Abstract
A combined hydrogen-carbon-isotope and microthermometric study has been carried out on CH4-bearing fluid inclusions in high-pressure jadeitites from the famous jadeite tract Myanmar. Two types of fluid inclusions were found in jadeites, large H2O-rich and CH4-poor inclusions and small H2O-poor and CH4-rich inclusions, thus indicating a possible entrapment of CH4-H2O fluids under unmixing conditions. Microthermometric results yield lower temperature limits for the entrapment of these fluid inclusions of ca. 300 to 400°C. The bulk composition of the fluid inclusions is mostly H2O (87 to 94 mol.% H2O) and the isotopic composition of methane and water in the inclusions is characterized by δ13C(CH4) values ranging from -30.1 to -25.5‰, and δD(H2O) values ranging from -56.3 to -49.8‰. The stable isotope data would be indicative of an abiogenic mechanism of CH4 formation; the occurrence of the jadeite veins in this paleo-subduction zone thus most likely point to the formation of these CH4-bearing fluid inclusions by abiogenic thermal maturation of subducted organic carbon. These data not only provide evidence for cycling of organic carbon in paleo-subduction zones but also show that CH4 not only occurs as shallow CH4-rich plumes in accretionary prisms of recent subduction zones but also occurs in deeper portions of at least the upper 20 km of paleo-subduction zones.
