Raffaele Sassi

The b0 lattice parameter and chemistry of phengites from HP/LT metapelites

The statistical b 0 method is based on the fact that the cumulative b 0 values of white K-micas (WKM) from metapelites of a specific bulk composition can be used, in the lowermost T range of metamorphism, as a semiquantitative P indicator: mean b 0 values (and related celadonite content) in WKM increase with increasing P, thus characterizing the baric type of metamorphism. The solid petrological framework of this method includes specific constraints on rock bulk composition and need of other conditions. In this way, a reference b 0 scale for LT metapelites has become available in the literature, covering the whole range of metamorphic P. However, after many publications based on it, which made a large amount of data available, revision of this b 0 scale is now opportune, and the primary need is a better definition of the reference b 0 values and WKM chemistry for HP/LT metapelites. Such a better definition is the aim of this paper. The considered HP/LT metapelites come from various terrains: Schistes Lustres of W Alps, Diablo Range (Franciscan Complex of California), Phyllite-Quartzite Unit of Crete and Peloponnese, UHP gneiss from Dora Maira, Attic-Cycladic Complex of Sifnos, Tavsanli Zone and Nilufer area of NW Turkey. In addition, WKM chemical and b 0 data were used from various sources. The new chemical and b 0 data (EMPA and XRPD) allowed us: ( i ) to establish the reference b 0 and chemistry of WKM from HP/LT metapelites: b 0 turns out to be around 9.040 A; ( ii ) to confirm and extend to HP/LT metapelites the petrological basis of the b 0 method, particularly the control of mineral assemblages on b 0 values; ( iii ) to glimpse more complex relationships between b 0 and Si than those currently known, and particularly the importance of the Fe 2+ /(Mg + Fe 2+ ) ratio; ( iv ) to ascertain the validity of the b 0 method for detecting generic HP/LT conditions; (ν) to ascertain the inability of the b 0 method to discriminate, within the above HP field, differences between various P/T regimes, mainly due to still poorly known crystal-chemical problems, and the fact that comparative b 0 interpretations of various HP/LT sample populations from different belts are impeded by poor understanding of the extent of reequilibration of WKM at higher T and lower P during exhumation.

Hydrogen-bond and cation partitioning in muscovite: a single-crystal neutron-diffraction study at 295 and 20 K

Abstract The crystal chemistry of a pegmatitic Fe-bearing muscovite (with FeO ~5.1 wt%) from Val di Crana, Arvogno (Verbania, Nothern Italy) has been investigated by means of wavelength-dispersive X-ray spectroscopy and constant-wavelength [λ = 0.9462(2) Å] single-crystal neutron diffraction at 295 and 20 K (2θmax = 90°). The structure refinement at 295 K shows no significant deviation from the full occupancy of the K site (using the scattering length of potassium alone), and a disordered Si/Aldistribution in the two independent tetrahedral sites (i.e., T1 and T2) and Fe/Al-distribution in the M2 octahedral site. The difference Fourier map of the nuclear density shows that only one independent H site occurs in the muscovite structure, at both room and low temperature. No evidence of disorder in two sub-sites, as observed in a previous study, was found. The thermal displacement of the H site is here described anisotropically. A trifurcated hydrogen bonding scheme was found: the O6-H bond distance corrected for “riding motion” is ~0.984 Å at 295 K, and three significantly weak hydrogen bonds to the O atoms O2, O4, and O6 occur, with H···O2 = 2.635(5), H···O4 = 2.657(4), and H···O5 = 2.647(4) Å and O6-H···O angles all similar in value (~138°). The structure refinements show that the structure configuration of muscovite is maintained at least down to 20 K: the cation disordering in the tetrahedral and octahedral site and the H-bonding scheme are preserved. The structural evolution induced by decreasing temperature is governed by the contraction along a direction perpendicular to (001), mainly due to the compression of the “inner” K-O bonds, which leads to an increase of the tetrahedral rotation angle (α) of the six-membered ring. A further effect in response to lowering the temperature is a significant reduction of the magnitude of the thermal displacement parameters.

