1,721,014 research outputs found
Polyphase inclusions in garnet pyroxenites from Sulu (China) as carriers of seawater at ultrahigh pressure
No full textUnravelling processes of fluid-mediated element exchange between slab lithologies and the mantle wedge is of primary importance in understanding element mobility in subduction zones. Several studies have addressed element transfer related to fluid release during prograde metamorphism in subduction zones [1-4]. Nevertheless, detailed studies documenting interactions between felsic, mafic and ultramafic rocks at ultrahigh-pressure (UHP) are still scarce [5]. For this reason, UHP metasomatised rocks represent ideal materials to study the element exchange at pressures corresponding to sub-arc depths in subduction zones.
We present preliminary results of Ca-rich garnet–clinopyroxenites from Suolushu, occurring as layers in a large serpentinite body at Hujialin, Rizhao County, in the Sulu UHP metamorphic terrane (eastern China). Both clinopyroxenites and hosting serpentinites are intercalated with coesite-eclogites and hosted by coesite-bearing gneiss. Similar garnet–clinpyroxene layers from Hujialin have been studied by [6] and interpreted as cumulates crystallised from a hydrous, subduction related magma at ~ 1 GPa and 1000 °C. They were subjected to minor Ca enrichment coeval with serpentinisation of the host ultramafic rocks and then subducted at UHP (4.8 ±0.6 GPa and 750 ±50 °C). Ca-rich garnet–clinopyroxenites are composed of centimeter-sized garnet porphyroblasts in a matrix of fine-grained green diopside, opaque minerals associated with green spinel, and garnet. Garnet porphyroblasts include rounded clinopyroxene, opaque minerals, and/or spinel grains. Aggregates of magnetite and spinel are abundant in some samples. Peak porphyroblastic garnets preserve primary polyphase inclusions in their cores, consisting of amphibole(s), chlorite, pyroxene, micas and spinel. We studied these inclusions with the Transmission Electron Microscope at the University of Milano. They show an inner part formed by amphibole and clinopyroxene surrounded by Al rich Mg-silicates. Amphibole and pyroxene grow coherently by sharing the [001] direction, the one parallel to the tetrahedral chains. At the grain boundary between amphiboles and pyroxene, or amphiboles and Al rich Mg silicates, smaller amphibole grains extremely enriched in both Cl (up to 8 at.%) and Sr (up to 1.5 at.%) occasionally occur. Such Cl-amphiboles grow coherently with the neighbouring amphibole. The Al rich Mg-silicate phases show electron diffraction patterns with several streaking, indicating possible polytypic disorder. They exhibit periodicities of 14.1 Å, characteristic of chlorite. These phases form a rim between the inclusion precipitates and the hosting garnet, whereas a direct contact between garnet and amphibole or pyroxene have never been observed.
Serpentinites act as carriers of oceanic Cl, B, Sr, Rb, Cs, and alkalis which are recycled into variably saline fluids within the stability field of antigorite serpentine [7]. Polyphase inclusions studied in Hujalin clinopyroxenites likely derive from the interaction with the hosting serpentinites at HP-UHP and could represent a snapshot of such fluid-mediated element recycling occurring in the slab at sub-arc depths.
References. [1] Bebout, G.E., Ryan, J.G., Leeman, W.P., Bebout, A.E. (1999): Earth Planet. Sci. Lett, 171, 63–81; [2] Becker, H., Jochum, K.P., Carlson, R.W. (2000): Geology, 163, 65–99; [3] Scambelluri, M., Philippot, P. (2001): Lithos, 55, 213–227; [4] Spandler, C.J., Hermann, J., Arculus, R.J., Mavrogenes, J.A. (2003): Contrib. Mineral. Petrol., 146, 205–222; [5] Malaspina, N., Hermann, J., Scambelluri, M., Compagnoni, R. (2006): Lithos, 90, 19–42; [6] Yang, J.J. (2006): J. Petrol., 47, 965–990; [7] Scambelluri, M., Fiebig, J., Malaspina, N., Müntener, O., Pettke, T. (2004): Int. Geol. Rev., 46, 595–613
Low temperature SR-XRPD study of akermanite-gehlenite solid solution
No full textLow temperature thermal expansion coefficients of members of the solid solution gehlenite (ge)-åkermanite (åk) were measured by synchrotron radiation X-Ray powder diffraction. The linear thermal expansion coefficient is maximum for a composition with about 50 % content of åk. In åk-rich compositions an incommensurate modulated structure is present. The ge-åk solid solution shows a non-ideal behaviour, with negative excess volume near the ge end-member
Thermal expansion and phase transitions in akermanite and gehlenite
