MR33A-01 INVITED
Grain-Grain Interfaces in Diffusion and Deformation
Grain-grain interfaces provide important paths for rapid diffusion of ions through rocks and critical sites for storage of incompatible elements within rocks. For an incompatible element for which the concentration in grain- grain interfaces far exceeds that in grain interiors, grain (homophase) boundaries and interphase (heterophase) boundaries provide the primary routes for mass transfer since flux is the product of concentration times diffusivity. For major elements, the rate of diffusion along boundaries depends on the composition of the interfaces. We investigate two kinetic properties, diffusion and deformation, to better understand the role of boundaries and their composition on kinetic properties of rocks. Mg-Co interdiffusion experiments were carried out using diffusion couples formed from fine-grained discs of Mg-olivine and Co-olivine. Results from undoped, Ca-doped, and Sc- doped samples permit us to quantify of the roles of ionic size and ionic charge of impurity ions on grain boundary diffusion. Addition of bi-valent Ca ions results in a small decrease in Mg-Co grain boundary interdiffusivity, possibly because these relatively large cations clog the boundaries by occupying sites that could have been used by diffusing bi-valent Mg and Co ions. In contrast, addition of tri-valent Sc ions leads to an increase in rate of diffusion of bi-valent Mg and Co ions along grain boundaries, suggesting that negatively charged defects such as metal vacancies are formed to charge balance excess positive charge introduced by tri-valent Sc ions occupying sites normally filled by Mg and Co. High-temperature creep experiments were performed on undoped and Sc- doped samples of forsterite, again to explore the effect of ionic charge on grain boundary diffusion. Deformation of our fine-grained samples in the diffusion creep regime was dominated by grain boundary diffusion (Coble creep). Samples enriched in Sc flowed a factor of at least two faster than their higher purity counterparts, again indicating that charge-compensating point defects form in response to the introduction of heterovalent cations. These results demonstrate that kinetic properties of rocks are sensitive not only to major element composition of grain interiors but also to trace element content of grain boundaries. Therefore, trace amounts of impurities along grain-grain interfaces will significantly influence properties such as viscosity and electrical conductivity in Earth's interior. Such impurities might be introduced through metasomatic processes or result from elemental segregation to grain boundaries during phase transitions where the impurity element is more incompatible in the new phase than in the original grains.
MR33A-02
Grain Boundary Diffusion of Carbon in Mantle Minerals
The total amount of carbon in the Earth's near surface reservoirs is considered negligible compared to the amount believed to be present in the mantle. However, carbon is incompatible in most major mantle minerals and the mode of carbon storage is largely unknown. Carbon is also a candidate for being present as one of the light elements in the outer core. In light of recent findings that siderophile elements are mobile in grain boundaries of MgO, we have investigated whether grain boundaries of periclase (MgO) and olivine (Fo90) represent potential fast pathways for diffusion (and storage) of carbon. The experimental strategy utilized a carbon 'source' (high-purity graphite wafers) and a metal 'sink' (either Ni or Fe wire or foil) separated by a wafer of polycrystalline periclase or olivine. Experiments were annealed at 1573 and 1773K and 1.5 GPa for 5-10 hr in a modified 19-mm piston-cylinder assembly. Results indicate that carbon is highly mobile in grain boundaries of both periclase and olivine. Estimated diffusivities range between 10-11 and 10-8 m2/s. There appears to be a correlation between fO2 and carbon diffusivity, as fluxes in experiments featuring a Fe sink were significantly lower than those featuring a Ni sink, where fO2 was approximately 5 log units higher. Preliminary data also suggests carbon is mobile in grain boundaries of β-quartz. These results suggest that an outward flux of carbon from the outer core and an upward flux in the mantle is a possibility, and that grain boundary diffusivities of carbon are fast enough to result in transport distances as great as ~100s of km over geologically relevant time scales.
MR33A-03
The role of transformation twins on enhanced diffusion in minerals.
