Estimation of Accumulation of Chromium Ion Impurities in the CaSiO3 and MgSiO3 Phases of the Earth’s Lower Mantle at Pressures of 18–25 GPa

Doklady Earth Sciences - Based on the data of atomic modeling, the different schemes of isomorphic incorporation of Cr3+ ions into the crystal structures of CaSiO3 and MgSiO3 were tested in the...     

Thermal properties of harzburgite and dunite at 0.8–3 GPa and 300–823 K and implications for the thermal evolution of Tibet

Thermal diffusivity(D)and thermal conductivity(κ)of harzburgite and dunite from Luobusha ophiolite were simultaneously measured up to 3 GPa and 823 K using the transient plane-source method in a multi anvil apparatus.The results show that the values of D andκof both samples systematically decrease with increasing temperature and increase with increasing pressure.By combination of the thermal physical data of rocks and minerals and geophysical constraints,we performed numerical simulation on the thermal evolution of Tibet vary over depth,distance and geologic ages.The present results provide new constraints on occurrence of partial melting and its geophysical significance beneath Tibetan crust.     

Crystal structures of Zn2SiO4 III and IV synthesized at 6.5–8 GPa and 1,273 K

We report the crystal structures determined under ambient condition for two Zn₂SiO₄ polymorphs synthesized at 6.5 GPa and 1,273 K (phase III) and 8 GPa and 1,273 K (phase IV) and also compare their ²⁹Si MAS NMR spectroscopic characteristics with those of other Zn₂SiO₄ polymorphs (phases I, II and V). Electron microprobe analysis revealed that both of phases III and IV are stoichiometric like the lower-pressure polymorphs (phases I and II), contrary to previous report. The crystal structures were solved using an ab initio structure determination technique from synchrotron powder X-ray diffraction data utilizing local structural information from ²⁹Si MAS NMR as constraints and were further refined with the Rietveld technique. Phase III is orthorhombic (Pnma) with a = 10.2897(5), b = 6.6711(3), c = 5.0691(2) Å. It is isostructural with the high-temperature (Zn_1.1Li_0.6Si_0.3)SiO₄ phase and may be regarded as a ‘tetrahedral olivine’ type that resembles the ‘octahedral olivine’ structure in the (approximately hexagonally close packed) oxygen arrangement and tetrahedral Si positions, but has Zn in tetrahedral, rather than octahedral coordination. Phase IV is orthorhombic (Pbca) with a = 10.9179(4), b = 9.6728(4), c = 6.1184(2) Å. It also consists of tetrahedrally coordinated Zn and Si and features unique edge-shared Zn₂O₆ dimers. The volumes per formula under ambient condition for phases III and IV are both somewhat larger than that of the lower-pressure polymorph, phase II, suggesting that the two phases may have undergone structural changes during temperature quench and/or pressure release.     

Growth of ringwoodite reaction rims from MgSiO3 perovskite and periclase at 22.5 GPa and 1,800 °C

The growth rate of ringwoodite reaction rims between MgSiO₃ perovskite and periclase was investigated at 22.5 GPa and 1,800 °C for 1–24 h using the Kawai-type high-pressure apparatus. The reaction was likely to proceed by a diffusion-controlled mechanism in which the dominant diffusion mechanism was grain-boundary diffusion. The reaction constant (the width of the ringwoodite reaction rim squared divided by time) determined from these experiments was between 1.3 × 10^?15 and 5.6 × 10^?15 m²/s. A Pt inert marker experiment indicated that the MgO component migrated faster than the SiO₂ component in ringwoodite. Thus, either Mg or O having the slower diffusion rate controlled the reaction. Because previous diffusion studies have shown that diffusion rates of O are slower than those of Mg, O would be a rate-controlling element for ringwoodite formation from MgSiO₃ perovskite and periclase. The growth rate appeared to be too fast to explain the observed topographic rise (~10 km) inside mantle plumes at the 660-km discontinuity.     

