Structure and viscosity of rhyolitic composition melts

The average local structure of a rhyolitic composition glass has been determined at 25°C using X-ray radial distribution analysis (RDA) and quasi-crystalline modelling and is best described as similar to that in a stuffed framework composed principally of six-membered rings of Si and Al tetrahedra (basically a stuffed tridymite-like model). Using this model it is possible to calculate a density (2.41 g/cm³) which compares well with the measured density (2.40 g/cm³); a structural model based on four-membered rings (an albite-like model) results in a substantially higher calculated density (2.60 g/cm³). We suggest that the rhyolite glass structural model is appropriate for rhyolitic melts, based on evidence from the recent literature. New viscosity data for an anhydrous rhyolite composition measured between 1200°C and 1500°C are presented and interpreted in terms of our proposed structural model and previous melt structure models for the major normative components of rhyolite. A mechanism for diffusion and viscous flow in framework silicate melts (including rhyolite composition) is proposed on the basis of recent molecular orbital calculations and molecular dynamics simulations of silicate and fluoride melts.     

The combined effects of chlorine and fluorine on the viscosity of aluminosilicate melts

The effect of fluorine and fluorine + chlorine on melt viscosities in the system Na₂O-Fe₂O₃-Al₂O₃-SiO₂ has been investigated. Shear viscosities of melts ranging in composition from peraluminous [(Na₂O + FeO) < (Al₂O₃ + Fe₂O₃)] to peralkaline [(Na₂O + FeO) > (Al₂O₃ + Fe₂O₃)] were determined over a temperature range 560-890 °C at room pressure in a nitrogen atmosphere. Viscosities were determined using the micropenetration technique in the range of 10^8.8 to 10^12.0 Pa s. The compositions are based on addition of FeF₃ and FeCl₃ to aluminosilicate melts with a fixed amount of SiO₂ (67 mol%). Although there was a significant loss of F and Cl during glass syntheses, none occurred during the viscometry experiments. The presence of fluorine causes a decrease in the viscosity of all melts investigated. This is in agreement with the structural model that two fluorines replace one oxygen; resulting in a depolymerisation of the melt and thus a decrease in viscosity. The presence of both chlorine and fluorine results in a slight increase in the viscosity of peraluminous melts and a decrease in viscosity of peralkaline melts. The variation in viscosity produced by the addition of both fluorine and chlorine is the opposite to that observed in the same composition melts, with the addition of chlorine alone (Zimova M. and Webb S.L. (2006) The effect of chlorine on the viscosity of Na₂O-Fe₂O₃-Al₂O₃-SiO₂ melts. Am. Mineral.91, 344-352). This suggests that the structural interaction of chlorine and fluorine is not linear and the rheology of magmas containing both volatiles is more complex than previously assumed.     

H2O diffusion in dacitic and andesitic melts

The diffusion of water in dacitic and andesitic melts was investigated at temperatures of 1458 to 1858 K and pressures between 0.5 and 1.5 GPa using the diffusion couple technique. Pairs of nominally dry glasses and hydrous glasses containing between 1.5 and 6.3 wt.% dissolved H₂O were heated for 60 to 480 s in a piston cylinder apparatus. Concentration profiles of hydrous species (OH groups and H₂O molecules) and total water (C_H2Ot = sum of OH and H₂O) were measured along the cylindrical axis of the diffusion sample using IR microspectroscopy. Electron microprobe traverses show no significant change in relative proportions of anhydrous components along H₂O profiles, indicating that our data can be treated as effective binary interdiffusion between H₂O and the rest of the silicate melt. Bulk water diffusivity (D_H2Ot) was derived from profiles of total water using a modified Boltzmann-Matano method as well as using fittings assuming a functional relationship between D_H2Ot and C_H2Ot. In dacitic melts D_H2Ot is proportional to C_H2Ot up to 6 wt.%. In andesitic melts the dependence of D_H2Ot on C_H2Ot is less pronounced. A pressure effect on water diffusivity could not be resolved for either dacitic or andesitic melt in the range 0.5 to 1.5 GPa. Combining our results with previous studies on water diffusion in rhyolite and basalt show that for a given water content D_H2Ot increases monotonically with increasing melt depolymerization at temperatures >1500 K. Assuming an Arrhenian behavior in the whole compositional range, the following formulation was derived to estimate D_H2Ot (m²/s) at 1 wt.% H₂O_t in melts with rhyolitic to andesitic composition as a function of T (K), P (MPa) and S (wt.% SiO₂):

