Maria Luce Frezzotti

Carbon dragged at sub-arc depths and sequestered in the asthenospheric upper mantle during cold subduction is potentially released after millions of years during the breakup of continental plates. However, it is unclear whether these deep-carbon reservoirs can be locally remobilized on shorter-term timescales. Here we reveal the fate of carbon released during cold subduction by analyzing an anomalously deep earthquake in December 2020 in the lithospheric mantle beneath Milan (Italy), above a deep-carbon reservoir previously imaged in the mantle wedge by geophysical methods. We show that the earthquake source moment tensor includes a major explosive component that we ascribe to carbon-rich melt/fluid migration along upper-mantle shear zones and rapid release of about 17,000 tons of carbon dioxide when ascending melts exit the carbonate stability field. Our results underline the importance of carbon-rich melts at active continental margins for emission budgets and suggest their potent...

ABSTRACT Here we discuss the fate of subducted carbonates and its implications for recycling of crustal carbon. Thermodynamic models predict little decarbonation along most subduction geotherms, and the mechanisms by which carbon is transferred from the subducting slab to the overlying mantle remain poorly constrained. Diamond-bearing fluid inclusions in garnet in oceanic metasedimentary rocks from Lago di Cignana (western Alps) represent the first occurrence of diamond from a low-temperature subduction complex of clearly oceanic origin (T ≤600°C; P ≥3.5 GPa). The presence of diamonds in and associated with fluid inclusions provides clear evidence of carbon transport by fluids at depths that are directly relevant to slab-mantle fluid transfer during subduction. At room temperature, the fluid inclusions contain aqueous fluid, a vapor bubble, and multiple solid daughter crystals. Daughter crystals identified by Raman spectroscopy and microprobe analysis include ubiquitous Mg-calcite/calcite and rutile, and less common diamond, quartz, paragonite, dawsonite, rhodochrosite, dypingite, and pentahydrite. Molecular CO2 is absent or in trace amounts. The aqueous liquid phase contains ≥0.2 wt%, HCO3-, CO32-, and SO42- ions. In Raman spectra, broad peaks at 773 and 1017 cm-1 point to the presence of both Si(OH)4(aq) and deprotonated monomers (e.g., SiO(OH)3-(aq), and SiO2(OH)22-(aq)), indicative of alkaline solutions. The absence of CO2 in the vapor, and the presence of carbonate daughter minerals, CO32-(aq), and HCO3-(aq) also show that the trapped fluids are alkaline at ambient conditions. High activities of aqueous carbon species reveal that carbonate dissolution is an important mechanism for mobilizing slab carbon at sub-arc depths (100-200 km) during oceanic subduction. Our results imply that the magnitude of carbon release and transport from the slab at sub-arc depths is greater than experimentally predicted on the basis of decarbonation reactions alone.

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: i) aqueous and/or non-polar gaseous fluid inclusions (FI), ii) multiphase solid inclusions (MSI), and iii) 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 chloride-bearing...

We report SEM-EDS, WDS, micro-Raman, IR-synchrotron-radiation imaging, and LA-ICP-MS data on minerals and multiphase solid inclusions (MSI) in equilibrium at UHP peak in Dora-Maira whiteschists. These rocks consist of pyrope-rich garnet, quartz/coesite, phengite, kyanite, talc, chlorite, and minor Mg-rich minerals. Three generations of pyrope occur: 1) prograde reddish megablasts (10-20 cm across); 2) zoned porphyroblasts (2-10 cm across) with prograde core and peak rim; and 3) peak smaller crystals (<2 cm across). Peak assemblage consists of pyrope (Prp82-98Alm1-16), coesite, phengite (Si=3.55-3.58 a.p.f.u.), kyanite, talc, and accessory rutile, zircon and monazite. P-T conditions of UHP peak have been estimated to be 4.3 GPa and 730°C. The fluid phase present at UHP conditions may be inferred from primary MSI (10-30 microns across) within peak pyrope, which contain Mg-Chl, Na-K-Phl, Cl-rich Ap, Zn-rich Py, and chlorides as daughter minerals, Tlc and Mgs as step-daughter minerals, and Rt, Zrn and Mnz as incidentally-trapped minerals. IR-synchrotron- radiation reveals the presence of an aqueous fluid in the inclusions and a significant water diffusion from MSI to host pyrope. Trace elements and REE patterns collected by LA-ICP-MS show substantial enrichment in LILE and depletion in HFSE in these fluids. Possible mechanisms for LILE-enriched fluid generation during the growth of pyrope at UHP conditions in the Dora Maira whiteschists are discussed.

