Shock Melting Research Articles

Petrogenesis of acapulcoites and lodranites: A shock-melting model

Acapulcoites are modeled as having formed by shock melting CR-like carbonaceous chondrite precursors; the degree of melting of some acapulcoites was low enough to allow the preservation of 3–6 vol % relict chondrules. Shock effects in acapulcoites include veins of metallic Fe–Ni and troilite, polycrystalline kamacite, fine-grained metal–troilite assemblages, metallic Cu, and irregularly shaped troilite grains within metallic Fe–Ni. While at elevated temperatures, acapulcoites experienced appreciable reduction. Because graphite is present in some acapulcoites and lodranites, it seems likely that carbon was the principal reducing agent. Reduction is responsible for the low contents of olivine Fa (4–14 mol %) and low-Ca pyroxene Fs (3–13 mol %) in the acapulcoites, the observation that, in more than two-thirds of the acapulcoites, the Fa value is lower than the Fs value (in contrast to the case for equilibrated ordinary chondrites), the low FeO/MnO ratios in acapulcoite olivine (16–18, compared to 32–38 in equilibrated H chondrites), the relatively high modal orthopyroxene/olivine ratios (e.g., 1.7 in Monument Draw compared to 0.74 in H chondrites), and reverse zoning in some mafic silicate grains. Lodranites formed in a similar manner to acapulcoites but suffered more extensive heating, loss of plagioclase, and loss of an Fe–Ni–S melt. Acapulcoites and lodranites experienced moderate post-shock annealing, presumably resulting from burial beneath material of low thermal diffusivity. The annealing process repaired damaged olivine crystal lattices, lending acapulcoites and lodranites the appearance of unshocked (i.e., shock-stage S1) rocks. Any high-pressure phases that may have formed during initial shock reverted to their low-pressure polymorphs during annealing. Some samples were subsequently shocked again; several acapulcoites reached shock-stage S2 levels, ALH 84190 reached S3, and the lodranite MAC 88177 reached S5.

Localized shock melting in lherzolitic shergottite Northwest Africa 1950: Comparison with Allan Hills 77005

Abstract— The lherzolitic Martian meteorite Northwest Africa (NWA) 1950 consists of two distinct zones: 1) low‐Ca pyroxene poikilically enclosing cumulate olivine (Fo70–75) and chromite, and 2) areas interstitial to the oikocrysts comprised of maskelynite, low‐ and high‐Ca pyroxene, cumulate olivine (Fo68–71) and chromite. Shock metamorphic effects, most likely associated with ejection from the Martian subsurface by large‐scale impact, include mechanical deformation of host rock olivine and pyroxene, transformation of plagioclase to maskelynite, and localized melting (pockets and veins). These shock effects indicate that NWA 1950 experienced an equilibration shock pressure of 35–45 GPa. Large (millimeter‐size) melt pockets have crystallized magnesian olivine (Fo78–87) and chromite, embedded in an Fe‐rich, Al‐poor basaltic to picro‐basaltic glass. Within the melt pockets strong thermal gradients (minimum 1 °C/μm) existed at the onset of crystallization, giving rise to a heterogeneous distribution of nucleation sites, resulting in gradational textures of olivine and chromite. Dendritic and skeletal olivine, crystallized in the melt pocket center, has a nucleation density (1.0 × 103 crystals/mm2) that is two orders of magnitude lower than olivine euhedra near the melt margin (1.6 × 105 crystals/mm2). Based on petrography and minor element abundances, melt pocket formation occurred by in situ melting of host rock constituents by shock, as opposed to melt injected into the lherzolitic target. Despite a common origin, NWA 1950 is shocked to a lesser extent compared to Allan Hills (ALH) 77005 (45–55 GPa). Assuming ejection in a single shock event by spallation, this places NWA 1950 near to ALH 77005, but at a shallower depth within the Martian subsurface. Extensive shock melt networks, the interconnectivity between melt pockets, and the ubiquitous presence of highly vesiculated plagioclase glass in ALH 77005 suggests that this meteorite may be transitional between discreet shock melting and bulk rock melting.

