Crustal CO2 liberation during the 2006 eruption and earthquake events at Merapi volcano, Indonesia

High-temperature volcanic gas is widely considered to originate from ascending, mantle-derived magma. In volcanic arc systems, crustal inputs to magmatic gases mainly occur via subducted sediments in the mantle source region. Our data from Merapi volcano, Indonesia imply, however, that during the April-October 2006 eruption significant quantities of CO₂ were added from shallow crustal sources. We show that prior to the 2006 events, summit fumarole gas δ¹³C_(CO2) is virtually constant (δ¹³C_1994-2005 = -4.1 ± 0.3‰), but during the 2006 eruption and after the shallow Yogyakarta earthquake of late May, 2006 (M6.4; hypocentres at 10-15 km depth), carbon isotope ratios increased to -2.4 ± 0.2‰. This rise in δ¹³C is consistent with considerable addition of crustal CO₂ and coincided with an increase in eruptive intensity by a factor of ∼3 to 5. We postulate that this shallow crustal volatile input supplemented the mantle-derived volatile flux at Merapi, intensifying and sustaining the 2006 eruption. Late-stage volatile additions from crustal contamination may thus provide a trigger for explosive eruptions independently of conventional magmatic processes.