Vol48/04/2005def 731 ANNALS OF GEOPHYSICS, VOL. 48, N. 4/5, August/October 2005 Key words Phlegraean Fields – Plinian eruptions – vesicle textures – magma ascent dynamics 1. Introduction Understanding the dynamics of explosive eruptive events has been the primary goal of the volcanological community for many years, since Plinian style volcanic eruptions represent the most powerful and dangerous expression of vol- canism on the Earth. The development of sophis- ticated numerical codes as well as the implemen- tation of experimental studies has allowed us to move several steps further in quantifying the complex physico-chemical properties of mag- mas with different compositions and rheological behaviour (Sigurdsson et al., 2000). Further- more, detailed investigations on recently eye- witnessed eruptions have contributed much to highlight the basic eruptive processes (Newhall and Punongbayan, 1996; Druitt and Kokeelar, 2002). However, monitoring active volcanoes provides geophysical and geochemical data mostly applying to specific eruptions, and nu- merical simulations or experimental runs are not yet able to reproduce the full range of natural phenomenologies being observed. Textural char- acterization of pumice clasts has proved to be a valid complementary approach in investigating magmatic processes not directly observable nor Constraining the dynamics of volcanic eruptions by characterization of pumice textures Margherita Polacci Istituto Nazionale di Geofisica e Vulcanologia, Sede di Pisa, Italy Abstract We have characterized the textures of pumice clasts from Phlegraean Fields to gain insights into the conduit flow-dynamics of alkaline explosive eruptions. Vesicularities, vesicle number densities, and vesicle sizes and shapes were measured to obtain the bulk and groundmass properties of the juvenile fraction of Campanian Ign- imbrite (CI) and Agnano Monte Spina (AMS) eruptions. The results report the coexistence of three end-mem- ber pumice types in the deposits of both eruptions, 1) microvesicular, 2) tube and 3) expanded, which differ ac- cording to clast morphology and the macro- to microscopic vesicle texture. Vesicularities (0.85-0.94 for CI, 0.51-0.91 for AMS) and vesicle number densities (2-4 × 105 cm−2 in CI, 3 × 105-106 cm−2 in AMS) span quite a wide range in all the three pumice types. Overall, tube pumices exhibit the highest bulk (0.89) and groundmass (CI 0.85, AMS 0.82) average vesicle volume fractions but the lowest average vesicle number densities (CI 2 × 105, AMS 4 × 105 cm−2). Comparison with textures of calc-alkaline pumices has revealed many similarities and points to a common origin and distribution of the products from both magma compositions within the volcanic conduit. In addition, the results of the textural analysis were interpreted in the light of the conduit flow model- ing of Phlegraean Fields eruptions. The comparison of textural observations with results from simulations of conduit magma ascent has exhibited a good agreement between measured and numerically calculated vesicular- ities for both compositions, helping to constrain the overall dynamics of alkaline versus calc-alkaline eruptions. Mailing address: Dr. Margherita Polacci, Istituto Na- zionale di Geofisica e Vulcanologia, Sede di Pisa, Via della Faggiola 32, 56126 Pisa, Italy; e-mail: polacci@pi.ingv.it Now at: Istituto Nazionale di Geofisica e Vulcanologia, Sezione di Catania, Piazza Roma 2, 95123 Catania, Italy. 732 Margherita Polacci recognizable from other studies of volcanic de- posits. Crystal and vesicle textures have been used as a proxy for the assessment of conditions related to the eruption dynamics (Klug and Cashman, 1996; Hammer et al., 1999; Polacci et al., 2001, 2003; Klug et al., 2002), and for constraining results from both numerical simula- tions (Polacci et al., 2004) and laboratory exper- iments of conduit magma flow (Mader et al., 1996, 1997; Blower et al., 2001). We have conducted detailed textural inves- tigations on pumice clasts from two alkaline ex- plosive eruptions from the Phlegraean Fields, namely Agnano Monte Spina (AMS) and Cam- panian Ignimbrite (CI). The dataset on pumice textures was then used to constrain results from numerical modeling of conduit magma ascent. The main goal of this research was to investi- gate conduit processes occurring during alka- line eruptions, and compare the findings with observations coming from calc-alkaline erup- tions typical of subduction volcanoes. Results from the investigation of the juve- nile fraction of CI fallout deposits have been re- ported in a recent paper (Polacci et al., 2003), and here will be summarized and combined with analyses referring to pumice products of AMS to provide a comprehensive picture of the dynamics of alkaline silicic eruptions. 