Charles E. Lesher

Isotope fractionation in silicate melts by thermal diffusion

The phenomenon of thermal diffusion (mass diffusion driven by a temperature gradient, known as the Ludwig–Soret effect) has been investigated for over 150 years, but an understanding of its underlying physical basis remains elusive. A significant hurdle in studying thermal diffusion has been the difficulty of characterizing it. Extensive experiments over the past century have established that the Soret coefficient, ST (a single parameter that describes the steady-state result of thermal diffusion), is highly sensitive to many factors. This sensitivity makes it very difficult to obtain a robust characterization of thermal diffusion, even for a single material. Here we show that for thermal diffusion experiments that span a wide range in composition and temperature, the difference in ST between isotopes of diffusing elements that are network modifiers (iron, calcium and magnesium) is independent of the composition and temperature. On the basis of this finding, we propose an additive decomposition for the functional form of ST and argue that a theoretical approach based on local thermodynamic equilibrium holds promise for describing thermal diffusion in silicate melts and other complex solutions. Our results lead to a simple and robust framework for characterizing isotope fractionation by thermal diffusion in natural and synthetic systems.

Evidence from the rare-earth-element record of mantle melting for cooling of the Tertiary Iceland plume

Widespread flood basalt volcanism and continental rifting in the northeast Atlantic in the early Tertiary period (∼55 Myr ago) have been linked to the mantle plume now residing beneath Iceland. Although much is known about the present-day Iceland plume, its thermal structure, composition and position in the early Tertiary period remain unresolved. Estimates of its temperature, for example, range from >1,600 °C in some plume models to ∼1,500 °C based on the volume and composition of basaltic crust. Several recent studies have emphasized similarities in the thermal and chemical structure of the Tertiary and present-day plumes to argue for stability of the mantle anomaly, whereas others, relate variations in basalt volumes and compositions to changes in plume flux. Moreover, some authors,, have assumed that the plume was rift-centred for its entire history, whereas others argue that it became ridge-centred only after plate separation,. Here we report compositional data for ∼6,000 metres of flood basalts erupted in east Greenland, close to the inferred plume axis, that we use to constrain the Tertiary plume structure. Rare-earth-element systematics place limits on the pressures and extents of mantle melting and show that the mantle was initially moderately hot (∼1,500 °C), but that its temperature declined during flood volcanism. These observations are difficult to reconcile with current plume-head models, and call for important lithospheric control,, on actively upwelling mantle along the rifted margin.

Self diffusion of network formers (silicon and oxygen) in naturally occurring basaltic liquid

Self diffusion coefficients (D*) for silicon and oxygen in anhydrous basaltic liquid [O/(Si + Al) = 2.5] were measured at 1 and 2 GPa and temperatures between 1320 and 1600°C. Simple diffusion couples were composed of isotopically normal basaltic glass synthesized from chemical reagents mated to chemically identical glass enriched in 18O and 30Si. Concentrations of 18O and 30Si across the interfacial region of the couples were analyzed by ion microprobe. At 1 and 2 GPa, Do* is consistently larger than DSi* for a given diffusion couple, but only at the highest temperature (1600°C) is the difference outside the small uncertainties for the analytical measurements. At 1 GPa the self diffusivities for both Si and O are well-described by the Arrhenius relationship InD(Si.O)*=(−12.5±0.2)−(170000±2000)/RT, where T is temperature in K, R is the gas constant in J K−1 mole−1, and D* is expressed in m2 s−1. Self diffusion coefficients at 2 GPa are a factor of 1.5 greater and at 1400°C the activation volume (Va) is −6.7 cm3 mol−1. The similarity in self diffusion coefficients, small activation energies (<50% of the Si-O bonding energy), and negative activation volumes for Si and O self-diffusion in basaltic liquid suggest that network former diffusion is a largely cooperative process involving local contraction of the anionic structure. An evaluation of the Eyring η-D relationship implies a mean translation distance for network former diffusion that is 2–3 times the diameter of the oxygen ion and of the order of the Si-Si separation distance. These features of network former diffusion are consistent with the formation of high-coordinated Si as a transition complex in melts populated by Q2, Q3, and Q4 species. In light of inferred changes in melt structure with increasing silica content, we further speculate that the dominant mode of network former diffusion changes from a cooperative process in basaltic liquid, perhaps involving SiO5 transition complexes, to an ionic process (Si4+ and O2−) in liquids approaching full polymerization.

