Experimental determination of REE partition coefficients between amphibole and basaltic to andesitic liquids at high pressure

Amphibole/liquid partition coefficients for the REE(D^amph/liq_REE) obtained from natural rocks increase systematically with bulk rock compositional change from basalt to rhyolite and are higher for the middle to heavy REE. Five new experimentally determined sets of D_REE (La, Sm, “Eu²⁺”, Ho, Lu)and 4 published sets are similar to these data and provide values for use in geochemical modelling of magmatic processes involving amphibole, over a range of temperature, pressure and oxygen fugacity. The partition coefficients increase significantly with decreasing temperature, and increase slightly with increasing oxygen fugacity. Pressure does not appear to have a major effect, although one data set suggests increased pressure lowers D^amph/liq_REE in a basaltic composition.     

Ferric-ferrous equilibria in natural silicate liquids at 1 bar

Ferric and ferrous iron concentrations have been measured in 57 silicate liquids equilibrated at temperatures (1,200°–1,330°C) above the liquidus and at oxygen fugacities close to those defined by quartz-fayalite-magnetite. The experimental results reported here span virtually the entire known compositional range of lavas. An empirical equation relating the mole fraction of Fe₂O₃ and FeO to oxygen fugacity, absolute temperature and liquid composition at 1 bar has been formulated, based on the present experimental results and published data. Extrapolating the proposed empirical relationship over several hundred degrees, for instance below the solidus of the 1965 Makaopuhi tholeiitic lava lake, yields calculated oxygen fugacities which are a little lower than those measured directly in drill holes; at 1,100°C this discrepancy is 0.4 log unit. However, the agreement between the oxygen fugacities calculated from our empirical equation and those indicated by the composition of Fe-Ti oxides in andesites and siliceous obsidians is much closer.     

Prediction of plagioclase-melt equilibria in anhydrous silicate melts at 1-atm

Many models for plagioclase-melt equilibria have been proposed over the past 30 years, but the focus is increasingly on the effects of water content and pressure. However, many geological and petrological applications concern low pressure and low water systems, such as the differentiation of large terrestrial basaltic magma chambers, and lunar and asteroidal magmatism. There is, therefore, a justified need to quantify the influence of anhydrous liquid composition on the composition of equilibrium plagioclase at 1-atm. With this in mind, a database of over 500 experimentally determined plagioclase-liquid pairs has been created. The selected low pressure, anhydrous, experiments include both natural and synthetic liquids, whose compositions range from basalt to rhyolite. Four equations are proposed, derived from this data. The first is based on a thermodynamically inspired formalism, explicitly integrating the effect of temperature. This equation uses free energies and activities of crystalline anorthite available from the literature. For the activity of anorthite in the liquid phase, it is found that current models of the activity of individual oxides are insufficient to account for the experimental results. We have therefore derived an empirical expression for the variation of anorthite activity in the liquid as a function of melt composition, based upon inversion of the experimental data. Using this expression allows the calculation of plagioclase composition with a relative error less than 10%. However, in light of the fact that temperature is not necessarily known for many petrological applications, an alternative set of T-independent equations is also proposed. For this entirely empirical approach, the database has been divided into three compositional groups, treated independently for regression purposes: mafic–ultramafic, alkali-rich mafic–ultramafic, and intermediate-felsic. This separation into distinct subgroups was found to be necessary to maintain errors below acceptable limits, but results across group boundaries were found to be comparable. Overall, 50% of plagioclase compositions are predicted to within 2% of the experimentally derived value, and 90% to within 5%, representing a significant improvement over existing models.     

A parameterized model for REE distribution between low-Ca pyroxene and basaltic melts with applications to REE partitioning in low-Ca pyroxene along a mantle adiabat and during pyroxenite-derived melt and peridotite interaction

