We present new thermometers and barometers based on clinopyroxene–liquid equilibria specific to alkaline differentiated magmas. The new models were calibrated through the regression analyses of ...experimental datasets obtained by merging phase equilibria experiments from the literature with new experiments performed by using trachytic and phonolitic starting compositions. The regression strategy was twofold: (1) we have tested previous thermometric and barometric equations and recalibrated these models using the new datasets; (2) we have calibrated a new thermometer and a new barometer including only regression parameters that closely describe the compositional variability of the datasets. The new models yield more precise estimates than previous thermometers and barometers when used to predict temperatures and pressures of alkaline differentiated magmas. We have tested the reliability of the new equations by using clinopyroxene–liquid pairs from trachytes and phonolites erupted during major explosive eruptions at the Phlegrean Fields and Mt. Vesuvius (central Italy). The test yielded crystallization conditions comparable to those determined by means of melt and fluid inclusion analyses and phase equilibria studies; this validates the use of the proposed models for precise estimates of crystallization temperatures and pressures in differentiated alkaline magmas. Because these magmas feed some of the most voluminous, explosive, and threatening volcanic eruptions in the world, a better understanding of the environmental conditions of their reservoirs is mandatory and this is now possible with the new models provided here.
Isothermal and undercooling experiments were conducted on one of the most primitive trachybasalts from Mt. Etna volcano in order to examine the crystallization mechanisms controlling the textural and ...compositional variability of clinopyroxene. Experiments were performed at 400–800 MPa, 1050–1200 °C, 0–4 wt.% H2O and at oxygen fugacity 2 log units above the Ni-NiO + 2 buffer. In isothermal experiments, the final resting temperature is approached from room temperature and clinopyroxene growth is dominated by an interface-controlled mechanism, leading to the formation of small (∼10 µm) and euhedral crystals with homogeneous compositions. Conversely, in undercooling experiments, the final resting temperature is approached after annealing at temperature above the liquidus, imposing an effective degree undercooling (ΔT) to the system. In presence of undercooling, the crystallization of clinopyroxene is dominated by a diffusion-controlled mechanism that determines the formation of large (>100 µm) crystals, constituted by two compositionally distinct domains, enriched in Al2O3 + TiO2 and SiO2 + MgO, respectively. The maximum growth rate (Gmax) decreases progressively from ∼10−7 to ∼10−8 cm/s as the degree of undercooling increases from ∼20 to ∼230 °C, due to the increase in nucleation rate. At low to moderate degrees of undercooling (ΔT = 23–41 °C) clinopyroxene is prevalently euhedral to subhedral, whereas at high degrees of undercooling, the crystal shape changes from prevalently subhedral (ΔT = 73–123 °C) to skeletal and dendritic (ΔT = 132–233 °C). Hourglass sector zoning similar to that documented for natural phenocrysts from eruptions at Mt. Etna volcano is observed only at low degrees of undercooling (ΔT = 23–32 °C). This type of zoning develops in the form of the cation exchange Si + Mg{-111} ↔ Al + Ti{100} and demonstrates that hourglass sector zoning is an effective indicator of sluggish kinetic effects caused by relatively low degrees of undercooling. In contrast, at increasing degrees of undercooling (ΔT > 32 °C), strong melt supersaturation determines the early formation of Al2O3 + TiO2-rich dendritic crystals and further SiO2 + MgO-rich overgrowths, as the bulk system attempts to return to a near-equilibrium state between the advancing crystal surface and the feeding melt.
The experimentally-determined relationship between ΔT and clinopyroxene chemistry is used to reconstruct the crystallization conditions of natural clinopyroxenes from 1974 and 2002–2003 eccentric eruptions at Mt. Etna volcano. Clinopyroxene rims record much higher degrees of undercooling (up to ∼110 °C) than crystal mantles associated with magma recharge at depth (mostly 0–40 °C). Hence, the rims track decompression-induced degassing and cooling during the ascent of magma towards the surface.
