Andrea Cavallo

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.

FTIR imaging in diffusion studies: CO2 and H2O in a synthetic sector-zoned beryl

In this work we investigate the strongly inhomogeneous distribution of CO2 and H2O in a synthetic beryl having a peculiar hourglass zoning of Cr due to the crystal growth. The sample was treated at 800°C, 500 MPa, in a CO2-rich atmosphere. High-resolution FESEM images revealed that the hourglass boundary is not correlated to physical discontinuities, at least at the scale of tens of nanometers. Polarized FPA-FTIR imaging, on the other side, revealed that the chemical zoning acts as a fast pathway for carbon dioxide diffusion, a feature never observed so far in minerals. The hourglass zone boundary may be thus considered as a structural defect possibly due to the mismatch induced by the different growth rates of each sector. High-resolution synchrotron-light FTIR imaging, in addition, also allows enhancement of CO2 diffusion along the hourglass boundary to be distinguished from diffusion along fractures in the grain. Therefore, FTIR imaging provides evidence that different diffusion mechanisms may locally combine, suggesting that the distribution of the target molecules needs to be be carefully characterized in experimental studies. This piece of information is mandatory when the study is aimed at extracting diffusion coefficients from analytical profiles. Combination of TOF-SIMS and FPA data shows a significant depletion of type II H2O along the hourglass boundary, indicating that water diffusion could be controlled by the distribution of alkali cations within channels, coupled to a plug effect of CO2.

Magmatic Evolution and plumbing system of ring-fault volcanism: the Vulcanello Peninsula (Aeolian Islands, Italy)

The Vulcanello peninsula is situated north of Vulcano, the southernmost island of the Aeolian Arc. It was built at the rim of La Fossa Caldera between 1000 and 1650 A.D. Erupted products are mafic to intermediate in composition, while the coeval products erupted inside the caldera are mainly rhyolitic. Therefore, Vulcanello’s activity represents an anomalous mafic post-caldera volcanism in a convergent setting. A petrographic and geochemical study was carried out on lavas and pyroclastic rocks representing the entire eruptive history of the volcanic centre. New data (major and trace elements and Sr isotope ratios on whole rocks, and major element compositions on mineral phases) and geochemical models were used to investigate shallow level differentiation processes ( i.e. , fractional crystallisation, fractional crystallisation plus crustal assimilation, degassing, magma mixing/recharge). The study suggests that the entire Vulcanello activity can be considered as the uninterrupted expulsion of a single deep magma batch of shoshonitic composition emitted from a NE–SW ring fault of La Fossa Caldera. The magma is genetically related to the shoshonitic basalts found as melt inclusions in the olivine crystals erupted in the products of the 1888–1890 “ La Fossa” activity. This points to a possible single deep plumbing system for both La Fossa Cone and Vulcanello centres, strongly controlled by NW–SE to N– S regional structures. The shoshonitic magma, undergoing fractional crystallisation, partly rose directly to the surface where two strombolian cones were constructed, while residual magma remained at depth, and, partially degassed and crystallised, it subsequently erupted both effusively to form a lava platform and explosively to form a third pyroclastic cone. The remaining magma evolved to latite by AFC process and was erupted both as a lava flow (Punta del Roveto) and in the form of pyroclastic products ( i.e. , the upper part of the third cone), controlled by shallow ring faults of La Fossa Caldera. Therefore the Vulcanello plumbing system is controlled by tectonic structures at depth and by shallower volcano-tectonic (caldera) fractures.

H2O and CO2 in minerals of the haüyne-sodalite group: an FTIR spectroscopy study

Abstract This paper reports an infrared spectroscopic study of a set of sodalite-group minerals. The specimens have been identified using a combination of X-ray diffraction and microchemical analysis. As expected, the Si/Al ratio is ~1; the extra framework cation content is characterized by a well-defined Na ⇌ (Ca+K) substitution. The lattice parameters of the studied samples range from sodalite (sample LM11) with a = 8.889(2) Å, to haüyne (sample HR3S) with a = 9.1265(2) Å. The specimens, having the SO42− group as a dominant anion, show a clear correlation between the a cell edge and the K content. Single-crystal FTIR spectroscopy shows that haüyne and nosean typically contain enclathrated CO2 molecules, inadditionto H2O and minor carbonate, while sodalite is virtually CO2-free. Detailed microspectrometric mappings show a non-homogeneous distribution of volatile constituents across the crystals, which may be related to the presence of fractures in the crystals. Because of such zoning, a relatively wide variation is observed when calibrating extinction coefficients on the basis of a bulk analytical method such as CHN elemental analysis.