Formation of a solid solution in the MgSiO3–MnSiO3 perovskite system

Experiments using laser-heated diamond anvil cells combined with synchrotron X-ray diffraction and SEM–EDS chemical analyses have confirmed the existence of a complete solid solution in the MgSiO₃–MnSiO₃ perovskite system at high pressure and high temperature. The (Mg, Mn)SiO₃ perovskite produced is orthorhombic, and a linear relationship between the unit cell parameters of this perovskite and the proportion of MnSiO₃ components incorporated seems to obey Vegard’s rule at about 50 GPa. The orthorhombic distortion, judged from the axial ratios of a/b and \( \sqrt{2}\,a/c, \) monotonically decreases from MgSiO₃ to MnSiO₃ perovskite at about 50 GPa. The orthorhombic distortion in (Mg_0.5, Mn_0.5)SiO₃ perovskite is almost unchanged with increasing pressure from 30 to 50 GPa. On the other hand, that distortion in (Mg_0.9, Mn_0.1)SiO₃ perovskite increases with pressure. (Mg, Mn)SiO₃ perovskite incorporating less than 10 mol% of MnSiO₃ component is quenchable. A value of the bulk modulus of 256(2) GPa with a fixed first pressure derivative of four is obtained for (Mg_0.9, Mn_0.1)SiO₃. MnSiO₃ is the first chemical component confirmed to form a complete solid solution with MgSiO₃ perovskite at the P–T conditions present in the lower mantle.     

Congruent melting of calcium carbonate in a static experiment at 3500 K and 10–22 GPa: Its role in the genesis of ultradeep diamonds

Resulting from static experiments performed to study the phase state of CaCO₃, it was found that its melting is congruent at 20–22 GPa and 3500 K. The obtained experiment data show that the field of congruent melting of calcium carbonate is rather broad (form 2300 to 3500–3800 K at 20–22 GPa). However, the potential presence of a high-temperature phase boundary at which CaCO₃ is decomposed into CaO and CO₂ is not ruled out. The existence of a wide area of congruent melting of calcium carbonate (a common primary inclusion in diamonds of the transition zone and lower mantle of the Earth) allow one to consider deep-seated melts as potential parental media for ultradeep diamonds.     

A Preliminary Study of the Solubility of Copper in Water Vapor at Elevated Temperatures and Pressures

INTRODUCTIONAs early as 1644 , Ren啨Descartes proposed thatsilver-,gold-,and lead-bearing veins were producedby the condensation of vapors emanating from theearth' s interior ( Williams-Jones et al ., 2002) . Butthe solubility of many metals is higher in liquid thanin vapor andtherefore the focus of research on hydro-thermal ore deposits was placed on hydrothermal sys-tems (Barnes ,1997) ,considering the fact that ore-forming elements may exist as complexes in hydro-thermal solutions . T…     

Formation of Water-Bearing Defects in Olivine in the Presence of Water–Hydrocarbon Fluid at 6.3 GPa and 1200°C

The main trends of water dissolution in Fe-bearing olivine have been investigated in the olivine–H₂O–hydrocarbon fluid system in experiments at a pressure of 6.3 GPa, a temperature of 1200°C, and hydrogen fugacity ( fH₂) buffered by the Mo–MoO₂ equilibrium. The content and contribution of ОH defects of different types in Fe-bearing olivines depend on the composition of reduced fluids in the system. As the fraction of hydrocarbons in the fluid increases, the H₂O content in olivine crystals decreases from 900 to 160–180 ppm, while the ОН absorption peaks become lower at high frequencies and occupy a larger part of the infrared spectrum in the low-frequency region. According to the experimental results, even the deepest seated mantle olivines with OH defects were not equilibrated with a fluid rich in light alkanes or oxygenated hydrocarbons.     

