Bernd Wunder

Antigorite: High-pressure stability in the system MgOSiO2H2O (MSH)

Abstract The breakdown reactions of antigorite: (1) forming talc + forsterite + water at low pressures and (2) forming forsterite + clinoenstatite + water at high pressures were determined in reversed equilibrium experiments. Results on reaction (1) were found to be in good agreement with former experimental determinations by both Johannes [Johannes, W., 1975. Zur Synthese und thermischen Stabilitat von Antigorit. Fortschr. Mineral. Beih. 53, 36.] and Evans et al. [Evans, B.W., Johannes, W., Oterdoom, H., Trommsdorff, V., 1976. Stability of crysotile and antigorite in the serpentinite multisystem. Schweiz. Mineral. Petrogr. Mitt. 56, 79–93.]. From our experiments the invariant point (I1), interconnecting the two reactions, can be located at about 15 kbar/650°C. This is consistent with the thermodynamic calculations using the dataset of Berman [Berman, R.G., 1988. Internally consistent thermodynamic data for minerals in the system J. Petrol. 29, 445–522.]; however, it is in contrast to recent experimental studies of Ulmer and Trommsdorff [Ulmer, P., Trommsdorff, V., 1995a. Serpentine stability to mantle depths and subduction-related magmatism. Science 268, 858-861.] who determined I1 at 21 kbar/730°C. Our PT-conditions for I1 could be confirmed by equilibrium experiments on reaction (10) talc + forsterite ↔ clinoenstatite + water, which is generated at I1 as well. Up to about 25 kbar the breakdown reaction (2) is nearly pressure-independent. Towards still higher pressures the dP/dT-slope of reaction (2) bends and becomes negative. Schreinemakers analysis as well as thermodynamic calculations of the upper pressure-stability of antigorite show that the possible antigorite breakdown reaction (3) antigorite ↔ clinoenstatite + brucite + water and reaction (4) brucite + clinoenstatite ↔ forsterite + water could originate at a new invariant point I2, provided that the reactions (2) and (11) brucite + antigorite ↔ forsterite + water intersect. Bracketing equilibrium (4) and combining these results with those on reaction (2), I2 was located at only about 51 kbar/490°C, compared to 77 kbar/680°C according to Bermans data. However, when taking into account the dense hydrous magnesium silicate (= DHMS)-phase A, Mg7Si2O8(OH)6, the phase relations of antigorite are changed resulting (i) in the metastability of I2 and reaction (3) and (ii) in a new invariant point I7 at about 44 kbar and 580°C generating the new antigorite breakdown-reaction (16) antigorite ↔ phase A + clinoenstatite + water. On the basis of these new data on the stability of antigorite, earlier conclusions about dehydration depths in subducted serpentine-bearing oceanic lithosphere have to be reconsidered. The maximum pressure stability of antigorite according to reaction (16) extends between 44 and 55 kbar, that is between about 130 and 160 km depths, as opposed to about 75 kbar (220 km) following Ulmer and Trommsdorff (see above). Because many different thermal regimes are possible in subduction zones, no specific dehydration depth can be expected but rather more continuous dehydration fronts in space and time.

Antigorite Pressure and temperature dependence of polysomatism and water content

The pressure and temperature dependence of antigorite polysomatism was investigated in the P,T-range of 350-710°C, 0.2-5.0 GPa within the system MgO-SiO2-H2O (MSH). TEM-study indicated that increasing temperature and decreasing pressure of MSH-antigorite formation are correlated with a shorter a-cell periodicity, i.e. smaller m- value (m = number of tetrahedra in a single chain along the wavelength a). For the P,T-conditions investigated, the compositional m-range of antigorite is rather narrow (14-18) corresponding to Mg2.79Si2O5(OH)3.57 - Mg2.83Si2O5(OH)3.67. The change in the crystal structure of antigorite is combined with a gradual partial dehydration process and loss of MgO. For the formulation of reactions that include antigorite, the P,T-dependence of the chemical composition of antig- orite has to be considered. However, our results support the conclusion of Mellini et al. (1987) of the limited appli- cability of a modulation-dependent geothermobarometer, because equilibrium states of antigorite modulation are rarely observed in natural occurrences and have not been reached in our experiments. The successive partial dehydration of antigorite during ongoing subduction of serpentine-bearing oceanic litho- sphere probably influences its rheological properties.

