Multi-anvil High-pressure Apparatus Research Articles

Experimental study of the interaction between garnets of eclogitic and lherzolitic parageneses and H2O-CO2 fluid under the P-T parameters of the lithospheric mantle

Experimental modeling of the interaction between garnets of mantle parageneses with H2O–CO2 fluid was performed in order to determine the characteristic features and trends in garnet compositions due to metasomatic alterations, and to study the patterns of formation of carbon and carbon-bearing phases in these processes. Experiments were carried out in the lherzolitic garnet–CO2–H2O–C and eclogitic garnet–CO2–H2O–C systems at 6.3 GPa in the temperature range of 950–1550 °C using double graphite+platinum ampoule on a multi-anvil high-pressure apparatus of a split-sphere type (BARS). The interaction of mantle garnets with H2O–CO2 fluid was found to cause garnet recrystallization and formation of new phases, namely magnesian carbonate, coesite, kyanite, corundum–eskolaite solid solution and silicate–carbonate melt, as well as metastable graphite (950–1550 °C) and diamond growth on seeds (1250–1550 °C). It was established that the garnet-fluid interaction was accompanied by the bivalent cations' redistribution between recrystallized garnet, carbonate and carbonate–silicate melt. In these processes calcium was preferentially partitioned in the melt, while magnesium was partitioned in garnet and carbonate. Recrystallization of garnets of lherzolitic and eclogitic parageneses under effect of H2O–CO2 fluid resulted not only in compositional changes, but in the formation of carbonate, kyanite, coesite, graphite, corundum–eskolaite solid solution and carbonate-silicate melt inclusions in these garnets. The revealed patterns of variations in garnet compositions, as well as the typical inclusions in them, can be regarded as indicators of metasomatic alteration of garnets of mantle parageneses involving H2O–CO2 fluid. The obtained results allow one to infer that H2O–CO2 fluids play a crucial role in redistribution of divalent cations under the P-T parameters of the lithospheric mantle and control over the composition of carbonate–silicate melts coexisting with the eclogitic and lherzolitic assemblages. Another important result of this study is the experimental demonstration that graphite capsules on their own fail to ensure reliable hermeticity with respect to the fluid and melt phases in the high-pressure, high-temperature experiments. It was substantiated that the procedure involving a hermetically sealed platinum ampoule with inner graphite capsule prevents uncontrollable loss of matter in long-term petrological experiments and is optimal for modeling systems containing melts and fluids.

Experimental Modeling of Decarbonation Reactions, Resulting in the Formation of CO2 Fluid and Garnets of Model Carbonated Eclogites under Lithospheric Mantle P,T-Parameters

First experimental modeling of decarbonation reactions resulting in the formation of CO2-fluid and Mg, Fe, Ca, and Mn garnets, with composition corresponding to the garnets of carbonated eclogites of types I and II (ECI and ECII), was carried out at a wide range of lithospheric mantle pressures and temperatures. Experimental studies were performed on a multi-anvil high-pressure apparatus of a “split sphere” type (BARS), in (Mg, Fe, Ca, Mn)CO3-Al2O3-SiO2 systems (with compositional variations according to those in ECI and ECII), in the pressure interval of 3.0–7.5 GPa and temperatures of 1050–1450 °C (t = 10–60 h). A specially designed high-pressure cell with a hematite buffering container—preventing the diffusion of hydrogen into the platinum capsule—was used, in order to control the fluid composition. Using the mass spectrometry method, it was proven that in all experiments, the fluid composition was pure CO2. The resulting ECI garnet compositions were Prp48Alm35Grs15Sps02–Prp44Alm40Grs14Sps02, and compositions of the ECII garnet were Prp57Alm34Grs08Sps01–Prp68Alm23Grs08Sps01. We established that the composition of the synthesized garnets corresponds strongly to natural garnets of carbonated eclogites of types I and II, as well as to garnets from xenoliths of diamondiferous eclogites from the Robert Victor kimberlite pipe; according to the Raman characteristics, the best match was found with garnets from inclusions in diamonds of eclogitic paragenesis. In this study, we demonstrated that the lower temperature boundary of the stability of natural garnets from carbonated eclogites in the presence of a CO2 fluid is 1000 (±20) °C at depths of ~90 km, 1150–1250 (±20) °C at 190 km, and 1400 (±20) °C at depths of about 225 km. The results make a significant contribution to the reconstruction of the fluid regime and processes of CO2/carbonate-related mantle metasomatism in the lithospheric mantle.

