Melting relations in the chloride–carbonate–silicate systems at high-pressure and the model for formation of alkalic diamond–forming liquids in the upper mantle

The experiments in the model system CaMgSi₂O₆–(Na₂CO₃, CaCO₃)–KCl are performed at 5 GPa and 1400–1600 °C in order to study the phase relations, including liquid immiscibility, in the chloride–carbonate–silicate systems with application to alkali and chlorine-rich liquids preserved in kimberlitic diamonds. Experiments in the boundary joins of the system demonstrated that both the carbonate–silicate and chloride–carbonate melts are homogeneous; while high-temperature (above 1800 °C) liquid immiscibility is assumed for the chloride–silicate join of the above system. Addition of silicate component into the chloride–carbonate melts and chloride component into the carbonate–silicate melts results in splitting of the homogeneous liquids into the immiscible chloride–carbonate brine and carbonate–silicate melt. Carbonate–silicate and chloride–carbonate branches of the miscibility gap converge within the carbonate-rich region of the system. Regular temperature evolution of the shape and size of the miscibility gap is deduced. With decreasing temperature, the convergence point moves toward more Si-rich compositions, expanding fields of homogeneous chloride–carbonate silica-saturated melts. This effect is governed by the precipitation of the silicate phases even from silica-bearing chloride–carbonate melts. In addition, experiments revealed regular evolution of both Cl-bearing carbonate–silicate melt and Si-bearing chloride–carbonate brine toward the low-temperature chlorine–bearing carbonatitic liquid with decreasing temperature. These trends are similar to the evolution of the melt and brine inclusions in some diamonds from Botswana, Brazil, Canada, and Yakutia, indicating their growth during cooling. The model for interaction of the chloride–carbonate brine with the mantle rocks is developed on the basis of the present experimental data. This model is applied to the chlorine-enriched kimberlites of the Udachnaya–East pipe.     

Spheroidal pyroxenite aggregates in the Bushveld Complex — a special case of silicate liquid immiscibility

Spherical aggregates of orthopyroxene are reported from some parts of the Bushveld Complex in a variety of host rocks.Detailed mapping has shown that these spherical aggregates, comprising pyroxenite spheroids in a quartz-norite matrix, are contact phenomena and not stratigraphic markers. Orthopyroxene, biotite and amphibole are enriched in spheroids relative to matrix; their mineral chemistry showing a fairly constant orthopyroxene and plagioclase composition through the spheroids and into the matrix, indicating in-situ formation.Bulk chemistry shows spheroid to matrix tie-lines orthogonal to those generally accepted for silicate liquid immiscibility, but other chemical information is consistent with the occurrence of immiscibility.The formation of the aggregates may be related to the industrial process of spherical agglomeration, by which spheroids are formed by the introduction of an immiscible “bridging liquid” to the melt — probably derived from the floor rocks in this case. The mechanism accounts for the field relationships, petrography and chemistry of the aggregate-matrix system. The petrology of the process equates with a special case of silicate liquid immiscibility induced by local contamination and ageing of the original magma.A similar “bridging liquid” mechanism could also account for the formation of the so-called “boulder bed” beneath the Merensky Reef.     

Characteristics of melt inclusions in skarn minerals from Fe,Cu(Au) and Au(Cu) ore deposits in the region from Daye to Jiujiang

The skarns and skarn deposits are widely distributed at home and abroad. The skarn deposits include many kinds of ores and higher ore grade. Some of them are broad in scale. Scientists of ore deposits from different countries have paid and are paying grea…     

Geological settings and metallogenesis of high-grade iron deposits in China

The predominant types of high-grade iron deposits in China include skarn, sedimentary metamorphic(banded ironformation, BIF-type), continental/submarine volcanic-hosted and magmatic Fe-Ti-Voxide deposits. Based on a comprehensive review of current studies on these deposits, this paper suggests that the oxygen concentration in atmosphere played an important role for the formation of BIFs, whereas the tectonic setting and deep magmatic differentiation processes are more important for the other types. Notably, both high temperature and high pressure experiments and melt inclusion studies indicate that during the differentiation, high temperature magmas could develop iron-rich magma via liquid immiscibility but not pure oxide melt("iron ore magma"). Fe-P melt could be generated directly by liquid immiscibility under hydrous and oxidized condition. The formation of high-grade iron deposits is mostly associated with the processes related to multiple stages of superimposition, e.g., desiliconization and iron enrichment, removal of impurity, and remobilization and re-precipitation of iron. According to the temporal evolution, the high-grade iron deposit could be divided into multi-episode superimposition type(temporally discontinuous mineralization) and multi-stage superimposition type(temporally continuous mineralization). The former is represented by the sedimentary metamorphic iron deposit, and the latter includes those related to magmatic-hydrothermal fluids(e.g., skarn,volcanic-hosted and magmatic types).     