Partitioning of Mg, Ca, and Na between carbonatite melt and hydrous fluid at 0.1–0.2 GPa

Experimental studies of the element distribution between carbonatite melts and hydrous fluids are hampered by the fact that neither the fluid nor the melt can be isochemically quenched in conventional high-pressure vessels. In order to overcome this problem, we used a double-capsule technique to separate immiscible fluid and melt phases during and after the runs. The inner platinum capsules were charged with carbonate mixtures (CaCO₃, MgCO₃ and Na₂CO₃) and placed inside the outer capsules charged with distilled water and diamond powder. The latter was used as an inert trap for solids precipitating from the fluid on quenching. Carbonate melt and hydrous fluid equilibrated through a small hole left in the upper end of the inner capsule. The runs were performed in rapid-quench cold-seal pressure vessels at 0.1–0.2 GPa and 700–900 °C in the two-phase (fluid + melt) stability region. Both quenched melt and quenched fluid were dissolved in dilute HCl and analysed by inductively coupled plasma atomic emission spectroscopy. The results show that under all conditions investigated, fluid/melt partition coefficients for Ca and Mg are similar and several times smaller than those for Na. At 0.1 GPa and a water/carbonatite ratio of 1 (by weight), the partition coefficients are D_Na = 0.35 ± 0.02, D_Ca = 0.09 ± 0.02, and D_Mg = 0.13 ± 0.01. Between 700 and 900 °C, the effect of temperature on partitioning is negligible. However, D_Na increases significantly with decreasing water/carbonatite ratio in the system. Our data show that the release of a hydrous fluid enriched in sodium and simultaneous crystallisation of calcite can transform an alkaline, vapour-saturated carbonatite melt into a body of pure calcite surrounded by zones of sodium metasomatism. Thus, it is quite possible that carbonate magmas with substantial amounts of alkalies were common parental liquids of plutonic carbonatites. Received: 6 May 1999 / Accepted: 31 August 1999     

Phase relations and formation of sodium-rich majoritic garnet in the system Mg3Al2Si3O12–Na2MgSi5O12 at 7.0 and 8.5 GPa

The pseudo-binary system Mg₃Al₂Si₃O₁₂–Na₂MgSi₅O₁₂ modelling the sodium-bearing garnet solid solutions has been studied at 7 and 8.5 GPa and 1,500–1,950°C. The Na-bearing garnet is a liquidus phase of the system up to 60 mol% Na₂MgSi₅O₁₂ (NaGrt). At higher content of NaGrt in the system, enstatite (up to ∼80 mol%) and then coesite are observed as liquidus phases. Our experiments provided evidence for a stable sodium incorporation in garnet (0.3–0.6 wt% Na₂O) and its control by temperature and pressure. The highest sodium contents were obtained in experiments at P = 8.5 GPa. Near the liquidus (T = 1,840°C), the equilibrium concentration of Na₂O in garnet is 0.7–0.8 wt% (∼6 mol% Na₂MgSi₅O₁₂). With the temperature decrease, Na concentration in Grt increases, and the maximal Na₂MgSi₅O₁₂ content of ∼12 mol% (1.52 wt% Na₂O) is gained at the solidus of the system (T = 1,760°С). The data obtained show that most of natural diamonds, with inclusions of Na-bearing garnets usually containing <0.4 wt% Na₂O, could be formed from sodium-rich melts at pressures lower than 7 GPa. Majoritic garnets with higher sodium concentrations (>1 wt% Na₂O) may crystallize at a pressure range of 7.0–8.5 GPa. However the upper pressure limit for the formation of naturally occurring Na-bearing garnets is restricted by the eclogite/garnetite bulk composition.     

Reactions of iron with calcium carbonate at 6 GPa and 1273–1873 K: implications for carbonate reduction in the deep mantle

Experimental data on Fe-CaCO₃ interaction at 6 GPa and 1273–1873 K are presented. The system models the hypothetical redox interaction in subducting slabs at the contact with the reduced mantle and a putative process at the core-mantle boundary. The reaction is accompanied by carbonatite melt formation. It also produces Fe3C and calcium wustite, which form solid or liquid phases depending on experimental conditions. In iron-containing systems at 6 GPa, calcium carbonate melts in the range 1473–1573 K, which is consistent with aragonite disappearance from complex carbonate systems. The composition of calcium carbonate liquid is not influenced by metallic Fe. It corresponds to nearly pure CaCO₃. Along the mantle adiabat or at slightly higher temperatures, nearly pure CaCO₃ coexists with metallic iron or calcium wustite. This hypothesis explains the coexistence of metallic iron and carbonate inclusions in lithospheric and superdeep diamonds.     

