The effect of oxygen fugacity on the solubility of carbon-oxygen fluids in basaltic melt

The solubility of CO₂CO fluids in a mid-ocean ridge basalt (morb) has been measured at 1200°C, 500–1500 bar, and oxygen fugacities between NNO and NNO-4. High oxygen fugacities, and thus CO₂-rich fluids, were produced by using a starting material equilibrated at NNO, and Ag₂C₂O₄ as the fluid source. Low oxygen fugacities were achieved by using graphite capsules, and MgCO₃ as the fluid source. These graphite-saturated fluids have the lowest possibleC/O₂CO ratio for a given pressure and temperature.

Experiments were run in a rapid-quench internally heated pressure vessel. Fluid compositions were measured using a simple vacuum technique and by Raman spectroscopy of fluid inclusions. The two techniques yielded comparable results. Fourier transform micro-infrared spectroscopy was used to identify and measure concentrations of dissolved volatiles in double-polished wafers of the quenched glasses. Carbonate was the only carbon-bearing species identified. Raman spectroscopic analysis of inclusion-free areas of glass confirmed the absence of dissolved molecular CO₂, CO and carbon. The measured concentrations of dissolved CO₂ in the glasses were proportional to the fugacity of CO₂ during the experiments, calculated from the measured fluid compositions. The data were fit to the equationX_CO2^melt(ppm)= 0.492 fCO₂ (bar).

The insolubility of CO, compared to CO₂, may be related to the fact that dissolution of CO requires reduction of another species in the melt, whereas dissolution of CO₂ does not. Due to the fact that CO will be an important component of natural CO fluids at low pressures and low oxygen fugacities, equilibrium dissolved CO₂ contents will be less than calculated assuming pure CO₂ fluids, but as theC/O₂CO ratio in a pure CO fluid at fixed pressure and temperature is a direct function of oxygen fugacity, measurement of the oxygen fugacity of quenched glasses or trapped fluids in natural samples should allow saturation concentrations to be calculated. Dissolved CO₂ contents of somemorb are less than expected if they were in equilibrium with pure CO₂. These samples must, therefore, have been more reduced than average if they were fluid-saturated. Together with results from other studies of CO₂ and H₂O solubilities in basalt, the results of this study provide a comprehensive framework for modelling CO₂ solution inmorb.     

Rare earth element systematics of hydrous liquids from partial melting of basaltic eclogite a re-evaluation

The origin of orogenic andesitic magmas is tested by calculations of REE fractionation in hydrous melts derived from partial melting of subducted ocean basalt in eclogite facies. New data on the subsolidus phase proportions of basaltic eclogite, the enrichment of LREE in altered ocean basalts, and experimentally determined REE partition coefficients (K_D's) between garnet and melt have been included in trace element fractionation equations. Non-modal melting of phases combined with variation inK_D's during melting is a unique feature of these calculations.Variation ofK_D, melting proportions, initial proportion of subsolidus phases, degree of melting, and initial REE concentrations yield a wide range of input parameters that produce REE profiles in partial melts of basaltic eclogite matching REE profiles of some orogenic andesites. The positive correlation of REE concentration with silica content for many andesitic suites can be accounted for by non-modal melting if quartz (or a similar phase with low REEK_D values) melts at a high melting proportion and garnet melts at a low melting proportion during the first stages of fusion. However, no mineralogic fractionation scheme can account for REE/silica systematics if REEK_D values are linearly decreasing with increasing melting. Earlier workers who have used similar calculations to discredit the eclogite fractionation model have set overly strict, and sometimes incorrect, constraints concerning the range in REEK_D values for garnet, the subsolidus proportions of phases in basaltic eclogite, and the relative concentrations of REE in subducted ocean crust undergoing partial melting.     

The solubility of iron sulphides in synthetic and natural waters at ambient temperature

