ANDAMAN MUD VOLCANOES—MINERALOGY AND GEOCHEMISTRY OF MUD AND A POTENTIAL HYDROCARBON SOURCE

Mud volcanoes are a common feature in the Baratang Island of Middle Andamans, India. The association of methane gas and adsorbed hydrocarbons in the mud has been cited widely by scientists working in Andamans and also by others working on other eruption areas around the world. This study examines the nature of these mud volcanoes, in terms of their geochemical and mineralogical composition, with particular reference to the clay minerals formed by sedimentary volcanism Such volcanic deposits may be exploited as potential hydrocarbon reserves. Preliminary analyses suggest that gas is venting at variable rates in different gas channels at the same site, and that the compositional differences in these vent gases are nearly as large as can be produced by hydrate crystallization. Our findings are compatible with the supposition that venting evolves from fast (mud volcano), to intermediate (hydrate crystallization), to slow (carbonate precipitation) rates, as venting becomes organized into discrete vents over time. The most realistic explanation of the observed effects is a rapid local emission of mud and/or water. Powder XRD of bulk mud samples enables identification of the complete suite of minerals, in addition to the clay minerals. Variability in modal mineralogy is tracked using the Rietveld Method of crystal structure refinement. In addition, micro-XRD of sample cores in situ provides mineral identification as well as textural information such as grain size and distribution, as well as preferred orientation. Stable isotopes of the separated clay minerals (smectite and illite-rich extruded mud) from the mud volcanoes will discriminate between fluid sources for volcanism in the Andamans. The most likely mechanism is re-hydration of shales by both hydrocarbons and a geochemically mature fluid from greater depth within the wedge. Trace element and REE studies of the muds will be conducted to test the above hypothesis. The overall results attest active local flow of geochemically different fluids along deep-seated faults penetrating the wedge, with the waters as well as the gases coming from below.