Evidence for a ∼ 200–100 ka meteorite impact in the Western Desert of Egypt

In this study, we present evidence, in the form of unusual silicate glasses, for a meteorite impact event ∼ 200–100 ka in the Dakhleh Oasis, Western Desert, Egypt. These glasses, known locally as Dakhleh Glass, were derived from the shock melting of a series of unconsolidated sediments underlain by interbedded carbonates, sandstones and phosphate-rich lithologies. Hypervelocity impact in to a volatile-rich target resulted in the production of impact glasses with CaO and Al 2O 3 contents of up to ∼ 25 and 18 wt.%, respectively. Other notable properties include the presence of globules of immiscible calcite and pyrrhotite melt phases, shattered quartz grains, and fragments of silicified plant matter. Dating of geoarchaeological artefacts associated with the Dakhleh Glass support preliminary 40Ar/ 39Ar data, indicating a ∼ 200–100 ka age for the impact event. Geoarchaeological evidence indicates that archaic Homo sapiens and early modern humans continually inhabited this region of the Western Desert during this period. While it is unclear at present whether the Dakhleh Glass was formed during a cratering event or a large aerial burst, the effect on the environment and inhabitants of Dakhleh would have been catastrophic.

Shock implantation of Martian atmospheric argon in four basaltic shergottites: A laser probe 40Ar/ 39Ar investigation

Spatially resolved argon isotope measurements have been performed on neutron-irradiated samples of two Martian basalts (Los Angeles and Zagami) and two Martian olivine-phyric basalts (Dar al Gani (DaG) 476 and North West Africa (NWA) 1068). With a ∼50 μm diameter focused infrared laser beam, it has been possible to distinguish between argon isotopic signatures from host rock (matrix) minerals and localized shock melt products (pockets and veins). The concentrations of argon in analyzed phases from all four meteorites have been quantified using the measured J values, 40Ar/ 39Ar ratios and K 2O wt% in each phase. Melt pockets contain, on average, 10 times more gas (7–24 ppb 40Ar) than shock veins and matrix minerals (0.3–3 ppb 40Ar). The 40Ar/ 36Ar ratio of the Martian atmosphere, estimated from melt pocket argon extractions corrected for cosmogenic 36Ar, is: Los Angeles (∼1852), Zagami (∼1744) and NWA 1068 (∼1403). In addition, Los Angeles shows evidence for variable mixing of two distinct trapped noble gas reservoirs: (1) Martian atmosphere in melt pockets, and (2) a trapped component, possibly Martian interior ( 40Ar/ 36Ar: 480–490) in matrix minerals. Average apparent 40Ar/ 39Ar ages determined for matrix minerals in the four analyzed meteorites are 1290 Ma (Los Angeles), 692 Ma (Zagami), 515 Ma (NWA 1068) and 1427 Ma (DaG 476). These 40Ar/ 39Ar apparent ages are substantially older than the ∼170–474 Ma radiometric ages given by other isotope dating techniques and reveal the presence of trapped 40Ar. Cosmic ray exposure (CRE) ages were measured using spallogenic 36Ar and 38Ar production. Los Angeles (3.1 ± 0.2 Ma), Zagami (2.9 ± 0.4 Ma) and NWA 1068 (2.0 ± 0.5 Ma) yielded ages within the range of previous determinations. DaG 476, however, yielded a young CRE age (0.7 ± 0.25 Ma), attributed to terrestrial alteration. The high spatial variation of argon indicates that the incorporation of Martian atmospheric argon into near-surface rocks is controlled by localized glass-bearing melts produced by shock processes. In particular, the larger (mm-size) melt pockets contain near end-member Martian atmospheric argon. Based on petrography, composition and argon isotopic data we conclude that the investigated melt pockets formed by localized in situ shock melting associated with ejection. Three processes may have led to atmosphere incorporation: (1) argon implantation due to atmospheric shock front collision with the Martian surface, (2) transformation of an atmosphere-filled cavity into a localized melt zone, and (3) shock implantation of atmosphere trapped in cracks, pores and fissures.