Recent discovery of multiphase solid inclusions (MSI) in peak minerals from ultra-high pressure (UHP) terranes opened up new prospects for understanding the fluid-rock interaction during deep subduction in both crust and mantle. The first report on MSI in UHP rocks was from Dora-Maira (DM; Case Parigi; western Alps) whiteschists, more than ten years ago (Philippot et al, 1995, CMP, 121, 29-44). Nevertheless, the nature of such a fluid, and its role on the origin of the unusual composition of these rocks is still matter of debate. We report data on inclusions in DM UHP pyropes and HP prograde kyanite, part of them from a new sampling site (SSW Case Parigi). Primary MSI (30 micron) are present only in small UHP pyropes (1 - 6 cm) and often show post-entrapment decrepitation. Each MSI contains Mg-chlorite, Na-phlogopite, minor Cl-rich apatite, talc, pyrite, magnesite, Ca-rich chlorides +/- liquid water. Maps of total water concentrations collected in MSI-rich pyropes by infrared synchrotron radiation show gradients that suggest considerable H diffusion from inclusions into the host garnet (Frezzotti et al, 2007, abstract ECROFI XIX). In prograde kyanite, rare fluid inclusions are high salinity brines, containing different salts. Present data indicate that at HP conditions brines were present in the rocks and that at UHP peak aqueous fluids were enriched in Si, Al, Mg, Na, Ca, but still containing significant amounts of Cl, P, S, C. DM whiteschists are commonly considered metasomatic rocks from a granitic protolith. Our data on MSI in UHP pyrope and on rare brines in prograde HP kyanite strongly support metasomatism by external high-Ca-Mg fluids, probably evolved during serpentinite dehydration as proposed by Sharp and Barnes (2004, EPSL, 226, 243-254). Present data support the model of Compagnoni and Hirajima (2001, Lithos, 57, 219-236), who proposed that metasomatic fluids were introduced into the system during prograde metamorphism, channelled along shear zones cutting across the precursor granitoid/orthogneiss.

Fluid inclusions in garnet pyroxenite xenoliths from Salt lake Crater (Oahu) preserve evidence for a high flux of deep (diamond C-O-H-S) fluids (or melts) in the mantle beneath Hawaii. Garnet pyroxenites are dry and consist of clinopyroxene, orthopyroxene, olivine and garnet; they are interpreted as magmatic segregations (cumulates) from alkali-basaltic magmas within upper mantle lherzolites, reequibrated within the Hawaiian lithosphere at 1000-1150°C and 1.6-2.5 GPa (~50-80 km). In all mineral phases, CO2-rich fluid inclusions are extremely abundant, forming intergranular and intragranular trails. In ortho- and clinopyroxene, a few isolated early fluid inclusions are preserved (3 -50 mu m), which were trapped at an earlier stage and before cooling of host rocks in the mantle. Early inclusions contain exceptionally high-density CO2-rich fluids (superdense; d = 1.21 g/cm3), corresponding to pressures of 1.8-2 GPa, at the inferred mantle temperatures (Frezzotti et. al., 1992). Raman analyses in selected high-density and superdense early CO2 inclusions reveals vibrational bands (3638 cm-1 and 2609 cm-1), which are characteristic of OH- stretching vibrations in isolated H2O molecules, and of H2S, respectively. A few among inclusions additionally show the typical vibration for diamond at 1332 cm-1. Fluids contained within inclusions are CO2-rich, and contain minor amounts of H2O and H2S ± diamond, defining a complex fluid mixture in the C-O-H-S system in the mantle beneath Hawaii, at P ~ 2 GPa. The non-systematic presence of diamonds within fluid inclusions indicates that diamonds were already present in mantle fluids at the time of trapping as inclusions, and constrains fluid origin at pressures above 5 GPa (~160 km). At such high pressures, the fluid phase may have been a CO2-rich melt (i.e. carbonatitic) containing water and H2S, or a CH4-H2O-H2S fluid, depending on local fO2 conditions (Green and Falloon, 1998; Gudfinnsson and Presnall, 2005). Present data suggest that significant amounts of (diamond) CO2-H2S-H2O fluids are present in the mantle beneath Hawaii. A high (C-O-H-S) volatile flux rising from the asthenosphere represents a strong metasomatic agent able to carry very high concentrations of incompatible trace elements, and to strongly increase mantle fusibility (Gudfinnsson and Presnall, 2005). Such rising fluids or melts could induce partial melting at the base of the lithosphere at normal mantle temperatures, obviating the need for concentrated hot jets localized under ``hotspot&#x27;&#x27; volcanoes. Preservation of diamonds within fluid inclusions further suggests a fluid phase evolution in a ``cold&#x27;&#x27; mantle environment. References. Frezzotti, M.L., Burke, E.A.J., De Vivo B., Stefanini B. &amp; Villa I.M. (1992) Eur. J. Mineral., 4, 1137-1153. Green, D. H. and Falloon, T. J. (1998) Pyrolite: A Ringwood concept and its current expression. pp 311-380 in The Earth&#x27;s Mantle; Composition, Structure, and Evolution, ed I.N.S. Jackson, Cambridge, Cambridge University Press, 566 pp. Gudfinnsson, G.H. and Presnall, D. C. (2005). J. Petrology, 46, 1645-1659.

