Tracing magma sources in an arc‐arc collision zone: Helium and carbon isotope and relative abundance systematics of the Sangihe Arc, Indonesia

Abstract

The Sangihe Arc is presently colliding with the Halmahera Arc in northeastern Indonesia, forming the world's only extant example of an arc‐arc collision zone. We report the first helium and carbon isotopic and relative abundance data from the Sangihe Arc volcanoes as a means to trace magma origins in this complicated tectonic region. Results of this study define a north‐south trend in 3 He/ 4 He, CO 2 / 3 He, and δ 13 C, suggesting that there are variations in primary magma source characteristics along the strike of the arc. The northernmost volcanoes (Awu and Karangetang) have higher CO 2 / 3 He and δ 13 C (up to 179 × 10 9 and −0.4‰, respectively) and lower 3 He/ 4 He (∼5.4 R A ) than the southernmost volcanoes (Ruang, Lokon, and Mahawu). Resolving the arc CO 2 into component structures (mantle‐derived, plus slab‐derived organic and carbonate CO 2 ), the northern volcanoes contain an unusually high (>90%) contribution of CO 2 derived from isotopically heavy carbonate associated with the subducting slab (sediment and altered oceanic basement). Furthermore, the overall slab contribution (CO 2 of carbonate and organic origin) relative to carbon of mantle wedge origin is significantly enhanced in the northern segment of the arc. These observations may be caused by greater volumes of sediment subduction in the northern arc, along‐strike variability in subducted sediment composition, or enhanced slab‐derived fluid/melt production resulting from the superheating of the slab as collision progresses southward.