Degassing during basalt eruption is the major pathway for mantle gases to enter the atmosphere and was also the major source for the formation and evolution of the early atmosphere. Carbon dioxide (CO2) is the second most abundant gas component in basaltic magmas at mid-ocean ridges and ocean islands, after water (H2O). Moreover, due to the low solubility of CO2 in melts compared to H2O, CO2 is the most major component in the gas phase during degassing of submarine basalts. Hence, bubble growth and degassing in submarine basaltic melts are largely controlled by CO2. Direct measurement of CO2 concentrations in submarine basaltic glasses often show oversaturation of CO2, meaning that basalt degassing is not an equilibrium process, but also controlled by CO2 diffusion and transport. Therefore, understanding CO2 diffusion in basaltic melt is essential to quantifying CO2 bubble growth and degassing, as well as the volatile budget of the mantle.
Diffusion couple experiments will be carried out to investigate CO2 diffusion in dry and wet haplobasaltic melts. For the two halves in each couple, the chemical compositions (including H2O content) will be similar, but CO2 concentration will be zero in one half and about 1000 parts per million in the other half. The experimental procedures will be similar to our previous diffusion couple experiments on H2O and Ar diffusion. The experimental conditions will be 1300-1700°C, 0.5-1.5 GPa, and 0-7 weight% H2O. After the experiments, CO2 concentration profiles will be measured using a microscope Fourier transform infrared spectrometer. The profiles will be fit by the theoretical solution to obtain diffusivity. The new data will be combined with previous data to assess the dependence of CO2 diffusivity on temperature, pressure, and H2O content. From diffusion data obtained from this grant, bubble growth in basaltic melts will be modeled using recently developed models. Furthermore, multicomponent bubble growth will be tackled. Hence, this work will provide a fundamental understanding to CO2 diffusion, bubble growth, degassing, and kinetic fractionation of gas components of basaltic magma at mid-ocean ridge, ocean islands and island arc settings.
This grant has supported our research on diffusion, kinetics and dynamics in geological systems. Ten papers have been published with support of this grant, and a few more are in the pipeline. Some discoveries and results are summarized below. 1. Diffusion in rocks. Diffusion in rocks and other heterogeneous solid media has been investigated for over forty years. However, the derived equations for bulk (or effective) diffusivity used similarity argument between diffusion and other physical properties such as electrical and thermal conductivity, and hence either implicitly or explicitly assumed that the concentrations are continuous across phase boundaries. That is, these derived relations, which have permeated through the literature and textbooks, can only be applied when the partition coefficients of the component between every pair of phases in the medium is 1, which is rarely the case. In this work, we include the effect of partitioning between different phases and present the general and simple method to derive the equations to relate bulk diffusivity to individual-phase diffusivities in heterogeneous media. Our new theory has applications not only in geology, but also in other branches of science and engineering, such as diffusion in composite materials and thin films. 2. Diffusion of water in andesitic melt. Water is an important volatile component in andesitic eruptions and deep-seated andesitic magma chambers. We investigated H2O speciation and diffusion by dehydrating haploandesitic melts at 743–873 K and 100 MPa in cold-seal pressure vessels. FTIR microspectroscopy was utilized to measure species and total H2O concentration profiles on the quenched glasses from the dehydration experiments. The equilibrium constant of the H2O speciation reaction varies with temperature as lnK = 1.547–2453/T where T is in K. Water diffusivity at the experimental conditions increases rapidly with H2O concentration, contrary to previous H2O diffusion data in an andesitic melt at 1608–1848 K. The diffusion profiles are consistent with the model that molecular H2O is the diffusion species. Based on the above speciation model, total diffusivity (in m2/s) in haploandesite is formulated and reported. By comparison with previous water diffusion studies, H2O diffusivity at T < 873 K in calc-alkaline silicate melts is found to increase with degree of polymerization (andesite < dacite < rhyolite), opposite to the trend at superliquidus temperatures. In a separate publication, we investigated H2O diffusion in a haploandesitic melt at 1 GPa in a piston-cylinder apparatus. The work extends the temperature and water concentration range of the previous study. More importantly, this study for the first time resolved OH diffusivity during water diffusion in a silicate melt. The obtained OH diffusivity is similar to fluorine diffusivity but is much higher than Eyring diffusivity. 3. Mechanism of coal outbursts. Thousands of mine workers die every year from mining accidents, and instantaneous coal outbursts in underground coal mines are one of the major killers. Various models for these outbursts have been proposed, but the precise mechanism is still unknown. We hypothesize that the mechanism of coal outbursts is similar to magma fragmentation during explosive volcanic eruptions; i.e., it is caused by high gas pressure inside coal but low ambient pressure on it, breaking coal into pieces and releasing the high-pressure gas in a shock wave. Hence, coal outbursts may be regarded as another type of gas-driven eruption, in addition to explosive volcanic, lake, and possible ocean eruptions. We verify the hypothesis by experiments using a shock-tube apparatus. Knowing the mechanism of coal outbursts is the first step in developing prediction and mitigation measures. The new concept of gas-driven solid eruption is also important to a better understanding of salt-gas outbursts, rock-gas outbursts, and mud volcano eruptions. 4. Calibration of IR technique for quantitative measurement of OH in apatite crystals. Recently, apatite is becoming an important mineral used to characterize its formation conditions, especially as a volatile indicator. For this purpose, it is critically to accurately measure the OH and other volatile concentrations in apatite. In this work, we have calibrated the infrared (IR) method for determining OH concentrations in apatite with absolute concentrations obtained through elastic recoil detection (ERD) analysis. IR spectra were collected on oriented, single-crystal apatite samples using polarized transmission infrared spectroscopy. Based on our calibration, the detection limit of H2O concentration in apatite by IR approaches parts per million level for wafers of 0.1 mm thickness. The accuracy based on our calibration is 5–10% relative.
