Degassing history of water, sulfur, and carbon in submarine lavas from Kilauea volcano, Hawaii

Major, minor, and dissolved volatile element concentrations were measured in tholeiitic glasses from the submarine portion (Puna Ridge) of the east rift zone of Kilauea Volcano, Hawaii. Dissolved H_(2)O and S concentrations display a wide range relative to nonvolatile incompatible elements at all depths. This range cannot be readily explained by fractional crystallization, degassing of H20 and S during eruption on the seafloor, or source region heterogeneities. Dissolved C0_2 concentrations, in contrast, show a positive correlation with eruption depth and typically agree within error with the solubility at that depth. We propose that most magmas along the Puna Ridge result from (I) mixing of a relatively volatile-rich, undegassed component with magmas that experienced low pressure (perhaps subaerial) degassing during which substantial H_(2)O, S, and C0_2 were lost, followed by (2) fractional crystallization of olivine, clinopyroxene, and plagioclase from this mixture to generate a residual liquid; and (3) further degassing, principally of C0_2 for samples erupted deeper than 1000 m, during eruption on the seafloor. The degassed end member may form at upper levels of the summit magma chamber (assuming less than lithostatic pressure gradients), during residence at shallow levels in the crust, or during sustained summit eruptions. The final phase of degassing during eruption on the seafloor occurs slowly enough to achieve melt/vapor equilibrium during exsolution of the typically CO_(2)-rich vapor phase. We predict that average Kilauean primary magmas with 16% MgO contain ~0.47 wt% H_(2)O, ~900 ppm S, and have δD values of ~-30 to -40‰. Our model predicts that submarine lavas from wholly submarine volcanoes (i.e., Loihi), for which there is no opportunity to generate the degassed end member by low pressure degassing, will be enriched in volatiles relative to those from volcanoes whose summits have breached the sea surface (i.e., Kilauea and Mauna Loa)

An Experimental Study of Water and Carbon Dioxide Solubilities in Mid-Ocean Ridge Basaltic Liquids. Part II: Applications to Degassing

Degassing processes in basaltic magmas rich in both water and carbon dioxide can be modeled using the solubilities of the endmember systems and the assumption of Henry's law. Suites of vapor-saturated basaltic melts having a range of initial CO_2/H_2O ratios and erupted over a narrow depth interval will define negatively sloped arrays on an H_2O vs CO_2 plot. It is important that all of the major volatile species be considered simultaneously when interpreting trends in dissolved volatile species concentrations in magmas. Based on measured concentrations of water and carbon dioxide in basaltic glasses, the composition of the vapor phase at 1200°C that could coexist with a basaltic melt and the pressure at which it would be vapor saturated can be calculated. The range in vapor compositions in equilibrium with submarine basalts reflects the range in water contents in the melts characteristic of each environment. The ranges in the molar proportion of CO_2 in vapor phases (X^ν_(CO2)) calculated to be in equilibrium with submarine tholeiitic glasses are 0⋅93–1⋅00 for mid-ocean ridge basalts (MORB), 0⋅60–0⋅99 for glasses from Kilauea [representative of ocean island basalts (OIB)] and 0–0⋅94 for glasses from back-arc basins (BABB). MORB glasses from spreading centers ranging from slow (e.g. the Mid-Atlantic Ridge) to fast (e.g. East Pacific Rise, 9–13°N) are commonly supersaturated with respect to CO_2-rich vapor, resulting from magma ascent rates so rapid that magmas erupt on the sea-floor without having been fully degassed by bubble nucleation and growth during ascent. In contrast to the MORB glasses, volatile contents in submarine glasses from Kilauea are consistent with having been in equilibrium with a vapor phase containing 60–100 mol% CO_2 at the pressure of eruption, reflecting differences in average magma transport rates during eruptions at mid-ocean ridges and hotspot volcanoes. Degassing during decompression of tholeiitic basaltic magma is characterized by strong partitioning of CO_2 into the vapor phase. During open system degassing, CO_2 is rapidly removed from the melt with negligible loss of water, until a pressure is reached at which the melt is in equilibrium with nearly pure water vapor. From this pressure downward, the water content of the melt follows the water solubility curve. During closed system degassing, water and CO_2 contents in vapor-saturated basaltic magmas will depend strongly on the vapor composition as determined by the initial volatile concentrations. Deviation from open system behavior, toward lower dissolved H_2O and CO_2 saturation concentrations at a given pressure, will be greatest in melts having high total volatile concentrations and high CO_2:H_2O ratios. Closed system degassing of basaltic melts having the low initial H_2O and CO_2 contents typical of MORB and OIB, however, are similar to the open system case

