Sylvain Petitgirard

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.

Status of the hard X-ray microprobe beamline ID22 of the European Synchrotron Radiation Facility

The ESRF synchrotron beamline ID22, dedicated to hard X-ray microanalysis and consisting of the combination of X-ray fluorescence, X-ray absorption spectroscopy, diffraction and 2D/3D X-ray imaging techniques, is one of the most versatile instruments in hard X-ray microscopy science. This paper describes the present beamline characteristics, recent technical developments, as well as a few scientific examples from recent years of the beamline operation. The upgrade plans to adapt the beamline to the growing needs of the user community are briefly discussed.

Solid–liquid iron partitioning in Earth’s deep mantle

Melting processes in the deep mantle have important implications for the origin of the deep-derived plumes believed to feed hotspot volcanoes such as those in Hawaii. They also provide insight into how the mantle has evolved, geochemically and dynamically, since the formation of Earth. Melt production in the shallow mantle is quite well understood, but deeper melting near the core–mantle boundary remains controversial. Modelling the dynamic behaviour of deep, partially molten mantle requires knowledge of the density contrast between solid and melt fractions. Although both positive and negative melt buoyancies can produce major chemical segregation between different geochemical reservoirs, each type of buoyancy yields drastically different geodynamical models. Ascent or descent of liquids in a partially molten deep mantle should contribute to surface volcanism or production of a deep magma ocean, respectively. We investigated phase relations in a partially molten chondritic-type material under deep-mantle conditions. Here we show that the iron partition coefficient between aluminium-bearing (Mg,Fe)SiO3 perovskite and liquid is between 0.45 and 0.6, so iron is not as incompatible with deep-mantle minerals as has been reported previously. Calculated solid and melt density contrasts suggest that melt generated at the core–mantle boundary should be buoyant, and hence should segregate upwards. In the framework of the magma oceans induced by large meteoritic impacts on early Earth, our results imply that the magma crystallization should push the liquids towards the surface and form a deep solid residue depleted in incompatible elements.

Terapascal static pressure generation with ultrahigh yield strength nanodiamond

Terapascal static pressure generation is enabled in laboratory due to implementation of nanocrystralline diamond microballs. Studies of materials’ properties at high and ultrahigh pressures lead to discoveries of unique physical and chemical phenomena and a deeper understanding of matter. In high-pressure research, an achievable static pressure limit is imposed by the strength of available strong materials and design of high-pressure devices. Using a high-pressure and high-temperature technique, we synthesized optically transparent microballs of bulk nanocrystalline diamond, which were found to have an exceptional yield strength (~460 GPa at a confining pressure of ~70 GPa) due to the unique microstructure of bulk nanocrystalline diamond. We used the nanodiamond balls in a double-stage diamond anvil cell high-pressure device that allowed us to generate static pressures beyond 1 TPa, as demonstrated by synchrotron x-ray diffraction. Outstanding mechanical properties (strain-dependent elasticity, very high hardness, and unprecedented yield strength) make the nanodiamond balls a unique device for ultrahigh static pressure generation. Structurally isotropic, homogeneous, and made of a low-Z material, they are promising in the field of x-ray optical applications.

The Extreme Conditions Beamline P02.2 and the Extreme Conditions Science Infrastructure at PETRA III

Performance description of the Extreme Conditions Beamline (ECB, P02.2) at PETRA III that is optimized for micro-diffraction at simultaneous high pressure and high and low temperatures created in different diamond anvil cells environments. Additional information of the capabilities of the Extreme Conditions Science Infrastructure for DAC work is provided.

Fate of MgSiO3 melts at core–mantle boundary conditions

Significance A new technique has been developed to measure in situ the density of amorphous material composed of light elements under extreme conditions of pressure using the X-ray absorption method. At core–mantle boundary (CMB) pressure, the densities of MgSiO3 glass and melts are similar to the one of the crystalline bridgmanite, within uncertainty. Due to the affinity of iron oxide for silicate liquids, melting in the MgSiO3–FeSiO3 system will produce dense melts that could accumulate above the CMB, leading to the formation of a dense basal magma ocean in the early Earths mantle. One key for understanding the stratification in the deep mantle lies in the determination of the density and structure of matter at high pressures, as well as the density contrast between solid and liquid silicate phases. Indeed, the density contrast is the main control on the entrainment or settlement of matter and is of fundamental importance for understanding the past and present dynamic behavior of the deepest part of the Earth’s mantle. Here, we adapted the X-ray absorption method to the small dimensions of the diamond anvil cell, enabling density measurements of amorphous materials to unprecedented conditions of pressure. Our density data for MgSiO3 glass up to 127 GPa are considerably higher than those previously derived from Brillouin spectroscopy but validate recent ab initio molecular dynamics simulations. A fourth-order Birch–Murnaghan equation of state reproduces our experimental data over the entire pressure regime of the mantle. At the core–mantle boundary (CMB) pressure, the density of MgSiO3 glass is 5.48 ± 0.18 g/cm3, which is only 1.6% lower than that of MgSiO3 bridgmanite at 5.57 g/cm3, i.e., they are the same within the uncertainty. Taking into account the partitioning of iron into the melt, we conclude that melts are denser than the surrounding solid phases in the lowermost mantle and that melts will be trapped above the CMB.

