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

A. Shaw, D. Hilton, T. Fischer, E. Hauri

Arenal Volcano

The MARGINS Subduction Factory:

1) How do forcing functions regulate production of magma and fluid from the Subduction Factory?

2) How does the volatile cycle (H2O and CO2) impact chemical, physical and biological processes from trench to deep mantle?

3) What is the mass balance of chemical species and material across the Subduction Factory?

Outline

1) Different methods for measuring gas fluxes and evaluating mass balance at arcs

2) Comparing fluxes from IBM and Central America

3) How well do PT models predict fluid behavior?

4) Summarize the strengths of different methods, their limitations and future directions

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, …

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

Volcanic output flux estimates:

1) Assumed 3He flux combined with direct measurements of a volcanic gas (x) relative to 3He

2) Remote sensing techniques

3) Melt inclusion studies combined with magma production rates

Volcanic Sampling

1) Fumaroles

2) geothermal wells

3) water springs

4) bubbling hot springs and mudpots

Momotombo volcano

Volcanic Sampling

1) Fumaroles

2) geothermal wells

3) water springs

4) bubbling hot springs and mudpots

100

0

400

200

300

500

Site 1039Costa Rica

Sano and Marty (1995); Sano and Williams (1996)Sano and Marty (1995); Sano and Williams (1996)

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

3) M+L+S = 1

crustaladditions

CO

2/3 H

e109

1010

1011

1012

1013

1014

13 C

-14

-12

-10

-8

-6

-4

-2

0

latitude (ºN)

10.0 10.5 11.0 12.0 12.5 13.0

3 He/

4 He

(Rc/

RA)

0

2

4

6

8

10

fumarolegeothermal wellwater springbubbling hot spring

COSTA RICA NICARAGUA

MORB

arcs worldwide

MORB

3) A higher slab component was observed in Nicaragua

Source of carbon:

1) Dominant source of CO2 is from a limestone/marine carbonate source (83-86%)

2) L/S of input = 10.5 was essentially indistinguishable from the output (9-6-11.1) – see Hilton’s poster for revised models

CO2 flux estimates:

1) 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)

2) 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?

3) Sediment-derived N flux : 28 × 108 mol/yr (Elkins et al., 2006) –

completely recycled through the arc

Remote sensing:

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!

1) Satellite-based measurements

2) COSPEC: correlation spectrometer or miniaturized versions such as the mini-DOAS or FlySPEC

Masaya volcano, Nicaragua

Ozone Mapping Instrument (OMI) on NASA’s Aura satellite is used to map and quantify sulfur dioxide gas (SO2) emitted by volcanoes

Satellite-based remote sensing:

10 000 tons SO2/day

Power Law Distribution of (SOPower Law Distribution of (SO22) fluxes) fluxes

N = afN = af-c -c (N= #volcanoes with flux (N= #volcanoes with flux f) f)

F = fF = f11 + f + f2 2 + f+ f33 + …….+ + …….+ ffNN{(c/(1-c))(N+1)(N/(N+1)){(c/(1-c))(N+1)(N/(N+1))1/c1/c}}

SOSO22 flux estimates for arc systems: flux estimates for arc systems:

Hilton et al., 2002 after Brantley & Hilton et al., 2002 after Brantley & Koepenick (1995)Koepenick (1995)

F = 2.5 x 1010 mol SO2/yr

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

Estimate primary volatile contents of melts and combine with magma production rates to derive fluxes

Melt inclusion studies:

100m

Analytical methods:

1) Pre-eruptive H, C, S, Fl and Cl contents are measured by SIMS

2) Major elements by electron microprobe (Fe-Mg exchange)

3) SEM imaging of inclusions (crystallization and size)

10m

Izu-Bonin: evidence for slab-derived fluids

0

20

40

60

80

100

120

140

0 10 20 30 40

B

Ba

/La

Oshima

Nijima

Hachijojima

Aogashima

addition of fluids

MORB

(ppm)

Fractional crystallization:

Volatile concentrations are thought to increase with fractional crystallization due to their incompatibility

1

10

100

1000

10000

0.65 0.7 0.75 0.8 0.85

Mg#

CO

2

Oshima

Nijima

Hachijojima

Aogashima

Fractionalcrystallization

Volatile loss through degassing

1

10

100

1000

10000

0 1000 2000 3000 4000

SO2 (ppm)

CO

2 (

pp

m)

Oshima

Nijima

Hachijojima

Aogashimadegassing

Vapor saturation curves: pressure of entrapment

1

10

100

1000

10000

0 2 4 6 8

H2O (wt %)

CO

2 (p

pm

) Oshima

Nijima

Hachijojima

Aogashima

P (kbar)

Depth (km)

50003000

1000500

1.5 3 9 15

Melt inclusions from Nijima volcano were trapped at the deepest depth (15km), based on solubility based vapor saturation curves

Degassing style:1) Open style degassing: exsolved vapor is lost

2) Closed system: vapor re-equilibrates with melt

1

10

100

1000

10000

0 2 4 6 8

H2O (wt %)

CO

2 (p

pm

) Oshima

Nijima

Hachijojima

Aogashima

P (kbar)

Depth (km)

50003000

1000500

1.5 3 9 15

closed

open

Source estimates:

1

10

100

1000

10000

0 2 4 6 8

H2O (wt %)

CO

2 (p

pm

) Oshima

Nijima

Hachijojima

Aogashima

P (kbar)

Depth (km)

50003000

1000500

1.5 3 9 15

closed

closed w / 5% exsolved

open

1200 ppm

3831 ppm

1) Highest concentration sample: 1200 ppm CO2

2) Extrapolating back for 5% vapor exsolution: 3831 ppm CO2

after Newman and Lowerstern, 2002

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

Method output

(109mol/yr)

% of sed

input

% of total input

Izu-Bonin highest content of MI: 1200 ppm

4.8 12.5 5

Modeled pre-degassing value: 3831 ppm

15.4 39.9 15

Central America Remote sensing 58 41 23

He-C relationships

71 50 28

P-T controls on the volcanic output

Phase equilibria predicts little CO2 recycling at cold subduction zones

high T

high T

low T

low T

H2O (wt %)

CO2 (wt %)

Kerrick and Connolly, 2001

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)

Thermodynamic modeling of decarbonation

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

(Gorman et al., 2006)

Pervasive infiltration model

CO2 recycling:

1) We find relatively low CO2 recycling efficiencies at the Izu-Bonin (5-15%) and Central America (23-28%) arc systems.

2) 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.

3) Decarbonation is indeed more limited in cooler regimes such as the Izu-Bonin arc as compared to Costa Rica.

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?

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