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Volatile-induced magma differentiation in the plumbing system of Mt. Etna volcano (Italy): evidence from glass in tephra of the 2001 eruption

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Abstract

Mount Etna volcano was shaken during the summer 2001 by one of the most singular eruptive episodes of the last centuries. For about 3 weeks, several eruptive fractures developed, emitting lava flows and tephra that significantly modified the landscape of the southern flank of the volcano. This event stimulated the attention of the scientific community especially for the simultaneous emission of petrologically distinct magmas, recognized as coming from different segments of the plumbing system. A stratigraphically controlled sampling of tephra layers was performed at the most active vents of the eruption, in particular at the 2,100 m (CAL) and at the 2,550 m (LAG) scoria cones. Detailed scanning electron microscope and energy dispersive x-ray spectrometer (SEM-EDS) analyses performed on glasses found in tephra and comparison with lava whole rock compositions indicate an anomalous increase in Ti, Fe, P, and particularly of K and Cl in the upper layers of the LAG sequence. Mass balance and thermodynamic calculations have shown that this enrichment cannot be accounted for by “classical” differentiation processes, such as crystal fractionation and magma mixing. The analysis of petrological features of the magmas involved in the event, integrated with the volcanological evolution, has evidenced the role played by volatiles in controlling the magmatic evolution within the crustal portion of the plumbing system. Volatiles, constituted of H2O, CO2, and Cl-complexes, originated from a deeply seated magma body (DBM). Their upward migration occurred through a fracture network possibly developed by the seismic swarms during the period preceding the event. In the upper portion of the plumbing system, a shallower residing magma body (ABT) had chemical and physical conditions to receive migrating volatiles, which hence dissolved the mobilized elements producing the observed selective enrichment. This volatile-induced differentiation involved exclusively the lowest erupted portion of the ABT magma due to the low velocity of volatiles diffusion within a crystallizing magma body and/or to the short time between volatiles migration and the onset of the eruption. Furthermore, the increased amount of volatiles in this level of the chamber strongly affected the eruptive behavior. In fact, the emission of these products at the LAG vent, towards the end of the eruption, modified the eruptive style from classical strombolian to strongly explosive.

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Acknowledgments

M.V. thanks G. Ottonello and M.S. Ghiorso for productive suggestions and discussions that significantly contributed to the quality of this work. Authors are grateful to M.A. Clynne, W.A. Bohrson, and an anonymous reviewer who provided perceptive and insightful reviews that improved the manuscript. R.A. Corsaro and L. Miraglia are greatly acknowledged for their technical assistance in SEM-EDS data acquisition at INGV-Sezione di Catania. Work supported by research grants from INGV-DPC (National Institute for Geophysics and Volcanology-Department of Civil Defence) and the University of Catania.

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Appendices

Appendix 1

Assuming that the isenthalpic mixing trend for each oxide (Fig. 6) is linear, we can define the slope of this line on each binary molar diagram as:

$$ {\operatorname{d} a} \mathord{\left/ {\vphantom {{\operatorname{d} a} {\operatorname{d} b}}} \right. \kern-\nulldelimiterspace} {\operatorname{d} b} \approx {\Delta a} \mathord{\left/ {\vphantom {{\Delta a} {\Delta b}}} \right. \kern-\nulldelimiterspace} {\Delta b} $$

with a = any of the considered oxide and b = SiO2.

Δa and Δb can be simply calculated as:

$$ a_{{{\text{LAG}} - {\text{RG}}1}} - a_{{{\text{LAG}} - {\text{RG}}2}} $$

where a LAG-RG1 is the concentration of any oxide in the measured LAG residual glass representative of a given proportion of mixing, whereas a LAG-RG2 is the concentration of any oxide of the theoretical mixed term obtained from isenthalpic mixing simulations with MELTS representative of the same mixing proportion. Glasses of LAG layers are representative of the mixing, and the proportion chosen for the simulation are: 70% ABT and 30% DBM, according to Viccaro et al. (2006). Calculations were performed for all measured and calculated LAG residual glass compositions. However, in Table 5 are shown results only from sample 9 LAG 7-RG with Δa and Δb values calculated for each oxide.

