※一部利用できない機能があります
| 1.
論文 |
論文
|
Gahlan, Hisham A. ; Arai, Shoji
概要:
金沢大学大学院自然科学研究科自然計測<br />The peculiar Co, Zn and Mn-rich chromian spinels are hosted by magnetite veins, serpentinites an
…
d chromitites of the mantle section of the Proterozoic Bou-Azzer ophiolite, Morocco. The spinel is complexly zoned either optically or chemically, and exhibits anomalously high MnO, ZnO and CoO contents (up to 22, 7.5 and 2 wt%, respectively). It has four distinct optical zones particularly in the magnetite veins and less typically in serpentinites. The highest level of these elements, probably divalent, is recorded within the ferritchromite zone and/or within the core zone if the ferritchromite zone is absent. These elements as well as Fe exhibit enrichment along grain boundaries and fractures of the altered spinels. Fe, Mn, Zn and Co were most probably supplied from olivine upon severe serpentinization during and after obduction of the ophiolite. The enrichment of the Bou-Azzer chromian spinel in Mn, Zn and Co was governed mainly by the fluid/mineral (spinel) ratio and partition coefficient between spinel and the relevant fluid among many factors. The Co-, Zn- and Mn-rich chromian spinels can be used as an exploration guide for Co-Ni-Zn-Cu sulfide mineralization associated with serpentinized peridotites.
続きを見る
|
||||
| 2.
論文 |
論文
|
Mizukami, Tomoyuki ; Yokoyama, Hironori ; Hiramatsu, Yoshihiro ; Arai, Shoji ; Kawahara, Hirokazu ; Nagaya, Takayoshi ; Wallis, Simon R.
概要:
It is known that plate boundaries in subduction zones show heterogeneous slip nature with strongly coupled seismogenic z
…
ones and various types of episodic tremor and slip (ETS) zones. In order to examine the petrological controls on the large-scale structure, we compared recent geophysical observations in the Shikoku area, southwest Japan with petrological models of the hanging wall mantle wedge. As a result, we found a close relationship between mineral assemblages in the mantle wedge and the characteristics of slow slip behaviour recorded in the Shikoku area: Short-term ETSs take place in the antigorite + olivine stability field and silent long-term slow slip events (SSEs) take place in the antigorite+brucite stability field. Based on observations of natural antigorite serpentinites, we propose a model that the dominant serpentinization reaction in the mantle wedge changes with increasing depth resulting in variable extents of pore fluid pressures along slip planes. The serpentinization reaction in the antigorite+brucite stability field (olivine + H2O → antigorite + brucite) proceeds at the expense of water. This is consistent with moderately elevated pore pressures inferred for long-term SSEs. The existence of weak brucite enhances the development of shear zones oblique to the main foliation. The resultant anastomosing network provides fluid pathways that may help reduce pore pressures on slip planes. In contrast, progress of the serpentinization reaction in the antigorite + olivine stability field (olivine + H2O + SiO2 → antigorite) results in a large amount of residual water that contributes to further increase pore fluid pressures on slip planes of short-term SSEs. Our results imply that understanding of serpentinization reactions and their contributions to fluid networks in mantle wedge is important in constructing quantitative 3-D models for strain evolutions along plate boundaries. © 2014 Elsevier B.V.
続きを見る
|
||||
| 3.
論文 |
論文
|
Morishita, Tomoaki ; Arai, Shoji ; Ishida, Yoshito
概要:
金沢大学理工研究域自然システム学系<br />Trace-element compositions of jadeite (±omphacite) in jadeitites from the Itoigawa-Ohmi district
…
of Japan, analyzed by a laser-ablation inductively coupled plasma mass spectrometry technique showed chemical zoning within individual grains and variations within each sample and between different samples. Primitive mantle-normalized patterns of jadeite in the samples generally showed high large-ion lithophile element contents, high light rare earth element/heavy rare earth element ratios and positive anomalies of high field strength elements. The studied jadeitites have no signatures of the protolith texture or mineralogy. Shapes and distributions of minerals coupled with chemical zoning within grains suggest that the jadeitites were formed by direct precipitation of minerals from aqueous fluids or complete metasomatic modification of the precursor rocks by fluids. In either case, the geochemical characteristics of jadeite are highly affected by fluids enriched in both large-ion lithophile elements and high field strength elements. The specific fluids responsible for the formation of jadeitites are related to serpentinization by slab-derived fluids in subduction zones. This process is followed by dissolving high field strength elements in the subducting crust as the fluids continue to circulate into the subducting crusts and serpentinized peridotites. The fluids have variations in chemical compositions corresponding to various degrees of water-rock interactions. © 2007 The Authors; Journal compilation © 2007 Blackwell Publishing Asia Pty Ltd.
続きを見る
|