Since the 1930's physical chemists have been aware that many solids show enhanced diffusion rates and reaction rates in the vicinity of a phase transition in one or other of the phases involved in a reaction or coupled diffusion process. This phenomenon has been termed the "Hedval effect", following Hedval's brief mention of it in one of his chemistry texts. The origins of this phenomenon have remained obscure, however. A number of recent examples now demonstrate that enhanced diffusion along transformation twin walls can be expected in certain silicate structures. Transformation twin walls in quartz show enhanced diffusion of cations such as Na+ and Li+, transport being limited only by charge balancing mechanisms. The density of such domain walls is known to be highly sensitive to temperature, showing a singularity as the transformation temperature is approached. In view of this, one origin of the Hedval effect, of enhanced transport close to the transition temperature of a mineral, can be attributed to the presence of domain walls and the increase in the number of these walls close to the transformation temperature.
MR33A-04 INVITED
Defect Interaction and Diffusion in Periclase (MgO)
Diffusion in minerals is the rate-limiting step in many mass transport processes that are fundamental to geochemistry and geophysics. Thus, there has been a long-standing interest in finding theoretical or empirical relationships that relate the diffusion coefficient in a mineral to well-known ionic properties such as radius and charge. In many silicate minerals the diffusion coefficient is negatively correlated with ionic charge. In periclase, and some other minerals with a strong component of ionic bonding, highly charged cations diffuse rapidly due to their association with cation vacancies. Experiments on Al3+ diffusion in MgO have been performed over a wide range of pressure (1 atm to 25 GPa) and temperature (1573 to 2273 K), and the association energy and Al- vacancy pair diffusivity inferred from each by applying a simple defect interaction model to the diffusion profiles. The binding energy for all but one experiment is in the range 0.45-0.62 eV, and shows no clear dependence on temperature or pressure. The activation energy for Al-vacancy pair diffusion is 2.1 eV, and the activation volume is 2.9 cm3/mol.
MR33A-05
Heat transport properties of mantle phases from ab initio calculations
In order to be able to model the thermal state and dynamics of Earth's interior it is essential to have accurate values for the thermal diffusivity and thermal conductivity of the relevant mineral phases. At present, very little is known from theory or experiment at pressure-temperature conditions typical of most of deep Earth. In view of this, we have implemented two schemes that allow us to determine the thermal conductivity and diffusivity of insulators from ab initio calculations: 1) a reverse perturbation steady-state non-equilibrium molecular dynamics method, and 2) a technique based on the transient decay of a periodic non-equilibrium thermal profile. In both cases the forces driving the dynamics are computed within density functional theory. The two methodologies will be outlined, and preliminary results for MgO and Mg2SiO4 presented.
MR33A-06
Temperature-Dependent Thermal Diffusivity of Crustal Minerals, Rocks and Melts: Implications for Positive Thermal Feedback During Crustal Anatexis
The thermal evolution of orogenic belts is governed by the rates of heat transfer by advection and conduction. The physical properties governing heat conduction are thermal diffusivity (κ) and conductivity (k = κρCP), where ρ is density and CP is heat capacity. Numerical models of orogenic belts typically assume constant values for κ (~10-6 m2s-1) and k (~4 Wm- 1K-1), but in fact κ, CP, and hence k), are significantly temperature-dependent. Thermal diffusivity data for orthoclase crystal and glass, haplogranitic glass, leucogranite and garnet schist, were acquired at temperatures up to 1000 °C, using the laser-flash method (LFA). This isolates the phonon component of heat transfer from radiative transfer and avoids thermal contact losses. In all cases, κ decreases rapidly with increasing temperature, asymptotically approaching a high-temperature limit. Glasses and melts have lower κ than chemically equivalent crystalline materials. Dissolved water also has a depressing effect on κ (Hofmeister et al. 2006 Geophys. Res. Lett.), so that hydrous granitic liquids should be particularly efficient insulators. Crustal melting will therefore produce an insulating layer, which will retain heat. This positive feedback between melting and thermal insulation may promote increased melt fraction, and may provide a mechanism for leucogranite generation by shear heating. Steady-state thermal gradients are governed by the conductivity, k. Calculations for orthoclase crystal, glass and liquid indicate that k of crystals decreases with T, while k of glass increases with T. At 1000°C, k is ~2 Wm-1K-1 for the crystal, but only ~1.5 Wm-1K-1 for glass/melt. The low thermal conductivity of melts suggests that steady-state thermal gradients will be higher across partially molten layers than in unmelted crust.
