Analytical models for bubble growth during decompression of high viscosity magmas

Analytical models for decompressional bubble growth in a viscous magma are developed to establish the influence of high magma viscosity on vesiculation and to assess the time-scales on which bubbles respond to decompression. Instantaneous decompression of individual bubbles, analogous to a sudden release of pressure (e.g. sector collapse), is considered for two end-member cases. The infinite melt model considers the growth of an isolated bubble before significant bubble interaction occurs. The shell model considers the growth of a bubble surrounded by a thin shell and is analogous to bubble growth in a highly vesicular magmatic foam. Results from the shell model show that magmas less viscous than 10⁹ Pa s can freely expand without developing strong overpressures. The timescales for pressure re-equilibration are shortened by increased ratios of bubble radius to shell thickness and by larger decompression. Time-scales for isolated bubbles in rhyolitic melts (infinite melt model) are significantly longer, implying that such bubbles could experience internal pressures greater than the ambient pressure for at least a few hours following a sudden release of pressure. The shell model is developed to assess bubble growth during the linear decompression of a magma body of constant viscosity. For the range of decompression rates and viscosities associated with actual volcanic eruptions, bubble growth continues at approximately the equilibrium rate, with no attendant excess of internal pressure. The results imply that viscosity does not have any significant role in preventing the explosive expansion of high viscosity foams. However, for viscosities of >10⁹ Pa s there is the potential for a viscosity quench under the extreme decompression rates of an explosive eruption. It is proposed that the typical vesicularities of pumice of 0.7–0.8 are a consequence of the viscosity of the degassing magmas becoming sufficiently high to inhibit bubble expansion over the characteristic time-scale of eruption. For fully degassed silicic lavas with viscosities in the range 10¹⁰ to 10¹² Pa s time-scales for decompression of isolated bubbles can be hours to many months.     

The viscosity of hydrous dacitic liquids: implications for the rheology of evolving silicic magmas

The viscosity of a series of six synthetic dacitic liquids, containing up to 5.04 wt% dissolved water, was measured above the glass transition range by parallel-plate viscometry. The temperature of the 10¹¹ Pa s isokom decreases from 1065 K for the anhydrous liquid, to 864 K and 680 K for water contents of 0.97 and 5.04 wt% H₂O. Including additional measurements at high temperatures by concentric-cylinder and falling-sphere viscometry, the viscosity (η) can be expressed as a function of temperature and water content w according to: where η is in Pa s, T is temperature in K, and w is in weight percent. Within the conditions of measurement, this parameterization reproduces the 76 viscosity data with a root-mean square deviation (RMSD) of 0.16 log units in viscosity, or 7.8 K in temperature. The measurements show that water decreases the viscosity of the dacitic liquids more than for andesitic liquids, but less than for rhyolites. At low temperatures and high water contents, andesitic liquids are more viscous than the dacitic liquids, which are in turn more viscous than rhyolitic liquids, reversing the trend seen for high temperatures and low water contents. This suggests that the relative viscosity of different melts depends on temperature and water content as much as on bulk melt composition and structure. At magmatic temperatures, rhyolites are orders of magnitude more viscous than dacites, which are slightly more viscous than andesites. During degassing, all three liquids undergo a rapid viscosity increase at low water contents, and both dacitic and andesitic liquids will degas more efficiently than rhyolitic liquids. During cooling and differentiation, changing melt chemistry, decreasing temperature and increasing crystal content all lead to increases in the viscosity of magma (melt plus crystals). Under closed system conditions, where melt water content can increase during crystallization, viscosity increases may be small. Conversely, viscosity increases are very abrupt during ascent and degassing-induced crystallization.     

