Japan's geologists discover metamorphic rocks 'microdiamonds' in Nagasaki Prefecture
Japan researchers have discovered microdiamonds in the Nishisonogi metamorphic rock formation in Nagasaki Prefecture, second region after Italian Alps.
Japan researchers have discovered microdiamonds in the Nishisonogi metamorphic rock formation in Nagasaki Prefecture. After the Italian Alps, this is the second region in the world, which shows that microdiamonds can form in the metamorphic rock through the subduction of oceanic plates.
A collaboration of researchers based in Kumamoto University stated that the microdiamonds in metamorphic rocks are vital minerals because they form in 'continental collision zones' and show that the crust has penetrated deeper than 120 km below the surface. Microdiamonds have gained massive attention in recent times because they have been discovered in metamorphic rocks around the world and it has become clear that they are formed in collisions between continents.
As Japan is an oceanic plate subduction zone and not a continental collision zone it was assumed that Japan would not produce such microdiamonds. However, the assumption changed after the first microdiamonds from metamorphic rocks in Japan were found in the Nishisonogi metamorphic rock formation on the west coast of Nagasaki Prefecture.
The microdiamonds in Japan were discovered in an area that is an approximately 100-million-year-old Cretaceous metamorphic rock formation. Blocks of pelitic rocks and basic schist rocks are scattered amongst serpentinite rocks that were created from mantle material, on the west coast of Saikai City in Nagasaki Prefecture. These rocks are called a 'serpentinite mélange' and show that they have risen from deep in the subduction zone.
Microdiamonds found in the serpentinite mélange
Researchers found microdiamonds in the serpentinite mélange and their formation conditions have been estimated to be a temperature of about 450 degrees Celcius and a pressure of about 2.8 GPa. This makes them the coldest diamonds ever formed. It has been assumed that the Nishisonogi metamorphic rock was formed under a pressure of about 1 GPa, however it is now clear that they were ultrahigh-pressure metamorphic rocks that rose after subducting to 120 km which is considered to be a very unexpected discovery by the scientists.
Professor Tadao Nishiyama, the leader of this study said that the discovery of microdiamonds from Japan's first metamorphic rocks will rewrite Japan's geological history.
The Nagasaki metamorphic rocks were said to belong to a low-temperature, high-pressure-type metamorphic rock belt, the "Sanbagawa Belt," which crosses the Japanese mainland, he said. It has become clear, however, that they are independently-formed ultrahigh-pressure metamorphic rocks, he added.
Published By : Brigitte Fernandes
Published On: 6 September 2020 at 11:26 IST