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Volatile Abundances in Basaltic Magmas & Their Degassing Paths Tracked by Melt Inclusions Nicole M é trich Laboratoire Pierre Sue CNRS-CEA, France Paul.

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Presentation on theme: "Volatile Abundances in Basaltic Magmas & Their Degassing Paths Tracked by Melt Inclusions Nicole M é trich Laboratoire Pierre Sue CNRS-CEA, France Paul."— Presentation transcript:

1 Volatile Abundances in Basaltic Magmas & Their Degassing Paths Tracked by Melt Inclusions Nicole M é trich Laboratoire Pierre Sue CNRS-CEA, France Paul Wallace Dept. of Geological Sciences University of Oregon, USA Volcan Colima, Mexico Photo by Emily Johnson

2 Outline Formation of melt inclusions & post-entrapment modification Application of experimental volatile solubility studies to natural systems The record of magma degassing preserved in melt inclusions & the effect of H 2 O loss on magma crystallization Eruption styles and volatile budgets: information from melt inclusions Unresolved questions & directions for future studies

3 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 enclosed. Formation mechanisms: 1. Skeletal or other irregular growth forms due to strong undercooling 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 & inclusion entrapment Melt inclusions can be affected by post-entrapment processes Roedder (1984); Lowenstern (1995)

4 100  m Experimental and natural polyhedral olivine with melt inclusions (slow cooling) Keanakakoi Ash, Kilauea, Hawaii Faure & Schiano (2005) Experimental & natural skeletal (hopper morphology) olivine with melt inclusions (faster cooling) Paricutin, Mexico 500  m Keanakakoi Ash Faure & Schiano (2005) 100  m Jorullo volcano, Mexico

5 100  m Blue Lake Maar, Oregon Cascades Experimental and natural closed dendritic olivine with melt inclusions (very fast cooling) Stromboli Volcano Faure & Schiano (2005)

6 Experiments in CMAS system. Effect of Growth Rate on Trapped Melt Compositions Rapid growth morphologies have inclusions that are moderately to strongly enriched in Al 2 O 3. This is caused by boundary layer enrichment due to slow diffusion of Al 2 O 3 relative to CaO.

7 Differences between Experimental & Natural Melt Inclusions Most natural melt inclusions show no evidence of anomalous enrichment in slowly diffusing elements, even in small inclusions and rapid growth forms like skeletal or hopper crystals. Volatile components have faster diffusivities than Al 2 O 3 and thus should not generally be affected by boundary layer enrichment effects. Data from Johnson et al. (2008)

8 Post-Entrapment Modification of Melt Inclusions Diffusive loss of H 2 or molecular H 2 O Crystal Melt inclusion Inclusion entrapment Cooling Shrinkage vapor bubble Crystallization along melt – crystal interface Fe Diffusive loss of H-species – Should be limited to <1 wt% H 2 O by redox equilibria & melt FeO if loss occurs by H 2 diffusion (Danyushevsky, 2001). – Leaves distinct textural features – magnetite dust – from oxidation. – Possible rapid diffusion of molecular H 2 O (Almeev et al., 2008).

9 Review of Experimentally Measured Solubilities for Volatiles Volatiles occur as dissolved species in silicate melts & also in a separate vapor phase if a melt is vapor saturated. In laboratory experiments, melts can be saturated with a nearly pure vapor phase (e.g., H 2 O saturated). In natural systems, however, multiple volatile components are always present (H 2 O, CO 2, S, Cl, F, plus noble gases, volatile metals, alkalies, etc.). 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 H 2 O saturated or CO 2 saturated is, strictly speaking, incorrect because the vapor phase always contains other volatiles. Some key things to remember:

10 Solubilities with 2 Volatile Components Present H 2 O and CO 2 contribute the largest partial pressures, so people often focus on these when comparing pressure & volatile solubility Solid lines show solubility at different constant total pressures Dashed lines show the vapor composition in equilibrium with melts of different H 2 O & CO 2 From Dixon & Stolper (1995)