The mechanism of Re enrichment in arc magmas: evidence from Lau Basin basaltic glasses and primitive melt inclusions

Rhenium and other trace element data were obtained in situ by laser ablation ICP-MS analysis of submarine-erupted volcanic glasses and olivine-hosted melt inclusions from the Valu Fa Ridge, the south tip of the Lau Basin, in the southwestern Pacific Ocean. The chemistry of the Lau Basin basaltic glasses changes systematically from compositions similar to MORB in the Lau Spreading Centers, to more arc-like compositions in the Valu Fa Ridge, providing geochemical profiles both along the Lau Spreading Centers (ridges) and across the Valu Fa Ridge. The east seamount samples of the Valu Fa Ridge have diagnostic trace element ratios (Ba/Nb, Nb/U, Ce/Pb) close to global arc averages, with high Ba/La, indicating addition of considerable amounts of subduction-released fluids. In contrast, samples from the west seamount and the Lau Spreading Centers show a smaller influence from subduction fluids. The variable degrees of subduction influences apparent in the chemistry of these suites provide an ideal means to explore the mechanisms of Re enrichment in undegassed arc magmas. All of the analyzed arc melts have significantly higher Re concentrations than previously published, largely subaerially erupted samples, confirming that high Re is a characteristic of undegassed arc magmas. The east seamount samples are characterized by higher Re and lower Yb/Re than the more MORB-like Lau Spreading Center lavas. The lack of correlation between Yb/Re and Fo of host olivine suggests that low Yb/Re is not due to magmatic differentiation. When the Lau Basin sample suite is plotted together with MORB data, Yb/Re is positively correlated with Ce/Pb and Nb/U, and negatively correlated with Ba/Nb, indicating that Re is much more mobile than Yb during dehydration of subducted slabs. Thus, Re enrichment in arc magmas is likely due to addition of Re via fluids released from subducted slabs; the recognition of high Re in arcs favors arguments for a slab origin of radiogenic ¹⁸⁷Os/¹⁸⁸Os components in arc rocks.     

A study of melt inclusions at Vulcano (Aeolian Islands, Italy): insights on the primitive magmas and on the volcanic feeding system

 This work presents the results of a microthermometric and EPMA-SIMS study of melt inclusions in phenocrysts of rocks of the shoshonitic eruptive complex of Vulcano (Aeolian Islands, Italy). Different primitive magmas related to two different evolutionary series, an older one (50–25 ka) and a younger one (15 ka to 1890 A.D.), were identified as melt inclusions in olivine Fo_88–91 crystals. Both are characterized by high Ca/Al ratio and present very similar Rb/Sr, B/Be and patterns of trace elements, with Nb and Ti anomalies typical of a subduction zone. The two basalts present the same temperature of crystallization (1180±20  °C) and similar volatile abundances. The H₂O, S and Cl contents are relatively high, whereas magmatic CO₂ concentrations are very low, probably due to CO₂ loss before low-pressure crystallization and entrapment of melt inclusions. The mineral chemistry of the basaltic assemblages and the high Ca/Al ratio of melt inclusions indicate an origin from a depleted, metasomatized clinopyroxene-rich peridotitic mantle. The younger primitive melt is characterized with respect to the older one by higher K₂O and incompatible element abundances, by lower Zr/Nb and La/Nb, and by higher Ba/Rb and LREE enrichment. A different degree of partial melting of the same source can explain the chemical differences between the two magmas. However, some anomalies in Sr, Rb and K contents suggest either a slightly different source for the two magmas or differing extents of crustal contamination. Low-pressure degassing and cooling of the basaltic magmas produce shoshonitic liquids. The melt inclusions indicate evolutionary paths via fractional crystallization, leading to trachytic compositions during the older activity and to rhyolitic compositions during the recent one. The bulk-rock compositions record a more complex history than do the melt inclusions, due to the syneruptive mixing processes commonly affecting the magmas erupted at Vulcano. The composition and temperature data on melt inclusions suggest that in the older period of activity several shallow magmatic reservoirs existed; in the younger one a relatively homogeneous feeding system is active. The shallow magmatic reservoir feeding the recent eruptive activity probably has a vertical configuration, with basaltic magma in the deeper zones and differentiated magmas in shallower, low-volume, dike-like reservoirs. Received: 11 March 1998 / Accepted: 14 July 1998     

A fragmentation criterion for highly viscous bubbly magmas estimated from shock tube experiments

