Volatile and N isotope chemistry of the Molucca Sea collision zone: Tracing source components along the Sangihe Arc, Indonesia

Abstract

Volcanic gases are sensitive indicators of subduction processes and are used to evaluate the contributions from various source components. Nitrogen isotope systematics in particular are a valuable tool for determining the fate of organic matter in subduction zones. We present the first arc‐wide survey of trace gas chemistry and nitrogen isotope variations from the Sangihe Arc of northeastern Indonesia, where the narrow Molucca Sea Plate subducts beneath the Sangihe Arc to the west and the Halmahera Arc to the east. Relative volatile abundances and N isotopic compositions of volcanic gases show systematic along‐arc variations. Northern volcanoes exhibit low N2/He ratios and δ15N values (northern minima 542 and −7.3‰, respectively), indicating minimal addition of sediment to source magmas. In contrast, the southern part of the arc is characterized by high N2/He and δ15N values (southern maxima 2000 and +2.1‰, respectively), consistent with greater sediment contributions in the formation of the magmas. These observations can be correlated with the complex tectonic setting of the region whereby oblique collision between the two arcs has caused sediment obduction, decoupling the accretionary wedges from the underlying oceanic plate. In the north, where the collision is more developed, the lack of trace gas and N isotope evidence of sedimentary inputs to the source of arc magmas is consistent with enhanced sediment decoupling. In the south, where collision and accretionary wedge decoupling are not yet taking place, sediments would presumably subduct normally, in agreement with higher N2/He and δ15N values. Awu volcano, at the northernmost extension of the arc, is anomalous and exhibits high N2/He (2852) coupled with low δ15N (−3.3‰). These values are suggestive of increased slab contribution in the northernmost arc, possibly by slab melting as collision stalls the progress of the subducting plate and allows it to become superheated.