Experimental Modeling of the Mantle–Slab Interaction in the Metal–Carbonate System, Conditions of Crystallization and Indicator Characteristics of Diamond

Abstract —Experimental studies aimed at determining the conditions for the formation of diamond and graphite as a result of the redox interaction of reduced mantle rocks and oxidized rocks of the slab in a wide temperature range, including the conditions of both “cold” and “hot” subduction, were carried out on a “split-sphere” multianvil high-pressure apparatus (BARS) in the (Fe,Ni)–(Mg,Ca)CO3 system, at 6.3 GPa and 800–1550 °C for 35–105 h, using the “sandwich” assembly. We have established that the interaction of Fe,Ni metal and carbonate is due to the creation and propagation of a redox front, at rates from 1.3 (800 °C) to 118 μm/h (1550 °C). At T < 1200 °С, this interaction leads to the formation of alternating reaction zones (from the reduced center to the oxidized periphery): metal → metal + wüstite/magnesiowüstite → magnesiowüstite + graphite ± Mg,Fe,Ca carbonates → magnesite + aragonite. In this case, in the reduced part of the samples, the formation of a Ni,Fe metal phase strongly enriched in Ni (up to 65–70 wt.% vs. the initial 10 wt.%) was recorded. At higher temperatures, the formation of Fe,Ni metal–carbon (≥1200 °C) and carbonate (≥1330 °C) melts was observed. We have found that the presence of nickel precludes the formation of carbides in the reduced part of the sample and ensures stable diamond crystallization at 1400–1550 °C both in metal–carbon and carbonate melts. Our experiments demonstrate that diamonds from the metal–carbon melt are characterized by inclusions of taenite and magnesiowüstite. The morphology of these diamonds is determined by the {111} layer-by-layer grown faces, and their indicator characteristics are nitrogen–vacancy and nickel-related (884 nm) centers at 1400 °C or nickel–nitrogen centers (S3, 598 nm, 727 nm, 746 nm, etc.) at 1550 °C. For diamonds formed in the carbonate melt, the morphology is determined by the {100} and {111} (vicinal-growth) faces; carbonates are identified as inclusions; and nitrogen–vacancy centers H3, NV0, and NV– are fixed in the photoluminescence spectra. Experiments show that the indicator of the metal–carbonate interaction temperature is the degree of structural perfection of graphite, which increases in the range of 800–1550 °C.

Experimental Modeling of the Interaction between Garnets of Mantle Parageneses and CO2 Fluid at 6.3 GPa and 950–1550 °C

