Simona Ferrando

The chemical behavior of fluids released during deep subduction based on fluid inclusions

Abstract This review combines fluid inclusion data from (HP-)UHP rocks with experimental research and thermodynamic models to investigate the chemical and physical properties of fluids released during deep subduction, their solvent and element transport capacity, and the subsequent implications for the element recycling in the mantle wedge. An impressive number of fluid inclusion studies indicate three main populations of fluid inclusions in HP and UHP metamorphic rocks: (1) aqueous and/or non-polar gaseous fluid inclusions (FI); (2) multiphase solid inclusions (MSI); and (3) melt inclusions (MI). Chemical data from preserved fluid inclusions in rocks match with and implement “model” fluids by experiments and thermodynamics, revealing a continuity behind the extreme variations of physico-chemical properties of subduction-zone fluids. From fore-arc to sub-arc depths, fluids released by progressive devolatilization reactions from slab lithologies change from relatively diluted chloridebearing aqueous solutions (±N2), mainly influenced by halide ligands, to (alkali) aluminosilicate-rich aqueous fluids, in which polymerization probably governs the solubility and transport of major (e.g., Si and Al) and trace elements (including C). Fluid inclusion studies point to a reconsideration of the petrological models explaining deep volatile liberation, and their flux into the mantle wedge.

Fluid-rock interaction in UHP phengite-kyanite-epidote eclogite from the Sulu orogen, eastern China

Combined petrographic, minerochemical, fluid inclusion, and stable isotope studies have been carried out on phengite-kyanite-epidote (Phe-Ky-Ep) eclogites from Hushan and Qinglongshan (Donghai area, UHP Sulu orogen, China) to unravel their metamorphic evolution and fluid-rock interaction. A complex metamorphic evolution, from coesite-eclogite (P = 3.5-4.0 GPa and T = 840 ± 50°C) to greenschist facies conditions (P ≈ 0.2 GPa and T ≈ 350°C), through quartz-eclogite and HT amphibolite-facies conditions, was recognized. The associated fluids are aqueous and show a progressive change in salinity and composition, reflecting different metamorphic stages. Stable isotope data confirm that, prior to subduction, the Sulu eclogite protolith experienced a meteorichydrothermal alteration, and indicate that during metamorphic evolution the rocks recrystallized without pervasive fluid infiltration.

X-ray single-crystal structure refinement of an OH-rich topaz from Sulu UHP terrane (Eastern China) – Structural foundation of the correlation between cell parameters and fluorine content

An OH-rich topaz, Al 2 SiO 4 F 1.4 OH 0.6 , occurring in a kyanite quartzite of the Sulu UHP terrane (eastern China) has been optically and chemically (electron microprobe) characterised and its crystal structure refined by single-crystal X-ray diffraction data. Anisotropic refinement converged to R = 0.024 in the Pbnm space group with cell parameters a = 4.6696(8) A, b = 8.8486(17) A, c = 8.3915(22) A. The position of the hydrogen atom has been detected; because the OH → F substitution is less than 50 %, the disorder of H does not lower the overall symmetry. The influence of the OH → F substitution on the lattice parameters is discussed in terms of crystal structure and shown to affect primarily the a and b cell edges; instead c is almost constant. A critical revision of earlier linear correlations of the F content ( w f ) vs the lattice parameters and the availability of new data recently published, allowed to obtain improved regression lines for w f vs a ( r 2 = 0.982) and b ( r 2 = 0.968) cell edges.

