Japan Geoscience Union Meeting 2019

Presentation information

[J] Oral

H (Human Geosciences ) » H-RE Resource and Engineering Geology

[H-RE16] Resource Geology

Wed. May 29, 2019 10:45 AM - 12:15 PM 106 (1F)

convener:Tsubasa Otake(Division of Sustainable Resources Engineering, Faculty of Engineering, Hokkaido University), Daisuke Araoka(Geological Survey of Japan, National Institute of Advanced Industrial Science and Technology), Ryohei Takahashi(Graduate School of International Resource Sciences, Akita University), Tatsuo Nozaki(Research and Development Center for Submarine Resources, Japan Agency for Marine-Earth Science and Technology), Chairperson:Tsubasa Otake(Faculty of Engineering, Hokkaido University), Daisuke Araoka

10:45 AM - 11:15 AM

[HRE16-01] The role of water in the concentration of Cr

★Invited Papers

*Shoji Arai1 (1.Department of Earth Sciences, Faculty of Science, Kanazawa University)

Keywords:Chromium, Chromitite, Water

The role of water in the formation of chromitites, which have been designated as a typical orthomagmatic ores, has been long controversial. Johan et al. (1983, 2017), for example, concluded that chromites are precipitated from water-rich fluids at relatively low temperatures (< 1000 ºC) to form podiform chromitites in the mantle. Those arguments are mainly based on two observations; (1) podiform chromitites and surrounding peridotites have been preferentially hydrated, which may mean the initial water enrichment, and (2) chromite grains contain fluid and hydrous mineral inclusions of apparently primary origin. I criticize the related interpretations based on those observations. The selective hydration in and around chromitite pods is due to the Mg-rich character of olivines, which has been obtained during a subsolidus cooling stage (Arai, 1978), and does not definitely show the involvement of water-rich fluid in chromitite formation. The origin of the chromite-hosted hydrous mineral inclusions has not been left unclear. Many authors a priori interpreted the inclusions as being representative of the trapped melts or fluids that precipitated the host chromites. This is, however, highly questionable. The inclusions are most probably formed during a stage of peridotite-melt reaction within a complicated igneous stage of podiform chromitite generation in the mantle. Hydrothermal fluids can transport Cr and precipitate chromite, but may not form most of the podifrom chromitites.