Preservation of methane generated during serpentinization of upper mantle rocks: Evidence from fluid inclusions in the Nidar ophiolite, Indus Suture Zone, Ladakh (India)

The Nidar Ophiolite Complex (NOC) within the Indus Suture Zone in Eastern Ladakh, India, represents a suprasubduction zone (SSZ) ophiolite from a fore-arc setting. The lower part of the ophiolite sequence is comprised of ultramafic upper mantle rocks that are Mg-rich (Fo in olivine > 90-92) and contain 2-7% Cr-spinel. Pure-methane (CH4) fluid inclusions occur in olivine from partially serpentinized harzburgite and dunite from the NOC. Homogenization temperatures range from - 160 degrees C to - 108 degrees C, and freezing behavior combined with Raman analyses indicate that the inclusions contain no other gaseous species. The majority of the inclusions appear to be of secondary origin although some isolated inclusions of indeterminate origin were observed. CH4 in the Nidar ophiolite was generated as a by-product of serpentinization of ultramafic rocks in the mantle wedge above the subducting slab, coupled with the complete consumption of water during hydration of serpentine. The presence of the lizardite polymorph of serpentine is consistent with formation in a rock-dominated system (low water activity) that was being deformed in a non-isotropic stress environment. The observed fluid inclusion isochores suggest various degrees of reequlibration during the history of the rocks, with the more extreme (high P) isochores most closely approximating the serpentinization conditions during prograde metamorphism at temperatures < 600 degrees C and pressures in excess of about 2 kbars. These results support previous studies that have shown that early-formed fluid inclusions in mantle-derived rocks may be preserved during tectonic uplift to the surface and maintain the original mantle chemical signature. (c) 2007 Elsevier B.V. All rights reserved.