Abstract —Experimental modeling of the interaction of eclogitic and lherzolitic garnets with CO2 fluid was carried out on a multianvil high-pressure apparatus of the “split-sphere” type (BARS) in platinum ampoules with inner graphite capsules, using a buffered high-pressure cell with a hematite container, at a pressure of 6.3 GPa and in the temperature range 950–1550 °C. It has been established that the main interaction processes at 6.3 GPa and 950–1250 °C are partial dissolution, recrystallization, and carbonation of garnet which lead to the formation of magnesian carbonate, kyanite, and coesite, a decrease in Mg contents in the recrystallized garnet, and the formation of carbonate, silicate, and oxide inclusions in it. Under these conditions, crystallization of metastable graphite and growth of diamond on the seed at ≥1250 °C were observed. In the temperature range 1350–1550 °C, the garnet underwent partial dissolution and recrystallization in CO2 fluid; no carbonation took place. These processes were accompanied by a decrease in the portion of the grossular component in the garnet and by the enrichment of the fluid phase with calcium. We have established the indicative characteristics of garnet that interacted with CO2 fluid: zoning, with low contents of CaO and MgO in the rims of crystals relative to the cores, and the presence of carbonate, kyanite, coesite, and CO2 inclusions. The compositions of the produced garnet and carbonates are consistent with the data on these minerals in mantle peridotite and eclogite parageneses and in inclusions in diamonds, which suggests a significant role of metasomatism with the participation of CO2 fluid in the evolution of deep-seated rocks and in the diamond formation. In this experimental research, we have first studied the processes of diamond crystallization and determined the boundary conditions for diamond growth in the system silicate–carbonate–CO2, which simulates natural diamond formation media. In general, the established regularities can be regarded as potential indicators of mantle metasomatism and mineral formation with the participation of CO2 fluid.

Experimental Study of Sulfur Solubility in Ca–Mg Carbonate Melt under P–T Parameters of Lithospheric Mantle

Abstract —Modeling the compositions, properties, and conditions of generation of natural agents of mantle metasomatism is one of the most topical subjects in experimental petrology. Particular attention is paid to the study of C- and S-bearing metasomatic agents and their role in the global carbon and sulfur cycles and in the processes of natural diamond formation. Experimental studies aimed at the estimation of sulfur solubility in carbonate melts under lithospheric mantle conditions were carried out on a multianvil high-pressure apparatus of the “split-sphere” type (BARS) in the carbonate–sulfur ((Mg,Ca)CO3–S) and carbonate–pyrite ((Mg,Ca)CO3–FeS2) systems at 6.3 GPa and 1050–1550 °C for 20–60 h. It has been experimentally established that the main processes occurring in the carbonate–sulfur system are the recrystallization of carbonate in a sulfur melt (1050–1350 °C) and the generation of a high-calcium carbonate melt with dissolved sulfur (5.0–6.5 wt.%) (1450–1550 °C) as well as graphite recrystallization and the initial stage of diamond growth (1550 °C) in this melt. The work demonstrates that the carbonate–pyrite interaction is accompanied by the recrystallization of carbonates and pyrite (1050–1250 °C) and the generation of two immiscible melts (sulfide one with dissolved oxygen and carbonate one with dissolved sulfur (1.7–2.5 wt.%) (1350–1550 °C)) along with the formation of graphite and the growth of diamond on seed crystals (1550 °C). It has been found that the solubility of sulfur in carbonate melts increases with temperature, which goes together with a decrease in CaO (±FeO) concentrations and an increase in MgO concentrations. The present study has shown for the first time that melts of alkaline-earth carbonates are capable of dissolving up to 6.5 wt.% sulfur and they are probable sulfur concentrators under the conditions of the lithospheric mantle.

Electrical conductivity of siderite and its implication for high conductivity anomaly in the slab-mantle wedge interface

Carbonate minerals as a dominant carbon host can be transported to the Earth’s deep interior via subduction of the oceanic lithosphere, and their physicochemical behavior potentially has a significant influence on the compositional heterogeneity and physical properties in the deep mantle. In this study, we measured the electrical conductivity of natural siderite at 1–3 GPa and 100–700°C using a complex impedance analyzer in a large volume multi-anvil high-pressure apparatus. A sharp increase in conductivity was observed at ∼400°C under various pressures, and subsequently, the electrical conductivity keeps anomalously high values in the whole temperature range owing to a small quantity of interconnected highly conductive phases (graphite and magnetite) produced from the low degree decarbonation of siderite. The change in electrical conductivity and activation enthalpy suggest that the conduction mechanisms before and after low degree decarbonation of siderite are the small polaron (electron hopping in Fe2+–Fe3+) and highly conductive phases, respectively. Our results indicate the incipient decarbonation temperatures at 1–3 GPa are considerably lower than the decomposition boundary of siderite determined by phase equilibrium experiments, implying the initial decarbonation reaction of Fe-bearing carbonates in the subducting oceanic crust occurs at a shallower depth. The 30 vol.% of siderite is required to enhance the electrical conductivity of (Mg, Fe)CO3 solid solutions. Magnetite and graphite generated from the decarbonation reaction of the siderite component of Fe-bearing carbonate make a significant contribution to the high conductivity anomaly observed in the slab-mantle wedge interface.