Low-Ca pyroxenes play an important role in mantle melting, melt-rock reaction, and magma differentiation processes. In order to better understand REE fractionation during adiabatic mantle melting and pyroxenite-derived melt and peridotite interaction, we developed a parameterized model for REE partitioning between low-Ca pyroxene and basaltic melts. Our parameterization is based on the lattice strain model and a compilation of published experimental data, supplemented by a new set of trace element partitioning experiments for low-Ca pyroxenes produced by pyroxenite-derived melt and peridotite interaction. To test the validity of the assumptions and simplifications used in the model development, we compared model-derived partition coefficients with measured partition coefficients for REE between orthopyroxene and clinopyroxene in well-equilibrated peridotite xenoliths. REE partition coefficients in low-Ca pyroxene correlate negatively with temperature and positively with both calcium content on the M2 site and aluminum content on the tetrahedral site of pyroxene. The strong competing effect between temperature and major element compositions of low-Ca pyroxene results in very small variations in REE partition coefficients in orthopyroxene during adiabatic mantle melting when diopside is in the residue. REE partition coefficients in orthopyroxene can be treated as constants at a given mantle potential temperature during decompression melting of lherzolite and diopside-bearing harzburgite. In the absence of diopside, partition coefficients of light REE in orthopyroxene vary significantly, and such variations should be taken into consideration in geochemical modeling of REE fractionation in clinopyroxene-free harzburgite. Application of the parameterized model to low-Ca pyroxenes produced by reaction between pyroxenite-derived melt and peridotite revealed large variations in the calculated REE partition coefficients in the low-Ca pyroxenes. Temperature and composition of starting pyroxenite must be considered when selecting REE partition coefficients for pyroxenite-derived melt and peridotite interaction.     

Experimental rare earth element partition coefficients for garnet,clinopyroxene and amphibole coexisting with andesitic and basaltic liquids

The partitioning of La, Sm, Dy, Ho and Yb between garnet, calcic clinopyroxene, calcic amphibole and andesitic and basaltic liquids has been studied experimentally. Glasses containing one or more REE in concentrations of 500–2000 ppm were crystallized at pressures of 10–35 kbar, and temperatures of 900–1520°C. Water was added to stabilize amphibole and to allow study of partition coefficients over wide temperature ranges. Major element and REE contents of crystal rims and adjacent glass were determined by EPMA, with limits of detection for individual REE of 100–180 ppm. Measured partition coefficients, D_REE^Cryst-liq, are independent of REE concentration over the concentration ranges used.D-values show an inverse dependence on temperature, best illustrated for garnet. At a given temperature, they are almost always higher for equilibria involving andesitic liquid. Garnet shows by far the greatest range of D-values, with e.g. D_La < 0.05 and D_Yb ~ 44 for andesitic liquid at 940°C. D_Yb falls to ~ 12 at 1420°C. D_Sm^Ga-liq also correlates negatively with temperature and positively with the grossular content of garnet. Patterns of D_ree^Cryst-Liq for calcic clinopyroxenes and amphiboles are sub-parallel, with D-values for amphibole generally higher. Both individual D-values and patterns for the crystalline phases studied are comparable with those determined for phenocryst-matrix pairs in natural dacites, andesites and basalts.D-values and patterns are interpreted in terms of the entry of REE³⁺ cations into mineral structures and liquids of contrasted major element compositions. The significance of the partition coefficients for models of the genesis of andesitic and Hy-normative basaltic magmas is assessed. Most magmas of these types in island arcs are unlikely to be produced by melting of garnet-bearing sources such as eclogite or garnet lherzolite.     

Experimental study into the petrogenesis of crystal-rich basaltic to andesitic magmas at Arenal volcano