     

俄罗斯Kurile-Kamchatka地区安山质熔体的成分、挥发分组成和微量元素

Melt inclusions in minerals from some volcanoes of the Kurile-Kamchatka region were examined.The studied basaltic andesites and andesites were sampled from volcanoes of the Central Kamchatka depression(Shiveluch and Bezymyannyi),Eastern Kamchatka volcanic belt(Avachinskii and Karymskii),and Iturup Island,Southern Kuriles(Kudryavyi).Basalts of the 1996 eruption of the Karymskii volcanic center and dacites of Dikii Greben'volcano,Southern Kamchatka were also studied.More than 260 melt inclusions from 31 rock samples were homogenized,and quenched glasses were analyzed using electron and ion microprobes.The compositions of melt inclusions in andesitic phenoerysts vary in silica contents from 56 to 80wt%.Al_2 O_3 ,FeO,MgO,CaO decrease and Na_2O and K_2O increase with increasing SiO_2.Many inclusions(about 80% )are dacitic or rhyolitic.However,the compositions of silicic glasses(>65wt% SiO_2)in andesites significantly differ in TiO2,FeO,MgO,CaO,and K_2O contents from those in dacites and rhyolites.High-potassium melts(K_2O 3.8～6.8wt% )with various SiO_2 from 51.4 to 77.2wt% were found in minerals of all volcanoes studied.This indicates a contribution of a component selectively enriched in potassium to magmas of the whole region.A great compositional diversity of melt inclusions in plagioelase phenocrysts from the Bezymyannyi andesites suggests a complex history of plagioclase crystallization and magma evolution in the andesite formation.Melts from different volcanoes strongly vary in volatile contents.The highest H_2O contents are found in the melts from Shiveluch(3.0～7.2wt%,4.7wt% on average)and Avachinskii (4.7～4.8wt%);while those are lower in melts of Kudryavyi(0.1～2.6wt% ),Dikii Greben'(0.4～1.8wt%),and Bezymyannyi (<1wt%).Chlorine contents are also variable.The lowest values are found in the Bezymyannyi melts(0.09wt% on average),the highest Cl contents are typical of melt inclusions in minerals from the Karymskii andesites(0.26wt% on average).The melts from Avachinskii,Dikii Greben',Kudryavyi,and Shiveluch show intermediate Cl contents(0.13～0.20wt% ).The pressure of 350～1600 bar determined by CO_2 fluid inclusions in plagioclase from the Shiveluch andesites suggests a magma chamber at a depth of 1.5～6 km. Concentrations of 17 elements were determined in glasses of melt inclusions in plagioclases from five volcanoes(Avachinskii, Bezymyannyi,Dikii Greben',Kudryavyi,and Shiveluch).The studied melts show similar trace-element patterns with Nb and Ti minima and B,K,Be,and Li maxima.The melts are close to typical island arc magmas by Sr/Y,La/Yb,K/Ti,and Ca/St ratios, and have some specific regional geochemical features.REE patterns sensitive to degree of magma differentiation indicate that Kudryavyi magmas are most primitive,while Shiveluch magmas are most evolved.     

In situ observations of bubble growth in basaltic,andesitic and rhyodacitic melts