The Cortlandt complex: evidence for large-scale liquid immiscibility involving granodiorite and diorite magmas

Granodiorite and diorite plutons of the Rosetown complex, N.Y., which are associated with the nearby Cortlandt complex, have chemical and textural characteristics indicating that large-scale liquid immiscibility played a major role in their petrogenesis. Rare earth element, zirconium, niobium and phosphorus abundances are much greater in the diorite precluding the possibility that the Rosetown diorite and granodiorite are related by fractional crystallization. The trace element data also eliminate the possibility that the granodiorite represents: (1) a partial melt of crustal rocks including basalt; (2) a granitic cumulate; or (3) a residue from an aqueous fluid derived either from a silicate melt or crustal rocks. Liquid immiscibility appears to be viable model for the origin of the Rosetown granodiorite and iron-rich diorite. This model is supported by the following: (1) the major element compositions occur in a two-liquid field on a Greig diagram; (2) both bodies have similar Sr isotope compositions; (3) common phases in the two rock types have overlapping compositions; (4) the major and trace element data of the diorite and granodiorite are similar to the experimentally determined partition data of immiscible liquid pairs; and (5) possible ocelli of iron-rich diorite are found in the granodiorite.     

Plagiogranites as late-stage immiscible liquids in ophiolite and mid-ocean ridge suites: An experimental study

Chemical studies of two ophiolite suites and of selected mid-oceanic rift (MOR) regions indicate the presence of certain magmatic compositions: basalt, Fe-enriched basalt, and sodium granite (plagiogranite). There is a notable lack of evidence for melts of intermediate composition (i.e. 50–60 wt.% SiO₂). To determine possible relationships between basic rocks (basalts and gabbros) and acidic rocks (plagiogranites) a primitive basalt was fractionated at low pressure, under anhydrous conditions, and at different oxygen fugacities near the iron-wustite buffer and slightly above the quartz-fayalite-magnetite buffer. Samples of this basalt were taken to slightly above liquidus temperatures and then cooled at rates ranging from 1 to 2°C/hr. A liquid line of descent characterized by an Fe enrichment was delineated by quenching these experiments from a final temperature in the range of 1200 to 1000°C and analyzing the residual liquid (glass). After 95% crystallization of olivine, plagioclase, calcium pyroxene, and ilmenite, the residual liquid was an Fe-enriched basalt. This Fe-enriched basalt became immiscible at a temperature of about 1010°C. The immiscible phases produced were a more Fe-enriched basaltic liquid and a granitic liquid. The granitic liquid is similar in composition to the naturally occurring plagiogranites found in small volumes in ophiolites and in certain MOR regions. It is therefore concluded that silicate liquid immiscibility could be the petrogenetic process responsible for producing plagiogranite in some MOR regions and in some ophiolites. On the other hand, plagiogranites in ophiolites and MOR rock suites having andesitic and dacitic composition rocks may have evolved under conditions more closely approximating equilibrium crystallization and/or they may have evolved at high water pressures. The available experimental data suggest that amphibole would crystallize early and yield SiO₂-enriched liquids at depths greater than 4.5 km for P_H2O's in the range 0.6–1.0 P_total.The major problem in interpreting any of the natural plagiogranites as products of silicate liquid immiscibility is the fact that neither the Fe-enriched conjugate liquid or its crystalline equivalent has been described in the ophiolite or MOR literature. The identification of this Fe-rich conjugate magma is essential in any rock suite if a completely convincing case for silicate liquid immiscibility is to be made.     

