Patrizia Fumagalli

The 10Å phase: a high-pressure expandable sheet silicate stable during subduction of hydrated lithosphere

H2O storage and release in deep subducting lithosphere is controlled by complex reaction suites involving a variety of hydrous phases. As a result of its relatively large thermal stability and intermediate composition, the 10A O phase (Mg3Si4O10(OH)2WnH2O) has been regarded as a relevant H2O reservoir in a wide range of rock compositions and mineral assemblages. High-pressure syntheses of the 10A O phase were carried out at 6.7 GPa and 650‡C under fluidsaturated conditions in a Walker-type multi-anvil apparatus, from 5 min to 430 h. X-ray powder diffraction of large platy hexagonal crystals of the 10A O phase (up to 100 Wm) were indexed on the basis of a trioctahedral-type structure. Long-term run products (s 110 h) reveal sensitivity of the 10A O phase to treatment with acetone leading to the appearance of diffractions at greater d-spacings (10.2^11.6 A O ) with respect to the basal peak of the 10A O phase (9.64^ 10.07 A O ). This swelling behavior is strongly related to synthesis run duration. The Raman spectrum of the 10A O phase at frequencies less than 800 cm 31 shows a strong similarity to talc. In the Si^O stretching region (800^1100 cm 31 ), the 10A O phase exhibits three modes (909, 992 and 1058 cm 31 ), as compared to two in talc. The bending mode of water (X2 )i s found at 1593 cm 31 . In the OH stretching region, peaks at 3593, 3622 and 3668 cm 31 were observed. The acetone treated sample shows a C^H stretching mode at 2923 cm 31 while the double bond CNO signal is absent. The swelling behavior of the 10A O phase is interpreted as due to intercalation of acetone with pre-existing interlayer water. The efficiency of this process is dependent on the amount of the interlayer water which in turn depends on run duration. The relation between the response to acetone treatment and run duration is therefore interpreted as a time-dependent hydration of the 10A O phase. The fractions transformed from non-expandable to expandable fractions was fitted to the Avrami empirical law which suggests that kinetics are mainly controlled by diffusion rather than phase boundary reactions. The ability to accommodate variable amounts of H2O makes the 10A O phase a major H2O sink whenever a hydrous phase such as chlorite and serpentine breaks down during prograde transformations in the subducted lithosphere. Under H2O-saturated conditions, a fully hydrated 10A O phase occurs; when H2O-undersaturated conditions prevail, a H2O-deficient 10A O phase incorporates the volatile component available. The exchange capacity of interlayer molecules in the 10A O phase structure opens new scenarios on the control of fluid compositions escaping from subducted

The 10 Å phase: Crystal structure from single-crystal X-ray data

Abstract Here we report the results of the first three-dimensional refinement of the 10 Å phase performed with single-crystal X-ray data. The 10 Å phase, Mg3Si4O10(OH)2H2O, is monoclinic, space group C2/m, a = 5.323(1)Å, b = 9.203(1)Å, c = 10.216(1)Å, β = 99.98(1)°, V = 492.9(2) Å3; the calculated density, assuming Z = 2, is 2.676 g.cm-3. The structure has been solved by direct methods and refined by least-squares method with anisotropic displacement parameters. The final agreement index (R1) was 0.088 for 54 refined parameters and 499 unique observed reflections collected with a diffractometer with a CCD detector. The structure of the 10 Å phase is very similar to that of a homo-octahedral, 1 M trioctahedral mica: it is a silicate consisting of 2:1 tetrahedral-octahedral layers parallel to (001). The mean Si-O, Mg1-O, and Mg2-O bond lengths are 1.626, 2.082, and 2.081 Å, respectively. The ditrigonal rotation angle α is 0.53°. The interlayer of the 10 Å phase is occupied by water molecules. According to the oxygen occupancy, 1 H2O p.f.u. is assumed in the investigated sample. Although the average water oxygen position is in the mid-plane, structural refinement suggests disorder along c*. Twelve hydrogen bonds are located between the water molecule and the 6 + 6 oxygen atoms of the basal rings of adjacent tetrahedral sheets (water-oxygen distances averaging 3.19 Å). Therefore there are six possible orientations for the water molecule, with six hydrogen bonds pointing toward the upper basal ring and six pointing toward the lower ring of tetrahedral sheets. The orientational disorder of water, in agreement with previous Raman spectroscopy data, is a feature relevant to the evaluation of thermodynamic functions and thermal stability of the 10 Å phase, which is a possible water carrier (9.1 wt%) in subducting slabs at high pressure.

Equation of state and compressibility of phlogopite by in-situ high-pressure X-ray powder diffraction

The elastic properties of a natural phlogopite have been studied by in-situ high-pressure X-ray powder diffraction experiments on the ID9 beamline at the ESRF, at room temperature. Several EoS models (Birch-Murnaghan, Vinet, Poirier-Tarantola) have been fitted to the experimental P-V data and the results obtained are presented and discussed. The third-order Birch-Murnaghan EoS, assuming V 0 fixed at its experimental value, yields K 0 = 49.7(±0.5) GPa, K’ 0 =8.59(±0.19), K’ 0 =-0.5953 GPa −1 (implied value). The axial compressibilities at room conditions, determined by the third-order Birch-Murnaghan EoS, result in β EoS a0 =3.48(5), β EoS b0 =3.2(1), β EoS c0 =13.2(1) 10 −3 GPa −1 . Equilibrium thermodynamic calculations have been carried out to show how the new elastic parameters here reported affect the stability field of phlogopite.

