Angele Ricolleau

Toward an internally consistent pressure scale.

Our ability to interpret seismic observations including the seismic discontinuities and the density and velocity profiles in the earths interior is critically dependent on the accuracy of pressure measurements up to 364 GPa at high temperature. Pressure scales based on the reduced shock-wave equations of state alone may predict pressure variations up to 7% in the megabar pressure range at room temperature and even higher percentage at high temperature, leading to large uncertainties in understanding the nature of the seismic discontinuities and chemical composition of the earths interior. Here, we report compression data of gold (Au), platinum (Pt), the NaCl-B2 phase, and solid neon (Ne) at 300 K and high temperatures up to megabar pressures. Combined with existing experimental data, the compression data were used to establish internally consistent thermal equations of state of Au, Pt, NaCl-B2, and solid Ne. The internally consistent pressure scales provide a tractable, accurate baseline for comparing high pressure–temperature experimental data with theoretical calculations and the seismic observations, thereby advancing our understanding fundamental high-pressure phenomena and the chemistry and physics of the earths interior.

Spin transition and equations of state of (Mg, Fe)O solid solutions

[1]xa0We have performed a series of experiments to investigate the compositional effect on the compression behavior of (Mg, Fe)O solid solutions at high pressure. The in-situ synchrotron X-ray diffraction data revealed abnormal volume contractions at about 40, 60, and 80 GPa for (Mg0.80, Fe0.20)O, (Mg0.61, Fe0.39)O, and (Mg0.42, Fe0.58)O, respectively. The volume contractions are associated with the reported electronic transition of high-spin to low-spin in Fe2+, and caused by the reduction of Fe2+ ionic radius across the transition. A least-squares fit of the compression data to the Birch-Murnaghan equation of state yielded bulk modulus K0 (GPa) = 160 − 10XFeO for the high-spin (Mg,Fe)O and K0 = 170(3) GPa for the low-spin (Mg,Fe)O. The equations of state of (Mg,Fe)O established in this study are directly applicable to the Earths lower mantle in composition and pressure ranges and provide essential data for modeling the density profile of the lower mantle.

Density profile of pyrolite under the lower mantle conditions

[1]xa0The pyrolite model is one of the possible compositions of the Earths lower mantle. The lower mantles composition is generally modelled by comparing seismic observations with mineral physics data of possible lower mantle end-member phases. Here, we report the compression behavior of a natural KLB-1 peridotite (a representative composition of the pyrolite model) in a quasi-hydrostatic environment at simultaneous high pressure (P) and temperature (T), covering the entire range of lower mantle P-T conditions up to 112 GPa. This is the first experimentally determined density profile of pyrolite under the lower mantle conditions. The results allow us to directly compare the measured density of peridotite mantle along the geotherm with the Preliminary Reference Earth Model (PREM) derived from seismic observations, without extrapolation. The comparison shows significant mismatch between the two, which calls for a re-evaluation of the PREM density model or a non-pyrolite lower mantle composition.

Raman characterization of synthetic magnesian calcites

Abstract Magnesian calcites are important components of sediments and biominerals. Although Raman spectra of calcite, dolomite, and magnesite are well known, those of magnesian calcites deserve further investigation. Nineteen syntheses of magnesian calcites covering the range 0–50 mol% MgCO3 have been carried out at high pressure and temperature (1–1.5 GPa, 1000–1100 ℃). The crystalline run products have been characterized by μ-Raman spectroscopy. For all lattice and internal modes (L, T, ν1, ν4, 2ν2) but ν3, wavenumbers align closer to the calcite– dolomite line than the calcite–magnesite line. The compositional dependence is strong and regression curves with high correlation coefficients have been determined. Full-width at half maximum (FWHM) plot along parabolas that depart from the calcite–dolomite or calcite–magnesite lines. The limited data dispersion of both shifts and FWHM allow using Raman spectral properties of magnesian calcites to determine the Mg content of abiotic calcites. A comparison with Raman data from the literature obtained on synthetic magnesian amorphous calcium carbonate (Mg ACC) shows that the wavenumber position of the ACC ν1 mode is systematically shifted toward lower values, and that their FWHM are higher than those of their crystalline counterparts. The FWHM parameters of crystalline and amorphous materials do not overlap, which allows a clear-cut distinction between crystalline and amorphous materials. In synthetic magnesian calcites, the shift and FWHM of Raman bands as a function of magnesium can be interpreted in terms of changes of metal-O bond lengths resulting from the replacement of calcium by magnesium. The facts that the wavenumber of magnesian calcites are close to the calcite–dolomite line (not calcite-magnesite), that the FWHM of the T, L, and ν4 modes reach a maximum around 30 ±5 mol% MgCO3, and that a peak specific to dolomite at 880 cm–1 is observed in high-magnesian calcites indicate that dolomite-like ordering is present above ~10 mol% MgCO3. Mg atom clustering in cation layers combined with ordering in successive cation basal layers may account for the progressive ordering observed in synthetic magnesian calcites.

