Silvio Mollo

Thermal decomposition along natural carbonate faults during earthquakes

Earthquake slip is facilitated by a number of thermally activated physicochemical processes that are triggered by temperature rise during fast fault motion, i.e., frictional heating. Most of our knowledge on these processes is derived from theoretical and experimental studies. However, additional information can be provided by direct observation of ancient faults exposed at the Earth’s surface. Although fault rock indicators of earthquake processes along ancient faults have been inferred, the only unambiguous and rare evidence of seismic sliding from natural faults is solidified friction melts or pseudotachylytes. Here we document a gamut of natural fault rocks produced by thermally activated processes during earthquake slip. These processes occurred at 2–3 km depth, along a thin (0.3–1.0 mm) principal slip zone of a regional thrust fault that accommodated several kilometers of displacement. In the slip zone, composed of ultrafine-grained fault rocks made of calcite and minor clays, we observe the presence of relict calcite and clay, numerous vesicles, poorly crystalline/amorphous phases, and newly formed calcite skeletal crystals. These observations indicate that during earthquake rupture, frictional heating induced calcite decarbonation and phyllosilicate dehydration. These microstructures may be diagnostic for recognizing ancient earthquakes along exhumed faults.

Clinopyroxene-liquid thermometers and barometers specific to alkaline differentiated magmas

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.

The partitioning of trace elements between clinopyroxene and trachybasaltic melt during rapid cooling and crystal growth

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 Aliv), 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 Aliv 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 Aliv 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 Alvi. 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.

Experimental constraints on evolution of leucite-basanite magma at 1 and 10-4 GPa: implications for parental compositions of Roman high-potassium magmas

The separate effects of pressure (10 −4 and 1.0 GPa), water, CO 2 , oxygen fugacity and calcium doping on the liquid line of descent of a primitive leucite-basanite magma (SiO 2 = 47.06 wt%, MgO = 12.76 wt% and Mg# = 75.1) from the Montefiascone Volcanic Complex (Vulsini volcanoes, central Italy) were experimentally investigated in the 1350–1160 °C temperature range. Results indicate that low-pressure liquidus temperatures are ≤1280 °C and that the high-pressure T liquidus is 1350 °C under anhydrous conditions; the latter is lowered to ~ 1275 °C by the addition of 3 wt% water. Cr-spinel is always the liquidus phase. At comparable f O 2 values, high and low pressure runs produced the same phase assemblage (spinel + olivine + clinopyroxene) up to 50 % crystallization, although olivine was partially or totally replaced by phlogopite in hydrous experiments. An increase in oxygen fugacity and the addition of CaO determine an increase in both the degree of melt crystallization and the stability field of clinopyroxene. These determine contrasting effects on the composition of residual liquids: the former increases SiO 2 content, whereas the latter induces the desilication of melts. The replacement of olivine by phlogopite, induced by increasing amounts of water, leads to the production of glass with lower potassium contents. Comparison of the natural and experimental melts shows that many of major and trace element variations exhibited by high-K primitive ( i.e. , high Mg/Mg + Fe) magmas at Montefiascone, are consistent with their derivation from a single parental leucite-basanite melt by fractional crystallization of different proportions of mineral phases, plus carbonate assimilation. The changes in phases stability and melt composition caused by carbonate assimilation may also have fundamental implications for the origin of the calcic high-magnesium leucitites and melilitites. In particular, the complex metasomatic interactions that can develop at the interface between potassic magmas and carbonate wall rocks, may lead to melting of calcite. This low-viscosity melt readily mixes with the surrounding magma inducing the crystallization of Ca-Tschermak-rich pyroxene and hercynitic spinel, affecting significantly the SiO 2 , CaO and alumina composition of the resulting hybrid melt. A key finding of our study is that magmas such as the studied leucite-basanite may be considered parental to the wide spectrum of mafic high-K compositions in the Roman Province, which have been traditionally considered as representing near primary magmas reflecting distinct mantle source compositions and/or processes.