2. Methods The samples analyzed in this study are alka- li trachytic pumice clasts from the B1 and D1 Plinian magmatic fallout layers of AMS erup- tion (Di Vito et al., 1999). Pumices were first classified into different types on the basis of macroscopic features. Subsequently, selected clasts from each pumice type were thin-sec- tioned for qualitative and quantitative investi- gation of pumice textures. Images were ac- quired via optical and scanning electron mi- croscopy at different magnification (30x, 125x, 250x, 500x, 1000x) to cover the full range of vesicle size, shape, spatial distribution and de- gree of interconnectivity. Backscattered elec- tron images (BSE) were then made binary and processed with shareware software for image analysis to obtain bulk and groundmass (only vesicles < 200 µm) crystal-free vesicle meas- urements. Details of the procedure can be found in Polacci et al. (2001, 2003). Two-dimension- al bulk and groundmass vesicularities (area fraction of vesicles), groundmass vesicle num- ber densities (number of vesicles per unit area), average vesicle equivalent diameter (d, the di- ameter of a circle whose area equals that of the vesicle) and deformation (D=l−b/l+b, where l and b are the major and minor axes of vesicle best fit ellipse) were computed. Conversion to volumetric vesicle number densities can easily be obtained dividing the areal number densities by the average vesicle diameter. These values were then compared with measurements made on the Campanian Ignimbrite pyroclasts (Po- lacci et al., 2003) and on pumice clasts from calc-alkaline eruptions from subduction volca- noes (Klug and Cashman 1994; Polacci et al. 2001; Klug et al., 2002; Rosi et al., 2004). 3. Results 3.1. Observation of pumice textures According to the morphological features and vesicle characteristics of the investigated sam- ples, we were able to distinguish different pumice types for which we followed the same classifica- tion also applied to the CI juvenile fraction (Po- lacci et al., 2003). 1) «Microvesicular» pumices are mostly equidimensional or angular clasts con- taining heterogeneous vesicles, and they are the most abundant juvenile component of the de- posit. 2) Fairly elongated, typical «tube» pumice clasts are characterized by alignments of highly stretched, deformed vesicles. Evidence of exten- sively sheared vesicles up to collapse is present in clasts having low aspect ratios (very flattened in the direction parallel to clast elongation). 3) Fi- nally, «expanded» pumices include all pumice clasts presenting extensive vesicle expansion fea- tures, and span from extremely vesicular retic- ulites (> 90% interconnected vesicles) to clasts with inflated-like textures (vesicular core and progressively denser outer margins). Textures intermediate between the three end-member pumice types are common, and transitional steps among different pumice types can be observed. 733 Constraining the dynamics of volcanic eruptions by characterization of pumice textures Table I summarizes the observed general macro- scopic and microscopic textural characteristics. 3.2. Quantification of pumice textures Bulk and groundmass textural parameters are reported as minimum and maximum, along with the average value, to provide the textural variability within each pumice type (table II). Owing to the difficulty of decoalescing vesicle textures where extreme interconnectivity oc- curs, highly expanded pumice clasts were not included in this analysis, and only results per- taining to pumices with inflated core and dense margins are reported in table II. Bulk and groundmass vesicularities, as well as ground- mass vesicle number densities, span quite a wide range of values for all three pumice types, with the lowest values applying to inflated pumices (table II). On average, tube pumices record the highest bulk (0.89) and groundmass (0.82) vesicularities, but the lowest vesicle number densities (4 × 105 cm−2). Similar values of vesicle size and deformation are exhibited by all pumice types, with the lowest values per- taining again to inflated pumices (table II). 