Understanding how shallow magma is emplaced within volcanoes is crucial for hazard assessment. The 2002–2003 Stromboli eruption provides the opportunity to investigate shallow magma emplacement ...resulting from orthogonal feeder dikes and its possible effects. Stromboli erupted in 2002–2003, when effusive activity replaced Strombolian activity. On December 28, a NE‐SW fissure propagated from the lava‐filled northernmost summit crater. On December 29, a NW‐SE fissure propagated north of the craters, feeding NW‐SE aligned vents. On December 30, this area collapsed, reaching the sea and generating a tsunami. In mid February 2003, the NW‐SE fissure became inactive, while the NE‐SW effusive fissure continued until July. A model for shallow magma emplacement is proposed. The lateral propagation of a NE‐SW dike from the northernmost crater was triggered. Below, a NW‐SE dike, propagating from the magma‐filled NE tip of the NE‐SW elongated conduit, fed the NW‐SE aligned vents. In February, the conduit periphery became solidified, freezing the NW‐SE dike, and the transport of magma was limited to the central part, focusing its rise below the craters. This fed the NE‐SW fissure until the supply decreased further (July), returning to the ordinary level sustaining Strombolian activity. Orthogonal dike emplacement followed the trajectories of the maximum (gravitational) stress σ1, partly controlled by the irregular topography of the uppermost edifice. The emplacement of orthogonal dikes in a limited area is feasible at non‐perfectly conical active volcanoes, where the maximum gravitational stress may show variations from a purely radial path.
Radon anomalies are commonly observed prior to dynamic failure in the crust and are interpreted as cracking of the medium, thus attracting considerable attention in understanding the precursory ...phenomena of earthquakes and volcanic activity. In this study we have compared the starting radon emissions from low porosity crystalline lava (phonolite) samples with those from damaged and failed samples. The damaged sample was loaded up to just beyond the end of the linear elastic phase, as evidenced by the output of AE energy, the increase in total porosity and a decrease in P‐wave and S‐wave velocity relative to the intact sample. Whereas, the failed sample showed deformation behaviour characteristically brittle with increasing values of AE output and porosity as the sample approached macroscopic failure. Radon measurements have evidenced that dilatational microcracking of deformed sample produced no significant variation in radon emanation with respect to the intact sample. In contrast, after macroscopic failure, radon emanation drastically increased. Therefore, major finding from this study is that, in the case of low porosity and relatively high strength crystalline lavas, the development of a macroscopic fracture provides new large exhaling surface resulting in a substantial increase in radon emission rate.
Key Points
Radon emission from rocks increases by means of macroscopic fractures
Radon emission does no change in deformed low‐porosity crystalline lavas
Radon emission is governed by the different rock types
We document for the first time the role played by natural solidification paths on the partitioning of rare earth elements (REE) between clinopyroxene and melt. To do this, we investigated the ...compositional variation of clinopyroxenes formed under increasing cooling rate conditions from core to rim of a dike at Mt. Etna volcano. As the rate of cooling increases, clinopyroxenes are progressively depleted in Si + Ca + Mg counter-balanced by enrichments in Al + Na + Ti. Consequently, the concentration of REE in clinopyroxene increases due to an increased ease of locally balancing the excess charge at the M2 site as the number of surrounding tetrahedral aluminium atoms increases. Since Al
iv
in clinopyroxene is a charge-balancing cation for REE, the partition coefficients (D
REE
) measured at the dike chilled margin are distinctly higher than those from the dike interior. We conclude that, in naturally solidifying magmas, kinetically controlled cation substitution reactions can be treated in terms of the energetics of the various charge-imbalanced configurations. This finding is corroborated by the near-parabolic dependence of D
REE
on cation radius due to charge-balance mechanisms described by the lattice strain model.
We present results from phase equilibria experiments conducted on the most primitive pahoehoe “cicirara” trachybasaltic lava flow ever erupted at Mt. Etna Volcano. This lava is characterized by a ...pahoehoe morphology in spite of its high content of phenocrysts and microphenocrysts (>40 vol%) with the occurrence of centimetre-sized plagioclases (locally named cicirara for their chick-pea-like appearance). Our experiments have been performed at 400 MPa, 1100–1150 °C and using H
2
O and CO
2
concentrations corresponding to the water-undersaturated crystallization conditions of Etnean magmas. Results show that olivine does not crystallize from the melt, whereas titanomagnetite is the liquidus phase followed by clinopyroxene or plagioclase as a function of melt–water concentration. This mineralogical feature contrasts with the petrography of pahoehoe cicirara lavas suggesting early crystallization of olivine and late formation of titanomagnetite after plagioclase and/or in close association with clinopyroxene. The lack of olivine produces MgO-rich melt compositions that do not correspond to the evolutionary behaviour of cicirara magmas. Moreover, in a restricted thermal path of 50 °C and over the effect of decreasing water concentrations, we observe abundant plagioclase and clinopyroxene crystallization leading to trace element enrichments unlikely for natural products. At the same time, the equilibrium compositions of our mineral phases are rather different from those of natural cicirara phenocrysts and microphenocrysts. The comparison between our water-undersaturated data and those from previous degassing experiments conducted on a similar Etnean trachybasaltic composition demonstrates that pahoehoe cicirara lavas originate from crystal-poor, volatile-rich magmas undergoing abundant degassing and cooling in the uppermost part of the plumbing system and at subaerial conditions where most of the crystallization occurs after the development of pahoehoe surface crusts.