11 Estimating Vapor-Saturation Pressures for Melt Inclusions Etna 3900 BP eruption Melt inclusions (12-14wt% MgO) in olivine Fo91 (Kamenetsky et al., Geology 2007) Etna 2001,2002 Ca,Mg-bearing carbonates Arc basalts (Wallace 2005) CO 2 diffuses into a shrinkage bubble during cooling CO 2 loss demonstrated in heating experiments on olivine (Fo88) from a Mauna Loa picrite. Melt inclusions re-homogenized at 1400°C for <10 min. As much as 80% of the initial CO 2 can be transferred to a shrinkage bubble over a cooling interval of ~ 100°C. Carbonate crystals lining bubble walls Total vapor pressure (PH 2 O+PCO 2 ) for an inclusion can be calculated assuming: Vapor saturation – how do we know melts were vapor saturated? – Large variations in ratios of bubble volume to inclusion volume – Presence of dense CO 2 liquid in bubbles – Homogenization not possible in heating experiments No post-entrapment loss of CO 2 or H 2 O to bubbles, no leakage, no H 2 O diffusive loss. CO 2 lost to bubbles lowers vapor saturation pressure. Cervantes et al., (2002)

12 Chlorine Solubility in Basaltic Melts In this simplified experimental system, basaltic melts are either saturated with H 2 O-Cl vapor or molten NaCl with dissolved H 2 O (hydrosaline melt) Natural basaltic melts typically have <0.25 wt% Cl. From Webster et al., (1995) Vapor saturated Hydrosaline melt (brine) saturated Continuous transition from vapor to hydrosaline melt as Cl concentration in vapor (% values) rapidly increases 2 kbar Cl (wt%) H 2 O (wt%)

13 Jugo et al. (2005) Sulfur Solubility Changes in f O 2 have a strong effect on solubility because S 6+ is much more soluble than S 2-. Basalt Trachyandesite Sulfide saturatedSulfate saturated Sulfur solubility depends on temperature, pressure, melt composition & oxygen fugacity. Thermodynamic model of Scaillet & Pichavant (2004) relates these variables to f S 2.

14 The record of magma degassing preserved in melt inclusions & the effect of H 2 O loss on magma crystallization Popocatépetl, Mexico

15 Melt inclusion data from a single volcano or even a single eruptive unit often show a range of H 2 O and CO 2 values. In most cases, this range reflects variable degassing during ascent before the melts were trapped in growing olivine crystals (i.e., polybaric crystallization) Melt inclusions from Keanakakoi Ash, Kilauea, Hawaii (Hart & Wallace, unpublished) H 2 O and CO 2 Variations in Basaltic Melt Inclusions ClosedOpen Open-system degassing Exsolved gas is continuously separated from melt Closed-system degassing Exsolved gas remains entrained in melt & maintains equilibrium.

16 H 2 O and CO 2 Contents of Basaltic Magmas Olivine-hosted melt inclusion pressure estimates rarely exceed ~400 MPa. In contrast, CO 2 -rich fluid inclusions commonly indicate higher pressures (Hansteen & Klügel). As much as 90% of the initial dissolved CO 2 in melts is lost when they reach crustal depths. Melt inclusion CO 2 provides information on degassing & crystallization processes. H 2 O, S and Cl are much more soluble than CO 2, and give information on degassing paths and the primary volatile contents of basaltic magmas & their mantle sources. Wallace (2005)

17 Open-system degassing Open-system degassing [ Exsolved gas is continuously separated from melt ]  Strong decrease of CO 2 and negligible H 2 O loss until the melt reaches vapor saturation pressure for pure H 2 O Mariana Trough samples Melt inclusions: CO 2 = 875  141 ppm Host glasses: CO 2 = 18  5 ppm Comparable H 2 O concentrations 2.23  0.07 vs 2.12  0.10 wt% Newman et al., 2000 G-cubed1 ! Studies of melt inclusions from basaltic tephra from explosive volcanic activity (e.g., lava fountains, strombolian activity) often show significant H 2 O loss that cannot be strictly explained by pure open- or closed-system degassing of magmas Closed-system degassing: Exsolved gas remains entrained in melt & maintains equilibrium.