Bubble growth strongly affects the physical properties of degassing magmas and their eruption dynamics. Natural samples and products from quench experiments provide only a snapshot of the final state of volatile exsolution, leaving the processes occurring during its early stages unconstrained. In order to fill this gap, we present in situ high-temperature observations of bubble growth in magmas of different compositions (basalt, andesite and rhyodacite) at 1,100 to 1,240 °C and 0.1 MPa (1 bar), obtained using a moissanite cell apparatus. The data show that nucleation occurs at very small degrees of supersaturaturation (<60 MPa in basalt and andesite, 200 MPa in rhyodacite), probably due to heterogeneous nucleation of bubbles occurring simultaneously with the nucleation of crystals. During the early stages of exsolution, melt degassing is the driving mechanism of bubble growth, with coalescence becoming increasingly important as exsolution progresses. Ostwald ripening occurs only at the end of the process and only in basaltic melt. The average bubble growth rate (G _R) ranges from 3.4 × 10^?6 to 5.2 × 10^?7 mm/s, with basalt and andesite showing faster growth rates than rhyodacite. The bubble number density (N _B) at nucleation ranges from 7.9 × 10⁴ mm^?3 to 1.8 × 10⁵ mm^?3 and decreases exponentially over time. While the rhyodacite melt maintained a well-sorted bubble size distribution (BSD) through time, the BSDs of basalt and andesite are much more inhomogeneous. Our experimental observations demonstrate that bubble growth cannot be ascribed to a single mechanism but is rather a combination of many processes, which depend on the physical properties of the melt. Depending on coalescence rate, annealing of bubbles following a single nucleation event can produce complex bubble size distributions. In natural samples, such BSDs may be misinterpreted as resulting from several separate nucleation events. Incipient crystallization upon cooling of a magma may allow bubble nucleation already at very small degrees of supersaturation and could therefore be an important trigger for volatile release and explosive eruptions.     

CO2 solubility and speciation in intermediate (andesitic) melts: the role of H2O and composition

We determined total CO₂ solubilities in andesite melts with a range of compositions. Melts were equilibrated with excess C-O(-H) fluid at 1 GPa and 1300°C then quenched to glasses. Samples were analyzed using an electron microprobe for major elements, ion microprobe for C-O-H volatiles, and Fourier transform infrared spectroscopy for molecular H₂O, OH⁻, molecular CO₂, and CO₃²⁻. CO₂ solubility was determined in hydrous andesite glasses and we found that H₂O content has a strong influence on C-O speciation and total CO₂ solubility. In anhydrous andesite melts with ∼60 wt.% SiO₂, total CO₂ solubility is ∼0.3 wt.% at 1300°C and 1 GPa and total CO₂ solubility increases by about 0.06 wt.% per wt.% of total H₂O. As total H₂O increases from ∼0 to ∼3.4 wt.%, molecular CO₂ decreases (from 0.07 ± 0.01 wt.% to ∼0.01 wt.%) and CO₃²⁻ increases (from 0.24 ± 0.04 wt.% to 0.57 ± 0.09 wt.%). Molecular CO₂ increases as the calculated mole fraction of CO₂ in the fluid increases, showing Henrian behavior. In contrast, CO₃²⁻ decreases as the calculated mole fraction of CO₂ in the fluid increases, indicating that CO₃²⁻ solubility is strongly dependent on the availability of reactive oxygens in the melt. These findings have implications for CO₂ degassing. If substantial H₂O is present, total CO₂ solubility is higher and CO₂ will degas at relatively shallow levels compared to a drier melt. Total CO₂ solubility was also examined in andesitic glasses with additional Ca, K, or Mg and low H₂O contents (<1 wt.%). We found that total CO₂ solubility is negatively correlated with (Si + Al) cation mole fraction and positively correlated with cations with large Gibbs free energy of decarbonation or high charge-to-radius ratios (e.g., Ca). Combining our andesite data with data from the literature, we find that molecular CO₂ is more abundant in highly polymerized melts with high ionic porosities (>∼48.3%), and low nonbridging oxygen/tetrahedral oxygen (<∼0.3). Carbonate dominates most silicate melts and is most abundant in depolymerized melts with low ionic porosities, high nonbridging oxygen/tetrahedral oxygen (>∼0.3), and abundant cations with large Gibbs free energy of decarbonation or high charge-to-radius ratio. In natural silicate melt, the oxygens in the carbonate are likely associated with tetrahedral and network-modifying cations (including Ca, H, or H-bonds) or a combinations of those cations.     

