Erro-Tobbio Peridotite Research Articles

Formation of Ultramylonites in an Upper Mantle Shear Zone, Erro-Tobbio, Italy

Deformation in the upper mantle is localized in shear zones. In order to localize strain, weakening has to occur, which can be achieved by a reduction in grain size. In order for grains to remain small and preserve shear zones, phases have to mix. Phase mixing leads to dragging or pinning of grain boundaries which slows down or halts grain growth. Multiple phase mixing processes have been suggested to be important during shear zone evolution. The importance of a phase mixing process depends on the geodynamic setting. This study presents detailed microstructural analysis of spinel bearing shear zones from the Erro-Tobbio peridotite (Italy) that formed during pre-alpine rifting. The first stage of deformation occurred under melt-free conditions, during which clinopyroxene and olivine porphyroclasts dynamically recrystallized. With ongoing extension, silica-undersaturated melt percolated through the shear zones and reacted with the clinopyroxene neoblasts, forming olivine–clinopyroxene layers. Furthermore, the melt reacted with orthopyroxene porphyroclasts, forming fine-grained polymineralic layers (ultramylonites) adjacent to the porphyroclasts. Strain rates in these layers are estimated to be about an order of magnitude faster than within the olivine-rich matrix. This study demonstrates the importance of melt-rock reactions for grain size reduction, phase mixing and strain localization in these shear zones.

The role of brucite in water and element cycling during serpentinite subduction – Insights from Erro Tobbio (Liguria, Italy)

The Erro Tobbio olivine-antigorite serpentinites and associated dehydration veins represent hydrated oceanic mantle rocks that escaped complete dehydration and recycling into the mantle after subduction to ~ 550–600 °C and 2.0–2.5 GPa. These rocks thus offer valuable insights into the petrological evolution of a slice of hydrated oceanic mantle and the geochemical cycling down to intermediate subduction zone depths. Our study emphasises the role of brucite upon rock-buffered hydration and subduction dehydration employing bulk and in situ chemical data sets combined with petrology.Bulk rock data reveal a coherent mantle peridotite slice affected by variable melt depletion and refertilisation. Subsequent fluid-rock interaction stages proceeded isochemically with respect to SiO2, i.e., without significant SiO2 enrichment characteristic for hydrothermal ocean floor serpentinisation. Relicts of low-T mesh textures after olivine and preservation of precursor mineral and low-T hydration geochemical features indicate a lack of subsequent fluid and metamorphic overprinting, even on scales of tens of micrometres. Fluid-mobile element enrichments are modest with exceptions for B and W. Enrichment signatures of U/Cs < 1 and Rb/Cs of 4–26 are characteristic of shallow forearc hydration within or atop the slab by fluids derived from breakdown of clays or first dehydration of altered oceanic crust with a subordinate sedimentary pore fluid component. Overall, the geochemical and petrological changes of the Erro Tobbio peridotites during fluid-rock interactions were rock-buffered, in contrast to fluid-buffered hydration accompanied with significant SiO2 metasomatism at, e.g., mid ocean ridges.Silica-neutral rock-buffered serpentinisation resulted in prominent brucite formation upon olivine hydration. In absence of excess SiO2, subsequent serpentine transformation of chrysotile/lizardite to antigorite likely produced even more brucite. Rock-buffered fluid-rock interactions thus provide a mechanism for stabilising brucite in subduction zone serpentinites, presumably along hydration fronts and within deeper sections of the oceanic lithospheric mantle. Finally, brucite + antigorite dehydration produced up to 40 vol% of metamorphic olivine and prominent olivine + Ti-clinohumite + magnetite vein networks at temperatures <550–600 °C, prior to complete antigorite breakdown. Wall rocks released alkali elements, B, Cr, As, Sb, and Ba into the dehydration fluids, along with substantial Sr, REE and HFSE redistribution into vein minerals.

Tectonic and magmatic evolution of the mantle lithosphere during the rifting stages of a fossil slow–ultraslow spreading basin: insights from the Erro–Tobbio peridotite (Voltri Massif, NW Italy)

