Patrick Cordier

Mineralogy and Petrology of Comet 81P/Wild 2 Nucleus Samples

The bulk of the comet 81P/Wild 2 (hereafter Wild 2) samples returned to Earth by the Stardust spacecraft appear to be weakly constructed mixtures of nanometer-scale grains, with occasional much larger (over 1 micrometer) ferromagnesian silicates, Fe-Ni sulfides, Fe-Ni metal, and accessory phases. The very wide range of olivine and low-Ca pyroxene compositions in comet Wild 2 requires a wide range of formation conditions, probably reflecting very different formation locations in the protoplanetary disk. The restricted compositional ranges of Fe-Ni sulfides, the wide range for silicates, and the absence of hydrous phases indicate that comet Wild 2 experienced little or no aqueous alteration. Less abundant Wild 2 materials include a refractory particle, whose presence appears to require radial transport in the early protoplanetary disk.

Pressure sensitivity of olivine slip systems and seismic anisotropy of Earth's upper mantle

The mineral olivine dominates the composition of the Earths upper mantle and hence controls its mechanical behaviour and seismic anisotropy. Experiments at high temperature and moderate pressure, and extensive data on naturally deformed mantle rocks, have led to the conclusion that olivine at upper-mantle conditions deforms essentially by dislocation creep with dominant [100] slip. The resulting crystal preferred orientation has been used extensively to explain the strong seismic anisotropy observed down to 250 km depth. The rapid decrease of anisotropy below this depth has been interpreted as marking the transition from dislocation to diffusion creep in the upper mantle. But new high-pressure experiments suggest that dislocation creep also dominates in the lower part of the upper mantle, but with a different slip direction. Here we show that this high-pressure dislocation creep produces crystal preferred orientations resulting in extremely low seismic anisotropy, consistent with seismological observations below 250 km depth. These results raise new questions about the mechanical state of the lower part of the upper mantle and its coupling with layers both above and below.

Densification involved in the UV-based photosensitivity of silica glasses and optical fibers

A comprehensive survey of photosensitivity in silica glasses and optical fiber is reviewed. Recent work on understanding the mechanisms contributing to germanium or aluminum doped fiber photosensitivity is discussed within the framework of photoelastic densification models.

Shear deformation experiments of forsterite at 11 Gpa - 1400°C in the multianvil apparatus

Synthetic forsterite samples were shear-deformed at 11 GPa, 1400°C in the multianvil apparatus. The deformation microstructures have been characterised by SEM, EBSD, X-ray diffraction peak broadening and strain anisotropy analysis, and TEM. Different time durations have been characterised with a view to follow the evolution of strain and stress in high-pressure deformation experiments. A high density of [001] dislocations is introduced during pressurization at room temperature although no significant macroscopic shear or crystal preferred orientations are induced at this stage. The deviatoric stress is probably on the order of 1.5 GPa. Heating at 1400°C leads to a rapid decrease of the density of these dislocations. The shear deformation at high-temperature leads to measurable strain and development of crystal preferred orientations after one hour. Stress and strain-rate continue to decrease with time, such that eight hour experiments exhibit microstructures where recovery is apparent. At this stage, the stress level is estimated at ca. 100 MPa from dislocation density measurements. Crystal preferred orientations and TEM characterisation show that glide of [001] dislocations on (100) or (010) is the dominant deformation mechanism. Further investigation is needed to determine whether inhibition of [100] glide in these experiments is due to the role of water or whether a physical effect of pressure is also contributing.

Dislocation creep in MgSiO3 perovskite at conditions of the Earth's uppermost lower mantle

