Piston-cylinder Experiments Research Articles

Chemical and isotopic fractionation of wet andesite in a temperature gradient: Experiments and models suggesting a new mechanism of magma differentiation

Piston–cylinder experiments were conducted to investigate the behavior of partially molten wet andesite held within an imposed temperature gradient at 0.5 GPa. In one experiment, homogenous andesite powder (USGS rock standard AGV-1) with 4 wt.% H 2O was sealed in a double capsule assembly for 66 days. The temperature at one end of this charge was held at 950 °C, and the temperature at the other end was kept at 350 °C. During the experiment, thermal migration (i.e., diffusion in a thermal gradient) took place, and the andesite underwent compositional and mineralogical differentiation. The run product can be broadly divided into three portions: (1) the top third, at the hot end, contained 100% melt; (2) the middle-third contained crystalline phases plus progressively less melt; and (3) the bottom third, at the cold end, consisted of a fine-grained, almost entirely crystalline solid of granitic composition. Bulk major- and trace-element compositions change down temperature gradient, reflecting the systematic change in modal mineralogy. These changes mimic differentiation trends produced by fractional crystallization. The change in composition throughout the run product indicates that a fully connected hydrous silicate melt existed throughout the charge, even in the crystalline, cold bottom region. Electron Backscatter Diffraction analysis of the run product indicates that no preferred crystallographic orientation of minerals developed in the run product. However, a significant anisotropy of magnetic susceptibility was observed, suggesting that new crystals of magnetite were elongated in the direction of the thermal gradient. Further, petrographic observation reveals alignment of hornblende parallel to the thermal gradient. Finally, the upper half of the run product shows large systematic variations in Fe–Mg isotopic composition reflecting thermal diffusion, with the hot end systematically enriched in light isotopes. The overall δ 56Fe IRMM-14 and δ 26Mg DSM-3 offsets are 2.8‰ and 9.9‰, respectively, much greater than the range of Fe–Mg isotope variation in high-temperature terrestrial samples. In contrast, no obvious chemical differentiation was observed in a similar experiment (of 33 days duration) where the temperature ranged from 550 to 350 °C, indicating the critical role of the melt in causing the differentiation observed in the 950–350 °C experiment. If temperature gradients can be sustained for the multi-million-year time scales implied by geochronology in some plutonic systems, thermal migration could play a heretofore unrecognized role in the development of differentiated plutons. Elemental distributions, dominated by phase equilibria, cannot be used to discriminate thermal migration from conventional magma differentiation processes such as fractional crystallization. However, the observation of Fe–Mg isotopic variations in partially molten portions of the experiment indicates that these isotopic systems could provide a unique fingerprint to this process. This result could also provide a possible explanation for the Fe–Mg isotope variations observed in high-temperature silicate rocks and minerals.

Oxygen fugacity, temperature reproducibility, and H2O contents of nominally anhydrous piston-cylinder experiments using graphite capsules

aB stra C t The Pt-graphite double-capsule technique is a very commonly used method in high-temperature, high-pressure experimental petrology, particularly for anhydrous experiments relevant to primitive basaltic magmas and mantle melting. We have performed a series of experiments that place better constraints on the range of oxygen fugacity imposed by this capsule material, on the Fe 3+ /Fe 2+ ratios in experimentally produced melts and minerals, and on the temperature reproducibility in Pt-graphite capsules. Oxygen fugacity in our piston-cylinder experiments using Pt-graphite capsules is CCO-0.7 (IW+1.5, QFM-2.2) at 1.5 GPa and 1360 °C. Comparison with other estimates and thermodynamic calculations indicate that a value of CCO-0.8 ± 0.3 can be used as a first approximation at least over the P-T range relevant for MORB and OIB magma generation (0.5–3.0 GPa, 1100–1500 °C). Under those conditions, the amount of Fe 3+ in silicate phases (pyroxenes, olivine, glass) and spinel is negligible (Fe 3+ / ΣFe < 0.05) and would not significantly affect thermodynamic properties. Significantly higher values of fO2 cannot be achieved using Pt-graphite or graphite only capsules, but fO2 can be tuned to lower values by using small pieces of PtFe alloys. The potential range of fO2 that can be reached in graphite or Pt-graphite capsules is CCO to CCO-4. Temperature reproducibility in piston-cylinder experiments has been examined and can be as low as ±10 °C. Finally, unless capsules are dried overnight at 400 °C before the experiment, small amounts of H 2O are always present in nominally dry experiments. These small amounts of H 2O should not, however, significantly change phase relations.