When a highly viscous bubbly magma is sufficiently decompressed, layer-by-layer fracturing propagates through the magma at a certain speed (fragmentation speed). On the basis of a recent shock tube theory by Koyaguchi and Mitani [Koyaguchi, T., Mitani, N. K., 2005. A theoretical model for fragmentation of viscous bubbly magmas in shock tubes. Journal of Geophysical Research 110 (B10), B10202. doi:10.1029/2004JB003513.], gas overpressures at the fragmentation surface are estimated from experimental data on fragmentation speed in shock tube experiments for natural volcanic rocks with various porosities. The results show that gas overpressure at the fragmentation surface increases as initial sample pressure increases and sample porosity decreases. We propose a new fragmentation criterion to explain the relationship between the gas overpressure at the fragmentation surface, the initial pressure and the porosity. Our criterion is based on the idea that total fragmentation of highly viscous bubbly magmas occurs when the tensile stress at the midpoint between bubbles exceeds a critical value. We obtain satisfactory agreement between our simulation and experiment when we assume that the critical value is inversely proportional to the square root of bubble wall thickness. This fragmentation criterion suggests that long micro-cracks or equivalent flaws (e.g., irregular-shaped bubbles) that reach the midpoints between bubbles are a dominant factor to determine the bulk strength of the bubbly magma.     

A tale of two magmas,Fuego, Guatemala

Fuego volcano in Guatemala erupted in 1974 in a basaltic sub-Plinian event, which has been well documented and studied. In 1999, after a period of quiescence lasting 20 years, Fuego erupted again, this time less violently, but with persistent low-level activity. This study investigates the link between these episodes. Previous melt inclusion studies have shown magma erupted in 1974 to have been a volatile-rich hybrid tapped from a vertically extensive system. By contrast, magma erupted in 1999 and 2003 is similar in composition to that erupted in 1974, but melt inclusions are more evolved. Although melt inclusions from the later period are CO₂ rich (up to ∼1,500 ppm), they have low H₂O concentration (max 1.5 wt.%, compared to ∼6 wt.% in 1974). These melt inclusions have a modified H₂O concentration due to diffusive re-equilibration at shallow pressures. Despite this diffusive exchange, both eruptions show evidence of recent mingling of the same low and higher K melts, one of which was slightly cooler than the other and as a result traversed the amphibole stability field. (²¹⁰Pb/²²⁶Ra) data on selected bulk rock samples from 1974 suggest that whereas the cooler, more evolved end-member may have been degassing since the last major eruption in the 1930s, the warmer end-member intruded at most a decade prior to the 1974 eruption. The two end-members are thus batches of the same magma emplaced shallowly ∼30 years apart during which time the older batch was cooled and differentiated before mixing with the younger influx. The presence of the same two melts in the later eruptions suggests that magma in 1999 and 2003 is partly residual from 1974. The current eruptive activity is clearing the system of this residual magma prior to an expected new magma batch.     

Permeability development in vesiculating magmas: implications for fragmentation

 Fragmentation, or the "coming apart" of magma during a plinian eruption, remains one of the least understood processes in volcanology, although assumptions about the timing and mechanisms of fragmentation are key parameters in all existing eruption models. Despite evidence to the contrary, most models assume that fragmentation occurs at a critical vesicularity (volume percent vesicles) of 75–83%. We propose instead that the degree to which magma is fragmented is determined by factors controlling bubble coalescence: magma viscosity, temperature, bubble size distribution, bubble shapes, and time. Bubble coalescence in vesiculating magmas creates permeability which serves to connect the dispersed gas phase. When sufficiently developed, permeability allows subsequent exsolved and expanded gas to escape, thus preserving a sufficiently interconnected region of vesicular magma as a pumice clast, rather than fully fragmenting it to ash. For this reason pumice is likely to preserve information about (a) how permeability develops and (b) the critical permeability needed to insure clast preservation. We present measurements and calculations that constrain the conditions (vesicularity, bubble size distribution, time, pressure difference, viscosity) necessary for adequate permeability to develop. We suggest that magma fragments explosively to ash when and where, in a heterogeneously vesiculating magma, these conditions are not met. Both the development of permeability by bubble wall thinning and rupture and the loss of gas through a permeable network of bubbles require time, consistent with the observation that degree of fragmentation (i.e., amount of ash) increases with increasing eruption rate. Received: 5 July 1995 / Accepted: 27 December 1995     