Composition and provenance of slab-derived hydrous fluids and melts represent a key parameter in understanding the geochemical variations of subduction-related magmas. Chemical and physical characteristics of the slab components may be strongly reliant upon both the thermal structure of the mantle wedge, and the nature of the subducted lithosphere. While fluids are well known to play a key role in element recycling during subduction of oceanic crust, it is less clear how dehydration and/or melting occur during subduction of continental crust. A direct approach to get information on the nature of fluids is provided by fluid inclusion analysis. Two examples of peak fluid inclusions in UHP metamorphic rocks from the Su-Lu Terrane (China) and the Dora-Maira Massif (Alps) are discussed. In the Su-Lu terrane, primary multiphase solid (MS) inclusions within UHP quartzites represent fluids internally generated through dehydration reactions at pressure above 3.5 GPa. Inclusions contain paragonite, muscovite, sulfates, carbonates, phosphates, and oxides. Calculated compositions indicate aqueous fluids (H2O = 25 - 50 wt %) containing tens of wt % of Si, Al, and alkalies. Trace element patterns show enrichments in large-ion lithophile elements (LILE; K, Sr, Rb, Cs, Ba, Pb, U) and light rare earth elements (LREE). In the Dora-Maira whiteschists, MS inclusions formed at 730°C and ≤ 4GPa have rather complex compositions and consist of Mg-chlorite, Na-phlogopite, Cl-rich apatite, Zn-rich pyrite, and chlorides, with subordinate talc and magnesite. Although water is absent, IR-synchrotron-radiation mapping revealed significant H+ diffusion from MS inclusions to host garnet. Peak aqueous solutions are Si, Al, and Na-rich, and contain significant amounts of Mg, Ca, P, S, and LILE, supporting an open system fluid generation with significant contribution from external brines derived from dehydration of oceanic lithosphere during subduction. Although the studied rocks have been collected from two distinct mountain belts, fluid data show remarkable similarities. Intermediate aqueous solutions (H2O ≤ 50 wt %) may be produced during dehydration reactions of subducted continental lithosphere at depths > 120-150 km. A remarkable feature of this fluid is represented by the dominant alkali-alumino-silicate character of the solutes. Further, solutions are concentrated, resulting in an unusual composition, which is intermediate between a fluid and a melt. Deep fluids show trace element distribution characterized by significant Ti and Nb (and Ta) depletions and LILE enrichments, resulting in patterns close to upper continental crust composition. Following subduction at depth, aqueous solutions released from continental lithologies may interact with the peridotitic mantle, imprinting their enriched signatures. Addition to mantle rocks of fluid-transported upper continental crust components could result in a composition close to EM2 (enriched mantle 2) mantle end-member, supporting a major role of continental subduction in fluid mediated element recycling.

Quartz-rich xenoliths in lavas (basalts to andesites; 90-30 ka) from Alicudi contain abundant melt and fluid inclusions. Two generations of CO2-rich fluid inclusions are present in quartz-rich xenolith grains: early (Type I) inclusions related to partial melting of the host xenoliths, and late Type II inclusions related to the fluid trapping during xenolith ascent. Homogenisation temperatures of fluid inclusions correspond to two density intervals: 0.93-0.68 g/cm3 (Type I) and 0.47-0.26 g/cm3 (Type II). Early Type I fluid inclusions indicate trapping pressures around 6 kbar, which are representative for the levels of partial melting of crustal rocks and xenolith formation. Late Type II fluid inclusions show lower trapping pressures, between 1.7 kbar and 0.2 kbar, indicative for shallow magma rest and accumulation during ascent to the surface. Data suggest the presence of two magma reservoirs: the first is located at lower crustal depths (about 24 km), site of fractional crystallizat...

