Chiara Groppo

Micro-Raman spectroscopy for a quick and reliable identification of serpentine minerals from ultramafics

Identifying serpentine minerals in rocks is generally accomplished by means of Scanning Electron Microscopy - Energy Dispersion Spectrometry (SEM-EDS) and Transmission Electron Microscopy (TEM), both of which require complicated sample preparation. In this work, we evaluate the use of micro-Raman spectroscopy, which requires no sample preparation, in identifying the different serpentine minerals contained in thin sections of serpentinized peridotites, where the various phases occur in different microstructural positions. The micro-Raman spectra were obtained from samples previously characterized by optical microscopy, SEM-EDS and TEM. Micro-Raman spectroscopy proved to be a quick, easy and reliable method for the identification of serpentine minerals.

POTENTIAL TOXICITY OF NONREGULATED ASBESTIFORM MINERALS: BALANGEROITE FROM THE WESTERN ALPS. PART 1: IDENTIFICATION AND CHARACTERIZATION

In the Italian western Alps, asbestos mineralization (both chrysotile and tremolite amphibole) takes place from serpentinites, together with other less common asbestiform minerals not regulated by the current legislation. In the context of a study on the evaluation of the asbestos risk in this area, the possible role played by the associated asbestiform minerals in the overall toxicity of the airborne fraction has been examined. The first mineral investigated was balangeroite [(Mg,Fe2+,Fe3+,Mn2+)42Si16O54(OH)36], an iron-rich asbestiform contaminant of chrysotile from the Balangero mine (Piedmont), which crystallizes as rigid and brittle fibers. In order to prepare a sample in a form appropriate for chemical and cellular tests, the fibers were separated from the rock and comminuted without damage to their crystalline structure and surface state (as confirmed by X-ray diffraction [XRD] and ultraviolet–visible [UV-Vis] spectroscopy). The first properties examined were durability in simulated body fluids (Gamble’s solution) and toxicity to epithelial cells. When compared to UICC crocidolite (the amphibole blue asbestos, regarded as the most pathogenic form), balangeroite appears even more durable than crocidolite. Balangeroite and UICC crocidolite showed a similar in vitro cytotoxic effect on a human epithelial cell line, as evidenced by leakage of intracellular lactate dehydrogenase (LDH) activity, which, observed after a 24-h incubation, was dose dependent and maximal at 12 μg/cm2 for each fiber type. Data show that chemical composition, form, durability, and cell toxicity indicate balangeroite as a potentially harmful fibrous mineral that needs to be examined by further chemical and cellular tests.

Exhumation History of the UHPM Brossasco-Isasca Unit, Dora-Maira Massif, as Inferred from a Phengite-Amphibole Eclogite

A well-preserved phengite-amphibole eclogite (Br2F) from the UHP Brossasco-Isasca Unit (BIU) of the Dora-Maira Massif was studied in detail. The eclogite consists of the peak assemblage omphacite, garnet, phengite, rutile, and quartz. A porphyroblastic blue-green amphibole statically overgrows the eclogitic foliation defined by the preferred orientation of phengite flakes, and by the alignments of abundant accessory rutile grains. Both omphacite and phengite are partially replaced by fine-grained symplectites, consisting of clinopyroxene + albite and biotite + oligoclase, respectively. The metamorphic evolution of eclogite Br2F was reconstructed combining microstructural observations, conventional thermobarometry, and pseudosection analysis. A first pseudosection was calculated in the NKCFMASH system in the pressure range 5-45 kbar to model the peak and early retrogressive conditions, whereas a second pseudosection, calculated in the NCFMASH system, was used to model the albite + clinopyroxene symplectite after omphacite. Peak metamorphic conditions of P = 37.7 kbar and T = 732°C were estimated. The decompressional P-T path is associated with significant cooling from about 730°C at 38 kbar to 630°C at 14 kbar. These data, obtained combining pseudosection analysis with conventional thermobarometric methods, are in agreement with the P-T paths estimated from other lithologies by Hermann (2003), Castelli et al. (2004), and Groppo et al. (2006), and confirm that the BIU equilibrated within the diamond stability field.