Multiple titanium substitutions in biotites from high-grade metapelitic xenoliths (Euganean Hills, Italy): Complete crystal chemistry and appraisal of petrologic control

Abstract Biotites from metapelitic xenoliths included within trachytes from the Euganean Hills (Italy) were analyzed by single-crystal X-ray diffraction (XRD), electron microprobe (EMP), scanning electron microscope (SEM), secondary ion mass spectrometry (SIMS), and Mössbauer spectroscopy. These biotites are Ti-rich and occur in gneissic xenoliths that underwent regional high-T/low-P metamorphism, at about 750 °C, followed by pyrometamorphism during incorporation in the melt at temperatures close to 950 °C. Biotites are zoned, with TiO2 content ranging from 6.79 (cores) to 8.14 wt% (rims). SIMS measurements show that the H2O content is in the range 2.88-4.08 wt%. The simultaneous occurrence of high-Ti and high-H2O contents, and the main cation substitutions based on EMP analyses suggest that the role of Ti-oxy in these biotites is less important than Ti-vacancy and Ti-Tschermak substitutions. Single-crystal XRD confirms that the Ti-oxy exchange was indeed effective but not the dominant substitution mechanism. Based on our data and those taken from literature on petrologically well-constrained systems, we propose that there is a petrologic control on the type of Ti-substitution mechanisms. We consider two types of petrologic groupings for biotites: (1) group A consisting of biotites from H2O-free or H2O-poor petrologic environments (e.g., volcanic rocks, ultrabasic xenoliths, and crustal xenoliths in which biotite underwent incongruent melting): Ti substitution in these biotites occurs via Ti-oxy predominantly, or more specifically Fe3+-Ti-oxy; and (2) group B consisting of biotites from H2O-rich petrologic environments (e.g., metamorphic rocks and crustal granitoids): Ti-vacancy, or more specifically Fe3+-Ti-vacancy, is the dominant mechanism in them. It is concluded that during high-grade metamorphism the dominating type of Ti substitution in biotite is controlled by H2O activity.

The baric character of the Patagonian basement as deduced from the muscovite d060,331̄ spacing a first contribution from Eastern Andean Metamorphic Complex (Andes, Chile)

In this work we present new data concerning the pressure character of metamorphism in the Aysen Eastern Andean Metamorphic Complex. The parameter d 060,331 of the K-white mica from basement politic rocks was measured as a barometric indicator. Three sample groups from different areas were considered, for a total of 69 samples. All measured values suggest a metamorphic setting characterised by intermediate to high pressure.

The octahedral sheet of metamorphic 2M1-phengites: A combined EMPA and AXANES study