Pressure dependence of viscosity of rhyolitic melts

Viscosity of silicate melts is a critical property for understanding volcanic and igneous processes in the Earth. We investigate the pressure effect on the viscosity of rhyolitic melts using two methods: indirect viscosity inference from hydrous species reaction in melts using a piston cylinder at pressures up to 2.8 GPa and direct viscosity measurement by parallel-plate creep viscometer in an internally-heated pressure vessel at pressures up to 0.4 GPa. Comparison of viscosities of a rhyolitic melt with 0.8 wt% water at 0.4 GPa shows that both methods give consistent results. In the indirect method, viscosities of hydrous rhyolitic melts were inferred based on the kinetics of hydrous species reaction in the melt upon cooling (i.e., the equivalence of rheologically defined glass transition temperature and chemically defined apparent equilibrium temperature). The cooling experiments were carried out in a piston-cylinder apparatus using hydrous rhyolitic samples with 0.8-4 wt% water. Cooling rates of the kinetic experiments varied from 0.1 K/s to 100 K/s; hence the range of viscosity inferred from this method covers 3 orders of magnitude. The data from this method show that viscosity increases with increasing pressure from 1 GPa to 3 GPa for hydrous rhyolitic melts with water content ?0.8 wt% in the high viscosity range. We also measured viscosity of rhyolitic melt with 0.13 wt% water using the parallel-plate viscometer at pressures 0.2 and 0.4 GPa in an internally-heated pressure vessel. The data show that viscosity of rhyolitic melt with 0.13 wt% water decreases with increasing pressure. Combining our new data with literature data, we develop a viscosity model of rhyolitic melts as a function of temperature, pressure and water content.     

The effect of melt composition on the wetting angle between silicate melts and olivine

The wetting angle between silicate melts containing Ca, Li, Na, or K and olivine single crystals have been measured as part of an investigation of the dependence of the solid-liquid interfacial energy on melt composition and olivine orientation. The wetting angle increases with increasing silica content of the melt on (100) surfaces, but decreases with increasing silica content on (010) and (001) surfaces. For a given silica content, the wetting angle on (100) decreases in going from Ca to Li to Na to K, while the wetting angle on (010) and (001) increases in going from Ca to K-bearing melts. Based on published values for liquid-vapor interfacial energies, the observed changes in wetting angle with changes in melt composition indicate that the solid-liquid interfacial energy increases with increasing silica content of the melt for the (100) surface. However, for (010) and (001) surfaces, the variation of the solid-liquid interfacial energy with silica content depends upon whether Ca or K is present in the melt. In addition, the solid-liquid interfacial energy depends upon the orientation of the olivine in the following manner: _sl ⁽⁰¹⁰⁾ _sl ⁽⁰⁰¹⁾ _sl ⁽¹⁰⁰⁾ .     

The influence of the local pressure gradient and of the metamorphic grade on the composition of pegmatites in metamorphic terrains

It is proposed that the composition of pegmatites in metamorphic terrains is dependent on the local pressure gradient of the zone in which they are formed. Pegmatites having a composition similar to that of the host rock are formed in areas of shallow pressure gradient, while the composition of pegmatites that developed under a steep pressure gradient differs from that of the host rock. Relationships between the metamorphic environment, pressure gradient and composition of the pegmatites are investigated.     

On the volume dependence of viscosity of some magmatic silicate melts

Summary Density and viscosity measurements of three melts of volcanic rock composition (basalt and andesite) at low temperatures were carried out to understand the role of free volume in the viscous behavior of a magma and to estimate the flow unit in the melts. The data combined with literature data suggest the following conclusion: free-volume theory is not applicable to these silicate melts; the relation between viscosity and the inverse of free volume does not yield a straight line in a wide temperature range from the glass-transion temperature to 1550°C. However, two depolymerized melts, diopside and Oki-Dozen alkali basalt (OAB), yield almost linear relationships. Thus, the free-volume theory should hold to a fairly good approximation for these two melts. Based on this approximation, the radius of flow unit for diopside melt was calculated to be about 4.7 Å, and that for Oki-Dozen alkali basalt to be about 4.2 Å. The three-dimensional silicate anions which may correspond to the flow unit are Si₁₄O₃₅ ^14– and Si₁₆O₄₀ ^16– for diopside melt, and Si₁₀O₂₅ ^10– and Si₁₂O₃₀ ^12– for OAB melt. The temperature effect on the initial slope of the viscosity-pressure relation has also been examined in the frame of free-volume theory. It was concluded that the relative increase of the initial slope of the relation with increasing temperature might be caused by the increase of free volume.With 6 Figures     