Arenal volcano is nearly unique among arc volcanoes with its 42 year long (1968–2010) continuous, small-scale activity erupting compositionally monotonous basaltic andesites that also dominate the entire, ~7000 year long, eruptive history. Only mineral zoning records reveal that basaltic andesites are the result of complex, open-system processes deriving minerals from a variety of crystallization environments and including the episodic injections of basalt. The condition of the mafic input as well as the generation of crystal-rich basaltic andesites of the recent, 1968–2010, and earlier eruptions were addressed by an experimental study at 200 MPa, 900–1,050 °C, oxidizing and fluid-saturated conditions with various fluid compositions [H₂O/(H₂O + CO₂) = 0.3–1]. Phase equilibria were determined using a phenocryst-poor (~3 vol%) Arenal-like basalt (50.5?wt% SiO₂) from a nearby scoria cone containing olivine (Fo₉₂), plagioclase (An₈₆), clinopyroxene (Mg# = 82) and magnetite (X_ulvö = 0.13). Experimental melts generally reproduce observed compositional trends among Arenal samples. Small differences between experimental melts and natural rocks can be explained by open-system processes. At low pressure (200 MPa), the mineral assemblage as well as the mineral compositions of the natural basalt were reproduced at 1,000 °C and high water activity. The residual melt at these conditions is basaltic andesitic (55 wt% SiO₂) with 5 wt% H₂O. The evolution to more evolved magmas observed at Arenal occurred under fluid-saturated conditions but variable fluid compositions. At 1,000 °C and 200 MPa, a decrease of water content by approximately 1 wt% induces significant changes of the mineral assemblage from olivine + clinopyroxene + plagioclase (5 wt% H₂O in the melt) to clinopyroxene + plagioclase + orthopyroxene (4 wt% H₂O in the melt). Both assemblages are observed in crystal-rich basalt (15 vol%) and basaltic andesites. Experimental data indicate that the lack of orthopyroxene and the presence of amphibole, also observed in basaltic andesitic tephra units, is due to crystallization at nearly water-saturated conditions and temperatures lower than 950 °C. The enigmatic two compositional groups previously known as low- and high-Al₂O₃ samples at Arenal volcano may be explained by low- and high-pressure crystallization, respectively. Using high-Al as signal of deeper crystallization, first magmas of the 1968–2010 eruption evolved deep in the crust and ascent was relatively fast leaving little time for significant compositional overprint by shallower level crystallization.     

Oxygen chemical diffusion in three basaltic liquids at elevated temperatures and pressures

The diffusivity of oxygen has been measured in three basaltic liquids from 1280 to 1450°C and 4 to 21 kilobars using a solid media piston-cylinder apparatus. The measurements were done by monitoring the reduction of ferric iron in previously oxidized spheres of basalt melt. The compositions studied were olivine nephelinite, alkali basalt, and 1921 Kilauea tholeiite.The isobaric temperature dependence of oxygen diffusivity is adequately described by Arrhenius relationships for the three liquids studied. Arrhenius activation energies were determined at 12 kilobars for olivine nephelinite (62± 6 kcal/mole) and tholeiite (51 ± 4 kcal/mole) and at 4, 12, and 20 kilobars for alkali basalt (70 ± 7, 86 ± 6, and 71 ± 14 kcal/mole, respectively). The Arrhenius parameters for the three compositions define a compensation law which is indistinguishable from those for oxygen diffusion in simple silicate melts (DUNN, 1982) and for divalent cation diffusion in basaltic melts (Hofmann, 1980). These results suggest that the principal species contributing to the total diffusivity of oxygen is the oxide anion (O^2?).The isothermal pressure dependence of oxygen diffusion is complex and quite different from that observed for cationic diffusion in silicate melts. All three compositions show a sharp decrease in oxygen diffusivity at approximately the same pressure as the change in the liquidus phase from olivine to pyroxene, but otherwise the pressure dependence can be described by Arrhenius type equations. The equations yield negative activation volumes for the olivine nehpelinite and the alkali basalt. The activation volumes determined for the tholeiite are near zero at low pressure and positive at high pressure. A negative activation volume represents a decrease in the average size of the principal diffusing species.The results of this study are consistent with a melt model which includes both continuous changes in the relative proportions of the various anionic species in the melt with pressure and the occurrence of anionic disproportionation reactions within narrow pressure ranges.     

Transfer and exchange equilibria in a portion of the pyroxene quadrilateral as deduced from natural and experimental data