The cooling kinetics of plagioclase feldspar as revealed by electron-microprobe mapping

Abstract In this study, we have used electron-microprobe mapping to investigate plagioclase compositional evolution due to cooling kinetics. We re-analyzed five run-products from a prior study (Iezzi et al. 2011), crystallized by cooling a natural andesitic melt from 1300 to 800 °C at 25, 12.5, 3, 0.5, and 0.125 °C/ min under atmospheric pressure and air redox state. As the cooling rate decreases, the texture of large plagioclases changes from skeletal to hollow to nearly equant. In this study, we use X‑ray map data to obtain a database of 12 275 quantitative chemical analyses. The frequency of An-rich plagioclases showing disequilibrium compositions substantially increases with increasing cooling rate. At 25 and 12.5 °C/min the distribution is single-mode and narrow, at 0.5 and 0.125 °C/min is single-mode but very broad, whereas at the intermediate cooling rate of 3 °C/min two distinct plagioclase populations are present. This intermediate cooling rate is fast enough to cause departure from equilibrium for the crystallization of the An-rich population but also sufficiently slow that An-poor plagioclases nucleate from the residual melt. We interpret our findings in the context of time-temperature-transformation (TTT) diagrams, and infer the crystallization kinetics of plagioclase in the experiments. Compositional trends and our inferences regarding TTT systematics are consistent with two discrete nucleation events that produced separate populations of plagioclase (i.e., An-rich and An-poor populations) at 3 °C/min. Using plagioclase-melt pairs as input data for the thermometric reaction between An and Ab components, we find that plagioclase mirrors very high- (near-liquidus) crystallization temperatures with increasing cooling rate. These results have important implications for the estimate of post-eruptive solidification conditions. Lava flows and intrusive bodies from centimeters to a few meters thick are characterized by a short solidification time and a significant thermal diffusion. Under such circumstances, it is possible to crystallize plagioclases with variable and disequilibrium chemical compositions simply by cooling a homogeneous andesitic melt. X‑ray element maps enrich the study of plagioclase compositional variations generated under conditions of rapid cooling.

An improved clinopyroxene-based hygrometer for Etnean magmas and implications for eruption triggering mechanisms

Abstract We have refined the clinopyroxene-based hygrometer published by Armienti et al. (2013) for a better quantitative understanding of the role of H2O in the differentiation of Etnean magmas. The original calibration data set has been significantly improved by including several experimental clinopyroxene compositions that closely reproduce those found in natural Etnean products. To verify the accuracy of the model, some randomly selected experimental clinopyroxene compositions external to the calibration data set have been used as test data. Through a statistic algorithm based on the Mallows’ CP criterion, we also check that all model parameters do not cause data overfitting, or systematic error. The application of the refined hygrometer to the Mt. Etna 2011–2013 lava fountains indicates that most of the decreases in H2O content occur at P < 100 MPa, in agreement with melt inclusion data suggesting abundant H2O degassing at shallow crustal levels during magma ascent in the conduit and eruption to the surface.

Tectonic control on the petrophysical properties of foredeep sandstone in the Central Apennines, Italy

Petrophysical properties of rocks and their applicability at larger scale are a challenging topic in Earth sciences. Petrophysical properties of rocks are severely affected by boundary conditions, rock fabric/microstructure, and tectonics that require a multiscale approach to be properly defined. Here we (1) report laboratory measurements of density, porosity, permeability, and P wave velocities at increasing confining pressure conducted on Miocene foredeep sandstones (Frosinone Formation); (2) compare the laboratory results with larger-scale geophysical investigations; and (3) discuss the effect of thrusting on the properties of sandstones. At ambient pressure, laboratory porosity varied from 2.2% to 13.8% and P wave velocities (Vp) from 1.5 km/s to 2.7 km/s. The P wave velocity increased with confining pressure, reaching between 3.3 km/s and 4.7 km/s at 100 MPa. In situ Vp profiles, measured using sonic logs, matched the ultrasonic laboratory measurement well. The permeability varied between 1.4 × 10−15 m2 and 3.9 × 10−15 m2 and was positively correlated with porosity. The porosity and permeability of samples taken at various distances to the Olevano–Antrodoco fault plane progressively decreased with distance while P wave velocity increased. At about 1 km from the fault plane, the relative variations reached 43%, 65%, and 20% for porosity, permeability, and P wave velocity, respectively. This suggests that tectonic loading changed the petrophysical properties inherited from sedimentation and diagenesis. Using field constraints and assuming overburden-related inelastic compaction in the proximity of the fault plane, we conclude that the fault reached the mechanical condition for rupture in compression at differential stress of 64.8 MPa at a depth of 1500 m.

