Marco Scambelluri

Deep fluids in subduction zones

The fluid inclusions preserved in high and ultrahigh pressure rocks provide direct information on the compositions of fluid phases evolved during subduction zone metamorphism, and on fluid–rock interactions occurring in such deep environments. Recent experiments and petrologic studies of eclogite–facies rocks demonstrate that stability of a number of hydrous phases in all rock systems allows fluid transport into the mantle sources of arc magmas, as well as into much deeper levels of the Earths mantle. In eclogite–facies rocks, the presence of large ion lithophile elements (LILE) and light rare earths (LREE)-bearing hydrous phases such as epidote and lawsonite, together with HFSE repositories as rutile and other Ti-rich minerals, controls the trace element budget of evolved fluids and fluid-mediated cycling of slab components into the overlying mantle. Studies of fluid inclusions in eclogite–facies terrains suggest that subduction mainly evolves aqueous solutions, melts being produced only locally. Eclogite-facies rocks diffusely record processes of fluid–melt–rock interactions that exerted considerable control on the element and volatile budget of subduction fluids. Trace element fractionation during such interactions needs to be tested and quantified in more detail to achieve the ultimate compositions actually attained by fluids leaving off the slab. Variably saline inclusions with minor CO2 and N2 are trapped in rock-forming high pressure minerals; brines with up to 50% by weight dissolved solute are diffusely found in veins. The latter inclusions are residues after fluid–rock interactions and deposition of complex vein mineralogies: this evidence suggests increased mineral solubility into the fluid and formation, at a certain stage, of silicate-rich aqueous solutions whose geochemical behaviour and transport capacity can approach that of a melt phase. This is supported by experimental work showing high solubility of silicate components in fluids at high pressures. However, natural examples of inclusions trapping such a fluid and quantitative analyses of its major and trace element composition are not yet available. Fluids in high and very high pressure rocks do not move over large scales and the channelways of fluid escape from the slab are not yet identified. This suggests that only part of the slab fluid is lost and returned to the surface via magmatism; the remaining trapped fraction being subducted into deeper levels of the upper mantle, to renew its budget of substances initially stored in the exosphere.

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% Na2O; 1–5 wt% K2O). 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

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.

Incompatible element-rich fluids released by antigorite breakdown in deeply subducted mantle

We present first trace element analyses of the fluid produced during breakdown of antigorite serpentine, a major dehydration reaction occurring at depth within subducting oceanic plates. Microinclusions filled with crystals+aqueous liquid are disseminated within olivine and orthopyroxene grown at pressures and temperatures beyond the stability field of antigorite. Despite hydrogen loss and significant major element changes that have affected the analyzed inclusions, their trace element composition still reflects characteristics of the subduction fluid released during serpentinite dehydration. The fluid is enriched in incompatible elements indicating either (1) interaction with fluids derived from crustal slab components, or (2) dehydration of altered (serpentinized) oceanic mantle previously enriched in incompatible elements. Several features of the analyzed fluid+mineral inclusions (high Pb/Th, Pb/U and Pb/Ce) are in agreement with available experimental work, as well as with the geochemical signatures of most arc lavas and of several ocean island basalt mantle sources. The trace element patterns of the fluid+mineral inclusions do not display relative enrichment in large ion lithophile elements compared to high field strength elements, thus suggesting that the latter elements may become soluble in natural subduction fluids. fl 2001 Elsevier Science B.V. All rights reserved.

Ophiolite mélange zone records exhumation in a fossil subduction channel

Recent models propose that the exhumation of high-pressure rocks occurs by means of return flow inside a low-viscosity channel of serpentinite situated between the plates. To test this hypothesis, we investigated a serpentinite melange in the Western Alps, which contains exotic mafic and metasedimentary tectonic blocks, recording heterogeneous metamorphic evolutions and variable high-pressure ages. The peak metamorphic conditions range from eclogite- to garnet-blueschist-facies. The structural evidence and the pressure-temperature paths of the different blocks suggest coupling between blocks and matrix, at least in the blue-schist facies. 39 Ar- 40 Ar dating indicates eclogite-facies peak at ca. 43 Ma and blueschist-facies peak at ca. 43 and 40 Ma in different blocks, respectively. These data point to diachronous metamorphic paths resulting from independent tectonic evolutions of the different slices. We therefore propose that this melange formed during exhumation of subducted rocks equilibrated at different depths inside a subduction channel. This mechanism can be extended to other serpentinite melanges in the Alps and other orogens (e.g., the Cyclades, the Coast Ranges of California) for which a growing heterogeneity in the timing of metamorphic equilibration and of pressure-temperature paths can be expected with further investigations.

