Geochemical and petrological indicators of volcanic behavior: Merapi volcano, Java, Indonesia

Gunung Merapi, one of Indonesia's most active volcanoes, is characterized by long periods of dome growth and intermittent explosive pyroclastic events. Merapi currently degasses continuously through high-T fumaroles (>200°C), and erupts crystal-rich basaltic-andesite that contains a large range of igneous and calc-silicate crustal inclusions. To evaluate mechanisms that trigger explosive eruptions, we sampled lavas, inclusions (xenoliths), and gas from active fumaroles. Additionally, we established a time-integrated experiment reaction series mimicking crustal assimilation at Merapi under magmatic conditions. Merapi lava contains abundant plagioclase crystals which show complex zoning and vary in anorthite (An) content between 40 and 95 mol% across resorption surfaces. A negative correlation between An mol% and other indicators of magmatic fractionation, such as MgO and FeO, has been observed. Moreover, Sr isotope analyses of discrete zones in plagioclase yields ⁸⁷Sr/⁸⁶Sr values that notably exceed those of the host lavas. Zones with the highest An content also tend to show the highest radiogenic Sr values, consistent with a Ca-rich, high-⁸⁷Sr/⁸⁶Sr crustal contaminant. Abundant metamorphosed limestone xenoliths contain compositionally identical feldspar to the high-An population in the lavas, demonstrating that magma-crust interaction is a significant process at Merapi. Carbon isotope ratios of fumarole CO₂ sampled during quiescent degassing periods form a baseline of δ¹³C_2001-2008 = -4.1%. The notable exceptions are the 2006 values, obtained immediately after the eruption and the 6.4 magnitude Yogyakarta earthquake, which show elevated δ¹³C values up to -2.4%. Notably, the rise in δ¹³C values coincided with an increase in eruptive intensity and volcano seismicity by a factor of 3 to 5 for several weeks after the earthquake. This is consistent with addition of a late-stage, crustal volatile component added to purely mantle and slab-derived volatile sources. This observation argues for extensive and ongoing magma-crust interaction beneath the volcano, especially during eruptive and/or seismic events. Our high P-T experiments show that interaction between Merapi magma and limestone can rapidly liberate crustal CO₂ on a timescale of only seconds to minutes. We therefore expect vigorous CO₂ bubble nucleation and growth on a scale of perhaps hours to days in nature. Late volatile input could therefore accelerate or trigger explosive eruptions independently of magmatic recharge and fractionation by sudden over-pressurization of the upper parts of the magma system. Such an event would provide shallow seismic warning signals immediately prior to an erratic, CO₂-driven, eruption crisis. Thus we conclude that crust-mantle interaction processes have serious implications for eruptive behavior, volatile emission, and hazard management at Merapi and similar systems elsewhere.