Shock Melting Research Articles

Nanoscale View of Shock Wave Propagation in Single Crystal Fe, W, and Ta for Nuclear Fusion Technology

We report non-equilibrium Molecular Dynamics simulations providing a nanoscale view for the modeling of shock wave generation, propagation and melting in single crystalline materials Fe, Ta, W, of clear interest for Nuclear Fusion Technology. Our methodology successfully uses massive parallel molecular dynamics in an attempt to cover similar times and length scales as laser-shock experiments. Response of the materials are analyzed in terms of modern atomistic visualization and evolution of their structural properties. Preliminary results point that Wand Ta behave more efficiently in terms of uniformity under shock propagation than lighter materials like Fe. This kind of materials must attract our attention in the short term as possible designs in inertial confinement fusion (ICF) targets.

Shock metamorphism investigations of quartz grains in clasts from impact breccia of the Eyreville B drill core, Chesapeake Bay impact structure, USA

Abstract– The Chesapeake Bay impact structure, approximately 85 km in diameter, has been drilled in 2005–2006 at Eyreville (Virginia, USA), to a total depth of 1766 m. In the drill cores, the abundance of shock metamorphosed material is very variable with depth. Shocked mineral and lithic clasts, as well as melt particles, are most abundant in suevitic impact breccia section (1397–1451 m depth). Shocked quartz (i.e., quartz grains with planar fractures and/or planar deformation features) and melt particles, although rare, are also dispersed in the Exmore Formation unit (444–867 m depth). Other lithologies in the Eyreville drill cores show no clear evidence of shock metamorphism. Here, we report on the investigations of 40 samples from the impact breccia section. A total of more than 27,000 quartz grains were examined in about 200 clasts. The abundance of highly shocked clasts tends to decrease with increasing depth. Crystalline clasts derived from the crystalline basement are commonly only slightly shocked (contain generally <10 rel% of shocked quartz grains). The clasts of metamorphosed sediments show a low proportion of shocked quartz grains (mostly <10 rel%). Sedimentary clasts show a wide range of proportions of shocked quartz grains, with several of them being highly shocked clasts (most values between 0 and 40 rel%). Conglomerates show the highest proportion of shocked quartz grains of all types of clasts (up to 83 rel%). Polycrystalline quartz clasts are also commonly highly shocked (contain mostly between 10 and 40 rel% of shocked quartz grains). These hard nonporous clasts are possibly more liable to show evidence of shock. The investigations suggest that the intensity of shock metamorphism is the result of several parameters, such as original position in the target (both horizontal and vertical) and the properties of each lithology (e.g., grain size, porosity, and amount of matrix). According to the universal‐stage investigations, the dominant orientations of planar deformation features in quartz are , , and also .

Natural dissociation of olivine to (Mg,Fe)SiO 3 perovskite and magnesiowüstite in a shocked Martian meteorite

We report evidence for the natural dissociation of olivine in a shergottite at high-pressure and high-temperature conditions induced by a dynamic event on Mars. Olivine (Fa(34-41)) adjacent to or entrained in the shock melt vein and melt pockets of Martian meteorite olivine-phyric shergottite Dar al Gani 735 dissociated into (Mg,Fe)SiO(3) perovskite (Pv)+magnesiowüstite (Mw), whereby perovskite partially vitrified during decompression. Transmission electron microscopy observations reveal that microtexture of olivine dissociation products evolves from lamellar to equigranular with increasing temperature at the same pressure condition. This is in accord with the observations of synthetic samples recovered from high-pressure and high-temperature experiments. Equigranular (Mg,Fe)SiO(3) Pv and Mw have 50-100 nm in diameter, and lamellar (Mg,Fe)SiO(3) Pv and Mw have approximately 20 and approximately 10 nm in thickness, respectively. Partitioning coefficient, K(Pv/Mw) = [FeO/MgO]/[FeO/MgO](Mw), between (Mg,Fe)SiO(3) Pv and Mw in equigranular and lamellar textures are approximately 0.15 and approximately 0.78, respectively. The dissociation of olivine implies that the pressure and temperature conditions recorded in the shock melt vein and melt pockets during the dynamic event were approximately 25 GPa but 700 °C at least.