A model for the viscosity of rhyolite as a function of H 2O-content and pressure: A calibration based on centrifuge piston cylinder experiments

The Newtonian viscosity of synthetic rhyolitic liquids with 0.15–5.24 wt% dissolved water was determined in the interval between 580 and 1640 °C and pressures of 1 atm and 5–25 kbar. Measurements were performed by combining static and accelerated (up to 1000 g) falling sphere experiments on water-bearing samples, with high temperature concentric cylinder experiments on 0.15 wt% H 2O melts. These methods allowed viscosity determinations between 10 2 and 10 7 Pa s, and cover the complete range of naturally occurring magmatic temperatures, pressures, and H 2O-contents for rhyolites. Our viscosity data, combined with those from previous studies, were modeled by an expression based on the empirical Vogel–Fulcher–Tammann equation, which describes viscosities and derivative properties (glass transition temperature T g , fragility m, and activation volume of viscous flow Va) of silicic liquids as a function of P- T- X (H2O). The fitted expressions do not account for composition-dependent parameters other than X (H2O) and reproduce the entire viscosity database for silicic liquids to within 3.0% average relative error on log η (i.e. std. error of estimate of 0.26 log units). The results yield the expected strong decrease of viscosity with temperature and water content, but show variable pressure dependencies. Viscosity results to be strongly affected by pressure at low pressures; an effect amplified at low temperatures and water contents. Fragility, as a measure for the deviation from Arrhenian behavior, decreases with H 2O-content but is insensitive to pressure. Activation volumes are always largely negative (e.g., less than −10 cm 3/mol) and increase strongly with H 2O-content. Variations in melt structure that may account for the observed property variations are discussed.

Temperature–pressure–volume equation of state of the B1 phase of sodium chloride

The temperature–pressure–volume ( T– P– V) equation of state (EOS) of the B1 phase of NaCl is simulated using the molecular dynamics (MD) method with a breathing shell model (BSM) for Cl ions. Quantum correction to the MD pressures is made using the Wigner–Kirkwood expansion of free energy. Required energy parameters, including Cl breathing parameters, are derived empirically to reproduce the observed molar volume, volume thermal expansion, elastic constants, T– P– V data from piston-cylinder experiments and the temperature dependence of bulk modulus, all as accurately as possible. The observed volume-compression data of the high pressure B2 phase of NaCl are also included to optimize the energy parameters. The MD simulation with BSM is found to accurately reproduce the available measured T– P– V data of both the B1 and the B2 phases. The MD simulated T– P– V EOS is presented up to 30 GPa and 1200 K. Compared with the present MD EOS, the Decker EOS, which has often been used as a practical pressure scale for many years, underestimates pressure by within 4% over the T and P ranges up to 30 GPa and 1200 K. The error in the Decker EOS increases with increasing pressure, with the differences in pressure reaching 0.8 GPa at 300 K and 20 GPa, and 0.7 GPa at 1200 K and 20 GPa.

Diffusion of Au, Pd, Re, and P in FeNi alloys at High Pressure

Multi-anvil and piston cylinder experiments were performed to determine the effect of both pressure and temperature on the diffusivities of several siderophile elements in a Fe 90Ni 10 core-analog alloy. Activation energies were calculated to be between 244 and 257 kJ/mol for Re, Pd, and Au at 10 GPa, and 264 kJ/mol for P at 1 GPa. It was found that pressure has a marked negative effect on the diffusivities of Au, Re, and Pd, and activation volumes for these elements were calculated to be between 3 and 6 cm 3/mol at 10 GPa. The effect of both temperature and pressure on P diffusion is noticeably less, and the absolute diffusivity of phosphorus is consistently higher than that of the other elements. It is inferred that the reason for this difference is because P is diffusing via an interstitial mechanism as opposed to Re, Pd, and Au which occupy regular lattice sites in the crystal. The effect of pressure and temperature together with depth in the Earth suggests that these elements may continue to exhibit different behavior at more extreme conditions. The significance of these new results lies in the ability to place constraints on many time-dependent processes that pertain to the formation and evolution of planetary cores, as well as the formation and cooling histories of other metal-rich bodies in the solar system, such as meteorites.

The effect of water on Si and O diffusion rates in olivine and implications for transport properties and processes in the upper mantle