Sulfur diffusion in basaltic melts Freda, Carmela; Baker, Don R.; Scarlato, Piergiorgio
Geochimica et cosmochimica acta,
11/2005, Letnik:
69, Številka:
21
Journal Article
Recenzirano
We measured the diffusion coefficients of sulfur in two different basaltic melts at reduced conditions (i.e., in the sulfide stability field), temperatures from 1225°C to 1450°C, pressures of 0.5 and ...1 GPa, and water concentrations of 0 and 3.5 wt%. Although each melt is characterized by slightly different sulfur diffusion coefficients, the results can be combined to create a general equation for sulfur diffusion in anhydrous basalts:
D
=
2.19
×
10
−
4
exp
(
−
226.3
±
58.3
R
T
)
where D is the diffusion coefficient in m
2s
-1, the activation energy is in kJ mol
-1, R is the gas constant, and T is the temperature in K. Sulfur diffusion in basalts with 3.5 wt% water is a factor of three to seven higher than in anhydrous melts and can be described by:
D
=
5.91
×
10
−
7
exp
(
−
130.8
±
82.6
R
T
)
At the conditions of this study the pressure does not measurably affect sulfur diffusion. Sulfur diffusion in dry basaltic melts is one order of magnitude higher than sulfur diffusion in dry andesitic and dacitic melts, whereas sulfur diffusion in hydrous basaltic and andesitic melts is within the same order of magnitude. When compared to the diffusivity of other volatile species in nominally dry basaltic melts, sulfur diffusion appears to be two times lower than CO
2 diffusion and two orders of magnitude lower than H
2O diffusion.
We report on the newly discovered lava flow that erupted in the Colli Albani Volcanic District, which is the most recent and, geochemically the most peculiar effusive event recognised in the entire ...ultrapotassic Roman Province (Central Italy).
This lava flow is associated with the Monte Due Torri scoria cone, located approximately 5km south of the Albano hydromagmatic centre (69–36ka). The Monte Due Torri scoria cone displays well-preserved morphological characteristics and the 40±7ka age determined for the associated lava flow indicates that its activity was nearly contemporaneous to the most recent, explosive activity that occurred at the Albano centre from 41 to 36ka.
By comparing chemical and petrological features of the Monte Due Torri lava flow, Albano products, and older products (>69ka), we show that the youngest Colli Albani eruptions were fed by two new batches of parental magmas that originated in a phlogopite-bearing metasomatised mantle, each one feeding one of the two youngest eruptive cycles (at 69ka and 41–36ka). The trace element signature, e.g., very low Pb content, of primitive (MgO>3wt.%) magmas feeding the initiation of the hydromagmatic activity at Albano (69ka) and the subsequent effusive activity at Monte Due Torri (40ka) indicates that a magma chamber located in the deep anhydrite-bearing dolomite formation was tapped. However, the polygenic activity, the changes in magma composition, and the variable thermometamorphic clasts occurring in the hydromagmatic deposits (recording variable substrata) suggest, particularly for the Albano eruptive centre, a more complex plumbing system consisting of at least two more magma chambers at a shallower depth, i.e., in the Mesozoic limestone and Pliocene pelite formations.
The large amount of stratigraphic, volcanological, and geochemical data collected for the Colli Albani Volcanic District, one of the main districts in the ultrapotassic Roman Province, enable us to contribute insights into the still open debate regarding the temporal variation of the metasomatised mantle source of the Italian potassic magmas. Based on our data, i.e., variation of radiogenic and trace elements over time, we suggest that the observed variation in the mantle source of the ultrapotassic magmas can be related to progressive consumption of the phlogopite component in the metasomatised source rather than the transition from lithosphere- to asthenosphere-derived magmatism and/or the transition from orogenic to anorogenic magmatism.
► Time-dependent geochemical variations in ultrapotassic magmas. ► Phlogopite exhaustion in the metasomatised mantle. ► Dolomite assimilation increasing sulphur activity. ► Pb trough in the spider diagram.
On 5 July 2014, an eruptive fissure opened on the eastern flank of Etna volcano (Italy) at ~3.000 m a.s.l. Strombolian activity and lava effusion occurred simultaneously at two neighbouring vents. In ...the following weeks, eruptive activity led to the build-up of two cones, tens of meters high, here named Crater N and Crater S. To characterize the short-term (days) dynamics of this multi-vent system, we performed a multi-parametric investigation by means of a dense instrumental network. The experimental setup, deployed on July 15-16th at ca. 300 m from the eruption site, comprised two broadband seismometers and three microphones as well as high speed video and thermal cameras. Thermal analyses enabled us to characterize the style of eruptive activity at each vent. In particular, explosive activity at Crater N featured higher thermal amplitudes and a lower explosion frequency than at Crater S. Several episodes of switching between puffing and Strombolian activity were noted at Crater S through both visual observation and thermal data; oppositely, Crater N exhibited a quasi-periodic activity. The quantification of the eruptive style of each vent enabled us to infer the geometry of the eruptive system: a branched conduit, prone to rapid changes of gas flux accommodated at the most inclined conduit (i.e. Crater S). Accordingly, we were able to correctly interpret acoustic data and thereby extend the characterization of this two-vent system.