Abstract We investigate the structural, petrological and compositional features recorded by strongly deformed and melt-percolated Erro–Tobbio peridotites (Voltri Massif, Ligurian Alps, NW Italy), in order to demonstrate that the processes of shear-zone formation and melt percolation are intimately linked by a positive feedback. We focus on spinel and plagioclase peridotites, and extensional shear zones that underwent infiltration by upwelling asthenospheric melts. Shear and porosity bands, which developed during extension prior to melt infiltration, represent important structural and rheological pathways to facilitate and enhance melt infiltration into the extending lithosphere and the ascent of such melts to shallower levels. Our results lend strong support to numerical models addressing the physical processes underlying extensional systems. These show that, in the case of slow–ultraslow continental extension and the subsequent formation of slow–ultraslow spreading oceans, porosity and shear-localization bands may develop in a previously unstructured lithosphere, prior to melt infiltration. Our studies on the Erro–Tobbio peridotites allow a model for the inception of continental extension and rifting to drifting of slow–ultraslow spreading oceans to be proposed. We suggest that integrated studies of on-land peridotites, coupled with geophysical–structural results from modern oceans, may provide clues to the geodynamic processes governing continental extension and passive rifting.

Subduction of a fossil slow–ultraslow spreading ocean: a petrology-constrained geodynamic model based on the Voltri Massif, Ligurian Alps, Northwest Italy

Slow–ultraslow spreading oceans are mostly floored by mantle peridotites and are typified by rifted continental margins, where subcontinental lithospheric mantle is preserved. Structural and petrologic investigations of the high-pressure (HP) Alpine Voltri Massif ophiolites, which were derived from the Late Jurassic Ligurian Tethys fossil slow–ultraslow spreading ocean, reveal the fate of the oceanic peridotites/serpentinites during subduction to depths involving eclogite-facies conditions, followed by exhumation. The Ligurian Tethys was formed by continental extension within the Europe–Adria lithosphere and consisted of sea-floor exposed mantle peridotites with an uppermost layer of oceanic serpentinites and of subcontinental lithospheric mantle at the rifted continental margins. Plate convergence caused eastward subduction of the oceanic lithosphere of the Europe plate and the uppermost serpentinite layer of the subducting slab formed an antigorite serpentinite-subduction channel. Sectors of the rather unaltered mantle lithosphere of the Adria extended margin underwent ablative subduction and were detached, embedded, and buried to eclogite-facies conditions within the serpentinite-subduction channel. At such P–T conditions, antigorite serpentinites from the oceanic slab underwent partial HP dehydration (antigorite dewatering and growth of new olivine). Water fluxing from partial dehydration of host serpentinites caused partial HP hydration (growth of Ti-clinohumite and antigorite) of the subducted Adria margin peridotites. The serpentinite-subduction channel (future Beigua serpentinites), acting as a low-viscosity carrier for high-density subducted rocks, allowed rapid exhumation of the almost unaltered Adria peridotites (future Erro–Tobbio peridotites) and their emplacement into the Voltri Massif orogenic edifice. Over in the past 35 years, this unique geologic architecture has allowed us to investigate the pristine structural and compositional mantle features of the subcontinental Erro–Tobbio peridotites and to clarify the main steps of the pre-oceanic extensional, tectonic–magmatic history of the Europe–Adria asthenosphere–lithosphere system, which led to the formation of the Ligurian Tethys. Our present knowledge of the Voltri Massif provides fundamental information for enhanced understanding, from a mantle perspective, of formation, subduction, and exhumation of oceanic and marginal lithosphere of slow–ultraslow spreading oceans.

Melt migration and intrusion in the Erro-Tobbio peridotites (Ligurian Alps, Italy): Insights on magmatic processes in extending lithospheric mantle

