Gabriele Lanzafame | Elettra- Sincrotrone Trieste (original) (raw)

Papers by Gabriele Lanzafame

Lithos, 2014

At the Mount Etna volcano (Italy) the massive release of magmatic gasses (especially H 2 O, CO 2 ... more At the Mount Etna volcano (Italy) the massive release of magmatic gasses (especially H 2 O, CO 2 and SO 2 ) during explosive eruptions and through the persistent gas plume, raises important issues: i) the volume of magma erupted at given periods is insufficient to feed the gas plume of the related periods; ii) gas-dominated, explosive eruptions do not emit differentiated products, but relatively Mg-rich magmas; iii) H 2 O measured in melt inclusions (~3.5 wt.% at about 0.4 GPa) exceeds what is expected from intraplate mantle-derived melts (b1.4 wt.%). Literature data on melt inclusions (MIs) along with textural and compositional analyses of plagioclase and phase stability constrained by MELT calculations, enabled us to build a model that reconciles the abovementioned discrepancies with the high variability of the water content along the Etnean feeding system. We propose that along an open magma conduit, continuous gas loss from the free surface of magma at depth promotes an almost steady stream of H 2 O-rich fluid extending well below the gas saturation depth. The velocity of volatile migration might be enhanced by the huge amount of CO 2 present at Mount Etna, because the CO 2 exsolved at high-pressure conditions may act as carrier for other volatile species. In this way the H 2 O-undersaturated primitive magma present in the plumbing system, may undergo a volatile "flushing" proportional to the residing time. This process is fundamental to increase the originally low H 2 O content of primitive magma (≤1.4 wt.%) allowing it to overcome the saturation threshold, to exsolve as gas and to promote the eruption of Mg-rich lavas. Such mechanism would also account for the unexpectedly high amount of magmatic water released during non-eruptive periods.

Earth-Science Reviews, 2014

Plagioclase is the most common phenocryst in all Etnean magmatic suites (~50% in volume), as well... more Plagioclase is the most common phenocryst in all Etnean magmatic suites (~50% in volume), as well as in most lavas erupted worldwide. Its stability field is strongly dependent on the physico-chemical conditions of the melt and, consequently, it can be used as a tool to record the processes occurring within the feeding system. With this aim, a detailed textural and compositional study of plagioclase was performed on the products emitted during the eruptions. Four distinct textures were recognized at the crystal cores: (1) clear and rounded (An 73-85 ), (2) dusty and rounded (An 73-85 ), (3) sieved (An 82-88 ) and (4) patchy (An 60-81 ), while two distinct textures are commonly observed at the crystal rim: (1) dusty (An 73-90 ) and (2) with melt inclusion alignments (An 70-76 ). Moreover all plagioclases present a thin (10-20 μm) outermost less calcic (An 53-76 ) rim. For each crystal a complex evolutionary path was reconstructed, and several growth and resorption episodes were identified. The fO 2 was estimated using Plag-Cpx/liquid equilibrium in order to calculate the Fe +3 /Fe 2+ ratio in the melt and, in turn, to reconstruct the primitive magma composition by adding a wehrlitic assemblage to the least evolved lava of the four eruptive episodes. MELTS modeling was then developed using this primary magma composition, as well as a trachybasaltic lava. Calculations were performed at variable pressures (400-50 MPa, step of 0.50 MPa) and H 2 O contents (3.5-0 wt.%, step 0.5 wt.%) in order to estimate the crystallization temperature of olivine, clinopyroxene, plagioclase and spinel, decreasing T from the liquidus down to 1000°C at steps of 20°C. P-T and water contents were also determined using geothermobarometers and plagioclase-melt hygrometers respectively, aiming at verifying the parameters used in the MELTS modeling. At this point plagioclase textural features and compositions were related to specific P-T-fO 2 -H 2 O conditions. Plagioclase stability models indicate that: (1) H 2 O strongly influences the plagioclase-melt equilibrium allowing the crystallizations of more calcic compositions only at shallow levels; (2) patchy cores form at high pressure (up to 350 MPa) and low water content (b 1.7 wt.%); (3) clear dissolved cores form at lower pressure (150 MPa) and higher water content (1.5-2.8 wt.%); (4) dusty rims form at even lower pressure straddling the H 2 O-saturation curve and, (5) melt alignments form during degassing. According to experimental works each of these textures can be related to a different process within the feeding system, such as multiple magma inputs (patchy core), volatile addition or increase in T (clear core), mixing (dusty rims) and rapid decompression and degassing (melt inclusion alignment at rims). These inferences were successfully compared with the eruptive evolution of each event as deduced from direct observations, and geophysical and petrological data. The overall picture shows that plagioclase crystallizes under polybaric conditions in a vertically extended and continuous feeding system in which at least two magma crystallization levels were identified. Plagioclase stability also indicates that a large variability in water content characterizes the magma within the feeding system.

