Sulfur and chlorine degassing from primitive arc magmas: temporal changes during the 1982-1983 eruptions of Galunggung (West Java, Indonesia)

The 1982-1983 eruptions of Galunggung represent a nine-month period of inte rmittent volcanic activity with significant changes in explosivity and emis sion of volatiles. Eruptions started with Vulcanian explosions but changed gradually to Strombolian activity. Compositions of juvenile material change d from basaltic andesite to high-Mg basalt, which are among the most primit ive rock types known in the Indonesian arc system. Although bulk compositio ns suggest a single evolution trend, we infer from the compositions of melt inclusions in olivine phenocrysts that the magmas represent derivatives of a complex spectrum of primary melts. Primitive inclusions in olivine pheno crysts from magma erupted during the Strombolian phase contain up to simila r to 2000 ppm sulfur, but concentrations decrease rapidly with increasing S iO2 down to matrix glass values (50-100 ppm). 'Vulcanian' inclusions appear to be degassed before eruption (similar to 200 ppm ). Chlorine concentrati ons increase from 750 to 2200 ppm in Strombolian, and from 800 to 1500 in V ulcanian magmas, whereas matrix glass contains about 1000 ppm in both cases . Ash leachates show two cycles of decreasing S/Cl ratios: from 9.7 to 5.6 at the start of the activity, and from 12.2 to 2.0 after four months. As th e second cycle follows upon increased seismic activity at shallow depth, it probably reflects degassing of fresh sulfur-rich magma arriving in the sha llow Galunggung reservoir. In contrast to the degassed state of Vulcanian m agma, the significant amounts of adsorbed sulfur on the ashes point to an e xcess source of sulfur, which was most likely derived from intruding Stromb olian magma. Hence, the observed sulfur flux of 2 Mt is not in accordance w ith a petrologic estimate of 0.09 Mt. Using a published value of 550 Mt of erupted material about 0.34km(3) fresh undegassed magma is needed to accoun t for the observed sulfur flux. This is close to the erupted volume of Vulc anian magma (0.26 kM(3)), which presumably was replaced completely by Strom bolian magma during the eruption. Using the petrologic method, we calculate a total release of 0.3 Mt chlorine, which agrees well with an output of 0. 47 Mt estimated independently from S/Cl ratios of the ash leachates and TOM S sulfur yields. Ash leachates show that about 35% of the sulfur and 30% of the chlorine was scavenged from the eruption plumes. Our results suggest t hat sulfur and chlorine were largely decoupled during degassing, which resu lted in considerable variations in S/Cl ratios during the Galunggung erupti ons. We infer that sulfur degassing reflects the arrival of fresh magma at shallow depth, whereas chlorine is largely derived from simultaneously erup ted material. As a consequence, the petrologic estimates are more consisten t with observed emissions for chlorine than for sulfur. (C) 2001 Elsevier S cience B.V. All rights reserved.