We performed piston cylinder experiments (1200–1350 °C, 2 GPa) to determine the diffusion rates of Si and O in mantle olivine under water undersaturated (brucite absent, 45 ppm H 2O in olivine) as well as close to water-saturated (brucite present, ∼370 ppm H 2O in olivine) conditions. Diffusion couples consisted of oriented and polished San Carlos olivine cylinders coated with thin (∼few 100 nm) films of the same composition enriched in 29Si and 18O, with a protective coating of ZrO 2 on top. Relationships between water solubility in olivine and water fugacity, combined with thermodynamic equilibrium calculations, indicate fH 2O ∼1 GPa, fO 2 ∼IW buffer for brucite absent and fH 2O ∼9 GPa, fO 2 ∼QFM buffer for brucite present experiments. We find that under hydrous conditions D Si ≈ D O and diffusion anisotropy is weak to non-existent. Fitting the raw data at 2 GPa and fH 2O ∼0.93 GPa yields Arrhenius parameters [ D o and E p in D = D o exp(− E p/ RT)] of: 1.68 (±3.52) × 10 −7 m 2 s −1 and 358 ± 28 kJ mol −1 for Si, and 1.43 (±1.80) × 10 −4 m 2 s −1 and 437 ± 17 kJ mol −1 for O, respectively (1 sigma errors). D (2 GPa, fH 2O = 0.97 GPa, 1200 °C): D (1 atm., dry, 1200 °C) is 1000 for Si and 10 for O, respectively. Equations incorporating explicitly the effect of water are discussed in the text. Analysis of our data suggests that O diffuses by an interstitial mechanism whereas Si diffuses via vacancy complexes. The relation between the water fugacity and the Si diffusion rates seems to obey a power law with a water fugacity exponent of 0.2–1. The amount of H incorporated into olivine at the experimental conditions is orders of magnitude higher than the likely concentration of Si vacancies. Therefore, a small fraction (∼0.01%) of the total incorporated H in olivine suffices to considerably enhance the concentration of Si vacancies, and hence diffusion rates. Activation energies for O diffusion under dry and wet conditions are similar, indicating that the mechanism of this diffusion does not change in the presence of water. This inference is consistent with results of computer simulations. Dislocation creep in olivine under wet conditions appears to be controlled by both, Si as well as O diffusion. Absolute creep rates can be calculated from the diffusion data if it is assumed that climb and glide of dislocations contribute equally to creep. Finally, analysis of the various transport properties indicate that <10 ppm of water in olivine is sufficient to cause a transition from “dry” to “wet” laws for most processes. As these water contents are even lower than the observed water contents in most mantle olivines (i.e. minimum values measured at the surface), we conclude that results of water present but undersaturated kinetic experiments are directly applicable to the mantle. Indeed, “wet” kinetic laws should be used for modeling geodynamic processes in the upper mantle, even if the mantle is thought to be undersaturated with respect to water.

The origin of the negative niobium tantalum anomaly in subduction zone magmas

In order to determine the distribution of niobium and tantalum between clinopyroxenes and aqueous fluids, we measured in two separate sets of experiments niobium and tantalum solubility in clinopyroxenes and in aqueous fluids, respectively. The solubility of niobium in clinopyroxenes was experimentally investigated in the system CaMgSi2O6–NaAlSi2O6–Nb2O5–H2O at 1.5 GPa and 700–1100 °C using piston-cylinder experiments. In the presence of excess Nb2O5, CaNb2O6 coexists with a clinopyroxene. The solubility of niobium in the pyroxene increases drastically with the aluminum content. While the solubility of niobium is in the order of 100–300 ppm by weight for Al-poor clinopyroxenes, it reaches 4 wt.% for clinopyroxenes containing 10 wt.% Al2O3. Microprobe analyses suggest that Nb is incorporated in the clinopyroxene as NaNbAl2O6 component. The solubility of CaNb2O6 in aqueous fluid was determined by the direct visual observation of the dissolution of CaNb2O6 crystals in an aqueous fluid using an externally-heated diamond anvil cell. At 1.5 to 1.7 GPa and 800–1000 °C, an aqueous fluid saturated with diopside dissolves only 20–100 ppm by weight of CaNb2O6, i.e. niobium solubility in the fluid is orders of magnitude below the solubility in aluminous clinopyroxenes. Experiments on the solubility of CaTa2O6 in clinopyroxenes and in aqueous fluid suggest that Ta behaves generally similar to Nb, but with the notable exception that the solubility of CaTa2O6 in aqueous fluids and aluminous pyroxenes is approximately a factor of five lower than the solubility of the corresponding niobium compound. Our results imply that (1) the fluid/clinopyroxene partition coefficient for Nb and Ta is between 0.1 and 0.001 for aqueous fluids containing little dissolved silicates and for clinopyroxenes of a composition realistic for the subducted slab or the subarc mantle. Only for very Al-poor clinopyroxenes may the partition coefficient approach unity. (2) The depletion of Nb and Ta in subduction zone magmas and fluids is related to the intrinsically low solubility of Nb and Ta in water-rich and solute-poor fluids in the shallow parts of a subduction zone. (3) The presence of rutile in the subducted slab is not a necessary requirement for the development of the negative Nb and Ta anomaly. (4) Fluid transport of niobium and tantalum will affect the Nb/Ta ratio in the subarc mantle only under exceptional circumstances. (5) Previous reports of high fluid/clinopyroxene partition coefficients for Nb and Ta are probably related to the Al-free or Al-poor composition of the systems studied.