The influence of primary magma composition,H2O and pressure on mid-ocean ridge basalt differentiation

MORB suites display variations in their chemical differentiation trends which are closely related to the incompatible element enrichment of the basalts. We examine suites of primitive to evolved basalts from the Pacific-Nazca Ridge at 28° S (mostly depleted); from the Juan Fernandez microplate region (depleted) and from the Explorer Ridge, northeast Pacific (mostly enriched). Trends for incompatible element enriched MORBs consistently show less depletion of Al₂O₃ and less enrichment of FeO when plotted on MgO variation diagrams.Least squares modeling indicates that enriched basalts have undergone less plagioclase crystallization than depleted basalts especially in the early stages of differentiation. Using thermodynamic modelling, we show that variations between MORB differentiation trends result largely from differences in the major element chemistry and H₂O content of primary magmas. Our chosen enriched and depleted near-primary magmas are similar in major element chemistry but the enriched near-primary magma has higher H₂O and lower Al₂O₃ than the depleted near-primary magma. The MORB crystallization sequence is: olivineolivine+plagioclase olivine+plagioclase+high-Ca pyroxene; and the separate and combined effects of lower Al₂O₃ and higher H₂O are to cause plagioclase to crystallize later (lower temperature), and to make the interval of olivine+plagioclase crystallization shorter. As a result, enriched differentiates have higher Al₂O₃ and lower FeO than depleted MORBs at a given MgO content, even though their parents' Al₂O₃ is lower. Crystallization of enriched basalts at higher pressure than depleted basalts is not able to account for differences between the differentiation trends because the proportion of plagioclase is higher during three-phase crystallization at high pressure.The variations in trends do not depend on geographic location and thus are superimposed on any regional variations in MORB chemistry or mantle source. Nor are they related to spreading rate. Depleted basalts from the fast-spreading 28° S and Juan Fernandez ridges have differentiation trends similar to depleted basalts from the medium-spreading Galapagos Spreading Center, whereas differentiation trends for enriched basalts from the medium-spreading Explorer Ridge are quite different. Fe³⁺/Fe_total is similar (and quite low) for enriched and depleted basalts, indicating that neither oxidation state nor early magnetite crystallization are important.     

Entropy dependence of viscosity and the glass-transition temperature of melts in the system diopside-anorthite

Viscosities of diopside-anorthite melts were measured over the wide range of temperature (near the glass-transition temperature–1580°C/1bar) and pressure (5–20 kb/above the liquidus temperature). The measurements were carried out by the fibre-elongation method for low temperature and the counter-balanced sphere method for high temperature at 1 bar, and the sinking and floating spheres method for high temperature at high pressure. Some of the values obtained deviated slightly from those in the literature. The data on viscosity and the glasstransition temperature have been interpreted on the basis of the configurational entropy theory, by which temperature and compositional effects on viscosity were explained well. The configurational entropies at the glasstransition temperature of magmatic silicate melts are almost constant if we use an average molecular weight (amw) or bead as a unit; 8.0±1.2 J/K·amw, 1.1 ±0.2cal/K·bead. The latter value coincides well with the value from the literature for organic polymers. The negative deviation from linearity of the glass-transition temperature of intermediate melts may be interpreted as the effect of the mixing entropy. The calculated glasstransition temperature-composition curve using the mixing entropy agreed well with the experimental values.     

Empirical models relating viscosity and tracer diffusion in magmatic silicate melts

