The Role of Magmatic Volatiles in Arc Magmas Paul Wallace University of Oregon

1. The Role of Magmatic Volatiles in Arc Magmas Paul Wallace University of Oregon

2. Complex reaction zone at slab-wedge interface Breeding et al. (2004) Volatile Recycling & Subduction Zone Magmatism Components in downgoing slab • Sediment • Altered oceanic crust • Serpentinized upper mantle (?)

3. Outline • How do we measure magmatic volatile concentrations? • Review of experimental studies of volatile solubility • Volatile contents of basaltic arc magmas based on melt inclusion data • A comparison of volatile inputs and outputs in subduction zones • Effect of H2O on melting of the mantle wedge, and a brief look at how fluids and melts move through the wedge.

4. Problem of Magma Degassing • Solubility of volatiles is pressure dependent • Volatiles are degassed both during eruption & at depth before eruption • Bulk analysis of rock & tephra are not very useful!

5. How do we measure volatile concentrations in magmas? • • Melt inclusions • • Submarine pillow glasses • • Experimental petrology 100 mm Moore & Carmichael (1998) Phase equilibria for basaltic andesite

6. How do we analyze glasses & melt inclusions for volatiles? • Secondary ion mass spectrometry (SIMS or ion microprobe) H2O, CO2, S, Cl, F • Fourier Transform Infrared (FTIR) spectroscopy H2O, CO2 • Electron microprobe Cl, S, F • Nuclear microprobe CO2 • Larger chips of glass from pillow rims or experimental charges can be analyzed for H2O and CO2 using bulk extraction techniques e.g., Karl-Fischer titration, manometry

7. What are melt inclusions & how do they form? • Primary melt inclusions form in crystals when some process interferes with the growth of a perfect crystal, causing a small volume of melt to become encased in the growing crystal. • This can occur from a variety of mechanisms, including: 1. skeletal or other irregular growth forms due to strong undercooling or non-uniform supply of nutrients 2. formation of reentrants by resorption followed by additional crystallization 3. wetting of the crystal by an immiscible phase (e.g. sulfide melt or vapor bubble) or attachment of another small crystal (e.g. spinel on olivine) resulting in irregular crystal growth and entrapment of that phase along with silicate melt • Melt inclusions can be affected by many post-entrapment processes: 1. Crystallization along the inclusion-host interface 2. Formation of a shrinkage bubble caused by cooling, which depletes the included melt in CO2.

8. Experimental and natural polyhedral olivine with melt inclusions (slow cooling) 100 mm Keanakakoi Ash, Kilauea, Hawaii Experimental & natural skeletal (hopper morphology) olivine with melt inclusions (faster cooling) Keanakakoi Ash 500 mm Paricutin, Mexico

9. Cooling Post-Entrapment Modification of Melt Inclusions Ascent & Eruption Slow Cooling Inclusion entrapment Vapor bubble Crystal Diffusive exchange Melt inclusion Crystallizaton along melt – crystal interface Volatile leakage if inclusion ruptures Crystallization & possible further leakage

10. Volcanic gases - another way to get information on volatiles • • Ground & airborne remote sensing • • Satellite-based remote sensing • • Direct sampling & analysis COSPEC at Masaya TOMS data for El Chichon & Pinatubo Sampling gases at Cerro Negro

11. Review of Experimentally Measured Solubilities for Volatiles Some key things to remember: • Volatile components occur as dissolved species in silicate melts, but they can also be present in an exsolved vapor phase if a melt is vapor saturated. • In laboratory experiments, it is possible to saturate melts with a nearly pure vapor phase (e.g., H2O saturated), though the vapor always contains at least a small amount of dissolved solute. • In natural systems, however, multiple volatile components are always present (H2O, CO2, S, Cl, F, plus less abundant volatiles like noble gases). • When the sum of the partial pressures of all dissolved volatiles in a silicate melt equals the confining pressure, the melt becomes saturated with a multicomponent (C-O-H-S-Cl-F-noble gases, etc.) vapor phase. • Referring to natural magmas as being H2O saturated or CO2 saturated is, strictly speaking, incorrect because the vapor phase is never pure and always contains more than one volatile component.