Differences in the chemical composition of metamorphic and igneous pyroxene minerals may be attributed to a transfer reaction, which determines the Ca content of the minerals, and an exchange reaction, which determines the relative Mg:Fe²⁺ ratios. Natural data for associated Ca pyroxene (Cpx) and orthopyroxene (Opx) or pigeonite are combined with experimental data for Fe-free pyroxenes, to produce the following equations for the Cpx slope of the solvus surface: $\text{> 1080°C: T =}\text{1000}\text{(0.468 + 0.246X}{}^{\mathrm{Cpx}}\text{? 0.123 ln (1–2 [Ca]))}$$\text{< 1080°C: T =}\text{1000}\text{(0.054 + 0.608X}{}^{\mathrm{Cpx}}\text{? 0.304 ln (1–2 [Ca]))}$, and the following equation for the temperature-dependence of the Mg-Fe distribution coefficient: $\text{T =}\text{1130}\text{(}\text{ln}\text{K}{}_{\mathrm{p}}\text{+ 0.505)}$, where T is absolute temperature, X is $\text{Fe}{}^{\mathrm{2+}}\text{(Mg + Fe}{}^{\mathrm{2+}}\text{))}$, [Ca] is $\text{Ca}\text{(Ca + Mg + Fe}{}^{\mathrm{2+}}\text{)}$ in Cpx, and K_D is the distribution coefficient, defined as $\text{X}{}^{\mathrm{Opx}}\text{/(1 ? X}{}^{\mathrm{Opx}}\text{) ÷ X}{}_{\mathrm{Cpx}}\text{/(1 ?}{}^{\mathrm{Cpx}}\text{)}$.The transfer and exchange equations form useful temperature indicators, and when applied to 9 sets of well-studied rocks, yield pairs of temperatures that are in good agreement. For example, temperatures obtained for the Bushveld Complex are 1020°C (solvus equation) and 980°C (exchange equation), based on 7 specimens. The uncertainty in these numbers, due to precision and accuracy errors, is estimated to be ±60°.     

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.     

The effect of composition on the ferric-ferrous ratio in basaltic liquids at atmospheric pressure

The effects on the ferric-ferrous ratio of varying individual components in a dry basaltic liquid have been determined at atmospheric pressure and constant oxygen fugacity (fO₂). Experiments were conducted by suspending 100 mg samples from pt loops at 1200°C (fO₂ = 10^?8atm) and 1360°C (fO₂ = 10^?6atm) in an atmosphere controlled by mixtures of CO₂ and H₂. A microanalytical wetchemical technique and the electron microprobe were used to determine the composition of the resulting basaltic glasses. In order of decreasing significance, the addition of oxides of K, Na, Si, Al, or Ca produces an increase in the ferric-ferrous ratio of the melt at 1200°C. The change in the ferric-ferrous ratio produced by component addition is less at 1360°C than at 1200°C.     

Reversed pyroxene phase equilibria in CaO-MgO-SiO2 from 925° to 1,175° C at one atmosphere pressure

Experiments using V₂O₅ as a high-temperature solvent have produced compositional reversals defining the miscibility gap between enstatite and diopside on the join Mg₂Si₂O₆-CaMgSi₂O₆ between 925° and 1,175° C at atmospheric pressure. These experiments locate an equilibrium near 1,000° C among diopside, protoenstatite, and orthoenstatite; they verify the stable coexistence of diopside and protoenstatite above 1,000° C and disprove the hypothesis that orthoenstatite has a stability field which is continuous from temperatures below 1,000° C to the solidus. The phase relations suggest that the orthorhombic low-Ca pyroxene on the solidus in this system (formerly identified as orthoenstatite) is a phase distinct from the orthoenstatite stable with diopside at low subsolidus temperatures. Data locating the orthoenstatite-diopside miscibility gap validate the use at low pressures of symmetric orthopyroxene and asymmetric clinopyroxene solution models in this system.     

Effects of experimental reheating of natural basaltic ash at different temperatures and redox conditions