18  Volatilecalc (Newman & Lowenstern 2002) computations assuming equilibrium conditions Spilliaert et al. 2006 JGR Etna : 2002 Lava fountain activity Photos: P. Allard Closed system ascent of magma coexisting with a CO 2 -rich gas phase at 400 MPa Closed-system degassing and gas fluxing  Etna (Sicily) 2002 flank eruption CO 2 diffusion in bubble Bulk rocks Melt inclusions Etna 2002 Métrich et al., 2007 Both major and trace elements of natural inclusions in Fo~82, match those of the basalt-trachybasalt bulk-rocks Not a pure closed-system degassing  a two-stage (multi-stage) process?

19 Closed-system degassing and gas fluxing Combined effect of open-system addition of CO 2 -rich gas to ascending/ponding magma Consistency with  high CO 2 in primary magmas (e.g. Kilauea, Gerlach et al., 2002; Etna, Allard et al., 1999), high CO 2 flux at basaltic volcanoes (Fisher & Marty 2005; Wallace 2005 for reviews), high CO 2 /SO 2 ratio in gas emissions with increasing explosivity of eruption (e.g. Burton et al., 2007; Aiuppa et al. 2007)  if true such a process should be the common case at open-conduit basaltic volcanoes Effect of disequilibrium degassing (Gonnermann & Manga 2005) - Need more data on diffusion of CO 2 relative to H 2 O ( see Baker et al., 2005) Need more data on natural samples combined with experiments on disequilibrium degassing Spilliaert et al., 2006, JGR CO 2 -flushed magma ponding zone Enhanced magma dehydration 2.6 wt% H 2 O 1175 ppm CO 2 2.7 wt% H 2 O 1140 ppm CO 2

20 20 50 100 150 200 300 CSD 2% CSD 1% Closed-system degassing and gas fluxing  Volatilecalc computations assuming equilibrium conditions  Irazù volcano (Costa Rica) - 1763 & 1963-65 eruptions: Closed-system degassing (CSD 2%), coupled with ascent, crystallization and cooling (1075-1045°C) (Benjamin et al. 2007, JVGR,168, 68-92) - Natural M.I. in (  1 mm) olivine Fo 87-79; with, on average, cp  host scoria (54wt% SiO 2 ; Ba/La = 17-20)  Arenal volcano (Costa Rica) - pre-historic eruptions: Closed-system degassing (CSD 1%) coupled with fractionation and ascent from 2 to 0.2 kbars (Wade et al. 2006, JVGR,157, 94-120) - Natural M.I. in 0.25-1 mm size olivine with Fo 79  ol-wr bulk equilibrium Wade et al. 2006  In both cases, the highest CO 2 and H 2 O contents are preserved in M.I.hosted in Mg- rich olivines Not a pure closed-system degassing  CO 2 -rich gas fluxing

21 Gas fluxing, H 2 O loss and crystallization Johnson et al., 2008, EPSL 269 High MgO, high H 2 O M.I. in Fo 88-91  Minimum pressure of olivine formation 400 MPa At 400MPa - H 2 O-undersaturated melt Total pressure > 200MPa - Melt interaction with CO 2 -rich gas  CO 2 -rich gas fluxing depletes melt in H 2 O and thereby causes olivine crystallization Jorullo (Mexico) monogenic basaltic cinder cone Central part of the subduction-related Trans-Mexican Volcanic Belt Phase diagram for early Jorullo melt composition (10.5 wt.% MgO) constructed using MELTS ( Ghiorso & Sack,1995; Asimow & Ghiorso,1998 ) and pMELTS ( Ghiorso et al., 2002).