Formation of pseudotachylitic breccias in the central uplifts of very large impact structures: Scaling the melt formation

Abstract– The processes leading to formation of sometimes massive occurrences of pseudotachylitic breccia (PTB) in impact structures have been strongly debated for decades. Variably an origin of these pseudotachylite (friction melt)‐like breccias by (1) shearing (friction melting); (2) so‐called shock compression melting (with or without a shear component) immediately after shock propagation through the target; (3) decompression melting related to rapid uplift of crustal material due to central uplift formation; (4) combinations of these processes; or (5) intrusion of allochthonous impact melt from a coherent melt body has been advocated. Our investigations of these enigmatic breccias involve detailed multidisciplinary analysis of millimeter‐ to meter‐sized occurrences from the type location, the Vredefort Dome. This complex Archean to early Proterozoic terrane constitutes the central uplift of the originally >250 km diameter Vredefort impact structure in South Africa. Previously, results of microstructural and microchemical investigations have indicated that formation of very small veinlets involved local melting, likely during the early shock compression phase. However, for larger veins and networks it was so far not possible to isolate a specific melt‐forming mechanism. Macroscopic to microscopic evidence for friction melting is very limited, and so far chemical results have not directly supported PTB generation by intrusion of impact melt. On the other hand, evidence for filling of dilational sites with melt is abundant. Herein, we present a new approach to the mysterium of PTB formation based on volumetric melt breccia calculations. The foundation for this is the detailed analysis of a 1.5 × 3 × 0.04 m polished granite slab from a dimension‐stone quarry in the core of the Vredefort Dome. This slab contains a 37.5 dm3 breccia zone. The pure melt volume in 0.1 m3 PTB‐bearing granitic target rock outside of the several‐decimeter‐wide breccia zone in the granite slab was estimated at 5.2 dm3. This amount can be divided into 4.6 dm3 melt (88%), for which we have evidenced a limited material transport (at maximum, ≈20 cm) and 0.6 dm3 melt (12%) with, at most, grain‐scale material transport, which we consider in situ formed shock melt. The breccia zone itself contains about 10 dm3 of matrix (melt). Assuming melt exchange over 20 cm at the slab surface, between breccia zone and surrounding melt‐bearing host rock volume, the outer melt volume is calculated to contain the same amount of melt as contained by the massive breccia zone. Meso‐ and microscopic observations indicate melt transport is more prominent from larger into smaller melt occurrences. Thus, melt of the breccia zone could have provided the melt fill for all the small‐scale PTB veins in the surrounding target rock. Extrapolating this melt capacity calculation for 1 m3 PTB‐bearing host rock shows that a host rock volume of this dimension is able to take up some 52 dm3 melt. Scaling up 1000‐fold to the outcrop scale reveals that exchange between a host rock volume of 2 m radius around a 37 m3 breccia zone could involve some 10 m3 melt. These results demonstrate that large melt volumes (i.e., large breccia zones) can be derived, in principle, from local reservoirs. However, strong decompression would have to apply in order to exchange these considerable melt volumes, which would only be realistic during the decompression phase of impact cratering upon central uplift formation, or locally where compressive regimes acted during the subsequent down‐ and outward collapse of the central uplift.

The distinct morphological and petrological features of shock melt veins in the Suizhou L6 chondrite

Abstract– The morphology and petrology of distinct melt veins in the Suizhou L6 chondrite have been investigated using scanning electron microscopy, electron microprobe analyses, and Raman spectroscopy, synchrotron energy‐dispersive diffraction, and transmission electron microscopy. It is found that the melt veins in the Suizhou meteorite morphologically are the simplest, straightest, and thinnest among all shock veins known from meteorites. At first glance, these veins look like fine fractures, but petrologically they are solid melt veins of chondritic composition and consist of fully crystalline materials of two distinct lithological assemblages, with no glassy material remaining. The Suizhou melt veins contain the most abundant high‐pressure mineral species when compared with all other veins known in chondrites. Thus, these veins in Suizhou are classified as shock veins. All rock‐forming and almost all accessory minerals in the Suizhou shock veins have been transformed to their high‐pressure polymorphs, and no fragments of the precursor minerals remain in the veins. Among the 11 high‐pressure mineral phases identified in the Suizhou veins, three are new high‐pressure minerals, namely, tuite after whitlockite, xieite, and the CF phase after chromite. On the basis of transformation of plagioclase into maskelynite, it is estimated that the Suizhou meteorite experienced shock pressures and shock temperatures up to 22 GPa and 1000 °C, respectively. Shearing and friction along shock veins raised the temperature up to 1900–2000 °C and the pressure up to 24 GPa within the veins. Hence, phase transition and crystallization of high‐pressure minerals took place only in the Suizhou shock veins. Fast cooling of the extremely thin shock veins is regarded as the main reason that up to 11 shock‐induced high‐pressure mineral phases could be preserved in these veins.

