Bruno Reynard

High-Pressure Creep of Serpentine, Interseismic Deformation, and Initiation of Subduction

The supposed low viscosity of serpentine may strongly influence subduction-zone dynamics at all time scales, but until now its role could not be quantified because measurements relevant to intermediate-depth settings were lacking. Deformation experiments on the serpentine antigorite at high pressures and temperatures (1 to 4 gigapascals, 200° to 500°C) showed that the viscosity of serpentine is much lower than that of the major mantle-forming minerals. Regardless of the temperature, low-viscosity serpentinized mantle at the slab surface can localize deformation, impede stress buildup, and limit the downdip propagation of large earthquakes at subduction zones. Antigorite enables viscous relaxation with characteristic times comparable to those of long-term postseismic deformations after large earthquakes and slow earthquakes. Antigorite viscosity is sufficiently low to make serpentinized faults in the oceanic lithosphere a site for subduction initiation.

Experimental evidence for carbonate stability in the Earth's lower mantle

We present experimental results on the stability of carbonates up to 50 GPa and at high temperatures (1500-2500 K). The experiments were conducted in a laser-heated diamond anvil cell and the run products were characterized by analytical transmission electron microscopy. Dolomite is shown to break down to a CaCO3 + MgCO3 assemblage at pressures between 20 and 50 GPa. No decarbonatation was evident, suggesting that carbonates remain stable under these conditions with respect to rocksalt oxide + CO2 assemblages. Equimolar mixed powders of dolomite + enstatite and dolomite + olivine were transformed into magnesite + calcic perovskite and into magnesite + calcic and magnesian perovskites + magnesiowustite, respectively. The very strong partitioning of Ca in silicates suggests that magnesite is the stable carbonate in the presence of silicates in the Earths lower mantle down to at least 1500 km. Finally, eutectoid or eutectic intergrowth of magnesiowustite and magnesite is observed, suggesting a possible mutual solubility between these two phases at high pressures and high temperatures. Lower mantle magnesiowustite may provide an alternative host for carbon in the Earths lower mantle.

Raman spectroscopic studies of carbonates part I: High-pressure and high-temperature behaviour of calcite, magnesite, dolomite and aragonite

The room-temperature Raman spectra of aragonite, magnesite and dolomite have been recorded up to 30 GPa and 25 GPa, respectively and no phase changes were observed during compression, unlike calcite. The effect of temperature on the room-pressure Raman spectra of calcite, aragonite, magnesite and dolomite is reported up to 800–1100 K. The measured relative pressure and temperature-shifts of the Raman lines are greater for the lattice modes than for the internal modes of the CO3 groups. These shifts are used to calculate the mode anharmonic parameters of the observed Raman modes; they are negative and their absolute values are smaller (close to 0) for the internal CO3 modes than for the lattice modes (4–17 10−5 K−1). The temperature shifts of the lattice modes in calcite are considerably larger than those for dolomite and magnesite, and a marked non-linear increase in linewidth is observed above 400° C for calcite. This is consistent with an increasing relaxational component to the libration of the CO3 groups about their threefold axes, premonitory to the rotational order-disorder transition at higher temperature. This behaviour is not observed for the other calcite structured minerals in this study. We examine systematic variations in the lattice mode frequencies and linewidths with composition, to begin to understand these differences in their anharmonic behaviour. Finally, we have used a simple Debye-Waller model to calculate atomic displacements in calcite, magnesite and dolomite with increasing temperature from the vibrational frequency data, to provide a direct comparison with atomic positional data from high-temperature structure refinements.