First-principles thermoelasticity of transition metals at high pressure: Tantalum prototype in the quasiharmonic limit

The thermoelastic properties of tantalum have been investigated over its theoretical high-pressure bcc solid phase (up to $26\phantom{\rule{0.2em}{0ex}}000\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ at $10\phantom{\rule{0.3em}{0ex}}\mathrm{Mbar}$) using an advanced first-principles approach that accurately accounts for cold, electron-thermal, and ion-thermal contributions in materials where anharmonic effects are small. Specifically, we have combined ab initio full-potential linear-muffin-tin-orbital electronic-structure calculations for the cold and electron-thermal contributions to the elastic moduli with phonon contributions for the ion-thermal part calculated using model generalized pseudopotential theory. For the latter, a summation of terms over the Brillouin zone is performed within the quasiharmonic approximation, where each term is composed of a strain derivative of the phonon frequency at a particular $\mathbf{k}$ point. At ambient pressure, the resulting temperature dependence of the Ta elastic moduli is in excellent agreement with ultrasonic measurements. The experimentally observed anomalous behavior of ${C}_{44}$ at low temperatures is shown to originate from the electron-thermal contribution. At higher temperatures, the main contribution to the temperature dependence of the elastic moduli comes from thermal expansion, but inclusion of the electron- and ion-thermal contributions is essential to obtain quantitative agreement with experiment. In addition, the pressure dependence of the moduli at ambient temperature compares well with recent diamond-anvil-cell measurements to $1.05\phantom{\rule{0.3em}{0ex}}\mathrm{Mbar}$. Moreover, the calculated longitudinal and bulk sound velocities in polycrystalline Ta at higher pressure and temperature in the vicinity of shock melting $(\ensuremath{\sim}3\phantom{\rule{0.3em}{0ex}}\mathrm{Mbar})$ agree well with data obtained from shock experiments. However, at high temperatures along the melt curve above $1\phantom{\rule{0.3em}{0ex}}\mathrm{Mbar}$, the ${B}^{\ensuremath{'}}$ shear modulus becomes negative, indicating the onset of unexpectedly strong anharmonic effects. Finally, the assumed temperature dependence of the Steinberg-Guinan strength model obtained from scaling with the bulk shear modulus is examined at ambient pressure.

Melting of bcc transition metals and icosahedral clustering

In contrast to polyvalent metals, transition metals have low melting slopes (dT/dP) that are due to partially filled d-bands that allow for a lowering of liquid phase energy through s–d electron transfer and the formation of local structures. In the case of bcc transition metals, we show that the apparent discrepancy of diamond-anvil cells (DAC) melting measurements with shock melting of Mo can be understood by reexamining the shock data for V and Ta and introducing the presence of an icosahedral short range order (ISRO) melt phase.

Impact melt sheet formation on Mars and its implication for hydrothermal systems and exobiology