Experimental determination of high-temperature elemental losses from biomass slag

Abstract The loss of alkali metal elements from high-temperature molten biomass slag (wood and rice straw) can be related to the extent of polymerization of the melt. If the alkali metals occur as network-modifying and charge-balancing cations in highly depolymerized melts, such as wood slag, they are easily evaporated during prolonged heating and subsequently deposited on heat exchangers. If the melt is highly polymerized, such as rice straw slag, where the alkali metals occur as network-modifying cations, they are strongly retained in the polymerized network. These differences can be related to the availability of large-sized and low-density charged melt positions. Rice straw ash melt is dominated by a relatively open polymerized network that will easily accommodate the large Na + and K + ions. Wood ash melt is highly depolymerized and does not easily accommodate the large K + ion and only to a certain extent the Na + ion, but will accommodate the smaller and more highly charged Ca 2+ ion. Therefore, the alkali metals in wood slag melt are strongly partitioned into the vapor phase, with K preferentially lost relative to Na from the liquid phase. It is a consequence of this study that the use of straw fuels, compared to wood fuels, may significantly reduce the alkali loss from high temperature molten slag. It is tentatively estimated that about 70% of potassium in rice ash may be retained in the slag. This is in contrast to wood ash where all potassium is lost to the combustion gas with prolonged heating. However, the highly polymerized nature of rice and wheat straw melts and their low melting points render these straws less attractive as fuels for many biomass-fueled power plants.

High-pressure viscometry of polymerized silicate melts and limitations of the Eyring equation

Abstract In situ falling-sphere measurements of viscosity have been performed to determine the viscosity of dacite melt (68 wt% SiO2) from 1.5 to 7.1 GPa at temperatures between 1730 and 1950 K, using the T-25 MA8 multianvil apparatus at the GSECARS 13-ID-D beamline at the Advanced Photon Source, Argonne National Lab. The viscosity of dacite melt decreases between 1.5 and 7.1 GPa. At 1.5 GPa and 1825 K the viscosity is 86.6 ± 17.3 Pa⋅s, whereas at 6.6 GPa and 1900 K it is 2.8 ± 0.6 Pa·s. The negative pressure dependence of viscosity results in an activation volume of .12.4 ± 1.4 cm3/mol at 1800 K and .5.1 ± 0.9 cm3/mol at 1900 K. These new data are compared with viscosities estimated from the Eyring equation using oxygen self-diffusion data for the same bulk composition at high pressures. The Eyring equation generally predicts viscosities that are greater than measured viscosities. In addition, the Eyring equation predicts a minimum viscosity at 5 GPa, but no minimum was seen in our falling sphere data set. These discrepancies suggest that the mechanisms for viscous flow and self-diffusion of oxygen in polymerized melts may differ at high pressures, thus limiting the utility of the Eyring equation for high-pressure extrapolations. Further development of the Adam-Gibbs theory may provide an alternative for relating self-diffusion and viscosity at high pressures.

High-pressure x-ray tomography microscope: Synchrotron computed microtomography at high pressure and temperature

A new apparatus has been developed for microtomography studies under high pressure. The pressure generation mechanism is based on the concept of the widely used Drickamer anvil apparatus, with two opposed anvils compressed inside a containment ring. Modifications are made with thin aluminum alloy containment rings to allow transmission of x rays. Pressures up to 8GPa have been generated with a hydraulic load of 25T. The modified Drickamer cell is supported by thrust bearings so that the entire pressure cell can be rotated under load. Spatial resolution of the high pressure tomography apparatus has been evaluated using a sample containing vitreous carbon spheres embedded in FeS matrix, with diameters ranging from 0.01to0.2mm. Spheres with diameters as small as 0.02mm were well resolved, with measured surface-to-volume ratios approaching theoretical values. The sample was then subject to a large shear strain field by twisting the top and bottom Drickamer anvils. Imaging analysis showed that detailed microst...

Spinel–garnet lherzolite transition in the system CaO-MgO-Al2O3-SiO2 revisited: an in situ X-ray study

Large discrepancies are reported for the near-solidus, pressure–temperature location of the spinel to garnet lherzolite univariant curve in the system CaO-MgO-Al2O3-SiO2 (CMAS). Experimental data obtained previously from the piston-cylinder apparatus indicate interlaboratory pressure differences of up to 30% relative. To investigate this disparity—and because this reaction is pivotal for understanding upper mantle petrology—the phase boundary was located by means of an independent method. The reaction was studied via in situ X-ray diffraction techniques in a 6-8 type multianvil press. Pressure is determined by using MgO as an internal standard and is calculated from measured unit cell volume by using a newly developed high-temperature equation of state for MgO. Combinations of real-time and quenched-sample observations are used to bracket the phase transition. The transition between 1350 and 1500°C was reversed, and the reaction was further constrained from 1207 to 1545°C. Within this temperature range, the transition has an average dT/dP slope of ∼40 ± 10°C/kbar, consistent with several previous piston-cylinder studies. Extrapolation of our curve to 1575°C, an established temperature of the P-T invariant point, yields a pressure of 25.1 ± 1.2 kbar. We also obtained a real-time reversal of the quartz–coesite transition at 30.5 ± 2.3 kbar at 1357°C, which is about 2 to 4 kbar lower in pressure than previously determined in the piston-cylinder apparatus.