Majoritic garnets monitor deep subduction fluid flow and mantle dynamics

The ultradeep mantle rocks of western Norway display three generations of majoritic garnet. The first two derive from incompatible element–depleted transition-zone mantle and exsolved pyroxene components during Archean upwelling, accretion to subcratonic lithosphere (M1 stage), and isobaric cooling until the Middle Proterozoic (M2). A subsequent Scandian (430–390 Ma) subduction cycle initiated diamond crystallization (M3). Here we report a third majoritic garnet crystallized at grain boundaries and in microfractures, and stable with pyroxene, phlogopite, and spinel in the M3 assemblage. The trace element signatures of M3 minerals indicate crustal metasomatism, phlogopite being the main large ion lithophile element repository. These features imply majorite crystallization from crust-derived subduction fluids at 200 km depth. Our finding fixes the deepest occurrence of free subduction fluid phases and indicates that garnet is a reliable monitor of deep mantle evolution and fluid-mediated chemical recycling.

Chlorine cycling during subduction of altered oceanic crust

Eclogitic rocks that have experienced devolatilization, with little or no interaction with external fluid sources, can be viewed as analogues of crustal material which may be transferred back into the mantle during subduction. Thus they can be used to evaluate the extent of the recycling of volatile elements, such as chlorine. We report new oxygen isotope ratios of omphacite, and fluid inclusion data determined from eclogitic metagabbros, of the Rocciavre massif (Italian Alps). The data are compared with those obtained for the Monviso, Cyclades and the Franciscan Complex high-pressure rocks. In all localities, relics of early dehydration fluids are preserved as primary fluid inclusions in the cores of omphacite megacrysts (Rocciavre, Monviso and Franciscan Complex) or garnet (Cyclades). Salinity estimates of the inclusion fluids range from 32 to 45 wt% NaCl in Rocciavre, 17 to 21 wt% NaCl in Monviso, and are similar to seawater in other areas. Omphacite and bulk-rock δ18O values of Rocciavre (5.1–6.8‰) and Monviso (3.0–5.3‰) metagabbros are markedly lower than those of Cyclades (6.8–14.3‰) and the Franciscan (6.7–13.1‰) metabasites. The fluid salinity–δ18O systematics of eclogitic rocks is similar to that documented along a typical section of the altered oceanic crust and unmetamorphosed ophiolites. This suggests that high-pressure metamorphism, and associated processes, did not modify significantly the variability in chlorine concentrations and oxygen isotope ratios, inherited from a stage of sea-floor hydrothermal alteration under low- (basaltic layer) and high-temperature (gabbroic layer) conditions, respectively. Extrapolating the estimated H2O and Cl contents of eclogitic rocks to a representative section of the subducted oceanic crust indicates that a minimum of 100 to 200 ppm Cl could be recycled into the mantle during subduction. This yields a Cl/H2O ratio of 3.6 to 7.5×10−2 for the subducted oceanic crust, which is similar to E-MORB. On the basis of available Cl isotopic data, we infer that a large proportion (70%) of the Cl stored in the altered crust should be recycled to the mantle to generate an isotopic composition of the subducted crust equivalent to the source of unaltered mid-ocean ridge basalt (δ37Cl = 4.7‰).

Boron isotope evidence for shallow fluid transfer across subduction zones by serpentinized mantle

Serpentinites formed by alteration of oceanic and forearc mantle are major volatile and fluid-mobile element reservoirs for arc magmatism, though direct proof of their dominance in the subduction-zone volatile cycles has been elusive. Boron isotopes are established markers of fluid-mediated mass transfer during subduction. Altered oceanic crust and sediments have been shown to release in the subarc mantle 11 B-depleted fluids, which cannot explain 11 B enrichment of many arcs. In contrast to these crustal reservoirs, we document high δ 11 B values retained in subduction-zone Alpine serpentinites. No 11 B fractionation occurs in these rocks with progressive burial: the released 11 B-rich fluids uniquely explain the elevated δ 11 B of arc magmas. B, O-H, and Sr isotope systems indicate that serpentinization was driven by slab fluids that infiltrated the slab-mantle interface early in the subduction history.