A critical evaluation of literature values for the solubility products, K _sp ^NBS = [Fe²⁺][HS^–] Fe²⁺ HS^– (H _NBS ⁺ )^–1, of various iron sulphide phases results in consensus values for the pKs of 2.95 ± 0.1 for amorphous ferrous sulphide, 3.6 ± 0.2 for mackinawite, 4.4 ± 0.1 for greigite, 5.1 ± 0.1 for pyrrhotite, 5.25 ± 0.2 for troilite and 16.4 ± 1.2 for pyrite.Where the analogous ion activity products have been measured in anoxic freshwaters in which there is evidence for the presence of solid phase FeS, the values lie within the range of 2.6–3.22, indicating that amorphous iron sulphide is the controlling phase. The single value for a groundwater of 2.65 (2.98 considering carbonate complexation) agrees. In seawater four values range between 3.85 to 4.2, indicating that mackinawite or greigite may be the controlling phase. The single low value of 2.94 is in a situation where particularly high fluxes of Fe (II) and S (–II) may result in the preferential precipitation of amorphous iron sulphide. Formation of framboidal pyrite in these sulphidic environments may occur in micro-niches and does not appear to influence bulk concentrations. Calculations show that the formation of Fe₂S₂ species probably accounts for very little of the iron or sulphide in most natural waters. Previously reported stability constants for the formation of Fe (HS)₂ and (Fe (HS)₃)^– are shown to be suspect, and these species are also thought to be negligible in natural waters. In completely anoxic pore waters polysulphides also have a negligible effect on speciation, but in tidal sediments they may reach appreciable concentrations and lead to the direct formation of pyrite. Concentrations of iron and sulphide in pore waters can be controlled by the more soluble iron sulphide phase. The change in the IAP with depth within the sediment may reflect ageing of the solid phase or a greater flux of Fe (II) and S (–II) nearer the sediment surface. This possible kinetic influence on the value of IAPs has implications for their use in geochemical studies involving phase formation.     

Volcanic and atmospheric controls on ash iron solubility: A review

The ash material produced by volcanic eruptions carries important information about the underground magma eruptive conditions and subsequent modifications in the volcanic plume and during atmospheric transport. Volcanic ash is also studied because of its impacts on the environment and human health. In particular, there is a growing interest from a multidisciplinary scientific community to understand the role that ash deposition over open ocean regions may play as a source of bioavailable Fe for phytoplankton production. Similar to aeolian mineral dust, the processes that affect the mineralogy and speciation of Fe in ash may promote solubilisation of Fe in ash, and thus may increase the amount of volcanic Fe supplied to ocean surface waters. Our knowledge of these controls is still very limited, a situation which has hindered quantitative interpretation of experimental Fe release measurements. In this review, we identify the key volcanic and atmospheric controls that are likely to modulate ash Fe solubility. We also briefly discuss existing data on Fe release from ash and make some recommendations for future studies in this area.     

He diffusion in olivine

Helium diffusion in olivine (dunite xenolith) has been measured in the temperature range 1180–1460°C; a linear Arrhenius function was obtained with an activation energy of 120₋₂₇⁺³² kcal/mole, and a pre-exponential factor (D₀) = 2.2 × 10⁸ cm²/s.

Diffusion mechanisms are not a viable means of degassing He from the mantle. Olivine phenocrysts can be expected to retain previously trapped He, during cooling in extrusive basalts, provided the flow units are thinner than 50 m; xenoliths will retain mantle He signatures only if magma transport times are less than 50 years, or if the He fugacity in the magma is high enough to prevent xenolith degassing. The lower oceanic crust is probably substantially degassed of He. Trapped He will be qualitatively retained in quenched submarine basalt glass only if the cooling rate is faster than 5 × 10¹⁴°C/m.y.; glass at several centimeters depth in a basalt flow (near the spherulite zone) will have cooling rates lower than this, so He loss may be significant in many basalt glass samples.     

Magma-water interactions in subaqueous and emergent basaltic

In the subaqueous growth and emergence of a basaltic volcano clasts are formed by one or a combination of (1) explosive release of magmatic volatiles; (2) explosive expansion and collapse of steam formed at magma-water contact surfaces; (3) explosive expansion of steam following enclosure of water in magma, or entrapment of water close to magma; and (4) cooling-contraction. These processes, named respectivelymagmatic explosivity, contact-surface steam explosivity, bulk interaction steam explosivity, andcooling-contraction granulation, can be enhanced by mutual interaction and feedback. The first three (explosive) processes are limited at certain water depths (hydrostatic pressures) and become increasingly vigorous at shallower levels. The depth of onset of magmatic explosivity depends largely on juvenile volatile content; it is up to 200 m for tholeiitic magmas and up to 1 km for alkalic magmas. At the depth where formation of clastic deposits becomes predominant over effusion of lavas, magmatic explosivity is subordinate to steam explosivity as a clast-forming process. The upward transition to accumulation of dominantly clastic deposits is not simply related to the onset of substantial exsolution of magmatic volatiles and can occur without it. Contact-surface explosivity commonly requires initiation by a vigorous impact between magma and water and, although no certain depth limit is known, likelihood of such explosivity decreases rapidly with depth. Clast generation by bulk interaction explosivity appears to be restricted to depths much shallower than that of the critical pressure of water, which in sea water is at about 3 km. Cooling-contraction granulation can occur in any depth of water, but at shallow levels may be replaced by contact-surface explosivity. During continuous eruption under water, tephra can be ejected and deposited within a cupola of steam such that rapid quenching does not occur. Emergent volcanoes are characterized by distinctive steam-explosive activity that results primarily from a bulk interaction between rapidly ascending magma and a highly mobile slurry of clastic material, water, and steam. The water gets into the vent by flooding across or through the top of the tephra pile, and violent explosions cease when this access is sealed. The eruptions during emergence of Surtsey and Capelinhos typify the distinctive explosive activity, the style and controls of which are different from those of maar volcanoes.     