Block-by-block and layer-by-layer growth modes in coral skeletons

Abstract Understanding the dynamics of biomineral growth is a challenging goal of biomineralogy that can be achieved in part by deciphering biomineral structures and chemistries. The morphology, structure, and chemistry of six skeletons of Corallium and Paracorallium species (C. rubrum, C. elatius, C. johnsoni, C. niobe, P. japonicum, and P. thrinax) from the Mediterranean, the Atlantic, and the Pacific oceans have been studied by X-ray micro-computed tomography, polarized light microscope, scanning electron microscope, and electron microprobe. All species have two types of biomineral structures: an inner skeleton and sclerites that are small grains of Mg-calcite found in the living tissues surrounding the skeleton. All skeletons display a central core surrounded by an annular domain. In the species studied by electron microprobe (C. rubrum, C. elatius, and P. japonicum), the central core and the annular domains display different chemical compositions with the core richer in magnesium and poorer in sulfur than the annular domain. In terms of structure, special emphasis has been put on central cores for which little data are available. The central cores are made of sclerites and sclerite aggregates within a cement consisting of fine layers of Mg-calcite. On the other hand, the annular parts are made of fine concentric layers of calcite crystallites with only rare sclerites. These contrasting features imply two different growth modes: (1) a “block and cement” mode taking place at the apex of a branch and associated with a fast axial growth rate (~2 mm/yr); and (2) a layer-by-layer mode occurring below the apex and associated with a slow radial growth (~0.2 mm/yr). The change from a growth mode to another is anatomically controlled by the presence of a continuous network of gastrodermal canals around the sub-apical skeleton, preventing to a large extent the aggregation of sclerites. It is generally accepted that the Coralliidae family exhibits different types of skeletogeneses. In contrast with this idea, we observe that all studied Corallium species display remarkable similarities in terms of skeletogenesis and a unifying growth model for the Corallium genus is proposed. Similarities and differences with previous models are discussed. The present study shows that the morphological criterion initially used to establish the genus Paracorallium in the Coralliidae family is inadequate.

Equation of state of the high-pressure Fe3O4 phase and a new structural transition at 70 GPa

Abstract We have investigated the high-pressure behavior of Fe3O4 by in situ X-ray diffraction measurements from 11 to 103 GPa. Up to 70 GPa, the previous observed high-pressure Fe3O4 phase (h-Fe3O4) is stable, with a CaTi2O4-type structure. The compression curve shows an abnormal volume contraction at about 50 GPa, likely associated with the magnetic moment collapse observed at that pressure. Fitting the compression data up to 45 GPa to the Birch-Murnaghan equation of state yields a bulk modulus, KT0 = 172 GPa, and V0 = 277 Å3, with fixed Kʹ = 4. At a pressure between 64 and 73 GPa, a new structural transition was observed in Fe3O4, which can be attributed to a martensitic transformation as described by Yamanaka et al. (2008) for post-spinel structural transition. The diffraction data can be best fitted with a Pnma space group. No breakdown of Fe3O4 was observed up to at least 103 GPa. The new high-pressure polymorph is about 6% denser than the h-Fe3O4 phase at 75 GPa.

Growth kinetics and distribution of trace elements in precious corals

The concentration and spatial distribution of major (Ca, Mg) and trace elements (Na, Sr, S, Li, Ba, Pb, and U) in different Corallium skeletons (C. rubrum, C. japonicum, C. elatius, C. konojoi) have been studied by electron microprobe (EMP) and laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS). EMP data show positive Na-Mg and negative Na-S and Mg-S correlations in all skeletons. LA-ICPMS data display additional Sr-Mg, Li-Mg, and U-Mg positive correlations. Medullar zones in the skeletons, corresponding to fast growing zones, are systematically richer in Mg, Na, Sr, Li, and U and poorer in S than the surrounding slow growing zones. These spatial distributions are mostly interpreted in terms of growth kinetics combined with steric effects influencing the incorporation of impurities in biogenic calcites. This interpretation is in agreement with available experimental data on kinetic effects on the incorporation of elements in calcite. At a different scale, annual growth rings in annular slow growing zones show oscillations in Mg, Na, Sr, and S. These chemical oscillations probably result from growth rate variations: fast growth would produce rings enriched in Mg, Sr, and Na, while slow growth would produce rings enriched in Ca, S and organic matter. From previous studies in C. rubrum, the Mg-rich rings would develop during the spring to fall period while the S-rich rings would form immediately after (late fall and winter). Analytical traverses performed in annular zones of different Corallium skeletons indicate that Mg, Na, Sr, Li, and U decrease from core to rim. This observation indicates that radial growth rate decreases as the colony gets older. Contrary to Mg, Na, Sr, Li, S, and U, barium and lead concentrations are identical in medullar and annular zones and appear independent of growth kinetics. Thus, concentrations in Corallium skeletons could provide indications on Ba and Pb contents in the oceans. Barium and lead concentrations are higher in Mediterranean than in Pacific precious corals, these two elements can be used to discriminate C. rubrum from C. japonicum, and contribute enforcing regulations on the trade of precious corals.