4. Discussion 4.1. Comparison with pumice textures from calc-alkaline eruptions and feed-back with numerical modeling of conduit magma ascent Table III reports data on vesicle measure- ments related to several well studied sustained Table I. Pumice types from AMS eruption. Pumice type Clast morphology Macroscopic texture Microscopic texture Microvesicular Equidimensional Moderately vesicular to vesicular; Heterogeneous vesicles, or angular. max vesicle size < 1 mm elongated/deformed vesicles coexisting with spherical vesicles. Tube Elongated; aspect Dense to vesicular, alignments Elongated, sheared vesicles. ratio 1:2 to 1:4. of large (> 1 mm) tube vesicles. Expanded Bread-crusted Vesicular to extremely vesicular, Highly interconnected vesicles. to reticulites. extensive expansion features. Table II. Summary of textural measurements. AMS fallout layer B1 and D1 Pumice type Number *Gm vesicularity **Bulk vesicularity *Gm vesicle ***Average Average measured range range n density gm d (µm) gm D range (cm−2) m 10136 0.69-0.84 (0.78) 0.80-0.88 (0.84) 328610- 17.5 0.390 -805330 (488354) t 1871 0.76-0.85 (0.82) 0.85-0.91 (0.89) 291290- 19.7 0.433 -621637 (402025) e 13024 0.41-0.69 (0.57) 0.51-0.87 (0.70) 275250 12.8 0.367 -1169230 (660697) m, t, and e indicate microvesicular, tube and expanded pumice types (here e refers only to inflated pumices); Gm is groundmass (average values in parentheses); * vesicles < 200 µm; ** including vesicles > 200 µm (phenocryst- free); *** calculated as d = 2(φ /Naπ)1/2, where φ is vesicularity and Na vesicle number density. 734 Margherita Polacci calc-alkaline explosive eruptions: the 1991 cli- mactic Pinatubo eruption (Philippines), the 800-year-BP Quilotoa eruption (Ecuador), the 7700-year-BP Mt. Mazama eruption (Crater lake, Oregon), and the 1980 Mt. St. Helens eruption (Washington), respectively. Data on the CI eruption are also included to allow full comparison with the textural dataset on alkaline eruptions from Phlegraean Fields. Detailed in- spections of the juvenile fraction of CI and AMS have revealed that clast morphologies and vesicle textures present close similarities with those exhibited by pumices from the above re- ported calc-alkaline eruptions, despite their dif- ferent bulk compositions, volatile and crystal contents. First of all, the bulk of the deposit in eruptions of both magmatic compositions is generally constituted by microvesicular pumice clasts, which contain heterogeneous vesicle populations at various steps in the coalescence process and with varying degrees of deforma- tion (m and white, tables II and III). A subordi- nate pumice type is represented by dense to vesicular tube pumices and/or pumices with fo- liated surfaces, characterized by alignments of highly elongated/stretched vesicles (t and foli- ated, tables II and III). Finally, expanded pumice textures, given by the growth and coa- lescence of poorly deformed vesicles, are also present in clasts of both magma types (e and gray, tables II and II). Specific measurements on pumice textures report overlapping ranges of vesicle number densities, and similar vesicle size and deformation. Yet vesicularities ob- tained with image analyses and density-derived vesicularities are higher overall in pumices from CI and AMS in comparison to those from the reported calc-alkaline pyroclasts (table III), with exceptions from Mt. Mazama and Mt. St. Helens helium picnometry values that were measured on a limited number of samples and may be not representative of the whole deposit. If pumice clasts do not exhibit evidence of post- fragmentation expansion features, vesicularities can be assumed to represent the state of magma at the time of fragmentation, and differences in vesicularities may therefore record different fragmentation conditions owing to the different magma properties characterizing alkaline and calc-alkaline compositions. Table III. Vesicle measurements from explosive silicic eruptions. Eruption Pumice type Bulk vesicularity Average density-derived Average bulk Average from image analysis bulk vesicularity vesicle n density (cm−3) gm D *CI m 0.85-0.94 (0.89) 0.78 1×108 0.390 t 0.87-0.92 (0.89) 0.75 1×108 0.591 e - 0.81 - - **Pinatubo White 0.76-0.85 (0.81) 0.60 1×108-109 0.371 Foliated 0.72-0.82 (0.77) - 1×109 0.383 Gray 0.68-0.82 (0.74) 0.66 1×108-109 0.234 ***Quilotoa White (0.80) 0.66 8.9 ×108 0.413 Gray (0.84) 0.71 8.1 ×108 0.274 ^Mt. Mazama Different - 0.82 1.0 ×109 - types ^^Mt. St. Helens White (0.80) 0.80 8.2 ×108 - Gray (0.60) 0.61 2.0 ×109 - m, t, and e indicate microvesicular, tube and expanded pumice types; Gm is groundmass (in the third column, average values are given in parentheses); * from Polacci et al. (2003); ** from Polacci et al. (2001), density-de- rived vesicularities from Pallister et al. (1996); *** from Rosi et al. (2004); ^ from Klug et al. (2002), 0.82 is he- lium picnometer vesicularity; ^^ from Klug and Cashman (1994), 0.80 and 0.61 are helium picnometer vesicu- larities. 