22  Crystallization recorded by melt inclusions mainly driven by H 2 O loss during magma ascent - At 400-200 MPa: Water loss likely due to gas fluxing – olivine crystallization - At low pressure: CO 2 -depleted melts lose H 2 O by its direct exsolution in the vapor phase Jorullo (Mexico) monogenic basaltic cinder cone H 2 O loss and crystallization Johnson et al., 2008, EPSL 269

23 Melt inclusion studies provide evidence for crystallization driven by H 2 O loss (+ cooling) at many volcanoes. Message can be difficult to decipher because of additional processes such as: - Mixing involving degassed and undegassed magmas (Popocatépetl & Colima; Atlas et al., 2006) - Mingling (e.g. Fuego, Roggensack 2001) - Assimilation (Paricutin, Lurh 2001; Jorullo, Mexico, Johnson et al., 2008) A case of efficient control of H 2 O degassing on magma crystallization is Stromboli - an open conduit volcanoe with low magma production rate and high degassing excess - where magmas share same chemical composition but have contrasting textures, crystal abundances (<10-50%) and viscosities (Métrich et al., 2001, Landi et al., 2004; Bertagnini et al., 2003, 2008) H 2 O loss and crystallization

24 Sulfur and halogen degassing 80% S is lost between 140 and 10 MPa, whereas Cl starts degassing at low pressure (Ptot<20-10MPa) and F at Ptot<10MPa ? Sulfur starts degassing at pressure (~150 MPa) in oxidized magmas in which sulfur is dissolved as sulfate > submarine sulfide-saturated basalts ( Dixon et al., 1991 ) Irazù: Benjamin et al. 2007, JVGR,168, 68-92 Arenal: Wade et al. 2006, JVGR,157, 94-120 Etna: Spilliaert et al., 2006, EPSL, 248, 772-786 Sulfide saturation

25 Eruption styles and degassing budget Information from melt inclusions What are the recent improvements? Stromboli - 2006

26 Basalt: LK: Laki 1783-84 eruption; K: Kilauea, annual average; ML Mauna Loa; PC Pacaya 1972 eruption; St: Stromboli annual average Volatile budget for basaltic fissure eruptions  Predicted relationship between SO 2 emissions and eruptive magma volume assuming that SO 2 released during eruption is provided by the sulfur dissolved in silicate melt  Compared to sulfur emissions measured by independent methods as ulraviolet correlation spectrometer (COSPEC), atmospheric turbidity and Total Ozone Mapping Spectrometer (TOMS) Uncertainties in SO 2 emission data are generally considered to be about  30% for the TOMS data and  20–50% for COSPEC. Pre-requisite: no differential transfer of gas  S = C S(M.I.) – C S(res) Wallace 2005, JVGR C S(M.I.) : S content in primitive melt (melt inclusion) C S(res.) : Residual S content in bulk lava or in matrix glass corrected for crystallization Petrologic estimates of the sulfur output

27 [1,3] Thordarson &Self: (1993) Bull Vocanol 93 and (1996) JVGR 74; [2] Thordarson et al., (2001), JVGR, 108 Eldgjà [2] Laki [1] Melt inclusions p-tephra* s-tephra lava M.I. and W.R. have comparable composition >95% of initial sulfur released Sulfur partly exsolved in gas phase during magma ascent at shallow depth prior to eruption  75% escaped at vents, lofted by the eruptive column (strong fire fountaining) to 5-15 km altitudes at the beginning of each eruptive phase and 25% during the lava flowing * p-tephra : quenched melts indicative of magma degassing during during ascent  Approach used for assessing the impact of large flood basalts on the atmosphere (Self et al; 2008 Science) Volatile budget for basaltic fissure eruptions

