Resolving volatile sources along the western Sunda arc, Indonesia

Abstract

We present the chemical and isotope (HeCN) characterization of active fumaroles and hydrothermal gases and waters from the summits and flanks of 19 volcanic centers along the western Sunda arc, Indonesia. Samples were collected over two field expeditions (1991 and 2010) and cover 13 volcanic centers in Sumatra, 5 in Java and one in Bali. In addition, we present data from three geothermal sites in Sumatra associated with active fault systems in-between volcanic centers (IBVC). The overall aim is to resolve volatiles associated with the sub-arc mantle (subducting slab and mantle wedge) from inputs derived from the over-riding arc crust. The western Sunda arc is a prime locality to assess controls on volatile provenance at subduction zones due to changes in composition and thickness of over-riding crust and variations in sediment input rates along the strike of the arc.The dry gas chemistry of the majority of volcanic samples is dominated by CO2 with inert gas variations (HeArN2) typical of subduction zone gases. However, there is a strong crustal control on the HeCO2 isotope and relative abundance systematics on a number of volcanic centers: this effect is most clearly observed at flank localities and in water phase samples. Filtering the entire database for modifications due to air contamination, degassing-induced fractionation (C-isotopes and CO2/3He ratios) and crustal contamination associated with volatile transport within shallow-level hydrothermal systems allows recognition of the magmatic volatile signature of individual volcanoes along the arc. Helium isotopes at all but two volcanic centers (Talang and Dempo on Sumatra) range from 5.3 RA to 8.1 RA (RA=air 3He/4He) pointing to the mantle wedge as the principal source of He but with a small input of crustal (radiogenic) He at some localities. Samples from Java and Bali span an even more limited range (6.6 to 7.9 RA) implying a relatively smaller input of crustal He. Carbon isotope and CO2/3He ratios vary from −1.4‰ to −6.4‰ and 4.38 to 150 (×109), respectively, with higher and more variable values to the north of Sumatra. This latitudinal effect is not apparent in air-corrected N-isotope values (δ15NC=−3.91 to +5.92‰) or various elemental ratios such as N2/Ar and N2/He. The three IBVC sites, all located in Sumatra, have significantly lower 3He/4He ratios (<3.6 RA) with CO2/3He values both higher and lower than volcanic centers. Their δ13C, δ15NC and gas ratios overlap with the volcanic centers.The elemental and isotopic characteristics of carbon and helium have been modified at IBVC sites due to crustal processes. However, based upon relationships between CH4 and 3He as well as co-variations between HeCN isotopes, the over-riding crust and associated sediments add minimal volatiles to volcanic centers throughout the western Sunda arc. In turn, subducted sediment, particularly the Nicobar Fan which supplies Himalayan-derived sediment to the slab off Sumatra, exerts a strong control on the magmatic CO2 characteristics although it is less influential for N2. In spite of large sections of incoming sedimentary material being off-scraped during subduction, our data suggest that a significant fraction must enter the trench, enhancing fluid/melt production in the mantle wedge. We propose that subduction-related source contamination plays the dominant role over thick/old crustal basement in supplying the major volatile output budget of the western Sunda arc.