Did Late Miocene (Messinian) gypsum precipitate from evaporated marine brines? Insights from the Piedmont Basin (Italy)

During the first stage of the Late Miocene Messinian salinity crisis (5.97–5.60 Ma), deposition of sulfates (the Primary Lower Gypsum) occurred in shallow silled peripheral subbasins of the Mediterranean undergoing restricted water exchange with the Atlantic Ocean. Fluid inclusions in Messinian selenite crystals from the Piedmont Basin (northwest Italy) have surprisingly low salinities (average of 1.6 wt% NaCl equivalent), suggesting that parent waters were depleted in Na + and Cl – compared to modern seawater. Modern gypsum from a Mediterranean salt work, in contrast, contains fluid inclusions with elevated salinities that match the normal evaporation trend expected for seawater. The salinity data indicate that the Messinian sulfate deposits from the Piedmont Basin formed from hybrid parent waters: seawater mixed with Ca 2+ and SO 4 2– enriched freshwaters that dissolved coeval marginal marine gypsum. Such mixed parent waters and complex recycling processes should be taken into account when explaining the genesis of other Messinian gypsum deposits across the Mediterranean Basin.

Late-Alpine rodingitization in the Bellecombe meta-ophiolites (Aosta Valley, Italian Western Alps): evidence from mineral assemblages and serpentinization-derived H2-bearing brine

We report on Alpine metamorphic and fluid inclusion evolution of a polyphase rodingite occurrence within the Bellecombe antigorite-serpentinite, exposed in the Piemonte zone of Aosta Valley, NW Italy. Fine-grained rodingitic rocks, derived from a protolith of basaltic dike(s), are cross-cut by a network of at least six vein generations, consisting of chlorite, diopside, and grossular garnet (Type I), andradite-grossular garnet + diopside (Type II), andradite-rich garnet + chlorite (Type III), grossular-rich garnet (Type IV), vesuvianite (Type V), and chlorite (Type VI). The fine-grained rodingite and associated veins reveal a tectono-metamorphic history similar to that of the hosting serpentinite and characterized by an earlier high pressure metamorphism, followed by decompression/re-equilibration under greenschist facies conditions and by final cooling. The fluid inclusion study, performed on primary fluid inclusions in vesuvianite from Type V veins and on secondary fluid inclusions in andradite-rich garnet from Type III veins, revealed that at P = 0.22 GPa and T = 400°C, an H2-bearing (XH2  = 0.010) brine (6 wt.% CaCl2 + 6 wt.% NaCl) with traces of CH4 (XCH4  = 0.002 was introduced into the rock. This fluid had a composition compatible with the Ca-rich H2-bearing, reducing aqueous solutions reported from the serpentinization front. These data point to an important event of rodingitization, probably triggered by hydration of metamorphic olivine, during the late greenschist facies Alpine evolution that was probably triggered by hydration of metamorphic olivine.

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.

Metamorphic history of HP mafic granulites from the Gesso-Stura Terrain (Argentera Massif, Western Alps, Italy)

At the Frisson Lakes (FL), a small layered mafic sequence surrounded by migmatitic orthogneisses is exposed as a tectonic window of the crystalline basement of the Gesso-Stura Terrain (GST) of the Argentera Massif (Western Alps, Italy) within its Meso-Cenozoic sedimentary cover. The mafic sequence is characterised by Pl-rich and Pl-poor layers of HP granulites consisting of porphyroclasts of Omp, Grt, Amp, Pl, and rare Qtz embedded in a fine-grained mylonitic matrix of neoblastic Amp, Pl, Cpx, Grt and minor Bt and Qtz. Symplectites of Amp + Pl partly replace porphyroclastic Grt and Omp. Chemical compositions of the same minerals from the Pl-rich and Pl-poor layers are different, suggesting very limited element diffusion between layers during metamorphic evolution. All samples show the following features: (i) porphyroclasts of Grt and Cpx characterised by cores rich in exsolved rutile and albite, respectively, (ii) zoned Cpx with Jd- and CaTi-Ts-richer core, and (iii) a systematic decrease in Al(VI) of Amp from the earliest porphyroclasts to the latest mylonitic neoblasts. Geological and petrological data indicate that the FL mafic sequence derived from the mylonitic deformation of a former layered mafic intrusion, which occurred before Carboniferous migmatization. The mafic sequence records a continuous metamorphic evolution characterised by: (i) a metamorphic peak (stage A) at HP granulite-facies conditions (735 ± 15 LC and 1.38 ± 0.05 GPa) in which coarse-grained minerals developed statically; (ii) an early decompression stage (stage B), which occurred at the transition between HP granulite- and HP-HT amphibolite-facies (709 ± 2 °C and 1.10 ± 0.02 GPa); (iii) a HT–MP amphibolite-facies stage (stage C) (665 ± 15 °C and 0.85 ± 0.15 GPa), which was accompanied by pervasive mylonitic deformation; and (iv) a final non-pervasive “symplectitic” stage (stage D) (500 < T < 625 °C and P < 0.59 GPa), which developed at shallow crustal levels, most likely favoured by a moderate influx of hydrous fluids. The P–T path reconstructed for the FL layered mafic sequence, for the first time quantitatively estimated for the Argentera Massif, allows to conclude that the granulites and eclogites previously reported in the GST belong to a single metamorphic stage, which developed at HP granulite-facies conditions. This metamorphic history, which appears to be similar to that recorded in other External Crystalline Massifs, is consistent with a subduction process followed by relatively rapid exhumation.

