Norikatsu Akizawa

We examined vertical variations of the petrological characteristics of a 33-m-thick peridotite section under the layered gabbro section along Wadi Fizh of the northern Oman ophiolite to understand the formation mechanism of the Mohorovicic discontinuity (Moho) beneath a spreading center. Here, we refer to the base of the layered gabbro section as “L-Moho” for the sake of simplicity. Network-like gabbro sills in peridotites increase in frequency upward to the L-Moho. The L-Moho is underlain by a

Dunite bands and dikes in ophiolitic mantle peridotites are interpreted as fossil melt channels within the suboceanic mantle. Concordant dunite bands (i.e. fossil melt channels transposed by outward transportation from the ridge axis via horizontal mantle flow) are particularly important as they possibly represent the melt channels through which the parental melts of mid-ocean ridge basalt (MORB) were transported to shallower depths beneath the paleo-ridge axis. We conducted field observations a

Abstract This paper explores the evolutional process of back-arc basin (BAB) magma system at final spreading stage of extinct BAB, Shikoku Basin (Philippine Sea) and assesses its tectonic evolution using a newly discovered oceanic core complex, the Mado Megamullion. Bulk and in-situ chemical compositions together with in-situ Pb isotope composition of dolerite, oxide gabbro, gabbro, olivine gabbro, dunite, and peridotite are presented. Compositional ranges and trends of the igneous and peridotit

This paper details a new discovery of anorthite-bearing diopsidites (= anorthite diopsidites) from the lowermost crust along Wadi Fizh in the northern Oman ophiolite. The anorthite diopsidites occur as networks within layered gabbros 50 m above the gabbro/peridotite boundary, and are mainly composed of high-Mg diopsidic clinopyroxenes and anorthites with various amounts of uvarovite. They are intermediate in mineral chemical characteristics between the crustal gabbros and the diopsidites, and in

Abstract Diopsidite, mainly composed of diopsidic clinopyroxene (diopside for simplicity), was precipitated from a high‐temperature hydrothermal solution, rich in silicate components mainly in the mantle peridotite. We present here a new type of diopsidite in the uppermost mantle section, which contains Cr ‐rich minerals such as chromite, Cr ‐rich diopside (<2.5 wt.% Cr 2 O 3 ) and Cr ‐rich grossular (<7 wt.% Cr 2 O 3 ). The chromites are varied in shape, subhedral to anhedral, and sometim

Platinum-group elements (PGEs) are one of the key tracers to reveal early differentiation processes of the Earth due to their preferential distribution into the metallic core. Meanwhile, informative evidence for early differentiation has been greatly disturbed through metasomatic PGE disturbance, which has been demonstrated through a number of PGE data for natural mantle peridotites as well as base-metal sulfide and platinum group mineral grains therein. The mechanism and process of metasomatic

A simple analytical procedure for determination of whole-rock major- and trace-element composition by micro-X-ray fluorescence (μXRF) spectrometer and inductively coupled plasma-mass spectrometer (ICP-MS) using fused-glass bead (sample + lithium tetraborate) is presented. In the case of peridotites, chromian spinel (Cr-spinel) is one of the minerals resistant to acids and interferes with accurate and reproducible determination of whole-rock composition. Such resistant minerals were not observed

地球表面に露出しているかんらん岩は，過去においてマントルに存在していた物質であり，マントル物質の化石である．かんらん岩が経験した圧力-温度-変形史（P-T-d-t経路）を復元することができれば，どのようにマントルが地球内部で運動し，それに伴いどのように温度変化と変形が進行したのかを明らかにすることができる．そのP-T-d-t経路を，かんらん岩の地表への上昇による“化石化”の時点から連続的かつできるだけ過去に遡ることができれば，長期間にわたるマントルダイナミクスの理解を深めることができる．日高変成帯南部に位置する幌満かんらん岩体では，かんらん岩に記録された過去のP-T-d-t経路を詳細に読み解く研究が盛んに行われてきた．本稿では，幌満かんらん岩体のP-T-d-t経路の研究に焦点をあてレビューを行い，定置に至るまでの最終上昇時からできるだけ過去に遡って幌満かんらん岩体が辿った運動・熱・変形史を整理・概観し，新たな提案をする．

ABSTRACT Base-metal sulfides (BMSs) are minerals that host platinum-group elements (PGE) in mantle peridotites and significantly control the bulk PGE content. They have been investigated in detail down to the sub-micrometer scale to elucidate PGE behavior in the Earth's interior. Base-metal sulfides are supposedly subjected to supergene and seawater weathering, leading to the redistribution of PGEs at low temperatures. Careful and thorough measurements of BMSs are thus required to elucidate PGE

Abstract Troctolites were recovered during Integrated Ocean Drilling Program Expedition 345 at the Hess Deep Rift, next to fast-spreading East Pacific Rise. These troctolites are divided into three groups based on textural differences: coarse-grained (1–10 mm in length) troctolite, fine-grained (~ 2 mm in length) troctolite, and skeletal olivine-bearing troctolite. All troctolites exhibit a magmatic fabric. The major-element compositions of olivine, plagioclase, and clinopyroxene in the troctoli

Rock and sediment samples were collected from petit-spots in the northwestern Pacific. The sampling was conducted using deep-submergence vehicle (DSV) Shinkai 6500 and its mother ship, research vessel (RV) Yokosuka during YK20-14S and YK21-07S cruises. The collected rock samples are basalt and peperite. Some of the basalts include small mantle xenoliths (∼3 cm in diameter). The dataset of rock and sediment samples from the petit-spots located on >130 Ma northwestern Pacific plate are presented h