Open AccessVolatile and N isotope chemistry of the Molucca Sea collision zone: Tracing source components along the Sangihe Arc, Indonesia
Author(s) -
Clor Laura E.,
Fischer Tobias P.,
Hilton David R.,
Sharp Zachary D.,
Hartono Udi
Publication year - 2005
Publication title -
geochemistry, geophysics, geosystems
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.928
H-Index - 136
ISSN - 1525-2027
DOI - 10.1029/2004gc000825
Subject(s) - geology , subduction , accretionary wedge , volcanic arc , island arc , ophiolite , geochemistry , earth science , paleontology , tectonics
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 N 2 /He ratios and δ 15 N 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 N 2 /He and δ 15 N 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 N 2 /He and δ 15 N values. Awu volcano, at the northernmost extension of the arc, is anomalous and exhibits high N 2 /He (2852) coupled with low δ 15 N (−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.