17:15 〜 18:30
[SEM13-P10] 沖縄トラフ海底熱水鉱床のisocubaniteの岩石磁気特性
キーワード：isocubanite、submarine hydrothermal deposits、low-temperature magnetometry
In recent years, submarine hydrothermal deposits have been attracting attention as a new mineral resource. To understand the nature of the deposits and constrain the possible area of hidden ore, it is necessary to gather information about the environment where deposits occur and grow based on investigations of mineral composition and texture of known deposits. Some of the ore minerals exhibit characteristic magnetic properties which potentially enable us a quick identification of such specific minerals. However, magnetic properties are not well documented except for a small number of minerals since they vary not only by mineral species but by chemical composition and texture.
In this study, we searched for ore minerals that can be identified by magnetic measurements from a series of samples collected at the Okinawa Trough submarine hydrothermal deposits and investigated the magnetic properties in detail. Samples were a suite of drilled subseafloor core samples and a block sample of a hydrothermal chimney collected from Noho site and Hakurei site in the Okinawa Trough under the framework of the Next-Generation Technology for Ocean Resources Exploration Project, Cross-ministerial Strategic Innovation Promotion Program (SIP) by the Japanese cabinet office.
First, low temperature magnetometry was conducted to seek for a characteristic signal which appear among different samples. In several samples we found a common feature that a magnetic transition occurs at ~100 K and below this temperature the magnetic moment is larger than at higher temperatures. On those samples or samples taken from nearby depth, we conducted XRD measurements, optical microscopic observation and EPMA analysis and found out that a Cu-Fe sulfide mineral isocubanite should be responsible for the magnetic feature. We further conducted detailed magnetic measurements and Mössbauer spectroscopy on a sample portion consisting almost exclusively of isocubanite that was separated from the chimney samples in which isocubanite appears most prominently. As a result, it was revealed that magnetic properties, Mössbauer spectrum, crystal structure, and the occurrence of intimate lamellar structure with chalcopyrite are generally consistent with ‘aged isocubanite’ which was originally collected at the Snake Pit, Mid-Atlantic Ridge, reported by Wintenberger et al. (1994). On the other hand, chemical composition and texture (with or without exsolution of chalcopyrite) varied among isocubanite samples indicating that the magnetic transition at 100 K is common to isocubanite in wide range of its solid solution, while the sharpness of the transition and the strength of the magnetic moment at low temperature seem to differ by Cu/Fe ratio.
From these results we conclude that low temperature magnetometry on ore samples may be effective to detect isocubanite and have the potential to make contribution to our understanding of the nature of submarine hydrothermal deposits.
In this study, we searched for ore minerals that can be identified by magnetic measurements from a series of samples collected at the Okinawa Trough submarine hydrothermal deposits and investigated the magnetic properties in detail. Samples were a suite of drilled subseafloor core samples and a block sample of a hydrothermal chimney collected from Noho site and Hakurei site in the Okinawa Trough under the framework of the Next-Generation Technology for Ocean Resources Exploration Project, Cross-ministerial Strategic Innovation Promotion Program (SIP) by the Japanese cabinet office.
First, low temperature magnetometry was conducted to seek for a characteristic signal which appear among different samples. In several samples we found a common feature that a magnetic transition occurs at ~100 K and below this temperature the magnetic moment is larger than at higher temperatures. On those samples or samples taken from nearby depth, we conducted XRD measurements, optical microscopic observation and EPMA analysis and found out that a Cu-Fe sulfide mineral isocubanite should be responsible for the magnetic feature. We further conducted detailed magnetic measurements and Mössbauer spectroscopy on a sample portion consisting almost exclusively of isocubanite that was separated from the chimney samples in which isocubanite appears most prominently. As a result, it was revealed that magnetic properties, Mössbauer spectrum, crystal structure, and the occurrence of intimate lamellar structure with chalcopyrite are generally consistent with ‘aged isocubanite’ which was originally collected at the Snake Pit, Mid-Atlantic Ridge, reported by Wintenberger et al. (1994). On the other hand, chemical composition and texture (with or without exsolution of chalcopyrite) varied among isocubanite samples indicating that the magnetic transition at 100 K is common to isocubanite in wide range of its solid solution, while the sharpness of the transition and the strength of the magnetic moment at low temperature seem to differ by Cu/Fe ratio.
From these results we conclude that low temperature magnetometry on ore samples may be effective to detect isocubanite and have the potential to make contribution to our understanding of the nature of submarine hydrothermal deposits.