The Adam-Gibbs equations describing relaxation in silicate melts are applied to diffusion of trace components of multicomponent liquids. The Adam-Gibbs theory is used as a starting point to derive an explicit relation between viscosity and diffusion including non-Arrhenian temperature dependence. The general form of the equation is D_iη = A_iexp{Δ(s_cE_i)/TS_c}, where D is diffusivity, η is melt viscosity, T is absolute temperature, Δ(s_cE_i) is the difference between the products of activation energies and local configurational entropies for viscous and diffusive relaxation, A_i is a constant that depends on the characteristics of the diffusing solute particles, and S_c is configurational entropy of the melt. The general equation will be impractical for most predictive purposes due to the paucity of configurational entropy data for silicate melts. Under most magmatic conditions the proposed non-Arrhenian behaviour can be neglected, allowing the general equation to be simplified to a generalized form of the Eyring equation to describe diffusion of solutes that interact weakly with the melt structure: D_iη/T = Q_iexp{ΔE_i/RT}, where Q_i and ΔE_i depend on the characteristics of the solute and the melt structure. If the diffusing solute interacts strongly with the melt structure or is a network-forming cation itself, then ΔE_i = 0, and the relation between viscosity and diffusion has the functional form of the classic Eyring and Stokes-Einstein equations; D_iη/T = Q_i. If the diffusing solute can make diffusive jumps without requiring cooperative rearrangement of the melt structure, the diffusivity is entirely decoupled from melt viscosity and should be Arrhenian, i.e., D_i = Q_iexp{B_i/T}. A dataset of 594 published diffusivities in melts ranging from the system CAS through diopside, basalt, andesite, anhydrous rhyolite, hydrous rhyolite, and peralkaline rhyolite to albite, orthoclase, and jadeite is compared with the model equations. Alkali diffusion is completely decoupled from melt viscosity but is related to melt structure. Network-modifying cations with field strength Z_i²/r between 1 and 10 interact weakly with the melt network and can be modelled with the extended form of the Eyring equation. Diffusivities of cations with high field strength have activation energies essentially equal to that of viscous flow and can be modelled with a simple reciprocal Eyring-type dependence on viscosity. The values of Q_i, ΔE_i and B_i for each cation are different and can be related to the cation charge and radius as well as the composition of the melt through the parameters Z_i²/r, M/O, and Al/(Na + K + H). I present empirical fit parameters to the model equations that permit prediction of cation diffusivities given only charge and radius of the cation and temperature, composition and viscosity of the melt, for the entire range of temperatures accessible to magmas near to or above their liquidus, for magmas ranging in composition from basalt through andesite to hydrous or anhydrous rhyolite. Pressure effects are implicitly accounted for by corrections to melt viscosity. Ninety percent of diffusivities predicted by the models are within 0.6 log units of the measured values.     

Aluminosilicate melts: structure, composition and temperature

The anionic structure of aluminosilicate melts of intermediate degree of polymerization (NBO/T = 0.5) and with along the composition join (LS4-LA4) has been examined in-situ to ˜1480 °C, and compared with recent data for melts along the analog composition join and with less polymerized melts along the join and O_{_5}. With , the anionic equilibrium, (1) , adequately describes the structure. With , a second expression, (2) , is required because an additional structural unit, Q¹, is stabilized in the melts. The enthalpy, , of reaction (1) increases from − 36 ±4 kJ/mol in the absence of aluminum to 34± 5 kJ/mol at and 64 ± 4 kJ/mol at Al/(Al + Si) = 0.45. Similar trends are reported for other alkali aluminosilicate melts. Least-squares fitting of abundance of structural units as a function of temperature and bulk composition has been conducted. The unit abundance is dominantly a function of temperature, Al/(Al +Si), and bulk melt polymerization. Configurational entropy and heat capacity of mixing of melts above their glass transition temperatures have been calculated with the aid of the least-squares fitted equations. The values of these parameters indicate that as the ionization potential of the metal cations increases, configurational heat capacity of alkali aluminosilicate melts becomes temperature dependent. As a result, transport properties (viscosity, diffusivity, and conductivity) of such melts will not show Arrhenian behavior even in the high-temperature range. Further, discontinuous changes in entropy and heat capacity of mixing results from temperature-induced changes in types of structural units in the melts. Such discontinuous changes would also be reflected in discontinuous changes of temperature-dependent transport properties. Received: 26 September 1996 / Accepted: 18 October 1996     

Density of peridotite melts at high pressure

Densities of ultramafic melts were determined up to 22 GPa by relative buoyancy experiments. Olivine and diamond were used as buoyancy markers. We confirmed that the density crossover of PHN 1611 melt and its equilibrium olivine (Fo94) occurs at around 13.5 GPa and 2030 °C and that olivine floats from deeper regions in the magma ocean of the primordial terrestrial mantle. The comparison of the compression curves of basic and ultrabasic melts implies that the basic melt is more compressible. This can be explained by the difference in the amount of compressible linkage of SiO_n and AlO_n polyhedra. The interstitial melt trapped by the density crossover can be the cause of the impedance anomaly of the seismic wave in the deep upper mantle.     