12. H2O and CO2 solubilities measured by experiment • Solubilities are strongly pressure dependent • Solubilities do not vary much with composition • CO2 has very low solubility compared to H2O (~30x lower)

13. Solubilities with more than 1 volatile component present Solid lines show solubility at different constant total pressures Dashed lines show the vapor composition in equilibrium with melts of different H2O & CO2 From Dixon & Stolper (1995) • In natural systems, melts are saturated with a multicomponent vapor phase • H2O and CO2 contribute the largest partial pressures, so people often focus on these when comparing pressure & volatile solubility

14. Chlorine Solubility Vapor saturated Continuous transition from vapor to hydrosaline melt as Cl concentration in vapor (% values) rapidly increases Hydrosaline melt (brine) saturated From Webster et al., (1995) • In this simplified experimental system, basaltic melts are either saturated with H2O-Cl vapor or molten NaCl with dissolved H2O (hydrosaline melt) • Real basaltic melts typically have <0.25 wt% Cl and thus are not saturated with hydrosaline melt

15. Chlorine in rhyolitic melts Note: x and y axes have been switched from previous figure • Cl solubility is much lower in rhyolitic melts compared to basaltic melts • Some rhyolitic melts (e.g., Augustine volcano) have high enough dissolved Cl for the melt to be saturated with hydrosaline melt before eruption

16. Sulfur Solubility • S solubility is more complicated because of multiple oxidation states • Dissolved S occurs as either S2- or S6+ • Solubility is limited by sat’n with pyrrhotite, Fe-S melt, anhydrite, or CaSO4 melt • S in vapor phase occurs primarily as H2S and SO2 Minerals Basaltic glasses From Jugo et al. (2005) • Fortunately we can measure the oxidation state of S in minerals & glasses by measuring the wavelength of S K radiation by electron microprobe

17. Effect of oxygen fugacity on S speciation in silicate melts From Jugo et al. (2005) • A rapid change from mostly S2- to mostly S6+ occurs over the oxygen fugacity range that is typical for arc magmas

18. Effect of oxygen fugacity on S solubility Jugo et al. (2005) • Changes in oxygen fugacity have a strong effect on solubility because S6+ is much more soluble than S2-.

19. Sulfur solubility – effects of temperature, pressure & composition S solubility at low oxygen fugacity S2- is the dominant species Solubility of both S2- and S6+ are temperature dependent

20. S solubility in intermediate to silicic melts ° • Because of strong temperature dependence of S solubility, low temperature magmas like dacite and rhyolite have very low dissolved S. • This led earlier workers to erroneously conclude that eruptions of such magma would release little SO2 to Earth’s atmosphere

21. Vapor–Melt Partitioning of Sulfur • Experiments show strong partitioning of S into vapor (Scaillet et al., 1998; Keppler, 1999) • Thermodynamic modeling allows calculation of vapor-melt partitioning at high fO2 SO2 (vapor) + O2– (melt) + 0.5 O2 (vapor) = SO42– (melt) Isopleths of Constant Svapor / Smelt Temperature (°C)

22. S Contents of Magmatic Vapor Phase for Intermediate to Silicic Magmas From Wallace (2003) STotal (mol%) in vapor • Because S strongly partitions into the vapor phase at lower temperatures, most of the SO2 released from eruptions of intermediate to silicic magma comes from a pre-eruptive vapor phase

23. What can melt inclusions tell us about volatiles if magmas are generally vapor saturated? • Only part of the story – melt inclusions tell us the concentrations of • dissolved volatiles • Information captured by melt inclusions depends on the vapor / melt • partition coefficient, and thus is different for each volatile component • Melt inclusions also provide information on magma storage depths • and vapor phase compositions (e.g., use of H2O vs. CO2 diagram) • • Diagrams in the next two figures show how much of the initial • amount of each volatile is still dissolved at the time inclusions are • trapped

24. Degassing of low-H2O basaltic magma (Kilauea) Fraction remaining (C / Cinitial) • When olivine crystallizes in the magma chamber beneath the summit of of Kilauea, most of the original dissolved CO2 has already been degassed from the melt.