The Alpine/Apennine ophiolites are largely thought to represent lithosphere sectors mostly formed at an ocean/continent transition and allow the direct observation of petrologic and geodynamic processes in extensional systems evolving from continental rifting to ultraslow spreading. The Alpine/Apennine peridotites experienced multiple melt/rock interaction and melt intrusion events occurred at different lithospheric depths, thus providing insights on mantle dynamics and lithosphere-asthenosphere interactions during progressive lithosphere extension. Here we present an overview of this multi-stage melt migration and intrusion history, as recorded in the Erro-Tobbio (ET) peridotites (Ligurian Alps, Italy). In the ET spinel peridotites, the oldest intrusion event is documented by the diffuse occurrence of cm-scale folded pyroxenite bands. They display variably fractionated REE spectra, marked by LREE depletion and absent Eu N anomaly. Unusual trace element signature (high Sc,V contents and low MREE/HREE ratios) in clinopyroxenes from one pyroxenite layer is witness of a precursor garnet-bearing magmatic assemblage. Spinel pyroxenites likely originated as high- P (> 15–20 kbar) intrusions that preceded the extension-related peridotite exhumation. In the spinel peridotites, field, textural and chemical evidence ( e.g . olivine embayment replacing pyroxene porphyroclasts, increasing modal olivine at constant bulk Mg values), points that they experienced open-system melt migration by reactive porous flow, subsequent to pyroxenite intrusion and folding. Melt/rock interaction (causing olivine crystallization and pyroxene dissolution) occurred at high melt volumes at deep lithospheric levels. At shallower lithospheric depths, the ET peridotites were impregnated by melts causing significant plagioclase enrichment and crystallization of poikilitic orthopyroxene replacing mantle olivine and clinopyroxene. Reacted clinopyroxenes preserve strong LREE depletion, indicating that impregnating melts originated as depleted melt fractions. After impregnation, peridotites underwent multiple gabbroic intrusion events. Structural and geochemical features of melt impregnation and melt intrusion products point to a progressive change in melt composition and dynamics. Peridotite impregnation was caused by diffuse migration of opx-saturated depleted melts, and is consistent with cooling and crystallization of migrating melts when the peridotites, due to lithosphere extension and thinning, became part of shallower and colder lithospheric environments. The subsequent intrusion events originated by MORB-type aggregated magmas that had not experienced significant compositional modifications during ascent. The transition from porous flow melt migration to emplacement of magmas in fractures reflects progressive change of the lithospheric mantle rheology, across the ductile to brittle transition, during extension-related uplift and cooling of the ET mantle.

Origin and emplacement of ultramafic–mafic intrusions in the Erro-Tobbio mantle peridotite (Ligurian Alps, Italy)

The Erro-Tobbio peridotites (Voltri Massif, Ligurian Alps) represent subcontinental lithospheric mantle tectonically exhumed during Permo–Mesozoic extension of the Europe–Adria lithosphere. Previous studies have shown that exhumation started during Permian times, and occurred along kilometer-scale lithospheric shear zones which enhanced progressive deformation and recrystallization from spinel- to plagioclase-facies conditions. Ongoing field and petrologic investigations have revealed that the peridotites experienced, during uplift, a composite history of diffuse melt migration and multiple episodes of ultramafic–mafic intrusions. In this paper we present the results of field, structural and petrologic–geochemical investigations into a sector of the Erro-Tobbio peridotite unit that preserves well this multiple intrusion history. Melt impregnation in the peridotites is evidenced by significant plagioclase enrichment and crystallization of unstrained orthopyroxene replacing kinked mantle olivine and clinopyroxene; impregnating melts were thus opx-saturated. Melt–rock interaction caused chemical changes in mantle minerals (e.g. Al decrease and REE increase in cpx; Ti and Cr# enrichment in spinel). Nevertheless, clinopyroxenes still exhibit LREE depletion (Ce N/Sm N = 0.006–0.011), indicating a depleted signature for the percolating melts. Melt impregnation was thus related to diffuse porous flow migration of depleted MORB-type melt fractions that modified their compositions towards opx saturation by mantle–melt interaction during ascent. The impregnated peridotites are intruded by a hectometer-scale stratified cumulate body, mostly consisting of troctolites and plagioclase wehrlites, showing gradational, interfingered contacts with the host mantle rocks. Subsequent intrusion events are revealed by the occurrence of olivine gabbros as decameter-wide lenses, variably thick (centimeter- to meter-scale) dykes and thin dykelets, which crosscut both the peridotite foliation and the magmatic layering in the cumulates. Overall, major and trace element compositions of minerals in the intrusives indicate that they represent variably differentiated cumulus products crystallized from rather primitive N-MORB-type aggregated melts. Slightly more evolved compositions are shown by olivine gabbros, relative to the troctolites and plagioclase wehrlites of the cumulate body. Mineral chemistry features (e.g. the Fo–An correlation and high Na, Ti, Mg# in cpx) indicate that the studied intrusive rocks crystallized at moderate pressure conditions (3–5 kbar, i.e. 9–15 km depth). Our study thus points to a progressive transition from porous flow melt migration to emplacement of magmas in fractures, presumably related to progressive change of lithospheric mantle rheology during extension-related uplift and cooling.