No full textThermal expansion has been measured by laboratory and synchrotron X-ray powder diffraction for end-member (a) over circle kermanite (ak, Ca2MgSi2O7) and gehlenite (ge, Ca2Al2SiO7) in the range 20-1,500 K. In ak in the range 340-390 K, there is a negative linear thermal expansion in [001] direction. This is related to the phase transition from an incommensurate modulated structure (IC) to a normal one (N). The volumetric mean thermal expansion coefficients for ak and ge, obtained with a linear fit of the experimental data in the temperature range 298-1,400 K, are respectively 32.1x10(-6) and 28.3x10(-6) K-1. The variation of the c/a ratio with temperature, due to different thermal expansion along the crystallographic axes, can be related to the different behaviour of the tetrahedral layers in the N and IC phases. Analysis of the variation of the superstructure peaks intensity across the phase transition confirms the tricritical behaviour of the IC/N transition in ak
The MnCO3-II high-pressure polymorph of rhodocrosite
No full textWe investigated the behavior of MnCO3 in the pressure range 0-50 GPa and ambient temperature by synchrotron X-ray single-crystal diffraction technique. MnCO3 maintains the calcite-type structure (R3c symmetry) up to 44 GPa. Above this pressure we observed a phase transition. The highpressure phase, MnCO3-II, is triclinic, with cell parameters a = 2.928(2), b = 4.816(4), c = 5.545(4) Å, α = 101.71(6)°, β = 94.99(6)°, γ = 89.90(6)°, and V = 76.28(10) Å3 at 46.8 GPa. The structure is solved with the charge flipping algorithm. MnCO3-II is isostructural with CaCO3-VI. The density increase on phase transition is 4.4%. The occurrence of CaCO3-VI structure in MnCO3 composition indicates that CaCO3-VI structure is also adopted by carbonates with cations smaller than calcium
Non-ideality and defectivity of the akermanite-gehlenite solid solution: An X-ray diffraction and TEM study
No full textThis paper reports a structure analysis of the åkermanite-gehlenite solid solution. This solution is non-ideal with a negative excess volume in the entire compositional range. X-ray diffraction shows anomalous behavior of the cell parameters close to the gehlenite end-member (åk08ge92, åk35ge65). This behavior is correlated with an excess of Si and deficiency of Al with respect to the Mg content, which implies a defective, non-stoichiometric structure with Ca vacancies. Electron microscopy images have confirmed an increase in the defectivity on the atomic scale for Al-rich compositions, and single-crystal structure refinements show a correlated decrease of the tetrahedral volume preferentially occupied by Si. The incommensurate modulation, characteristic of åkermanite, has been observed also in åk95ge05, and it is still visible as diffuse scattering in åk75ge25. (210) twinning has been observed in the entire compositional range
High-pressure behavior of akermanite and gehlenite and phase stability of the normal structure in melilites
No full textÅkermanite (Ca 2MgSi 207) and gehlenite (Ca 2Al 2Si0 7) have been studied at high pressure by synchrotron radiation powder and single-crystal diffraction up to 30 GPa. At about 2 GPa, the incom-mensurately modulated structure (IC) transforms to a nornal structure (N). The bulk modulus for the N structure, fitted with a Birch Murihagan EoS on powder data, is 93.5(5) GPa. The compressibility is anisotropic, and it is greater along the c axis, in the direction perpendicular to the tetrahedral layers of the structure. Above 15 GPa, a phase transition is observed, marked by a discontinuity in the elastic behavior and a small change in intensity and in the full-width at half maximum (FWHM) of the powder diffraction peaks. The diffraction patterns are indexed with respect to tetragonal cell of the N-melilite structure up to 30 GPa. A hysteresis in the elastic behavior is observed during decompression. In contrast, single-crystal data show a new monoclinic phase appearing above 15 GPa. The unit-cell parameters are a = 8.82(1) Å, b = 7.34(1) Å, c = 9.13(1) Å, β = 115.1(2)°. This unit cell is similar to that of Ca 2ZnGe 1.25Si o.75O 7.reported in the literature. A refinement using the corresponding model in space group P2 1/n fits the single-crystal data with a reasonable R Bragg = 15%, considering that the crystal is twinned and the mosaicity is large. Gehlenite has a higher bulk modulus, 106.1(4) GPa, than does åkermanite. The compressibility is anisotropic, and the behavior is similar to that of åkermanite, but the presence of Al in tetrahedral sites decreases the compressibility parallel to the (001) plane. The structure of gehlenite is stable up to 25 GPa, when a phase transition occurs
The high-pressure stability of chlorite and other hydrates in subduction mélanges: experiments in the system Cr2O3–MgO–Al2O3–SiO2–H2O