Hydrothermal deposits are a prime exobiological research target on Mars and impact craters are potentially a significant source for these deposits. Key factors in the longevity of terrestrial impact crater hydrothermal systems are the volume of shock melt produced and the degree to which this melt is quenched by mixing with un-melted clasts. Mixing occurs when the shock melt is smeared along the crater floor, and the degree of mixing is inversely proportional to the thickness of this smeared layer. To investigate the potential development of thick melt sheets on Mars, we apply an analytical impact model to asteroid and long-period (LP) comet impacts that produces martian craters with final diameters of 30–640 km. We combine our model shock melt volumes with estimates of the surface area of the transient cavity to estimate initial shock melt thickness, which we use as a proxy measure of the amount of mixing between shock melt and un-melted clasts. We calculate this shock melt thickness parameter for a range terrestrial craters with and without thick melt sheets, from which we infer the melt sheet characteristics for the model Mars craters. The results demonstrate that most asteroid impacts on Mars do not produce thick, clast-free melt sheets. Only asteroid impact craters ⩾ 580 km in diameter on Mars produce shock melt thicknesses comparable to Earth's 200-km-diameter Sudbury crater, and martian craters ⩽ 65 km in diameter produce shock melt thicknesses comparable to terrestrial craters that lack melts sheets entirely. LP comet impacts on Mars are more likely to produce thick melt sheets. LP comet impact craters ⩾ 280 km in diameter on Mars produce shock melt thicknesses comparable to that of the Sudbury crater. Given that LP comet impacts on Mars are rare compared to asteroid impacts, we conclude that large, long-lived impact hydrothermal systems are likewise rare on Mars. Comparisons of the Mars asteroid impact rate with the potential longevity of impact hydrothermal systems indicate that such systems were active < 1 % of the time in the last 3.5 Ga, which raises questions about the exobiological importance of impact crater hydrothermal systems on Mars after the Noachian.

Iron at earth-core conditions

We have carried out electronic structure calculations for iron under high pressure using pseudopotential plane wave methods. There is a controversy regarding the structure of iron at moderate pressures (30-100 GPa) and temperatures (1000-2400 K), with different experiments suggesting different structures, such as orthorhombic, double hexagonal close packed (dhcp), etc. Our earlier calculations using the linear muffin-tin orbital method within atomic-sphere-approximation had argued against the stability of the orthorhombic phase. The more accurate calculations presented here predict qualitatively the same results. We have additionally studied the stability of various phases of Fe at different compressions by calculating phonon frequencies. These rule out the stability of the orthorhombic phase. To validate our zero-temperature electronic-structure results at finite temperature, we have compared the shock Hugoniot and melting properties of iron with the results of our electronic structure calculations. Though we have used simplified models for these estimates, our predictions compare well with the experimental data. We thus propose that these models can be used to obtain the information about the high-pressure melting curves of planetary materials.

Crystallization rates of shock melts in three martian basalts: Experimental simulation with implications for meteoroid dimensions

Dynamic crystallization experiments have been performed on synthetic glasses representative of shock-generated melts observed in Los Angeles, Sayh al Uhaymir 150 and Dar al Gani 476 martian basalts. On the basis of qualitative (texture) and quantitative (fractal analysis) results, we show that melt pockets in Los Angeles cooled at a rate of 1040–1560 °C/h. Sayh al Uhaymir 150 and Dar al Gani 476 melt pockets cooled at 780 °C/h. Conductive cooling models, for a range of meteoroid diameters (10–50 cm), indicate that the minimum meteoroid diameter was small, on the order of 10–15 cm and that melt pockets cooled from post-shock temperatures within minutes. Our results also have bearing on shock implanted martian atmospheric components because it is during cooling that the melt pockets have the potential to lose gases. Modeling of argon diffusion in a spherical melt pocket indicates that during cooling and quench crystallization ∼4–60% of trapped martian atmospheric argon may be lost from the melt pocket through diffusive transport.

The melt rocks of the Vredefort impact structure – Vredefort Granophyre and pseudotachylitic breccias: Implications for impact cratering and the evolution of the Witwatersrand Basin