Toward a general viscosity equation for natural anhydrous and hydrous silicate melts

A new and empirical viscosity equation for anhydrous and hydrous natural silicate melts has been developed using the following formulation:

     

Solubility of carbon dioxide in melts of andesite,tholeiite, and olivine nephelinite composition to 30 kbar pressure

Carbon dioxide solubilities in H₂O-free hydrous silicate melts of natural andesite (CA), tholeiite (K 1921), and olivine nephelinite (OM1) compositions have been determined employing carbon-14 beta-track mapping techniques. The CO₂ solubility increases with increasing pressure, temperature, and degree of silica-undersaturation of the silicate melt. At 1650° C, CO₂ solubility in CA increases from 1.48±0.05 wt % at 15 kbar to 1.95±0.03 wt % at 30 kbar. The respective solubilities in OM1 are 3.41±0.08 wt % and 7.11±0.10 wt %. The CO₂ solubility in K1921 is intermediate between those of CA and OM1 compositions. At lower temperatures, the CO₂ contents of these silicate melts are lower, and the pressure dependence of the solubility is less pronounced. The presence of H₂O also affects the CO₂ solubility (20–30% more CO₂ dissolves in hydrous than in H₂O-free silicate melts); the solubility curves pass through an isothermal, isobaric maximum at an intermediate CO₂/(CO₂+H₂O) composition of the volatile phase. Under conditions within the upper mantle where carbonate minerals are not stable and CO₂ and H₂O are present a vapor phase must exist. Because the solubility of CO₂ in silicate melts is lower than that of H₂O, volatiles must fractionate between the melt and vapor during partial melting of peridotite. Initial low-temperature melts will be more H₂O-rich than later high-temperature melts, provided vapor is present during the melting. Published phase equilibrium data indicate that the compositional sequence of melts from peridotite +H₂O+CO₂ parent will be andesite-tholeiite-nephelinite with increasing temperature at a pressure of about 20 kbar. Examples of this sequence may be found in the Lesser Antilles and in the Indonesian Island Arcs.     

The influence of marine and terrestrial source material on the composition of petroleum

This paper consists of two interrelated parts. In the first part, the influence of the composition of sediment organic matter on crude oil composition is discussed. The second part deals with the origin of normal paraffins in petroleum.Source beds with abundant terrestrial plant matter generate heavy hydrocarbons rich in five-ring naphthenes. Unless such source beds are exposed to a high temperature for a prolonged time, the oils released are also rich in five-ring naphthenes. Such oils are rare; thus far the only examples found are some Eocene Wilcox oils from the Texas Gulf Coast and some Eocene Green River oils from the Uinta Basin, Utah. Normally, oil source beds are not rich in terrestrial plant matter and the five-ring naphthene content of the source bed hydrocarbons, as well as that of the produced oils, is low.The n-paraffins generated by oil source beds rich in terrestrial plant matter are characterized by abnormally low (C₂₁ + C₂₂)/(C₂₈ + C₂₉) ratios of 0.6–1.2. In oils of dominantly marine origin, this ratio is in the range 1.5–5.0. The ratio of marine to terrestrial organic matter in source beds appears to influence both the naphthene composition and the n-paraffin composition of the generated oils.Evidence is presented that petroleum n-parainns originate from slow thermal cracking of fatty acids contained in fats and waxes. Reaction equations are discussed which explain the major geochemical observations, including the difference in carbon-number distribution of the assumed parental fatty acids and of their descendant n-paraffins. In normal oils, which originate mostly from fat rich marine organic matter, the n-paraffin concentration tapers off above C₂₀. The molecular weight range of the fatty acids of plant waxes is considerably higher than that of fats. If plant waxes contribute strongly to the oil source material, the molecular weight distribution of the petroleum n-paraffins formed is abnormal and high carbon numbers in the C₂₄-C₃₂ range dominate.