Table 5 Compositions (molar proportion) chosen for “MINUIT©” calculations

As four mineral phases (excluding amphibole) characterize products of the 2001 eruption, we can express the slope on each diagram (Fig. 6) with the general relation:

$$ {\left( {{\Delta a} \mathord{\left/ {\vphantom {{\Delta a} {\Delta b}}} \right. \kern-\nulldelimiterspace} {\Delta b}} \right)}_{{{\text{M}}1}} X_{{{\text{M}}1}} + {\left( {{\Delta a} \mathord{\left/ {\vphantom {{\Delta a} {\Delta b}}} \right. \kern-\nulldelimiterspace} {\Delta b}} \right)}_{{{\text{M}}2}} X_{{{\text{M}}2}} + {\left( {{\Delta a} \mathord{\left/ {\vphantom {{\Delta a} {\Delta b}}} \right. \kern-\nulldelimiterspace} {\Delta b}} \right)}_{{{\text{M}}3}} X_{{{\text{M}}3}} + {\left( {{\Delta a} \mathord{\left/ {\vphantom {{\Delta a} {\Delta b}}} \right. \kern-\nulldelimiterspace} {\Delta b}} \right)}_{{{\text{M}}4}} X_{{{\text{M}}4}} = {\Delta a} \mathord{\left/ {\vphantom {{\Delta a} {\Delta b}}} \right. \kern-\nulldelimiterspace} {\Delta b} $$
(1)

where M1 = plagioclase (Plg), M2 = augite (Cpx), M3 = olivine (Ol), M4 = Ti-magnetite (Ti-mt), and X M1X M4 represent their molar proportions. An example on the nine LAG 7-RG of mineral phase compositions in molar proportion used in the calculation are reported in Table 5 (source of data from Viccaro et al. 2006). Taking as an example TiO2, from Eq. 1 we have:

$${\left( {{\Delta {\text{a}}_{{{\text{TiO2}}}} } \mathord{\left/ {\vphantom {{\Delta {\text{a}}_{{{\text{TiO2}}}} } {\Delta b}}} \right. \kern-\nulldelimiterspace} {\Delta b}_{{{\text{SiO}}2}} } \right)}X_{{{\text{Plg}}}} + {\left( {{\Delta {\text{a}}_{{{\text{TiO2}}}} } \mathord{\left/ {\vphantom {{\Delta {\text{a}}_{{{\text{TiO2}}}} } {\Delta b}}} \right. \kern-\nulldelimiterspace} {\Delta b}_{{{\text{SiO}}2}} } \right)}X_{{{\text{Cpx}}}} + {\left( {{\Delta {\text{a}}_{{{\text{TiO2}}}} } \mathord{\left/ {\vphantom {{\Delta {\text{a}}_{{{\text{TiO2}}}} } {\Delta b}}} \right. \kern-\nulldelimiterspace} {\Delta b}_{{{\text{SiO}}2}} } \right)}X_{{{\text{Ol}}}} + {\left( {{\Delta {\text{a}}_{{{\text{TiO2}}}} } \mathord{\left/ {\vphantom {{\Delta {\text{a}}_{{{\text{TiO2}}}} } {\Delta b}}} \right. \kern-\nulldelimiterspace} {\Delta b}_{{{\text{SiO}}2}} } \right)}X_{{{\text{Ti}} - {\text{mt}}}} = {\Delta {\text{a}}_{{{\text{TiO2}}}} } \mathord{\left/ {\vphantom {{\Delta {\text{a}}_{{{\text{TiO2}}}} } {\Delta b}}} \right. \kern-\nulldelimiterspace} {\Delta b}_{{{\text{SiO}}2}} $$