High-pressure ammonium-bearing silicates: Implications for nitrogen and hydrogen storage in the Earth’s mantle

Abstract The ammonium analogues of the high-pressure potassium-bearing silicate phases K-hollandite, K-Si-wadeite, K-cymrite, and phengite were synthesized in the system (NH4)2O(-MgO)-Al2O3-SiO2- H2O [N(M)ASH] using multi-anvil and piston-cylinder equipment. Syntheses included NH4-hollandite (NH4AlSi3O8) at 12.3 GPa, 700 °C; NH4-Si-wadeite [(NH4)2Si4O9] at 10 GPa, 700 °C; NH4-cymrite (NH4AlSi3O8⋅H2O) at 7.8 GPa, 800 °C; and NH4-phengite [NH4(Mg0.5Al1.5)(Al0.5Si3.5)O10(OH)2] at 4 GPa, 700 °C. Run products were characterized by SEM, FTIR, and powder XRD with Rietveld refinements. Cell parameters of the new NH4 end-members are: a = 9.4234(9) Å, c = 2.7244(3) Å, V = 241.93(5) Å3 (NH4-hollandite); a = 6.726(1) Å, c = 9.502(3) Å, V = 372.3(1) Å3 (NH4-Si-wadeite); a = 5.3595(3) Å, c = 7.835(1) Å, V = 194.93(5) Å3 (NH4-cymrite). NH4-phengite consisted of a mixture of 1M, 2M1, 2M2, 3T, and 2Or polytypes. The most abundant polytype, 2M1, has cell dimensions a = 5.2195(9) Å, b = 9.049(3) Å, c = 20.414(8) Å, β = 95.65(3)°, V = 959.5(5) Å3. All unit-cell volumes are enlarged in comparison to the potassium analogues. Substitution of NH4 for K does not cause changes in space group. NH4 incorporation was confirmed by the appearance of NH4-vibration modes ν4 and ν3 occurring in the ranges of 1397-1459 and 3223-3333 cm-1, respectively. Ammonium in eclogite facies metasediments is mainly bound in micas and concentrations may reach up to a few thousand parts per million. It can be stored to greater depths in high-pressure potassium silicates during ongoing subduction. This possibly provides an important mechanism for nitrogen and hydrogen transport into the deeper mantle.

Island-arc basalt alkali ratios: Constraints from phengite-fluid partitioning experiments

In order to constrain the large ion lithophile (LIL) element distribution for subduction-zone environments, exchange coefficients \(K^{phe-fluid}_{D}\ {=}\ (\mathit{X}^{phe}_{Rb,Cs}\ {\cdot}\ \mathit{X}^{fluid}_{K}){/}(\mathit{X}^{phe}_{K}\ {\cdot}\ \mathit{X}^{fluid}_{Rb,Cs})\) for K, Rb, and Cs between aqueous fluids and phengite (phe) have been determined experimentally at 2.0 and 4.0 GPa. Derived K D values for the Rb-K exchange slightly increase from 1.62 ± 0.10 at 2 GPa, 600 °C, to 1.84 ± 0.15 at 4 GPa, 700 °C. For the Cs-K exchange, much lower K D values of 0.22 ± 0.06 (at 2 GPa, 600 °C) and 0.37 ± 0.10 (at 4 GPa, 700 °C) were determined. The results show that, for the pressure-temperature range investigated, Rb preferentially fractionates into phengite, whereas Cs partitions into the fluid. Assuming a one-step model of perfect Rayleigh fractionation for continuous decomposition of phengite during subduction, varying alkali ratios observed for island-arc basalts as a function of slab depth may be explained by the LIL-fractionation behavior between fluids and phengites determined in this study. Our data indicate that previously derived models for metasomatic mass transfer during subduction processes need to be reconsidered.

Ab initio prediction of equilibrium boron isotope fractionation between minerals and aqueous fluids at high P and T

Abstract Over the last decade experimental studies have shown a large B isotope fractionation between materials carrying boron incorporated in trigonally and tetrahedrally coordinated sites, but the mechanisms responsible for producing the observed isotopic signatures are poorly known. In order to understand the boron isotope fractionation processes and to obtain a better interpretation of the experimental data and isotopic signatures observed in natural samples, we use first principles calculations based on density functional theory in conjunction with ab initio molecular dynamics and a new pseudofrequency analysis method to investigate the B isotope fractionation between B-bearing minerals (such as tourmaline and micas) and aqueous fluids containing H 3 BO 3 and H 4 BO 4 - species. We confirm the experimental finding that the isotope fractionation is mainly driven by the coordination of the fractionating boron atoms and have found in addition that the strength of the produced isotopic signature is strongly correlated with the B O bond length. We also demonstrate the ability of our computational scheme to predict the isotopic signatures of fluids at extreme pressures by showing the consistency of computed pressure-dependent β factors with the measured pressure shifts of the B O vibrational frequencies of H 3 BO 3 and H 4 BO 4 - in aqueous fluid. The comparison of the predicted with measured fractionation factors between boromuscovite and neutral fluid confirms the existence of the admixture of tetrahedral boron species in neutral fluid at high P and T found experimentally, which also explains the inconsistency between the various measurements on the tourmaline–mica system reported in the literature. Our investigation shows that the calculated equilibrium isotope fractionation factors have an accuracy comparable to the experiments and give unique and valuable insight into the processes governing the isotope fractionation mechanisms on the atomic scale.

