Volatile fluxes at arc volcanoes: comparing different techniques and evaluating mass balance

1. Volatile fluxes at arc volcanoes: comparing different techniques and evaluating mass balance A. Shaw, D. Hilton, T. Fischer, E. Hauri Arenal Volcano

2. The MARGINS Subduction Factory: • How do forcing functions regulate production of magma and fluid from the Subduction Factory? • How does the volatile cycle (H2O and CO2) impact chemical, physical and biological processes from trench to deep mantle? • What is the mass balance of chemical species and material across the Subduction Factory?

3. Outline • Different methods for measuring gas fluxes and evaluating mass balance at arcs • Comparing fluxes from IBM and Central America • How well do PT models predict fluid behavior? • Summarize the strengths of different methods, their limitations and future directions

4. Volatile Recycling: Subduction zones cycle material between the Earth’s mantle and its exospheric reservoirs Major Volatiles: CO2, H2O, sulfur species (SO2 & H2S) Trace Volatiles : N2, noble gases (He, Ne, Ar), H2, CH4, …

5. Volatile Recycling: Subduction zones cycle material between the Earth’s mantle and its exospheric reservoirs Major Volatiles: CO2, H2O, sulfur species (SO2 & H2S) Trace Volatiles : N2, noble gases (He, Ne, Ar), H2, CH4, … 5

6. Volcanic output flux estimates: • Assumed 3He flux combined with direct measurements of a volcanic gas (x) relative to 3He • Remote sensing techniques • Melt inclusion studies combined with magma production rates

7. Volcanic Sampling1) Fumaroles2) geothermal wells3) water springs4) bubbling hot springs and mudpots Momotombo volcano

8. Volcanic Sampling1) Fumaroles2) geothermal wells3) water springs4) bubbling hot springs and mudpots

9. 0 100 200 300 400 500 Site 1039Costa Rica

10. Source of gases: three component end-member mixing: • L : marine carbonate/limestone (δ13C= 0‰, C/3He = 1013 ) • M : mantle (δ13C = -6.5‰, C/3He = 2×109) • S : organic-rich sediment(δ13C = -30‰, C/3He = 1013) • Mass balance: • 1) (13C/12C)OBS = M(13C/12C)M + L(13C/12C)L + S(13C/12C)S • 2) 1/(12C/3He)OBS = M/(12C/3He)M + L/(12C/3He)L + S/(12C/3He)S • M+L+S = 1 Sano and Marty (1995); Sano and Williams (1996)

11. crustal additions

12. Source of carbon: • Dominant source of CO2 is from a limestone/marine carbonate source (83-86%) • L/S of input = 10.5 was essentially indistinguishable from the output (9-6-11.1) – see Hilton’s poster for revised models • A higher slab component was observed in Nicaragua

13. CO2 flux estimates: • Average CO2/3He for Central America = 2.3 × 1010 mol/yr combined with an assumed 3He flux ~ 3 mol/yr (based on global subaerial flux scaled to arc length): CO2 flux : 7.1 × 1010 mol/yr (4% of global volcanic arc flux) • Mass balance: this represents 23-28% of the CO2 input to the arc, using estimates from Li and Bebout, 2005 – a significant fraction is cycled to the deep mantle or is lost in the forearc region – limited by fluid availability? • Sediment-derived N flux : 28 × 108 mol/yr (Elkins et al., 2006) – completely recycled through the arc

14. Remote sensing: • Satellite-based measurements • COSPEC: correlation spectrometer or miniaturized versions such as the mini-DOAS or FlySPEC Masaya volcano, Nicaragua Measure absorption of UV light by SO2, corresponding to a SO2 concentration. Wind speed and plume geometry are considered to derive an SO2 flux. SO2 flux * xi/SO2 = flux of xi, the gas of interest Limitation: you need a fairly large flux of gas!