MR33A-07 INVITED
What can we really learn about the mantle from water contents of minerals in xenoliths?
Mantle xenoliths are pieces of the interior of the Earth that have been entrained by magma and brought to the surface, frequently quite rapidly. As such they often contain geochemical signatures characteristic of their source regions, although interactions with the magma prior to and during transport and depressurization can result in alteration and overprinting. A number of recent studies have investigated the water contents in the nominally anhydrous minerals within xenoliths from a range of localities and attempted to provide constraints on the water content of the mantle source regions. They have made use of the experimentally observed rates of water loss from the various minerals to argue that several minerals retain their water contents during transport, while olivine grains, in particular, are significantly dehydrated. Using these observations, estimates have been made of rates of ascent from the mantle source region and of mantle water content. At issue is the robustness of the arguments for water loss based on experimental studies and their true applicability to the issue of xenolith entrainment and transport. In olivine, the subject of most experimental study for water diffusion and solubility, diffusive transport of water-derived species occurs by 2 processes: a redox process that is rate limited by self-diffusion of hydrogen, and a slower process whereby defect associates composed of a hydrogen ion and a metal vacancy diffuse cooperatively, rate limited by the diffusivity of the metal vacancies. Water loss by the redox process is limited by the number of iron atoms that can change from ferrous to ferric; many more hydrogen ions can be lost as hydrogen-vacancy associates. It is anticipated that both processes also occur in the pyroxenes and garnet, although only the redox process has been studied in detail. A further complication to the use of experimentally determined hydrogen diffusivities results from the demonstrated compositional effects on diffusion. In low-iron clinopyroxene, diffusion is much slower than in high-iron clinopyroxene, resulting from lower fluxes of polarons at lower iron concentrations. While these issues must be considered in any analysis of mantle xenoliths, judicious use of experimental data does provide useful constraints on mantle water content and on xenolith ascent rates.
MR33A-08
Oxidation Kinetics and Textures for Natural and Synthetic MORB Glasses and Subliquidus Melts: RBS Analyses
The dynamics of oxidation of basaltic melts and glasses involves the diffusive motion of network-modifying cations out of the material (i.e., to the free surface) where they react with atmospheric oxygen to produce oxide precipitates or thin films. The cation flux is countered by an inward flux of electron holes; thus the material, modified by the loss of cations, experiences, too, oxidation at an internal front [e.g., Cook & Cooper, 2000]. The texture so produced is a complex, non-equilibrium one. For glasses of similar polymerization to basalts, the internal oxidation front often involves the precipitation of nanometer-scale ferrites; in the oxidation dynamic, because of the persistent, metastable texture, there is no reason to believe that these ferrites should have compositions predicted by chemical equilibrium. The effect on understanding paleomagnetic field strength recorded in MORBs could be profound: this is the long-range goal of our experiments. We report here on oxidation textures produced for three glasses and melts produced from (i) a natural MORB and (ii) mixed oxides to produce MORB analogs \--- one free of all alkali cations. All materials were melted at ambient pressure at 1430°C in an atmosphere buffered at FMQ. Glasses so prepared had their glass-transition temperatures (Tg) characterized by thermal analysis. Oxidation experiments were performed on well- annealed glass specimens at temperatures Tg ± ~25°C as well as on crystallizing melts at 900°C; oxidation was done in dry air at ambient pressure. Ion (Rutherford) backscattering spectrometry (RBS) was used to characterize the near-surface texture (chemical distribution) of the oxidized specimens. The textures are found to be sensitive to oxidation temperature, initial melt polymerization, and the bulk chemistry (particularly as it affects the summed partial molar volumes of network-modifying oxides). A general result \--- applicable to all compositions \--- is that out-diffusion of ionic Fe and ionic Mg effects oxidation at higher temperatures, giving way to kinetic domination of ionic Ca at lower temperatures. Specific results will be presented. We are initiating transmission electron microscopy studies to characterize the compositions and distributions of internally-nucleated ferrites. Cook, G.B. and R.F. Cooper, Amer. Mineral. 85, 397-406 (2000).