Iron and aluminum substitution mechanism in the perovskite phase in the system MgSiO3-FeAlO3-MgO

AbstractFe,Al-bearing MgSiO3 perovskite (bridgmanite) is considered to be the most abundant mineral in Earth’s lower mantle, hosting ferric iron in its structure as charge-coupled (Fe2O3 and FeAlO3) and vacancy components (MgFeO2.5 and Fe2/3SiO3). We examined concentrations of ferric iron and aluminum in the perovskite phase as a function of temperature (1700–2300 K) in the MgSiO3-FeAlO3-MgO system at 27 GPa using a multi-anvil high-pressure apparatus. We found a LiNbO3-structured phase in the quenched run product, which was the perovskite phase under high pressures and high temperatures. The perovskite phase coexists with corundum and a phase with (Mg,Fe3+,☐)(Al,Fe3+)2O4 composition (☐ = vacancy). The FeAlO3 component in the perovskite phase decreases from 69 to 65 mol% with increasing temperature. The Fe2O3 component in the perovskite phase remains unchanged at ~1 mol% with temperature. The A-site vacancy component of Fe2/3SiO3 in the perovskite phase exists as 1–2 mol% at 1700–2000 K, whereas 1 mol% of the oxygen vacancy component of MgFeO2.5 appears at higher temperatures, although the analytical errors prevent definite conclusions. The A-site vacancy component might be more important than the oxygen vacancy component for the defect chemistry of bridgmanite in slabs and for average mantle conditions when the FeAlO3 charge-coupled component is dominant.

Synthesis and characterization of cubic boron nitride (Al)-Al2O3 composites under high pressure and high temperature conditions

Cubic boron nitride (cBN) (Al)-Al2O3 composites were synthesized by Chinese multi-anvil high pressure apparatus (CHPA) sintering at 5.0 GPa and 1200–1500 °C using Al-coated cBN (cBN (Al)) and Al2O3 powders. The effects of Al coating, phase composition, densification, Vickers hardness and thermogravimetry of the composites were investigated. Al coating on cBN powder activated the surface of cBN in the composites and reacted with cBN to produce AlN under high pressure and high temperature (HPHT) environment, which helped to promote the bonding strength of cBN and Al2O3. The hardness of cBN (Al) + 15 vol% Al2O3 is 29.7 GPa when sintered at 5.0 GPa and 1350 °C. Compared to cBN-Al2O3 composites, the hardness of cBN (Al)-Al2O3 composites increased by 37.5%. AlN, the reaction product of cBN with Al under HPHT environments, acts as a bridge phase and joins the two phases of cBN and Al2O3 in the form of chemical bonds. This is the intrinsic mechanism of hardness improvement of cBN (Al)-Al2O3 composites.

In situ measurements of electrical resistivity of metals in a cubic multi-anvil apparatus by van der Pauw method.

On the basis of the van der Pauw method, we developed a new technique for measuring the electrical resistivity of metals in a cubic multi-anvil high-pressure apparatus. Four electrode wires were introduced into the sample chamber and in contact with the pre-pressed metal disk on the periphery. The sample temperature was measured with a NiCr-NiSi (K-type) thermocouple, which was separated from the sample by a thin hexagonal boron nitride layer. The electrodes and thermocouple were electrically insulated from each other and from the heater by an alumina tube as well. Their leads were in connection with cables through the gap between the tungsten carbide anvils. We performed experiments to determine the temperature dependence of electrical resistivity of pure iron at 3 and 5 GPa. The experiments produce reproducible measurements and the results provide an independent check on electrical resistivity data produced by other methods. The new technique provides reliable electrical resistivity measurements of metallic alloys and compounds at high pressure and temperature.