Phase transition induced by solid solution: The BCa-BMg substitution in richteritic amphiboles

Abstract Eleven compositions along the join Na(NaMg)Mg5Si8O22(OH)2-Na(NaCa)Mg5Si8O22(OH)2 (“magnesiorichterite”-richterite) have been synthesized at T = 800-850 °C and PH₂O = 0.35-0.5 GPa. The run products have been characterized by electron probe microanalysis (EPMA), synchrotron and conventional X-ray powder diffraction (XRPD), Fourier transformed infrared (FTIR) spectroscopy, and selected area electron diffraction (SAED-TEM). Nominally, the chemical variation along the join can be expressed as BMgxBCa1-x with 0 ≤ x ≤ 1. A combination of EPMA and FTIR data in the OH-stretching region show that a complete solid solution is obtained under the conditions used. Nevertheless, a slight deviation from the nominal compositions involving a limited loss of Na at A and B sites, balanced by an increase of Ca at the B site, is present. Several indications of a displacive and coelastic P21/m → C2/m transformation induced by the Ca-Mg chemical substitution are observed. The phase transition occurs at B-site composition (Xc) close to B(Na1Mg0.7Ca0.3). C2/m samples with a Ca content of 0.34, 0.45, and 0.54 apfu show a significant strain tail related to local compositional inhomogeneities. This residual strain disappears as the amount of BCa significantly increases with respect to that of BMg. The transformation behavior observed here mirrors that of pyroxenes along the join diopside (CaMgSi2O6)-enstatite (Mg2Si2O6). The cell parameters of amphiboles with CMg5, TSi8, and W(OH)2 and variable A- and B-site populations follow almost linear and continuous trends, indicative of small amounts of spontaneous strain accompanying these monoclinic phase transitions and the absence of significant miscibility gaps among different amphibole groups when quenched from higher temperatures of crystallization.

Spectroscopy and X-ray structure refinement of sekaninaite from Dolní Bory (Czech Republic)

Abstract The crystal chemistry of sekaninaite from Dolní Bory, Czech Republic, was characterized by a multimethod approach. Particular emphasis was put on the characterization of the channel constituents (i.e. H2O and CO2). Electron microprobe analysis shows the sample to be close to the Fe endmember [XFe = Fe/(Fe+Mg) = 94%) with significant Mn (1.48 wt.%) present; laser ablation mass-spectrometry showed the presence of 0.42 wt.% Li2O. H2O and CO2 contents (1.48 and 0.17 wt.%, respectively) were determined via secondary-ion mass-spectrometry. Sample homogeneity was checked by Fourier-transform infrared (FTIR) imaging using a microscope equipped with a focal plane array detector. Single-crystal FTIR spectroscopy confirmed the presence of two types of H2O groups in different orientations (with prevalence of the type II orientation), and that CO2 is oriented preferentially normal to the crystallographic c axis. Using the Beer-Lambert relation, integrated molar coefficients, Ɛi, were calculated for both types of H2O (Ɛi H2O[I] = 6000±2000; Ɛi H2O[II] = 13000±1000) and for CO2 (Ɛi CO2 = 2000±1000).

Solid solution along the synthetic LiAlSi2O6-LiFeSi2O6 (spodumene-ferri-spodumene) join: A general picture of solid solutions, bond lengths, lattice strains, steric effects, symmetries, and chemical compositions of Li clinopyroxenes