We present the variation in trace element partition coefficients measured at the interface between rapidly cooled clinopyroxene crystals and co-existing melts. Results indicate that, as the cooling ...rate is increased, clinopyroxene crystals are progressively depleted in Si, Ca and Mg counterbalanced by enrichments in Al (mainly tetrahedral Al
iv
), Na and Ti. Partition coefficients (Ds) for rare earth elements (REE), high field strength elements (HFSE) and transition elements (TE) increase with increasing cooling rate, in response to clinopyroxene compositional variations. The entry of REE into the M2 site is facilitated by a coupled substitution where either Na substitutes for Ca on the M2 site or Al
iv
substitutes for Si in the tetrahedral site. The latter substitution reflects an increased ease of locally balancing the excess charge at M2 as the number of surrounding Al
iv
atoms increases. Due to the lower concentration of Ca in rapidly cooled clinopyroxenes, divalent large ion lithophile elements (LILE) on M2 decrease at the expense of monovalent cations. Conversely, higher concentrations of HFSE and TE on the M1 site are facilitated as the average charge on this site increases with the replacement of divalent-charged cations by Al
vi
. Although crystallization kinetics modify clinopyroxene composition, deviations from equilibrium partitioning are insufficient to change the tendency of a trace element to be compatible or incompatible. Consequently, there are regular relationships between ionic radius, valence of the trace element and D. At both equilibrium and cooling rate conditions, Ds for isovalent cations define parabola-like curves when plotted against ionic radius, consistent with the lattice strain model, demonstrating that the partitioning of trace elements is driven by charge balance mechanisms; cation substitution reactions can be treated in terms of the energetics of the various charge-imbalanced configurations.
Open‐conduit conditions characterize several of the most hazardous and active volcanic systems of basaltic composition worldwide, persistently refilled by magmatic inputs. Eruptive products with ...similar bulk compositions, chemically buffered by continual mafic inputs, nevertheless exhibit heterogeneous glass compositions in response to variable magma mixing, crystallization, and differentiation processes within different parts of the plumbing system. Here, we document how multivariate statistics and magma differentiation modeling based on a large data set of glass compositions can be combined to constrain magma differentiation and plumbing system dynamics. Major and trace elements of matrix glasses erupted at Stromboli volcano (Italy) over the last 20 years provide a benchmark against which to test our integrated petrological approach. Principal component analysis, K‐means cluster analysis, and kernel density estimation reveal that trace elements define a multivariate space whose eigenvectors are more readily interpretable in terms of petrological processes than major elements, leading to improved clustering solutions. Comparison between open‐ and closed‐system differentiation models outlines that steady state magma compositions at constantly replenished and erupting magmatic systems approximate simple fractional crystallization trends, due to short magma residence times. Open‐system magma evolution is associated with magma storage crystallinities that are lower than those associated with closed‐system scenarios. Accordingly, open‐system dynamics determine the efficient crystal‐melt separation toward the top of the reservoir, where eruptible melts continuously supply the ordinary activity. Conversely, a mush‐like environment constitutes the bottom of the reservoir, where poorly evolved magmas result from mixing events between mush residual melts and primitive magmas injected from deeper crustal levels.
Plain Language Summary
Volcanoes characterized by continuous eruptive activity are typified by constant replenishment of new magma, rising from deeper regions of the crust. The volcanic glass (supercooled silicate melt), represents the residual liquid of magma crystallization, and is found as the intracrystalline matrix of eruptive products. The study of its chemical composition may provide insight into the processes occurring at depths beneath the volcanic vent, where magma compositional changes result from crystallization and mixing with new magma rising from depth. We combine statistical analyses and analytical equations based on the chemical composition of the matrix glasses from Stromboli volcano, in order to constrain the processes which produce their chemical variations, identifying different environments where magmas are stored at depth. Our results also show that when magma is stored for a short period of time, the chemical changes to which the magma is subjected in a constantly replenished system are similar to those occurring in a system which is closed to new inputs of magma.
