Dislocation-creep controlled flow of upper mantle materials causes the developments of crystallographic preferred orientation (CPO) of olivine and seismic anisotropy. Olivine CPO pattern controls the relationship between mantle flow geometry and resultant seismic anisotropy in the upper mantle. It has been known that types of olivine CPO are mainly controlled by stress and water fugacity under the upper mantle conditions. Recent experimental studies at high pressures have revealed that pressure is also an important parameter that controls the types of olivine CPO patterns. This review covers experimental studies on the deformation of olivine single crystals and polycrystalline aggregates. The easiest slip system under various conditions can be estimated from the flow laws for oriented single crystals and is consistent with the dominant pattern of olivine CPO. Moreover, experimental techniques for deformation experiments are reviewed. To evaluate the magnitude of seismic anisotropy based on the experimental data, achievements of the steady-state fabric strength in deformation experiments would be important. Deformation of olivine samples with large shear strains under the upper mantle conditions will make a significant contribution to understanding the origin of seismic anisotropy in the upper mantle.
The origin of within-plate volcanism in the eastern margin of the Eurasian Plate has been attributed to the influence from the underlying Pacific stagnant slab, asthenospheric mantle flow independent of the stagnant slab, and combination of them. In this study, relatively undifferentiated alkaline basalts erupted in eastern China younger than 15 Ma, which is after the opening of the Japan Sea, were chosen to discuss spatial geochemical variation of the upper mantle beneath this area. Low-FeO alkaline basalts are depleted in FeO* and TiO2, enriched in SiO2, Al2O3, and fluid-mobile trace elements, such as Rb, Ba, K, and Pb, show enriched isotopic compositions, and are enriched in radiogenic Pb. These basalts are sporadically distributed in eastern and northeastern China. High-FeO alkaline basalts are extremely enriched in FeO* and TiO2, depleted in SiO2, Al2O3, and fluid-mobile trace elements, show depleted isotopic compositions, and are depleted in radiogenic Pb. These basalts are distributed at approximately 119°E between 30 and 40°N. These geochemical characteristics as well as geophysical investigations on the upper mantle beneath these areas suggest that fluid derived from subducted sediment and oceanic crust dehydrated at subduction zone contribute to the origin of low-FeO and high-FeO alkaline basalts, respectively.
Melts from asthenospheric depths, such as silica-deficient alkali basalts beneath mid-ocean ridges, pyroxinite melts beneath ocean islands, and felsic melts from subducted slabs beneath arcs, are usually not in equilibrium with mantle peridotite at the shallower depths. Reactions between deep melts and shallow peridotites have drawn attention because they are immediately related to the geochemical features of mid-ocean ridge basalts (MORBs), ocean island basalts (OIBs), island arc basalts (IABs), and high-Mg andesites (HMAs). Reactions between the deep melts and the shallow peridotites are peritectic in many cases, whereby they consume and precipitate particular mineral phases as part of the reaction process. This type of reaction is highly complex because the open behavior of the reaction system contains a significant amount of variability. The dynamics of the melt transport alter the mass balance in the melt-peridotite reaction system by affecting its temperature and the chemical stoichiometry via heat transfer and chemical disequilibrium caused by melt inflow-outflow. Differences in source rocks and degrees of melting cause considerable fluctuation in the chemistry of deep melts, which also affects the reaction. For example, deep alkali basalt melts dissolve orthopyroxene in the shallow mantle peridotite, whereas felsic slab melts precipitate orthopyroxene through the consumption of olivine. As such, the melt-peridotite reaction in an open system clearly affects the petrology and chemistry in both residual mantle peridotites and reacted melts. Moreover, evidence of these changes should be recorded in the major and trace element chemistries of residual mantle rocks and erupted melts. In this article, the author summarizes previous research and examines the role of the melt-peridotite reaction in an open system to determine the origins of geochemical diversity in residual peridotites and melts obtained from the various magmatic settings.