Abstract Seven clinopyroxene compositions along the join M2LiM1AlTSi2O6 (spodumene) to M2LiM1Fe3+TSi2O6 (ferri-spodumene) were synthesized at 2 GPa, 800 °C under highly oxidizing conditions (using H2O2 fluid) in an end-loaded piston cylinder. In addition, the LiFe3+Si2O6 composition was also synthesized under the intrinsically reducing conditions in a piston cylinder, to check the effect of fO2 on iron speciation. The run products were characterized by field emission scanning electron microscope (FE-SEM), Rietveld refinements on XRPD synchrotron data, and space groups were assigned using SAED-TEM patterns. Run products are composed mainly of lithium clinopyroxene (Li-Cpx), plus minor amounts of hematite (magnetite under reducing condition) and corundum, as independently detected by image analysis (area%) and Rietveld refinements (wt%); moreover, Rietveld results were used to derive cell parameters, M1-site occupancy (Al vs. Fe3+), atomic positions, and average bond lengths of all these Li-Cpx indexed in the C2/c space groups according to SAED-TEM. Li-Cpx with Al and Fe3+ amounts close to 50:50 are actually slightly richer in Al apfu than nominal; the LiFe3+Si2O6 grown under very oxidized and reducing conditions have very similar cell parameters, indicating that fO2 is unable to induce a significant incorporation of Fe2+ in these Li-Cpx. The replacement of Al with Fe3+ induces a linear (%) increase of the cell edges following b > a > c, whereas β is roughly constant and the cell volume increases linearly. Furthermore, the substitution of Al with Fe3+ only weakly affects the T-O average length (<1%), whereas M2-O and M1-O bonds increase linearly of 2.3 and 5.0%, respectively. These new experimental data have been compared with other available on Li-, Na-, and Ca-Cpx, i.e., M2(Li,Na,Ca,Mg,Fe2+)M1(Mg,Fe2+Al,Ni,Cr,Ga,V,Fe3+,Mn,Sc,In)TSi2O6, to model lattice strain, bond lengths, steric effects, and phase transitions behaviors. The replacement of Al with progressively larger cations in LiM3+Si2O6 Cpx (M3+: Ni, Cr, Ga, V, Fe3+, Ti, Sc, and In) results in a linear increase following V > b > a > c, whereas β is roughly constant except for Ti-end-member and P21/c compositions. Lattice strains induced by X, T, and P for Li-Cpx in the C2/c stability field show that when the M1 site is progressively filled with a large cation, ε1 axis (ε1 > ε2 > ε3) increases along b, whereas ε2 and ε3 are nearly parallel to a and at about 30° from c. Conversely, T will provoke a similar enlargement of ε1 and ε2 along b and a edges, respectively, whereas ε3 is again oriented at about 30° from c; the increasing of P will instead shorten all strain tensor components (ε1, ε2, and ε3) with a similar percentage amount; notably, high-P is the only stress that induces a strain component to be almost parallel to c edge. Moreover, finite lattice strains and orientation in C2/c LiMe3+Si2O6 Li-Cpx induced by Me3+: Al-Fe3+, Fe3+-Sc, Sc-In are slightly different, with ε1 invariably lying along b; conversely, Li-Na cation substitution is completely different with the highest and lowest deformations on the ac plane and ε2 along b; ε3 vector is negative and oriented at about 30° from T-chains. The ideal replacement of Al with larger cations up to In in Li-Cpx induces the M1-O, M2-O, and T-O average bond lengths to increase by 10.6, 4.3, and <0.5%. Steric effects in LiM1Me3+Si2O6 and NaM1Me3+Si2O6 Cpx are significant and very similar, whereas several other Me1+ and Me2+ substitutions in Cpx at both the M1 and M2 site, keeping fixed the other site, display less or even the absence of steric effects. Our new data also better elucidate relationships between Li-Cpx composition, symmetry at room and non-ambient conditions and Tc. The aggregate cation radii at the M1 site does not exclusively control the stability of C2/c and P21/c polymorphs; instead valence electrons can profoundly favor the stabilization of a polymorph.

Nanomechanical characterization of K-basalt from Roman comagmatic province: A preliminary study

Based on atomic force microscopy (AFM), contact resonance AFM (CR-AFM) is a nondestructive technique that allows one to perform single point measurements as well as surface mapping of the indentation modulus of a material. In this work we exploit the possibility to use CR-AFM to study synthetic materials representative of K-basalt from Roman comagmatic Province. Having observed the presence of subsurface voids and inclusions at micrometer and sub-micrometer scale, a preliminary study has been conducted to verify the capability of CR-AFM nanomechanical mapping to nondestructively detect these features.Based on atomic force microscopy (AFM), contact resonance AFM (CR-AFM) is a nondestructive technique that allows one to perform single point measurements as well as surface mapping of the indentation modulus of a material. In this work we exploit the possibility to use CR-AFM to study synthetic materials representative of K-basalt from Roman comagmatic Province. Having observed the presence of subsurface voids and inclusions at micrometer and sub-micrometer scale, a preliminary study has been conducted to verify the capability of CR-AFM nanomechanical mapping to nondestructively detect these features.