*Physical Volcanology and Facies Analysis of Silicic Lavas: Monte Amiata Volcano (Italy) DOI: http://dx.doi.org/10.5772/intechopen.108348*


*magmatic enclave. Labels of SLLF units as in Figure 2.*

#### **Table 4.**

*Microscopic petrographic characteristics of Monte Amiata SLLFs.*

lava flow dynamics. Our results all show that the lavas emitted at Monte Amiata represent, in terms of temperature, an extreme end member for each of the compositions and mineral assemblages investigated here.

On the basis of the results provided in **Table 5**, we can distinguish two sample subsets: (1) a first group (PES, preeruptive stage) characterized by crystalline liquid balances associated with a phase of initial crystallization, where the liquid is the total rock, representative of a preeruptive or initial phase of ascent; (2) a second group (ES, eruptive stage) where the liquid phase in equilibrium with a specific crystalline phase is the interstitial fluid. This group has been related to a late phase of the crystallization occurred during the ascent of the magma and/or during the mass emplacement of lavas. The PES group shows temperature estimates between 900°C and 1070°C, whereas ES group has temperature range between 800°C and 900°C. These temperature ranges constitute the entire temperature interval provided by the employment of all the geothermometers analyzed and constitute therefore an enlarged estimate of the real intervals of expected temperature.

The estimation of dissolved water content (hereafter reported as H2O and expressed in wt%) is of fundamental importance to comprehend the storage and *Physical Volcanology and Facies Analysis of Silicic Lavas: Monte Amiata Volcano (Italy) DOI: http://dx.doi.org/10.5772/intechopen.108348*

#### **Figure 11.**

*(a) Highly porphyritic texture: The phenocrysts are fragmented crystals of K-feldspar, plagioclase, orthopyroxene, biotite set in a glassy groundmass (optical microscopic image crossed polarized, unit Q, sample AMT 14–51); (b) broken crystals of K-feldspar, zoned plagioclase, orthopyroxene, biotite, and resorbed quartz set in a perlitic glassy groundmass (BSE-SEM image, unit L, sample AMT 14–71b); (c) glassy groundmass, microlite-free, with perlitic texture (BSE-SEM image, unit L, sample AMT 13–10b); (d) glassy groundmass with ultra-microlites (probably Fe-Ti oxides) aligned to the flow direction (BSE-SEM image, unit L, sample AMT 13–10b); (e) devitrified groundmass with spherulites (optical microscopic image plane polarized, unit P, sample AMT 14–54); (f) heterogeneous glassy groundmass with perlitic texture and darker flow bands (optical microscopic image plane polarized, unit L, sample AMT 14-71b); (g) glassy groundmass with perlitic texture and flow band enriched in crystal fragments (BSE-SEM image, unit L, sample AMT 13–10); (h) highly vesicular glassy groundmass with large flattened vesicles (BSE-SEM image, unit L, sample AMT 13–10). Labels of SLLFs as in Figure 2. bt: biotite; gdm: groundmass; K-fs: K-feldspar; opx: orthopyroxene; plg: plagioclase; qz: quartz, xts: crystals.*


*Temperature calculated by using ([103, 104]; Eqs. 23, 24a, 26) plagioclase model for only the Plg-liquid couples which passed equilibrium test (2.2.1.). GM and TR are the groundmass liquids and the total rock liquids, respectively. Talk1, Talk2,Talk3, and Talk4 are the temperature intervals calculated for each Cpx-liquid equilibrium combination which passed the equilibrium tests. Temperature values referred to as Talk1,Talk2,Talk3, and Talk4 correspond to the recalibration of [105] models of [106]. The apatite geothermometer, reported as apatite, is the value calculated for the [104] geothermometer. More details can be found in [102]. A temperature interval between 900°C and 1070°C is assumed to be ideal for the initial crystallization at depth, whereas a lower temperature interval (800°C and 900°C) is likely characteristic of the late crystallization condition at the shallow levels, emplacement, or post-emplacement. \* Reported equations are as from [103, 104].*

#### **Table 5.**

*Summary of the minimum and maximum temperature values estimated by using the geothermometers proposed by [102] for the SLLF of the BAS and MAS.*

ascent conditions of magma, as well as the eruption style and emplacement of the volcanic products. H2O, in fact, more than any other component, affects the physical properties of the magmatic and volcanic materials as well as the associated processes (diffusivity, crystallization, and degassing). One weight percent of H2O dissolved in a SiO2-rich magma may change its viscosity of ca. 6 orders of magnitude and its glass transition temperature (i.e., the temperature at which, upon certain stress conditions, there a transition between a viscous to a brittle mechanical behavior occurs) of 200°C.

Since there are no data on the CO2 concentration of melt inclusions, the H2O-CO2 saturation model of [108] (https://melts.ofm-research.org/CORBA\_CTserver/GG-H2O-CO2.html) was used to estimate the fluid content of the magma stored in the Monte Amiata magma chamber positioned at 6 km depth [49, 109, 110], where the expected external pressure is 106 MPa and temperature is 900–1070°C (see before). The average oxide percentage was considered (**Table 3**) in calculations. The H2O and CO2 mole fractions in the fluid phase were varied to compute the H2O and CO2 in the coexisting melt at 900–950°C, whereas no results were obtained at higher temperatures. Outcomes are reported in **Figure 12**, showing that maximum H2O concentration is 3.96–3.98 wt% for zero CO2 and maximum CO2 concentration is ca. 500 ppm for zero H2O.

Of course, the H2O and CO2 concentrations in the deep magma might be everywhere along these lines. Nevertheless, it must be recalled that in the Monte Amiata edifice and surrounding area, a strong degassing of magmatic/mantellic CO2 occurs as indicated by high CO2 fluxes from soil [111, 112], the presence of CO2 in the fluid inclusions [113], and the composition of geothermal fluids ([114]; and references therein).

Therefore, owing to this occurrence of magmatic/mantellic CO2, the magma in the Monte Amiata chamber might be saturated with CO2 and consequently water concentration would be low or even very low, although this possibility is a hypothesis to be proven or rejected by means of melt inclusions data. If this hypothesis is true, CO2 would be readily lost upon degassing and H2O concentration would weakly decrease, thus explaining the lack of explosive activity at Monte Amiata.

*Physical Volcanology and Facies Analysis of Silicic Lavas: Monte Amiata Volcano (Italy) DOI: http://dx.doi.org/10.5772/intechopen.108348*

**Figure 12.**

*CO2 vs. H2O coexistence in melt calculated by means of the saturation model of [108] for temperature of 900°C (blue line and dots) and 950°C (red line and dots).*