An Experimental Study of Water and Carbon Dioxide Solubilities in Mid-Ocean Ridge Basaltic Liquids. Part I: Calibration and Solubility Models

Experiments were conducted to determine the solubilities of H_2O and CO_2 and the nature of their mixing behavior in basaltic liquid at pressures and temperature relevant to seqfloor eruption. Mid-ocean ridge basaltic (MORB) liquid was equilibrated at 1200°C with pure H_2O at pressures of 176–717 bar and H_2O—CO_2 vapor at pressures up to 980 bar. Concentrations and speciation of H_2O and CO_2 dissolved in the quenched glasses were measured using IR spectroscopy. Molar absorptivities for the 4500 cm^(−1) band of hydroxyl groups and the 5200 and 1630 cm^(−1) bands of molecular water are 0⋅67±0⋅03, 0⋅62±0⋅07, and 25±3 l/mol-cm, respectively. These and previously determined molar absorptivities for a range of silicate melt compositions correlate positively and linearly with the concentration of tetrahedral cations (Si+Al). The speciation of water in glass quenched from vapor-saturated basaltic melt is similar to that determined by Silver & Stolper (Journal of Petrology 30, 667–709, 1989) in albitic glass and can be fitted by their regular ternary solution model using the coefficients for albitic glasses. Concentrations of molecular water measured in the quenched basaltic glasses are proportional to fH_2O in all samples regardless of the composition of the vapor, demonstrating that the activity of molecular water in basaltic melts follows Henry's law at these pressures. A best fit to our data and existing higher-pressure water solubility data (Khitarov et al., Geochemistry 5, 479–492, 1959; Hamilton et al., Journal of Petrology 5, 21–39, 1964), assuming Henrian behavior for molecular water and that the dependence of molecular water content on total water content can be described by the regular solution model, gives estimates for the V^(o,m)_(H2O) of 12±1 cm^3/mol and for the 1-bar water solubility of 0⋅11 wt%. Concentrations of CO_2 dissolved as carbonate in the melt for pure CO_2-saturated and mixed H_2O-CO_2-saturated experiments are a simple function of f_(CO2). These results suggest Henrian behavior for the activity of carbonate in basaltic melt and do not support the widely held view that water significantly enhances the solution of carbon dioxide in basaltic melts. Using a ΔV^(o,m)_r of 23 cm^3/mol (Pan et al., Geochimica et Cosmochimica Acta 55, 1587–1595, 1991), the solubility of carbonate in the melt at 1 bar and 1200°C is 0⋅5 p.p.m. Our revised determination of CO_2 solubility is ∼20% higher than that reported by Stolper & Holloway (Earth and Planetary Science Letters 87, 397–408, 1988)

