Erro Tobbio Research Articles

Formation of Olivine Veins by Reactive Fluid Flow in a Dehydrating Serpentinite

AbstractMany exposed high‐pressure meta‐serpentinites comprise a channelized network of olivine‐rich veins that formed during dehydration at depth and allowed the fluid to escape from the dehydrating rock. While previous studies have shown that chemical heterogeneities in rocks can control the formation of olivine‐enriched vein‐like interconnected porosity networks on the sub‐millimeter scale, it is still unclear how these networks evolve toward larger scales and develop nearly pure olivine veins. To explore this, we study the effect of reactive fluid flow on a dehydrating serpentinite. We use thermodynamic equilibrium calculations to investigate the effect of variations in the bulk silica content in serpentinites on the dehydration reaction of antigorite + brucite = olivine + fluid and the silica content of this fluid phase. Further, we develop a numerical model that combines the effects of intrinsic chemical heterogeneities with reactive transport with dissolved silica as metasomatic agent. Our model shows how reactive transport can lead to vein widening and olivine enrichment within a vein in an antigorite‐rich matrix, such as observed in the veins of the Erro Tobbio meta‐serpentinites. This is a critical step in the evolution toward larger‐scale vein systems and in the evolution of dynamic porosity, as this step helps account for the chemical feedback between the dehydrating rock and the liberated fluid.

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.

Multi-stage Reactive Formation of Troctolites in Slow-spreading Oceanic Lithosphere (Erro–Tobbio, Italy): a Combined Field and Petrochemical Study

Abstract Many recent studies have investigated the replacive formation of troctolites from mantle protoliths and the compositional evolution of the percolating melt during melt–rock interaction processes. However, strong structural and geochemical constraints for a replacive origin have not yet been established. The Erro–Tobbio impregnated mantle peridotites are primarily associated with a hectometre-size troctolitic body and crosscutting gabbroic dykes, providing a good field control on melt–rock interaction processes and subsequent magmatic intrusions. The troctolitic body exhibits high inner complexity, with a host troctolite (Troctolite A) crosscut by a second generation of troctolitic metre-size pseudo-tabular bodies (Troctolite B). The host Troctolite A is characterized by two different textural types of olivine, corroded deformed millimetre- to centimetre-size olivine and fine-grained rounded undeformed olivine, both embedded in interstitial to poikilitic plagioclase and clinopyroxene. Troctolite A shows melt–rock reaction microstructures indicative of replacive formation after percolation and impregnation of mantle dunites by a reactive melt. The evolution of the texture and crystallographic preferred orientation (CPO) of olivine are correlated and depend on the melt/rock ratio involved in the impregnation process. A low melt/rock ratio allows the preservation of the protolith structure, whereas a high melt/rock ratio leads to the disaggregation of the pre-existing matrix. The mineral compositions in Troctolite A define reactive trends, indicative of the buffering of the melt composition by assimilation of olivine during impregnation. The magmatic Troctolite B bodies are intruded within the pre-existing Troctolite A and are characterized by extreme textural variations of olivine, from decimetre-size dendritic to fine-grained euhedral crystals embedded in poikilitic plagioclase. This textural variability is the result of olivine assimilation during melt–rock reaction and the correlated increase in the degree of undercooling of the percolating melt. In the late gabbroic intrusions, mineral compositions are consistent with the fractional crystallization of melts modified after the reactive crystallization of Troctolites A and B. The Erro–Tobbio troctolitic body has a multi-stage origin, marked by the transition from reactive to fractional crystallization and diffuse to focused melt percolation and intrusion, related to progressive exhumation. During the formation of the troctolitic body, the melt composition was modified and controlled by assimilation and concomitant crystallization reactions occurring at low melt supply. Similar processes have been described in ultraslow-spreading oceanic settings characterized by scarce magmatic activity.

Fluid-mobile elements in serpentinites: Constraints on serpentinisation environments and element cycling in subduction zones

Fluid-mobile element (FME) systematics in serpentinites are key to unravel the environments of mantle rock hydration, dehydration, and element recycling in subduction zones. Here we compile serpentinite geochemical data and, for the first time, report discriminative FME enrichment trends for mid ocean ridge vs. forearc serpentinisation by applying alkali element-U ratios. Characteristic element fractionations are thereby governed by redox-dependent differential U mobility at mid ocean ridges and in forearcs, and by high Cs input in forearcs due to fluids equilibrated with sediments. Simple modelling reproduces the observed enrichment trends in serpentinites that range over several orders of magnitude. From these systematics, first constraints on potentially discriminative fractionation trends for unconventional fluid tracers such as B, As, and Sb can be deduced. Prominent W enrichments that correlate with FMEs suggest significant W mobility in low-temperature serpentinising environments.Application of the alkali element-U systematics to the subducted metaperidotites of Erro Tobbio (recording initial brucite+antigorite breakdown during subduction) and Almirez (recording final antigorite breakdown) reveal that pre-subduction FME enrichment signatures are retained in progressively subducted hydrous mantle rocks to beyond subarc levels. Associated dehydration veins and fluid inclusions reveal subordinate alkali element-U fractionation trends during dehydration. Subducted hydrous mantle rocks therefore may introduce characteristic element signatures and thus contribute towards mantle geochemical heterogeneities.