Earth-Science Reviews, 2014

Plagioclase is the most common phenocryst in all Etnean magmatic suites (~50% in volume), as well... more Plagioclase is the most common phenocryst in all Etnean magmatic suites (~50% in volume), as well as in most lavas erupted worldwide. Its stability field is strongly dependent on the physico-chemical conditions of the melt and, consequently, it can be used as a tool to record the processes occurring within the feeding system. With this aim, a detailed textural and compositional study of plagioclase was performed on the products emitted during the eruptions. Four distinct textures were recognized at the crystal cores: (1) clear and rounded (An 73-85 ), (2) dusty and rounded (An 73-85 ), (3) sieved (An 82-88 ) and (4) patchy (An 60-81 ), while two distinct textures are commonly observed at the crystal rim: (1) dusty (An 73-90 ) and (2) with melt inclusion alignments (An 70-76 ). Moreover all plagioclases present a thin (10-20 μm) outermost less calcic (An 53-76 ) rim. For each crystal a complex evolutionary path was reconstructed, and several growth and resorption episodes were identified. The fO 2 was estimated using Plag-Cpx/liquid equilibrium in order to calculate the Fe +3 /Fe 2+ ratio in the melt and, in turn, to reconstruct the primitive magma composition by adding a wehrlitic assemblage to the least evolved lava of the four eruptive episodes. MELTS modeling was then developed using this primary magma composition, as well as a trachybasaltic lava. Calculations were performed at variable pressures (400-50 MPa, step of 0.50 MPa) and H 2 O contents (3.5-0 wt.%, step 0.5 wt.%) in order to estimate the crystallization temperature of olivine, clinopyroxene, plagioclase and spinel, decreasing T from the liquidus down to 1000°C at steps of 20°C. P-T and water contents were also determined using geothermobarometers and plagioclase-melt hygrometers respectively, aiming at verifying the parameters used in the MELTS modeling. At this point plagioclase textural features and compositions were related to specific P-T-fO 2 -H 2 O conditions. Plagioclase stability models indicate that: (1) H 2 O strongly influences the plagioclase-melt equilibrium allowing the crystallizations of more calcic compositions only at shallow levels; (2) patchy cores form at high pressure (up to 350 MPa) and low water content (b 1.7 wt.%); (3) clear dissolved cores form at lower pressure (150 MPa) and higher water content (1.5-2.8 wt.%); (4) dusty rims form at even lower pressure straddling the H 2 O-saturation curve and, (5) melt alignments form during degassing. According to experimental works each of these textures can be related to a different process within the feeding system, such as multiple magma inputs (patchy core), volatile addition or increase in T (clear core), mixing (dusty rims) and rapid decompression and degassing (melt inclusion alignment at rims). These inferences were successfully compared with the eruptive evolution of each event as deduced from direct observations, and geophysical and petrological data. The overall picture shows that plagioclase crystallizes under polybaric conditions in a vertically extended and continuous feeding system in which at least two magma crystallization levels were identified. Plagioclase stability also indicates that a large variability in water content characterizes the magma within the feeding system.

Lithos, 2014

ABSTRACT Basaltic lava flows can form tubes in response to the cooling of the outer surface. We c... more ABSTRACT Basaltic lava flows can form tubes in response to the cooling of the outer surface. We collected lava stalactites (frozen lava tears) and sampled lava from the ceilings of three lava tubes on Mount Etna. Comparison of the petrographic characters between ceiling lavas and relative stalactites reveal surprising differences in the groundmass textures and crystal compositions. Major and trace element contents in stalactites show only a slight increase in alkali and SiO2 compared to ceiling lava, whereas significant differences exist in composition and textures between plagioclases within the ceiling lava and those within the stalactites, being in the last case definitively more An-rich. We advance the hypothesis that the high temperature reached in the cave caused the exsolution of the volatiles still trapped in the dripping melt. The volatiles, mainly H2O, formed bubbles and escaped from the melt; such a water-loss might have promoted the silicate polymerization in the stalactites resulting in the growth of An-rich plagioclase phenocrysts. Our results have important implications: in fact plagioclase phenocrysts are usually associated with intratelluric growth and are often considered as the main petrologic evidence for the existence of a magma chamber. The textural and chemical features of plagioclases in stalactites prove that phenocrysts growth in syn to post-eruptive conditions is plausible and clearly explain the relatively low viscosity of many phenocrysts-rich lava flows on Mount Etna, as well as on many other volcanoes around the world. Therefore, we can conclude that plagioclase phenocrysts cannot exclusively be considered as having originated within a magma chamber.