Experimental Modeling of Ankerite–Pyrite Interaction under Lithospheric Mantle P–T Parameters: Implications for Graphite Formation as a Result of Ankerite Sulfidation

Experimental modeling of ankerite–pyrite interaction was carried out on a multi-anvil high-pressure apparatus of a “split sphere” type (6.3 GPa, 1050–1550 °C, 20–60 h). At T ≤ 1250 °C, the formation of pyrrhotite, dolomite, magnesite, and metastable graphite was established. At higher temperatures, the generation of two immiscible melts (carbonate and sulfide ones), as well as graphite crystallization and diamond growth on seeds, occurred. It was established that the decrease in iron concentration in ankerite occurs by extraction of iron by sulfide and leads to the formation of pyrrhotite or sulfide melt, with corresponding ankerite breakdown into dolomite and magnesite. Further redox interaction of Ca,Mg,Fe carbonates with pyrrhotite (or between carbonate and sulfide melts) results in the carbonate reduction to C0 and metastable graphite formation (±diamond growth on seeds). It was established that the ankerite–pyrite interaction, which can occur in a downgoing slab, involves ankerite sulfidation that triggers further graphite-forming redox reactions and can be one of the scenarios of the elemental carbon formation under subduction settings.

Diamond formation during sulfidation of metal–carbon melts

Experimental studies modeling the mechanism of diamond crystallization during sulfidation of metal–carbon melts were performed on a multi-anvil high-pressure apparatus BARS at pressure of 6.3 GPa, temperature range of 1270–1470 °C, and run duration of 20 to 35 h. Sandwich experiments with layer filling were carried out using schemes FeS2-[Fe(Fe90Ni10) + C]-FeS2, FeS2-Ol-[Fe90Ni10 + C]-Ol-FeS2 and S-Ol-[Fe90Ni10 + C]-Ol-S. The experiments have revealed that crystallization of diamond and/or graphite occurs during sulfidation of a metal–carbon melt either due to mixing of metal–carbon and sulfide melts or due to influx of sulfur from external sources through a polycrystalline olivine layers. Transport of a sulfur fluid or a metal–sulfide–carbon melt through a silicate matrix occurs through the intergranular space and microcracks as well as by recrystallization of olivine. Sulfidation is accompanied by a decrease in the solubility of carbon and leads to supersaturation that is necessary for crystallization of diamond or graphite. The inhibitory effect of the sulfide component is the cause for changes in the diamond morphology and transition of carbon phase crystallization from diamond to metastable graphite. The synthesized diamond crystals are classified as low-nitrogen or nitrogen-free diamonds. The revealed regularities may be used to explain the genesis of diamonds with central inclusions and nitrogen-free CLIPPIR diamonds.

Structural and Compositional Features of Garnet-Containing Samples Synthesized in a System with Samarium at High Pressure and High Temperature

A distinctive feature of garnets associated with diamonds is specific containing of “light” rare earth elements. In the paper, the garnet-containing samples obtained at high pressure and high temperature in the system introduced with samarium (Sm) are studied. The experiments are carried out using a multianvil high-pressure apparatus of the “split-sphere” type (BARS) at a pressure of 5 GPa and a temperature of 1300 °С. The accuracy of measuring the pressure and temperature is ± 0.2 GPa и ± 25 °С, respectively. As a result, pyrope grains are synthesized with a CaO content no higher than 0.15 wt.% and Cr2O3 concentration within the range of 3.61-7.55 wt.%. The garnets are characterized by the stable presence of an impurity in the form of the Sm constituent. The garnets contain a significant amount of olivine inclusions. Crystals of the synthesized spinel are observed mainly in the interstices. This study demonstrates that the interaction of the components in the serpentine — chromite — corundum — Sm system leads to the crystallization of pyrope garnet, which forms large intergrowths of individual grains. The zoning observed in garnet is due to the transfer of components by fluid during the experiment. It is concluded that the Sm content in garnet can significantly increase depending on its content in the system.

