Origin of gases and water in mud volcanoes of Andaman accretionary prism: implications for fluid migration in forearcs

Extensive mud volcanism on the Andaman accretionary prism occurs above a complex network of faults and is caused by the convergence of the Indian plate and the Burmese microplate. Mud volcanoes of the Andaman forearc have received little attention in spite of the fact that they are one of the important features of this tectonic setting, located within an ocean basin that has one of the highest sedimentation rates in the world, and that the materials emitted by them present a unique opportunity to study the chemistry of the detachable parts of the subducting slab. In this study we present mineralogical, chemical and isotopic data for argillaceous matter (mud matrix), gases and water emitted by these mud volcanoes and attempt to understand the variations in terms of their sources and processes within the forearc. The mud matrix is composed of smectite–illite–kaolinite–chlorite–plagioclase–quartz–calcite assemblage derived both from sediments and altered oceanic crust and originates from a deep-burial diagenetic environment. The modes of δ13C distributions for methane (>− 42‰), ethane (>− 27‰) and CO2 (<− 3‰) indicate thermogenic origin for these gases, with TOC and N of associated mud suggesting marine organic matter as the source. The water ejected at these mud volcanoes is much fresher (Cl− = 45 to 135 mM) than seawater and its δ18O (− 0.2 to 2.6‰) and δD (− 24 to − 14‰) isotopic compositions fall well below the global meteoric water line. From their trace element contents and stable isotopic compositions we infer that the mud water is a mixture of sediment pore water (ancient seawater) and water released from dehydration of clay minerals. The 87Sr/86Sr of mud water (~ 0.7071) confirms the above inference and points out that altered oceanic crust plays a significant role in controlling the chemistry of water. The formation temperatures of mud ejecta, derived from mineralogical (smectite/illite), chemical (K+/Na+) and isotopic (δD/δ18O) geothermometers, lie in the range of 50 °C–120 °C — which corresponds to a depth zone of 2 to 6 km within the Andaman forearc. Based on all the evidences we conclude that the sampled mud volcano ejecta originate at the plate-boundary décollement zone, from the sediments and altered oceanic crust of the subducting Indian plate.