Water and carbon dioxide in basaltic magmas

Experiments were conducted in which basaltic melts were equilibrated with a vapor phase consisting of pure water, pure carbon dioxide, and water-carbon dioxide mixtures at 1200°C and 200 to 980 bars in order to develop a basis for interpreting the behavior of these volatiles during the evolution and degassing of submarine magmas. Molar absorptivities for the 4500 cm^(-1) band for hydroxyl groups and for the 5230 and 1630 cm^(-1) bands of molecular water were calibrated to be 0.67 ± 0.04, 0.62 ± 0.08, and 25 ± 3 l/mole-cm, respectively. The solubility of water in MORE liquid was determined from the experiments in which MORB melt was equilibrated with pure H_20 vapor. Results are in agreement with the higher-pressure results of Hamilton et al. (1964) on Columbia River basalt. Trends observed in the concentrations of molecular water and hydroxyl groups with respect to total water concentration in the quenched, experimental, basaltic glasses are similar to those observed in albitic glasses (Silver and Stolper, 1989). Moreover, the concentration of molecular water measured in the quenched basaltic glasses is approximately proportional to water fugacity in all samples regardless of the composition of the vapor (X_CO_2), demonstrating that molecular water solubility in basaltic melts is closely approximated by Henry's law at pressures less than 1 kbar. Total water concentrations and the speciation of water in vapor-saturated basaltic melt are fit by a regular ternary solution model with the coefficients for albitic glasses (Silver and Stolper, 1989), where the activity of water in the melt is given by Henry's law for molecular water. At pressures higher than about 1 kbar, the effect of the molar volume of water in the melt (V^(0,m)_(H_2O)) on the activity of water in vapor-saturated melts is no longer negligible; a (V^(0,m)_(H_2O)) ~12 cm^3/mole fits the data of Hamilton et al. (1964). Concentrations of CO_2 dissolved as carbonate in the experimental glasses range from 63 to 315 ppm CO_2. Carbonate was the only species of dissolved carbon observed. The mole fraction of CO_2 in the vapor varied from 0.39 to 0.93. Concentrations of CO_2 dissolved as carbonate in the melt for all the experiments are proportional to fCO_2. The data for pure CO_2-saturated and mixed H_2O-CO_2-saturated experiments are fit with a straight line through the origin with a slope of 40 ppm/100 bar fCO_2 (equivalent to 47 ppm/km water depth). These results suggest Henrian behavior for CO_2; that is, the solubility of CO_2 in the basaltic melt is essentially proportional to the fugacity of CO_2 with the same constant of proportionality whether the vapor contains pure CO_2 or H_2O + CO_2. These results do not support the widely held view that water enhances the solubility of carbon dioxide in basaltic melts. Results of degassing calculations show that the vapor phase in equilibrium with MORB magmas at typical midoceanic eruption depths is CO_2 -rich and that the dissolved CO_2 contents should vary linearly with depth of eruption. Basaltic magmas containing &lt; 1.0 wt. % H_2O will not degas significant quantities of water until pressures &lt; 100 bars are reached. As water contents increase either through fractional crystallization or variations in the initial water contents. an inverse correlation is predicted between dissolved CO_2 and H_20 contents in melts saturated with a mixed H_2O-CO_2 vapor phase. These predictions were tested by examining the water and carbon dioxide concentrations in suites of basaltic glasses from the Juan de Fuca Ridge and Hawaii. Concentrations of dissolved H_2O and CO_2 were measured in a suite of basaltic glasses from the Juan de Fuca Ridge. CO_2 contents dissolved as carbonate range from about 45 to 360 ppm by weight. In contrast to the predictions based on vapor-saturated degassing samples empted at a given depth exhibit a large range in dissolved CO_2 contents that we interpret to be the result of variable amounts of degassing. The lowest CO_2 contents at each depth are in reasonable agreement with the experimentally determined CO_2 solubility curve for basalt at low pressures. All glasses with CO_2 values higher than the experimentally determined solubility at the emption depth are oversaturated because of incomplete degassing. The highest CO_2 contents are spatially associated with the local topographic highs for each ridge segment. Lavas from relatively deep areas may have had greater opportunity to degas duIing ascent from a relatively deeper magma chamber or during late ral flow in dikes or seatloor lava flows. The highest observed CO_2 concentrations are from the axial seamount and lead to an estimate of a minimum depth to the magma chamber of 2.7 kilometers beneath the ridge axis. Water contents were not modified during degassing and were found to behave incompatibly duIing partial melting and crystal fractionation. Variations in ratios of water to other incompatible elements suggest that water has a bulk partition coefficient similar to La duIing partial melting (~D0.010). Major, minor, and dissolved volatile element concentrations were measured in tholeiitic glasses from the submarine portion (puna Ridge) of the east lift zone of Kilauea Volcano, Hawaii. Dissolved H_2O and S concentrations display a wide range relative to nonvolatile incompatible elements at all depths. This range cannot be readily explained by fractional crystallization, degassing of H_2O and S during eruption on the seafloor, or source region heterogeneities. Dissolved CO_2 concentrations, in contrast, show a positive correlation with eruption depth and typically agree within error with the solubility at that depth. Magmas along the Puna Ridge can be modelled as resulting from (1) mixing of a relatively volatile-rich, undegassed component with magmas that experienced low pressure (perhaps subaerial) degassing during which substantial H_2O, S, and CO_2 were lost, followed by (2) fractional crystallization of olivine, clinopyroxene, and plagioclase from this mixture to generate a residual liquid; and (3) further degassing, principally of CO_2 for samples erupted deeper than 1000 m, during eruption on the seafloor. The degassed end member may form at upper levels of the summit magma chamber (assuming less than lithostatic pressure gradients), during residence at shallow levels in the crust, or during sustained summit eruptions. The final phase of degassing during eruption on the seafloor occurs slowly enough to achieve melt/vapor equilibrium during exsolution of the typically CO_2-rich vapor phase. According to the model, an average Kilauean primary magma with 16.0 % MgO should contain ~0.47 wt. % H_20 and ~900 ppm S. The model predicts that submarine lavas from wholly submarine volcanoes (i.e., Loihi), for which there is no opportunity to generate the degassed end member by low pressure degassing, will be enriched in volatiles relative to those from volcanoes whose summits have breached the sea surface (i.e., Kilauea and Mauna Loa).</p