Homologous temperature of olivine: Implications for creep of the upper mantle and fabric transitions in olivine

The homologues temperature of a crystalline material is defined as T/T_m, where T is temperature and T_m is the melting(solidus) temperature in Kelvin. It has been widely used to compare the creep strength of crystalline materials. The melting temperature of olivine system,(Mg,Fe)_2SiO_4, decreases with increasing iron content and water content, and increases with confining pressure. At high pressure, phase transition will lead to a sharp change in the melting curve of olivine. After calibrating previous melting experiments on fayalite(Fe_2SiO_4), the triple point of fayalite-Fe_2SiO_4 spinel-liquid is determined to be at 6.4 GPa and 1793 K. Using the generalized means, the solidus and liquidus of dry olivine are described as a function of iron content and pressure up to 6.4 GPa. The change of T/T_m of olivine with depth allows us to compare the strength of the upper mantle with different thermal states and olivine composition. The transition from semi-brittle to ductile deformation in the upper mantle occurs at a depth where T/T_m of olivine equals 0.5. The lithospheric mantle beneath cratons shows much smaller T/T_m of olivine than orogens and extensional basins until the lithosphere-asthenosphere boundary where T/T_m 0.66, suggesting a stronger lithosphere beneath cratons. In addition, T/T_m is used to analyze deformation experiments on olivine. The results indicate that the effect of water on fabric transitions in olivine is closely related with pressure. The hydrogen-weakening effect and its relationship with T/T_m of olivine need further investigation. Below 6.4 GPa(200 km), T/T_m of olivine controls the transition of dislocation glide from [100] slip to [001] slip. Under the strain rate of 10~(-12)–10~(-15) s~(-1) and low stress in the upper mantle, the [100](010) slip system(A-type fabric) becomes dominant when T/T_m 0.55–0.60. When T/T_m 0.55–0.60, [001] slip is easier and low T/T_m favors the operation of [001](100) slip system(C-type fabric). This is consistent with the widely observed A-type olivine fabric in naturally deformed peridotites, and the C-type olivine fabric in peridotites that experienced deep subduction in ultrahigh-pressure metamorphic terranes. However, the B-type fabric will develop under high stress and relatively low T/T_m. Therefore, the homologues temperature of olivine established a bridge to extrapolate deformation experiments to rheology of the upper mantle. Seismic anisotropy of the upper mantle beneath cratons should be simulated using a four-layer model with the relic A-type fabric in the upper lithospheric mantle, the B-type fabric in the middle layer, the newly formed A- or B-type fabric near the lithosphere-asthenosphere boundary, and the asthenosphere dominated by diffusion creep below the Lehmann discontinuity. Knowledge about transition mechanisms of olivine fabrics is critical for tracing the water distribution and mantle flow from seismic anisotropy.     

Point-defect formation parameters in olivine

The formation internal energy and volume of the point-defect species in olivine are derived for a number of approximate charge-neutrality conditions. The formation parameters of an effectively charged point defect are a function of the approximate condition of charge neutrality. Since the approximate charge-neutrality condition depends upon the partial pressure of oxygen and the activity of enstatite in olivine, changes in the magnitude of these thermodynamic-state variables result in different values of the formation parameters if the charge-neutrality regime is altered. The fractional change in the formation internal energies of the energetically unfavorable defects, such as Si vacancies, is small between the charge-neutrality regions while the fractional change for the favorable defects, such as Mg vacancies, is large.     

橄榄石矿物弹性研究进展

橄榄石是上地幔含量最丰富的矿物,对其弹性性质的研究对解释地球内部的物质组成和动力学过程至关重要.近年来,对橄榄石矿物弹性的研究,无论是实验测试还是模拟计算都取得了很大的进展.本文首先介绍了研究橄榄石弹性的主要方法,然后从铁、镁不同配分比例下橄榄石的弹性性质,水对橄榄石弹性的影响以及橄榄石各向异性等三个方面,对近年来橄榄石弹性研究所取得的主要进展做了介绍.实验方法方面主要介绍了1)超声共振谱法(Resonant Ultrasound Spectroscopy,RUS);2)X射线衍射法;3)X射线照相法;4)超声干涉法;5)布里渊散射法.     

