Ahmed El Goresy

The Majorite-Pyrope + Magnesiowüstite Assemblage: Constraints on the History of Shock Veins in Chondrites

Shock veins in the Sixiangkou (L6) chondrite contain two high-pressure assemblages: (i) majorite-pyrope solid solution plus magnesiowüstite that crystallized at high pressures and temperatures from a shock-induced silicate melt of bulk Sixiangkou composition and (ii) ringwoodite plus low-calcium majorite that were produced by solid-state transformation of olivine and low-calcium pyroxene. The morphology and chemistry of the majorite-pyrope garnet and the size of the magnesiowüstite crystals indicate a longer duration at high pressure and temperature than predicted by impact scenarios. This pressure-temperature regime is constrained by the olivine-ringwoodite and orthopyroxene-majorite phase transformations, fusion of the meteorite constituents, and crystallization of majorite-pyrope solid solution plus magnesiowüstite from that melt under high pressure.

A new natural high-pressure (Na, Ca)-hexaluminosilicate [(CaxNa1-x)Al3+xSi3-xO11] in shocked Martian meteorites

A (Ca,Na)-hexaluminosilicate, whose Ca end member was previously synthesized in numerous high-pressure experiments, has been identified by Raman spectroscopy in heavily shocked Martian meteorites. This mineral has a structural formula close to (CaxNa1-x)Al3+xSi3-xO11 and is similar to the calcium aluminum silicate phase previously synthesized in high-pressure experiments performed on anorthite and rocks of basaltic composition. This new mineral occurs in shock melt pockets in two distinct settings and is intimately intergrown with SiO2-stishovite. The first setting, encountered in Zagami, consists of idiomorphic equant crystals overgrown by acicular stishovite that crystallized from a melt of labradorite composition. The second setting contains the (Na, Ca)-hexaluminosilicate phase intergrown with stishovite and hollandite and was formed during partial melting at high pressures. The mineralogical association (Na,Ca)-hexaluminosilicate+stishovite was observed in shock melt pockets, which have distinct bulk compositions in seven Martian shergottites. This new mineral represents, after majorite, the second natural occurrence of a silicate mineral with silicon in both four and six coordination. The assemblage stishovite+(Na,Ca)-hexalummosilicate sets constraints on the pressure and temperature conditions that prevailed during shock in some of the studied meteorites. The (Na, Ca)-hexaluminosilicate mineral is a potential carrier of Al and Na during subduction of oceanic crust in the lower mantle of the Earth

Two forsterite-bearing FUN inclusions in the Allende meteorite

We have discovered two FUN inclusions, CG-14 and TE, among a group of five forsterite-rich inclusions in Allende, two of which are described for the first time herein. All five consist of euhedral forsterite and spinel crystals poikilitically enclosed by fassaite. Forsterite and spinel are usually segregated from one another, sometimes into a spinel-rich mantle and a forsterite-rich core. Some inclusions contain vesicles, indicating that they were once molten. The crystallization sequence inferred from textures is: spinel, forsterite, fassaite and, finally, Mg-rich melilite. One concentrically-zoned inclusion contains melilite in its mantle whose composition lies on the opposite side of the liquidus minimum in the melilite binary from that in its core. This suggests that segregation of forsterite from spinel in all of these inclusions could be due to volatilization of MgO and SiO2 relative to Al2O3 and CaO from the outsides of droplets. CG-14 is relatively uniformly enriched in refractory elements relative to Cl chondrites by a factor similar to that for Ca-, Al-rich coarse-grained inclusions except for Ca, Al and Hf which are unusually low. No Ce anomaly such as found in FUN inclusions Cl and HAL is present in CG-14. Whole-rock samples of CG-14 and TE are more strongly mass-fractionated in oxygen relative to “normal” Allende inclusions than the FUN inclusion EK 1-4-1 and less so than Cl. Relative to bulk Allende, both inclusions have strongly massfractionated magnesium and silicon and 25Mg excesses or deficits of 24Mg or 26Mg. CG-14 has a 29Si excess or a deficit of 28Si or 30Si. Volatilization loss cannot be responsible for the magnesium and silicon isotope fractionations, as this would require prohibitively large mass loss from these magnesium-rich inclusions. The remarkable similarity in textures between FUN and non-FUN inclusions implies similar thermal histories, arguing against different rates of evaporative loss of major elements. Sputtering alone may be insufficient to account for the magnitude and direction of oxygen isotope fractionation in FUN inclusions.