25. Degassing of H2O-rich rhyolitic magma Fraction remaining (C / Cinitial) • When rhyolitic melt inclusions are trapped in quartz or feldspar at typical magma chamber depths, most of the original CO2 and S has been degassed

26. Volatile contents of mafic arc magmas based on melt inclusions 100 mm 100 mm Blue Lake Maar, Oregon Cascades Jorullo volcano, Mexico Photos by Emily Johnson, Univ. of Oregon

27. H2O & CO2 in Melt Inclusions from Jorullo Volcano, Mexico Vapor saturation isobars from Newman & Lowenstern (2002) All data by FTIR CO2 (ppm) Avg. error H2O (wt.%) Johnson et al. (in press) • Early – wide range of olivine crystallization pressures (mid-crust to surface) • Middle & Late – all olivine crystallized at very shallow depths • Degassing and crystallization occurred simultaneously during ascent

28. Degassing Paths During Magma Ascent & Crystallization Degassing paths calculated using Newman & Lowenstern (2002) Initial melt CO2 (ppm) H2O (wt.%) Johnson et al. (in press) • Some data cannot be explained by simple degassing models

29. Effects of degassing • Melt inclusion data from a single volcano or even a single eruptive unit often show a range of H2O and CO2 values. • In most cases, this range reflects variable degassing during ascent before the melts were trapped in growing olivine crystals. • S can also be affected by this variable degassing, but Cl and F solubilities are so high that they tend to stay dissolved in the melt. • From a large number of analyzed melt inclusions (preferably 15-25), the highest analyzed volatile values provide a minimum estimate of the primary volatile content of the melt before any degassing. • The data shown on the following slides are for the least degassed melt inclusions from a number of different volcanoes.

30. Estimate based on magma flux & CO2 flux Arc basaltic magmas CO2 = 0.6–1.3 wt.% Minimum for arc magmas based on global CO2 flux Mariana arc • H2O contents of arc basaltic magmas are quite variable • CO2 contents are lower than estimates based on global arc CO2 flux

31. Melts from mantle wedge + subducted sediment Melts from mantle wedge + subducted oceanic crust Minimum for arc magmas based on global CO2 flux Arc basaltic magmas CO2 = 0.6–1.3 wt.% Mariana arc • Subducted oceanic crust and sediments contain abundant C in the form of carbonate sediment/limestone and buried organic C • This figure shows simple mass balance for bulk addition of H2O & CO2 from slab to wedge, and for addition of H2O-rich, CO2-poor fluid to the wedge from the slab

32. Minimum for arc magmas based on global CO2 flux Arc basaltic magmas CO2 = 0.6–1.3 wt.% Melts from mantle wedge + low-CO2 fluid from slab Mariana arc • Subducted oceanic crust and sediments contain abundant C in the form of carbonate sediment/limestone and buried organic C • This figure shows simple mass balance for bulk addition of H2O & CO2 from slab to wedge, and for addition of H2O-rich, CO2-poor fluid to the wedge from the slab

33. Melts from mantle wedge + subducted sediment Melts from mantle wedge + subducted oceanic crust Chlorine in Arc and Back-arc Basaltic Magmas • Cl contents in arc and back-arc magmas (Lau Basin, Marianas) are much higher than in MORB • This indicates substantial recycling of seawater-derived Cl into the mantle wedge