The unique combination of its large size (250–300 km diameter), deep levels of erosion (>7 km), and widespread regional mining activity make the Vredefort impact structure in South Africa an exceptional laboratory for the study of impact-related deformation phenomena in the rocks beneath giant, complex impact craters. Two types of impact-generated melt rock occur in the Vredefort Structure: the Vredefort Granophyre – impact melt rock – and pseudotachylitic breccias. Along the margins of the structure, mining and exploration drilling in the Witwatersrand goldfields has revealed widespread fault-related pseudotachylitic breccias linked to the impact event. There, volumetrically limited melt breccia occurs in close association with cataclasite or mylonitic zones associated with bedding-parallel normal dip-slip faults that formed during inward slumping of the crater walls, and in rare subvertical faults oriented radially to the center of the structure. This association is consistent with formation of pseudotachylites by frictional melting. On the other hand, rocks in the Vredefort Dome – the central uplift of the impact structure – contain ubiquitous melt breccias that range in size from sub-millimeter pods and veinlets to dikes up to tens of meters wide and hundreds of meters long. Like fault-related pseudotachylites in the goldfields and elsewhere in the world, they display a close geochemical relationship to their wallrocks, indicating local derivation. However, although mm/cm- to, rarely, dm-scale offsets are commonly found along their margins, they do not appear to be associated with broader fault zones, are commonly considerably more voluminous than most known fault-related pseudotachylites, and show no consistent relationship between melt volumes and slip magnitude. Recent petrographic observations indicate that at least some of these melt breccias formed by shock melting, with or without frictional melting. Consequently, the non-genetic term “pseudotachylitic breccia” has been adopted for these Vredefort occurrences. These breccias formed during the impact in rocks at temperatures ranging from greenschist to granulite facies, and were subsequently annealed to varying degrees during cooling of the central uplift. In addition to the pseudotachylitic breccias, nine clast-laden impact melt dikes (Vredefort Granophyre), each up to several kilometers long, occur in vertical radial and tangential fractures in the Vredefort Dome. Unlike the pseudotachylitic breccias, they display a remarkably uniform bulk composition and clast populations that are largerly independent of their wallrocks, and they contain geochemical traces of the impactor. They represent intrusive offshoots of the homogenized impact melt body that originally lay within the crater. U–Pb single zircon and Ar–Ar dating indicates that the Vredefort Granophyre and pseudotachylitic breccias, and the Witwatersrand pseudotachylites all formed at 2020±5 Ma – the age of the impact event, making the breccias a convenient time marker in the evolution of the structurally complex Witwatersrand basin with its unique gold deposits.

Shock melts in QUE 94411, Hammadah al Hamra 237, and Bencubbin: Remains of the missing matrix?

Abstract— We have studied the CB carbonaceous chondrites Queen Alexandra Range (QUE) 94411, Hammadah al Hamra (HH) 237, and Bencubbin with an emphasis on the petrographical and mineralogical effects of the shock processing that these meteorite assemblages have undergone. Iron‐nickel metal and chondrule silicates are the main components in these meteorites. These high‐temperature components are held together by shock melts consisting of droplets of dendritically intergrown Fe,Ni‐metal/sulfide embedded in silicate glass, which is substantially more FeO‐rich (30–40 wt%) than the chondrule silicates (FeO &lt;5 wt%). Fine‐grained matrix material, which is a major component in most other chondrite classes, is extremely scarce in QUE 94411 and HH 237, and has not been observed in Bencubbin. This material occurs as rare, hydrated matrix lumps with major and minor element abundances roughly similar to the ferrous silicate shock melts (and CI). We infer that hydrated, fine‐grained material, compositionally similar to these matrix lumps, was originally present between the Fe,Ni‐metal grains and chondrules, but was preferentially shock melted. Other shock‐related features in QUE 94411, HH 237, and Bencubbin include an alignment and occasionally strong plastic deformation of metal and chondrule fragments.The existence of chemically zoned and metastable Fe,Ni‐metal condensates in direct contact with shock melts indicates that the shock did not substantially increase the average temperature of the rock. Because porphyritic olivine‐pyroxene chondrules are absent in QUE 94411, HH 237, and Bencubbin, it is difficult to determine the precise shock stage of these meteorites, but the shock was probably relatively light (S2–S3), consistent with a bulk temperature increase of the assemblages of less than ˜300 °C. The apparently similar shock processing of Bencubbin, Weatherford, Gujba (CBa) and QUE 94411/HH 237 (CBb) supports the idea of a common asteroidal parent body for these meteorites.