New zircon shock phenomena and their use for dating and reconstruction of large impact structures revealed by electron nanobeam (EBSD, CL, EDS) and isotopic U–Pb and (U–Th)/He analysis of the Vredefort domeThis article is one of a series of papers published in this Special Issue on the theme of Geochronology in honour of Tom Krogh.

Integrated electron nanobeam (EBSD, CL, EDS) and isotopic measurements (U–Pb, (U–Th)/He) of zircon from the collar and centre of the 80 km wide central uplift of the 2020 ± 3 Ma Vredefort impact structure reveal new shock features in a microstructural progression related to impact basin formation and degree of U–Pb age resetting: (1) planar fractures in {1K0} and {1K2} orientation during initial shock wave compression; (2) curviplanar fractures in {1K1} orientation, now annealed, which host glassy inclusions of partial melt of the host rock; (3) microtwin lamellae in an orientation of 65° about [110], attributed to shock wave rarefaction; (4) nucleation of impact-age crystallites, possibly on microtwins, during post-shock heating by impact melt; and (5) crystal-plastic deformation linked to crater modification of the core of the central uplift. Planar fracturing and microtwinning ≥20 GPa in “cold shock” zircon in granitoid at a radial distance of 25 km failed to reset zircon age. Single-grain ID–TIMS data extend between pre-impact age of 2077 ± 11 Ma and a secondary Pb-loss event at ca. 1.0 Ga — the latter reflecting Kibaran igneous activity between 1.110 and 1.021 Ga. Age resetting by the impact event operated in an ∼15 km wide “hot shock” zone of impact-elevated temperatures ≥700 °C at the core of the central uplift. Mechanisms include internal recrystallization, defect-accelerated Pb diffusion via shock microstructures and melt films, and late crystal-plastic deformation. Igneous zircons from a 2019 ± 2 Ma foliated norite impact melt yield a mean (U–Th)/He date of 923 ± 61 Ma, indicating exposure of the present surface after this time.

Sound velocity and shock melting of low porosity aluminum

Shock loading-release is performed on the porous aluminum with micropore and porosity m=1.04. Time-resolved interfacial velocity between the porous aluminum and LiF window is measured with diatance interferometer system for any reflector (DISAR) under five pressures ranging from 53 GPa to 99 GPa . From the interfacial velocity, the Euler longitudinal sound velocities under five pressures and the bulk sound velocities under 53 GPa, 72 GPa and 91 GPa are obtained. The melting pressure of the material is about 81 GPa. The poisson ratio before shock melting is about 0.37. From the analysis, the existence of the micropore in the material reduces the the shock melting obviously. So, its induced anharmonic effect cannot be neglected. Considering anharmonic effect into the equation of state, the anharmonic parameter is calculated to be about 30.