Pressure-induced amorphization (PIA) in the solid state (i.e., without melting) is a well-known process that has been experimentally observed for several materials and minerals1-2, including alpha-quartz1-4. Apart from meteorite impacts5, it has been predicted as irrelevant under natural P-T conditions because it can develop only under high pressures (e.g., 15-40 GPa for alpha-quartz1-4,6), in metastable materials, either at very low

Composition and provenance of slab-derived hydrous fluids and melts represent a key parameter in understanding the geochemical variations of subduction-related magmas. Chemical and physical characteristics of the slab components may be strongly reliant upon both the thermal structure of the mantle wedge, and the nature of the subducted lithosphere. While fluids are well known to play a key role in

Several mantle xenoliths from the island of Sao Miguel (Azores, Portugal) have been studied to investigate the nature of the mantle beneath the Azorean archipelago. Ultramafic xenoliths are porphyroclastic spinel harzburgite and subordinate clinopyroxene-poor lherzolite, range between 3 and 10 cm in size and show clear signs of plastic deformation. In harzburgites, olivine porphyroclasts have Fo89-91, while in neoblasts it ranges between 80 and 87. Mg# in orthopyroxene and clinopyroxene spans between 85-91 and 84-92 respectively, while spinel is characterized by Cr# between 64 and 78. Two harzburgites show phlogopite. In lherzolite, olivine porphyroclasts show Fo89-91, orthopyroxenes and clinopyroxenes have Mg# 91-92 and 90-92 respectively, and spinel has Cr# 76-84. Abundant silica- and alkali-rich glass is present as intergranular micro-veins, and as primary melt inclusions in both porphyroclasts and neoblasts. Orthopyroxene porphyroclasts (1 - 6 mm) have exsolution-free rims, stro...

ABSTRACT Pure CO2 fluid inclusions are observed in fifteen quartz-rich xenoliths collected in basaltic-andesitic to andesitic volcanic products relevant to the older evolutionary stages of Lipari Island (223-105 ka). In volcanics forming central composite volcanoes (M. Mazzacaruso, 223-127 ka; M. S.Angelo, 105 ka), fluid inclusions are trapped during two distinct events: early Type I inclusions formed before host magma transport, and late (i.e. secondary) Type II inclusions trapped during magma ascent. Early Type I inclusions show homogenization temperatures corresponding to densities from 0.9 to 0.6 g/cc, while Type II inclusions record a considerably lower density interval between 0.38 and 0.1 g/cc. At the estimated trapping temperatures between 950 and 1090°C, obtained density values correspond to pressures of 0.58- 0.25 GPa (22-10 km) for Type I, and 0.13-0.03 GPa (5.5-1 km) for Type II inclusions, respectively. In those magmas erupted from fissural eruptive vents aligned along the main regional NNW-SSE and E-W faults systems (Timpone Ospedale, Monterosa and M. Chirica; 223-127 ka) only early Type I inclusions are observed. Density values form to two distinct intervals between 0.87 and 0.6 g/cc (0.53-0.25 GPa; 20-10 km; M. Chirica), and between 0.68-0.18 g/cc (0.32-0.05 GPa; 12-2 km; Timpone Ospedale and Monterosa). Fluid inclusion data together with tectonic features outline a complex magma storage and ascent evolution during the Lipari&amp;#39;s older evolutionary stages. Beneath the central volcanoes of M. Mazzacaruso, M. S.Angelo and the M. Chirica, two magma reservoirs, located at lower crustal depths (~22 km; close to the Moho) and at very shallow levels (5.5-1 km), are present. Mantle-Derived magmas are accumulated into the deep magma chamber and may then reside in the shallower reservoir for a short period of time before being erupted to the surface. Such a magma feeding system is similar to those outlined for the Alicudi and Stromboli volcanoes, and for most of the Vulcano&amp;#39;s eruptive stages. Conversely, an intermediate magma reservoir at middle crustal levels (~12 km) is shown to play an important role in storage and differentiation processes of mafic magmas relevant to the fissural eruptive vents of Timpone Ospedale, Monterosa, and M. Chirica. Magmas more likely arise from the deepest magma storage level located close to the Moho, as outlined by M. Chirica eruptive vent. The proposed scenario is that the regional fault systems control the magma storage evolution, contributing to create a zone of preferential accumulation at Mid- Crustal levels. Fault systems may also influence magma ascent and determine the upward magma movement directly to the eruptive system without an effective ponding in the shallow reservoirs located at 5-1 km depth. At middle crustal depths, magmas may reside for long time, and low rates of fractional crystallization occur. The occurrence of an intermediate storage level at similar depths beneath the rhyolitic Lentia domes at Vulcano, which are aligned along the main NNW-SSE tectonic trend, supports present model.