“New” lunar meteorites: Impact melt and regolith breccias and large-scale heterogeneities of the upper lunar crust

Abstract— We have analyzed nine highland lunar meteorites (lunaites) using mainly INAA. Several of these rocks are difficult to classify. Dhofar 081 is basically a fragmental breccia, but much of its groundmass features a glassy-fluidized texture that is indicative of localized shock melting. Also, much of the matrix glass is swirly-brown, suggesting a possible regolith derivation. We interpret Dar al Gani (DaG) 400 as an extremely immature regolith breccia consisting mainly of impact-melt breccia clasts; we interpret Dhofar 026 as an unusually complex anorthositic impact-melt breccia with scattered ovoid globules that formed as clasts of mafic, subophitic impact melt. The presence of mafic crystalline globules in a lunar material, even one so clearly impact-heated, suggests that it may have originated as a regolith. Our new data and a synthesis of literature data suggest a contrast in Al2O3-incompatible element systematics between impact melts from the central nearside highlands, where Apollo sampling occurred, and those from the general highland surface of the Moon. Impact melts from the general highland surface tend to have systematically lower incompatible element concentration at any given Al2O3 concentration than those from Apollo 16. In the case of Dhofar 026, both the bulk rock and a comparatively Al-poor composition (14 wt% Al2O3, 7 μg/g Sm) extrapolated for the globules, manifest incompatible element contents well below the Apollo 16 trend. Impact melts from Luna 20 (57°E) distribute more along the general highland trend than along the Apollo 16 trend. Siderophile elements also show a distinctive composition for Apollo 16 impact melts: Ni/Ir averaging ˜1.8x chondritic. In contrast, lunaite impact-melt breccias have consistently chondritic Ni/Ir. Impact melts from Luna 20 and other Apollo sites show average Ni/Ir almost as high as those from Apollo 16. The prevalence of this distinctive Ni/Ir ratio at such widely separated nearside sites suggests that debris from one extraordinarily large impact may dominate the megaregolith siderophile component of a nearside region 2300 km or more across. Highland polymict breccia lunaites and other KREEP-poor highland regolith samples manifest a strong anticorrelation between Al2O3 and mg. The magnesian component probably represents the chemical signature of the Mg-suite of pristine nonmare rocks in its most “pure” form, unaltered by the major KREEP-assimilation that is so common among Apollo Mg-suite samples. The average composition of the ferroan anorthositic component is now well constrained at Al2O3 ˜29–30 wt% (implying about 17–19 wt% modal mafic silicates), in good agreement with the composition predicted for flotation crust over a “ferroan” magma ocean (Warren 1990).

Synchrotron radiation X-Ray diffraction in situ study of fine-grained minerals in shock veins of Suizhou meteorite

The synchrotron radiation X-ray diffraction technique developed for in situ study at high pressure and temperature has also been used to investigate microscopic mineral inclusions in ultra-high-pressure metamorphic rocks and deep-mantle samples. Present study added two more examples of successful utilization of synchrotron radiation X-ray diffraction technique for in situ investigations of fine-grained (0.5―30 μm in size) minerals in very thin shock melt veins of the Suizhou meteorite: (i) X-ray diffraction measurement of extremely small-sized vein matrix minerals, and (ii) identification of the micron-sized new mineral tuite embedded in the vein matrix. It has been revealed that the fine-grained vein matrix consists of well crystallized garnet, kama-cite and troilite, and the powder diffraction pattern consisting of 17 lines with d-values, intensities ( I ), relative intensities ( I / I o ) and Miller indices, as well as the cell parameters for the new mineral tuite has also been successfully obtained. The result of present investigations has enriched the content of dynamic high-pressure mineralogy and that of Earth’s mantle geochemistry.