Key Points
The combination of multivariate statistics with geochemical modeling provides new constraints on magma differentiation processes
Multivariate statistics based on trace elements allow better retrieval of petrological information than those based on major elements
Magma differentiation in open systems approximates that occurring in closed systems when magma residence timescales are short
We present results from magma–carbonate interaction experiments designed to shed light on the geochemical evolution of clinopyroxene in the Roman Province (central Italy). Atmospheric pressure ...experiments were performed at 1140, 1160 and 1180°C under NNO, MH and air oxygen buffering conditions. The starting materials were a shoshonite and a phonotephrite doped with different amounts of CaO and CaO+MgO whose stoichiometric proportions reproduced the assimilation by magmas of calcite and dolomite, respectively. The results show that clinopyroxenes, spinels and residual glasses are ubiquitous phases in all run-products. Calcite-doped runs crystallize more clinopyroxene than dolomite-doped runs at the same conditions. This leads to the formation of strong desilicated CaO-rich melts showing compositions comparable to those of magmatic skarns. During magma–carbonate interaction, the content of Fe3+ in clinopyroxene increases with increasing fO2 promoting the substitution of Al for Si in tetrahedral site. Local charge imbalances are also compensated by the incorporation of highly charged cations, such as Ti, into the crystal lattice. According to this cation substitution, Al–Ca–Fe3+–Ti-rich clinopyroxenes of the skarn environment testify to continuous CO2 fluxes produced by the thermal decomposition of carbonate wall-rocks. Nevertheless, the oxidative capacity of CO2 progressively decreases from the skarn shells towards the interior of the magma chamber driving the crystallization of Si–Fe2+–Mg-rich clinopyroxenes.
•The geochemical evolution of clinopyroxene is a clear indicator of the degree of magma–carbonate interaction.•The crystal chemistry progressively changes from skarn shells to the interior of the magma chamber.•Al–Ca–Fe3+–Ti-rich clinopyroxenes testify to very high oxidizing fO2 conditions.
For a better understanding of the final solidification history of eruptions at Mt. Etna volcano (Sicily, Italy), we have investigated cation redistributions at the interface between ...sub-millimetre-sized clinopyroxene and titanomagnetite crystal rims and coexisting melts. The studied products were scoria clasts from lava fountains and rock samples from pahoehoe and aa lava flows. Our data indicate that scoria clasts from lava fountaining were rapidly quenched at the contact with the atmosphere, preserving the original crystal textures and compositions inherited during magma dynamics within the plumbing system. Kinetics and energetics of crystallization were instantaneously frozen-in and post-eruptive effects on mineral chemistry were negligible. The near-equilibrium compositions of clinopyroxene and titanomagnetite indicate that lava fountain episodes were supplied by high-temperature, H2O-rich magmas ascending with velocities of 0.01–0.31m/s. In contrast, magmas feeding lava flow eruptions underwent a more complex solidification history where the final stage of the crystal growth was mostly influenced by volatile loss and heat dissipation at syn- and post-eruptive conditions. Due to kinetic effects associated with magma undercooling, clinopyroxenes and titanomagnetites formed by crystal attachment and agglomeration mechanisms leading to intricate intergrowth textures. The final compositions of these minerals testify to closure temperatures and melt–water concentrations remarkably lower than those estimated for lava fountains. Kinetically-controlled cation redistributions at the crystal–melt interface suggest that the solidification of magma was driven by degassing and cooling processes proceeding from the uppermost part of the volcanic conduit to the surface.
•Cation redistributions have been measured in clinopyroxenes and titanomagnetites at the final stage of crystal growth.•These compositional variations reflect the cooling and thermal path of magmas ascending from the conduit to the surface.•Remarkable implications are found for the final solidification history of lava fountains and lava flows at Mt. Etna.