Phase Relations and Melting of Anhydrous K-bearing Eclogite from 1200 to 1600 C and 3 to 5 GPa

To investigate eclogite melting under mantle conditions, we have performed a series of piston-cylinder experiments using a homogeneous synthetic starting material (GA2) that is representative of altered mid-ocean ridge basalt. Experiments were conducted at pressures of 30, 40 and 50 GPa and over a temperature range of 1200^16008C. The subsolidus mineralogy of GA2 consists of garnet and clinopyroxene with minor quartz^coesite, rutile and feldspar. Solidus temperatures are located at 12308C at 30 GPa and 13008C at 50GPa, giving a steep solidus slope of 30^408C/GPa. Melting intervals are in excess of 2008C and increase with pressure up to 50GPa. At 30 GPa feldspar, rutile and quartz are residual phases up to 408C above the solidus, whereas at higher pressures feldspar and rutile are rapidly melted out above the solidus. Garnet and clinopyroxene are the only residual phases once melt fractions exceed 20% and garnet is the sole liquidus phase over the investigated pressure range. With increasing melt fraction garnet and clinopyroxene become progressively more Mg-rich, whereas coexisting melts vary from K-rich dacites at low degrees of melting to basaltic andesites at high melt fractions. Increasing pressure tends to increase the jadeite and Ca-eskolaite components in clinopyroxene and enhance the modal proportion of garnet at low melt fractions, which effects a marked reduction in the Al2O3 and Na2O content of the melt with pressure. In contrast, the TiO2 and K2O contents of the low-degree melts increase with increasing pressure; thus Na2O and K2O behave in a contrasted manner as a function of pressure. Altered oceanic basalt is an important component of crust returned to the mantle via plate subduction, so GA2 may be representative of one of many different mafic lithologies present in the upper mantle. During upwelling of heterogeneous mantle domains, these mafic rock-types may undergo extensive melting at great depths, because of their low solidus temperatures compared with mantle peridotite. Melt batches may be highly variable in composition depending on the composition and degree of melting of the source, the depth of melting, and the degree of magma mixing. Some of the eclogite-derived melts may also react with and refertilize surrounding peridotite, which itself may partially melt with further upwelling. Such complex magma-genesis conditions may partly explain the wide spectrum of primitive magma compositions found within oceanic basalt suites.

Magmatic implications of mantle wedge plumes: Experimental study

Numerical and laboratory experiments beside natural observations suggest that hydration and partial melting along the subducting slab can trigger Rayleigh–Taylor instabilities that evolve into partially molten diapiric structures (“cold plumes”) that rise through the hot asthenospheric wedge. Mixed cold plumes composed of tectonic melanges derived from subduction channels can transport the fertile subducted crustal materials towards hotter zones of the suprasubduction mantle wedge leading to the formation of silicic melts. We investigate magmatic consequences of this plausible geodynamic scenario by using an experimental approach. Melt compositions, fertility and reaction between silicic melts and the peridotite mantle (both hydrous and dry) were tested by means of piston–cylinder experiments at conditions of 1000°C and pressures of 2.0 and 2.5GPa. The results indicate that silicic melts of trondhjemite and granodiorite compositions may be produced in the ascending mixed plume megastructures. Our experiments show that the formation of an Opx-rich reaction band, developed at the contact between the silicic melts and the peridotite, protect silicic melts from further reaction in contrast to the classical view that silicic melts are completely consumed in the mantle. The mixed, mantle-crust isotopic signatures which are characteristic of many calc-alkaline batholiths are also expected from this petrogenetic scenario.

The effect of oxygen fugacity on hydroxyl concentrations and speciation in olivine: Implications for water solubility in the upper mantle

The effect of oxygen fugacity on hydroxyl speciation and solubility in San Carlos olivine was investigated in a series of piston cylinder experiments at 2.0 GPa and 1100 and 1300 °C. The presence of enstatite was intended to fix silica activity and experimental fO 2 in most runs was regulated by lining the inner walls of the sample-containing Pt capsules with either Fe, Ni or Re foil. Polarised infrared absorbance spectra show that olivine annealed in capsules lined with Fe foil only contain hydroxyl defects with vibrational energies between 3650 and 3400 cm − 1 (relatively high frequencies). Spectra of crystals annealed in capsules lined with Ni or Re foil show that these samples contain hydroxyl defects with vibrational energies between 3650 and 3400 cm − 1 (high frequency), and at 3355 and 3325 cm − 1 (low-frequency). The concentrations of hydroxyl defects responsible for absorptions at 3355 and 3325 cm − 1 change as a function of oxygen fugacity and are, therefore, interpreted as Fe 3+-related OH centres. Hydroxyl solubility in our experiments is substantially less than current solubility laws might suggest. These differences may be attributed to variations in experimental procedure, but they also demonstrate that the incorporation of hydroxyl in olivine must be better understood before an accurate model to describe water solubility in the deep Earth can be constructed. Our data show that, when experiments conducted at identical P– T conditions are compared, olivine annealed at low fO 2 (below Fe–FeO) contains less than half as much water as crystals from relatively more oxidised experiments. Because natural mantle conditions approximate to those of our low fO 2 experiments, current solubility laws – which are based on data from relatively high fO 2 experiments – overestimate the amount of water that can be hosted in olivine in the upper mantle. From a consideration of defect equilibria we propose a mechanism for the incorporation of OH in olivine in a way that is appropriate for the actual range of conditions expected in the mantle.

