◆Research findings by a group led by Visiting Assistant Professor Ken Nakayama of the International Research Institute for Marine Core Science have been published in *Scientific Reports*
Release Date:
Dating of Volcanic Massive Sulfide Deposits Using the Rhenium-Osmium Method
―Accurately constraining the timing of mid-ocean ridge subduction in the Japanese archipelago―
A research group led by Ken Nakayama, a visiting assistant professor at the International Research Institute for Marine Core Studies, has determined the formation ages of volcanic massive sulfides found in Nobeoka City, Miyazaki Prefecture, and Shimokawa Town, Hokkaido, using the rhenium-osmium (Re-Os) method (*1), revealing that the Makimine deposit in Miyazaki Prefecture formed approximately 89 million years ago, while the Shimokawa deposit formed approximately 48 million years ago.
As a result, we have successfully and accurately determined when subduction of the nearest mid-ocean ridge (the Izanagi-Pacific Ridge) (Note 2) occurred beneath the Japanese archipelago based on the formation ages of mineral deposits. This research is expected to contribute to a better understanding of the structural history of Hokkaido and the Japanese archipelago (Note 3), where the formation of geological zones is complex.
The findings of this study were published in the journal *Scientific Reports*, published by the international academic publisher Nature Research, on Tuesday, December 3, 2024 (local time).
*1 Rhenium-Osmium (Re-Os) Method
Re and Os are the elements with atomic numbers 75 and 76, respectively, and Os is one of the six platinum group elements. Re has two isotopes, 185Re and 187Re; the half-life of 187Re —the time it takes for its quantity to decrease by half—is 41.6 billion years, during which it emitsbeta raysto produce 187Os.The Re-Os method is a dating technique that utilizes this radioactive decay series. By plottingthe 187Re/188Os ratio and the 187Os/188Os ratio —both obtained through chemical analysis—on a graph, an approximate straight line (isochron) can be drawn, and the age (Re-Os isochron age) can be calculated from the slope of this line.
*2 Mid-Ocean Ridge
An underwater mountain range running through the middle of the ocean, with elevations of 2 to 3 km and lengths of several thousand kilometers. Mid-ocean ridges have high rates of geothermal heat flow, and most of the thermal energy released from the Earth’s solid interior is discharged as volcanic activity along these ridges. Therefore, mid-ocean ridges are, so to speak, the sites where oceanic plates are generated.
Note 3: History of Architecture
The history describing the processes through which a geological formation or the rocks that compose it have evolved to their present state.
[ [Paper Information]
Title: Re-Os dating of the Makimine and Shimokawa VMS deposits for new
Age constraints on ridge subduction beneath the Japanese Islands