Seismic anisotropy provides an important observational constraint on flow in the Earths deep interior. The quantitative interpretation of anisotropy, however, requires knowledge of the slip geometry of the constitutive minerals that are responsible for producing rock fabrics. The Earths lower mantle is mostly composed of (Mg, Fe)SiO3 perovskite, but as MgSiO3 perovskite is not stable at high temperature under ambient pressure, it has not been possible to investigate its mechanical behaviour with conventional laboratory deformation experiments. To overcome this limitation, several attempts were made to infer the mechanical properties of MgSiO3 perovskite on the basis of analogue materials. But perovskites do not constitute an analogue series for plastic deformation, and therefore the direct investigation of MgSiO3 perovskite is necessary. Here we have taken advantage of recent advances in experimental high-pressure rheology to perform deformation experiments on coarse-grained MgSiO3 polycrystals under pressure and temperature conditions of the uppermost lower mantle. We show that X-ray peak broadening measurements developed in metallurgy can be adapted to low-symmetry minerals to identify the elementary deformation mechanisms activated under these conditions. We conclude that, under uppermost lower-mantle conditions, MgSiO3 perovskite deforms by dislocation creep and may therefore contribute to producing seismic anisotropy in rocks at such depths.

Structural and chemical alteration of crystalline olivine under low energy He + irradiation

We present the results of irradiation experiments on crystalline olivine with He + ions at energies of 4 and 10 keV and fluences varying from 5 10 16 to 10 18 ions/cm 2 . The aim of these experiments is to simulate ion implantation into interstellar grains in shocks in the ISM. Irradiated samples were analysed by transmission electron microscopy (TEM). The irradiation causes the amorphization of the olivine, at all He + fluences considered. The thickness of the amorphized region is 40 15 nm and 90 10 nm for the 4 keV and 10 keV experiments, respectively. The amorphization of the olivine occurs in conjunction with an increase in the porosity of the material due to the formation of bubbles. In addition, the amorphized layer is decient in oxygen and magnesium. We nd that the O/Si and Mg/Si ratios decrease as the He + fluence increases. These experiments show that the irradiation of dust in supernova shocks can eciently alter the dust structure and composition. Our result are consistent with the lack of crystalline silicates in the interstellar medium and also with the compositional evolution observed from olivine-type silicates around evolved stars to pyroxene-type silicates around protostars.

Formation mechanisms of planar deformation features in naturally shocked quartz

Abstract Shock waves induce peculiar defects in quartz: shock mosaics, high-pressure polymorphs (coesite and stishovite), diaplectic glass and ‘planar deformation features’ (PDFs). Together, these features are the indices of ‘shock metamorphism’, PDFs appear under the optical microscope as straight and narrow defects parallel to the { 10 1n } rhombohedral planes with n = 3 and 2 as most frequent values, although n = 1, 4 and ∞ (i.e. basal plane) are also found. We report in the first part of this paper a detailed investigation by transmission electron microscopy of the fine structure of PDFs in shocked quartz grains from a variety of sites (Slate Islands, La Malbaie and Manson in North America; the Vredefort complex in South Africa; the Ries Crater in Germany and the Cretaceous-Tertiary boundary layer at Raton Basin, Colorado). We distinguish four PDF fine structures: (1) bands of dislocations; (2) lamellae of mixtures in various proportions of amorphous silica and small crystallites; (3) thin Brazil twin lamellae; (4) short, parallel lamellae forming serrated ladder structures. The possible formation mechanisms of these PDFs are discussed in the second part of the paper. It is suggested that the dislocation bands are probably not original PDF structures, but stem from a later recrystallization stage. The Brazil twins have been produced by relatively large deviatoric stresses which accompanied the shock wave. The other PDF structures, which consist of partial amorphization in the { 10 1 n } planes, must result from elastic instabilities of the shear modulus of the quartz structure at high pressure in these planes. Theoretical estimates of the elastic stiffness coefficients indicate that amorphization should start at approximately 10 GPa. The growth of the straight and narrow amorphous lamellae would be driven by the front of the shock wave.