Metamorphic CO2 production from calc-silicate rocks via garnet-forming reactions in the CFAS–H2O–CO2 system

The type and kinetics of metamorphic CO2-producing processes in metacarbonate rocks is of importance to understand the nature and magnitude of orogenic CO2 cycle. This paper focuses on CO2 production by garnet-forming reactions occurring in calc-silicate rocks. Phase equilibria in the CaO–FeO–Al2O3–SiO2–CO2–H2O (CFAS–CO2–H2O) system are investigated using P–T phase diagrams at fixed fluid composition, isobaric T–X(CO2) phase diagram sections and phase diagram projections in which fluid composition is unconstrained. The relevance of the CFAS–CO2–H2O garnet-bearing equilibria during metamorphic evolution of calc-silicate rocks is discussed in the light of the observed microstructures and measured mineral compositions in two representative samples of calc-silicate rocks from eastern Nepal Himalaya. The results of this study demonstrate that calc-silicate rocks may act as a significant CO2 source during prograde heating and/or early decompression. However, if the system remains closed, fluid–rock interactions may induce hydration of the calc-silicate assemblages and the in situ precipitation of graphite. The interplay between these two contrasting processes (production of CO2-rich fluids vs. carbon sequestration through graphite precipitation) must be considered when dealing with a global estimate of the role exerted by decarbonation processes on the orogenic CO2 cycle.

Fragments of the Western Alpine Chain as Historic Ornamental Stones in Turin (Italy): Enhancement of Urban Geological Heritage through Geotourism

In Piemonte, stone has always been the most widely used raw material for buildings, characterizing the architectural identity of the city of Turin. All kinds of rocks, metamorphic, igneous, and sedimentary, are represented, including gneisses, marbles, granitoids, and, less commonly, limestones. The great variety of ornamental stones is clearly due to the highly composite geological nature of the Piemonte region related to the presence of the orogenic Alpine chain and the sedimentary Tertiary Piemonte Basin. This paper provides a representative list of the most historic ornamental stones of Piemonte, which have been used over the centuries in buildings and architecture. The main stones occurring in Turin have been identified and described from a petrographic and mineralogical point of view in order to find out the corresponding geological units and quarry sites, from which they were exploited. This allows the associated cultural and scientific interest of stones to be emphasized in the architecture of a town which lies between a mountain chain and a hilly region.

Petrological constraints of the ‘Channel Flow’ model in eastern Nepal

Abstract The metamorphic architecture of eastern Nepalese Himalaya is characterized by a well-documented inverted metamorphic field gradient, with metamorphic grade increasing northward from lower (LHS) to higher (HHC) structural levels across the north-dipping Main Central Thrust Zone (MCTZ). Peak metamorphic conditions experienced by units at different structural levels have been investigated extensively, but their P–T–(t) evolution could be constrained better. A synthesis of our recent petrological studies in eastern Nepal is based on selected geotraverses across the Dudh–Kosi, Arun, and Tamur tectonic windows, where the LHS is exposed beneath MCTZ and HHC. To define the entire P–T evolution experienced by lithotectonic units, detailed petrological investigations were focused on metapelites. P–T trajectories were constrained combining microstructural observations and isochemical phase diagrams modelling. The uniformity of the approach applied is a robust method to quantitatively compare the resulting P–T paths. These P–T paths are compared with the petrological constraints inferred from the ‘Channel Flow’ model, one of the most popular paradigms to explain the tectonometamorphic evolution and the first-order geological features of the Himalaya. The overall geometries of our P–T paths match the results of the numerical model, suggesting that ‘Channel Flow’ is compatible, from a petrological viewpoint, as the main process operating during the exhumation of eastern Himalaya.