The electrical conductivity of albite feldspar: Implications for oceanic lower crustal sequences and subduction zones

Abstract Volatile-sensitive electrical soundings are becoming more widely adopted, with large nationwide arrays currently being acquired globally. This boom in new data is despite several key uncertainties relating to the electrical responses of a wide range of minerals that make up crustal regions. Complications include the influence of mineral chemistry, hydrous or nominally hydrous phases, and oxygen fugacity on charge-carrying ion activity within a mineral substrate. Feldspars are the most abundant mineral group in the Earth’s crust, comprising about 60% of its mineral assemblages and are particularly prevalent within subduction zones and lower crustal sequences. These areas are known locations where ore systems are commonly rooted, making them among the most widely studied regions in the Earth. To date, few studies exist that cover the electrical behavior of the feldspar mineral albite. To help address some of these issues and complications, we have undertaken electrical conductivity investigations on a single crystal of gem-quality albite (Ab49An48Or3) from Nuevo Casas Grande, Chihuahua, Mexico. Electrical conductivity measurements using impedance spectroscopy were performed at a pressure of 1 GPa over a temperature range of 373–1273 K in a multi-anvil high-pressure apparatus. Experiments were carried out using different metal electrodes: molybdenum, nickel, and rhenium to vary the oxygen fugacity during the experiments. FTIR measurements of the starting and final materials confirm that the initial samples are completely dry but absorb an average of 67 ppm H2O by mass during the experiments from the surrounding pressure medium materials. We observe no correlation in the amount of water absorbed in the feldspar to the oxygen fugacity under water-undersaturated conditions. Our investigations show that the activation enthalpy increases from ~0.77 to ~1.0 eV from the nominally hydrous to the completely dry feldspar. The activation enthalpy decreases with increasing oxygen fugacity for comparable water contents. An oxygen fugacity exponent of –0.069 is calculated at the nominal water content measured in the experiments, indicating an electrical conductivity mechanism that also involves the mobility of hydrogen.

Low frequency bulk modulus of partially molten peridotite, KLB-1

We report experimental results on the bulk modulus of partially molten peridotite at mantle conditions, using a multi-anvil high pressure apparatus coupled with synchrotron X-ray radiation. A sinusoidal pressure at an amplitude of 180 MPa with period of 360 s is applied to the sample as a function of temperature. The sample volume derived from the X-ray radiograph also demonstrated sinusoidal variation at the same frequency. The derived bulk modulus from the amplitude of the stress and strain shows that the bulk modulus is drastically reduced at the onset of the melting, as predicated by dynamic melting model.

К вопросу о вхождении редкоземельных элементов в структуру хромпироповых гранатов