Abstract Two types of metamorphic phengites are known: one is linked to high pressure and is 3T; the other is 2M1, and its composition is linked to rock-compositional constraints. This work investigates the octahedral sheet crystal-chemical differences between the two phengite types. Seven dioctahedral micas were studied: (1) one 3T phengite from an ultrahigh-pressure metagranitoid in the Dora Maira massif, Italy (P ~ 4.3 GPa, T ~ 730 °C); (2) five 2M1 phengites from medium-P orthogneisses in the Eastern Alps metamorphic basement, Italy (P ≤ 0.7 GPa, T ~ 500-600 °C); and (3) one 2M1 ferroan muscovite from pegmatite in Antarctica (P ≤ 0.2 GPa, T ~500 °C). All micas display significant extents of celadonite substitution. In particular, the 2M1-phengite formulae (calculated on the basis of 11 O) have 0.68 < IVAl < 0.82 atoms per formula unit (apfu); octahedral atoms are dominated by Al (1.6-1.8 apfu), with minor and variable Fe (0.20-0.35 apfu) and Mg (0.05-0.17 apfu), and very minor Ti, Mn, and Cr. Total octahedral occupancies are slightly above 2.00 apfu, i.e., there seems to be partial occupancy of the third M site. For all micas, we recorded XAFS spectra on mosaics of carefully separated flakes oriented flat on a plastic support that could be rotated so as to account for the polarization of the synchrotron radiation beam, and we processed them on the basis of the AXANES theory. Spectra show angle-dependent absorption variations for Al and Fe, which can be deconvoluted and fitted by dichroic effects. Pre-edges consistently show most Fe to be Fe3+ and little angle-dependent intensity variations. The 2M1-ferroan muscovite from Antarctica displays the same AXANES behavior as 2M1-phengites. By contrast, the ultrahigh-pressure 3T-phengite from Dora Maira (having IVAl = 0.42 apfu, and Al and Mg as the dominant octahedral constituents) has XAFS spectra that differ significantly. Not only is the IVAl feature strongly reduced, in agreement with the increased Si content, but also Fe XAFS spectra show one broad feature only, indicating that all Fe is Fe2+ in a fully disordered distribution with no angle-dependent variations. We conclude that this 3T-phengite is actually contaminated by exsolved Fe-bearing pyrope platelets, which cannot be resolved under SEM examination; by contrast, the 2M1-phengites, unrelated to high-pressure, suggest Al/Fe3+ order over the M1 and (M2, M3) sites, as also does the 2M1 pegmatitic muscovite.

Trachyte from the Roman aqueducts of Padua and Este (north-east Italy): a provenance study based on petrography, chemistry and magnetic susceptibility

This study presents the results of archaeometric analysis of the stones used to build the Roman aqueduct in Padua (northeast Italy), in order to identify the petrography and the quarry/ies of provenance from which the stones were exploited. The combined approach, involving petrographic, geochemical and magnetic susceptibility data, shows that the stones used for building the Roman aqueduct were of volcanic origin, with a trachyte composition, were quarried from two different localities in the Euganean Hills (Mt. Merlo and Mt. Oliveto) and that construction of the aqueducts, particularly of its various segments, both in the present-day city centre of Padua and in the Euganean Hills area, was centralised and managed by a single authority.

Experimental study of the seismic properties of the Eastern Alps (Italy) along the Aurina–Tures–Badia Valleys transect

Abstract In order to better constrain the interpretation and the nature of the seismic reflectors, experimental measurements at high confining pressure (up to 300 MPa) and room temperature of the compressional wave velocity (Vp) on 10 samples representative of the most common lithologies along the Aurina (Ahrntal), Tures (Tauferer Tal), and Badia (Abtei Tal) Valleys profile (Eastern Alps, Italy) have been performed. For each sample, the speed of ultrasonic waves was measured in three mutually perpendicular directions, parallel and normal to the rock foliation and lineation. The main results are: (a) Good agreement between the calculated vs. measured modal compositions of the considered rocks, indicating that they were presumably equilibrated at the estimated P – T conditions; therefore, the seismic properties are representative of the crustal level indicated by the thermobarometry. (b) Measured and calculated average Vp are in good agreement, and are typical of mid-crustal level (6.0–6.5 km/s). Only the amphibolites show Vp typical of the lower crust (7.2 km/s). (c) The seismic anisotropy of metapelites is very high (12–27%), both with orthorhombic and transverse isotropy symmetry; amphibolites are transversely isotropic with an anisotropy of 8%; orthogneisses and granitoids are isotropic or weakly anisotropic. (d) The contacts between amphibolites and all other rock types may generate good reflections, provided they are not steeply inclined. Although the metamorphic foliation remains steeply inclined, discordant buried sub-horizontal igneous contacts may be detected.