A diamond anvil cell for x-ray fluorescence measurements of trace elements in fluids at high pressure and high temperature.

We present a new diamond anvil cell (DAC), hereafter called the fluoX DAC, dedicated for x-ray fluorescence (XRF) analysis of trace elements in fluids under high pressure and high temperature to 10 GPa and 1273 K at least. This new setup has allowed measurement of Rb, Sr, Y, Zr, with concentrations of 50 ppm to 5.6 GPa and 1273 K. The characteristics of the fluoX DAC consist in an optimized shielding and collection geometry in order to reduce the background level in XRF spectrum. Consequently, minimum detection limits of 0.3 ppm were calculated for the abovementioned elements in this new setup. This new DAC setup coupled to the hard x-rays focusing beamline ID22 (ESRF, France) offers the possibility to analyze in situ at high pressure and high temperature, ppm level concentrations of heavy elements, rare earth elements, and first transition metals, which are of prime importance in geochemical processes. The fluoX DAC is also suitable to x-ray diffraction over the same high pressure-temperature range.

The crystal structures of Mg2Fe2C4O13, with tetrahedrally coordinated carbon, and Fe13O19, synthesized at deep mantle conditions

Abstract We simulated the redox decomposition of magnesium-siderite at pressures and temperatures corresponding to the top of the Earth’s D″ layer (135 GPa and 2650 K). It transforms into new phases, with unexpected stoichiometry. We report their crystal structure, based on single-crystal synchrotron radiation diffraction on a multi-grain sample, using a charge-flipping algorithm. Mg2Fe2(C4O13) is monoclinic, a = 9.822(3), b = 3.9023(13), c = 13.154(5) Å, β = 108.02(3)°, V = 479.4(3) Å3 (at 135 GPa). It contains tetrahedrally coordinated carbon units, corner-shared in truncated C4O13 chains. Half of the cations are divalent, and half trivalent. The carbonate coexists with a new iron oxide, Fe13O19, monoclinic, a = 19.233(2), b = 2.5820(13), c = 9.550(11) Å, β = 118.39(3)°, V = 417.2(5) Å3 (at 135 GPa). It has a stoichiometry between hematite, Fe2O3, and magnetite, Fe3O4. The formation of these unquenchable phases indicates, indirectly, the formation of reduced-carbon species, possibly diamond. These structures suggest the ideas that the mineralogy of the lower mantle and D″ region may be more complex than previously estimated. This is especially significant concerning accessory phases of fundamental geochemical significance and their role in ultra-deep iron-carbon redox coupling processes, as well as the iron-oxygen system, which certainly play an important role in the lower mantle mineral phase equilibria.

Stability of iron-bearing carbonates in the deep Earth’s interior

The presence of carbonates in inclusions in diamonds coming from depths exceeding 670 km are obvious evidence that carbonates exist in the Earth’s lower mantle. However, their range of stability, crystal structures and the thermodynamic conditions of the decarbonation processes remain poorly constrained. Here we investigate the behaviour of pure iron carbonate at pressures over 100 GPa and temperatures over 2,500 K using single-crystal X-ray diffraction and Mössbauer spectroscopy in laser-heated diamond anvil cells. On heating to temperatures of the Earth’s geotherm at pressures to ∼50 GPa FeCO3 partially dissociates to form various iron oxides. At higher pressures FeCO3 forms two new structures—tetrairon(III) orthocarbonate Fe43+C3O12, and diiron(II) diiron(III) tetracarbonate Fe22+Fe23+C4O13, both phases containing CO4 tetrahedra. Fe4C4O13 is stable at conditions along the entire geotherm to depths of at least 2,500 km, thus demonstrating that self-oxidation-reduction reactions can preserve carbonates in the Earth’s lower mantle.

Density measurements and structural properties of liquid and amorphous metals under high pressure

We have implemented an in situ X-ray diffraction analysis method suitable for the determination of pressure–volume–temperature equations of state in the critical case of liquid and amorphous materials over an extended thermodynamic range (T>2000 K and P>40 GPa). This method is versatile, it can be applied to data obtained using various angle-dispersive X-ray diffraction high pressure apparatus and, contrary to in situ X-ray absorption techniques, is independent from the sample geometry. Further advantage is the fast data acquisition (between 10 and 300 s integration time). Information on macroscopic bulk properties (density) and local atomic arrangement (pair distribution function g(r)) can be gathered in parallel. To illustrate the method, we present studies on liquid Fe–S alloys in the Paris Edinburgh press and in laser-heated diamond anvil cell (DAC), and measurements on Ce glass in DAC at room temperature.