Viscosities of shoshonitic and latitic melts, relevant to the Campi Flegrei caldera magmas, have been experimentally determined at atmospheric pressure and 0.5
GPa, temperatures between 840
K and ...1870
K, and H
2O contents from 0.02 to 3.30
wt.%.
The concentric cylinder technique was employed at atmospheric pressure to determine viscosity of nominally anhydrous melts in the viscosity range of 10
1.5
−
10
3
Pa
s. The micropenetration technique was used to determine the viscosity of hydrous and anhydrous melts at atmospheric pressure in the high viscosity range (10
10
Pa
s). Falling sphere experiments were performed at 0.5
GPa in the low viscosity range (from 10
0.35 to 10
2.79
Pa
s) in order to obtain viscosity data of anhydrous and hydrous melts. The combination of data obtained from the three different techniques adopted permits a general description of viscosity as a function of temperature and water content using the following modified VFT equation:
l
o
g
η
=
−
a
+
b
T
−
c
+
d
T
−
e
⋅
e
x
p
g
⋅
w
T
where
η is the viscosity in Pa·s,
T the temperature in K,
w the H
2O content in wt.%, and a, b, c, d, e, and g are the VFT parameters. This model reproduces the experimental data (95 measurements) with a 1σ standard deviation of 0.19 and 0.22 log units for shoshonite and latite, respectively. The proposed model has been applied also to a more evolved composition (trachyte) from the same area in order to create a general model applicable to the whole compositional range of Campi Flegrei products.
Moreover, speed data have been used to constrain the ascent velocity of latitic, shoshonitic, and trachytic melts within dikes. Using petrological data and volcanological information (geometrical parameters of the eruptive fissure and depth of magma storage), we estimate a time scale for the ascent of melt from 9
km to 4
km depth (where deep and shallow reservoirs, respectively, are located) in the order of few minutes. Such a rapid ascent should be taken into account for the hazard assessment in the Campi Flegrei area.
► Viscosities of shoshonitic and latitic melts relevant to the Campi Flegrei caldera magmas have been measured. ► Micropenetration, concentric cylinder and falling sphere were adopted to determine viscosity of dry and hydrous melts. ► The combination of experimental data allowed us create a general model of viscosity using the modified VFT equation. ► The proposed model has been applied to Campi Flegrei products.►Viscosity data have been used to constrain the ascent velocity of melts within dikes.
Eruptive scenarios associated with the possible reactivation of maar-forming events in the Quaternary, ultrapotassic Colli Albani Volcanic District (CAVD) provides implications for volcanic hazard ...assessment in the densely populated area near Rome. Based on detailed stratigraphy, grain size, componentry, ash morphoscopy and petro-chemical analyses of maar eruption products, along with textural analysis of cored juvenile clasts, we attempt to reconstruct the eruptive dynamics of the Prata Porci and Albano maars, as related to pre- and syn-eruptive interactions between trachybasaltic to K-foiditic feeder magmas and carbonate–silicoclastic and subvolcanic country rocks. Magma volumes in the order of 0.5–3.1
×
10
8 m
3 were erupted during the monogenetic Prata Porci maar activity and the three eruptive cycles of the Albano multiple maar, originating loose to strongly lithified, wet and dry pyroclastic surge deposits, Strombolian scoria fall horizons and lithic-rich explosion breccias. These deposits contain a wide range of accessory and accidental lithic clasts, with significant vertical stratigraphic variations in the lithic types and abundances. The two maar study cases hold a record of repeated transitions between magmatic (i.e, Strombolian fallout) and hydromagmatic (wet and dry pyroclastic surges) activity styles. Evidence of phreatic explosions, a common precursor of explosive volcanic activity, is only found at the base of the Prata Porci eruptive succession. The quantitative evaluation of the proportions of the different eruptive styles in the stratigraphic record of the two maars, based on magma vs. lithic volume estimates, reveals a prevailing magmatic character in terms of erupted magma volumes despite the hydromagmatic footprint. Different degrees of explosive magma–water interaction were apparently controlled by the different hydrogeological and geological–structural settings. In the Prata Porci case, shifts in the depth of magma fragmentation are proposed to have accompanied eruption style changes. In the Albano case, a deeply dissected geothermal aquifer in peri-caldera setting and variable mass eruption rates were the main controlling factors of repeated shifts in the eruptive style. Finally, textural evidence from cored juvenile clasts and analytical modeling of melt–solid heat transfer indicate that the interacting substrate in the Prata Porci case was at low, uniform temperature (~ 100 °C) as compared to the highly variable temperatures (up to 700–800 °C) inferred for the geothermal system beneath Albano.
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