Research of Professor Youxue Zhang

As of September 2019

Earth scientists aim at understanding the processes and history of Earth and other planets. Earth processes are diverse, ranging from microscopic processes occurring at the atomic level such as diffusion and chemical reactions, to macroscopic processes such as earthquakes, volcanic eruptions, and Earth differentiation. Earth history is long, from the violent beginning at about 4.5 billion years ago, to the appearance and demise of many species (including dinosaurs), and to the rise of humankind. Earth science is becoming increasingly broad, interdisciplinary and relevant to society.

Professor Zhang has contributed to a range of disciplines of Earth and planetary sciences: experimental petrology, volcanology, and geochemistry. More specifically, his contributions focus on kinetics and dynamics of magmatic and volcanic processes, early history of Earth, as well as recent new endeavor into Earth's surface environment. As a high-temperature experimentalist and a theoretician/modeler, one characteristic of his work is the combination of careful experiments with thorough theoretical analysis and modeling. Their experiments are guided by theoretical understanding, and followed by new and thorough theoretical analysis and quantitative modeling. They have found that such a combination is ideal in gaining fundamental insights of Earth processes. In recent years, Professor Zhang and his group have focused on diffusion in silicate melts and in rocks, kinetics of mineral dissolution and growth, and the origin and evolution of H2O and other volatiles in the Moon.

Diffusion in silicate melts and in rocks

Diffusion due to thermally excited motion of atoms, ions and clusters plays an essential role in magmatic and volcanic processes. For example, diffusion of H2O controls bubble growth in silicic magmas, which, in turn, affects the dynamics of volcanic eruptions. Diffusion of other elements may be important during mineral growth and dissolution or during ore formation. Professor Zhang and coworkers have investigated various diffusion problems. Some specific contributions are listed below.

H2O diffusion. Due to its importance in volcanology, the diffusion of H2O in natural silicate melts has been investigated since 1970's. One special feature of H2O diffusion is that the diffusion coefficient depends on H2O concentration even at very low concentration. H2O dissolves in silicate melts as two different species: molecular H2O and hydroxyl groups. By directly measuring the concentrations of both molecular H2O and hydroxyl groups, Zhang et al. (1991) showed that molecular H2O and hydroxyl groups have very different diffusivities. The diffusion of the H2O component is essentially due to molecular H2O diffusion, whereas diffusion coefficient of OH in rhyolite melt was too small to be resolved. Zhang and Behrens (2000) found that molecular H2O diffusivity depends exponentially on total H2O concentration. Ni et al. (2013) resolved the small OH diffusivity in andesite melt. In addition, Zhang and coworkers reported experimental data and developed models for H2O diffusion in various silicate melts (Zhang et al., 1991; Zhang and Stolper, 1991; Zhang and Behrens, 2000; Liu et al., 2004; Ni and Zhang, 2008; Ni et al., 2009a, 2009b; Behrens and Zhang, 2009; Wang et al., 2009; Fanara et al., 2013; Ni et al., 2013).

Multi-phase diffusion. Rocks and magmas typically consist of multiple phases. For example, an upper mantle rock consists of olivine, orthopyroxene, clinopyroxene, and spinel or garnet. A magma may contain phenocrysts and/or bubbles. When diffusion distances are longer than grain sizes, then diffusion in a multi-phase material can be described by a bulk (or effective) diffusivity. Hence, it is not only necessary to know the diffusion coefficient in individual phases, but also necessary to know how the bulk diffusivity is related to individual diffusivities. Previous treatment on relating bulk diffusivity and individual-phase diffusivities typically ignored the effect of elemental partitioning on the bulk diffusivity. For example, even if the diffusivity in a mineral is high, if the concentration of the element is low, then diffusion in the mineral may not contribute much to the bulk diffusive flux and hence to the bulk diffusivity. Zhang and Liu (2012) incorporated the effect of elemental partitioning and developed a new method and a new set of equations relating bulk diffusivity and individual-phase diffusivities. The method may be applied to treat diffusion of volatiles (H2O, noble gases, etc) in the mantle as well as the effect of phenocrysts and bubbles on the diffusion in magmas.

Multicomponent diffusion. Natural silicate melts typically contain five or more major components. Because the concentration gradient of one component may cause diffusive flux of another component, diffusion in multicomponent systems is complicated. For example, often there is uphill diffusion. When uphill diffusion occurs in natural melts, geochemists usually note that there is uphill diffusion, and then ignore it. Guo and Zhang (2016, 2018) investigated the difficult problem of multicomponent diffusion in a seven-component haplobasalt and an eight-component basalt. Diffusion-couple experiments were carried out and numerous uphill diffusion profiles were observed. All the data can be well fit using the diffusion matrix. We are continuing the research with the hope to reach practical applications such as predicting multicomponent diffusion during mineral dissolution.

Dissolution and growth of minerals in silicate melts

        Igneous rocks are complicated multi-mineralic materials formed by the crystallization of magmas. The thermodynamics of magma crystallization and sub-solidus equilibrium have been investigated for more than 100 years, so that it is now possible to roughly predict the crystallization sequence, the crystallization of each mineral, as well post-crystallization re-equilibration. On the other hand, the kinetics of mineral growth and dissolution and the associated diffusion in the melts are less studied. Professor Zhang and his group have investigated the general aspects of convective dissolution and growth of minerals and bubble (Zhang and Xu, 2003; Zhang, 2013), and the dissolution kinetics of olivine (Chen and Zhang, 2008), diopside (Chen and Zhang, 2009), anorthite (Yu et al., 2016), quartz (Yu et al., 2019), zircon (Zhang and Xu, 2016) and cassiterite (Yang et al., 2016) in natural basalt and/or rhyolite melts. The results have been applied to quantify post-entrapment olivine growth toward a melt inclusion (Newcombe et al., 2014) as well as zircon growth geospeedometer (Zhang and Xu, 2016).

Origin and evolution of H2O and other volatiles in the Moon

       Knowledge of H₂O and other volatiles abundances in the Moon are fundamental in constraining the origin of the Moon, in understanding lunar evolution, and in assessing future human exploration of the Moon. In the last decade, the lunar interior was found to have much higher abundances of H₂O and other volatiles than previously thought (e.g., Saal et al., 2008, 2013; Hauri et al., 2011, 2015, 2017; Hui et al., 2013, 2017; Chen et al., 2015; Lin et al., 2017; Mills et al., 2017; Ni et al., 2017, 2019). Zhang and coworkers have determined H2O and D/H ratio in lunar soil glass and found that H2O content of the order 100 ppm originates mostly from solar wind (Liu et al., 2012), measured H2O and D/H ratio in lunar anorthite crystals from highland anorthosites and inferred that the primitive lunar mantle contain about 100 ppm H2O (Hui et al., 2013, 2017), and evaluated the abundances of H2O and other volatiles in pre-eruptive lunar basalts and primitive lunar mantle using olivine-hosted melt inclusions (Chen et al., 2015; Ni et al., 2017, 2019). Many volatiles in the Moon seem to be depleted relative to the Earth by a factor of 3 to 6, and the depletion factor does not seem to correlate with the condensation temperature. One explanation of the data is that the Moon received a smaller fraction of the late veneer. However, there is still intense debate about how to interpret the observed data by different groups.

Other recent research topics and contributions by Professor Zhang's group include mantle evolution (Zhang, 2014a,b), diffusion and sulfide formation during ore formation (Zhang, 2015; Ni and Zhang, 2016, 2017), H2O and CO2 in apatite (Wang et al., 2011; Clark et al., 2016).