The solidified surface of a lava flow reflects the viscosity of its molten fraction and the cryst... more The solidified surface of a lava flow reflects the viscosity of its molten fraction and the crystal content during flow; crystal-poor basaltic lavas produce pahoehoe fields, whereas crystal-rich ones solidify with aa carapaces. At Mount Etna, volcano aa morphologies are very common, whereas pahoehoe lavas are rare. The latter are locally named "cicirara" due to the presence of centimeter-sized plagioclase phenocrysts much more abundant than in aa lavas. The phenocryst content of "cicirara" lavas contrasts with the low viscosity generally associated with pahoehoe morphology. Therefore, to reconcile the discrepancy between textural and volcanic observations, we have studied the most primitive pahoehoe "cicirara" lava sampled until now. Two samples at 0.5 and 1 m from the bottom of the 2m thick lava flow were investigated on the basis of their mineral compositional variations and textural features, i.e., size frequency and crystal size distribution (CSD). Results coupled with rheological models indicate that only large phenocrysts of plagioclase (>1 mm) and clinopyroxene have grown before eruption. Thermobarometric models and petrological computations based on the composition of plagioclase and clinopyroxene phenocryst cores highlight that only a small amount (10-15 vol.%) of crystals equilibrated at 12 km of depth. Cumulative size frequency and CSD data also indicate that plagioclase and clinopyroxene phenocryst rims grew heterogeneously and coalesced around their cores at depths <1 km, before eruption. In this view, the "cicirara" lava was erupted with a low crystalline content that favoured the formation of its pahoehoe surface; however, crystals with a size <1 mm (~75 vol.%) solidified at post-eruptive conditions. Our findings underline that the emplacement of high-viscosity aa or low-viscosity pahoehoe lavas is driven by the degree of undercooling imposed by the volatile exsolution rate in the shallowest portion of the Etnean plumbing system. A slow magma ascent rate promotes significant intratelluric degassing and widespread nucleation; consequently, the viscosity of the suspension significantly increases leading to an aa morphology. In contrast, pahoehoe "cicirara" lavas are associated with a rapid rise to the surface of poorly degassed, undercooled magmas.

Lithos, 2014

At the Mount Etna volcano (Italy) the massive release of magmatic gasses (especially H 2 O, CO 2 ... more At the Mount Etna volcano (Italy) the massive release of magmatic gasses (especially H 2 O, CO 2 and SO 2 ) during explosive eruptions and through the persistent gas plume, raises important issues: i) the volume of magma erupted at given periods is insufficient to feed the gas plume of the related periods; ii) gas-dominated, explosive eruptions do not emit differentiated products, but relatively Mg-rich magmas; iii) H 2 O measured in melt inclusions (~3.5 wt.% at about 0.4 GPa) exceeds what is expected from intraplate mantle-derived melts (b1.4 wt.%). Literature data on melt inclusions (MIs) along with textural and compositional analyses of plagioclase and phase stability constrained by MELT calculations, enabled us to build a model that reconciles the abovementioned discrepancies with the high variability of the water content along the Etnean feeding system. We propose that along an open magma conduit, continuous gas loss from the free surface of magma at depth promotes an almost steady stream of H 2 O-rich fluid extending well below the gas saturation depth. The velocity of volatile migration might be enhanced by the huge amount of CO 2 present at Mount Etna, because the CO 2 exsolved at high-pressure conditions may act as carrier for other volatile species. In this way the H 2 O-undersaturated primitive magma present in the plumbing system, may undergo a volatile "flushing" proportional to the residing time. This process is fundamental to increase the originally low H 2 O content of primitive magma (≤1.4 wt.%) allowing it to overcome the saturation threshold, to exsolve as gas and to promote the eruption of Mg-rich lavas. Such mechanism would also account for the unexpectedly high amount of magmatic water released during non-eruptive periods.

