New Constraints of Global Sulfur and Carbon Geodynamic Cycles

Recent work published in Scientific Reports describes a new experimental method for measuring the sulfur content of mid-ocean ridge basalt glasses and vesicles, together with their helium isotope values, to obtain MORB pre-degassing S/3He ratios.

Volcanism at mid-ocean ridges (MORs) and volcanic arcs arguably represents the main pathway of mantle volatiles to the atmosphere and oceans. Sulfur flux from arc volcanoes (ARC) has been measured for decades (left), and global arc volcanic sulfur flux is quite well constrained between 200 and 560 x 109 mol/yr (13 to 36 Mt/yr SO2) [1,2]. However, only indirect estimates of MOR sulfur flux exist. These estimates are based upon seawater-basalt sulfur exchange during hydrothermal alteration (78 x 109 mol/yr) [3] and from oceanic crust production (1640 x 109 mol/yr) [4], giving an uncertainty of two orders of magnitude. 

Recent work by Takanori Kagoshima, Yuji Sano, and Naoto Takahata (University of Tokyo, Japan), Teruyuki Maruoka (University of Tsukuba, Japan), Tobias Fischer (University of New Mexico, USA), and Keiko Hattori (University of Ottawa, Canada) published in Scientific Reports [5] describes a new experimental method for measuring the sulfur content of MORB (mid-ocean ridge basalt) glasses and vesicles, together with their helium isotope values, to obtain MORB pre-degassing S/3He ratios. Using global MOR 3He flux of 530±100 mol/yr [6], the authors obtained estimates of global sulfur flux from MOR of 100 x109 mol/yr.  The authors also report S/3He ratios from ARC high temperature, magmatic gases, and calculate global ARC sulfur flux of 720 x 109 mol/yr, somewhat higher than the range of previous estimates. High-temperature volcanic gas sulfur isotope ratios allowed the authors to estimate the relative contributions of upper mantle, subducted pyrite, and subducted sulfate to ARC SO2 emissions. The results show that, on a global scale, only 3% of emitted sulfur is from the upper mantle, with the rest contributed from the subducted slab (see figure right, a).  

Using this combination of high-temperature magmatic ARC gas estimates (with an average 3He/4He ratio of 6.8 RA, where RA is the ratio of air), CO2/3He ratios, and MOR and ARC 3He fluxes, the authors also calculated a global carbon flux of 3400 x109 mol/y (150 Mt/yr CO2) from MOR and ARC. For CO2, 11% of the ARC flux (2200 x109 mol/yr) originates in the mantle, with the remainder coming from organic matter and carbonates in the subducting slab (see figure right, b), similar to recently reported estimates [7]. Therefore, taking these estimates at face value, about 120 x 109 mol of S and 1400 x 109 mol of C are subducted past the zones of magma generation into the deeper mantle. 

This work illustrates the value of combining volatile measurements obtained from erupted glasses with those from fumarole emissions and provides new and detailed insights into global volatile budgets and sources.  Much more work remains to quantify the effects of volatile ingassing on mantle geochemistry through time.

 

Featured image: Kawah Ijen Volcano, East Java hosts a hyperacid crater lake, high temperature fumaroles, and extensive sulfur deposits.  This night photo shows sulfur burning in the fumarole field. Credit: Olivier Grunewald

Figure: a) Mid-ocean ridge and arc sulfur sources and fluxes calculated using MORB glasses and vesicles S/3He, δ34S, 3He flux and high-temperature arc volcanic gases S/3He, δ34S, and arc 3He flux, respectively. b) Mid-ocean ridge and arc carbon sources and fluxes calculated using MORB glasses and vesicles C/3He, δ13C, 3He flux and high-temperature arc volcanic gases C/3He, δ13C, and arc 3He flux, respectively (Kagoshima et al., 2015).

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