11B/10B analysis of geological materials by ICP–MS Plasma 54: Application to the boron fractionation between brachiopod calcite and seawater

Abstract The use of MC-ICP-MS for B isotopic measurements offers some advantages compared with other techniques such as the absence of interferences with atomic masses 10 and 11 and an enhanced speed of data acquisition. We have developed a fast and simple protocol for the separation by ion-exchange resins and concentration of B from natural waters, carbonates, phosphates, and silicates. Normalization of sample isotopic ratios to the NIST reference NBS 951 leads to an external reproducibility of ±0.3δ unit at 95% confidence level. The ionization yield is about 2 ions per 106 atoms of B in the analyzed solution. At least 2 μg of boron is required to obtain the above good-quality external reproducibility. Replicated analyses of Mediterranean seawater provided a δ11B of 40.26±0.29 (n=11). Aragonite-secreting fauna (corals and bivalves) living in tropical seawater have δ11B close to 25‰ that are much higher than calcite-secreting invertebrates such as brachiopods from tropical to high latitudes with δ11B ranging from 16.8‰ to 19.6‰. The first application of this method was to reevaluate the boron isotopic fractionation between calcite and water as a function of pH. We analyzed the calcitic shell of present-day brachiopods living in marine waters (surface down to 450 m) with pH varying from 7.6 to 8.4. By combining our data with those obtained by Sanyal et al. [Paleoceanography 11 (1996) 513] on cultured foraminifera, we derived an isotopic fractionation curve (αcalcite–seawater=0.023X[B(OH)4−]+0.976), resulting in the following relationship between the pH of seawater and the δ11B values of biogenic carbonates pH =− log 0.023 δ 11 B calcite +1000 δ 11 B seawater +1000 −0.976 −1 +8.9 This equation has a slope equal to that of the experimental clay–water fractionation curve [Palmer et al., 1987, Geochim. Cosmochim. Acta 51 (1987) 2319], which precludes the use of the boron isotope fractionation among calcite–clay pairs as a “paleo-pH meter”.

Early Archean serpentine mud volcanoes at Isua, Greenland, as a niche for early life

The Isua Supracrustal Belt, Greenland, of Early Archean age (3.81–3.70 Ga) represents the oldest crustal segment on Earth. Its complex lithology comprises an ophiolite-like unit and volcanic rocks reminiscent of boninites, which tie Isua supracrustals to an island arc environment. We here present zinc (Zn) isotope compositions measured on serpentinites and other rocks from the Isua supracrustal sequence and on serpentinites from modern ophiolites, midocean ridges, and the Mariana forearc. In stark contrast to modern midocean ridge and ophiolite serpentinites, Zn in Isua and Mariana serpentinites is markedly depleted in heavy isotopes with respect to the igneous average. Based on recent results of Zn isotope fractionation between coexisting species in solution, the Isua serpentinites were permeated by carbonate-rich, high-pH hydrothermal solutions at medium temperature (100–300 °C). Zinc isotopes therefore stand out as a pH meter for fossil hydrothermal solutions. The geochemical features of the Isua fluids resemble the interstitial fluids sampled in the mud volcano serpentinites of the Mariana forearc. The reduced character and the high pH inferred for these fluids make Archean serpentine mud volcanoes a particularly favorable setting for the early stabilization of amino acids.

Raman study of β-irradiated glasses

Abstract The ageing processes of nuclear glasses under irradiation have been simulated on three simple glass compositions (4-, 5- and 6-oxide glasses) by external β-irradiation. The structure of these glasses has been studied by Raman spectroscopy, and all three exhibit similar evolution under irradiation. The main findings are: (i) an increase of the network polymerization relative to the non-irradiated samples, (ii) an increase in concentration of molecular O 2 and (iii) a decrease of the average Si–O–Si bond angle. These data confirm the structural effects of the migration of sodium ions under external irradiation.

Pressure-induced exfoliation of inorganic fullerene-like WS2 particles in a Hertzian contact

Nanoparticles are potential additives for the improvement of lubricant properties, because of the structural modifications they undergo under high pressures in mechanical contacts. The behavior of inorganic fullerene-like WS2 nanoparticles (IF-WS2) under high isotropic pressures of up to 20GPa generated in a diamond anvil cell was studied and compared to the response of the lamellar 2H phase of WS2. The same materials were then subjected to static uniaxial pressures in a Hertzian contact in the GPa range. The evolution of the particles as a function of pressure was studied by in situ Raman spectroscopy and transmission electron microscopy at the end of the test. Data analysis shows that IF-WS2 particles resist high hydrostatic pressures well, but they are totally exfoliated by uniaxial compression in a Hertzian contact under low pressure. These results explain the excellent tribological properties at ambient temperature of IF-WS2 nanolubricant that have previously been attributed to the nested nanospheres...