Self diffusion of Si and O in dacitic liquid at high pressures

Abstract Laboratory experiments have been conducted to determine simultaneously the self diffusivities of Si and O in synthetic dacite melt (NBO/T = 0.1) from 1 to 5.7 GPa and from 1355 to 1662 °C. Glasses enriched in 18O and 28Si were synthesized and mated to their isotopically normal counterparts to form diffusion couples used in the piston cylinder device (1 and 2 GPa) and multi-anvil apparatus (4 to 5.7 GPa). Profiles of isotope abundances were measured by secondary ion mass spectrometry. Self-diffusion coefficients for Si (D*Si) are significantly lower than self-diffusion coefficients for O (D*0) at all run conditions; for example, D*0 = 6.45 ± 0.65 × 10-14 m2/s and D*Si = 1.45 ± 0.45 × 10-14 m2/s at 1 GPa and 1355 °C. The temperature dependence is similar, but not identical, for Si and O self diffusion at all pressures, yielding activation energies of 293-380 kJ/mol at 1 GPa, 264-305 kJ/mol at 2 GPa, and 155-163 kJ/mol at 4 GPa. The pressure dependence is similar for Si and O at all temperatures, giving activation volumes for Si and O that are -14.5 to -17.1 cm3/mol at 1460 °C, -9.8 to -8.7 cm3/mol at 1561 °C, and -8.8 to -9.3 cm3/mol at 1662 °C. Self-diffusion coefficients for Si and O reach maximum values at roughly 5 GPa. The mode of Si and O self diffusion in dacitic liquids is constrained by the large activation volumes, D*0 ≈ 2 D*Si, and predictions using the Eyring equation, which suggest that Si and O diffuse as molecular species at 1460 °C. At 1561 and 1662 °C, less negative activation volumes and predictions of the Eyring equation are consistent with diffusion of Si and O by a combination of mechanisms, including the formation of a high-coordinated intermediate species.

Self-diffusion of Si and O in diopside-anorthite melt at high pressures

Self-diffusion coefficients for Si and O in Di58An42 liquid were measured from 1 to 4 GPa and temperatures from 1510 to 1764°C. Glass starting powders enriched in 18O and 28Si were mated to isotopically normal glass powders to form simple diffusion couples, and self-diffusion experiments were conducted in the piston cylinder device (1 and 2 GPa) and in the multianvil apparatus (3.5 and 4 GPa). Profiles of 18O/16O and 29,30Si/28Si were measured using secondary ion mass spectrometry. Self-diffusion coefficients for O (D(O)) are slightly greater than self-diffusion coefficients for Si (D(Si)) and are often the same within error. For example, D(O) = 4.20 ± 0.42 × 10−11 m2/s and D(Si) = 3.65 ± 0.37 × 10−11 m2/s at 1 GPa and 1662°C. Activation energies for self-diffusion are 215 ± 13 kJ/mol for O and 227 ± 13 kJ/mol for Si. Activation volumes for self-diffusion are −2.1 ± 0.4 cm3/mol and −2.3 ± 0.4 cm3/mol for O and Si, respectively. The similar self-diffusion coefficients for Si and O, similar activation energies, and small, negative activation volumes are consistent with Si and O transport by a cooperative diffusion mechanism, most likely involving the formation and disassociation of a high-coordinated intermediate species. The small absolute magnitudes of the activation volumes imply that Di58An42 liquid is close to a transition from negative to positive activation volume, and Adam-Gibbs theory suggests that this transition is linked to the existence of a critical fraction (∼0.6) of bridging oxygen.

The transition from sedimentation to flood volcanism in the Kangerlussuaq Basin, East Greenland: basaltic pyroclastic volcanism during initial Palaeogene continental break-up

Primary basaltic pyroclastic deposits compose volumetrically c. 35–50% of the Lower Palaeogene Vandfaldsdalen, Mikis and Haengefjeldet Formations within the lower volcanic series of the East Greenland flood basalt province. The eruption and emplacement mechanisms of these deposits can be used to evaluate the volcanotectonic evolution of continental break-up and initiation of flood basalt volcanism. We present a revised stratigraphy for the pre-volcanic sedimentary rocks and synvolcanic pyroclastic and epiclastic deposits and lavas from the Kangerlussuaq Basin, East Greenland. Primary basaltic pyroclastic deposits range from airfall tuffs to bomb beds, surge deposits and vent sites with cinder cones. Early volcanism was apparently restricted to localized basins and formed shield volcano structures 30–40 km in diameter that are characterized by a heterogeneous sequence of intercalated hyaloclastites, sediments, pyroclastic deposits and lavas. The lateral variation of deposit morphology reflects distance from source and indicates a migrating locus of volcanism along the axis of rifting towards the NE, paired with continued down-dropping of inactive volcanic centres. The complex stratigraphic architecture of the initial volcanic deposits reflects local tectonic and sedimentary processes interacting with regional flood basalt volcanism and continental rifting.