Fluid evolution from metamorphic peak to exhumation in Himalayan granulitised eclogites, Ama Drime range, southern Tibet

A fluid inclusion study has been carried out on granulitised eclogites and associated amphibolite-facies veins from the eastern Himalaya (Ama Drime range, southern Tibet) in order to better characterise the fluid evolution of continental crust involved in Cenozoic subduction and continent-continent collision processes. Six distinct events of fluid influx have been characterised on the basis of petrographic observations and microthermometric measurements: 1) a high-density - medium-salinity aqueous fluid with CO 2 , trapped at the eclogitic peak (metamorphic stage M1); 2) pure CO 2 , present at the granulitic stage M2; 3) a low-salinity – low-density aqueous fluid with minor CO 2 in equilibrium with amphibole-bearing LP-M/HT mineral assemblage (stage M4); 4) a low-density aqueo-carbonic fluid responsible of vein formation (stage M5); 5) a subsequent influx of a low-density CO 2 -rich fluid; 6) a late influx of very-low salinity aqueous fluid. Comparing these data with those obtained from other localities of the Himalayas and from other collisional orogens, an internal origin is proposed for eclogitic and granulitic fluids, whereas an origin from the underlying metasediments of the Lesser Himalaya is suggested for the subsequent types of fluid, safe for the meteoric origin proposed for the latest fluid influx.

Are the large filamentous microfossils preserved in Messinian gypsum colorless sulfide-oxidizing bacteria?

The thick gypsum deposits formed in the Mediterranean Basin during the Messinian salinity crisis incorporate dense mazes of filamentous fossils, which were interpreted as algae or cyanobacteria, thus pointing to a shallow-marine subtidal or intertidal environment. The data presented here reveal that these filaments represent remains of colorless, vacuolated sulfide-oxidizing bacteria. This interpretation is supported by the presence of small crystal aggregates of iron sulfide (pyrite) and associated polysulfide within the filamentous fossils. Pyrite and polysulfide are considered to result from early diagenetic transformation of original zero-valent sulfur globules stored within the cells, which is a clade-diagnostic feature of living and degraded sulfur bacteria. In addition to filamentous fossils, the studied gypsum crystals contain remains of euryhaline and stenohaline diatoms and clay-rich aggregates interpreted as alteration products of marine snow floccules. This peculiar fossil assemblage reflects conditions of increased productivity in the water column, triggered by high fluxes of nutrients into the basin during phases of enhanced riverine runoff and freshwater discharge. This study confirms that gypsum evaporites have great potential to preserve the early stages of the taphonomic alteration of bacterial cells, shedding light on the paleoecology of ancient hypersaline environments.

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.