Sulla variability, compositiva dell’ amiche nellöambito di uno stesso affioramento di metapeliti di basso grado: un esempio. 1: Le biotiti

RiassuntoSono state analizzate 140 lamelle di miche (76 muscoviti e 64 biotiti) da 4 campioni di metapeliti di basso grado provenienti da un grande affioramento di basamento sudalpino in Pusteria. Le lamelle sono state scelte:(i) in ski microstrutturali diversi (fianchi e cerniere di micropieghe);(ii) in microsistemi chimici diversi (domini granoblastici e bande lepidoblastiche; lamelle a contatto con fasi diverse); e(iii) in modo da poter essere riferite per caratteri microstrutturali o allo stadio di cristallizzazione sincinematica Cr2 o a quello postcinematico Cr3. Vengono qui riportati i risultati analitici relativi alle biotiti, mentre si rimanda ad uno specifico lavoro per quanto riguarda le muscoviti e le considerazioni conclusive. Le biotiti dei letti granoblastici risultano sostanzialmente uguali a quelle dei letti lepidoblastici, così come le biotiti situate in zone di cerniera rispetto a quelle lungo i fianchi. Invece le biotiti cristallizzate durante Cr3 sono compositivamente diverse da quelle cristallizzate durante Cr2, e denotano temperature lievemente piu alte (pur essendo rimaste invariate le compatibilità mineralogiche). Sono state inoltre rilevate alcune peculiarità compositive delle biotiti in funzione delle fasi mineralogiche a contatto con le lameile analizzate.Abstract140 mica flakes (76 muscovites and 64 biotites) have been analyzed in 4 samples of lowgrade metapelites from a large outcrop of the Southalpine basement in Pusteria (Pustertal, Eastern Alps). The analyzed flakes have been selected:(i) in different microtextural sites (hinges vs. limbs of microfolds);(ii) in different chemical microsystems (granoblastic vs. lepidoblastic domains; flakes in contact with different phases); and(iii) in such a way that each flake could be either referred to the synkinematic crystallization stage Cr2 or to the postkinematic stage Cr3. The present paper reports the results obtained from the biotites, while the muscovites are considered in a separate paper, which also proposes some concluding remarks concerning both micas. Biotites from granoblastic domains are substantially identical to those from lepidoblastic domains, as well as biotites flakes located in the hinges of microfolds are identical to the flakes in the limbs. On the contrary, mica flakes crystallized during Cr2 compositionally differ from those crystallized during Cr3; in the latter case, biotite chemistry suggests slightly higher temperatures although the mineral compatibilities are unchanged. Some other chemical peculiarities of biotite flakes have been also detected: they depend on the different mineral phases in contact with the analyzed flake.

Characterization of a novel dual-core elliptical hollow optical fiber with wavelength decreasing differential group delay

The modal distribution of a novel elliptical hollow optical fiber is experimentally and numerically characterized. The fiber has a central elliptical air hole surrounded by a germanosilicate lanceolate ring core. Experiments reveal that the fiber behaves like a dual core waveguide and it is found that the differential group delay of each core decreases with wavelength with a PMD coefficient slope of ~10(-2) ps/m/THz. Experimental results are also compared with numerical modeling based on scanning electron microscopy images.

What drives the distribution in nature of 3T vs. 2M1 polytype in muscovites and phengites? A general assessment based on new data from metamorphic and igneous granitoid rocks

Abstract Petrologic, chemical, and polytype data are presented for dioctahedral potassic micas from Kfeldspar- bearing metamorphic and igneous rocks of acidic composition unaffected by high-pressure (HP) conditions. The paper aims to demonstrate that: (1) under non-HP conditions, in both metamorphic and igneous plutonic environments, a given bulk-rock compositional constraint imposes a more or less marked phengitic composition to dioctahedral potassic mica; and (2) this muscovite crystallizes as 2M1, notwithstanding its phengitic composition. The samples (157 in number) are from widespread provenances. We conclude that the growth of 3T polytype of muscovite is not a function of mica composition. This is consistent with the recent crystallographic knowledge on polytypism, cation ordering, elastic properties, and structural deformational mechanisms of muscovite, which address the stabilization of 3T with pressure.