Determination of pre-eruptive H2O,F and Cl contents of silicic magmas using melt inclusions: Examples from Taupo volcanic center,New Zealand

Water, F, and Cl contents of melt inclusions in phenocrysts from the 2-ka-old Taupo and Hatepe plinian tephras, and the 22-ka-old Okaia tephra from the Taupo volcanic center, New Zealand, were measured by electron and ion microprobe. Major and trace element chemistry of the inclusions is similar to that of bulk rock, supporting our assumption that volatile contents of inclusions are representative of the magma in which the crystals grew. Inclusions in the 2-ka Taupo plinian tephra contain a mean of 4.3 wt% H₂O, 450 ppm F, and 1700 ppm Cl; from the Hatepe plinian tephra 4.3 wt% H₂O, 430 ppm F, and 1700 ppm Cl; and from the Okaia tephra 5.9 wt% H₂O, 470 ppm F, and 2100 ppm Cl. Sulfur was below the detection limit of 200 ppm. The constant H₂O, F and Cl from a number of stratigraphic horizons in the tephra deposits suggest that the Taupo and Hatepe plinian tephras (>8.2 km³ magma volume) were derived from a magma body that did not contain a strong volatile gradient. By inference, there is no pre-eruptive volatile difference between these plinian eruptions and a phrea-toplinian eruption which occurred between the two. Virtually no major element zonation is seen in this eruptive sequence. Although the Okaia tephra was also erupted from the Taupo volcanic center, probably from a similar vent area, its higher volatile contents and distinct composition as compared to the Taupo tephras show that it was derived from a different, and possibly deeper, magma body.     

Petrological variation of large-volume felsic magmas from Hakkoda-Towada caldera cluster: Implications for the origin of high-K felsic magmas in the Northeast Japan Arc

Abstract The Hakkoda‐Towada caldera cluster (HTCC) is a typical Late Cenozoic caldera cluster located in the northern part of the Northeast Japan Arc. The HTCC consists of five caldera volcanoes, active between 3.5 Ma and present time. The felsic magmas can be classified into high‐K (HK‐) type and medium‐ to low‐K (MLK‐) type based on their whole‐rock chemistry. The HK‐type magmas are characterized by higher K₂O and Rb contents and higher ⁸⁷Sr/⁸⁶Sr ratios than MLK‐type magmas. Both magmas cannot be derived from fractional crystallization of any basaltic magma in the HTCC. Assimilation‐fractional crystallization model calculations show that crustal assimilation is necessary for producing the felsic magmas, and HK‐type magmas are produced by higher degree of crustal assimilation with fractional crystallization than MLK‐type magmas. Although MLK‐type magmas were erupted throughout HTCC activity, HK‐type magmas were erupted only during the initial stage. The temporal variations of magma types suggest the large contribution of crustal components in the initial stage. A major volcanic hiatus of 3 my before the HTCC activity suggests a relatively cold crust in the initial stage. The cold crust probably promoted crustal assimilation and fractional crystallization, and caused the initial generation of HK‐type magmas. Subsequently, the repeated supply of mantle‐derived magmas raised temperature in the crust and formed relatively stable magma pathways. Such a later system produced MLK‐type magmas with lesser crustal components. The MLK‐type magmas are common and HK‐type magmas are exceptional during the Pliocene–Quaternary volcanism in the Northeast Japan Arc. This fact suggests that exceptional conditions are necessary for the production of HK‐type magmas. A relatively cold crust caused by a long volcanic hiatus (several million years) is considered as one of the probable conditions. Intensive crustal assimilation and fractional crystallization promoted by the cold crust may be necessary for the generation of highly evolved HK‐type felsic magmas.     