The crystal structure of Mg 8 (Mg 2 Al 2 )Al 8 Si 12 (O,OH) 56 pumpellyite and its relevance in ultramafic systems at high pressure

Abstract Multianvil experiments in the model system MgO-Al2O3-SiO2-H2O at 680 °C and 5.2-6.0 GPa yield a variety of Mg,Al-rich mineral assemblages containing pumpellyite-type phases. Run products include the assemblages pumpellyite-coesite, pumpellyite-forsterite-orthoenstatite and pumpellyite-10 Å phase. The pumpellyite-coesite sample contains pumpellyite with nominal composition Mg8(Mg2Al2)Al8Si12 (O,OH)56 and about 6 wt% coesite. Powder diffraction data obtained using synchrotron radiation, a parallel-beam Debye geometry, a capillary sample mount, and an Image Plate were refined by Rietveld structure analysis. The structure was modeled in space group P21/m [cell a = 8.5759(4), b = 5.7295(2), c = 18.5376(9) Å, β = 97.691(3)°, V = 902.66(9) Å3] and shows extensive stacking disorder along the [001] direction, which can be described as layers of X-octahedra shifted by (1/2,0,0) vectors. The shifted layers locally show a sursassite-type arrangement. No disorder was detected in the Y-octahedra, which are fully occupied by Al ions, and in the W positions, commonly hosting sevenfold-coordinated Ca atoms in naturally occurring pumpellyite minerals, whereas in the synthetic Mg-rich pumpellyite they are fully occupied by Mg atoms in a distorted sixfold-coordination. The structure can then be described as a pumpellyite-type structure having a certain amount of sursassite domains. Alternative structure models involving: (1) possible presence of ardennite-domains; (2) a sursassite-type structure with pumpellyite domains; (3) cell doubling along c due to long range ordering of the layers containing the X octahedra; or (4) a statistically disordered structure in the centered A2/m parent space group and in other t subgroups, were all tested and discarded on the basis of the full-profile Rietveld refinements of powder diffraction data. The selected model indicates that the polytypic behaviour observed in pumpellyite and related structures (sursassite, and possibly ardennite) is also possible in the high pressure members of the group. The volume of the reaction, MgSiO3 (enstatite) + 2 Mg2SiO4 (forsterite) + Mg8(Mg2Al2)Al8Si12O42OH14 (pumpellyite) ↔ 5 Mg3Al2Si3O12 (pyrope) + 7 H2O re-evaluated using the new crystallographic data, gave a negative dP/dT slope. The assemblage chlorite-enstatite-pyrope acts as compositional barrier for the occurrence of pumpellyite in most ultramafic compositions for H2O-undersaturated conditions. In more complex Fe,Ca-containing systems, the preferential partitioning of Fe,Ca in pumpellyite compared to garnet enhances its potential petrological importance in ultramafic subducting slabs.

High-temperature and high-pressure behavior of carbonates in the ternary diagram CaCO3-MgCO3-FeCO3

Abstract We report the thermal expansion and the compressibility of carbonates in the ternary compositional diagram CaCO3-MgCO3-FeCO3, determined by in situ X-ray powder and single-crystal diffraction. High-temperature experiments were performed by high-resolution X-ray synchrotron powder diffraction from ambient to decarbonation temperatures (25–850 °C). Single-crystal synchrotron X ray diffraction experiments were performed in a variable pressure range (0–100 GPa), depending on the stability field of the rhombohedral structure at ambient temperature, which is a function of the carbonate composition. The thermal expansion increases from calcite, CaCO3, α0 = 4.10(7) ×10–5 K–1, to magnesite, MgCO3, α0 = 7.04(2) ×10–5 K–1. In the magnesite-siderite (FeCO3) join, the thermal expansion decreases as iron content increases, with an experimental value of α0 = 6.44(4) ×10–5 K–1 for siderite. The compressibility in the ternary join is higher (i.e., lower bulk modulus) in calcite and Mg-calcite [K0 = 77(3) GPa for Ca0.91Mg0.06Fe0.03(CO3)] than in magnesite, K0 = 113(1) GPa, and siderite, K0 = 125(1) GPa. The analysis of thermal expansion and compressibility variation in calcite-magnesite and calcite-iron-magnesite joins clearly shows that the structural changes associated to the order-disorder transitions [i.e., R3c calcite-type structure vs. R3 CaMg(CO3)2 dolomite-type structure] do not affect significantly the thermal expansion and compressibility of carbonate. On the contrary, the chemical compositions of carbonates play a major role on their thermo-elastic properties. Finally, we use our P-V-T equation of state data to calculate the unit-cell volume of a natural ternary carbonate, and we compare the calculated volumes to experimental observations, measured in situ at elevated pressure and temperatures, using a multi-anvil device. The experimental and calculated data are in good agreement demonstrating that the equation of state here reported can describe the volume behavior with the accuracy needed, for example, for a direct chemical estimation of carbonates based on experimental unit-cell volume data of carbonates at high pressures and temperatures.

Sessione S 10. Controls of mantle depletion/enrichment on geological processes

Abstract from 88th Congress of the Italian Geological Society, 2016-09-07 - 2016-09-09, NaplesAbstract from 88th Congress of the Italian Geological Society, 2016-09-07, 2016-09-09, Naplesbook Edited by D. Calcaterra, S. Mazzoli, F.M. Petti, B. Carmina & A. Zuccari doi: 10.3301/ROL.2016.79