THE COMPOSITE MANTLE EVOLUTION OF THE ERRO-TOBBIO PERIDOTITE (VOLTRI MASSIF, LIGURIA ALPS, ITALY)

THE COMPOSITE MANTLE EVOLUTION OF THE ERRO-TOBBIO PERIDOTITE (VOLTRI MASSIF, LIGURIA ALPS, ITALY)

THE EXHUMATION OF THE ERRO-TOBBIO MANTLE PERIDOTITES (LIGURIAN ALPS, ITALY) AS RECORDED BY MULTIPLE MELT INTRUSION EVENTS

THE EXHUMATION OF THE ERRO-TOBBIO MANTLE PERIDOTITES (LIGURIAN ALPS, ITALY) AS RECORDED BY MULTIPLE MELT INTRUSION EVENTS

Chronology, Petrology and Isotope Geochemistry of the Erro-Tobbio Peridotites (Ligurian Alps, Italy): Records of Late Palaeozoic Lithospheric Extension

Mantle peridotites from the Erro–Tobbio (ET) ophiolitic unit (Voltri Massif, Ligurian Alps) record a tectono-metamorphic decompressional evolution, indicated by re-equilibration from spinel- to plagioclase- to amphibole-facies conditions, and progressive deformation from granular to tectonite to mylonite fabrics. The peridotites are considered to represent subcontinental lithospheric mantle that was tectonically denuded during rifting and opening of the Jurassic Ligurian Tethys ocean, similar to the Northern Apennine (External Ligurides) ophiolitic peridotites. We performed chemical and isotopic investigations on selected granular and tectonite spinel peridotites and plagioclase tectonites and mylonites, with the aim of defining the nature of the mantle protoliths, and to date the onset of exhumation of the ET peridotites. Spinel- and plagioclase-bearing tectonites and mylonites exhibit heterogeneous bulk-rock major and trace element composition, despite rather homogeneous mineral chemistry, thus indicating that the ET mantle protoliths record a composite history of partial melting and melt migration by reactive porous flow. The lack of correlation between the observed geochemical heterogeneity and the structural type (granular, tectonite, mylonite) indicates that the inferred reactive porous flow event preceded the exhumationrelated lithospheric history of the Erro–Tobbio mantle. The tectonometamorphic evolution caused systematic chemical changes in minerals: (1) Al decrease in orthopyroxene; (2) Al decrease, and Cr and Ti increase in spinels; (3) Al and Sr decrease, Cr, Ti, Zr, Sc, V and middle to heavy rare earth element increase and development of a negative Eu anomaly in clinopyroxene. The studied samples have Nd isotope compositions consistent with a mid-ocean ridge basalt mantle reservoir. Sm/Nd isotope data on plagioclase and clinopyroxene separates (and corresponding whole rocks) from two plagioclase peridotites, representative of the plagioclase-bearing mylonitic extensional shear zone, have yielded ages of 273 � 16Ma and 313 � 16Ma, for the plagioclase-facies recrystallization stage, significantly older than the expected Jurassic age. This indicates that the Erro–Tobbio peridotites represent subcontinental lithospheric mantle that was tectonically exhumed from spinel-facies depths to shallower lithospheric levels during Late Carboniferous– Permian times. Our results are consistent with the previously documented evidence for an extensional regime in the Europe–Adria lithosphere during Late Palaeozoic time, and they represent the first record that extensional mechanisms were also active at lithospheric mantle levels.

Contrasting bulk and mineral chemistry in depleted mantle peridotites: evidence for reactive porous flow

A series of recent papers have indicated that reconstructed bulk compositions of abyssal peridotites define chemical correlations, namely increasing FeO tot and decreasing SiO 2 with increasing MgO, which cannot be produced by simple extraction of partial melts. However, no general consensus exists on the reliability of these trends, because they could be artifacts of the adopted calculations, and on the origin of abyssal peridotites. Specifically, it has been inferred that abyssal peridotite compositions are consistent with combined histories of partial melting and subsequent melt migration which caused either olivine addition, or dissolution and precipitation reaction, by equilibrium porous flow [Niu et al., Earth Planet. Sci. Lett. 152 (1997) 251–265; Asimow, Earth Planet. Sci. Lett. 169 (1999) 303–319]. We report combined bulk-rock and mineral chemical data for ophiolitic peridotites from the Erro-Tobbio (ET) Unit (Voltri Massif, Ligurian Alps), which represent lithosphere remnants of the Jurassic Ligurian Tethys embryonic ocean. These peridotites include sp-lherzolites and sp-harzburgites, and display overall depleted geochemical signature. However, comparison between bulk rock and corresponding mineral compositions reveals that these rocks cannot be residues of simple (equilibrium or fractional) melt extraction. Mineral compositions are similar in all the samples. By contrast, the bulk rock compositions define striking correlations, i.e. increasing FeO tot, Ni, Co, and decreasing Al 2O 3, SiO 2, CaO, Sc, Cr, Yb N, with increasing MgO: the MgO–FeO tot and MgO–SiO 2 correlations are similar to those recognized in abyssal peridotites. Thus, the ET peridotites provide evidence that the above trends are indeed consistent with similar variations in on-land peridotites; also, these trends cannot simply result from progressive melt depletion, because constituent minerals in the different ET samples have rather uniform composition. Calculated bulk modes indicate that the observed chemical variations are coupled to systematic modal changes, namely decrease in cpx and opx, and increase in olivine, at increasing bulk MgO. The ET peridotites also display decrease of the cpx/opx ratio at increasing bulk MgO, and this argues against a process of simple olivine addition. By contrast, some peculiar bulk-mineral compositional features – e.g. (i) nearly constant olivine Mg* [=Mg/(Mg+Fe tot)] values, at increasing modal olivine, (ii) opposite bulk Cr and Ni correlations, (iii) bulk Cr and Yb decrease, and parallel Ni increase, strikingly correlated with progressive modal cpx decrease and concomitant modal olivine increase – are consistent with the expected chemical and modal effects, during a process of interaction between depleted peridotites and melts migrating by equilibrium porous flow (involving pyroxene dissolution and olivine precipitation reactions). Bulk-rock and mineral chemistry data in the ET peridotites thus indicate that the major element correlations inferred by Niu et al. for the abyssal peridotites are reliable, and most likely result from a combined history of partial melting and melt interaction by reactive porous flow.