High-temperature alteration of oceanic gabbros by seawater (Hess Deep, Ocean Drilling Program Leg 147): Evidence from oxygen isotopes and elemental fluxes

Two major stages of high-temperature water rock interactions have been identified in gabbros of oceanic layer 3 exposed in the Hess Deep Rift Valley at the East Pacific Rise (2°N, 101°W). The 154 m of plutonic rocks cored at site 894G show mineralogies and textures that suggest that they represent the roof of an approximate 1 Myr magma chamber. A late stage of hydrothermal alteration produced mineral assemblages typical of lower amphibolite - upper greenschist facies conditions. The hydration of the gabbros occurred locally through the development of well-crystallized green amphiboles. Significant chemical fluxes were associated with the fluid flow and precipitation-dissolution mechanisms. Oxygen isotope data suggest an earlier cryptic stage of hydrothermal alteration of the Hess Deep gabbros at temperatures above 500°C. Hydration was limited with only minute amphibole lamellae in pyroxenes that trapped Cl, Na, and minor K from seawater. Ca-plagioclases remained stable and preserved their magmatic cationic compositions and zoning patterns. We propose that this high-temperature event lowered the δ18O of the gabbros by isotopic exchange with a hydrothermal fluid through solid state oxygen diffusion. Although it is unclear whether the 18O depletion of Hole 894G gabbros is representative of the entire lower oceanic crust, it is nevertheless thought to contribute to the oxygen isotope buffering of the oceans. During this stage, water rock ratios calculated using Sr isotopes are underestimated (0.1 – 0.5) because of insignificant diffusion of this cation in plagioclases. Better estimates of water rock ratios are provided by oxygen isotopes (0.2 – 1) which are important in unraveling the cooling history of the oceanic crust.

High-pressure, high-temperature Raman spectroscopic study of ilmenite-type MgSiO 3

Abstract High-pressure and high-temperature Raman spectra of ilmenite-type MgSiO3 have been collected up to 7 GPa and 1030 K, respectively. The observed Raman frequency shifts with pressure and temperature were used to calculate the isothermal, isobaric, and isochoric (or intrinsic) anharmonic mode parameters. The low values of the intrinsic mode parameters indicate that ilmenite-type MgSiO3 has a nearly quasi-harmonic behavior, and intrinsic anharmonic corrections to the entropy and relative enthalpy of ilmenite-type MgSiO3 do not exceed 4(3) J/(mol·K) and 4(3) kJ/mol at 2000 K, respectively. These corrections are lower than those inferred for other mantle minerals. Above 900 K, a backtransformation to an MgSiO3 glass was first observed, prior to recrystallization to enstatite near 1000 K. Back-transformation systematics for high-pressure silicate phases are discussed.

Migration and segregation of sodium under β-irradiation in nuclear glasses

Raman, 11B Nuclear Magnetic Resonance (NMR) and X-ray Photoelectron (XPS) spectroscopy investigations have been made on a series of β-irradiated aluminoborosilicate glasses. This work shows the following changes of the glass structure under β-irradiation: (i) an increase of glass polymerization, (ii) an increase of boron in a trigonal environment and (iii) no modification in the aluminum environment during irradiation. These glass modifications under β-irradiation can be correlated with the migration of sodium outside of the Si and B glass network formers. Moreover, XPS experiments show a strong decrease of the sodium concentration at the surface of these irradiated glasses. This study presents therefore strong evidences of migration and segregation processes of sodium during a β-irradiation in the bulk for these simplified aluminoborosilicate glass compositions.