The physical integrity of a sub-volcanic basement is crucial in controlling the stability of a volcanic edifice. For many volcanoes, this basement can comprise thick sequences of carbonates that are ...prone to significant thermally-induced alteration. These debilitating thermal reactions, facilitated by heat from proximal magma storage volumes, promote the weakening of the rock mass and likely therefore encourage edifice instability. Such instability can result in slow, gravitational spreading and episodic to continuous slippage of unstable flanks, and may also facilitate catastrophic flank collapse. Understanding the propensity of a particular sub-volcanic basement to such instability requires a detailed understanding of the influence of high temperatures on the chemical, physical, and mechanical properties of the rocks involved. The juxtaposition of a thick carbonate substratum and magmatic heat sources makes Mt. Etna volcano an ideal candidate for our study. We investigated experimentally the effect of temperature on two carbonate rocks that have been chosen to represent the deep, heterogeneous sedimentary substratum under Mt. Etna volcano. This study has demonstrated that thermal-stressing resulted in a progressive and significant change in the physical properties of the two rocks. Porosity, wet (i.e., water-saturated) dynamic Poisson's ratio and wet Vp/Vs ratio all increased, whilst P- and S-wave velocities, bulk sample density, dynamic and static Young's modulus, dry Vp/Vs ratio, and dry dynamic Poisson's ratio all decreased. At temperatures of 800°C, the carbonate in these rocks completely dissociated, resulting in a total mass loss of about 45% and the release of about 44wt.% of CO2. Uniaxial deformation experiments showed that high in-situ temperatures (>500°C) significantly reduced the strength of the carbonates and altered their deformation behaviour. Above 500°C the rocks deformed in a ductile manner and the output of acoustic emissions was greatly reduced. We speculate that thermally-induced weakening and the ductile behaviour of the carbonate substratum could be a key factor in explaining the large-scale deformation observed at Mt. Etna volcano. Our findings are consistent with several field observations at Mt. Etna volcano and can quantitatively support the interpretation of (1) the irregularly low seismic velocity zones present within the sub-volcanic sedimentary basement, (2) the anomalously high CO2 degassing observed, (3) the anomalously high Vp/Vs ratios and the rapid migration of fluids, and (4) the increasing instability of volcanic edifices in the lifespan of a magmatic system. We speculate that carbonate sub-volcanic basement may emerge as one of the decisive fundamentals in controlling volcanic stability.
► High temperatures strongly influence sub-volcanic carbonate successions. ► Thermal weakening could encourage large-scale deformation at Mt. Etna. ► Geophysical anomalies/observations can be explained by such thermal degradation. ► Decarbonation should not be dismissed in CO2 budgets for Mt. Etna. ► Carbonate basements may emerge as a decisive fundamental in volcanic stability.
Abstract
Systematic variations in the crystal cargo and whole-rock isotopic compositions of mantle-derived basalts in the intraplate Dunedin Volcano (New Zealand) indicate the influence of a complex ...mantle-to-crust polybaric plumbing system. Basaltic rocks define a compositional spectrum from low-alkali basalts through mid-alkali basalts to high-alkali basalts. High-alkali basalts display clinopyroxene crystals with sector (hourglass) and oscillatory zoning (Mg#61–82) as well as Fe-rich green cores (Mg#43–69), whereas low-alkali basalts are characterized by clinopyroxenes with unzoned overgrowths (Mg#69–83) on resorbed mafic cores (Mg#78–88), coexisting with reversely zoned plagioclase crystals (An43–68 to An60–84 from core to rim). Complex magma dynamics are indicated by distinctive compositional variations in clinopyroxene phenocrysts, with Cr-rich zones (Mg#74–87) indicating continuous recharge by more mafic magmas. Crystallization of olivine, clinopyroxene and titanomagnetite occurred within a polybaric plumbing system extending from upper mantle to mid-crustal depths (485–1059 MPa and 1147–1286°C), whereas crystallization of plagioclase with subordinate clinopyroxene and titanomagnetite proceeded towards shallower crustal levels. The compositions of high-alkali basalts and mid-alkali basalts resemble those of ocean island basalts and are characterized by FOZO-HIMU isotopic signatures (87Sr/86Sri = 0.70277–0.70315, 143Nd/144Ndi = 0.51286–0.51294 and 206Pb/204Pb = 19.348–20.265), whereas low-alkali basalts have lower incompatible element abundances and isotopic compositions trending towards EMII (87Sr/86Sri = 0.70327–70397, 143Nd/144Ndi = 0.51282–0.51286 and 206Pb/204Pb = 19.278–19.793). High- and mid-alkali basalt magmas mostly crystallized in the lower crust, whereas low-alkali basalt magma recorded deeper upper mantle clinopyroxene crystallization before eruption. The variable alkaline character and isotope composition may result from interaction of low-alkaline melts derived from the asthenosphere with melts derived from lithospheric mantle, possibly initiated by asthenospheric melt percolation. The transition to more alkaline compositions was induced by variable degrees of melting of metasomatic lithologies in the lithospheric mantle, leading to eruption of predominantly small-volume, high-alkali magmas at the periphery of the volcano. Moreover, the lithosphere imposed a filtering effect on the alkalinity of these intraplate magmas. As a consequence, the eruption of low-alkali basalts with greater asthenospheric input was concentrated at the centre of the volcano, where the plumbing system was more developed.