O–H isotope ratios of high pressure ultramafic rocks: implications for fluid sources and mobility in the subducted hydrous mantle

We examine the O–H isotope signatures of Alpine ultramafic rocks and eclogitic metagabbros of the Erro–Tobbio peridotite Unit (western Italian Alps), which record a subduction and exhumation cycle. Localization of subduction-related deformation along serpentinite mylonite shear zones favoured preservation of pre-subduction mantle and low temperature (oceanic) alteration assemblages within undeformed (meta)peridotite that underwent partial static recrystallization to high-pressure metamorphic parageneses. Bulk rock and mineral separate (clinopyroxene and serpentine) oxygen isotope ratios of the serpentinized mantle peridotites (5–8‰) are slightly enriched in 18O compared with those of the high-pressure metaperidotites and the serpentinite mylonites (4.4–7.6‰). The lowest values occur in high-pressure veins (3.5–5.7‰) and eclogitic metagabbros (3.1–5.3‰). These variations are comparable to variations observed in modern oceanic rocks and in non-subducted ophiolites. Preservation of pre-eclogitic δ18O signatures of the Erro–Tobbio rocks and a lack of oxygen isotope re-equilibration between different shear zones imply local-scale fluid flow at low water/rock ratios and closed system behaviour during high-pressure metamorphism. Different serpentine generations show a bimodal distribution in δD values: pre-eclogitic lizardite and chrysotile range from –102 to –77‰; high-pressure antigorite in the mylonites and in low strain metaperidotites range from –71 to –57‰ and –83 to –60‰, respectively. Comparable ranges occur in antigorite in the associated high-pressure veins, suggesting that the hydrogen signatures were acquired prior to veining. We propose that the isotopic variations reflect multiple events of fluid uptake in different geodynamic environments. The H- and O-isotope ratios in the eclogitic mylonites suggest that initial hydration occurred over a range of temperatures during local interaction with altered seawater along oceanic shear zones. The 18O-enriched and H-depleted compositions of chrysotile and lizardite in the mantle peridotites suggest that a second hydration event may have occurred as a result of interaction with metamorphic fluids at the early stages of burial in a forearc setting, where slabs undergo large-scale, low-temperature fluid fluxing. The oceanic mantle is thus a candidate for continuous hydration during its oceanic and early subduction history. The Erro–Tobbio unit thus represents an example of cycling of internally-derived fluids, whereby the different structural and textural domains behaved as relatively closed systems to fluid circulation during high-pressure metamorphism.

STRUCTURES AND MICROSTRUCTURES IN A THRUST-RELATED, GREENSCHIST FACIES TECTONIC MÉLANGE, VOLTRI GROUP (NW ITALY)