Lithos, 2010

The enrichment in potassium shown by the basic lavas erupted at Mount Etna volcano after 1971 (K ... more The enrichment in potassium shown by the basic lavas erupted at Mount Etna volcano after 1971 (K 2 O max~2.2 wt.%) has been considered by previous researchers to be too high to be related to simple crystal fractionation and instead linked this high K 2 O content to either crustal assimilation or changes in the magma source. Unfortunately all existing models for the post-1971 K 2 O enrichment fail to explain the phenomenon satisfactorily leaving the question still open. We present a critical re-examination of published data for major elements (633 whole rock analyses), trace elements (376 whole rock analyses) and isotopic ratios (136 87 Sr/ 86 Sr analyses), for historical and prehistorical lavas. Potassium enrichment is not limited to the products of the last 35 years. A comparable increase in potassium is noticed in lavas erupted during the pre-historic phase of the recent Mongibello (K 2 O max~2.5 wt.%) and in lavas related to the early phase of the ancient Mongibello (K 2 O max~3 wt.%). Moreover, data from melt inclusions in olivines from the 2001 and 2002 eruptions, reveal that potassium contents remain constant for melts with entrapment pressure between 490 to 100 MPa and increase significantly in melts entrapped at pressures below~100 MPa. We propose that supercritical fluids coming from deeper magmas and carrying alkali Cl-complexes migrate through basic to intermediate magmas residing in the shallow feeding system. As chlorine exsolves and leaves the system alkalis are released contributing to the observed potassium enrichment of the shallow magma. Fluctuations of the volatiles influx throughout time are likely related to the magma supply rate. Considering that the amount of magma entering a plumbing system is determined by the rate of regional extension, the flux of alkali Cl-complexes entering the melt might be related to an extensional regime acting in the Etnean area.

Chemical Geology, 2011

Seven rock samples were systematically collected from innermost to the outermost portion of a dik... more Seven rock samples were systematically collected from innermost to the outermost portion of a dike outcropping at Mt. Etna volcano. Results show that, from dike core-to-rim, plagioclase, clinopyroxene and titanomagnetite show compositional variations due to increasing cooling rate. Plagioclase is progressively enriched in An from innermost to the outermost part of the dike. Similarly, clinopyroxene components En + CaTs + CaFeTs increase, whereas Di + Hd decrease. The Usp content in titanomagnetite also systematically decrease from dike core-to-rim. Partition coefficients and thermometers based on the crystal-liquid exchange reaction indicate that, due to rapid cooling rates at the dike outer portions, early-formed crystal nuclei do not re-equilibrate with the melt. The chemistry of minerals progressively deviates from that of equilibrium; consequently, from dike core-to-rim, mineral compositions resemble those of high-temperature formation. The chemical variations of clinopyroxene and plagioclase in dike samples mirror those obtained from cooling experiments carried out on alkaline basalts. Accordingly, we used an experimental equation based on clinopyroxene compositional variation as a function of cooling rate to determine the cooling conditions experienced by the crystals during dike emplacement. The estimated cooling rates are comparable to those predicted by thermal modeling based on an explicit finite-difference scheme.

ABSTRACT We report results from geochemical and isotopic analyses conducted on nine samples colle... more ABSTRACT We report results from geochemical and isotopic analyses conducted on nine samples collected from the vertical section of a pahoehoe lava flow unit from Mt. Etna volcano. Textural observations in the field show that, during lava emplacement, volatile exsolution and degassing allowed the nucleation and growth of bubbles, which subsequently rose from the base towards the uppermost lava crust. The amount of phenocrysts is low (about 12 vol.%) and their compositions are comparable to intratelluric crystals analyzed for from historical and recent products. In contrast, from the basal zone to the uppermost lava crust, groundmass microlites are characterized by progressively more primitive compositions, i.e., olivines and clinopyroxenes show increasing Mg#, plagioclases are enriched in anorthite, and the ulvospinel content in titanomagnetites increases. Calculations based on thermodynamic models, thermometers and oxygen barometers indicate that, during post-eruption conditions, crystals formed at higher crystallization temperatures with increasing vertical height. The redox state of the melt progressively increased from the base towards the uppermost crust of the lava flow as is attested by increasing Fe2O3/FeO ratios in clinopyroxene and titanomagnetite. The lowest fO2 is recorded at the basal zone and suggests that this part of the lava was inaccessible to diffusion of atmospheric oxygen at the time of emplacement; whereas, the highest fO2 measured for the uppermost lava crust testifies to post-eruptive hydrogen loss, mainly transported by carrier gas phases.