Geological map of the ultra-high pressure Brossasco-Isasca unit (Western Alps, Italy)

In the southern Dora-Maira Massif, Western Alps, slivers of continental crust with similar lithologies, but recrystallized during the Alpine orogeny at different peak-P conditions, are exposed. They include the Brossasco-Isasca Unit (BIU) where coesite was first discovered in continental crust. A new 1:20,000-scale geologic map and related cross-sections of the whole BIU and adjoining units is presented, in which the most significant features useful to infer the pre-Alpine history and the Alpine tectonic and metamorphic evolution, are summarized. Thanks to detailed petrography and petrology, the geologic map shows the precise location of ultra-high pressure (UHP) minerals (such as coesite), and the locations of the most significant mineral assemblages (such as kyanite + jadeite). This innovative approach is used to distinguish the BIU from the adjacent units. Relict pre-Alpine structures (such as igneous intrusive contacts with basement xenoliths and metagranitoids) are summarized in a sketch illustrating the geologic setting of the UHP metamorphic unit as inferred before the Alpine orogeny.

Dissolving dolomite in a stable UHP mineral assemblage: Evidence from Cal-Dol marbles of the Dora-Maira Massif (Italian Western Alps)

Abstract In deep and cold subduction such as that experienced by the UHP Units of the Western Alps, carbon dissolution is a relevant mechanism for carbon transfer from the slab into the mantle. The UHP impure Cal-Dol-marbles from the Dora-Maira Massif are studied to investigate the poorly known evolution of dolomite during deep subduction. Dolomite shows four stages of growth, from pre-Alpine to early-retrograde Alpine, coupled with chemical variations and distinct included mineral assemblages. To explain the evidence for growth and partial reabsorption of dolomite through HP prograde, UHP peak, and UHP early-retrograde Alpine metamorphism, a chemically simple marble (Cal, Dol, Di, Fo, and retrograde Atg, Tr, Mg-Chl) has been studied in detail. Microstructural relationships, coupled with mineral chemistry, indicate the growth of the assemblage dolomite+diopside+forsterite±aragonite during HP prograde, UHP peak, and UHP early-retrograde evolution. Mixed-volatile P-T projection modeled in the simple CaO-(FeO)-MgO-SiO2-H2O-CO2 system and T-P-XCO2 petrogenetic grids and pseudosections predict the prograde (1.7 GPa, 560 °C) growth of dolomite in equilibrium with diopside and forsterite through the breakdown of antigorite+aragonite. In a H2O-CO2-saturated system, the subsequent HP-UHP evolution is predicted in the Di+Fo+Dol+Arg stability field in equilibrium with a dominantly aqueous COH fluid [0.0003 < XCO2 < 0.0008], whose composition is internally buffered by the equilibrium assemblage. Thermodynamic modeling indicates that neither the consumption nor the growth of new dolomite generations at UHP conditions can have been induced by metamorphic reactions. The abundant primary H2O+Cal+Dol+Cl-rich Tr+Cl-rich Tlc±chloride fluid inclusions present in UHP Cpx indicate that a dominantly aqueous, saline (salinity >26.3 wt% of NaCleq) COH fluid, containing Ca, Mg, and Si as dissolved cations was present during the growth of the UHP assemblage Dol+Cpx+Ol+Arg. The complex zoning of dolomite is therefore interpreted as due to protracted episodes of dissolution and precipitation in saline aqueous fluids at HP/UHP conditions. Kinetics of dolomite dissolution in aqueous fluids is poorly known, and experimental and thermodynamic data under HP conditions are still lacking. Data on calcite indicate that dissolution at HP is enhanced by a prograde increase in both P and T, by high salinity in aqueous fluids, and/or low-pH conditions. In the studied marble, the P-T path and the occurrence of free high-saline fluids represent favorable conditions: (1) for the inferred dissolution-precipitation processes of the stable dolomite in a closed system, and (2) for possible migration of the dissolved carbonate, if the system would have been open during subduction.