The ophiolitic Voltri Group in the eastern part of the Ligurian Alps (NW Italy) is made up of a number of thrust sheets emplaced during Alpine collision. These thrust sheets include (1) the Voltri-Rossiglione calcschist unit of Mesozoic high-pressure calcareous micaschists, metavolcanics and slices of serpentinite, overlain by (2) the Beigua serpentinite unit, of mainly antigorite serpentinite and eclogitic metagabbro, in turn overlain by (3) the Erro-Tobbio peridotites. In the northern part of the Voltri Massif, a conspicuous melange-type lithology occurs along the contact of the Beigua serpentinite unit and the Voltri-Rossiglione calcschist unit. In the vicinity of the contact, the structure in the hanging-wall serpentinites is dominated by kink-type crenulations. Towards the base of the serpentinite nappe these crenulations become intense, and veins and patches of talc + chlorite + tremolite + carbonate replace the original antigorite-dominated assemblage. The thrust itself is marked by a layer, at least several tens of metres thick, of intensely deformed and foliated antigorite-bearing talc-chlorite-tremolite schist, enclosing rounded and lense-shaped (phacoid) blocks, up to 25 metres across, of retrogressed eclogitic metagabbro, antigorite serpentinite, metabasic rock, calcschist and schistose micaceous marble. The main features of this chaotic lithology meet the descriptive criteria of a tectonic melange. The structures and assemblages in the wall rock units and those in the melange indicate that the melange lithology developed at a relatively late stage of greenschist facies ductile thrusting, emplacing the Beigua unit onto the rocks of the Voltri-Rossiglione unit. The structures indicate that the blocks and lenses were formed during localized deformation in a relatively narrow zone along the thrust plane via intense stretching and boudinage of the various lithologies in the foot- and hanging wall. The development of talc-chlorite-tremolite-carbonate assemblages at the expense of the overriding antigorite serpentinites require significant calciummetasomatism, hence extensive fluid activity, whilst the microstructures in the melange matrix suggest that the talc-chlorite-tremolite-carbonate assemblage was mechanically weak. It is suggested that both fluid activity and the associated metamorphic reactions strongly facilitated ductile to semi-brittle deformation along the thrust, leading to progressive fragmentation and mixing of the different lithologies and development of a tectonic melange.

Fluid and Element Cycling in Subducted Serpentinite: a Trace-Element Study of the Erro-Tobbio High-Pressure Ultramafites (Western Alps, NW Italy)

cycling of exogenic components. Diopside and Ti-clinohumite are The oceanic serpentinization of peridotites and the influence of such the high-pressure minerals acting as repositories for REE and Sr, an alteration on element cycling during their subduction dewatering and for high field strength elements (HFSE), respectively. The are here investigated in a mantle slice (Erro–Tobbio peridotite), first aqueous fluid equilibrated with such an assemblage is enriched in exposed to oceanic serpentinization and later involved in alpine Cl and alkaline elements but strongly depleted in REE and subduction, partial dewatering and formation of a high-pressure HFSE (less than chondrite abundances). Sr is low [(0·2–1·6) olivine+ titanian-clinohumite+ diopside+ antigorite assemblage × chondrites], although selectively enriched relative to light REE. in the peridotites and in veins. Previous work indicates that highpressure veins include primary brines, representing a residue after crystallization of the vein assemblage and containing recycled oceanic Cl and alkalis. To reconstruct the main changes during oceanic

FLUID AND ELEMENT CYCLING IN SUBDUCTED SERPENTINITES

Serpentinized peridotites are important components of the oceanic lithosphere and can contain 10-13 wt% bulk water fixed in serpentine and other associated hydrous phases. Since serpentine can survive to considerable depths during cold subduction, serpentinites are considered as the most effective transporters of water into the mantle. Prograde dewatering of subducted serpentinites produces aqueous fluids which contain appreciable amounts of solute. Thus, investigations on the compositional relationships between the oceanic assemblages and the high-pressure minerals + fluids become crucial to assess the fluid and element cycling occurring in serpentinites during subduction. These aspects have been investigated in the Western Alpine serpentinites (Erro-Tobbio peridotite unit), which represent fragments of mantle emplaced at high crustal levels during opening of the Mesozoic Alpine Tethys, and successively recrystallized at eclogite-facies conditions during alpine plate convergence and subduction. Eclogitization was accompanied by dewatering reactions and development of olivine + diopside + Ticlinohumite + antigorite in the rocks and in vein systems. During this history, shear localization into mylonitic domains, allowed survival of undeformed serpentinized peridotite blocks showing incomplete high-pressure recrystallization and preserving abundant relics of pre-subduction assemblages. Previous studies emphasized that the high-pressure breakdown of chlorine and alkali-bearing hydrothermal serpentines and phyllosilicates was accompanied by fluid production and by partitioning of these elements into the coexisting fluid phase. This has suggested deep cycling of shallow fluids during eclogitization. In this study we analyze the major and trace element compositions of bulk rocks and of key high-pressure minerals in order to test wether, besides chlorine and alkalies, other elements uptaken during oceanic alteration can be later released during subduction. The bulkrock analyses were made on in situ profiles from ‘preserved’ peridotites to intensely serpentinized ultramafites, and from host high-pressure metaperidotite to high-pressure veins. Our results indicate that the shallow serpentinization was accompanied by decrease in bulk-rock Ca, coupled with increase in H2O and Sr as a function of the intensity of alteration. The Rare Earth Element compositions of peridotite was not affected significantly by the oceanic alteration. The high-pressure veins display much higher bulk Sr concentrations than the precursor serpentinites and than their host rocks, thereby indicating Sr mobilization in the high-pressure fluid. Vein-filling diopside has trace element compositions comparable with those of primary mantle clinopyroxene still preserved in the host rocks, the main difference being related to Sr positive anomaly shown by the vein diopside. Apart their bulk-Sr enrichment, the veins do not display relevant differences in their trace element patterns compared to their host rocks. This indicates that the fluid drained by the vein systems equilibrated with the surrounding rocks. Since dewatering reaction take place in these rocks during ongoing eclogitization, we infer that the fluid was internally produced and buffered. The above results suggests close system behaviour of these serpentinites at eclogitic conditions and the internal cycling of fluid and elements such as chlorine, alkalis and strontium, originally uptaken during alteration at surface environments close to the seafloor.