Taking into consideration the contribution of each mineral phase for each oxide we obtain the following matrix:

$$\begin{array}{*{20}l} {{{\text{TiO}}_{2} } \hfill}&{{{\left( {0.010} \right)}X_{{{\text{Plg}}}} + {\left( {0.100} \right)}X_{{{\text{Cpx}}}} + {\left( {0.010} \right)}X_{{{\text{Ol}}}} + {\left( {0.950} \right)}X_{{{\text{Ti}} - {\text{mt}}}} = 10} \hfill} \\ {{{\text{Al}}_{2} {\text{O}}_{3} } \hfill}&{{{\left( { - 4.728} \right)}X_{{{\text{Plg}}}} + {\left( { - 0.840} \right)}X_{{{\text{Cpx}}}} + {\left( 0 \right)}X_{{{\text{Ol}}}} + {\left( { - 0.984} \right)}X_{{{\text{Ti}} - {\text{mt}}}} = - 24} \hfill} \\ {{{\text{FeO}}} \hfill}&{{{\left( {0.352} \right)}X_{{{\text{Plg}}}} + {\left( {3.124} \right)}X_{{{\text{Cpx}}}} + {\left( {7.392} \right)}X_{{{\text{Ol}}}} + {\left( {34.408} \right)}X_{{{\text{Ti}} - {\text{mt}}}} = 44} \hfill} \\ {{{\text{MgO}}} \hfill}&{{{\left( 0 \right)}X_{{{\text{Plg}}}} + {\left( { - 0.573} \right)}X_{{{\text{Cpx}}}} + {\left( { - 1.482} \right)}X_{{{\text{Ol}}}} + {\left( { - 0.228} \right)}X_{{{\text{Ti}} - {\text{mt}}}} = - 3} \hfill} \\ {{{\text{CaO}}} \hfill}&{{{\left( { - 2.394} \right)}X_{{{\text{Plg}}}} + {\left( { - 3.206} \right)}X_{{{\text{Cpx}}}} + {\left( { - 0.056} \right)}X_{{{\text{Ol}}}} + {\left( { - 0.014} \right)}{\text{X}}_{{{\text{Ti}} - {\text{mt}}}} = - 14} \hfill} \\ {{{\text{Na}}_{2} {\text{O}}} \hfill}&{{{\left( { - 0.812} \right)}X_{{{\text{Plg}}}} + {\left( { - 0.028} \right)}X_{{{\text{Cpx}}}} + {\left( 0 \right)}X_{{{\text{Ol}}}} + {\left( 0 \right)}X_{{{\text{Ti}} - {\text{mt}}}} = - 28} \hfill} \\ {{{\text{K}}_{2} {\text{O}}} \hfill}&{{{\left( {0.044} \right)}X_{{{\text{Plg}}}} + {\left( {0.011} \right)}X_{{{\text{Cpx}}}} + {\left( {0.011} \right)}X_{{{\text{Ol}}}} + {\left( 0 \right)}X_{{{\text{Ti}} - {\text{mt}}}} = 11} \hfill} \\ {{{\text{P}}_{2} {\text{O}}_{5} } \hfill}&{{{\left( 0 \right)}X_{{{\text{Plg}}}} + {\left( 0 \right)}X_{{{\text{Cpx}}}} + {\left( 0 \right)}X_{{{\text{Ol}}}} + {\left( 0 \right)}X_{{{\text{Ti}} - {\text{mt}}}} = 2} \hfill} \\ \end{array} $$

Several minimization cycles performed with “MINUIT-Function minimization and error analysis (version 2.77 ©copyright CERN Geneva 1994–1998)” solve this matrix for the composition LAG 7-Sample 9 giving a total value of precipitated solid = 0.28 g for each gram of melt (plagioclase 92 wt%; olivine 8 wt%). Calculated masses of precipitated solid for LAG residual glasses range between 0.24 and 0.34 g. Microlites of plagioclase and olivine in the groundmass are in equilibrium with the compositions measured at phenocrysts rim. On the whole, plagioclase constitutes ∼92% of solid and entirely balances Al, Ca, and Na concentrations, whereas the scarce olivine balances the slight differences observed for Mg (Fig. 6). Repeated cycles of function minimization do not balance the concentrations for other oxides, thus implying that a late fractionation of any of the considered mineral phases cannot be responsible to produce such variability.