Lithos, 2014

At the Mount Etna volcano (Italy) the massive release of magmatic gasses (especially H 2 O, CO 2 ... more At the Mount Etna volcano (Italy) the massive release of magmatic gasses (especially H 2 O, CO 2 and SO 2 ) during explosive eruptions and through the persistent gas plume, raises important issues: i) the volume of magma erupted at given periods is insufficient to feed the gas plume of the related periods; ii) gas-dominated, explosive eruptions do not emit differentiated products, but relatively Mg-rich magmas; iii) H 2 O measured in melt inclusions (~3.5 wt.% at about 0.4 GPa) exceeds what is expected from intraplate mantle-derived melts (b1.4 wt.%). Literature data on melt inclusions (MIs) along with textural and compositional analyses of plagioclase and phase stability constrained by MELT calculations, enabled us to build a model that reconciles the abovementioned discrepancies with the high variability of the water content along the Etnean feeding system. We propose that along an open magma conduit, continuous gas loss from the free surface of magma at depth promotes an almost steady stream of H 2 O-rich fluid extending well below the gas saturation depth. The velocity of volatile migration might be enhanced by the huge amount of CO 2 present at Mount Etna, because the CO 2 exsolved at high-pressure conditions may act as carrier for other volatile species. In this way the H 2 O-undersaturated primitive magma present in the plumbing system, may undergo a volatile "flushing" proportional to the residing time. This process is fundamental to increase the originally low H 2 O content of primitive magma (≤1.4 wt.%) allowing it to overcome the saturation threshold, to exsolve as gas and to promote the eruption of Mg-rich lavas. Such mechanism would also account for the unexpectedly high amount of magmatic water released during non-eruptive periods.

Earth-Science Reviews, 2014

Plagioclase is the most common phenocryst in all Etnean magmatic suites (~50% in volume), as well... more Plagioclase is the most common phenocryst in all Etnean magmatic suites (~50% in volume), as well as in most lavas erupted worldwide. Its stability field is strongly dependent on the physico-chemical conditions of the melt and, consequently, it can be used as a tool to record the processes occurring within the feeding system. With this aim, a detailed textural and compositional study of plagioclase was performed on the products emitted during the eruptions. Four distinct textures were recognized at the crystal cores: (1) clear and rounded (An 73-85 ), (2) dusty and rounded (An 73-85 ), (3) sieved (An 82-88 ) and (4) patchy (An 60-81 ), while two distinct textures are commonly observed at the crystal rim: (1) dusty (An 73-90 ) and (2) with melt inclusion alignments (An 70-76 ). Moreover all plagioclases present a thin (10-20 μm) outermost less calcic (An 53-76 ) rim. For each crystal a complex evolutionary path was reconstructed, and several growth and resorption episodes were identified. The fO 2 was estimated using Plag-Cpx/liquid equilibrium in order to calculate the Fe +3 /Fe 2+ ratio in the melt and, in turn, to reconstruct the primitive magma composition by adding a wehrlitic assemblage to the least evolved lava of the four eruptive episodes. MELTS modeling was then developed using this primary magma composition, as well as a trachybasaltic lava. Calculations were performed at variable pressures (400-50 MPa, step of 0.50 MPa) and H 2 O contents (3.5-0 wt.%, step 0.5 wt.%) in order to estimate the crystallization temperature of olivine, clinopyroxene, plagioclase and spinel, decreasing T from the liquidus down to 1000°C at steps of 20°C. P-T and water contents were also determined using geothermobarometers and plagioclase-melt hygrometers respectively, aiming at verifying the parameters used in the MELTS modeling. At this point plagioclase textural features and compositions were related to specific P-T-fO 2 -H 2 O conditions. Plagioclase stability models indicate that: (1) H 2 O strongly influences the plagioclase-melt equilibrium allowing the crystallizations of more calcic compositions only at shallow levels; (2) patchy cores form at high pressure (up to 350 MPa) and low water content (b 1.7 wt.%); (3) clear dissolved cores form at lower pressure (150 MPa) and higher water content (1.5-2.8 wt.%); (4) dusty rims form at even lower pressure straddling the H 2 O-saturation curve and, (5) melt alignments form during degassing. According to experimental works each of these textures can be related to a different process within the feeding system, such as multiple magma inputs (patchy core), volatile addition or increase in T (clear core), mixing (dusty rims) and rapid decompression and degassing (melt inclusion alignment at rims). These inferences were successfully compared with the eruptive evolution of each event as deduced from direct observations, and geophysical and petrological data. The overall picture shows that plagioclase crystallizes under polybaric conditions in a vertically extended and continuous feeding system in which at least two magma crystallization levels were identified. Plagioclase stability also indicates that a large variability in water content characterizes the magma within the feeding system.