High salinity fluid inclusions formed from recycled seawater in deeply subducted alpine serpentinite

The origin of high-pressure brines has been investigated in the Erro-Tobbio peridotite (Western Alps), a mantle slice that was exposed to the seafloor of the Mesozoic Ligurian-Piedmontese Tethys and was later involved in Alpine subduction. Hydrothermal alteration by seawater-derived fluids led to replacement of the mantle assemblage by Cl-bearing serpentine (0.35 wt% Cl), brucite (0.2 wt%), Cl- and alkali-bearing phyllosilicates (0.2 wt% Cl; 0.2–0.55 wt% Na 2O; 1–5 wt% K 2O). Relics of these hydrous phases occur in olivine + titanian clinohumite + antigorite assemblages formed at 2.5 GPa and 550–600°C during partial devolatilization and veining of the hydrothermally altered peridotite. The high-pressure phases lack chlorine and alkalis and are coeval with fluid inclusions trapped in the syn-eclogitic veins. The inclusions are salt-saturated and contain up to 50 wt% Cl, Na, K, Mg and Fe. High fluid chlorinity was probably achieved during rehydration of relict mantle minerals and deposition of hydrous phases in the veins. The data presented suggest that the seafloor hydrothermal signature was inherited by the eclogitic fluid due to partitioning of chlorine and alkalis into the fluid phase. The presence of salty brines in eclogitized hydrous peridotites can indicate deep recycling of seawater-derived fluids. Hydrous ultramafic systems can therefore act as large-scale carriers of seawater into the mantle.

