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.