Experimental study of the K2ZrSi3O9 (wadeite)–K2TiSi3O9 and K2(Zr,Ti)Si3O9–phlogopite systems at 2–3 GPa

Experiments ranging from 2 to 3 GPa and 800 to 1300 °C and at 0.15 GPa and 770 °C were performed to investigate the stability and mutual solubility of the K₂ZrSi₃O₉ (wadeite) and K₂TiSi₃O₉ cyclosilicates under upper mantle conditions. The K₂ZrSi₃O₉–K₂TiSi₃O₉ join exhibits complete miscibility in the P–T interval investigated. With increasing degree of melting the solid solution becomes progressively enriched in Zr, indicating that K₂ZrSi₃O₉ is the more refractory end member. At 2 GPa, in the more complex K₂ZrSi₃O₉–K₂TiSi₃O₉–K₂Mg₆Al₂Si₆O₂₀(OH)₄ system, the presence of phlogopite clearly limits the extent of solid solution of the cyclosilicate to more Zr-rich compositions [Zr/(Zr + Ti) > 0.85], comparable to wadeite found in nature, with TiO₂ partitioning strongly into the coexisting mica and/or liquid. However, at 1200 °C, with increasing pressure from 2 to 3 GPa, the partitioning behaviour of TiO₂ changes in favour of the cyclosilicate, with Zr/(Zr + Ti) of the K₂(Zr,Ti)Si₃O₉ phase decreasing from ∼0.9 to ∼0.6. The variation in the Ti content of the coexisting phlogopite is related to its degree of melting to forsterite and liquid, following the major substitution ^VITi+^VI□=2^VIMg. Received: 26 January 1999 / Accepted: 10 January 2000     

Time Variations in the Fluxes of Suprathermal Ions and Their Relative Abundances at 1 AU during 1998–2017

Astronomy Reports - Time variations in the fluxes of the suprathermal (0.04–2 MeV/nucleon) ions 3He, 4He, C, O, and Fe are studied together with their relative abundances in the 23rd and 24th...     

Estimation of the 3-D velocity structure of the Sorachi–Yezo and Oshima Belts in the western part of Hokkaido, Japan

We adopted the seismic tomography technique to refine the three-dimensional velocity structure model of the western part of Hokkaido, Japan. Using the P-wave first arrival data listed by Japan Meteorological Agency from 2002 to 2005, we could estimate a 3-D inhomogeneous velocity structure model with a low velocity at a depth of 14 km beneath Asahikawa. The crustal structure near Sapporo was characterized by lateral velocity change toward the southern seaside. The low-velocity zone near Urakawa, proposed by previous research, was also clarified. In general, the present model showed lower-velocity values for most of the crustal layers in the area concerned. The results of this study were affected by less number of higher magnitude events (M?≥?0.5) in the central part of the area of interest. However, the perturbation results for comparatively shallow layers (6–50 km) were good in resolution. It was found that the source region of the Rumoi–Nanbu earthquake of December 14, 2004 was characterized by a low-velocity zone, located between high velocity zones. Such an inhomogeneous crustal structure might play an important role in the relatively high seismic activity in the Rumoi–Nanbu earthquake source region.     

The effects of K2O on the compositions of near-solidus melts of garnet peridotite at 3 GPa and the origin of basalts from enriched mantle