Earth-Science Reviews, 2014

Plagioclase is the most common phenocryst in all Etnean magmatic suites (~50% in volume), as well... more Plagioclase is the most common phenocryst in all Etnean magmatic suites (~50% in volume), as well as in most lavas erupted worldwide. Its stability field is strongly dependent on the physico-chemical conditions of the melt and, consequently, it can be used as a tool to record the processes occurring within the feeding system. With this aim, a detailed textural and compositional study of plagioclase was performed on the products emitted during the eruptions. Four distinct textures were recognized at the crystal cores: (1) clear and rounded (An 73-85 ), (2) dusty and rounded (An 73-85 ), (3) sieved (An 82-88 ) and (4) patchy (An 60-81 ), while two distinct textures are commonly observed at the crystal rim: (1) dusty (An 73-90 ) and (2) with melt inclusion alignments (An 70-76 ). Moreover all plagioclases present a thin (10-20 μm) outermost less calcic (An 53-76 ) rim. For each crystal a complex evolutionary path was reconstructed, and several growth and resorption episodes were identified. The fO 2 was estimated using Plag-Cpx/liquid equilibrium in order to calculate the Fe +3 /Fe 2+ ratio in the melt and, in turn, to reconstruct the primitive magma composition by adding a wehrlitic assemblage to the least evolved lava of the four eruptive episodes. MELTS modeling was then developed using this primary magma composition, as well as a trachybasaltic lava. Calculations were performed at variable pressures (400-50 MPa, step of 0.50 MPa) and H 2 O contents (3.5-0 wt.%, step 0.5 wt.%) in order to estimate the crystallization temperature of olivine, clinopyroxene, plagioclase and spinel, decreasing T from the liquidus down to 1000°C at steps of 20°C. P-T and water contents were also determined using geothermobarometers and plagioclase-melt hygrometers respectively, aiming at verifying the parameters used in the MELTS modeling. At this point plagioclase textural features and compositions were related to specific P-T-fO 2 -H 2 O conditions. Plagioclase stability models indicate that: (1) H 2 O strongly influences the plagioclase-melt equilibrium allowing the crystallizations of more calcic compositions only at shallow levels; (2) patchy cores form at high pressure (up to 350 MPa) and low water content (b 1.7 wt.%); (3) clear dissolved cores form at lower pressure (150 MPa) and higher water content (1.5-2.8 wt.%); (4) dusty rims form at even lower pressure straddling the H 2 O-saturation curve and, (5) melt alignments form during degassing. According to experimental works each of these textures can be related to a different process within the feeding system, such as multiple magma inputs (patchy core), volatile addition or increase in T (clear core), mixing (dusty rims) and rapid decompression and degassing (melt inclusion alignment at rims). These inferences were successfully compared with the eruptive evolution of each event as deduced from direct observations, and geophysical and petrological data. The overall picture shows that plagioclase crystallizes under polybaric conditions in a vertically extended and continuous feeding system in which at least two magma crystallization levels were identified. Plagioclase stability also indicates that a large variability in water content characterizes the magma within the feeding system.

Lithos, 2014

ABSTRACT Basaltic lava flows can form tubes in response to the cooling of the outer surface. We c... more ABSTRACT Basaltic lava flows can form tubes in response to the cooling of the outer surface. We collected lava stalactites (frozen lava tears) and sampled lava from the ceilings of three lava tubes on Mount Etna. Comparison of the petrographic characters between ceiling lavas and relative stalactites reveal surprising differences in the groundmass textures and crystal compositions. Major and trace element contents in stalactites show only a slight increase in alkali and SiO2 compared to ceiling lava, whereas significant differences exist in composition and textures between plagioclases within the ceiling lava and those within the stalactites, being in the last case definitively more An-rich. We advance the hypothesis that the high temperature reached in the cave caused the exsolution of the volatiles still trapped in the dripping melt. The volatiles, mainly H2O, formed bubbles and escaped from the melt; such a water-loss might have promoted the silicate polymerization in the stalactites resulting in the growth of An-rich plagioclase phenocrysts. Our results have important implications: in fact plagioclase phenocrysts are usually associated with intratelluric growth and are often considered as the main petrologic evidence for the existence of a magma chamber. The textural and chemical features of plagioclases in stalactites prove that phenocrysts growth in syn to post-eruptive conditions is plausible and clearly explain the relatively low viscosity of many phenocrysts-rich lava flows on Mount Etna, as well as on many other volcanoes around the world. Therefore, we can conclude that plagioclase phenocrysts cannot exclusively be considered as having originated within a magma chamber.