EFFECTS OF P2O5 AND TiO2 ON THE PARTIAL MELTING OF SPINEL LHERZOLITE IN THE SYSTEM CaO-MgO-Al2O3-SiO2 AT 1.1 GPa

TheeffectsofsmallamountsofP2O5�onthepartialmeltingofmodelspinellherzoliteinthesystemCaO-MgO-Al2O3-SiO2� +� P2O5 have been investigated at 1.1 GPa in piston-cylinder experiments, in order to constrain the influence of this minor componentonmantlemelting.�Aconcentrationof�1�wt%�P2O5�inthemeltdepressesthesolidusofspinellherzolitebyonly�2°C,� reducesSiO2�inmeltsby�1�wt%�andAl2O3�by�0.44�wt%,�butincreasesCaOby�0.3�wt%�andMgOby�0.14�wt%.�Apreliminary� investigationoftheeffectofTiO2�inthesystemCaO-MgO-Al2O3-SiO2�+�TiO2�showsthatitdecreasesthesolidusby�~1°C/wt%,� anditseffectonthemeltcompositionissmall,�beingmainlyoneofdilution.

New high-pressure and high-temperature metal/silicate partitioning of U and Pb: Implications for the cores of the Earth and Mars

In order to quantify possible fractionation of U and Pb into a metallic core, we have performed piston cylinder and multi-anvil press experiments at high pressure (up to 20 GPa) and high temperature (up to 2400 °C) and obtained the distribution coefficient D metal–silicate and the exchange partition coefficient K metal–silicate for these elements between metal and silicates (mineral or liquid). D Pb metal–silicate and D U metal–silicate depend strongly on the S content of the metallic phase, and also on the oxygen fugacity, in agreement with an effective valence state of 4 for U in silicates and 2 for Pb in silicates. K Pb d metal–silicate and K U d metal–silicate show no discernable pressure and temperature trend. U remains lithophile even at high pressure and high temperature but its lithophile nature decreases at very low oxygen fugacity. From our experimental data, it was possible to calculate the U and Pb contents of the cores of Mars and Earth under core-mantle equilibrium conditions at high pressure and high temperature. From the D metal–silicate of the present study, we obtained that: 0.008 ppm < Pb in the core <4.4 ppm, and 0.0003 ppb < U in the core < 0.63 ppb, depending on whether the metal is S-free or S-saturated respectively, and if the mantle was molten or solid during the segregation process of the Earth’s core around ΔIW-2. For Mars, based on a core segregation process around ΔIW-1, we obtained that: 0.005 ppm < Pb in the core < 3 ppm, and 0.00002 ppb < U in the core < 0.05 ppb, depending on the metallic composition: S-free or S-saturated respectively. Our results suggest that the low concentration of Pb in the terrestrial mantle could not be explained by an early Pb sequestration in the Earth’s core even if S is the dominant light element of the core. If we assume a magma ocean scenario, U might produced a maximum value of 1.5% of the total heat budget of the core with a segregation occurring below ΔIW-3. The values found in the present study for U in the Martian core suggest that the magnetic field activity of Mars before ∼0.5 b.y. after its formation would be difficult to ascribe to the decay of U alone.

Experimental constraints on element mobility from subducted sediments using high-P synthetic fluid/melt inclusions

A series of hydrothermal piston-cylinder experiments have been performed to determine the composition of representative fluids and fluid/melt/rock interaction in subduction zones. Experiments were conducted under H 2O saturated conditions at 2.2 GPa over a temperature range from 600–750 °C. The experiments contained synthetic, trace-element-doped pelitic starting material and fractured quartz chips to trap and preserve synthetic fluid/melt inclusions. Pelite residues from the subsolidus experiments (600–650 °C) consist of an eclogite-facies mineral assemblage including quartz, phengite, epidote, rutile, garnet, amphibole, apatite, and zircon. Coexisting hydrous fluids are expected to be completely buffered for trace elements by this mineral assemblage. At 2.2 GPa the wet solidus for the pelitic starting material is located at approximately 675 °C and hydrous fluid and melt coexist as immiscible phases at least up to 750 °C. Residue phases in the supersolidus experiments (700–750 °C) are garnet, rutile, and zircon, which suggest that HREE and HFSE are largely retained in slab residues during very high degrees of H 2O saturated melting. Laser ablation ICPMS analysis and quantification of trapped fluid inclusions from the experiments indicate that subsolidus hydrous fluids released from subducted sediments have relatively high LILE contents compared to REE and HFSE, but overall are remarkably dilute. Total solute contents are approximately 5 wt.%, of which > 75% is SiO 2 and around 15% is Na 2O + Al 2O 3. The experimental results are used to show that subducting sedimentary rocks do not undergo significant element loss during metamorphic dehydration up to eclogite facies. If these fluids are representative of aqueous fluids released at sub-arc depths then simple slab dehydration models may be unable to account for element transfer from the slab to arc magmas. Instead, element recycling through subduction zones may be a product of complex fluid-melt-rock interaction processes involving multiple slab components.