Thermal diffusivity of olivine

The thermal diffusivity of a naturally occurring polycrystalline olivine (Fo₉₁Fa₀₉) was measured by the Flash technique in the temperature range of 450–1500 K. At 450 K the thermal diffusivity was 10.7 × 10^?7 m²/s and decreased as a function of reciprocal temperature to 7.0 × 10^?7 m²/s at 800 K. From that temperature, the values gradually increased to a maximum of 7.8 × 10^?7 m²/s at 1000 K, and then steadily decreased to 5.6 × 10^?7 m²/s at 1500 K. The unusual decrease above 1000 K was caused by a reduction of the previously oxidized samples. The olivine's oxidation state plays a significant role in the value of thermal diffusivity at high temperatures.     

Clinker formation in basaltic and trachybasaltic lava flows

Clinker is a term used to describe massive or scoriaceous fragments commonly associated with ‘a‘ā lava flows. Clinker is generally considered to form by fragmentation of an upper vesiculated crust, due to an increase in apparent viscosity and/or to an increase in shear strain rate. Surface clinker is considered to be transported to the flow front and incorporated at the base by caterpillar motion. Clinker that we have observed on a variety of lava flows has very variable textures, which suggests several different mechanisms of formation. In order to study clinker formation, we examined several lava flows from the Chaîne des Puys Central France, where good sections, surface morphology and surface textures are widespread and clearly visible. We observed basal and surface ‘a‘ā clinker that has fragmentation textures similar to those observed in ash formed in eruptions under dry conditions. In two pāhoehoe flows we have observed basal clinker that formed in-situ. Two other flows display clinker features identical to those commonly observed in phreatomagmatic ash, such as adhering particles, blocky shapes, spherical glass and attached microphenocrysts. Another pāhoehoe flow has a flakey, angular basal breccia, with microfaulted and abraded clasts. These were probably formed at a cooled lava base by large amounts of simple shear and consequent intra-lava brittle faulting. Using these observations we propose three different ways of fragmentation. (1) Clinker can form at the surface and eventually produce roll-over basal breccia. (2) Water/lava interactions can form basal clinker by phreatomagmatic fragmentation. Water/lava ratio variations may produce different clinker structures, in a manner similar to observed textural changes in phreatomagmatic eruptions. (3) Clinker can be formed by brittle brecciation during basal simple shear. The different clinker can provide information about the mechanisms and environmental conditions during lava flow emplacement.     

Segregation structures in vapor-differentiated basaltic flows

 Vesicle cylinders represent a spectacular kind of segregation structure involving residual liquids formed in situ during the cooling of lava flows. These vertical pipes, commonly found within basalt flows typically 2–10 m thick, are interpreted as the product of a vapor-driven differentiation process. The olivine phenocrysts and the earliest generation of groundmass olivines found in cylinder-bearing basalts appear to have been generally affected by magmatic oxidation, resulting in high-temperature iddingsite (HTI) alteration. This feature is also observed within cylinder-free basalt flows which exhibit other kinds of vesicular segregation structures, such as vesicle-rich pegmatoid segregation sheets and/or segregation vesicles. Detailed textural, petrological, and geochemical characteristics of two types of cylinders, three types of vesicle sheets, and five types of segregation vesicles are described, based on the study of 12 occurrences of HTI-bearing basalt flows from oceanic shield volcanoes or continental basalt plateaus. We propose a general classification of these segregation structures likely to derive from vapor differentiation. Flow thickness is probably the main factor influencing their morphology. Finally, we suggest that the concomitant occurrence of olivine oxidation and vapor-differentiation effects results from the late persistence of water oversaturation after eruption, perhaps due to a high rate of magma ascent. Received: 27 March 1999 / Accepted: 15 February 2000     