The solidified surface of a lava flow reflects the viscosity of its molten fraction and the cryst... more The solidified surface of a lava flow reflects the viscosity of its molten fraction and the crystal content during flow; crystal-poor basaltic lavas produce pahoehoe fields, whereas crystal-rich ones solidify with aa carapaces. At Mount Etna, volcano aa morphologies are very common, whereas pahoehoe lavas are rare. The latter are locally named "cicirara" due to the presence of centimeter-sized plagioclase phenocrysts much more abundant than in aa lavas. The phenocryst content of "cicirara" lavas contrasts with the low viscosity generally associated with pahoehoe morphology. Therefore, to reconcile the discrepancy between textural and volcanic observations, we have studied the most primitive pahoehoe "cicirara" lava sampled until now. Two samples at 0.5 and 1 m from the bottom of the 2m thick lava flow were investigated on the basis of their mineral compositional variations and textural features, i.e., size frequency and crystal size distribution (CSD). Results coupled with rheological models indicate that only large phenocrysts of plagioclase (>1 mm) and clinopyroxene have grown before eruption. Thermobarometric models and petrological computations based on the composition of plagioclase and clinopyroxene phenocryst cores highlight that only a small amount (10-15 vol.%) of crystals equilibrated at 12 km of depth. Cumulative size frequency and CSD data also indicate that plagioclase and clinopyroxene phenocryst rims grew heterogeneously and coalesced around their cores at depths <1 km, before eruption. In this view, the "cicirara" lava was erupted with a low crystalline content that favoured the formation of its pahoehoe surface; however, crystals with a size <1 mm (~75 vol.%) solidified at post-eruptive conditions. Our findings underline that the emplacement of high-viscosity aa or low-viscosity pahoehoe lavas is driven by the degree of undercooling imposed by the volatile exsolution rate in the shallowest portion of the Etnean plumbing system. A slow magma ascent rate promotes significant intratelluric degassing and widespread nucleation; consequently, the viscosity of the suspension significantly increases leading to an aa morphology. In contrast, pahoehoe "cicirara" lavas are associated with a rapid rise to the surface of poorly degassed, undercooled magmas.

Lithos, 2014

At the Mount Etna volcano (Italy) the massive release of magmatic gasses (especially H 2 O, CO 2 ... more At the Mount Etna volcano (Italy) the massive release of magmatic gasses (especially H 2 O, CO 2 and SO 2 ) during explosive eruptions and through the persistent gas plume, raises important issues: i) the volume of magma erupted at given periods is insufficient to feed the gas plume of the related periods; ii) gas-dominated, explosive eruptions do not emit differentiated products, but relatively Mg-rich magmas; iii) H 2 O measured in melt inclusions (~3.5 wt.% at about 0.4 GPa) exceeds what is expected from intraplate mantle-derived melts (b1.4 wt.%). Literature data on melt inclusions (MIs) along with textural and compositional analyses of plagioclase and phase stability constrained by MELT calculations, enabled us to build a model that reconciles the abovementioned discrepancies with the high variability of the water content along the Etnean feeding system. We propose that along an open magma conduit, continuous gas loss from the free surface of magma at depth promotes an almost steady stream of H 2 O-rich fluid extending well below the gas saturation depth. The velocity of volatile migration might be enhanced by the huge amount of CO 2 present at Mount Etna, because the CO 2 exsolved at high-pressure conditions may act as carrier for other volatile species. In this way the H 2 O-undersaturated primitive magma present in the plumbing system, may undergo a volatile "flushing" proportional to the residing time. This process is fundamental to increase the originally low H 2 O content of primitive magma (≤1.4 wt.%) allowing it to overcome the saturation threshold, to exsolve as gas and to promote the eruption of Mg-rich lavas. Such mechanism would also account for the unexpectedly high amount of magmatic water released during non-eruptive periods.