Effect of melt structure on trace-element partitioning between clinopyroxene and silicic, alkaline, aluminous melts

We have investigated trace-element partitioning between clinopyroxene and silicic melts ranging from 62 to 69 wt% SiO 2 in piston-cylinder experiments at 1.1 GPa and 1145-1155 °C. Run products consist of diopsidic clinopyroxene (with 3-5 wt% Al 2 O 3 and ∼1 wt% Na 2 O) coexisting with silicic, aluminous, and alkaline melts. Clinopyroxene-melt partition coefficients for rare earth elements (REE) are significantly higher (e.g., D Sm = 0.76 to 3.1) than previous studies of clinopyroxene-basaltic melt partitioning. The maximum D REE is also shifted toward middle REE (e.g., Tb). Although the partition coefficients of Sr and Ti (D Sr = 0.23-0.39, D Ti = 0.28-1.0) are also elevated relative to clinopyroxene-basaltic melt, partition coefficients for Zr and Li (0.14-0.23 and 0.16-0.25, respectively) are within the range of literature values. The high partition coefficients for REE, Sr, and Ti in this study do not reflect crystal-compositional effects. For instance, no relationship is observed between IV Al and these partition coefficients. REE and Ti partition coefficients correlate poorly with jadeite component of the clinopyroxene; however, the elevated Na 2 O contents of both the melt and clinopyroxene make it difficult to discriminate fully the effect of jadeite component from the effect of changes in melt structure. The non-bridging oxygen to tetrahedral cation ratio (NBO/T; Mysen et al. 1985) varies from 0.08 to 0.17 and roughly correlates with epx/melt D REE , consistent with a recent study (Gaetani 2004) that concluded that melt structure significantly affects partitioning when NBO/T < 0.49. Notably, we find a strong correlation between the molar Ca 2+ /(M + + M 2+ ) of the melt and the optimum D (D 0 ) for the REE, indicating that melt structure, and not jadeite content, is the dominant control on clinopyroxene-melt partitioning in these experiments. The partition coefficients for the REE, Ti, Zr, and Sr are in good agreement with observed clino-pyroxene-silicic glass partitioning within mantle xenoliths (Chazot et al. 1996; Vannucci et al. 1998), indicating these results may apply to lithospheric melting. Furthermore, the origin of hump-shaped REE patterns observed in residual clinopyroxene from abyssal peridotites could be explained if shallow depleted melts of peridotite are silica-rich. A two-stage fractional melting model assuming clinopy-roxene-silicic melt partitioning at shallow mantle depths from 10 to 30 km beneath a mid-ocean ridge can reproduce the observed hump-shaped REE patterns. If so, hump-shaped REE patterns may provide evidence for the existence and extraction of silica-rich melt created in the shallow mantle.

Phase equilibrium experiments at 0.5 GPa and 1100–1300 °C on a basaltic andesite from Arenal volcano, Costa Rica

We present results from piston–cylinder experiments on a synthetic composition of basaltic andesite that corresponds to lavas erupted from the ongoing eruption at Arenal volcano, Costa Rica, in order to shed light on magmatic processes at upper crustal depths beneath Arenal. The starting composition represents the least evolved basaltic andesite from the initiation of stage 2 of the current eruption. Anhydrous and hydrous experiments were conducted at 0.5 GPa and 1100–1300 °C: the principal phases encountered were melt, plagioclase, orthopyroxene and clinopyroxene of variable CaO content. Glass and plagioclase compositions change in a consistent manner with decreasing temperature for both hydrous and anhydrous experiments. The phase equilibria dictate that Arenal magmas must have contained > 2 wt.% H 2O in order for the erupted rocks to have once represented liquid compositions at a relatively high temperature (1200 °C) and > 4 wt.% H 2O if the melt was at the lower temperatures (≤ 1150 °C) that are more likely for the Arenal system. However, anorthite-rich plagioclase phenocrysts (> An 85) commonly found in Arenal lavas cannot be accounted for by any reasonable permutation of higher temperature and water content. The close correspondence of the phase compositions (rims of plagioclase, orthopyroxene) and crystallinity observed in stage 2 lavas from Arenal and a hydrous experiment with 2 wt.% water in the melt provides evidence for Arenal magmas ponding and equilibrating at 1150 °C and ∼ 12–14 km depth. The conclusion that Arenal lavas reflect equilibration between observed minerals and a melt with ∼ 2 wt.% H 2O at 0.5 GPa, ∼ 1150 °C, argues that these bulk compositions are unlikely to have ever reflected fully molten liquids.