34. High Salinity Fluids 17–45 % NaCl Fluid Inclusions in Eclogites as Analogues for Subduction Zone Fluids Data from Philippot et al. (1998) Low Salinity Fluids 3.1–4.0 % NaCl • Eclogites from exhumed subduction complexes contain fluid inclusions that represent samples of fluids released during dehydration of metabasalt

35. Melts from mantle wedge + subducted oceanic crust + sediment • S contents of arc magmas are typically higher than for MORB, but in most cases not nearly as enriched as is observed for Cl

36. Sulfur concentrations in melt inclusions & submarine basaltic glasses 5970 S (ppm) • The higher S contents of arc magmas relative to MORB are even more clear on this plot Data sources: Anderson (1974); Wallace & Carmichael (1992); Métrich et al. (1996; 1999); Cervantes & Wallace (2002)

37. Comparing inputs and outputs of volatiles in subduction zones Measuring volatile fluxes from arc volcanism - one method Modified from Fischer et al. (2002) Volcanic Gases • Measure SO2 flux by remote sensing • Collect & analyze fumarole gases • Use fumarole gas ratios (e.g., CO2/SO2) to calculate fluxes of other components

38. Measuring volatile fluxes - another method Melt Inclusions • Use magmatic volatile concentrations in melt inclusions • Combine with magma flux (mantle to crust) estimates from: – seismic studies of intraoceanic arcs – isotope systematics for crustal growth – geochronology & field mapping

39. Fluxes of Major Volatiles from Subduction-related Magmatism Gas Flux & Composition W Assuming 2–4 km3/yr magma flux

40. Input vs. Output for Major Volatiles in Subduction Zones Amount recycled to surface reservoir by magmatism H2O 40–120% of dike/gabbro H2O 20–80% of total CO2 ~ 50% S ~ 20% Cl ~ 100% • Inputs include structurally bound volatiles in subducted sediment & altered oceanic crust (Hilton et al., 2002; Jarrard, 2003)

41. CO2 Input vs. Output for Individual Arcs Data from Hilton et al. (2002)

42. How does addition of H2O to the mantle wedge cause melting? Experimental determinations of the effect of H2O on the peridotite solidus From Grove et al. (2006) Wet solidus Dry solidus

43. Effect of H2O on Isobaric Partial Melting of Peridotite Hirschmann et al. (1999) 1 GPa Xitle • Increasing H2O has a linear effect on degree of melting (Hirose & Kawamoto, 1995; Hirschmann et al., 1999)

44. Effect of H2O on Isobaric Partial Melting of Peridotite Hirschmann et al. (1999) 1 GPa Mariana Trough data from Stolper & Newman (1994)

45. Effect of H2O on Isobaric Partial Melting of Peridotite Max. H2O for amphibole-bearing peridotite • To get the high H2O contents of arc magmas, H2O must be added to the mantle either by aqueous fluid or hydrous melt

46. A model for hydrous flux melting of the mantle wedge • Fluids and/or hydrous melts percolate upward through the inverted thermal gradient in the mantle wedge • A small amount of very H2O-rich melt forms when temperatures reach the wet peridotite solidus • This wet melt continues to rise into hotter parts of the wedge, and becomes diluted with basaltic components melted from the peridotite • H2O-poor magmas form by upwelling induced decompression melting driven by corner flow From Grove et al. (2006)

47. From slab to surface – some complications • Hydrous minerals are also stable in the mantle wedge just above the slab & act like a ‘sponge’ • H2O released from the slab migrates into the wedge, reacts, & gets locked up in these phases • Chlorite is stable to ~135 km depth, then breaks down & again releases H2O upwards

48. Do fluids and melts move vertically upward through the mantle wedge? No, solid mantle flow deflects hydrous fluids from buoyant vertical migration through the wedge Solid mantle flow also deflects partial melts formed in the hottest part of the wedge back towards the trench From Cagnioncle et al. (2006)

49. And finally, mafic arc magmas have enough H2O to cause explosive eruptions (violent strombolian, sub-plinian, and occasionally plinian) that produce large amounts of ash and lapilli