Jiddat al Harasis 422: A ureilite with an extremely high degree of shock melting

Abstract–The Jiddat al Harasis (JaH) 422 ureilite was found in the Sultanate of Oman; it is classified as a ureilitic impact melt breccia. The meteorite consists of rounded polycrystalline olivine clasts (35%), pores (8%), and microcrystalline matrix (57%). Clasts and matrix have oxygen isotopic values and chemical compositions (major and trace elements) characteristic of the ureilite group. The matrix contains olivine (Fo83–90), low‐Ca pyroxene (En84–92Wo0–5), augite (En71–56Wo20–31), graphite, diamond, Fe‐metal, sulfides, chromite, and felsic glass. Pores are partly filled by secondary Fe‐oxihydroxide and desert alteration products. Pores are surrounded by strongly reduced silicates. Clasts consist of fine‐grained aggregates of polygonal olivine. These clasts have an approximately 250 μm wide reaction rim, in which olivine composition evolves progressively from the core composition (Fo79–81) to the matrix composition (Fo84–87). Veins crossing the clasts comprise pyroxene, Fe‐oxihydroxide, C‐phases, and chromite. Clasts contain Ca‐, Al‐, and Cr‐rich glass along olivine grain boundaries (<1 μm wide). We suggest that a significant portion of JaH 422, including olivine and all the pyroxenes, was molten as a result of an impact. In comparison with other impact‐melted ureilites, JaH 422 shows the highest melt portion. Based on textural and compositional considerations, clasts and matrix probably originated from the same protolith, with the clasts representing relict olivine that survived, but was recrystallized in the impact melt. During the melt stage, the high availability of FeO and elevated temperatures controlled oxygen fugacity at values high enough to stabilize olivine with Fo∼83–87 and chromite. Along pores, high Mg# compositions of silicates indicate that in a late stage or after melt crystallization FeO became less available and fO2 conditions were controlled by C−CO + CO2.

Coesite and stishovite in a shocked lunar meteorite, Asuka-881757, and impact events in lunar surface

Microcrystals of coesite and stishovite were discovered as inclusions in amorphous silica grains in shocked melt pockets of a lunar meteorite Asuka-881757 by micro-Raman spectrometry, scanning electron microscopy, electron back-scatter diffraction, and transmission electron microscopy. These high-pressure polymorphs of SiO(2) in amorphous silica indicate that the meteorite experienced an equilibrium shock-pressure of at least 8-30 GPa. Secondary quartz grains are also observed in separate amorphous silica grains in the meteorite. The estimated age reported by the (39)Ar/(40)Ar chronology indicates that the source basalt of this meteorite was impacted at 3,800 Ma ago, time of lunar cataclysm; i.e., the heavy bombardment in the lunar surface. Observation of coesite and stishovite formed in the lunar breccias suggests that high-pressure impact metamorphism and formation of high-pressure minerals are common phenomena in brecciated lunar surface altered by the heavy meteoritic bombardment.

Petrology and mineralogy of the shock-melted H chondrites Yamato–791088 and LaPaz Ice Field 02240

We studied the petrology and mineralogy of two types of shock-melted H chondrites: Yamato (Y)–791088 and LaPaz Ice Field (LAP) 02240. Y–791088, which consists of numerous coarse-grained relict phases (40%) and euhedral fine-grained minerals solidified from the shock melt (60%), experienced incomplete melting; a quiescent melt is indicated by the existence of abundant relict phases, pseudomorphed chondrules, and two types of glass. LAP 02240, which consists of small amounts of coarse-grained relict phases (∼10%) and fine-grained minerals (∼90%), experienced near-complete melting; a rapidly cooled mobilized melt is indicated by the homogeneous compositions of glass and opaque veins. The homogeneous compositions of relict olivines indicate that the precursors of both chondrites were equilibrated H chondrites. The melting features of Y–791088 and LAP 02240 are very similar to those of Y–790964 (LL) and the fine-grained lithology of Y–790519 (LL), respectively. These two types of shock-melted ordinary chondrites possibly formed in situ during dike formation. The quiescent melt is thought to have originated from the injection of shock-heated chondrite blocks into mobilized melt. These two types of melting could have occurred during dike formation on the H chondrite parent body. The textures of the two types of shock melts were not simply affected by the degree of shock melting: they were also controlled by the degree of shear stress.

Melting of Sn at high pressure: Comparisons with Pb

Measurements for Sn, made using the laser-heated diamond cell, are reported that extend the melting curve to 68 GPa and 2300 K. Initially the melting temperature of Sn increases linearly with increasing pressure (dT/dP approximately 40 K/GPa) and near 38 GPa (2200 K) the melting curve flattens (dT/dP approximately 0), indicating a zero volume phase change at melting. The results are in good agreement with previously reported shock melting studies. In comparison to Sn the melting curve of Pb is relatively linear to 100 GPa, the highest pressure at which measurements have been made.