Carbon and its isotopes in mid-oceanic basaltic glasses

Three carbon components are evident in eleven analyzed mid-oceanic basalts: carbon on sample surfaces (resembling adsorbed gases, organic matter, or other non-magmatic carbon species acquired by the glasses subsequent to their eruption), mantle carbon dioxide in vesicles, and mantle carbon dissolved in the glasses. The combustion technique employed recovered only reduced sulfur, all of which appears to be indigenous to the glasses. The dissolved carbon concentration (measured in vesicle-free glass) increases with the eruption depth of the spreading ridge, and is consistent with earlier data which show that magma carbon solubility increases with pressure. The total glass carbon content (dissolved plus vesicular carbon) may be controlled by the depth of the shallowest ridge magma chamber. Carbon isotopic fractionation accompanies magma degassing; vesicle CO₂ is about 3.8‰ enriched in ¹³C, relative to dissolved carbon. Despite this fractionation, δ¹³C_PDB values for all spreading ridge glasses lie within the range ?5.6 and ?7.5, and the δ¹³C_PDB of mantle carbon likely lies between ?5 and ?7. The carbon abundances and δ¹³C_PDB values of Kilauea East Rift glasses apparently are influenced by the differentiation and movement of magma within that Hawaiian volcano. Using ³He and carbon data for submarine hydrothermal fluids, the present-day mid-oceanic ridge mantle carbon flux is estimated very roughly to be about 1.0 × 10¹³ g C/yr. Such a flux requires 8 Gyr to accumulate the earth's present crustal carbon inventory.     

Fracture surface energy of olivine

A relatively simple indentation technique for the rapid measurement of fracture surface energy, , of small samples is described. The reliability of this technique is assessed by testing soda-lime glass for which there are good independent fracture mechanics determinations of fracture surface energy. The indentation technique gives a value for of 4.33 J m^–2 which compares favourably with the accepted value of 3.8 J m^–2. Fracture surface energies of the {010} and {001} cleavage planes of single crystal olivine (modal composition Fo₈₈Fa₁₂) are then determined and compared with theoretical estimates of the thermodynamic surface energy, , calculated from atomistic parameters ( is equal to in the absence of dissipative processes during crack extension). The experimental values for _{010} and _{001} are respectively 0.98 J m^–2 and 1.26 J m^–2. The calculated values of _{010} and _{001} are respectively in the range from 0.37 J m^–2 to 8.63 J m^–2 and 12.06 J m^–2. The particular advantages of the indentation technique for the study of the fracture surface energies of geological materials are outlined.     

Characteristics of some basaltic pyroclastics

The pyroclastic deposits of strombolian/hawaiian-type basaltic activity are readily distinguished from those ofsurtseyan type (i.e., the type exemplified by the opening stages of the Surtsey eruption of 1963–4) by being coarsergrained and better sorted. Moreover the fragments in the former have an external form in part controlled by surface tension — the termachnelith is proposed for such particles — whereas in the latter the fragments are clasts bounded by fracture surfaces. Other differences include the common occurrence of accretionary lapilli and « bomb sags » and the generally more thin-bedded character of the surtseyan deposits. The distinction between these two types of basaltic activity is important in palaeogeographic reconstructions.     

Giant Barkhausen jumps in Kamchatka basaltic lavas

Igneous rocks are shown to possess a magnetic fabric that admits the occurrence of giant Barkhausen jumps. It is also demonstrated that magnetic minerals of rocks can preserve information on a few magnetic fields (differing even in origin) and a few thermal magnetization temperatures.     

Composition of the core,II. Effect of high pressure on solubility of FeO in molten iron

An experimental and theoretical investigation of the effect of pressure on the solubility of FeO in molten iron has been carried out. Analyses of shock-wave compression data on iron oxides combined with measurements of the FeO bond length in “metallic” oxides suggest that the partial molar volume of FeO(V^*) dissolved in molten iron is substantially smaller than that of molten wüstite. Hence the effect of high pressure should be to increase the solubility of FeO in molten iron at a given temperature. This inference is confirmed by an experimental investigation of the effect of pressure on the position of the FeFeO eutectic. Thermodynamic calculations based on these experiments yield an estimate forV^* which is in reasonable agreement with the theoretical estimates. The experimental value ofV^* is used to calculate the effect of high pressure upon the FeFeO phase diagram. Solubility of FeO in molten iron increases sharply with pressure, the liquid immiscibility region contracts and disappears around 20 GPa and it is predicted that the FeFeO phase diagram should resemble a simple eutectic system above about 20 GPa. Analogous calculations predict that the solubility of FeO in molten iron in equilibrium with magnesiowüstite (Mg_0.8Fe_0.2)O at 2500°C increase from 14 mol.%(P = 0) to above 25 mol.% at 20 GPa. If the core formed by segregation of metallic iron originally dispersed throughout the earth, it seems inevitable that it would dissolved large amounts of FeO, thereby accounting for the observation that the density of the outer core is substantially smaller than that of pure iron under correspondingP, T conditions.