Rare earth elements (REE) in garnet are of interest in various fields of modern geology. The geochemistry of REEs in magmatic minerals is widely used in determining the distribution coefficients of crystal/melt and crystal/fluid, modeling the processes of melting and crystallization of magmatic rocks, studying deep mantle processes, age estimates and other issues of petrogenesis. The aim of the present work was a synthesis of a peridotite mineral association including the garnet containing REE at high pressure and high temperature. The initial sample consisted mainly of natural serpentine collected from ophiolites of the Eastern Sayan (Russia). As is known, the extreme stage of the regressive metamorphism of peridotites is serpentinization. It is depleted in calcium, but can recrystallize at high PT conditions into a harzburgite paragenesis, and at the initial stage of the experiment the chemical composition of the sample was a model harzburgite depleted in calcium and chromium, as well as a fluid of predominantly aqueous composition. As a source of chromium, chromite grains of 1–2 mm in size from peridotite xenoliths of the Udachnaya kimberlite pipe (Yakutia) were used. REE were added to the initial charge in the form of water-soluble salts. The experiment at a pressure of 5 GPa and temperature 1300 was performed on a multi-anvil high-pressure apparatus of the “split sphere” type (BARS) designed and developed at the V.S. Sobolev Institute of Geology and Mineralogy SB RAS. A container based on refractory oxide ZrO2 was used as a highpressure medium. The pressure in the cell before sample heating was estimated using the reference substances Bi and PbSe. The temperature was measured by a platinum-platinum-rhodium thermocouple PtRh30-PtRh6. The quenched was performed by switching off the voltage in the heater circuit. The experiment products contain an association of olivine + garnet + orthopyroxene + newly formed spinel. The predominant phase was olivine of a forsterite composition. A low-Fe orthopyroxene (1.49 – 1.68 wt% FeO) was found in elongated grains uniformly distributed throughout the sample. The newly formed spinel shows the faceted grains. The chromium content in the spinel significantly exceeds that of the initially added to the initial charge, 61.63 and 54.04 wt% Cr2O3, respectively. The garnet is characterized by a purple color, and was identified in the sample volume between olivine grains in the form of individual faceted crystals or their clusters. The largest garnets reached 0.5 mm in size. The synthesized garnet was determined as a high-Cr low-Ca pyrope variety. The contents of Cr2O3 and CaO are 10.15-11.21 and 0.06-0.11 wt%, respectively. The total content of REE in the garnet identified by the microprobe analysis is relatively high reaching 5-7 wt%. As a result of the work a mineral association corresponding to the peridotite paragenesis was obtained, including the subcalcic Crrich pyrope containing rare earth elements in significant amounts. It was estimated that their content in garnet mainly depends on the size of the ionic radius and, accordingly, on the atomic weight. This is consistent with the known facts about the preferable position of heavy REEs into the garnet structure compared to the light REEs.

Bulk modulus measurement at mHz frequencies and high-pressure using synchrotron x-ray radiation.

We report an experimental protocol for measuring the frequency dependence of the bulk modulus carried out in a synchrotron x-ray facility based multi-anvil high-pressure apparatus. An oscillating pressure perturbation characterized by x-ray diffraction produces a volume strain measured by imaging. Together, these yield the bulk modulus of the sample. Here, we report data at 3 mHz as an example of the possibility of providing these data for the frequency range of 1 mHz-100 mHz.

Experimental Investigation of Diopside Melt Viscosity at High Pressure

This paper presents the experimental evaluation of viscosity of the diopside-based model composition conducted at high P-T parameters (at the pressure of 4 GPa and in the temperature range of 1750°C - 1800°C) in the presence of olivine crystals. The experiments are carried out using the multi-anvil high-pressure apparatus of the “split-sphere” type (Russian acronym — BARS) according to the falling sphere method. The traveling time of a platinum (Pt) sphere in a melt is one of the parameters measured in experiments. Measurement of this parameter starts when the given P-T values are attained and stops when the electric current is turned off. There are three main positions of the Pt sphere observed in the experiments. Viscosity is calculated using the Stokes’ Law. It is found out that the Pt sphere velocity decreases expectedly as the relative viscosity of such heterogeneous compositions (liquid + solid phase) increases (in contrast to homogeneous melts). Viscosity values remain low when there is up to 7-10 wt-% of solid phase crystals in magma. The increase of olivine xenocrysts in magma leads to the progressive increase of viscosity values of the melt: by 1.5-2 orders of magnitude at 20-25 wt-%, by 3-3.5 orders of magnitude at 35-40 wt-%. The obtained experimental results allow concluding that the amount of solid phase in magma should be sufficiently low (less than 20-30 wt-%), otherwise, melts of the investigated composition can be moved only by explosive processes.