Viscosity of magmas containing highly deformable bubbles

The shear viscosity of a suspension of deformable bubbles dispersed within a Newtonian fluid is calculated as a function of the shear rate and strain. The relative importance of bubble deformation in the suspension is characterized by the capillary number (Ca), which represents the ratio of viscous and surface tension stresses. For small Ca, bubbles remain nearly spherical, and for sufficiently large strains the viscosity of suspension is greater than that of the suspending fluid, i.e. the relative viscosity is greater than 1. If Ca>O(1) the relative viscosity is less than one. In the limit that Ca→∞ (surface tension is dynamically negligible), numerical calculations for a suspension of spherical bubbles agree well with the experimental measurements of Lejeune et al. (1999, Rheology of bubble-bearing magmas. Earth Planet. Sci. Lett., vol. 166, pp. 71–84). In general, bubbles have a modest effect on the relative viscosity, with viscosity changing by less than a factor of about 3 for volume fractions up to 50%.     

Rutile saturation in magmas: implications for TiNbTa depletion in island-arc basalts

The TiO₂ contents of rutile-saturated melts ranging from basalt to rhyodacite have been investigated at P = 8–30 kbar and T = 1000–1300°C under hydrous, CO₂-saturated, and volatile-absent conditions. Dissolved TiO₂ is positively correlated with T and not strongly dependent on P_total. For fixed P and T, TiO₂ content decreases markedly as the melts become more felsic. The distribution of TiO₂ between rutile and liquid, expressed as a wt.% concentration ratio, D (rut/liq), is given by: In D = −3.16 + (9373T) + 0.026P − 0.152FM where T is in Kelvins, P in kbar and FM is a melt composition parameter, FM = [Na+K+ 2(Ca+Fe+Mg)]/Al· 1/Si in which the chemical symbols represent cation fractions. The first term expresses the competition of aluminate and titanate anions for charge-compensating cations, and the second term expresses the inverse dependence of dissolved TiO₂ on SiO₂ content. There is no apparent dependence of rutile solubility on water content.For ranges of probable solidus conditions, rutile saturation in basaltic, andesitic, and dacitic liquids requires 7–9, 5–7, and 1–3 wt.% TiO₂, respectively. These concentrations are well in excess of those found in the respective rock types, so depletion in Nb, Ta, and Ti and reduced Nb/U and Nb/Th ratios in volcanic rocks erupted at convergent plate margins cannot be attributed to residual rutile in their source regions. Thus, Nb, Ta and Ti depletion must be an inherent property of the source region.We suggest that the island-arc source region has been depleted in Nb and Ta by a previous episode of melt extraction (MORB), zoning refining, or equilibration with a percolating melt or fluid. Such a process markedly depletes the LILE and HFSE element concentrations in the residuum, but ratios such as Nb/U, Nb/Th and U/Th remain relatively constant due to similar solid-melt partition coefficients. The depletion of Nb relative to Th in the source regions of island-arc magmas occurs during hybridization of the source by rutile-saturated (Nb/Ta-depleted) melts or aqueous fluids. If the hybridizing agent is a melt, a relatively felsic composition, produced under low T (900°) hydrous conditions, is required.     

Fractionation trends of basalt magmas in lava flows

Chemical compositions of schlieren in basalt flows are compared with those of the host rocks for tracing the fractionation trends of basalt magmas under extrusive conditions. In the Warner high-alumina basalt of California and in the tholeiite of Hawaii and Japan, total iron increases markedly from the host rock to the schlieren whileSiO ₂ is nearly constant. In the high-alumina basalt of Huzi Volcano and in the tholeiite near Catania, Italy, total iron is nearly constant during fractionation whileSiO ₂ increases. In basalts of the hypersthenic rock series or calc-alkali rock series from California, total iron is also nearly constant whileSiO ₂ increases. The difference in fractionation trend in these flows is attributable to the difference of the state of oxidation of iron in the original magmas. Oxygen partial pressure of the magmas would not be maintained constant during the fractionation of extrusive bodies.     

Convective heat transfer in magmas near the liquidus

Natural convection in magmas at subliquidus temperatures is analyzed using Bingham plastic and power-law rheology models. Heat flux measurements were obtained at liquidus and subliquidus temperatures for degassed basaltic lava at atmospheric pressure. These measurements of heat flux ranged from 2 to 40 kW/m² and were obtained using two different types of convective heat flux probes. The agreement between the two different instruments and the theoretical calculations is excellent. A noticeable change in the trend of the convective heat flux data is observed in the vicinity of the liquidus temperature. Subliquidus convective heat flux rates (6–15 kW/m²) are attractive for energy extraction applications.     