Pre-1991 sulfur transfer between mafic injections and dacite magma in the Mt. Pinatubo reservoir

Before the 1991–1992 activity, a large andesite lava dome belonging to the penultimate Pinatubo eruptive period (Buag ∼ 500 BP) formed the volcano summit. Buag porphyritic andesite contains abundant amphibole-bearing microgranular enclaves of basaltic–andesite composition. Buag enclaves have lower K₂O and incompatible trace element (LREE, U, Th) contents than mafic pulses injected in the Pinatubo reservoir during the 1991–1992 eruptive cycle. This study shows that Buag andesite formed by mingling of a hot, water-poor and reduced mafic magma with cold, hydrous and oxidized dacite. Depending on their size, enclaves experienced variable re-equilibration during mixing/mingling. Re-equilibration resulted in hydration, oxidation and transfer of mobile elements (LILE, Cu) from the dacite to the mafic melts and prompted massive amphibole crystallization. In Buag enclaves, S-bearing phases (sulfides, apatite) and melt inclusions in amphibole and plagioclase record the evolution of sulfur partition among melt, crystal and fluid phases during magma cooling and oxidation. At high temperature, sulfur is partitioned between andesitic melt and sulfides (Ni-pyrrhotite). Magma cooling, oxidation and hydration resulted in exsolution of a S–Cl–H₂O vapor phase at the S-solubility minimum near the sulfide–sulfate redox boundary. Primary magmatic sulfide (pyrrhotite) and xenocrystic sulfide grains (pyrite), recycled together with olivines and pyroxenes from old mafic intrusives, were replaced by Cu-rich phases (chalcopyrite, cubanite) and, partially, by Ba–Sr sulfate. Sulfides degassed and transformed into residual spongy magnetite in response to fS₂ drop during final magma ascent and decompression. Our research suggests that a complete evaluation of the sulfur budget at Pinatubo must take into account the en route S assimilation from the country rocks. Moreover, this study shows that the efficiency of sulfur transfer between mafic recharges and injected magmas is controlled by the extent and rate of mingling, hydrous flushing and melt oxidation. Vigorous mixing/mingling and transformation of the magmatic recharge into a spray of small enclaves is required in order to efficiently strip their primary S-content that otherwise remains locked in the sulfides. Hydrous flushing increases the magma oxidation state of the recharges and modifies their primary volatile concentrations that cannot be recovered by the study of late-formed mineral phases and melt inclusions. Conversely, S stored in both late-formed Cu-rich sulfides and interstitial rhyolitic melt represents the pre-eruptive sulfur budget immediately available for release from mafic enclaves during their decompression.     

The effect of refreezing on the isotopic composition of melting snowpack

The isotopic composition of solid and liquid portions of natural melting snowpack is investigated in detail by the separating of liquid water from snow grains at different depths of the snowpack. The slope of the δD–δ¹⁸O line for the liquid phase is found to be lower than for the solid phase. This is proved to be due to the isotopic fractionation occurring in the melt–freeze mass exchange within the snowpack. Melting of the snowpack has no clear impact on the δD–δ¹⁸O line for the solid phase, but the slope of the δD–δ¹⁸O line for the liquid shows an overall slight decrease in the melting period. When the snowpack is refrozen, the refreezing process would inevitably cause the slope of the solid phase to decrease because of the discrepancy between the slopes of the two phases. Thus the slope of the solid would become lower and lower as the diurnal melt–freeze episodes cycle throughout the melting season. This effect is then demonstrated by looking into the isotopic composition changes of glacier firn. The extent of the effect depends on the snowpack properties and environmental conditions. The slope changes also result in a decreasing trend in deuterium excess. Copyright © 2007 John Wiley & Sons, Ltd.     