Decarbonation Reactions Involving Ankerite and Dolomite under upper Mantle P,T-Parameters: Experimental Modeling

An experimental study aimed at the modeling of dolomite- and ankerite-involving decarbonation reactions, resulting in the CO2 fluid release and crystallization of Ca, Mg, Fe garnets, was carried out at a wide range of pressures and temperatures of the upper mantle. Experiments were performed using a multi-anvil high-pressure apparatus of a “split-sphere” type, in CaMg(CO3)2-Al2O3-SiO2 and Ca(Mg,Fe)(CO3)2-Al2O3-SiO2 systems (pressures of 3.0, 6.3 and 7.5 GPa, temperature range of 950–1550 °C, hematite buffered high-pressure cell). It was experimentally shown that decarbonation in the dolomite-bearing system occurred at 1100 ± 20 °C (3.0 GPa), 1320 ± 20 °C (6.3 GPa), and 1450 ± 20 °C (7.5 GPa). As demonstrated by mass spectrometry, the fluid composition was pure CO2. Composition of synthesized garnet was Prp83Grs17, with main Raman spectroscopic modes at 368–369, 559–562, and 912–920 cm−1. Decarbonation reactions in the ankerite-bearing system were realized at 1000 ± 20 °C (3.0 GPa), 1250 ± 20 °C (6.3 GPa), and 1400 ± 20 °C (7.5 GPa). As a result, the garnet of Grs25Alm40Prp35 composition with main Raman peaks at 349–350, 552, and 906–907 cm−1 was crystallized. It has been experimentally shown that, in the Earth’s mantle, dolomite and ankerite enter decarbonation reactions to form Ca, Mg, Fe garnet + CO2 assemblage at temperatures ~175–500 °C lower than CaCO3 does at constant pressures.

Formation of Spessartine and CO2 via Rhodochrosite Decarbonation along a Hot Subduction P-T Path

Experimental simulation of rhodochrosite-involving decarbonation reactions resulting in the formation of spessartine and CO2-fluid was performed in a wide range of pressures (P) and temperatures (T) corresponding to a hot subduction P-T path. Experiments were carried out using a multi-anvil high-pressure apparatus of a “split-sphere” type (BARS) in an MnCO3–SiO2–Al2O3 system (3.0–7.5 GPa, 850–1250 °C and 40–100 h.) with a specially designed high-pressure hematite buffered cell. It was experimentally demonstrated that decarbonation in the MnCO3–SiO2–Al2O3 system occurred at 870 ± 20 °C (3.0 GPa), 1070 ± 20 °C (6.3 GPa), and 1170 ± 20 °C (7.5 GPa). Main Raman spectroscopic modes of the synthesized spessartine were 349–350 (R), 552(υ2), and 906–907 (υ1) cm−1. As evidenced by mass spectrometry (IRMS) analysis, the fluid composition corresponded to pure CO2. It has been experimentally shown that rhodochrosite consumption to form spessartine + CO2 can occur at conditions close to those of a hot subduction P-T path but are 300–350 °C lower than pyrope + CO2 formation parameters at constant pressures. We suppose that the presence of rhodocrosite in the subducting slab, even as solid solution with Mg,Ca-carbonates, would result in a decrease of the decarbonation temperatures. Rhodochrosite decarbonation is an important reaction to explain the relationship between Mn-rich garnets and diamonds with subduction/crustal isotopic signature.

A rapid-quench technique for multi-anvil high-pressure-temperature experiments.

In order to extend the pressure and compositional range where silicate melts can be quenched to form glass in a multi-anvil high-pressure and high-temperature apparatus, a rapid-quench technique, which includes an external cooling system and a low thermal-inertia assembly, was developed. This technique allows much higher cooling rates (6000-7000 °C/s) than regular piston-cylinder (130 °C/s) apparatus and multi-anvil (650 °C/s) apparatus, which are widely used in solid Earth science. Such high cooling rates are critical to avoid unwanted changes in a sample, such as melt crystallization and volatile loss, during quenching.