Enrichment in K₂O in oceanic island basalts (OIB) is correlated with high SiO₂, low CaO/Al₂O₃, and radiogenic isotopic signatures indicative of enriched mantle sources (EM1 and EM2). These are also chemical characteristics of the petit-spot lavas, which are highly enriched in K₂O (3–4 wt%) compared to other primitive oceanic basalts. We present experimentally derived liquids with varying concentrations of K₂O in equilibrium with a garnet lherzolite residue at 3 GPa to test the hypothesis that the major element characteristics of EM-type basalts are related to their enrichment in K₂O. SiO₂ is known to increase with K₂O at pressures less than 3 GPa, but it was previously unknown if this effect was significant at the high pressures associated with partial melting at the base of the lithosphere. We find that at 3 GPa for each 1 wt% increase in the K₂O content of a garnet lherzolite saturated melt, SiO₂ increases by ~0.5 wt% and CaO decreases by ~0.5 wt%. MgO and $K_{D}^{{{\text{Fe}} - {\text{Mg}}}}$ K D Fe - Mg each decrease slightly with K₂O concentration, as do Na₂O and Cr₂O₃. The effect of K₂O alone is not strong enough to account for the SiO₂ and CaO signatures associated with high-K₂O OIB. The SiO₂, CaO, and K₂O concentrations of experimentally derived partial melts presented here resemble those of petit-spot lavas, but the Al₂O₃ concentrations from the experimental melts are greater. Partitioning of K₂O between peridotite and melt suggests that petit spots, previously considered to sample ambient asthenosphere, require a source more enriched in K₂O than the MORB source.     

Phase Relations in the Harzburgite–Hydrous Carbonate Melt at 5.5–7.5 GPa and 1200–1350°С

Phase relations are studied experimentally in the harzburgite–hydrous carbonate melt system, the bulk composition of which represents primary kimberlite. Experiments were carried out at 5.5 and 7.5 GPa, 1200–1350°С, and \({{X}_{{{\text{C}}{{{\text{O}}}_{2}}}}}\) = 0.39–0.57, and lasted 60 hours. It is established that olivine–orthopyroxene–garnet–magnesite–melt assemblage is stable within the entire range of the studied parameters. With increase of temperature and \({{X}_{{{\text{C}}{{{\text{O}}}_{2}}}}}\) in the system, Ca# in the melt and the olivine fraction in the peridotite matrix significantly decrease. The composition of silicate phases in run products is close to those of high-temperature mantle peridotite. Analysis of obtained data suggest that magnesite at the base of subcontinental lithosphere could be derived by metasomatic alteration of peridotite by asthenospheric hydrous carbonate melts. The process is possible in the temperature range typical of heat flux of 40–45 mW/m², which corresponds to the conditions of formation of the deepest peridotite xenoliths. Crystallization of magnesite during interaction with peridotite matrix can be considered as experimentally substantiated mechanism of CO₂ accumulation in subcratonic lithosphere.     

Experimental study of the pyroxene-garnet phase transition in the Na2MgSi5O12 system at pressures of 13–20 GPa: First synthesis of sodium majorite

Doklady Earth Sciences -     

Stability of methane in reduced C–O–H fluid at 6.3 GPa and 1300–1400°C

The composition of a reduced C–O–H fluid was studied by the method of chromatography–mass spectrometry under the conditions of 6.3 GPa, 1300–1400°C, and fO₂ typical of the base of the subcratonic lithosphere. Fluids containing water (4.4–96.3 rel. %), methane (37.6–0.06 rel. %), and variable concentrations of ethane, propane, and butane were obtained in experiments. With increasing fO₂, the proportion of the CH₄/C₂H₆ peak areas on chromatograms first increases and then decreases, whereas the CH₄/C₃H₈ and CH₄/C₄H₁₀ ratios continually decrease. The new data show that ethane and heavier HCs may be more stable to oxidation, than previously thought. Therefore, when reduced fluids pass the “redox-front,” carbon is not completely released from the fluid and may be involved in diamond formation.     

Effect of sulfur concentration on diamond crystallization in the Fe–C–S system at 5.3–5.5 GPa and 1300–1370°C

The paper presents results of experiments aimed at diamond synthesis in the Fe–C–S system at 5.3–5.5 GPa and temperatures of 1300–1370°C and detailed data on the microtextures of the experimental samples and the composition of the accompanying phases (Fe₃C and Fe₇C₃ carbides, graphite, and FeS). It is demonstrated that diamond can be synthesized after temperatures at which carbides are formed are overcome and can crystallize within the temperature range of 1300°C (temperature of the peritectic reaction melt + diamond = Fe₇C₃) to 1370°C (of thermodynamically stable graphite) under the appearance experimental pressure. The possible involvement of natural metal- and sulfur-bearing compounds in the origin of natural diamond is discussed.