Solubility and partitioning of water in synthetic forsterite and enstatite in the system MgO–SiO2–H2O±Al2O3

The solubility of water in coexisting enstatite and forsterite was investigated by simultaneously synthesizing the two phases in a series of high pressure and temperature piston cylinder experiments. Experiments were performed at 1.0 and 2.0 GPa at temperatures between 1,100 and 1,420°C. Integrated OH absorbances were determined using polarized infrared spectroscopy on orientated single crystals of each phase. Phase water contents were estimated using the calibration of Libowitzky and Rossman (Am Mineral 82:1111–1115, 1997). Enstatite crystals, synthesized in equilibrium with forsterite and an aqueous phase at 1,350°C and 2.0 GPa, contain 114 ppm H2O. This is reduced to 59 ppm at 1,100°C, under otherwise identical conditions, suggesting a strong temperature dependence. At 1,350°C and 1.0 GPa water solubility in enstatite is 89 ppm, significantly lower than that at 2.0 GPa. In contrast water solubility in forsterite is essentially constant, being in the range 36–41 ppm for all conditions studied. These data give partition coefficients \( {\text{D}}_{{{\text{H}}_{2} {\text{O}}}} ^{{{\text{en}}/{\text{fo}}}} \) in the range 2.28–3.31 for all experiments at 1,350°C and 1.34 for one experiment at 1,100°C. The incorporation of Al2O3 in enstatite modifies the OH stretching spectrum in a systematic way, and slightly increases the water solubility.

Diffusion–reaction in a thermal gradient: Implications for the genesis of anorthitic plagioclase, high alumina basalt and igneous mineral layering

Piston-cylinder experiments investigating the interaction between basaltic andesite melt and partially molten gabbro in a thermal gradient provide insight into melt-rock reaction processes occurring during magma differentiation in the crust. In two experiments juxtaposing basaltic andesite and gabbro at 0.5 GPa pressure for durations of either 13 or 26 days, diffusive chemical exchange between the two materials results in mineral layering and notable mineral compositions such as anorthitic plagioclase. Specifically, the basaltic andesite gains Al 2O 3, MgO and CaO from the gabbro and loses Na 2O, K 2O, SiO 2 and FeO to it with a plagioclase-rich layer developing at the interface between the two materials in a process termed diffusion–reaction. The percent crystallinity of the basaltic andesite increases during the process and the plagioclase crystals within the interface region develop anorthitic cores (up to An 90) that abruptly shift in composition to thin rims that are in Na–Ca exchange equilibrium with the co-existing melt. Both the mineralogical layering and bulk compositional change occurring at the interface are reproduced in model simulations of diffusion–reaction. Isotopic tracers ( 45Ca, 6Li, 84Sr and 136Ba) initially deposited at the basaltic andesite–gabbro interface in the 13-day experiment were detected in the cores of the anorthitic plagioclase after the experiment, demonstrating that the melt chemically communicates with the plagioclase cores over the duration of the diffusion–reaction experiment. The formation of anorthitic plagioclase during diffusion–reaction may explain its widespread occurrence in terrestrial volcanic rocks without requiring the presence of ultra-calcic melts. Textures and mineralogical changes in the gabbro indicate that chemical transport occurs throughout the experiments despite temperatures at the cold end of the experimental capsule approaching 500 °C. For instance, apatite, FeNiS, olivine and almost pure albite occur at distinct, specific horizons in the gabbro within the 26-day experiment. Because the bulk element profiles indicating chemical transport reflect analyses of almost completely solid gabbro, equilibration between minerals and fluids/melts must be rapid. The overall effect of the diffusion–reaction process is to make an ascending magma more primitive in composition (and in this case, produce anorthitic plagioclase) while making surrounding crustal wall rocks more evolved. Several observations within igneous rocks support the occurrence of this process, suggesting that the genesis of porphyritic high alumina basalt, ubiquitously observed at convergent margins, could reflect a diffusion–reaction process in the crust.

Experimental constraints on the formation of high-P/high-T granulites in the Southern Bohemian Massif