735 Constraining the dynamics of volcanic eruptions by characterization of pumice textures Recent determination of viscosities of Phle- graean Fields trachytic melts have shown that CI and AMS viscosities may be up to two or- ders of magnitude lower than those of a typical rhyolite at anhydrous conditions (Giordano et al., 2004). The lower viscosity of these tra- chytic melts, together with the higher solubili- ties (Polacci et al., 2004), translate into a differ- ent distribution of the flow variables within the conduit and different fragmentation conditions. Figure 1 reports an example of the numerical modeling of magma ascent and fragmentation for a typical rhyolite composition and for tra- chytic compositions corresponding to the CI and AMS eruptions. Details of the conduit flow model are described elsewhere (Papale, 2001). The gas volume fraction distribution of the rhy- olitic magma is different from that of the two trachytic compositions. The rhyolitic magma fragments at a much deeper level in the conduit and at lower gas volume fraction (0.70 versus 0.82 and 0.83, in this specific case). A similar behaviour is obtained by changing the water content and/or the conduit diameter. If we take into account vesicularities pertain- ing to microvesicular pumice clasts, by far the most abundant juvenile component in the de- posits from both magma types, differences in cal- culated trachytic (alkaline) and rhyolitic (calc-al- kaline) vesicularities match quite well with re- sults of vesicularities directly measured on natu- ral pumice clasts (table III). Density-derived vesicularities are the best way to statistically rep- resent the vesicularity distribution of the whole deposit (Klug et al., 2002; Polacci et al., 2003), averaging 0.78 for CI pumice clasts, whereas lower values apply to Pinatubo and Quilotoa pumices, 0.60 and 0.66 respectively. The higher vesicularities displayed by both Mt. Mazama and Mt. St. Helens products may not only be the out- come of a limited number of processed samples but, alternatively, may result from magmatic mix- tures whose viscosity is lower than that of a typi- cal rhyolite or dacite, due to specific conditions like very poor crystal content (former case) or lower evolved chemical composition (latter case). By combining the dataset of textural meas- urements with observations from the numerical Fig. 1. Calculated distribution of the gas volume fraction along the conduit with the model of Papale (2001). Input parameters are 5 km conduit length, 122.5 MPa stagnation pressure, 6 wt% water content, 60 m conduit diameter and compositions corresponding to that of AMS, CI (Romano et al., 2003; Giordano et al., 2004, re- spectively) and a typical rhyolite (Innocenti et al., 1982). 736 Margherita Polacci simulations, we have been able to ascribe dif- ferences in pumice vesicularity to the different rheological properties of alkaline versus calc- alkaline magmas, which, in turn, may affect the flow property distributions within the volcanic conduit. The lower viscosity of alkaline mag- mas, together with their higher solubilities, im- plies that the Phlegraean Fields trachytic mag- ma needs to run a longer section of the conduit before reaching the conditions necessary to fragment, eventually allowing further gas exso- lution and magmatic mixture expansion in com- parison to highly viscous rhyolitic to dacitic magmas. 