Abstract
In this paper I provide an overview of previous studies of crustal fluids around the Ryukyu Islands and a summary of the interpreted origin and implications of the methane present in those fluids. The chemical compositions of crustal fluids around the Ryukyu Islands fall into two groups: one group with methane contents as high as 80%, and another with methane contents of about 60%. Methane in crustal fluids can originate either from basement rocks or from organic-rich layers overlying basement rocks. If the basement rocks of the Ryukyu Islands are the source rock of the methane in crustal fluids, there might be huge accumulations of natural gas around the Ryukyu Islands, requiring information about the areal extent and depth of such accumulations to quantify possible gas resources. On the other hand, organic-rich layers overlying basement can be also the origin of the methane in hydrothermal fluids in the southern Okinawa Trough. Thus, understanding the distribution of crustal fluids on land can lead to a better understanding of the distribution of crustal fluids below the nearby deep sea floor. Increasing our knowledge of the origin and distribution of crustal fluids will improve our understanding of mass flux in the Earth's crust.