Metamorphic CO2 Production in Collisional Orogens: Petrological Constraints from Phase Diagram Modeling of Himalayan, Scapolite-bearing, Calc-silicate Rocks in the NKC(F)MAS(T)-HC system

A reliable quantitative estimate of the metamorphic CO2 flux from collisional orogens is fundamental to our understanding of the deep carbon cycle, but it is still far from being constrained. Among major uncertainties are the poor knowledge of the nature of metamorphic CO2-producing processes and the amount of CO2 potentially released through these reactions. Previous studies of metamorphic decarbonation reactions in metacarbonate rocks mainly used simple model reactions between end-members in simplified model systems. However, fully quantitative modelling of calcsilicate rocks requires an investigation of very complex systems with more than six components. Moreover, scapolite solid solution has been rarely included in previous studies, although this mineral is often a major constituent of calc-silicate rocks. This study focuses on (1) the CO2-producing processes occurring in scapolite-bearing calc-silicate rocks and (2) the discussion of a methodological approach suitable to understand and quantify these processes. In this framework, phase relations and devolatilization reactions in the Na2O–K2O–CaO–(FeO)–MgO–Al2O3– SiO2–(TiO2)–H2O–CO2 [NKC(F)MAS(T)-HC] system are considered, with application to high-grade clinopyroxeneþ calciteþK-feldsparþ scapoliteþplagioclaseþ zoisite calc-silicate rocks from the Himalaya. The NKC(F)MAS(T)-HC equilibria involving scapolite and plagioclase solid solutions are investigated using (1) isobaric T–X(CO2) phase diagram sections and pseudosections and (2) mixed-volatile P–T phase diagram projections. This phase diagram approach allowed us to identify scapolite-bearing, CO2-producing, univariant (i.e. isobaric invariant) equilibria that have never been recognized before, and that could not be detected without considering Na–Ca solid solutions in the calculations. It is demonstrated that the investigated calc-silicate rocks behaved as a nearly closed, internally buffered, system during prograde metamorphism and that most of the observed key microstructures correspond to isobaric univariant or invariant assemblages. In such a nearly closed system, the fluid was mostly produced during prograde heating at the isobaric invariant points, where abrupt changes in mineral modes also occurred. The proposed phase diagram approach further allows quantitative estimation of the amount and composition of the fluid produced at such isobaric invariant points. On average, 2 5 mol of CO2 (110 g) per 1000 cm of reacting rock were produced during prograde metamorphism of this calc-silicate rock-type. Because similar scapolitebearing calc-silicate rocks are abundant in the Himalayan orogen, it is suggested that this calc-silicate rock-type might have produced large amounts of CO2-rich fluids during Himalayan metamorphism. A preliminary estimate of these amounts at the scale of the whole orogen suggests a total metamorphic CO2 production of (2–7) 10 mol, corresponding to (1–3) 10 Mt of CO2. Integrated over 20 Myr (i.e. the maximum duration of prograde metamorphism), the VC The Author 2017. Published by Oxford University Press. All rights reserved. For Permissions, please e-mail: journals.permissions@oup.com 53 J O U R N A L O F P E T R O L O G Y Journal of Petrology, 2017, Vol. 58, No. 1, 53–83 doi: 10.1093/petrology/egx005

Metamorphic CO2 Degassing in the Active Himalayan Orogen: Exploring the Influence of Orogenic Activity on the Long-Term Global Climate Changes

A number of studies suggest that mountain ranges have strong impact on the global carbon cycle; metamorphic degassing from active collisional orogens supplies a significant fraction of the global solid-Earth derived CO2 to the atmosphere, thus playing a fundamental role even in today’s Earth carbon cycle. The Himalayan belt, a major collisional orogen still active today, is a likely candidate for the production of a large amount of metamorphic CO2 that may have caused changes in long-term climate of the past, present and near future. Large metamorphic CO2 fluxes are facilitated by rapid prograde metamorphism of big volumes of impure carbonate rocks coupled with facile escape of CO2 to the Earth’s surface. So far, the incomplete knowledge of the nature, magnitude and distribution of the CO2-producing processes hampered a reliable quantitative modeling of metamorphic CO2 fluxes from the Himalayan belt. This study, integrated in the framework of the Ev-K2-CNR SHARE (Stations at High Altitude for Research on the Environment) Project, focuses on the metamorphic decarbonation processes occurring during the Himalayan collision. We hereby present preliminary results focusing on the distribution of different types of metacarbonate rocks in the Eastern Himalaya, their petrographic description and the first reported petrological data about the nature of the CO2-producing reactions in garnet-bearing calc-silicate rocks. These results represent a contribution toward a better understanding of the influence exerted by orogenic processes on climatic changes at global scale.