Dynamic response of materials on subnanosecond time scales, and beryllium properties for inertial confinement fusion

During the past few years, substantial progress has been made in developing experimental techniques capable of investigating the response of materials to dynamic loading on nanosecond time scales and shorter, with multiple diagnostics probing different aspects of the behavior. These relatively short time scales are scientifically interesting because plastic flow and phase changes in common materials with simple crystal structures—such as iron—may be suppressed, allowing unusual states to be induced and the dynamics of plasticity and polymorphism to be explored. Loading by laser-induced ablation can be particularly convenient: this technique has been used to impart shocks and isentropic compression waves from ∼1to200GPa in a range of elements and alloys, with diagnostics including line imaging surface velocimetry, surface displacement (framed area imaging), x-ray diffraction (single crystal and polycrystal), ellipsometry, and Raman spectroscopy. A major motivation has been the study of the properties of beryllium under conditions relevant to the fuel capsule in inertial confinement fusion: magnetically driven shock and isentropic compression shots at Z were used to investigate the equation of state and shock melting characteristics, complemented by laser ablation experiments to investigate plasticity and heterogeneous response from the polycrystalline microstructure. These results will help to constrain acceptable tolerances on manufacturing, and possible loading paths, for inertial fusion ignition experiments at the National Ignition Facility. Laser-based techniques are being developed further for future material dynamics experiments, where it should be possible to obtain high quality data on strength and phase changes up to at least 1TPa.

Calculation of optical absorption in Al across the solid-to-liquid transition

We calculate the optical-absorption spectrum of solid and liquid Al for temperatures between 0 and 1800 K with an empirical pseudopotential technique. It is found that a prominent peak in the spectrum of the solid (at $\ensuremath{\hbar}\ensuremath{\omega}\ensuremath{\sim}1.5\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$) smoothly disappears as the temperature is raised above ${T}_{\mathrm{melt}}=938\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. This is in agreement with earlier measurements on solid and liquid Al, but is in disagreement with more recent measurements on the liquid, in which the peak was seen to remain at $T=1550\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. Our results suggest that in situ optical constants measurements may be used as a diagnostic for the shock melting of Al.

Mechanism of Formation of Glass with Embedded Metal and Sulfides

GEMS (glass with embedded metal and sulfides) grains in interplanetary dust particles (IDPs) were examined using 200 keV analytical transmission electron microscopy. The morphologies and crystallography of embedded relict grains reveal that GEMS are pseudomorphs formed by irradiation processing of crystals free-floating in space. Some GEMS retain a compositional and morphological memory of the crystal from which they formed. Pseudomorphism may rule out condensation, annealing, flash heating, or shock melting as alternative mechanisms of GEMS formation. A significant and often dominant fraction of the atoms in GEMS were sputter-deposited from other grains. Therefore, a normal (solar) isotopic composition is not a reliable indicator of whether GEMS formed in the solar system or in presolar interstellar or circumstellar environments.

High‐pressure phases in shock‐induced melt veins of the Umbarger L6 chondrite: Constraints of shock pressure

Abstract— We report a previously undocumented set of high‐pressure minerals in shock‐induced melt veins of the Umbarger L6 chondrite. High‐pressure minerals were identified with transmission electron microscopy (TEM) using selected area electron diffraction and energy‐dispersive X‐ray spectroscopy. Ringwoodite (Fa30), akimotoite (En11Fs89), and augite (En42Wo33Fs25) were found in the silicate matrix of the melt vein, representing the crystallization from a silicate melt during the shock pulse. Ringwoodite (Fa27) and hollandite‐structured plagioclase were also found as polycrystalline aggregates in the melt vein, representing solid state transformation or melting with subsequent crystallization of entrained host rock fragments in the vein. In addition, Fe2SiO4‐spinel (Fa66‐Fa99) and stishovite crystallized from a FeO‐SiO2‐rich zone in the melt vein, which formed by shock melting of FeO‐SiO2‐rich material that had been altered and metasomatized before shock. Based on the pressure stabilities of the high‐pressure minerals, ringwoodite, akimotoite, and Ca‐clinopyroxene, the melt vein crystallized at approximately 18 GPa. The Fe2SiO4‐spinel + stishovite assemblage in the FeO‐SiO2‐rich melts is consistent with crystallization of the melt vein matrix at the pressure up to 18 GPa. The crystallization pressure of ˜18 GPa is much lower than the 45–90 GPa pressure one would conclude from the S6 shock effects in melt veins (Stöffler et al. 1991) and somewhat less than the 25–30 GPa inferred from S5 shock effects (Schmitt 2000) found in the bulk rock.

