マントル溶融過程の多様性: 特にメルト移動の場におけるメルト/マントル相互反応の重要性について

取得学位：博士(理学)，学位授与番号：博甲第255号，学位授与年月日：平成10年3月25日,学位授与年：199

Phase relation and Equation of State of Iron-titanium oxyhydroxides with α-PbO2 type crystal structure at deep mantle conditions

data sets of Paper "Phase relation and Equation of State of Iron-titanium oxyhydroxides with α-PbO2 type crystal structure at deep mantle conditions" by Matsukage et al

Chemical composition of hydrous and other minerals and their chromian spinel hosts in chromitite and troctolite from the Hess Deep

Primary hydrous and other minerals enriched with incompatible components were found in ocean-floor ultramafic-mafic plutonic rock suites recovered from two contrasting ridge systems, i.e., the East Pacific Rise (Hess Deep, equatorial Pacific), a typical fast-spreading system, and Mid-Cayman Trough, a typical slow-spreading system. They are characteristically associated with chromian spinel, enriched with Cr, one of compatible elements. The hydrous minerals can be formed through interaction between depleted mid-ocean ridge basalts (MORBs) and oceanic peridotite. Primary MORB produced in the deeper part inevitably react with shallower mantle peridotite; the magma selectively dissolves orthopyroxene with simultaneous olivine precipitation. Chromium is supplied to the melt from orthopyroxene, which is enriched with Cr over Al relative to ordinary basaltic melts. The effects of zone refining are also important for concentrations of the incompatible components, especially H20, Na, and Ti, in the modified magma, which in the extreme case is able to precipitate hydrous minerals. This mechanism is common to both fast- and slow-spreading ridges, and is more effective in stagnant or failed melt conduits. Some ultramafic rocks from upper mantle or transition-zone members of ophiolites have primary hydrous minerals, usually included by chromian spinel. Despite the often a priori assumption that slab-derived components are necessary for the formation of the chromitite with inclusions of primary hydrous minerals, this is clearly not necessary

Major element composition of an Early Enriched Reservoir: Constraints from 142Nd/144Nd isotope systematics in the early Earth and high pressure melting experiments of a primitive peridotite

The Accessible Silicate Earth (ASE) has a higher 142Nd/144Nd ratio than most chondrites. Thus, if the Earth is assumed to have formed from these chondrites, a complement low-142Nd/144Nd reservoir is needed. Such a low-142Nd/144Nd reservoir is believed to have been derived from a melt in the early Earth and is called the Early Enriched Reservoir (EER). Although the major element composition of the EER is crucial for estimating its chemical and physical properties (e.g., density) and is also essential for understanding the origin and fate of the EER, which are both major factors that determine the present composition of the Earth, it has not yet been robustly established. In order to determine the major element composition of the EER, we estimated the age and pressure–temperature conditions to form the EER that would best explain its Nd isotopic characteristics, based on Sm–Nd partitioning and its dependence on pressure, temperature, and melting phase relations. Our estimate indicates that the EER formed within 33.5 Myr of Solar System formation and at near-solidus temperatures and shallow upper-mantle pressures. We then performed high-pressure melting experiments on primitive peridotite to determine the major element composition of the EER at estimated temperature at 7 GPa and calculated the density of the EER. The result of our experiments indicates that the near-solidus melt is iron-rich komatiite. The estimated density of the near-solidus melt is lower than that of the primitive peridotite, suggesting that the EER melt would have ascended in the mantle to form an early crust. Given that high mantle potential temperatures are assumed to have existed in the Hadean, it follows that the EER melt was generated at high pressure and, therefore, its composition would have been picritic to komatiitic. As the formation age of the EER estimated in our study precedes the last giant, lunar-forming impact, the picritic to komatiitic crust (EER) would most likely have been ejected from the Earth by the last giant impact or preceding impacts. Thus, the EER has been lost, leaving the Earth more depleted than its original composition