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.

Postcumulus processes in oceanic-type olivine-rich cumulates: the role of trapped melt crystallization versus melt/rock interaction

Combined microstructural and geochemical investigations on MORB-type primitive olivine-rich cumulates intruded in the Erro–Tobbio (ET) mantle peridotites (Voltri Massif, Ligurian Alps, Italy) revealed that significant chemical changes in minerals were caused by postcumulus crystallization. This is indicated by the occurrence of accessory interstitial minerals (Ti-pargasite, orthopyroxene and Fe–Ti oxides) and by systematic chemical zoning in intercumulus clinopyroxene, resulting in marked trace element (e.g. REE, Ti and Zr) enrichment at constant high Mg-numbers (0.88–0.91) and LREE depletion. Geochemical modelling shows that low trapped melt amounts (<5%) are sufficient to produce the observed trace element enrichments. Chemical zoning in large (mm-size) clinopyroxenes was dominantly caused by in situ fractional crystallization of trapped interstitial liquid rather then porous flow migration of externally derived evolved melts. Zr enrichment relative to REEs in vermicular clinopyroxene and pargasitic amphibole point to small-scale migration and interaction between residual evolved low melt fractions and the olivine cumulus matrix at final stage of crystallization.

Oxygen and nitrogen isotopes as tracers of fluid activities in serpentinites and metasediments during subduction

N and O isotope systematics of a suite of high-pressure (HP) and ultrahigh-pressure (UHP) metasediments of the Schistes Lustres nappe and metaperidotites of the Erro Tobbio Massif from the Alpine-Appennine system are compared with their unmetamorphosed or hydrothermally-altered equivalent from the same localities and from the South West Indian Ridge (SWIR). The HP and UHP rocks studied represent a sequence of pelagic sediments and altered ultramafic rocks subducted to different depths of down to 90 km along a cold geothermal gradient (8 °C/km). Unmetamorphosed and HP metasediments show the same range in δ15N values irrespective of their metamorphic grade and bulk nitrogen concentrations. Together with several other geochemical features (K, Rb and Cs contents, δD), this indicates that δ15N values were unaffected by metamorphism and N was not released during subduction. N isotope analysis of serpentinites coupled with δ18O systematics suggests the involvement of a mafic (crustal) component during partial deserpentinization of the subducted oceanic mantle at the depth locus of island arc magmatism. This does not imply large-scale fluxes as the metagabbros are spatially associated with the analyzed serpentinites. It rather indicates preservation of presubduction chemical and isotopic heterogeneities on a local scale as documented for the metasediments.

The pre-oceanic evolution of the Erro-Tobbio peridotite (Voltri Massif, Ligurian Alps, Italy)

Abstract This paper presents the results of field, structural, petrologic and geochemical investigations on the Erro-Tobbio (E-T) ophiolitic peridotite (Voltri Massif, Ligurian Alps, Italy). This massif represents a mantle section equilibrated at spinel-facies conditions in the subcontinental lithosphere of the Europe-Adria system prior to the Early Jurassic that has been exhumed and emplaced at the sea-floor during rifting and opening of an ocean basin. The E-T massif comprises km-scale volumes of peridotites with structural and compositional characteristics pointing to melt–peridotite interaction. Their formation is thought to result from the interaction of pristine lithospheric peridotites with MORB-type melts ascending by porous flow, leading to the development of reactive spinel harzburgites, impregnated plagioclase peridotites and replacive spinel dunites. The melt-related events were followed by MORB melt intrusion. Field relationships between sheared lithospheric peridotites, including coarse tectonites as well as fine-grained mylonites developed during lithosphere extension, and melt-modified peridotites suggest that melt-related processes occurred during exhumation of the E-T mantle. These melt-related processes likely included both diffuse percolation and focused intrusion and are considered to be a consequence of MORB-forming partial melting of the asthenosphere induced by near-adiabatic decompressional upwelling related to lithosphere extension and thinning. Field, structural and petrological data allow us to conclude that the entire pre-oceanic evolution of deformation, metamorphism and magmatism recorded by the E-T mantle started during the Early-Middle Jurassic and was related to lithospheric extension leading to the Late Jurassic opening of the Ligurian Tethys ocean.

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

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)

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.

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