Fluid+melt intrusions in lechatelierite from the Popigai suevites: A product of dynamic interaction melts and fluids during the shock melting of the target gneiss

The paper presents data on lechatelierite form suevites of the Daldyn Formation in the Popigai astrobleme. Some of the lechatelierite samples show a complicated structure and contain block of diaplectic quartz glass and dynamic “intrusions” of glasses of types I, II, and III. The glasses of types I and II abound in fluid inclusions and display evidence of partial homogenization with lechatelierite. The glasses of type III are clearly separated from all other glasses but show evidence of dynamic interaction with them in the molten state. Fluid inclusions in the glasses of types I and II are syngenetic but have notably different densities from those of completely liquid or gaseous inclusions at 20°C. As is indicated by cryometric data, the liquid phase of the inclusions is aqueous solution of low salinity (5–8 wt % NaClequiv). The bulk petrochemistry of the glasses of type I characterizes them as highly silicic (96.04 wt % SiO2 on average), with elevated K and Na concentrations (Na2O + K2O = 0.72 wt % on average), with 0.73 wt % Al2O3 (on average) and analytical totals 1.97 wt % less than 100%. The glasses of type II are also rich in SiO2 (91.51 wt % SiO2 on average) but contain a broader spectrum of concentrations of major oxides (totaling 5.53 wt % on average) and deficient analytical totals (by 2.96 wt % on average). The glasses of type III are completely equal to impactites produced by melting gneisses of the Popigai astrobleme. The glasses of type I are interpreted to be the intrusion products of the “early” highly mobile and H2O-rich fluid+melt mixtures, whose protolithic material was K-Na feldspars of the target rocks. The derivation of these melts was associated with the capturing of much silica and water at a highly mobile behavior of K and Na and an inert behavior of Al. The glasses of type II were produced by the extensive mixing of silica and water at the limited involvement of apogneiss melts, and these glasses are sometimes deficient in Al. The glasses of type III are usual mixed apogneiss melts. Excess silica in the glasses of types I and II and their richness in water and deficiency in Al suggest impact anatexis and the selective separation of components during their derivation; the parental fluid-melt mixtures of these glasses were derived from such “hydrous” varieties of the target gneisses as diaphthorized and fractured rocks. The evolution and partial vitrification of lechatelierite and the glasses of types I and II proceeded under residual shock pressures, as follows from data on the dense (from ∼0.5 to 1 g/cm3) aqueous inclusions in these glasses, which suggest that the inclusions were captured in the glasses under pressures from ∼0.8 to 3.3 GPa. It follows that our lechatelierite samples have a complex multistage genesis, and their quenching facilitated the preservation of “intrusions” of various stages of shock melting, including the products of the “early” impact anatexis of the gneisses with the selective separation of components at the active participation of water.

Martian regolith in Elephant Moraine 79001 shock melts? Evidence from major element composition and sulfur speciation

Shock veins and melt pockets in Lithology A of Martian meteorite Elephant Moraine (EETA) 79001 have been investigated using electron microprobe (EM) analysis, petrography and X-ray Absorption Near Edge Structure (XANES) spectroscopy to determine elemental abundances and sulfur speciation (S 2− versus S 6+). The results constrain the materials that melted to form the shock glasses and identify the source of their high sulfur abundances. The XANES spectra for EETA79001 glasses show a sharp peak at 2.471 keV characteristic of crystalline sulfides and a broad peak centered at 2.477 keV similar to that obtained for sulfide-saturated glass standards analyzed in this study. Sulfate peaks at 2.482 keV were not observed. Bulk compositions of EETA79001 shock melts were estimated by averaging defocused EM analyses. Vein and melt pocket glasses are enriched in Al, Ca, Na and S, and depleted in Fe, Mg and Cr compared to the whole rock. Petrographic observations show preferential melting and mobilization of plagioclase and pyrrhotite associated with melt pocket and vein margins, contributing to the enrichments. Estimates of shock melt bulk compositions obtained from glass analyses are biased towards Fe- and Mg- depletions because, in general, basaltic melts produced from groundmass minerals (plagioclase and clinopyroxene) will quench to a glass, whereas ultramafic melts produced from olivine and low-Ca pyroxene megacrysts crystallize during the quench. We also note that the bulk composition of the shock melt pocket cannot be determined from the average composition of the glass but must also include the crystals that grew from the melt – pyroxene (En 72–75Fs 20–21Wo 5–7) and olivine (Fo 75–80). Reconstruction of glass + crystal analyses gives a bulk composition for the melt pocket that approaches that of lithology A of the meteorite, reflecting bulk melting of everything except xenolith chromite. Our results show that EETA79001 shock veins and melt pockets represent local mineral melts formed by shock impedance contrasts, which can account for the observed compositional anomalies compared to the whole rock sample. The observation that melts produced during shock commonly deviate from the bulk composition of the host rock has been well documented from chondrites, rocks from terrestrial impact structures and other Martian meteorites. The bulk composition of shock melts reflects the proportions of minerals melted; large melt pockets encompass more minerals and approach the whole rock whereas small melt pockets and thin veins reflect local mineralogy. In the latter, the modal abundance of sulfide globules may reach up to 15 vol%. We conclude the shock melt pockets in EETA79001 lithology A contain no significant proportion of Martian regolith.