In situ discovery of shock-induced graphite-diamond phase transition in gneisses from the Ries Crater, Germany

Abstract Reflected-light microscopy and fine-scale laser microRaman spectroscopy of shocked garnetcordierite- sillimanite gneisses in suevites of the Ries meteorite impact crater, Germany, led to the discovery of impact diamonds in their pristine graphite-diamond assemblages. Graphite-diamond textural relations permit a clear determination of the solid-state nature of the formation of diamond from graphite, which is estimated to have occurred at a peak-shock pressure between 30 and 40 GPa. Shock-induced transformations were promoted only in unkinked and undeformed graphite booklets at the graphite-garnet, graphite-sillimanite, or graphite-rutile interfaces, where the difference in shock impedance is very high. Reverberations of shock waves with short wavelengths similar to the grain sizes at the phase boundaries are probably important constraints for dynamic graphite-diamond phase transformation. Raman spectroscopic investigations of hard transparent carbon platelets intercalated between fine-grained diamond and deformed graphite revealed the platelets to be Raman inactive. The platelets are either dense amorphous carbon or an unknown dense crystalline carbon phase that is Raman inactive.

Elemental and isotopic fractionations produced through evaporation of the Allende CV chondrilte: Implications for the origin of HAL-type hibonite inclusions

Abstract Through evaporation of samples from the Allende carbonaceous chondrite we have produced a series of residues that show correlated variations in mineralogy, chemistry, and isotopic compositions. Major and minor elements are evaporated in the order (Fe, Mn, Cr) → (Mg, Si) → (Ca, Ti) → (Al) and their loss is reflected in the mineralogy of the remaining samples. Residues of low to moderate degrees of evaporation consist of increasingly Mg-rich olivine and silicate glass. After complete evaporation of Fe, Mg, and Si at approximately 96% mass loss, the residues consist of very fine-grained Ca aluminates. Evaporation at higher temperatures produced three residues that contain hibonite and a less refractory CaAl glass. Magnesium, Si, Ca, Ti, and O isotopes show mass-dependent fractionations that are consistent with Rayleigh-type distillation. The rare earth elements and other refractory trace elements are enriched in the residues up to ∼100 × CI, although several elements (V, Ba, Ce) are depleted due to their increased volatilities under oxidizing conditions. The most refractory residues also exhibit depletions in Eu, an element that is volatile under reducing conditions, but is as refractory as the other light rare earth elements under oxidizing conditions. The apparently contradictory presence of both Ce and Eu depletions in the residues is a result of changing evaporation dynamics in the course of the experiments: the release of large amounts of O during evaporation of the major element oxides creates “locally oxidizing” conditions in the samples; later, after most major elements have been vaporized, the local sample environments become more “reducing.” The three hibonite-bearing residues share many chemical and isotopic characteristics with five HAL-type hibonite inclusions for which an origin as distillation residues has been proposed; our data show that many of the unique features of these inclusions can be produced in a single evaporation event. Strong similarities between the hibonite-bearing residues and the hibonite inclusions HAL and DH-H1 suggest that the evaporation histories of these inclusions may be roughly comparable to those of our residues.

Evidence for fractional crystallization of wadsleyite and ringwoodite from olivine melts in chondrules entrained in shock-melt veins.

Peace River is one of the few shocked members of the L-chondrites clan that contains both high-pressure polymorphs of olivine, ringwoodite and wadsleyite, in diverse textures and settings in fragments entrained in shock-melt veins. Among these settings are complete olivine porphyritic chondrules. We encountered few squeezed and flattened olivine porphyritic chondrules entrained in shock-melt veins of this meteorite with novel textures and composition. The former chemically unzoned (Fa24–26) olivine porphyritic crystals are heavily flattened and display a concentric intergrowth with Mg-rich wadsleyite of a very narrow compositional range (Fa6–Fa10) in the core. Wadsleyite core is surrounded by a Mg-poor and chemically stark zoned ringwoodite (Fa28–Fa38) belt. The wadsleyite–ringwoodite interface denotes a compositional gap of up to 32 mol % fayalite. A transmission electron microscopy study of focused ion beam slices in both regions indicates that the wadsleyite core and ringwoodite belt consist of granoblastic-like intergrowth of polygonal crystallites of both ringwoodite and wadsleyite, with wadsleyite crystallites dominating in the core and ringwoodite crystallites dominating in the belt. Texture and compositions of both high-pressure polymorphs are strongly suggestive of formation by a fractional crystallization of the olivine melt of a narrow composition (Fa24–26), starting with Mg-rich wadsleyite followed by the Mg-poor ringwoodite from a shock-induced melt of olivine composition (Fa24–26). Our findings could erase the possibility of the resulting unrealistic time scales of the high-pressure regime reported recently from other shocked L-6 chondrites.