K–Rb–Cs partitioning between phlogopite and fluid: experiments and consequences for the LILE signatures of island arc basalts

The distribution of Rb–K and Cs–K between phlogopite and 1- to 2-m aqueous (K,Rb,Cs)–chloride solutions was investigated at 800 °C and pressures of 0.2, 2 and 4 GPa. Phlogopite of the solid solution binaries KAlMg3Si3O10(OH)2 (phlogopite)–RbAlMg3Si3O10(OH)2 (Rb–phlogopite) and KAlMg3Si3O10(OH)2–CsAlMg3Si3O10(OH)2 (Cs–phlogopite) formed within the experiments according to the chemical exchange vectors XIIK−1+XIIRb1+ and XIIK−1+XIICs1+ involving the interlayer sites. The compositions of phlogopite and coexisting fluids were determined from EMP and ICP analyses, respectively. The following K–Rb and K–Cs exchange coefficients KDphl–fluid between phlogopite and fluid were derived by extrapolating the measured Rb and Cs concentrations to the parts per million range, for which Henrys law is valid: KDphl–fluid(Rb–K): 1.71±0.06 at 0.2 GPa, 2.73±0.10 at 2 GPa and 2.76±0.15 at 4 GPa and KDphl–fluid(Cs–K): 0.57±0.05 at 0.2 GPa, 0.73±0.09 at 2 GPa and 0.93±0.26 at 4 GPa. Based on the contrasting and slightly pressure-dependent fractionation behaviour of Rb and Cs relative to K between phlogopite and hydrous fluids—Rb preferentially partitions into phlogopite, whereas Cs fractionates into the fluid—variations in the large ion lithophile element (LILE) ratios within rocks and infiltrated fluids, as a result of formation or breakdown of phlogopite, are discussed by applying models of Rayleigh fractionation and ion exchange processes operating in a one-dimensional chromatographic rock column. Assuming significant amounts of metasomatically formed phlogopite within the mantle wedge, calculations using the chromatographic column model, led to Cs/K ratios significantly lower than those observed for island arc basalts (IAB). We therefore propose that metasomatic formation of phlogopite within the mantle wedge should be limited and does not significantly influence the LILE characteristics of IAB.

Temperature distribution in piston-cylinder assemblies Numerical simulations and laboratory experiments

Knowledge of the temperature distribution in piston-cylinder assemblies is desirable for equilibrium studies and experiments under transient conditions. The accurate determination of equilibrium properties needs a homogeneous temperature within the sample and transient experiments often require a defined thermal gradient. Knowledge of the temperature difference between the sample and thermocouple is another important constraint for quantitative experiments. To this end, the temperature distribution within various different piston-cylinder assembly designs was modeled with a specially designed 3D-Finite-Difference (3D-FD) program and compared to laboratory observations. For the 3D-FD simulation, the temperature and pressure dependence of the thermal properties of piston-cylinder-assembly materials (NaCl, CaF2, pyrophyllite, Au, graphite, NiCr-alloy) was considered. Furthermore, the T -dependent resistivity of graphite was used to model the local heat generation of the graphite heater. Experimentally determined and modeled temperature distributions are in good agreement. This indicates that the 3D-FD program is useful and an appropriate tool in the design of virtual piston-cylinder assemblies to be used in homogeneous temperature-distribution or defined thermal gradient experiments. The influences of temperature, pressure, assembly design, assembly materials (CaF2, NaCl), stepped versus straight wall heater, and presence versus absence of gold capsules on the temperature distribution within piston-cylinder assemblies are modeled and discussed. Different piston-cylinder configurations are presented, optimized for equilibrium studies and transient experiments, focusing on low and predefined T -gradients, respectively.