A set of experiments have been performed on volcanic materials from Etna, Stromboli and Vesuvius in order to evaluate how the exposure to thermal and redox conditions close to that of active craters affects the texture and composition of juvenile pyroclasts. Selected samples were placed within a quartz tube, in presence of air or under vacuum, and kept at T between 700 and 1,130 °C, for variable time (40 min to 12 h). Results show that reheating reactivates the melt, which, through processes of chemical and thermal diffusion, reaches new equilibrium conditions. In all the experiments performed at T = 700–750 °C, a large number of crystal nuclei and spherulites grows in the groundmass, suggesting conditions of high undercooling. This process creates textural heterogeneities at the scale of few microns but only limited changes of groundmass composition, which remains clustered around that of the natural glasses. Reheating at T = 1,000–1,050 °C promotes massive groundmass crystallization, with a different mineral assemblage as a function of the redox conditions. Morphological modifications of clasts, from softening to sintering as temperature increases, occur under these conditions, accompanied by progressive smoothing of external surfaces, and a reduction in size and abundance of vesicles, until the complete obliteration of the pre-existing vesicularity. The transition from sintering to welding, characteristic of high temperature, is influenced by redox conditions. Experiments at T = 1,100–1,130 °C and under vacuum produce groundmass textures and glass compositions similar to that of the respective starting material. Collapse and welding of the clasts cause significant densification of the whole charge. At the same temperature, but in presence of air, experimental products at least result sintered and show holocrystalline groundmass. In all experiments, sublimates grow on the external surfaces of the clasts or form a lining on the bubble walls. Their shape and composition is a function of temperature and fO₂ and the abundance of sublimates shows a peak at 1,000 °C. The identification of the features recorded by pyroclasts during complex heating–cooling cycles allows reconstructing the complete clasts history before their final emplacement, during weakly explosive volcanic activity. This has a strong implication on the characterization of primary juvenile material and on the interpretation of eruption dynamics.     

The solubility of olivine in basaltic liquids: an ionic model

A simple ionic model which describes the solution of the forsterite component of olivine in silicate liquids is reported. The melting relation is represented: (Mg₂SiO₄)_ol = 2(Mg²⁺)_L + (SiO₄^4?_L and is extended to all silicate liquids by normalizing their compositions to 4 oxygens. At 1 bar, the temperature at which olivine is in equilibrium with any alkali-depleted basaltic composition can be calculated to within ±30°C. This error is increased considerably when applied to terrestrial basalts which contain several weight percent alkalis. Alkalis interfere with the equilibrium by generating strongly repulsive interionic forces which can be crudely modelled in a manner consistent with constraints imposed by regular solution theory.The model quantifies the reduced activity of SiO₄^4? monomers due to increasing SiO₂ concentrations in the melt. This is a consequence of polymerization which does not appear to operate gradually over the entire spectrum of mafic and ultramafic compositions. The coordination of alumina in melts which precipitate olivine only appears to be dominantly octahedral. Titanium acts as a polymerizing agent by interconnecting previously isolated SiO₄^4? monomers. Calcium associated with normative diopside tends to exhibit small but perceptible repulsive forces involving Mg²⁺.     

The distribution of Fe and Mg between olivine and lunar basaltic liquids

We have examined the Fe and Mg distribution between coexisting olivine and lunar basaltic liquids produced by equilibrium partial melting of natural lunar samples. In agreement with the findings of Roeder and Emslie (Contrib. Mineral. Petrol. 29, 275–289) on terrestrial compositions, the logarithms of the conventional distribution coefficients, K_ol-L^Fe and K_ol-L^Fe-Mg, are nearly linear functions of inverse temperature; and the exchange coefficient, K_D = K_ol-L^Fe-Mg, is nearly independent of temperature and composition within a given magma group. There are, however, small but significant differences in conventional and exchange distribution coefficients from one magma group to another, e.g. low-Ti vs high-Ti lunar basalts. It is possible to achieve slightly greater precision for the inverse temperature functions by including terms approximating silica activity in the conventional distribution coefficients. The term ($\text{2Si}\text{O}\text{)}{}_{\mathrm{L}}$ is apparently the best simple approximation for silica activity in olivine-saturated liquids based upon data for Fe, Mg, Mn, Ca, Ti and Cr. Pressure has noticeable effects upon Fe and Mg distribution between olivine and liquid only above 5 kbar.The excellent linear correlation of the logarithms of the distribution coefficients with inverse temperature allows calculation of approximate values of $\text{Δ}\text{H}\text{?}{}^0$ for the reactions : 2MgO_L + SiO_2LaiMg₂SiO_4ol and 2FeO_L + SiO_2LaiFe₂SiO_4ol. Values obtained, approx ?26 kcal/mole, are comparable with values of the heats of fusion of forsterite and fayalite calculated by Bradley (Am. J. Sci.260, 550–554) and measured by Orr (J. Am. Chem. Soc. 75, 528–529).The exchange distribution coefficient for Fe and Mg, K_D, is sensitive to large changes in liquid chemistry. Although K_D is explicitly independent of silica activity, K_D apparently changes with silica concentration. This change is a reflection of changes in the mixing properties of Fe and Mg in liquids with different chemistry and hence structure. Regular solution theory predicts that as the mixing properties of an element in a solution change, the most radical changes in activity coefficients occur in the range of dilute concentrations. Therefore, the distribution coefficients for trace elements will also be dependent upon large changes in liquid chemistry, even if corrections for silica and other liquid component activities are applied.     

