How much do we know the Earth's mantle? We would like to make clear what we want to know, what we can know and what we will be able to know the Earth's mantle. This special issue focuses on recent scientific advances in serpentinization, pressure-temperature paths including thermal history of the earth, tectono-magmatic processes, geochemical recycling, rheology and chemical compositions of the Earth's mantle based on analyses of mantle-derived materials including volcanic rocks. These topics provide useful information for a wide range of earth scientific communities.
Serpentinization of peridotites involves the production of hydrogen, which is a source of vital energy for chemosynthetic communities and abiotic methane or other hydrocarbons. Serpentinite-hosted hydrothermal vent fields that discharge fluids with hydrogen have been widely noticed as a possible environment for the generation of life on the early Earth and other terrestrial planets. In this context, it is important for us to understand petrological constraints on serpentinization processes related to hydrogen production. Magnetite formation by oxidation of iron in olivine is the most effective process for hydrogen production during serpentinization. Recent petrological studies have revealed that the magnetite formation is controlled by silica activity and Fe-Mg diffusion rate in olivine crystal, as well as temperature and water/rock ratio during serpentinization. Without local elevation of silica activity via fluid infiltration, magnetite forms at temperatures ranging approximately from 150 to 350 °C with most favorable condition at around 300 °C, but fails to form because of increasing diffusion rate in olivine crystal at higher temperatures and Fe-serpentine or Fe-brucite formation at lower temperatures. It should be kept in mind, however, that the formation of oxidized serpentine could produce hydrogen as well.
Serpentinization is a hydration process that causes significant changes in physical and chemical properties of the oceanic lithosphere. Based on hydrothermal experiments, the reaction rates of serpentinization have been empirically obtained for typical reaction (i.e., Olivine+H2O → Serpentine+Brucite) as a function of temperature and initial grain size of the reactant mineral. However, the rate equations used for these analyses take quite empirical forms, in which the solution chemistry (saturation state) is not taken into consideration; therefore, it is difficult to extrapolate the results to different conditions and to predict evolution of the fluid chemistry in the hydrothermal systems. Serpentinization reactions are characterized by coupled dissolution of primary minerals (Olivine, Pyroxenes) and formation of secondary minerals (Serpentine, Brucite, Talc, Magnetite); therefore, the rates of elementary reactions between individual minerals and solution will be required for estimating the rate of overall hydration reaction. I also discuss the effects of competitive processes among grain surface reactions, element diffusion, water supply and structural development during serpentinization.
The Godzilla Megamullion is the largest known oceanic core complex, located in the Parece Vela Basin, an extinct backarc basin in the Philippine Sea. The previous studies argued that the basin was active from 26 Ma to 12 Ma at an intermediate-spreading rate of 8.8-7.0 cm/year full-rate, although the basin shows the characteristics typical for slower spreading ridges. For example, many peridotites in the Parece Vela Basin are much less depleted than those exposed at comparable spreading rates on other mid-ocean ridge systems. The tectono-magmatic characteristics of the Parece Vela Basin were thus thought unusual and paradoxical. However, the recent studies, based on the high-density samplings on the Godzilla Megamullion, show the evidences that the basin became slow to ultraslow environment in its terminal phase. Zircon U-Pb dating of gabbroic rocks from the Godzilla Megamullion reveals that the estimated slip rate of the Godzilla Megamullion detachment fault was ∼ 2.5 cm/y; significantly slower than the previous estimate. The morphology and geology of the termination area are similar to those observed in ultraslow-spreading ridges. Decreasing degree of partial melting of the peridotites as well as increased amount of plagioclase-bearing peridotites (showing melt stagnation in the shallow lithospheric mantle) are observed towards the termination of the Godzilla Megamullion. Based on the recent observations at the Godzilla Megamullion, it would be argued that the terminal phase of a backarc basin development will go through an ultraslow-spreading environment, erupting alkaline basalts. There will be an overlap period of the terminal alkaline basalt magmatism and the rifting of a succeeding backarc basin.
Three enigmas of highly siderophile elements (HSE) in Earth's mantle, manifested by recent advances in HSE geochemistry of mantle peridotites, are reviewed in this paper. They are (1) the apparent overabundance of the HSE in the Earth's mantle compared to metal-silicate equilibrium, (2) supra-chondritic Pd/Ir and Ru/Ir ratios in primitive upper mantle (PUM) estimates, and (3) contrasting HSE patterns between massif- and xenolithic-peridotites. More studies on both natural and experiment-based data are clearly required for resolving these problems.
Osmium isotope ratios of abyssal peridotites are of great interest to geoscientists because they provide direct information about the present-day Earth's mantle. This is based on the fact that Os is a highly siderophile and compatible element and so concentrates in the core in preference to the mantle, and in the mantle in preference to the crustal materials. In addition, Os is not susceptible to metasomatism and alteration by sea water. In this review, we explain the advantages and details of using the model age of Re-Os system as a timing of Re depletion. We also present Os isotope ratios in abyssal peridotite whole rocks, and chromites and sulphides within abyssal peridotites from the Atlantic, Indian and Pacific Oceans, and young ophiolites.