in the degree of impact‐related resetting of (U‐Th)/He systematics and a range of different possible sources. The two youngest zircon grains yield an inverse‐variance weighted mean (U‐Th)/He age of 33.99 ± 0.71 Ma (2σ uncertainties n = 2; mean square weighted deviation = 2.6; probability [P] = 11%), which is interpreted to be the (U‐Th)/He age of formation of the Chesapeake Bay impact structure. This age is in agreement with K/Ar, 40Ar/39Ar, and fission track dates for tektites from the North American strewn field, which have been interpreted as associated with the Chesapeake Bay impact event.

Shock physics mesoscale modeling of shock stage 5 and 6 in ordinary and enstatite chondrites

Shock-darkening, the melting of metals and iron sulfides into a network of veins within silicate grains, altering reflectance spectra of meteorites, was previously studied using shock physics mesoscale modeling. Melting of iron sulfides embedded in olivine was observed at pressures of 40–50 GPa. This pressure range is at the transition between shock stage 5 (CS5) and 6 (CS6) of the shock metamorphism classification in ordinary and enstatite chondrites. To better characterize CS5 and CS6 with a mesoscale modeling approach and assess post-shock heating and melting, we used multi-phase (i.e. olivine/enstatite, troilite, iron, pores, and plagioclase) meshes with realistic configurations of grains. We carried out a systematic study of shock compression in ordinary and enstatite chondrites at pressures between 30 and 70 GPa. To setup mesoscale sample meshes with realistic silicate, metal, iron sulfide, and open pore shapes, we converted backscattered electron microscope images of three chondrites. The resolved macroporosity in meshes was 3–6%. Transition from shock CS5 to CS6 was observed through (1) the melting of troilite above 40 GPa with melt fractions of ~0.7–0.9 at 70 GPa, (2) the melting of olivine and iron above 50 GPa with melt fraction of ~0.001 and 0.012, respectively, at 70 GPa, and (3) the melting of plagioclase above 30 GPa (melt fraction of 1, at 55 GPa). Post-shock temperatures varied from ~540 K at 30 GPa to ~1300 K at 70 GPa. We also constructed models with increased porosity up to 15% porosity, producing higher post-shock temperatures (~800 K increase) and melt fractions (~0.12 increase) in olivine. Additionally we constructed a pre-heated model to observe post-shock heating and melting during thermal metamorphism. This model presented similar results (melting) at pressures 10–15 GPa lower compared to the room temperature models. Finally, we demonstrated dependence of post-shock heating and melting on the orientation of open cracks relative to the shock wave front. In conclusion, the modeled melting and post-shock heating of individual phases were mostly consistent with the current shock classification scheme (Stöffler et al., 1991, 2018).

New shock microstructures in titanite (CaTiSiO5) from the peak ring of the Chicxulub impact structure, Mexico

Accessory mineral geochronometers such as apatite, baddeleyite, monazite, xenotime and zircon are increasingly being recognized for their ability to preserve diagnostic microstructural evidence of hypervelocity-impact processes. To date, little is known about the response of titanite to shock metamorphism, even though it is a widespread accessory phase and a U–Pb geochronometer. Here we report two new mechanical twin modes in titanite within shocked granitoid from the Chicxulub impact structure, Mexico. Titanite grains in the newly acquired core from the International Ocean Discovery Program Hole M0077A preserve multiple sets of polysynthetic twins, most commonly with composition planes (K1) = ~ {1¯11}, and shear direction (η1) = , and less commonly with the mode K1 = {130}, η1 = ~ . In some grains, {130} deformation bands have formed concurrently with the deformation twins, indicating dislocation slip with Burgers vector b = can be active during impact metamorphism. Titanite twins in the modes described here have not been reported from endogenically deformed rocks; we, therefore, propose this newly identified twin form as a result of shock deformation. Formation conditions of the twins have not been experimentally calibrated, and are here empirically constrained by the presence of planar deformation features in quartz (12 ± 5 and ~ 17 ± 5 GPa) and the absence of shock twins in zircon (< 20 GPa). While the lower threshold of titanite twin formation remains poorly constrained, identification of these twins highlight the utility of titanite as a shock indicator over the pressure range between 12 and 17 GPa. Given the challenges to find diagnostic indicators of shock metamorphism to identify both ancient and recent impact evidence on Earth, microstructural analysis of titanite is here demonstrated to provide a new tool for recognizing impact deformation in rocks where other impact evidence may be erased, altered, or did not manifest due to generally low (< 20 GPa) shock pressure.

Olivine melting at high pressure condition in the chassignite Northwest Africa 2737

Shock-induced melting textures and assemblages in the chassignite Northwest Africa (NWA) 2737 were investigated. NWA 2737 consists mainly of coarse-grained olivine (Fa19.6–20.5) and minor amounts of low-Ca pyroxene, plagioclase (now maskelynite), and chromite. Several melt inclusions formed through pre-igneous activity are enclosed in the olivine grains, and some melt inclusions were melted again by the impact event. Several olivine fragments are entrained in the melt inclusion and dissociated into spherulitic ferroan–periclase and interstitial fine-grained clinopyroxene assemblages. The olivine fragments were melted once by the shock metamorphism, and ferroan–periclase crystallized from the olivine melt. We expect that bridgmanite crystallized from the residual melt and inverted to clinopyroxene through the subsequent high-temperature event, although we cannot rule out the possibility that the ferroan–periclase and clinopyroxene assemblage is the partial melting product of olivine. Parts of the melted olivine in the melt inclusion are mixed with the glass (having low-Ca pyroxene + a small amount of silica components) filling the matrix portion of the melt inclusion. Then, olivine, clinopyroxene, and tridymite crystallized from the mixed melts. We infer that the olivine–clinopyroxene–tridymite assemblage is the crystallization product from the mixed melts during rapid cooling at low- or ambient-pressure conditions. Olivine grains adjacent to the melt inclusion dissociated to clinopyroxene + ferroan–periclase assemblage. As the distance from the melt inclusion increased, polycrystalline olivine crystal assemblage formed. We expect that the polycrystalline olivine assemblage is the remnant of ringwoodite: metastable ringwoodite formed in the olivine grain by the shock metamorphism, and back transformed into olivine again through high temperature and low pressure conditions. Mineral assemblages in and adjacent to the melt inclusion show evidence for high pressure and high temperature conditions and for low pressure and high temperature conditions. The pressure–temperature–time path recorded in NWA 2737 is not simple, i.e., the ejection path from the impact site is complex.