4.2. Inferences with eruption dynamics Alkaline and calc-alkaline eruptions are char- acterized by similar eruptive phenomenologies. This indicates that differences in fragmentation conditions do not translate into significantly dif- ferent flow conditions at the eruptive vent (Polac- ci et al. 2004). Indeed, the observed similarities in clast morphologies and textures indicate that the origin of pumices from alkaline and calc-al- kaline eruptions may be ascribed to the same magmatic processes. Previous studies have inter- preted the origin of different pumice types within the same eruptive event as due to changes in the eruptive regime (convective versus buoyant) (Klug et al., 2002), or to the development of hor- izontal gradients of the magma properties within the conduit (Polacci et al., 2001; Rosi et al., 2004). In agreement with the last hypothesis the following horizontal zonation of flow conditions in volcanic conduits is outlined (fig. 2). Assum- ing a velocity profile, the central part of the con- duit is occupied by microvesicular pumice clasts, where vesicles are free to grow only subjected to elongational stresses. Pumices with tube and foli- ated textures are thought to originate in the region between the center and the conduit walls, where, owing to the exerted shear stress and velocity gradient, vesicles are extensively sheared and de- formed until they eventually collapse. Pumices with expansion features are located at the conduit walls, where the shear stress is maximum and where the local temperature and velocity rise by viscous heating (Costa and Macedonio, 2003) produces several related effects from crystal re- sorption (in crystal-rich eruptions as Pinatubo and Quilotoa) to enhanced volatile exsolution and vesicle growth and coalescence (in low vis- cosity, high solubility alkaline eruptions as CI and AMS) (Polacci et al., 2001, 2003). As the three end-member pumice types are related to conduit zones characterized by differ- ent magma properties, quantification of their proportion may provide insights into mecha- nisms of magma exsolution (volatile release from the melt), degassing (volatile separation from the melt), and development of permeabil- ity, all of which deeply affect the whole erup- tion dynamics. Tube pumices and pumices with extensive expansion features have the highest vesicularities and degree of vesicle intercon- nectivity (Klug et al., 2002; Polacci et al., 2003, and this paper), and their presence in vol- canic deposits may be used as a proxy to infer the ability of the magmatic mixture to develop a permeable network of vesicles through which the gas phase may eventually escape non-ex- plosively. The increase or decrease in the pro- portion of the different pumice types is there- fore likely to reflect a change in the eruption dynamics. This simplified scheme may apply to both alkaline and calc-alkaline eruption dynam- ics and constrain theoretical results of magma Fig. 2. Zonation of different pumice types within the volcanic conduit, after Polacci et al. (2003). Not to scale. 737 Constraining the dynamics of volcanic eruptions by characterization of pumice textures flow in volcanic conduits with observations on natural pumice clast textures. 5. Perspectives We have investigated the dynamics of Phle- graean Fields eruptions by combining textural characterization of pumice clasts with observa- tions from numerical modeling. Comparison with data and observations on pumice clasts from calc-alkaline eruptions has allowed us to provide a general scheme of the rhyolitic versus trachytic eruption dynamics. This interdiscipli- nary approach applied to other alkaline and calc-alkaline eruption compositions promises to represent a tool to constrain the overall dy- namics of explosive eruptions. Textural meas- urements available to date are however limited and do not cover the whole range of crystal and vesicle content present in natural volcanic prod- ucts. Moreover, numerical simulations of con- duit magma flow present several restrictions since, for example, they can be applied to spe- cific eruption conditions (steady or sub-steady), and/or do not model the along-the conduit ki- netics of crystal and bubble nucleation and growth or the multiphase magma rheology. Fu- ture efforts should therefore be focused on strengthening the existing textural dataset and on implementing numerical codes aimed at re- producing better the observed phenomenolo- gies. Particular emphasis should be placed on measuring textural parameters, like vesicle in- terconnectivity, specifically linked to the devel- opment of magma permeability, which is known to profoundly affect the eruption dy- namics (Klug and Cashman, 1996; Papale, 2001). 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