Seifertite, a dense orthorhombic polymorph of silica from the Martian meteorites Shergotty and Zagami

Seifertite is a dense orthorhombic polymorph of silica with the scrutinyite (α-PbO2) type structure that was found as lamellae occurring together with dense silica glass lamellae in composite silica grains in the heavily shocked Martian meteorite Shergotty. The mineral is also intergrown in some grains with minor stishovite and a new unnamed monoclinic dense silica polymorph with a ZrO2-type structure. Seifertite has also been found in the Martian shergottite Zagami and is a minor constituent in other Martian shergottites. Chemical analyses of seifertite in Shergotty indicate major SiO2 with minor concentrations of Al2O3 and Na2O. Selected-area electron diffraction (SAED) and X-ray diffraction can be interpreted in terms of an orthorhombic pattern from a scrutinyite (α-PbO2) structure. The cell parameters are a = 4.097(1) A, b = 5.0462(9) A, c = 4.4946(8) A, V = 92.92 A3, Z = 4, and the space group is Pbcn or Pb 2 n . Density is (calc.) = 4.294 g/cm3 (with pure SiO2), 4.309 g/cm3 (with empirical formula). It is inferred that seifertite was formed by shock-induced solid-state transformation of either tridymite or cristobalite on Mars at an estimated minimum equilibrium shock pressure in excess of 35 GPa. The new mineral is named after Friedrich A. Seifert (b. 1941), founding Director of the Bayerisches Geoinstitut, Universitat Bayreuth, Germany, for his seminal contributions to high-pressure geoscience.

The Proton Microprobe: A Powerful Tool for Nondestructive Trace Element Analysis

A proton microprobe capable of focusing proton beams with energies up to 6 million electron volts to a spot size of 2 x 2 square micrometers has been used for chemical analysis of small grains of minerals in lunar samples by proton-induced x-ray emission. The proton microprobe is preferable to the electron microprobe for analyzing trace elements whose concentrations are below the detection limit of the latter and for analyzing objects with numerous major and trace elements with a wide range of atomic numbers. Application of the proton microprobe to biological samples is feasible.

Natural dissociation of olivine to (Mg,Fe)SiO3 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 (Fa34-41) 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)SiO3 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)SiO3 Pv and Mw have 50–100 nm in diameter, and lamellar (Mg,Fe)SiO3 Pv and Mw have approximately 20 and approximately 10 nm in thickness, respectively. Partitioning coefficient, KPv/Mw = [FeO/MgO]/[FeO/MgO]Mw, between (Mg,Fe)SiO3 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.

A comparative study of naturally and experimentally shocked chondrites

Samples of the Jilin H5 chondrite were experimentally shock-loaded at the peak pressures of 12, 27, 39, 53, 78, 83, 93, and 133 GPa. The aim of this study is to compare experimentally shock-induced phenomena with those in naturally shocked chondrites and to test the feasibility of experimentally calibrating naturally induced shock phenomena in Hand L-chondrites. Planar fractures, mosaicism, brecciation in olivine and pyroxene, as well as transformation of plagioclase into diaplectic glass were observed in the Jilin samples shocked at pressures lower than 53 GPa. Shock-induced chondritic melts were first obtained at P > 78 GPa and more than 60% of the whole-rock melting was achieved at P similar to 133 GPa, and that shook-induced silicate melt consists of quenched microcrystalline olivine and pyroxene, metal, troilite and vesicular glass. No high-pressure phases were observed in any of the experimentally shocked samples, neither in the deformed nor in the molten regions. Deformation features in Jilin samples shock-loaded below 53 GPa are comparable to those found in H- and L-chondrites. The mineral assemblages in the molten regions in the shocked Jilin samples are also comparable to those encountered in the heavily shocked Yanzhuang (H6) and some Antarctic H-chondrites, but differ considerably from those found in heavily shocked Sixiangkou and many other L6 chondrites. Shock melt veins in L6 chondrites contain high-pressure polymorphs of olivine, pyroxene, plagioclase and high-pressure liquidus phases, whereas shock melt veins in heavily shocked H-chondrites contain mainly low-pressure mineral assemblages. The differences in the mineral constituents of shock melt veins in L- and H-chondrites clearly indicate differences in the shock histories of these meteorites. While crystallization in the shook melt veins in L-chondrites took place at high pressures, crystallization in shock-induced melt in most H-chondrites took place after decompression. It is evident that the thickness and abundance of shock melt veins and size of melt regions is not necessarily a quantitative measure of the degree of shock. The duration of the high-pressure regime, the time of the cooling and the P-T regime during the crystallization path, and the post-shock temperatures are stringent parameters that control the evolution of the shock-induced melt. So, scaling from shock experiments on millimeter-sized samples to natural shock features on kilometer-sized asteroids poses considerable problems in quantifying the P-T conditions during natural shock events on asteroids