Investigating the Lake Bosumtwi impact structure: Insight from numerical modeling

The current topography of the Lake Bosumtwi crater and some of its structural dimensions have been determined by geophysical methods. We combine these data with sophisticated numerical models to evaluate the cratering process itself (for example, melt and tektite generation) as well as to test the modeling code. The geophysical maps show some asymmetry in plan view, with the main anomaly north of the crater center. The simulations of the early stage show asymmetric patterns only in ejecta and tektite distributions, while the late stage is modeled for the vertical impact without any asymmetry. We estimate the projectile size from scaling laws and then, varying material properties, reproduce a crater, which is similar to the Bosumtwi, but too deep. Bulking allows us to reconcile differences between the model results and the observed topography. Shock melt estimates are in good agreement with the Bosumtwi magnetic signature. Modeled distribution of tektites assumes an impact angle of 30°–45° and an impact direction from the N‐NE. The combination of numerical models and field evidence not only provides necessary information for upcoming scientific drilling of the structure but also suggests interesting and well‐suited drill sites. Besides the central uplift and the annular moat with a suggested thick breccia cover, drilling at the location of the geophysical anomalies and comparison of downrange and transversal locations will provide new insight into preimpact and impact‐induced asymmetries.

Shock Melting Temperature of Initially Porous Iron andIndication for Melting Curve of Iron at High Pressures

The melting curve of iron is crucial for modelling of the earth's internal heatstructures and to understand melting of solids at high pressures.However, the measured melting temperatures of iron at high pressuresare disparate so far. We measured the shocked interface (porousiron/sapphire window) temperatures of a kind of porous iron. By using amodel for shock temperature measurement [High Pressures Res. 2 (1990)159] and the previous results of sound velocity measurements [Chin.Phys. Lett. 18 (2001) 852], we determine the melting temperatures ofiron at shock compression high pressures of 145 and 171 GPa. They areconsistent with the results reported by other shock compressionexperiments. Based on the possible different melting mechanisms of ironin diamond anvil cell and in shock compression, the corrected meltingtemperatures of iron at high pressures become more consistent.

Meteoritical evidence and constraints on asteroid impacts and disruption

Impact events have played a central role in the life of meteorites. They compacted and lithified the dust from which meteorites are made; produced shock minerals, shock melting, and shock blackening of meteoritic minerals on their parent bodies; turned their parent bodies into rubble; and dispersed at least some pieces of this rubble, sending them to Earth as meteorites. Thus, as well as owing their very existence to the occurrence of catastrophic disruptions, meteorites contain physical ground truth concerning the impact and disruption environment of the solar system. Reviewing these aspects of the impact–meteorite connection, we conclude that impacts severe enough to disrupt asteroids were rare in the earliest stages of the solar nebula, when meteorite parent bodies accreted and were lithified. Likewise, though catastrophic disruptions clearly have occurred over the past several billion years, the small number of exposure events seen in the meteoritic cosmic ray age record indicates that such disruptions at these times also were rare. However, catastrophic disruptions must have been very prevalent during the first billion years of the solar system, resulting in the widespread asteroid macroporosity inferred from the comparison of asteroid bulk densities to meteorite grain densities.