15. Satellite-based remote sensing: 10 000 tons SO2/day Ozone Mapping Instrument (OMI) on NASA’s Aura satellite is used to map and quantify sulfur dioxide gas (SO2) emitted by volcanoes

16. SO2 flux estimates for arc systems: Power Law Distribution of (SO2) fluxes N = af-c (N= #volcanoes with flux ³ f) F = f1 + f2 + f3 + …….+ fN{(c/(1-c))(N+1)(N/(N+1))1/c} F = 2.5 x 1010 mol SO2/yr Hilton et al., 2002 after Brantley & Koepenick (1995) Mather et al., 2006 Compiled new flux data from Nicaragua with published data since 1997 4360 Mg/day or 2.5 ± 0.8 × 1010 mol SO2/yr 12% of global volcanic SO2 flux

17. Melt inclusion studies: Estimate primary volatile contents of melts and combine with magma production rates to derive fluxes 100mm

18. Analytical methods: • Pre-eruptive H, C, S, Fl and Cl contents are measured by SIMS • Major elements by electron microprobe (Fe-Mg exchange) • SEM imaging of inclusions (crystallization and size) 10mm

20. Izu-Bonin: evidence for slab-derived fluids addition of fluids MORB (ppm)

21. Fractional crystallization: Volatile concentrations are thought to increase with fractional crystallization due to their incompatibility Fractional crystallization

22. Volatile loss through degassing degassing

23. Vapor saturation curves: pressure of entrapment Melt inclusions from Nijima volcano were trapped at the deepest depth (15km), based on solubility based vapor saturation curves

24. Degassing style: • Open style degassing: exsolved vapor is lost • Closed system: vapor re-equilibrates with melt

25. Source estimates: • Highest concentration sample: 1200 ppm CO2 • Extrapolating back for 5% vapor exsolution: 3831 ppm CO2 3831 ppm 1200 ppm after Newman and Lowerstern, 2002

26. Volatile budgets for CO2: Izu-Bonin output calculated assuming a magma production rate of 60 km3/Ma/km along with pre-eruptive CO2 contents and a trench length of 1050 km

27. P-T controls on the volcanic output Phase equilibria predicts little CO2 recycling at cold subduction zones low T high T H2O (wt %) low T high T CO2(wt %) Kerrick and Connolly, 2001

28. Thermodynamic modeling of decarbonation Subduction was modeled by stepwise variation of pressure and temperature along a path prescribed by a selected thermal model for a given arc Revised modeling considered the effect of fluid flow (pervasive vs. channelized) (Gorman et al., 2006)

29. Pervasive infiltration model (Gorman et al., 2006) Modeled output fluxes of CO2 match fluxes derived by direct gas measurements (Shaw et al, 2003) and remote sensing (Hilton et al, 2002) Output fluxes for the Izu-Bonin are also in very good agreement with melt inclusion derived estimates – less CO2 recycling A significant amount of CO2 is released in the fore-arc

30. CO2 recycling: • We find relatively low CO2 recycling efficiencies at the Izu-Bonin (5-15%) and Central America (23-28%) arc systems. • Implication is that either a significant fraction of C is being supplied to the deep mantle, or that CO2 loss in the fore-arc is substantial – as models suggest. • Decarbonation is indeed more limited in cooler regimes such as the Izu-Bonin arc as compared to Costa Rica.

31. Limitations and future directions for volatile fluxes: CO2: focus on the fore-arc (and back-arc) flux – melt inclusion from the volcanic arc for Central America SO2: measured using various techniques - outputs can be quantified, but inputs are poorly constrained H2O: melt inclusions are the only method for quantifying fluxes, due to additional water meteoric waters fluxed through the volcanic system – H isotopes can be used to identify source N2: direct gas measurements combined with isotopic analyses – ion probe techniques for N in glasses are very difficult Cl: both melt inclusions and direct gas measurements – what can the isotopes tell us? Source vs. degassing?

32. Acknowledgements: Margins-NSF, Guillermo Alvarado, Carlos Ramirez (ICE-UCR), Willi Strauch (INETER) Kohei Kazahaya, Masaaki Takahashi, Noritoshi Morikawa (GSJ), Aya Shimizu (University of Tokyo)