Crystallization, melt inclusion, and redox history of a Martian meteorite: Olivine-phyric shergottite Larkman Nunatak 06319

The Larkman Nunatak (LAR) 06319 olivine-phyric shergottite is composed of zoned megacrysts of olivine (Fo 76–55 from core to rim), pyroxene (from core to rim En 70Fs 25Wo 5, En 50Fs 25Wo 25, and En 45Fs 45Wo 10), and Cr-rich spinel in a matrix of maskelynite (An 52Ab 45), pyroxene (En 30–40Fs 40–55Wo 10–25,), olivine (Fo 50), Fe–Ti oxides, sulfides, phosphates, Si-rich glass, and baddeleyite. LAR 06319 experienced equilibration shock pressures of 30–35 GPa based on the presence of localized shock melts, mechanical deformation of olivine and pyroxene, and complete transformation of plagioclase to maskelynite with no relict birefringence. The various phases and textures of this picritic basalt can be explained by closed system differentiation of a shergottitic melt. Recalculated parent melt compositions obtained from melt inclusions located in the core of the olivine megacrysts (Fo >72) resemble those of other shergottite parent melts and whole-rock compositions, albeit with a lower Ca content. These compositions were used in the MELTS software to reproduce the crystallization sequence. Four types of spinel and two types of ilmenite reflect changes in oxygen fugacity during igneous differentiation. Detailed oxybarometry using olivine-pyroxene-spinel and ilmenite-titanomagnetite assemblages indicates initial crystallization of the megacrysts at 2 log units below the Fayalite-Magnetite-Quartz buffer (FMQ – 2), followed by crystallization of the groundmass over a range of FMQ – 1 to FMQ + 0.3. Variation is nearly continuous throughout the differentiation sequence. LAR 06319 is the first member of the enriched shergottite subgroup whose bulk composition, and that of melt inclusions in its most primitive olivines, approximates that of the parental melt. The study of this picritic basalt indicates that oxidation of more than two log units of FMQ can occur during magmatic fractional crystallization and ascent. Some part of the wide range of oxygen fugacities recorded in shergottites may consequently be due to this process. The relatively reduced conditions at the beginning of the crystallization sequence of LAR 06319 may imply that the enriched shergottite mantle reservoir is slightly more reduced than previously thought. As a result, the total range of Martian mantle oxygen fugacities is probably limited to FMQ − 4 to − 2. This narrow range could have been generated during the slow crystallization of a magma ocean, a process favored to explain the origin of shergottite mantle reservoirs.