天然麻粒岩高温流变实验研究

本实验在气体介质三轴高温流变仪上,采用怀安瓦窑口麻粒岩,在温度900～1200℃、围压300 MPa、应变速率10~(-4)～10~(-6)/s条件下,开展高温流变实验.实验样品麻粒岩由斜长石(52%)、单斜辉石和斜方辉石(40%)、石英(3%)、磁铁矿和钛铁矿(5%)组成,矿物平均粒度为:斜长石294μm、单斜辉石和斜方辉石282μm、石英97μm、磁铁矿和钛铁矿109μm.利用傅里叶变换红外光谱仪分析获得变形后样品的水含量约为0.17±0.05wt%.实验样品的强度随温度升高而降低,随应变速率降低而降低.基于力学数据,采用稳态流变方程,获得实验样品在900～1000℃时的应力指数为8.1～12.9,在1050～1150℃时的应力指数为4.8～5.8,平均值5.2.应力指数随着温度升高而降低.显微结构和成分分析表明,在900℃时麻粒岩出现矿物压扁与定向拉长特征,样品以位错滑移和微破裂变形为主;在950～1000℃时,麻粒岩样品中颗粒边界变得圆滑,表现出位错攀移特征,辉石和磁铁矿边缘出现微量熔体;在1050～1200℃时麻粒岩出现部分熔融,而且随着温度和实验时间(应变)增加,熔体含量增加,熔体结晶出微粒斜长石、辉石和橄榄石,部分辉石通过固体反应生成橄榄石.颗粒边界熔体和矿物反应促进了扩散作用,导致位错攀移和熔体引起的扩散蠕变共同控制了麻粒岩的高温流变.     

Ascending droplets in the earth's core

This paper is concerned with some new problems of the dynamics and energetics of the Earth's core. The model of the so-called gravitationally-powered dynamo is investigated under the assumption of liquid immiscibility in the FeS system as a possible core material. In this way the growing inner core causes nucleation of small FeS-droplets that ascend under the release of gravitational potential energy. This energy is enough to drive a dynamo with a toroidal magnetic field of mean size.     

Role of water in the origin of podiform chromitite deposits

We report experiments in basalt oversaturated with water to duplicate the nodular ore textures of podiform chromitite ores. In immiscible basalt-water systems saturated with olivine and chromite, olivine will reside in the melt while chromite will collect in the fluid phase. Fractionation is physical and is driven by differential wetting properties of melt and fluid against silicate and oxide surfaces. There is no need to suppress olivine from the liquidus of a primitive basaltic melt as suggested by Irvine [Irvine, T.N., Geology 5 (1977) 273-277], to achieve chromite accumulations as observed in natural podiform ore deposits. The results imply that podiform chromitite ores will form where a primitive olivine-chromite-saturated mantle melt is sufficiently water-rich to exsolve a fluid phase during passage through the uppermost mantle. The most likely geodynamic environment for podiform chromite mineralization to take place is a supra-subduction zone setting.     

Evidence that bio-metallic mineral precipitation enhances the complex conductivity response at a hydrocarbon contaminated site

The complex conductivity signatures of a hydrocarbon contaminated site, undergoing biodegradation, near Bemidji, Minnesota were investigated. This site is characterized by a biogeochemical process where iron reduction is coupled with the oxidation of hydrocarbon contaminants. The biogeochemical transformations have resulted in precipitation of different bio-metallic iron mineral precipitates such as magnetite, ferroan calcite, and siderite. Our main objective was to elucidate the major factors controlling the complex conductivity response at the site. We acquired laboratory complex conductivity measurements along four cores retrieved from the site in the frequency range between 0.001 and 1000 Hz. Our results show the following: (1) in general higher imaginary conductivity was observed for samples from contaminated locations compared to samples from the uncontaminated location, (2) the imaginary conductivity for samples contaminated with residual and free phase hydrocarbon (smear zone) was higher compared to samples with dissolved phase hydrocarbon, (3) vadose zone samples located above locations with free phase hydrocarbon show higher imaginary conductivity magnitude compared to vadose zone samples from the dissolved phase and uncontaminated locations, (4) the real conductivity was generally elevated for samples from the contaminated locations, but not as diagnostic to the presence of contamination as the imaginary conductivity; (5) for most of the contaminated samples the imaginary conductivity data show a well-defined peak between 0.001 and 0.01 Hz, and (6) sample locations exhibiting higher imaginary conductivity are concomitant with locations having higher magnetic susceptibility. Controlled experiments indicate that variations in electrolytic conductivity and water content across the site are unlikely to fully account for the higher imaginary conductivity observed within the smear zone of contaminated locations. Instead, using magnetite as an example of the bio-metallic minerals in the contaminated location at the site, we observe a clear increase in the imaginary conductivity response with increasing magnetite content. The presence of bio-metallic mineral phases (e.g., magnetite) within the contaminated location associated with hydrocarbon biodegradation may explain the high imaginary conductivity response. Thus, we postulate that the precipitation of bio-metallic minerals at hydrocarbon contaminated sites impacts their complex conductivity signatures and should be considered in the interpretation of complex conductivity data from oil contaminated sites undergoing intrinsic bioremediation.     