Degassing of Alkalic Basalts

In order to model quantitatively exsolution of volatiles over the range of basaltic melt compositions found on oceanic islands, I present compositional parameterizations of H2O and CO2 solubilities and use these parameterizations to develop vapor saturation and degassing models for alkalic basaltic liquids. Vapor-saturation diagrams generated as a function of melt composition are used to determine the pressure at which the melt was last in equilibrium with a vapor and the composition of the vapor phase based on measured H2O and CO2 contents in basaltic glasses. These models allow the calculation of the pressure at which a magma of known initial volatile content reaches vapor saturation and begins to exsolve a vapor phase. The higher solubility of CO2 in alkalic magmas causes vapor saturation in CO2-bearing alkalic magmas to be reached at lower pressures than in CO2-bearing tholeiitic magmas having identical volatile contents. However, if variations in major element and volatile concentrations were linked by variations in the extent of melting, then volatile-rich, strongly alkalic magmas would begin to exsolve a vapor at slightly higher pressures than volatile-poor alkali olivine basalts or tholeiites. Partitioning of H2O and CO2 into the vapor during volatile exsolution is controlled by the difference between H2O and CO2 solubilities. As melts become more alkalic, the relative difference between H2O and CO2 solubilities decreases, thus diminishing the preferential partitioning of CO2 into the vapor. Exsolution of volatiles from tholeiites is characterized by strong partitioning of CO2 into the vapor such that most or all CO2 is lost before any significant loss of H2O. In contrast, the combination of higher CO2 solubility and higher volatile contents (and perhaps higher CO2/H2O ratio) in alkalic melts results in less fractionation between CO2 and H2O during volatile exsolution

Determination of the Molar Absorptivity of Dissolved Carbonate in Basanitic Glass

Basanitic glasses with known dissolved C concentrations have been analyzed using infrared spectroscopy to calibrate the molar absorptivity of carbonate. C is dissolved as carbonate complexes in the glass resulting in absorption bands at 1525 and 1425 cm−1. Molar absorptivities of 283 ± 8 L/(mol·cm) were determined for both 1525 and 1425 cm−1 absorption bands. An integrated molar absorptivity of 60000 ± 1700 L/(mol·cm2) was determined using the integrated area under the doublet. These values are about 20–25% lower than those determined for tholeiitic and leucititic glasses and are intermediate between values characteristic of Na- and Ca-rich silicate glasses. Carbonate molar absorptivities for a range of basaltic glass compositions correlate with the molar ratio of Na/(Na + Ca)

Volatiles in Basaltic Glasses from Loihi Seamount, Hawaii: Evidence for a Relatively Dry Plume Component

New H2O, CO2 and S concentration data for basaltic glasses from Loihi seamount, Hawaii, allow us to model degassing, assimilation, and the distribution of major volatiles within and around the Hawaiian plume. Degassing and assimilation have affected CO2 and Cl but not H2O concentrations in most Loihi glasses. Water concentrations relative to similarly incompatible elements in Hawaiian submarine magmas are depleted (Loihi), equivalent (Kilauea, North Arch, Kauai–Oahu), or enriched (South Arch). H2O/Ce ratios are uncorrelated with major element composition or extent or depth of melting, but are related to position relative to the Hawaiian plume and mantle source region composition, consistent with a zoned plume model. In front of the plume core, overlying mantle is metasomatized by hydrous partial melts derived from the Hawaiian plume. Downstream from the plume core, lavas tap a depleted source region with H2O/Ce similar to enriched Pacific mid-ocean ridge basalt. Within the plume core, mantle components, thought to represent subducted oceanic lithosphere, have water enrichments equivalent to (KEA) or less than (KOO) that of Ce. Lower H2O/Ce in the KOO component may reflect efficient dehydration of the subducting oceanic crust and sediments during recycling into the deep mantle

Glass Cplume‐ridge Interaction, and Hydrous Melting along the Galápagos Spreading Center, 90.5°W to 98°W