Appendix 2

An attempt to quantify the volatiles needed to cause the observed variability can be done considering the ΔCl in moles (0.003) between averaged values of chlorine in the non “volatile differentiated” ABT (0.19 wt% in CAL 1) and the “volatile differentiated” LAG (0.29 wt% in LAG 7) on the basis of concentrations reported in Table 1. The amount of “volatile differentiated” magma in the ABT chamber can be conservatively assumed as the total volume of the material erupted in the very final phase of the event after the July 30 at the Laghetto vent. The volume of the last lava flow corresponds to about 240 × 103 m3 (Ferlito and Siewert 2006). The DRE volume of tephra emitted in the last eruptive phase can be conservatively estimated assuming the thickness of the LAG 7 layer (10 m) and the dimension of the Laghetto scoria cone (Calvari and Pinkerton 2004) as no less than 260 × 103 m3. The total volume of the “volatile differentiated” magma (500 × 103 m3) corresponds to 6.1 × 1013 moles, on the basis of averaged oxide weight percent concentrations and the weighted proportion of mineral phases in LAG magma (Tables 2 and 3). For this amount of magma, the ΔCltot (the Cl needed to attain a concentration of 0.29 wt% starting from Cl 0.19 wt%) is equal to 1.8 × 1011 moles. The gas present in the system is here approximated to a multicomponent phase constituted by H2O + CO2 + Cl. The molar proportion of H2O (∼64 mol%) and CO2 (∼36 mol%) are derived using VOLATILECALC on the basis of volatiles concentration in melt inclusions (see “The role of volatiles migration” and Métrich et al. 2004) at P of 200 MPa and T 1,100°C. The Cl proportion in the gas phase is calculated considering the fluid/melt partitioning of Cl. The distribution of Cl between the fluid and the melt is generally expressed in the form of Nernst-type partition coefficients (fluid/melt D Cl) as:

$$ ^{{{{\text{fluid}}} \mathord{\left/ {\vphantom {{{\text{fluid}}} {{\text{melt}}}}} \right. \kern-\nulldelimiterspace} {{\text{melt}}}}} D_{{{\text{Cl}}}} = {^{{{\text{fluid}}}} C_{{{\text{Cl}}}} } \mathord{\left/ {\vphantom {{^{{{\text{fluid}}}} C_{{{\text{Cl}}}} } {^{{{\text{melt}}}} C_{{{\text{Cl}}}} }}} \right. \kern-\nulldelimiterspace} {^{{{\text{melt}}}} C_{{{\text{Cl}}}} } $$

Many authors have experimentally derived fluid/melt D Cl for mafic melts at pressure of our interest (200 MPa) providing a wide range of values (0.9–18; Webster et al. 1999; Signorelli and Carroll 2000; Métrich et al. 2001; Wehrmann 2005; Alletti et al. 2006; Bhalla et al. 2002). For mass balance calculations we chose an averaged value of fluid/melt D Cl = 10, similar to what was found by Signorelli and Carroll (2000) and Métrich et al. (2001), from which by using the average Cl concentration in CAL 1 glass we obtain a fluid C Cl of 1.9, representing the Cl concentration (mol%) in the fluid phase. As the Cl-enrichment should exclusively be induced by Cl in the fluid phase, we obtain the following relation:

$$ {\text{Fluid}}_{{{\text{mol}}}} = {\left( {{\Delta {\text{Cl}}_{{{\text{tot}}}} } \mathord{\left/ {\vphantom {{\Delta {\text{Cl}}_{{{\text{tot}}}} } {^{{{\text{fluid}}}} C_{{{\text{Cl}}}} }}} \right. \kern-\nulldelimiterspace} {^{{{\text{fluid}}}} C_{{{\text{Cl}}}} }} \right)} \times 100 = 1.0 \times 10^{{12}} {\text{moles}} $$

which can be assumed as the amount of volatiles needed to differentiate 6.1 × 1013 moles of magma emitted in the last phase of the Laghetto eruption, resulting in a fluid/melt molar ratio of ∼1:7.

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Ferlito, C., Viccaro, M. & Cristofolini, R. Volatile-induced magma differentiation in the plumbing system of Mt. Etna volcano (Italy): evidence from glass in tephra of the 2001 eruption. Bull Volcanol 70, 455–473 (2008). https://doi.org/10.1007/s00445-007-0149-y

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