A Study of the Thermodynamics and Kinetics of Copper Adsorption Using Chemically Modified Rice Husk

The use of rice husk as a low cost adsorbent for the removal of copper from wastewater has been explored in a laboratory scale experiment. The rice husk used for the study was treated with alkali to increase the sorption properties. The influence of metal ion concentration, weight of biosorbent, stirring rates, temperature and pH were also evaluated, and the results are fitted using adsorption isotherm models. From the experimental results it was observed that almost 90–98% of the copper could be removed using treated rice husk. The Langmuir adsorption isotherm, Freundlich isotherm and Tempkin isotherm models were used to describe the distribution of copper between the liquid and solid phases in batch studies, and it was observed that the Langmuir isotherm better represented the adsorption phenomenon. The experimental rate constant, activation energy, Gibbs free energy, enthalpy and entropy of the reaction were calculated in order to determine the mechanism of the sorption process.     

Laboratory verification of submicron magnetite production in pseudotachylytes: relevance for paleointensity studies

Pseudotachylytes generally possess stable remanent magnetizations but the processes by which pseudotachylytes are magnetized remain poorly understood. Magnetic hysteresis and scanning electron microscope studies reveal that experimental frictional melting of granites produces dispersed submicron inclusions of weakly interacting pseudo-single-domain (PSD) magnetite, in artificial pseudotachylyte. The magnetite inclusions are absent in the undeformed granite protolith and result from oxidation of Fe in melt-susceptible mafic minerals during the melt-quenched event. The pseudotachylytes acquired a stable thermal remanence in fine-grained PSD magnetites during the rapid cooling of the melt, implying that fine-grained magnetite has the potential for paleointensity determinations of contemporaneous magnetic fields with co-seismic faulting in granitoids.     

A mathematical model of magma transport in the asthenosphere

Abstract

We present a mathematical model for the flow of a partial melt through its solid phase. The model is based on the conservation laws of two-phase flow, which reduce to a generalization of porous flow in a permeable medium, when the solid matrix deforms very slowly. The continuity equation for the melt contains a source term (due to melting), which is determined by the energy equation. In addition, the melt fraction is unknown, and a new equation, representing conservation of pore space, is introduced. This equation may also be thought of as a constitutive law for the melt pressure (which is not lithostatic).

The model is non-dimensionalized and simplified. Some simple solutions are considered, and it is suggested that the occurrence of high fluid pressures in the solutions may initiate fractures in the lithosphere, thus providing a starting-up mechanism for magma ascent to the surface.     

Electromagnetic Radiation Generated by Acoustic Excitation of Rock Samples

The paper presents an experiment on acoustic excitation of electromagnetic radiation (EMR) signals in skarn, sandstone, and magnetite ore samples. For the skarn and sandstone samples, the EMR signal amplitude was observed to decrease with increasing ultimate strength. Supposedly, this effect can be explained by assuming that EMR is generated when an acoustic wave propagates through an electrical double layer. The presence of piezoelectric inclusions (e.g., quartz) in the magnetite ore enhances the analog EMR signal and its spectral components.     

Studies of Brazilian meteorites: III. Origin and history of the Angra dos Reis achondrite

The Angra dos Reis meteorite fell in 1869 and is a unique achondrite. It is an ultramafic igneous rock, pyroxenite, with 93% fassaite pyroxene which has 15.7% Ca-Tschermak's molecule, plus calcic olivine (Fo_53.1; 1.3% CaO), green hercynitic spinel, whitlockite (merrillite), metallic Ni-Fe, troilite, as well as magnesian kirschsteinite (Ks_62.3Mo_37.7), within olivine grains, and celsian (Cs_90.2An_7.7Ab_1.7Or_0.4) which are phases reported in a meteorite for the first time, and plagioclase (An_86.0), baddeleyite, titanian magnetite (TiO₂, 21.9%), and terrestrial hydrous iron oxide which are phases reported for the first time in this meteorite. Petrofabric analysis shows that fassaite has a preferred orientation and lineation which is interpreted as being due to cumulus processes, possibly the effect of post-depositional magmatic current flow or laminar flow of a crystalline mush. The mineral chemistry indicates crystallization from a highly silica-undersaturated melt at low pressure. Since the meteorite formed as a cumulate, pyroxene crystals may have gravitationally settled from a melt which crystallized melilite first. Plagioclase would be unstable in such a highly undersaturated melt, and feldspathoids would be rare or absent due to the very low alkali contents of the melt. The presence of rare grains of plagioclase and celsian may be the result of late-stage crystallization of residual liquids in local segregations. Thus, the Eu anomaly in Angra dos Reis may be the result of pyroxene separation from a melt which crystallized melilite earlier, rather than plagioclase as previously suggested.     