ABSTRACT The Alicudi composite volcano (western Aeolian archipelago) was constructed between c. 106 and 28 ka by lava flows, domes and strombolian scoriae erupted during six Eruptive Epochs, interrupted by periods of dormancy and three caldera-type collapses in the summit area. Marine Isotope Stage (MIS) 5a (81 ka) terrace deposits and widespread Brown Tuffs of external origin are recognized and provide important marker beds for regional stratigraphic correlations. Volcanism was of central type, under control of the summit caldera collapses with negligible influence of regional tectonic trends. Alicudi rocks are basaltic to high-K andesitic and have the most primitive petrological compositions (high MgO, Ni, Cr contents), the lowest Sr–O and the highest Nd–He isotope ratios (87Sr/86Sr=0.70352 to 0.70410; 143Nd/144Nd=0.51289 to 0.51279; δ18O=+5.0 to 5.6; 3He/4He–R/Ra=c. 6.5 to 7.1) over the entire Aeolian archipelago. Their composition and variation through time are the result of polybaric crystal–liquid fractionation of parental calc-alkaline basalts to give basaltic andesitic and andesitic derivative melts. These underwent crustal assimilation during ascent, with basalts being contaminated more strongly than andesitic magmas. Sr–Nd–Pb isotopes suggest source metasomatic modification by fluids from an oceanic-type slab, with a minor role for subducted sediments. DVD: The 10 000 scale geological map of Alicudi is included on the DVD in the printed book and can also be accessed online at http://www.geolsoc.org.uk/Memoir37-electronic. Also included is a geochemical dataset for Alicudi.

We report SEM-EDS, WDS, micro-Raman, IR-synchrotron-radiation imaging, and LA-ICP-MS data on minerals and multiphase solid inclusions (MSI) in equilibrium at UHP peak in Dora-Maira whiteschists. These rocks consist of pyrope-rich garnet, quartz/coesite, phengite, kyanite, talc, chlorite, and minor Mg-rich minerals. Three generations of pyrope occur: 1) prograde reddish megablasts (10-20 cm across); 2) zoned porphyroblasts (2-10 cm across) with prograde

Silicate-melt inclusions in igneous rocks provide important information on the composition and evolution of magmatic systems. Such inclusions represent accidentally trapped silicate melt (±immiscible H2O and/or CO2 fluids) that allow one to follow the evolution of magmas through snapshots, corresponding to specific evolution steps. This information is available on condition that they remained isolated from the enclosing magma after their entrapment. The following steps of investigation are discussed: (a) detailed petrographic studies to characterise silicate-melt inclusion primary characters and posttrapping evolution, including melt crystallisation; (b) high temperature studies to rehomogenise the inclusion content and select chemically representative inclusions: chemical compositions should be compared to relevant phase diagrams.Silicate-melt inclusion studies allow us to concentrate on specific topics; inclusion studies in early crystallising phases allow the characterisation of primary magmas, while in more differentiated rocks, they unravel the subsequent chemical evolution. The distribution of volatile species (i.e., H2O, CO2, S, Cl) in inclusion glass can provide information on the degassing processes and on recycling of subducted material. In intrusive rocks, silicate melt inclusions may preserve direct evidence of magmatic stage evolution (e.g., immiscibility phenomena). Melt inclusions in mantle xenoliths indicate that high-silica melts can coexist with mantle peridotites and give information on the presence of carbonate melt within the upper mantle. Thus, combining silicate-melt inclusion data with conventional petrological and geochemical information and experimental petrology can increase our ability to model magmatic processes.