Water and magmas: Thermal effects of exsolution

The temperature changes caused by water exsolution from magma have been determined through calorimetric measurements performed on phonolitic and albitic compositions. The enthalpies of mixing of water with these melts have been derived from HF solution calorimetry, made at 323 K on glass samples containing up to 5 wt.% water, together with heat capacity data for the same series of samples. Mixing between aluminosilicate melts and water appears nearly ideal at magmatic temperatures, with small enthalpies of mixing that are negative for both melts at low pressures but can become positive for albite at high pressure. Regardless of the endothermic or exothermic nature of the process, water exsolution is associated with negligible temperature changes of only a few degrees even when 5 wt.% H₂O is degassed. However, thermal effects might be greater for more depolymerized melts such as basalts and related compositions.     

Non-Newtonian effects during injection in partially crystallised magmas

Injection of Newtonian crystal-free magmas into a partially crystallised host which may exhibit non-Newtonian properties produces magmatic structures such as pipes, syn-plutonic dikes or dendritic structures. Field relationships between the structure and the host rock commonly indicate what the rheological contrasts during the injection were. The manner in which a magma deforms in response to injection is mainly linked to crystal content and strain rate (i.e., injection rate). Three kinds of behaviour can be distinguished: (1) Newtonian at low crystal contents; (2) Non-Newtonian at intermediate (40–60%) crystal contents, or at high crystal contents if the strain rate is small; and (3) brittle failure at high crystal content or strain rates.Petrologic observations indicate that injection can take place when the host magma still behaves as a fluid. To investigate the physics of the injection process we review the results of injection experiments in non-Newtonian fluids. These experiments were performed to study viscous fingering in 2-D Hele Shaw cells. They provide the first step to establishing the main non-Newtonian effects during the formation of interfacial instabilities arising when a Newtonian fluid is injected into a more viscous fluid or paste. The qualitative comparison of the morphological features of the interfaces between the fluids in the experiments with those in nature suggests that, in magmas, irregularities of the interfaces (dikes and dendrites) result from non-Newtonian properties of the host. We conclude that fluid-like deformation, rather than brittle behaviour of the host, during injection is likely to produce the general features observed on the field. Cooling effects might be responsible for the widespread phenomenon of fragmentation. We emphasise that the main effect of non-Newtonian properties in partially crystallised magmas is to generate strongly heterogeneous media producing discontinuities which could explain the main morphological features of syn-plutonic injection structures.     

Tectonic stress controls on ascent and emplacement of magmas

The tectonic stresses can significantly affect the propagation of a magma-filled crack. It has been pointed out that the rheological boundaries control the emplacement of magmas through the effect of stress. However, it has not been clarified how the role of rheological boundaries depends on the regional tectonic and thermal states. We have evaluated the role of rheological boundaries under various tectonic and thermal conditions and found that the level of magma emplacement may jump according to the changes in the tectonic force or the surface heat flow. The stress profiles were estimated by a simple model of lithospheric deformation. We employed a three-layer model of the lithosphere; the upper crust, the lower crust and the upper mantle have different rheological properties. A constant horizontal force is applied to the lithosphere, and the horizontal strain is assumed to be independent of depth. When realistic tectonic forces (>10¹¹ N/m) are applied, the rheological boundaries mainly control the emplacement of magma. The emplacement is expected at the MOHO, the upper–lower crust boundary, and the brittle–ductile boundary. For lower tectonic forces (<10¹¹ N/m), the tectonic stress no longer plays an important role in the emplacement of magmas. When the tectonic stress controls the emplacement, the roles of rheological boundaries strongly depend on the surface heat flow. When the surface heat flow is relatively high (>80 mW/m²), the stress in the mantle is quite low and the MOHO cannot trap ascending magmas. For relatively low heat flow (<80 mW/m²), on the other hand, the MOHO acts as a magma trap, and the upper–lower crust boundary acts as a magma trap only when the magma supply rate is sufficiently high. Our results suggest that the emplacement depth can change responding to the change in the tectonic force and/or that in the surface heat flow. This may provide us a key to understand the relation between the evolution of a volcanic region and its tectonic and/or thermal history.