Pressure-induced slip-system transition in forsterite: Single-crystal rheological properties at mantle pressure and temperature

Abstract Deformation experiments were carried out in a Deformation-DIA high-pressure apparatus (D-DIA) on oriented Mg2SiO4 olivine (Fo100) single crystals, at pressure (P) ranging from 2.1 to 7.5 GPa, in the temperature (T) range 1373.1677 K, and in dry conditions. These experiments were designed to investigate the effect of pressure on olivine dislocation slip-system activities, responsible for the lattice-preferred orientations observed in the upper mantle. Two compression directions were tested, promoting either [100] slip alone or [001] slip alone in (010) crystallographic plane. Constant applied stress (σ) and specimen strain rates (ε) were monitored in situ using time-resolved X-ray synchrotron diffraction and radiography, respectively. Transmission electron microscopy (TEM) investigation of the run products reveals that dislocation creep assisted by dislocation climb and cross slip was responsible for sample deformation. A slip transition with increasing pressure, from a dominant [100]-slip to a dominant [001]-slip, is documented. Extrapolation of the obtained rheological laws to upper-mantle P, T, and σ conditions, suggests that [001]-slip activity becomes comparable to [100]-slip activity in the deep upper mantle, while [001] slip is mostly dominant in subduction zones. These results provide alternative explanations for the seismic anisotropy attenuation observed in the upper mantle, and for the “puzzling” seismic-anisotropy anomalies commonly observed in subduction zones

Planar deformation features in shocked quartz; a transmission electron microscopy investigation

Abstract We present a detailed investigation by transmission electron microscopy (TEM) of the fine structure of planar deformation features (PDF) in quartz grains within which shock metamorphism has been detected optically and which originate from various sites: Slate Islands, La Malbaie and Manicouagan Lake, Canada; Ries Crater, Germany; the Toba caldera, Sumatra; the Vredefort complex in South Africa and the K/T boundary at Raton Basin, Colorado. PDFs appear on TEM micrographs as straight and very narrow ( TEM reveals many more PDFs than are optically detected, probably because only concentrations of such defects in narrow areas can induce an optically detectable contrast. Caution should therefore be exercised when applying criteria based on the optical observation of single or multiple sets of PDFs. PDFs in samples from Slate Islands and Raton Basin exhibit a peculiar morphology, a high density of very tiny bubbles precipitated on the dislocation lines. Comparison with the behavior of wet quartz strongly suggests that these materials experienced lengthy annealing at moderate pressure and temperature.

Shock metamorphism of quartz with initial temperatures −170 to + 1000° C

Shock experiments on quartz single crystals with initial temperatures −170 to +1000°C showed that ambient temperature does not affect the type of defects formed but can lower the pressure of complete amorphization. The amount of glass recovered increases with both pressure and temperature, and the shock-induced phase transformation of quartz is temperature-activated with an apparent activation energy of <60 kJ/ mol. The phase transformation is localized along three types of transformation lamellae (narrow, s-shaped, and wide) which contain fractured and/or high-pressure phases. Transformation lamellae are inferred to form by motion of linear collapse zones propagating near the shock front. Equilibrium phases, such as stishovite, were not recovered and are probably not formed at high shock pressures: the dominant transformation mechanism is inferred to be solid-state collapse to a dense, disordered phase. Melting occurs separately by friction along microfaults, but no high-pressure crystal phases are quenched in these zones. Shock of quartz thus produces two types of disordered material, quenched melt (along microfaults) and diaplectic glass (in transformation lamellae); the quenched melt expands during P-T release, leaving it with a density lower than quartz, while recovered diaplectic glass has a density closer to that of quartz. At low pressures (< 15 GPa), quartz transforms mostly by shear melting, while at higher pressures it converts mostly along transformation lamellae. We find that shock paleopiezometers using microstructures are nominally temperature-invariant, so that features observed at impact craters and the K/T boundary require in excess of 10 GPa to form, regardless of the target temperature. Shock comminution will be much more extensive for impacts on cold surfaces due to lack of cementation of fragments by melt glass; shock on hot surfaces could produce much more glass than estimated from room-temperature experiments. Because of the shock-impedance mismatch between quartz specimen and steel capsule, the incident shock wave reverberates up to a final pressure. The dynamic compression process is quasi-isentropic with high strain rates. Preheating and precooling achieves final shock pressures and temperatures representative of single-shock states of room temperature quartz and of quartz on known planetary surfaces. Stress histories were calculated by detailed 1- and 2-dimensional computer simulations. The stress history throughout the sample is relatively uniform, with minor variations during unloading. Significant differences between impact pressures calculated by the shock-impedance-match method and specimen pressures calculated by computer simulations indicate the importance of modeling shock recovery experiments computationally.