Subduction of water into the mantle: History of an Alpine peridotite

Research Article| May 01, 1995 Subduction of water into the mantle: History of an Alpine peridotite Marco Scambelluri; Marco Scambelluri 1Dipartimento di Scienze della Terra, Corso Europa 26, 16132 Genova, Italy Search for other works by this author on: GSW Google Scholar Othmar Müntener; Othmar Müntener 2Institut für Mineralogie und Petrographie, Eidgenössische Technische Hochschule Zentrum, Zürich, Switzerland Search for other works by this author on: GSW Google Scholar Jörg Hermann; Jörg Hermann 2Institut für Mineralogie und Petrographie, Eidgenössische Technische Hochschule Zentrum, Zürich, Switzerland Search for other works by this author on: GSW Google Scholar Giovanni B. Piccardo; Giovanni B. Piccardo 1Dipartimento di Scienze della Terra, Corso Europa 26, 16132 Genova, Italy Search for other works by this author on: GSW Google Scholar Volkmar Trommsdorff Volkmar Trommsdorff 2Institut für Mineralogie und Petrographie, Eidgenössische Technische Hochschule Zentrum, Zürich, Switzerland Search for other works by this author on: GSW Google Scholar Author and Article Information Marco Scambelluri 1Dipartimento di Scienze della Terra, Corso Europa 26, 16132 Genova, Italy Othmar Müntener 2Institut für Mineralogie und Petrographie, Eidgenössische Technische Hochschule Zentrum, Zürich, Switzerland Jörg Hermann 2Institut für Mineralogie und Petrographie, Eidgenössische Technische Hochschule Zentrum, Zürich, Switzerland Giovanni B. Piccardo 1Dipartimento di Scienze della Terra, Corso Europa 26, 16132 Genova, Italy Volkmar Trommsdorff 2Institut für Mineralogie und Petrographie, Eidgenössische Technische Hochschule Zentrum, Zürich, Switzerland Publisher: Geological Society of America First Online: 02 Jun 2017 Online ISSN: 1943-2682 Print ISSN: 0091-7613 Geological Society of America Geology (1995) 23 (5): 459–462. https://doi.org/10.1130/0091-7613(1995)023<0459:SOWITM>2.3.CO;2 Article history First Online: 02 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Marco Scambelluri, Othmar Müntener, Jörg Hermann, Giovanni B. Piccardo, Volkmar Trommsdorff; Subduction of water into the mantle: History of an Alpine peridotite. Geology 1995;; 23 (5): 459–462. doi: https://doi.org/10.1130/0091-7613(1995)023<0459:SOWITM>2.3.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract The Erro-Tobbio peridotite (western Alps) is a slice of subcontinental mantle that underwent pre-Jurassic passive rifting, followed by sea-floor hydration and Cretaceous (Alpine) subduction. Analysis of its extension- and subduction-related structures and metamorphism suggest the dip below the Adria plate of both the extensional detachments and the subduction plane. High-pressure boudinage structures of rigid metarodingite within ductile antigorite serpentinite prove that antigorite survived subduction and remained stable at eclogite facies conditions. Subduction of serpentinized peridotites from ophiolite slabs is here considered the most effective mechanism of bringing water to great depth within the mantle. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Dehydration-induced fracturing of eclogite-facies peridotites: Implications for the mechanical behaviour of subducting oceanic lithosphere

Structural analysis of eclogite facies peridotites from the Voltri Massif in northwest Italy shows that deformation in the upper 60 km of the Alpine, serpentinite/peridotite-dominated subduction zone was strongly localized in antigorite-rich serpentinite mylonites. Deformation in the peridotite wall rock was dominated by the development of sub-planar shear-fracture systems and veins with olivine + titanian clinohumite-bearing assemblages. Fracturing of the peridotite wall rock was closely associated with synkinematic antigorite breakdown in serpentinite mylonites during peak subduction (H P-L T) metamorphism, and is inferred to have occurred as a result of hydraulic fracturing. The deformational structures observed allow to place some constraints on the mechanical behaviour of the Erro-Tobbio peridotites during subduction. It is inferred that fracturing of the peridotite may have occurred at transient fluid pressures in excess of 0.9 times the lithostatic pressure. The shear stresses along the subducting plate contact were controlled by the rheology of serpentinite mylonites, which suggests that maximum values of about 60 MPa were reached at 30 km depth, while below 40 km depth they decreased to less than 5 MPa. These shear stresses are distinctly lower than those inferred for basalt-dominated subduction zones.

Hydration-induced climb dissociation of dislocations in naturally deformed mantle olivine

Widely dissociated dislocations have been observed in mantle olivine from the Erro-Tobbio peridotite in N.W. Italy. Analysis of diffraction contrast in transmission electron microscopy (TEM) indicates that the dissociation reaction involves the climb dissociation on (001) and {021} planes of b=[001] unit dislocations into partial dislocations with Burgers vectors approximately equal to 1/x 〈011〉. In the most extreme case a unit dislocation dissociates into four partials which bound three planar defects. The unusually wide dissociation and the greater extent of dissociation in olivine from amphibole-bearing rocks suggests that the dissociation is related to hydration. The occurrence of fluid inclusions along the dislocations confirms that the partials and planar defects are saturated with volatiles. Analysis of possible planar defect structures in Fo90 shows that; (i) the most likely partial Burgers vectors are b=〈0 3/11 1/4〉; (ii) two of the planar defects are cation-deficient and can be stabilised by segregation of H+ to produce (Mg, Fe) (OH)2 layers which are iso-structural with the OH-rich interlayer of the humite group minerals; (iii) the central planar defect is formed by removing a stoichiometric olivine (002) layer so does not produce any local chemical changes. The climb dissociation provides a possible mechanism for the transformation of olivine to a humite group mineral. OH-rich interlayers may nucleate on dislocations and extend into the crystal by climb resulting in a gradual increase of (Mg, Fe) (OH)2 content. The only addition of material required is hydrogen which can rapidly diffuse into olivine. If the dissociation is stable and occurs at high temperatures and pressures it may significantly influence the nature and kinetics of deformation mechanisms and the olivine — spinel shear transformation mechanism in hydrated olivine.