新疆富蕴县索尔库都克铜矿床含矿辉石安山玢岩地球化学及年代学特征

新疆富蕴县索尔库都克铜矿产于中泥盆统北塔山组第二亚组中,前人报导的该矿床内含矿的蚀变安山岩Rb-Sr等时线年龄、安山岩全岩40Ar-40Ar年龄以及安山质凝灰岩K-Ar年龄显示其时代主要集中于早二叠世,与赋矿地层时代差距较大.对赋矿岩石安山玢岩进行锆石LA-ICP-MS U-Pb定年,获得了锆石U-Pb谐和年龄为396.6±1.2 Ma,表明其成矿时代为中泥盆世,且在二叠纪有一次热液活动.索尔库都克铜矿床赋矿岩石辉石安山玢岩及安山玢岩具高硅、高钛、高铝特征,岩石为高钾钙碱性系列;稀土元素特征显示轻稀土较重稀土富集,轻微的Eu负异常,安山玢岩的锆石Hf同位素的两阶段模式年龄为456~778 Ma,εHf(t)>0.研究认为该矿床应属于火山热液层控型铜矿床,其成矿安山玢岩体具有亏损地幔源特征.     

Igneous garnet and amphibole fractionation in the roots of island arcs: experimental constraints on andesitic liquids

To evaluate the role of garnet and amphibole fractionation at conditions relevant for the crystallization of magmas in the roots of island arcs, a series of experiments were performed on a synthetic andesite at conditions ranging from 0.8 to 1.2 GPa, 800–1,000°C and variable H₂O contents. At water undersaturated conditions and fO₂ established around QFM, garnet has a wide stability field. At 1.2 GPa garnet + amphibole are the high-temperature liquidus phases followed by plagioclase at lower temperature. Clinopyroxene reaches its maximal stability at H₂O-contents ≤9 wt% at 950°C and is replaced by amphibole at lower temperature. The slopes of the plagioclase-in boundaries are moderately negative in space. At 0.8 GPa, garnet is stable at magmatic H₂O contents exceeding 8 wt% and is replaced by spinel at decreasing dissolved H₂O. The liquids formed by crystallization evolve through continuous silica increase from andesite to dacite and rhyolite for the 1.2 GPa series, but show substantial enrichment in FeO/MgO for the 0.8 GPa series related to the contrasting roles of garnet and amphibole in fractionating Fe–Mg in derivative liquids. Our experiments indicate that the stability of igneous garnet increases with increasing dissolved H₂O in silicate liquids and is thus likely to affect trace element compositions of H₂O-rich derivative arc volcanic rocks by fractionation. Garnet-controlled trace element ratios cannot be used as a proxy for ‘slab melting’, or dehydration melting in the deep arc. Garnet fractionation, either in the deep crust via formation of garnet gabbros, or in the upper mantle via formation of garnet pyroxenites remains an important alternative, despite the rare occurrence of magmatic garnet in volcanic rocks.     

Mixing properties and stability of jadeite-acmite pyroxene in the presence of albite and quartz

The stability of synthetic jadeite-acmite pyroxene coexisting with albite and quartz has been determined at 600, 700, and 900° C. The end-member reaction: albite = jadeite + quartz has been determined to lie between 1.67 and 1.70 GPa at 600° C, 1.88 and 1.90 GPa at 700° C, and 2.44 and 2.48 GPa at 900° C. Jd₇₈Acm₂₂ + quartz is stable above 1.58, 1.78, and 2.33 GPa at 600, 700, and 900° C, respectively. Jd₆₁Acm₃₉ + quartz is stable above 1.47, 1.67, and 2.18 GPa at 600, 700, and 900° C, respectively. Addition of as much as 40% of acmite component in jadeite extends pyroxene stability by less than 300 MPa at 900° C. Unit-cell parameters measured for synthetic jadeite-acmite pyroxenes indicate linear volume-composition relations. The data are consistent with ideal mixing in jadeite-acmite solutions.     