Shock and thermal history of Northwest Africa 4859, an annealed impact‐melt breccia of LL chondrite parentage containing unusual igneous features and pentlandite

Abstract– Northwest Africa 4859 (NWA 4859) is a meteorite of LL chondrite parentage that shows unusual igneous features and contains widely distributed pentlandite. The most obvious unusual feature is a high proportion of large (≤3 cm diameter) igneous‐textured enclaves (LITEs), interpreted as shock melts that were intruded into an LL chondrite host. One such LITE appears to have been produced by whole rock melting of LL chondrite, initial rapid partial crystallization, and subsequent slow cooling of the residual melt in the host to produce a differentiated object. Other unusual features include mm‐sized “overgrowth objects,” fine‐grained plagioclase‐rich bands, and coarse troilite (≤7 mm across) grains. All these features are interpreted as having crystallized from melts produced by a single transient shock event, followed by slow cooling. A subsequent shock event of moderate (S3) intensity produced veining and transformed some of the pyroxene into the clinoenstatite polytype. Pentlandite (together with associated troilite) in NWA 4859 probably formed by the breakdown of a monosulfide precursor phase at low temperature (≤230 °C) following the second shock event. NWA 4859 is interpreted to be an unusual impact‐melt breccia that contains shock melt which crystallized in different forms at depth within the parent body.

Geochemical and microstructural evidence for in situ formation of pseudotachylitic Sudbury breccia by shock-induced compression and cataclasis

Pseudotachylitic breccias occur in the crater floor of large impact structures. Their origin is widely attributed either to shock melting and cataclasis during propagation of an impact shock wave, or to frictional melting and cataclasis during large slip displacements along crater collapse superfaults. By contrast, some models propose that the breccia matrices are allochthonous superheated melts from overlying impact melt sheets or from sites within the crater floor, which leaked into dilational fractures during collapse of the crater. In an effort to test these models, the compositions of pseudotachylite Sudbury breccias in the South Range of the Sudbury impact structure, Canada, were here compared to those of its associated impact melt sheet, the Sudbury Igneous Complex (SIC). The studied breccias occur along the contact between sandstone and a gabbro stock, and additional breccia bodies are located solely within these two lithologies. Major, trace element and lead isotope systematics suggest that the contact breccias are derived by mixing of comminuted gabbro and sandstone and that breccias hosted exclusively by sandstone or gabbro have compositions similar to their host rocks. The SIC has distinct chemical and isotopic compositions which argue strongly against the involvement of the impact melt sheet in the makeup of the breccias. Quartz and plagioclase in clasts and matrix of the breccias are transected by multiple microfractures and they have pervasive mosaic textures characterized by rolling and mottled undulatory extinction of micron-sized subgrain-like domains and new grains. These textures and strong local host control on chemical and clast composition of the breccias suggest that the breccias formed in situ by shock-induced compression and cataclasis of their host rocks during propagation of the shock wave.

Shock melt veins of Tenham chondrite as a possible paleomagnetic recorder: Rock magnetism and high-pressure minerals

Heavily shocked meteorites of shock stages S5 and S6 contain shock-induced melt veins (SMVs). SMVs might have reset the remanence of an asteroidal metamorphism at the time of giant collisions against a chondrite parent body. Here we present micropaleomagnetic and petrologic studies of SMVs in L6S5 Tenham chondrite with ∼500 μm thick black veins enclosing high-pressure minerals such as ringwoodite. Paleomagnetic data show that the high-temperature (HT, 200°C–650°C) and high-coercivity (HC, 20–100 mT) stable components of SMVs formed a cluster even from different portions of SMV, whereas the stable HT component of surrounding host rock showed a scattered orientation under stereonet projection. The host rock HC components form a girdle between the mixing of SMVs and unknown overprints, tracing the magnetic susceptibility foliation. Magnetic force microscopy and backscattered electron images confirmed kamacite and taenite assemblages in iron sulfides as remanence-carrying minerals in SMVs. Hysteresis data of SMVs revealed the presence of single-domain (SD) FeNi metals with Mrs/Ms = ∼0.1 and Hrc/Hc = ∼2, although these parameters are only applicable to magnetite. Because the metastable ringwoodite in SMVs transforms back to olivine at 188°C for 1000 Myr (metamorphism) or at 900°C for 1 h (postshock heating), the preservation of ringwoodite suggests that SMVs have not experienced either thermal condition. The magnetic time-temperature relation for SD FeNi metals suggested that 200°C unblocking temperature corresponds to the storage time of 100 years for kamacite and 4500 Myr for taenite at room temperature. The difference of HT components discards the possibility of postshock heating. Therefore, the SMV's remanence is a characteristic shock-induced thermal remanence that has newly been acquired during hypervelocity collision under a cryptic magnetic field.