Large bodies of felsic high- P /high- T granulites with the assemblage quartz + ternary feldspar (mesoperthite) + garnet + rutile ± kyanite occur in the Southern Bohemian Massif. They are thought to have formed during the Variscan orogeny in a Carboniferous subduction setting, at 950–1050°C and 1.5–1.9 GPa, from granitic protoliths. Deformation and retrograde reactions during subsequent, near-isothermal, exhumation lead to considerable difficulties in the precise reconstruction of the original peak P-T phase relations and mineral compositions, and hence the processes and timing of granulite formation in the Southern Bohemian Massif are presently not fully understood and still quite controversially discussed. In order to assess the processes that occurred close to the P-T peak, fluid-absent piston cylinder experiments were conducted. We used a granitic gneiss as starting material with the assemblage K-feldspar + plagioclase + quartz + biotite + muscovite, whose chemical composition almost perfectly matches the main granulite type of the Southern Bohemian Massif. The experimental conditions were chosen to simulate the metamorphic P-T path determined for the granulites, with runs at 750°C/1.6 GPa, 800°C/1.6 GPa, 850°C/1.6 GPa, 900°C/1.6 GP and 1000°C/1.6 GPa (prograde path) and 950°C/1.4 GPa, 900°C/1.2 GPa and 800°C/1.2 GPa (retrograde path). The experiments in the temperature range of 850–1000°C all yielded the typical granulite assemblage garnet + ternary feldspar + quartz ± kyanite ± rutile. At 1000°C/1.6 GPa roughly 30–40 vol.% partial melt was present, and large melt volumes persisted also on the retrograde path (>40 vol. % at 950°C/1.4 GPa, ~25 vol. % at 900°C/1.2 GPa). The melt-forming reaction observed in the experiments is: biotite + plagioclase + quartz = garnet + ternary feldspar + melt. At pressures of 1.6 GPa, this reaction commences at temperatures >750°C and goes to completion between 800°C and 850°C. The melts obtained are strongly peraluminous granitic in composition with A/CNK ranging from 1.11 to 1.43. K-rich feldspars are close to Ab 28 Or 72 An 0 at 750°C/1.6 GPa and change to Ab 48 Or 47 An 5 at 1000°C/1.6 GPa. Coexisting Na-rich feldspars are stable up to 850°C/1.6 GPa and change from Ab 92 Or 2 An 6 at 750°C/1.6 GPa to Ab 63 Or 30 An 7 at 850°C/1.6 GPa. Garnets are almandine-pyrope solid solutions with high Ti contents ranging from ~0.2 wt. % TiO 2 at 750°C/1.6 GPa to ~1.8 wt. % TiO 2 at 1000°C/1.6 GPa, thus indicating a strong increase of the Ti content with temperature, whereas only a slight correlation with pressure was found in the experiments at 800°C. Notably, the TiO 2 contents of the natural granulite garnets are generally low ( 2 contents in the range of ca. 0.3 wt. %, which is two to three times the whole rock content. As a consequence, almost no rutile formed in the runs at T>900°C. This leads to the important conclusion that the big (0.2-1 mm) euhedral accessory rutiles, which are very characteristic for the granulites of the Southern Bohemian Massif, did not form at peak P-T conditions, but most likely during the decompression and cooling stage, in the presence of melt. This observation supports previous numerical models according to which accessory minerals like zircon, monazite or rutile form in melt-bearing granulites after the P-T peak and date cooling histories rather than the metamorphic climax.

Piston-cylinder experiments on H2O undersaturated Fe-bearing systems: An experimental setup approaching fO2 conditions of natural calc-alkaline magmas

In this study, we present a modified double-capsule technique to perform experiments on H 2 O undersaturated, Fe-bearing systems at elevated pressures and temperatures and oxygen fugacities (f O₂ ) relevant for natural calc-alkaline magmas. Welded shut, Fe-preconditioned Au 90 Pd 10 capsules were placed in an outer Pt capsule that contains the same starting material. Experiments were performed at 1.0 GPa and 1200 °C using a synthetic, hydrous basalt and run with either boron nitride (BN) or MgO surrounding the welded capsules. Optimum results were obtained by using Fe-preconditioned Au 90 Pd 10 inner capsules in combination with MgO assemblies. The application of the modified doublecapsule technique with Fe-preconditioned inner AuPd capsules reduced Fe loss to less than 3% relative, conserved H 2 O within the error of ion-microprobe analyses, and kept the f O₂ (QFM+1.1) within 1 log unit of the initial value constrained by the Fe 2 O 3 /FeO ratio of the starting material (QFM+0.43). These conditions are similar to estimates of f O₂ during the crystallization of natural calc-alkaline magmas.

The crystal data and stability of calcite III at high pressures based on single-crystal X-ray experiments

Single-crystal X-ray experiments have been performed for calcite, calcite II, and calcite III at 300 K and pressures up to 4.1 GPa. The volume decrease from calcite to calcite II is estimated to be 0.54 cm3/mol (1.5 %) at 1.5 GPa, which compares well with the value 0.48 cm3/mol in the calcite to calcite II transition at 1.4 GPa reported previously based on piston-cylinder volume-compression experiments. The crystal data of calcite III have been determined using single-crystal X-ray analyses at 300 K and pressures between 2.3 and 4.1 GPa. The calcite III crystal is found to be monoclinic with the space group either C2, Cm, or C2/m from the systematic absence of reflections, and with the refined cell parameters of a = 8.707(7), b = 9.315(6), c = 6.154(4) A, and β = 115.18(2)° at 2.3 GPa. These crystal data of calcite III at 2.3 GPa yield a density of 2.94 g/cm3 for Z = 8, which is much less than the previously estimated densities, ranging from 3.11 to 3.28 g/cm3 at pressures from 1.9 to 2.9 GPa, using powder X-ray analyses based on orthorhombic structural models for calcite III, but is generally consistent with the reported data by the piston-cylinder experiments. Thus the present study reveals the density of calcite III is substantially less than that of aragonite (3.02 g/cm3 at 2.3 GPa), indicating that calcite III is metastable with respect to aragonite at high pressures.