A new hydrothermal scenario for the 2006 Lusi eruption, Indonesia. Insights from gas geochemistry

The 29th of May 2006 gas and mud eruptions suddenly appeared along the Watukosek fault in the north east of Java, Indonesia. Within a few weeks several villages were submerged by boiling mud. The most prominent eruption site was named Lusi. To date (November 2011) Lusi is still active and a similar to 7 km(2) area is covered by the burst mud breccia. The mechanisms responsible for this devastating eruption remain elusive. While there is consensus about the origin of the erupted mud, the source of water is uncertain, the origin of the gas is unknown and the trigger of the eruption is still debated. In order to shed light on these unknowns, we acquired a wide set of data of molecular and isotopic composition of gas sampled in several Lusi vents, in the surrounding mud volcanoes, in the closest natural gas field (Wunut), and in the hydrothermal vents at the neighbouring volcanic complex in the period 2006-2011. The boiling fluids erupted in the crater zone are apparently CO2-dominated, while colder CH4-dominated and C-2-C-3 bearing fluids are identified at several sites around the crater zone. Gas genetic diagrams, maturity plots and gas generation modelling suggest that the hydrocarbons are thermogenic (delta C-13(1) up to 35%circle; delta C-13(2) up to -20%circle), deriving from marine kerogen with maturity of at least 1.5%Ro, for instance in the similar to 4400 m deep Ngimbang source rocks. CO2 released from the crater and surrounding seeps is also thermogenic (delta C-13 from -15 to -24%0) related to kerogen decarboxylation or thermal CH4 oxidation in deep rocks, although three vents just outside the crater showed an apparent inorganic signature (-7.5%.4000 m) magmatic intrusions and hydrothermal fluids responsible for the enhanced heat that altered source rocks and/or gas reservoirs. The neighbouring magmatic Arjuno complex and its fluid-pressure system combined with high seismic activity could have played a key role in the Lusi genesis and evolution. Within this new model framework. Lusi is better understood as a sediment-hosted hydrothermal system rather than a mud volcano. (C) 2011 Elsevier B.V. All rights reserved.