Interlayer vacancy characterization of synthetic phlogopitic micas by IR spectroscopy

Phlogopitic micas of the solid solution binaries \(KMg_{3}[AlSi_{3}O_{10}](OH)_{2}\ (phlogopite)\ -\ RbMg_{3}[AlSi_{3}O_{10}](OH)_{2}\ (Rbphlogopite)\) , \(phlogopite\ -\ CsMg_{3}[AlSi_{3}O_{10}](OH)_{2}\ (Cs-phlogopite)\) , and \(phlogopite\ -\ BaMg_{3}[Al_{2}Si_{2}O_{10}](OH)_{2}\) (kinoshitalite) have been synthesized at temperatures of 700 and 800°C and pressures of 0.2 and 2.0 GPa. The run products have been investigated by optical microscopy, X-ray powder diffraction, electron microprobe, and infrared spectroscopy. All runs yielded between 81 and 100 wt.% of phlogopitic micas, beside traces of quartz, sanidine, and in one run talc. Celsian and cymrite formed as additional phases in the runs of the (K-Ba)-series. The synthetic phlogopitic micas often consist of mixtures of the three polytypes 1M, 2M 1 and 2M 2 , with 1M being the most abundant polytype. Based on electron microprobe analyses, interlayer vacancy concentrations of up to 0.29 (p.f.u.) were determined, indicating a significant talc component within the synthesized phlogopitic micas. In addition to the known characteristic phlogopite OH-stretching vibrational bands, the infrared spectra of the synthetic micas with incompletely filled interlayer sites exhibit a further OH-band, centered in the spectral range 3674 - 3678 cm −1 . The intensity of this band is correlated with the amount of vacancies. The vacancy concentration of phlogopitic micas was determined quantitatively from the intensity of this infrared band by using the intensity of the principal OH-band of synthetic talc (Mg 3 [Si 4 O 10 ](OH) 2 ) as a standard. The vacancy concentration of the interlayer site as determined in such a way by infrared spectroscopy corresponds to those independently derived by electron microprobe analyses.

Preserved near ultrahigh-pressure melt from continental crust subducted to mantle depths

Remnants of hydrous melt formed at mantle depths have been identified and characterized within high-pressure leucogranulites of the Orlica-Snieznik Dome (Bohemian Massif, central Europe). They occur as nanogranites in garnet formed via partial melting of granitoids during the Variscan orogeny. Melt composition and H2O content have been investigated in situ after experimental re-homogenization of the nanogranites, and are consistent with melts produced experimentally from crustal lithologies at mantle depths. This is the first geochemical study of melt inclusions from natural crustal rocks equilibrated close to the stability field of coesite, shedding light on how continental crust melts during deep subduction. Whereas decompressional melting is commonly invoked for deeply subducted crustal lithologies, melting occurred near or at the metamorphic peak pressure in the Orlica-Snieznik granulites. Melting of deeply subducted crustal rocks significantly modifies the rheology and thus promotes fast exhumation: this process has a critical influence on the geodynamic evolution of subduction-collision orogens as well as crustal differentiation at depth.

Li-isotope fractionation between silicates and fluids: Pressure dependence and influence of the bonding environment

Isotope fractionation experiments and molecular simulations were performed to determine the relation of Li-isotope fractionation between silicates and fluids and the corresponding cation coordination environments. The effect of pressure-induced changes of Li hydration in aqueous fluids on solid-fluid Li-isotope fractionation was studied by performing experiments in the system spodumene-fluid at three different pressures of 1, 4 and 8 GPa at temperatures ranging from 500 to 625 °C. 7 Li preferentially partitioned into the fluid in all three experiments. The Li-isotope fractionation of experiments at 1 and 4 GPa does not show a significant P dependence in comparison to previously published data at 2 GPa. At 8 GPa the Li-isotope fractionation is slightly decreased compared to the low-pressure data. In addition, the fractionation of lithium isotopes between Li-bearing amphibole and fluid was determined experimentally at 700 °C and 2 GPa, which resulted in a Δ 7 Li (Li-amph-fluid) of −1.7 ‰. Our experiments are complemented by ab initio molecular dynamics simulations of Li-bearing aqueous fluids aimed to determine structural properties at high P and T . Despite the increase in Li coordination from 4.0 to 5.4 with pressure at isothermal conditions, the mean Li-O distance of the fluid is almost unchanged between 1 and 8 GPa at 727 °C. This might explain the insignificant effect of pressure over a large P range observed experimentally. The new experimental results indicate a partial inapplicability of the coordination-principle on isotope fractionation. Therefore, we additionally analyzed the relation of isotope fractionation and Li-O bond length and applied the bond valence model. Using the available structural data of solids and fluids, in a first approximation, the bond valence model seems to be more appropriate to relate the local atomic structure to isotope fractionation than the simple coordination-dependent principle.