Lithos, 2010

The enrichment in potassium shown by the basic lavas erupted at Mount Etna volcano after 1971 (K ... more The enrichment in potassium shown by the basic lavas erupted at Mount Etna volcano after 1971 (K 2 O max~2.2 wt.%) has been considered by previous researchers to be too high to be related to simple crystal fractionation and instead linked this high K 2 O content to either crustal assimilation or changes in the magma source. Unfortunately all existing models for the post-1971 K 2 O enrichment fail to explain the phenomenon satisfactorily leaving the question still open. We present a critical re-examination of published data for major elements (633 whole rock analyses), trace elements (376 whole rock analyses) and isotopic ratios (136 87 Sr/ 86 Sr analyses), for historical and prehistorical lavas. Potassium enrichment is not limited to the products of the last 35 years. A comparable increase in potassium is noticed in lavas erupted during the pre-historic phase of the recent Mongibello (K 2 O max~2.5 wt.%) and in lavas related to the early phase of the ancient Mongibello (K 2 O max~3 wt.%). Moreover, data from melt inclusions in olivines from the 2001 and 2002 eruptions, reveal that potassium contents remain constant for melts with entrapment pressure between 490 to 100 MPa and increase significantly in melts entrapped at pressures below~100 MPa. We propose that supercritical fluids coming from deeper magmas and carrying alkali Cl-complexes migrate through basic to intermediate magmas residing in the shallow feeding system. As chlorine exsolves and leaves the system alkalis are released contributing to the observed potassium enrichment of the shallow magma. Fluctuations of the volatiles influx throughout time are likely related to the magma supply rate. Considering that the amount of magma entering a plumbing system is determined by the rate of regional extension, the flux of alkali Cl-complexes entering the melt might be related to an extensional regime acting in the Etnean area.

Chemical Geology, 2011

Seven rock samples were systematically collected from innermost to the outermost portion of a dik... more Seven rock samples were systematically collected from innermost to the outermost portion of a dike outcropping at Mt. Etna volcano. Results show that, from dike core-to-rim, plagioclase, clinopyroxene and titanomagnetite show compositional variations due to increasing cooling rate. Plagioclase is progressively enriched in An from innermost to the outermost part of the dike. Similarly, clinopyroxene components En + CaTs + CaFeTs increase, whereas Di + Hd decrease. The Usp content in titanomagnetite also systematically decrease from dike core-to-rim. Partition coefficients and thermometers based on the crystal-liquid exchange reaction indicate that, due to rapid cooling rates at the dike outer portions, early-formed crystal nuclei do not re-equilibrate with the melt. The chemistry of minerals progressively deviates from that of equilibrium; consequently, from dike core-to-rim, mineral compositions resemble those of high-temperature formation. The chemical variations of clinopyroxene and plagioclase in dike samples mirror those obtained from cooling experiments carried out on alkaline basalts. Accordingly, we used an experimental equation based on clinopyroxene compositional variation as a function of cooling rate to determine the cooling conditions experienced by the crystals during dike emplacement. The estimated cooling rates are comparable to those predicted by thermal modeling based on an explicit finite-difference scheme.

ABSTRACT We report results from geochemical and isotopic analyses conducted on nine samples colle... more ABSTRACT We report results from geochemical and isotopic analyses conducted on nine samples collected from the vertical section of a pahoehoe lava flow unit from Mt. Etna volcano. Textural observations in the field show that, during lava emplacement, volatile exsolution and degassing allowed the nucleation and growth of bubbles, which subsequently rose from the base towards the uppermost lava crust. The amount of phenocrysts is low (about 12 vol.%) and their compositions are comparable to intratelluric crystals analyzed for from historical and recent products. In contrast, from the basal zone to the uppermost lava crust, groundmass microlites are characterized by progressively more primitive compositions, i.e., olivines and clinopyroxenes show increasing Mg#, plagioclases are enriched in anorthite, and the ulvospinel content in titanomagnetites increases. Calculations based on thermodynamic models, thermometers and oxygen barometers indicate that, during post-eruption conditions, crystals formed at higher crystallization temperatures with increasing vertical height. The redox state of the melt progressively increased from the base towards the uppermost crust of the lava flow as is attested by increasing Fe2O3/FeO ratios in clinopyroxene and titanomagnetite. The lowest fO2 is recorded at the basal zone and suggests that this part of the lava was inaccessible to diffusion of atmospheric oxygen at the time of emplacement; whereas, the highest fO2 measured for the uppermost lava crust testifies to post-eruptive hydrogen loss, mainly transported by carrier gas phases.