No full textThe solubility of chromium in chlorite as a function of pressure, temperature, and bulk composition was investigated in the system Cr2O3–MgO–Al2O3–SiO2–H2O, and its effect on phase relations evaluated. Three different compositions with X Cr = Cr/(Cr + Al) = 0.075, 0.25, and 0.5 respectively, were investigated at 1.5–6.5 GPa, 650–900 °C. Cr-chlorite only occurs in the bulk composition with X Cr = 0.075; otherwise, spinel and garnet are the major aluminous phases. In the experiments, Cr-chlorite coexists with enstatite up to 3.5 GPa, 800–850 °C, and with forsterite, pyrope, and spinel at higher pressure. At P > 5 GPa other hydrates occur: a Cr-bearing phase-HAPY (Mg2.2Al1.5Cr0.1Si1.1O6(OH)2) is stable in assemblage with pyrope, forsterite, and spinel; Mg-sursassite coexists at 6.0 GPa, 650 °C with forsterite and spinel and a new Cr-bearing phase, named 11.5 Å phase (Mg:Al:Si = 6.3:1.2:2.4) after the first diffraction peak observed in high-resolution X-ray diffraction pattern. Cr affects the stability of chlorite by shifting its breakdown reactions toward higher temperature, but Cr solubility at high pressure is reduced compared with the solubility observed in low-pressure occurrences in hydrothermal environments. Chromium partitions generally according to XspinelCr ≫ XopxCr > XchloriteCr ≥ XHAPYCr > XgarnetCr . At 5 GPa, 750 °C (bulk with X Cr = 0.075) equilibrium values are XspinelCr = 0.27, XchloriteCr = 0.08, XgarnetCr = 0.05; at 5.4 GPa, 720 °C XspinelCr = 0.33, XHAPYCr = 0.06, and XgarnetCr = 0.04; and at 3.5 GPa, 850 °C XopxCr = 0.12 and XchloriteCr = 0.07. Results on Cr–Al partitioning between spinel and garnet suggest that at low temperature the spinel- to garnet-peridotite transition has a negative slope of 0.5 GPa/100 °C. The formation of phase-HAPY, in assemblage with garnet and spinel, at pressures above chlorite breakdown, provides a viable mechanism to promote H2O transport in metasomatized ultramafic mélanges of subduction channels
In situ study of methane anaerobic combustion on iron oxides: Towards a clean hydrogen production process
No full textThermal expansion and de-hydroxylation of phengite micas
No full textPhengite samples (2M 1 and 3T politypes) and a synthetic end-member muscovite specimen were studied by in situ high-temperature synchrotron radiation X-ray diffraction. The measured volume thermal expansion of 2M 1 phengite ( ≈ 36.6 × 10−6 K−1) was systematically greater than of the 3T polytype (≈33.3 × 10−6 K−1). A positive linear correlation between the average thermal expansion on (001) plane and the mean tetrahedral rotation angle at ambient condition is proposed on the ground of new measurements and literature data. Dehydroxylation processes were observed in 2M 1, starting at 1,000 K in 3T at 800 and 945 K in synthetic muscovite. Rietveld refinements allowed a determination of structural variations upon heating of phengite samples and their dehydroxylate phases. The phengite structure expands by regularizing the tetrahedral sheet and by reducing the bond length differences between the outer and inner coordination shell of the interlayer site. The dehydroxylate phase derived from 2M 1 is characterized by fivefold polyhedra in the low temperature form as a consequence of two OH groups reacting to form H2O + O (residual). The dehydroxylate exhibits an increase of the cation–cation distances along the M–Or–M bonds with respect to low-temperature phengite structures. For the 3T phase, we were unable to achieve completion of dehydroxylation. The refined structural model of the dehydroxylate phase shows two hydroxyl sites, but at a short distance from one another. This result suggests that the dehydroxylation reaction did not proceed to completion
Electron diffraction determination of 11.5 Å and HySo structures: candidate water carriers to the Upper Mantle
No full textThe dehydration reactions of minerals in subduction zones strongly control geological processes, such as arc volcanism, earthquakes, serpentinization, or geochemical transport of incompatible elements. In aluminum-bearing systems, chlorite is considered the most important hydrous phase at the top of the subducting plate, and significant amount of water is released after its decomposition. However, the dehydration mechanism is not fully understood, and additional hydrates are stabilized by the presence of Al beyond the stability field of chlorite. We applied here a cutting-edge analytical approach to characterize the experimental rocks synthesized at the high pressures and temperatures matching with deep subduction conditions in the upper mantle. Fast electron diffraction tomography and high-resolution synchrotron X‐ray diffraction allowed the identification and the successful structure solution of two new hydrous phases formed as dehydration product of chlorite. The 11.5 Å phase, Mg6Al(OH)7(SiO4)2, is a hydrous layer structure. It presents incomplete tetrahedral sheets and face-sharing magnesium and aluminum octahedra. The structure has a higher Mg/Si ratio compared to chlorite, and a significantly higher density (r0 = 2.93 g/cm3) and bulk modulus [K0 = 108.3(8) GPa], and it incorporates 13 wt% of water. The HySo phase, Mg3Al(OH)3(Si2O7), is a dense layered sorosilicate, [r0 = 3.13 g/cm3 and K0 = 120.6(6) GPa] with an average water content of 8.5 wt%. These phases indicate that water release process is highly complex, and may proceed with multistep dehydration, involving these layer structures whose features well match the high-shear zones present at the slab-mantle wedge interface
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