Heterogeneity of volatile sources along the Halmahera arc, Indonesia
Introduction
The Halmahera arc is situated in the northeastern part of Indonesia and extends in a roughly north-south direction between 3°N-1°S and 127°-128°E. It is the smallest of the four volcanic arcs that constitute the archipelago of Indonesia, with five active volcanoes, namely Dukono, Ibu, Gamkonora, Gamalama, and Makian (also known as Kie besi) from north to south (Fig. 1). Due to its comparative remoteness, difficulty of access and political, ethnic, and inter-faith unrest that persisted until the early 2000s (Goss, 2000; Bertrand, 2003), the Halmahera arc has been little studied, despite the fact that its volcanoes are among the most active in Indonesia. This situation is starting to be redressed with several recent studies highlighting the strong volcanic degassing source on Dukono (Carn et al., 2017; Bani et al., 2018), the fast-growing rate of Ibu lava dome since 1998 (Agustan et al., 2010; Saing et al., 2014), the magmatic signature of weak degassing at Gamkonora (Saing et al., 2020), and the interplay of hydrothermal, magmatic, and tectonic processes controlling recurrent eruptive activity of Gamalama volcano (Kunrat et al., 2020). Here, we report the first observations of gas compositions for Ibu volcano that, in combination with recently available data, enable an assessment of arc-scale gas emission budgets. We also highlight the compositional variability of magmas along the arc, based on analyses of recently erupted products.
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Geodynamic setting and volcanic activity
The geodynamics of the Halmahera arc are intimately linked to the tectonic activity in the Molucca Sea, where the Sangihe forearc is overriding the Halmahera forearc (e.g., Hall and Wilson, 2000). The process constitutes a unique present-day example of an arc-to-arc collision arising from the double subduction of the Molucca Sea plate, which dips west beneath the Eurasian plate and east under the Philippine Sea plate (Fig. 1, Baker et al., 1994; Forde, 1997; Hall and Wilson, 2000; Zhang et al.,
Field measurements
SO2 flux measurements were made with passive ultraviolet spectrometers that scanned the plume from a fixed position and using the retrieval method of differential optical absorption spectroscopy (DOAS) (Fig. 2). Spectra were obtained with a variable step angle, depending on the plume size and the distance from the plume. The spectrometer used was an Ocean Optic USB2000+ with a spectral range of 290–440 nm and spectral resolution of 0.5 FWHM. The SO2 column amounts (ppm.m) were retrieved using
Arc degassing budget
Table 1 reports our SO2 flux estimates for Ibu volcano based on UV DOAS. Our new SO2 flux results for Ibu range between 50 and 140 Mg/d with a daily mean value of 105 Mg. This is an order of magnitude higher than the estimate reported by Saing et al. (2014) that corresponded to SO2 released by explosions only. Our new estimate integrates both passive and eruptive discharges, and as such may be considered more representative of Ibu's bulk plume flux of SO2 to the atmosphere.
Table 2 presents the
Arc-scale degassing budget
The volcanic emission budget from the Indonesia arcs has so far been estimated based on inferences and extrapolations (Andres and Kasgnoc, 1998; Halmer et al., 2002; Aiuppa et al., 2019; Fischer et al., 2019; Bani et al., 2020), because direct measurements are available for only a few volcanoes. The most extensive volcanic degassing inventory for the archipelago is that of Carn et al. (2017), which is based on satellite observations. However, it represents only 20 out of 78 volcanoes in
Conclusions
We have reported the first arc-scale gas emission budget for the Halmahera arc, indicating daily fluxes of the order of 96,300 Mg/d of H2O, 2093 Mg/d CO2, 944 Mg/d SO2, 79 Mg/d of H2S, and 15 Mg/d of H2. The main source of SO2 and H2 from the arc is Dukono, whereas Ibu releases the greatest quantities of H2O and H2S. Gamalama and Ibu are the main sources of CO2. Dukono, situated about 80 km from the trench, is a CO2-poor gas source compared with the other volcanoes, although hydrothermal
Credit authorship contribution statement
P. Bani: Conceptualization, Methodology, Investigation, Formal analysis, Writing original draft, Writting reviews & editing. F. Nauret: Conceptualization, Methodology, Formal analysis, Writting original draft. C. Oppenheimer: Methodology, Resources, Writting review & editing. A. Aiuppa: Methodology, Resources, Writting review & editing. B.U. Saing: Investigation. N. Haerani: Investigation. H. Alfianti: Investigation. M. Marlia: Investigation. V. Tsanev: Methology, Resources, Software.
Declaration of Competing Interest
None.
Acknowledgments
This research was supported by IRD under the JEAI-COMMISSION program in collaboration with CVGHM. It has also benefited from the “Science & Impact” program from the French Embassy in Indonesia. A.A. acknowledges funding from the DECADE Initiative of the Deep Carbon Observatory and from the MIUR (under grant PRIN2017-2017LMNLAW). We thank M. Benbakkar, C. Fonquernie, C. Boseg, D. Auclair, M. Gannoun, K. Suchorski and C. Liorzou for the lab. Support and rock analyses in the Laboratoire Magmas et
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