Salt-rich aqueous fluids formed during eclogitization of metabasites in the Alpine continental crust (Austroalpine Mt. Emilius unit, Italian western Alps)

Abstract The metabasite bodies of the Mt. Emilius continental unit (western Italian Alps) underwent a stage of Alpine eclogite-facies metamorphism (1.1–1.3 GPa and 450–550°C) accompanied by polyphase deformation and recrystallization. The metabasites consist of two main rock types: (1) eclogites (omphacite+garnet+glaucophane+epidote+phengite/paragonite) preserving no relics of their precursors; (2) eclogitized granulites, i.e. rocks whose incomplete eclogitic recrystallization (clinopyroxene II+garnet II+epidote+amphibole+chlorite) allowed survival of textural and mineralogical relics of pre-Alpine granulitic assemblages (clinopyroxene I+garnet I+plagioclase+amphibole). In this latter case the pre-Alpine granulites were converted to eclogites as the result of infiltration of aqueous fluids during eclogitization. In both eclogites and eclogitized granulites hydrated high-pressure foliations are cut by eclogitic metamorphic veins. The bulk rock chemistry of the metabasites influenced the compositions of both the vein- and rock-forming clinopyroxenes and the compositional correlation between the vein- and rock-forming clinopyroxenes indicates that the syn-eclogitic fluids have re-equilibrated with the metabasite hosts. The predominant vein minerals (omphacite, epidote and garnet) contain primary high-salinity fluid inclusions. The fluids consist of two-phase (liquid+vapour) and of multiphase (liquid+vapour+salt+additional quartz) salty aqueous inclusions containing NaCl, CaCl 2 and MgCl 2 as the main chloride species. The vein inclusions show a salinity range from 17 to 45 wt.% salts in eclogites and from 20 to 50 wt.% salts in eclogitized granulites. In contrast, fluid inclusions in matrix minerals of the eclogitized granulites contain primary two-phase inclusions displaying a salinity range between 10 and 25 wt.% salts. The marked difference in fluid salinities recorded by the inclusions in the eclogitic veins and those in the partially re-equilibrated eclogitized granulites is interpreted in terms of progressive hydration during eclogitization of granulite-facies rocks, which caused an increase in the salt content of the residual inclusion fluids.

Redistribution of high-pressure fluids during retrograde metamorphism of eclogite-facies rocks (Voltri Massif, Italian Western Alps)

Abstract High-pressure clinopyroxenes in metasomatic mafic rocks (rodingites and metagabbros containing titanoclinohumite) and eclogites from the Voltri Massif (Ligurian Western Alps) retain primary, two-phase aqueous inclusions (H 2 O + NaCl ± KCl). In the metasomatized rocks the inclusions align with the c axes and the cleavage planes of diopside. The eclogites display mylonitic textures: primary fluid inclusions are hosted in early porphyroclasts of omphacite (omphacite I) and are absent in later fine-grained omphacite (omphacite II) that forms the mylonitic foliation. Omphacite I has been frequently replaced by retrograde symplectites (clinopyroxene + plagioclase ± amphibole); in such cases the fluid inclusions are either absent, or occur in much lower number. The textural observations suggest that formation of the symplectites has been controlled by availability of the fluid inclusions inside omphacite I. The symplectites have been in turn overgrown by glaucophanic and winchitic amphiboles, indicating that breakdown of the omphacite I to symplectites still occurred at relatively high-pressure. Post-entrapment changes have caused a large range of densities in the analyzed inclusions. Although the textural evidence constrains the entrapment of fluid inclusions to high-pressure conditions, the highest density isochores in the eclogites indicate pressure-temperature conditions (8 kbar and 550 °C) lower than those attained during the high-pressure stage. Such P-T estimates approach those of symplectite formation after omphacite I. Thus, the fluid inclusions preserved in omphacite were formed by low-pressure re-equilibration of a fluid presumably trapped at eclogite-facies conditions. The inclusions kinetically triggered the retrograde breakdown of the eclogitic clinopyroxenes. The compositional overlap between the analyzed inclusions and those related to the latest (greenschist) evolutionary stage, suggests further redistribution of the high-pressure fluids and close-system behaviour of these rocks during their overall exhumation path.