The Galápagos Spreading Center (GSC) between 90.5°W and 98°W manifests its interaction with the nearby Galápagos plume by way of variations in lava geochemistry, crustal thickness, and morphology along the ridge axis. Natural glasses from stations with ∼9 km average spacing were analyzed for major and minor elements, H2O, and CO2. Samples can be classified as enriched mid‐ocean ridge basalts (E‐MORB), transitional MORB (T‐MORB), or normal MORB (N‐MORB) on the basis of K/Ti ratios. E‐MORB dominate the GSC east of 92.6°W. T‐MORB are mainly found between 92.6°W and 95.5°W. West of the propagating rift tip at 95.5°W, N‐MORB dominate. High K/Ti E‐MORB also have higher H2O, Al2O3, and Na2O and lower FeO*, SiO2, and CaO/Al2O3 relative to N‐MORB at similar values of MgO, characteristics consistent with lower mean extents of partial melting relative to N‐MORB. We examine the melting process along this section of the GSC with a set of equations that simulate a deep zone of hydrous melting related to the depression of the mantle solidus by H2O. This model constrains the range of mantle source compositions, the depth of the additional hydrous melting zone, the melt productivity in the hydrous region, and the ratio of mantle flow rate through the hydrous zone relative to the anhydrous zone (Uw/U0) that can explain the measured crustal thickness as well as the fractionation‐corrected concentrations of K, Na2O, H2O, and Ti along the GSC. Far from the hot spot, the measured crustal thickness and N‐MORB compositions are explained by passive mantle upwelling (Uw/U0 = 1), mean melt fraction () ∼ 0.06, and a source with ∼35 ppm K, 130 ppm H2O, 2300 ppm Na2O, and 1050 ppm Ti. The transitional zone has a source enriched in K and could have a slight excess plume‐driven flow through the hydrous melting zone (Uw/U0 ≤ 1.5). The crustal thickness and glass compositions in the “enriched” region of the GSC nearest the hot spot are best explained by only a slight increase in the temperature of the mantle (\u3c∼20°C), coupled with a mantle source moderately enriched (relative to N‐MORB source) and plume‐driven flow through the hydrous zone of Uw/U0 = 1.5–3.5

Infrared Spectroscopic Measurements of CO\u3csub\u3e2\u3c/sub\u3e and H\u3csub\u3e2\u3c/sub\u3eO in Juan de Fuca Ridge Basaltic Glasses

Dissolved H2O and CO2 contents in basaltic glasses from the Juan de Fuca Ridge and neighboring seamounts were determined by infrared spectroscopy. CO2 contents range from about 45 to 360 ppm by weight, with carbonate ion complexes the only detectable form of dissolved carbon. Samples erupted at a given depth exhibit a large range in dissolved CO2 contents that we interpret to be the result of variable amounts of degassing. The lowest CO2 contents at each depth are in reasonable agreement with the experimentally determined CO2 solubility curve for basalt at low pressures. All glasses with CO2 values higher than the experimentally determined solubility at the eruption depth are oversaturated because of incomplete degassing. The highest CO2 contents are spatially associated with the local topographic highs for each ridge segment. Lavas from relatively deep areas may have had greater opportunity to degas during ascent from a magma chamber or during lateral flow in dikes or seafloor lava flows. The highest observed CO2 concentrations are from the axial seamount and lead to an estimate of a minimum depth to the magma chamber of 2.7 km beneath the ridge axis. H2O contents vary from 0.07 to 0.48 wt.%, with hydroxyl groups the only detectable form of dissolved water. Water contents correlate positively with FeO*/MgO and the highest water contents are found in the incompatible element-enriched Endeavour segment lavas. Variations in ratios of water to other incompatible elements suggest that water has a bulk partition coefficient similar to La during partial melting (D ∼ 0.01)

Implications of Subduction Rehydration for Earth\u27s Deep Water Cycle

The “standard model” for the genesis of the oceans is that they are exhalations from Earth’s deep interior continually rinsed through surface rocks by the global hydrologic cycle. No general consensus exists, however, on the water distribution within the deeper mantle of the Earth. Recently Dixon et al. [2002] estimated water concentrations for some of the major mantle components and concluded that the most primitive (FOZO) are significantly wetter than the recycling associated EM or HIMU mantle components and the even drier depleted mantle source that melts to form MORB. These findings are in striking agreement with the results of numerical modeling of the global water cycle that are presented here. We find that the Dixon et al. [2002] results are consistent with a global water cycle model in which the oceans have formed by efficient outgassing of the mantle. Present-day depleted mantle will contain a small volume fraction of more primitive wet mantle in addition to drier recycling related enriched components. This scenario is consis-tent with the observation that hotspots with a FOZO-component in their source will make wetter basalts than hotspots whose mantle sources contain a larger fraction of EM and HIMU components