The physical characterization of carbonate‐bearing normal faults is fundamental for resource development and seismic hazard. Here we report laboratory measurements of density, porosity, Vp, Vs, ...elastic moduli, and permeability for a range of effective confining pressures (0.1–100 MPa), conducted on samples representing different structural domains of a carbonate‐bearing fault. We find a reduction in porosity from the fault breccia (11.7% total and 6.2% connected) to the main fault plane (9% total and 3.5% connected), with both domains showing higher porosity compared to the protolith (6.8% total and 1.1% connected). With increasing confining pressure, P wave velocity evolves from 4.5 to 5.9 km/s in the fault breccia, is constant at 5.9 km/s approaching the fault plane and is low (4.9 km/s) in clay‐rich fault domains. We find that while the fault breccia shows pressure sensitive behavior (a reduction in permeability from 2 × 10−16 to 2 × 10−17 m2), the cemented cataclasite close to the fault plane is characterized by pressure‐independent behavior (permeability 4 × 10−17 m2). Our results indicate that the deformation processes occurring within the different fault structural domains influence the physical and transport properties of the fault zone. In situ Vp profiles match well the laboratory measurements demonstrating that laboratory data are valuable for implications at larger scale. Combining the experimental values of elastic moduli and frictional properties it results that at shallow crustal levels, M ≤ 1 earthquakes are less favored, in agreement with earthquake‐depth distribution during the L'Aquila 2009 seismic sequence that occurred on carbonates.
Key Points
We investigate and upscale the physical and transport properties of an exceptionally exposed carbonate‐bearing normal fault in central Italy
Physical and transport properties of the investigated fault zone are controlled by the protolith and the different deformation processes
Laboratory physical and mechanical data are in good agreement with in situ measurements and well‐match seismological data
Carbonate-bearing normal faults are important structures for controlling fluid flow and seismogenesis within the brittle upper crust. Numerous studies have tried to characterize fault zone structure ...and earthquake slip processes along carbonate-bearing faults. However, due to the different scales of investigation, these studies are not often integrated to provide a comprehensive fault image. Here we present a multi-scale investigation of a normal fault exhumed from seismogenic depths. The fault extends for a length of 10 km with a maximum width of about 1.5 km and consists of 5 sub-parallel and interacting segments. The maximum displacement (370–650 m) of each fault segment is partitioned along sub-parallel slipping zones extending for a total width of about 50 m. Each slipping zone is characterized by slipping surfaces exhibiting different slip plane phenomena. Fault rock development is controlled by the protolith lithology. In massive limestone, moving away from the slip surface, we observe a thin layer (<2 cm) of ultracataclasite, cataclasite (2–10 cm) and fault breccia. In marly limestone, the fault rock consists of a cataclasite with hydrofractures and smectite-rich pressure solution seams. At the micro-nanoscale, the slip surface consists of a continuous and thin (<300 μm) layer composed of coarse calcite grains (∼5–20 μm in size) associated with sub-micrometer grains showing fading grain boundaries, voids and/or vesicles, and suggesting thermal decomposition processes. Micrometer-sized calcite crystals show nanoscale polysynthetic twinning affected by the occurrence of subgrain boundaries and polygonalized nanostructures. Investigations at the kilometres-tens of meter scale provide fault images that can be directly compared with high-resolution seismological data and when combined can be used to develop a comprehensive characterization of seismically active fault structures in carbonate lithologies. Micro and nanoscale investigations along the principal slipping zone suggest that different deformation processes, including plastic deformation and thermal decomposition, were active during seismic slip.
•We describe a normal fault from the kilometre down to the nanoscale.•The fault has a length of 10 km, a maximum width of about 1.5 km and consists of 5, sub-parallel and interacting segments.•Each fault segment consists of sub-parallel slipping zones extending for a total width of about 50 m.•Micro-nanostructures suggest plastic deformation and thermal decomposition during seismic slip.
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