The role of natural solidification paths on REE partitioning between clinopyroxene and melt

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 Aliv in clinopyroxene is a charge-balancing cation for REE, the partition coefficients (DREE) 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 DREE on cation radius due to charge-balance mechanisms described by the lattice strain model.

Trace element behaviour during interaction between basalt and crustal rocks at 0.5–0.8 GPa: an experimental approach

We experimentally investigate the major and trace elements behavior during the interaction between two partially molten crustal rocks (meta-anorthosite and metapelite) and a basaltic melt at 0.5–0.8 GPa. Results show that a hybrid melt is formed at the basalt-crust contact, where plagioclase crystallizes. This contact layer is enriched in trace elements which are incompatible with plagioclase crystals. Under these conditions, the trace element diffusion coefficients are one order of magnitude larger than those expected. Moreover, the HFSE diffusivity in the hybrid melt is surprisingly higher than the REE one. Such a feature is related to the plagioclase crystallization that changes the trace elements liquid-liquid partitioning (i.e. diffusivity) over a transient equilibrium that will persist as long as the crystal growth proceeds.These experiments suggests that the behaviour of the trace elements is strongly dependent on the crystallization at the magma-crust interface. Diffusive processes like those investigated can be invoked to explain some unusual chemical features of contaminated magmatic suites.

Solidification and Turbulence (Non-laminar) during Magma Ascent: Insights from 2D and 3D Analyses of Bubbles and Minerals in an Etnean Dyke

Solidification, emplacement and fluid dynamics of a sub-volcanic rock at Mt Etna have been investigated through two-dimensional (2D) and three-dimensional (3D) textural analyses of the hosted bubbles and minerals. This rock is a 4 3 m thick aphyric dyke (DK) that solidified at a depth of 100–300 m, below the pristine surface level. Seven samples from the dyke rim (DK1) to core (DK7) have been analysed in two dimensions by using a high-resolution scanner, a transmission optical microscope and scanning electron microscopy imaging with back-scattered electrons, and in three dimensions by microfocus X-ray computed tomography. Field observations and mesoscopic polished rock surfaces show bubble-rich, -poor and -free patches even in rock pieces of a few cubic centimetres, with changes in sizes and shapes; even so, their shapes and spatial arrangement can never be attributed to high degrees of strain. In parallel, the amount of bubbles irregularly varies from dyke rim to core, whereas plagioclase (plg), clinopyroxene (cpx), titanomagnetite (timt), and olivine (ol) show only limited variations. The fabric of bubbles retrieved by 3D orientation of their maximum length (i.e. elongation) is invariably random in space for each DK sample. These bubble features have been attributed to transitional to turbulent flows; that is, non-laminar regimes (Reynolds number> 1000), predicted for a long time from numerical models and that occurred before the crystallization of minerals. Water solubility, volume of bubbles, magma density and viscosity models indicate that, at pressure higher than 10 MPa, 1 wt % H2O was dissolved in the original trachybasaltic magma, which, in turn, was close to its liquidus temperature. As the pressure decreased at very shallow levels, the magma significantly degassed and volatile exsolution induced marked crystallization (mostly plg followed by cpx). The viscosity of the system increased, decelerating and halting the magmatic suspension. The textures and fabrics of bubbles were suddenly frozen in, despite crystals continuing to grow under the effect of cooling rate variables from the inner (DK7) to outer (DK1) dyke portions. Fluid-dynamic computations suggest that the DK trachybasaltic magma ascended with a velocity of few metres per second in a transitional to turbulent regime, before the growth of minerals.