High-pressure polymorphs of magnesian orthopyroxene from a shock vein in the Yamato-000047 lherzolitic shergottite

Abstract– We found a simple thin shock vein, less than or equal to about 60 μm in width and 1.8 mm in length, in the poikilitic area in the Yamato (Y-) 000047 lherzolitic shergottite. The shock vein occurs only in magnesian Ca-poor clinopyroxene, which may have transformed from orthopyroxene during the pressure increase at the shock event. The shock vein consists of (Mg0.8,Fe0.2)SiO3 pyroxene polymorphs, such as columnar akimotoite, two kinds of pyroxene glasses, dendritic akimotoite, and framboidal pyroxene glass, in the order from the periphery to the center. The compositions and textures suggest that columnar akimotoite in the periphery of the shock vein crystallized from solid-state phase transition of clinopyoroxene during the cooling of the vein, and the remains in the shock vein solidified from shock-produced melt. The glass includes two kinds of massive glass in the vein and framboidal glass in the vein center. The framboidal glass is the most magnesian and may have been vitrified from perovskite crystallized from high-pressure melt produced at high temperature ≥3000 °C and high-pressure 23–40 GPa. Dendritic akimotoites in the vein center metastably crystallized from residual shock melt. The formation sequences of the constituent phases in the shock vein happen in the following order: columnar akimotoites, rim glass, center glass, framboidal glass, and dendritic akimotoites. The increase of the Raman intensity of 660–670 cm−1 in the order of rim glass, center glass, and framboidal glass suggests that the formation of the pyroxene chain proceeds faster in the vein center than in the vein rim due to its slower cooling. The finding of the shock vein consisting merely of high-pressure polymorphs of pyroxene, akimotoite, and framboidal glass (vitrified perovskite) is the first reported among all Martian meteorites.

First-principles calculations of solid and liquid aluminum optical absorption spectra near the melting curve: Ambient and high-pressure results

We present ab initio calculations of the linear optical conductivity of heated Al at ambient pressure and at the conditions relevant for shock melting $(P\ensuremath{\sim}125\text{ }\text{GPa},\text{ }T\ensuremath{\sim}5000\text{ }\text{K})$. It is shown that the visible and near-UV optical spectrum is very sensitive to the phase (fcc solid versus liquid) of Al for both $P=0$ and 125 GPa. The ambient-$P$ results confirm an earlier prediction and the results of a recent experiment while the high-$(P,T)$ results allow us to conclude that in situ measurements of optical constants should be able to diagnose the shock melting of Al.

Transformation textures, mechanisms of formation of high‐pressure minerals in shock melt veins of L6 chondrites, and pressure‐temperature conditions of the shock events

Abstract— The high‐pressure polymorphs of olivine, pyroxene, and plagioclase in or adjacent to shock melt veins (SMVs) in two L6 chondrites (Sahara 98222 and Yamato 74445) were investigated to clarify the related transformation mechanisms and to estimate the pressure‐temperature conditions of the shock events. Wadsleyite and jadeite were identified in Sahara 98222. Wadsleyite, ringwoodite, majorite, akimotoite, jadeite, and lingunite (NaAlSi3O8‐hollandite) were identified in Yamato 74445. Wadsleyite nucleated along the grain boundaries and fractures of original olivine. The nucleation and growth of ringwoodite occurred along the grain boundaries of original olivine, and as intracrystalline ringwoodite lamellae within original olivine. The nucleation and growth of majorite took place along the grain boundaries or fractures in original enstatite. Jadeite‐containing assemblages have complicated textures containing “particle‐like,” “stringer‐like,” and “polycrystalline‐like” phases. Coexistence of lingunite and jadeite‐containing assemblages shows a vein‐like texture. We discuss these transformation mechanisms based on our textural observations and chemical composition analyses. The shock pressure and temperature conditions in the SMVs of these meteorites were also estimated based on the mineral assemblages in the SMVs and in comparison with static high‐pressure experimental results as follows: 13–16 GPa, >1900 °C for Sahara 98222 and 17–24 GPa, >2100 °C for Yamato 74445.