Zoned Cr,Fe-spinel from the La Perouse layered gabbro,Fairweather Range,Alaska

Zoned spinel of unusual composition and morphology has been found in massive pyrrhotite-chalcopyrite-pent-landite ore from the La Perouse layered gabbro intrusion in the Fairweather Range, southeastern Alaska. The spinel grains show continuous zoning from cores with up to 53 wt.% Cr₂O₃ to rims with less than 11 wt.% Cr₂O₃. Their composition is exceptional because they contain less than 0.32 wt.% MgO and less than 0.10 wt.% Al₂O₃ and TiO₂. Also notable are the concentrations of MnO and V₂O₃, which reach 4.73 and 4.50 wt.%, respectively, in the cores. The spinel is thought to have crystallized at low oxygen fugacity and at temperatures above 900°C, directly from a sulfide melt that separated by immiscibility from the gabbroic parental magma.     

The mechanism of basaltic explosions: Experimental modeling

An instrument package for simulating basaltic eruptions (IPSBE) with a height of 18 m has been developed for investigating the processes that occur during Strombolian eruptions. The device follows the geometrical ratio between the actual plumbing system of a volcano, with the ratio of conduit diameter to conduit height being 1 to 1000. For the first time in physical modeling studies, we created conditions in which a moving gas-saturated model liquid enters the conduit; this enabled us to study bubble nucleation, expansion, and coalescence, the generation and transformation of gas structures, and the kinetic features shown by the evolution of the gas phase. These experiments revealed a novel (previously unknown) flow pattern of two-phase mixtures in a vertical column, viz., a cluster flow that involves the regular alternation of compact clusters of gas bubbles that are separated by a fluid that does not involve a free gas phase. It is shown that the liquid, bubble, cluster, and slug flow patterns are mutually transformed under certain conditions; they are polymorphous modifications of a gas-saturated liquid moving in a vertical pipe. The data thus acquired suggested a new model for the gas-liquid movement of a magma melt in a conduit: depending on the type of gas-liquid flow behavior at the vent, the crater will exhibit different types of explosive activity, including actual explosions.     

Origin and development of the Star Mountain rhyolite

The Star Mountain rhyolite covers 500 square miles in the Davis Mountains, 100 miles east of El Paso, Texas. It appears to be the largest known rhyolitic mass to have been emplaced as a liquid flow rather than as a « froth flow » or nuée ardente. That it was emplaced as a liquid is shown by the lineation of phenocrysts, the vesicular and autobrecciated zones, baking of underlying tuffs, and by the complete absence of shard structures and evidence for welding. Three flow units with an aggregate thickness of 800 feet can be distinguished on the basis of field, chemical, and petrographic criteria. With the exception of some vesicular zones the rock is either finely crystalline and massive, or glassy and brecciated. Minor structures indicate that the flow of each unit was highly irregular. Anorthoclase feldspar is the only significant phenocryst species in the unit. Phenocryst-groundmass compositional relationships cannot be explained on the basis of conventional granite system phase relationships, nor do recently proposed pantellerite relationships fit this case. An experimental investigation of this unusual case is under way. A high temperature of extrusion of the rhyolite is indicated by the lack of measurable triclinicity in the feldspar. A low water vapor pressure is demonstrated by the presence of apparently primary magnetite and the scarcity of vesicular zones. Despite the low water content, high temperature caused the low viscosity necessary for the melt to spread over the 500 square mile outcrop area, presumably with approximately the same fluidity as a typical basalt flow.