28  Petrologic estimates commonly used for assessing the degassing budget of other volatiles in particular Cl and F  The 94 days long flank eruption that occurred in 2002 at Mt Etna:  Modelling of the pressure related behavior of sulfur at Etna (2002 eruption)  ~80% sulfur released in the gas phase during magma ascent (between 140 and 10 MPa) in agreement with conclusions drawn by Self, Thordarson and co-authors  SO 2 flux: 6.9  10 8 kg (Petrologic estimates, Spilliaert et al. 2006) / 8.6  10 8 kg (COSPEC, Caltabiano et al. 2006)  Comparable S/Cl molar ratio (~5) in vapor phase derived from melt inclusion data and measured in gas emissions  no differential degassing of S (or Cl)  Arenal (COSPEC  0.41 Mt of SO 2 released since 1968 ) Better agreement with COSPEC when considering the S content (>2000 ppm) of olivine-hosted melt inclusions representative of the undegassed basaltic andesitic magma rather than partly degassed melt trapped in Plag & Cpx Petrologic estimates even > COSPEC a part of sulfur could be lost? Sulfur partly exsolved in gas phase during magma ascent at shallow depth without differential transfer of sulfur  Consistency between petrologic estimates of SO 2 budget and independent estimates (COSPEC or others) (Wade et al., 2007) Volatile budget for basaltic fissure eruptions

29 Differential transfer of gas bubbles – Excessive degassing - Izu-Oshima in Japan (Kazahaya et al 1994) - Villarica in Chile (Witter et al., 2004), - Popocatepetl in Mexico (Delgado-Granados et al., 2001; Witter et al., 2005) - Etna & Stromboli in Italy (Allard., 1997; Burton et al., 2007) - Masaya in Nicaragua (Delmelle et al., 1999, Stix, 2007)…. Stromboli Magma supply rate is assessed to be 0.001 km 3 y -1, 15  4 higher than the magma extrusion rate Assuming 0.22 wt% S dissolved in magma as derived from M.I.  <10% of magma is extruded given that quiescent degassing contributes to 95% total SO 2 degassing (Allard et al., 2008) Excessive degassing at persistently active basaltic volcanoes such as: e.g. Jaupart et Vergniolle, 1988, Vergniolle, 1996; Philips and Wood 1998  Differential transfer of gas bubbles MI data used for assessing the mass (volume) of unerupted magma when combined with gas flux measurements Qm =  SO 2 /2  S Qm : Mass flux of magma 2  S = SO 2 degassed from the magma  SO 2 = SO 2 flux measured by COSPEC or other techniques

30 Unresolved questions and directions for future studies Benbow (Ambrym, Vanuatu)

31  M ost suitable melt inclusions for volatile studies  quenched pyroclastites  Efforts dedicated in the last 15 years  basic data for assessing: - the SO 2 output from syn-eruptive degassing of basaltic magmas ascending in closed system conditions, with no differential gas transfer (gas loss) prior to eruption - the volume of non-erupted magma that has degassed in volcanic systems undergoing quiescent degassing - the degasssing paths of magmas - volatiles in arc magma mantle sources  A new idea  magma fluxed by CO 2 -rich gas causing magma dehydration Question: Effect of disequilibrium degassing ?  More data on basaltic melt inclusions in pyroxenes and comparison with data of olivine-hosted melt inclusions  Critical view of natural and experimental data on melt inclusions  Studies that include both melt inclusion & fluid inclusion analysis from the same samples

32 VGP special session: Model solubility, diffusive bubble growth, disequilibrium degassing, conduit processes Monday 15 December, 16h00, Oral session V14a, MC 3003 Tuesday 16 December, Poster session V21B, MC Hall D Efforts to improve the modeling of:  CO 2 -H 2 O evolution during decompression: - experimental and thermodynamic data on the solubility of CO 2 in H 2 O-bearing basaltic melts -more data on natural systems during well monitored eruptions allowing the combination of MI data with gas emission chemistry & seismic records  Magma ascent in the conduits by combining M.I. data with matrix textures & bubble distribution Integrating melt inclusion data with Accurate studies of their host olivines and the mineralogy of the host magmas Experimental data on volatile solubility Degassing models that include both thermodynamic and physical aspects Field work (gas measurements, acoustic and seismic) is a necessity and represents a main challenge for the next few years.

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