Experimental petrology of alkalic lavas: constraints on cotectics of multiple saturation in natural basic liquids

Experimental study of natural alkalic lava compositions at low pressures (pO₂QFM) reveals that crystallization of primitive lavas often occurs in the sequence olivine, plagioclase, clinopyroxene, nepheline without obvious reaction relation. Pseudoternary liquidus projections of multiply saturated liquids coexisting with plagioclase (±olivine±clinopyroxene±nepheline) have been prepared to facilitate graphical analysis of the evolution of lava compositions during hypabyssal cooling. Use of (TiAl₂)(MgSi₂)_–1 and Fe³⁺ (Al)_–1 exchange components is a key aspect of the projection procedure which is succesful in reducing a wide range of compositions to a systematic graphical representation. These projections, and the experiments on which they are based, show that low pressure fractionation plays a significant role in the petrogenesis of many alkalic lava suites from both continental and oceanic settings. However, the role of polybaric fractionation is more evident in the major element chemistry of these lava suites than in many tholeiitic suites of comparable extent. For example, the lavas of Karisimbi, East Africa, show a range of compositions reflecting a polybaric petrogenesis from primitive picrites at 1360° C/18 kb and leading to advanced low pressure differentiates. Evolved leucite-bearing potassic members of this and other suites may be treated in a nepheline-diopside-kspar (+olivine+leucite) projection. Compositional curvature on the plagioclase+clinopyroxene+olivine+leucite cotectic offers a mechanism to explain resorption of plagioclase in alkalic groundmass assemblages and the incompatibility of albite and leucite. This projection is useful for evaluating the extent of assimilation of the alkalic portions of crustal granulites. Assimilation appears to have played some role in the advanced differentiates from Karisimbi.     

Tracer diffusion of some alkali,alkaline-earth and transition element ions in a basaltic and an andesitic melt,and the implications concerning melt structure

The diffusion properties of Na, Cs, Sr, Ba, Co, Mn, Fe and Sc ions in a basaltic and an andesitic melt have been determined experimentally using the radiotracer residual-activity method, and narrow platinum capillaries, over the temperature range 1,300–1,400° C. Diffusion of all cations follows an Arrhenius relationship; the values of the activation energies range from 24 kcal mol^–1 for Na to 67 kcal mol^–1 for Co in the andesitic melt, and from 39 kcal mol^–1 for Na to 65 kcal mol^–1 for Cs in the basaltic melt. Relative diffusivities in the basaltic melt, but not in the andesitic melt, correlate with assumed ionic radii values. Each cation, except Na⁺, diffuses faster in the basaltic melt than in the andesitic melt over the studied temperature range. Sodium shows similar diffusivity in the two melts.Compensation diagrams incorporating new and some previously-published data indicate that Cs probably diffuses by different mechanisms in different silicate glass and melt systems. Iron has a relatively high activation energy which is consistent with its part occupancy of tetrahedral co-ordination polyhedra.     

砂-黏土混合物高压压实性能试验研究

利用自行设计高压固结仪对5种不同含砂率的饱和砂-黏土混合物开展一系列高压压缩试验,研究砂-黏土混合物压缩性和渗透性演化规律及其对矿物组分的依赖性。试验结果表明,饱和砂-黏土混合物的孔隙比-压力(e-lgP)曲线随着含砂率增高呈现出指数函数模式向双曲线函数模式转化,而e-P曲线在压力高于8.1 MPa则呈线性变化,其斜率依赖于含砂率。最优含砂率随固结压力的增大而逐渐从75%移向30%。不同含砂率的砂-黏土混合物的孔隙率、固结压力和含砂率存在统一的幂函数关系。渗透系数与含砂率有关,渗透系数与孔隙率符合指数函数关系。从砂-黏土混合物骨架结构和孔隙类型的角度出发,从本质上分析含砂率、固结压力对混合物压缩性和渗透性的作用规律。结合SEM微细观结构观测结果,验证砂-黏土混合物骨架结构与含砂率之间的相关性。