122,687 research outputs found
Polyphase inclusions in garnet pyroxenites from Sulu (China) as carriers of seawater at ultrahigh pressure
No full textUnravelling processes of fluid-mediated element exchange between slab lithologies and the mantle wedge is of primary importance in understanding element mobility in subduction zones. Several studies have addressed element transfer related to fluid release during prograde metamorphism in subduction zones [1-4]. Nevertheless, detailed studies documenting interactions between felsic, mafic and ultramafic rocks at ultrahigh-pressure (UHP) are still scarce [5]. For this reason, UHP metasomatised rocks represent ideal materials to study the element exchange at pressures corresponding to sub-arc depths in subduction zones.
We present preliminary results of Ca-rich garnet–clinopyroxenites from Suolushu, occurring as layers in a large serpentinite body at Hujialin, Rizhao County, in the Sulu UHP metamorphic terrane (eastern China). Both clinopyroxenites and hosting serpentinites are intercalated with coesite-eclogites and hosted by coesite-bearing gneiss. Similar garnet–clinpyroxene layers from Hujialin have been studied by [6] and interpreted as cumulates crystallised from a hydrous, subduction related magma at ~ 1 GPa and 1000 °C. They were subjected to minor Ca enrichment coeval with serpentinisation of the host ultramafic rocks and then subducted at UHP (4.8 ±0.6 GPa and 750 ±50 °C). Ca-rich garnet–clinopyroxenites are composed of centimeter-sized garnet porphyroblasts in a matrix of fine-grained green diopside, opaque minerals associated with green spinel, and garnet. Garnet porphyroblasts include rounded clinopyroxene, opaque minerals, and/or spinel grains. Aggregates of magnetite and spinel are abundant in some samples. Peak porphyroblastic garnets preserve primary polyphase inclusions in their cores, consisting of amphibole(s), chlorite, pyroxene, micas and spinel. We studied these inclusions with the Transmission Electron Microscope at the University of Milano. They show an inner part formed by amphibole and clinopyroxene surrounded by Al rich Mg-silicates. Amphibole and pyroxene grow coherently by sharing the [001] direction, the one parallel to the tetrahedral chains. At the grain boundary between amphiboles and pyroxene, or amphiboles and Al rich Mg silicates, smaller amphibole grains extremely enriched in both Cl (up to 8 at.%) and Sr (up to 1.5 at.%) occasionally occur. Such Cl-amphiboles grow coherently with the neighbouring amphibole. The Al rich Mg-silicate phases show electron diffraction patterns with several streaking, indicating possible polytypic disorder. They exhibit periodicities of 14.1 Å, characteristic of chlorite. These phases form a rim between the inclusion precipitates and the hosting garnet, whereas a direct contact between garnet and amphibole or pyroxene have never been observed.
Serpentinites act as carriers of oceanic Cl, B, Sr, Rb, Cs, and alkalis which are recycled into variably saline fluids within the stability field of antigorite serpentine [7]. Polyphase inclusions studied in Hujalin clinopyroxenites likely derive from the interaction with the hosting serpentinites at HP-UHP and could represent a snapshot of such fluid-mediated element recycling occurring in the slab at sub-arc depths.
References. [1] Bebout, G.E., Ryan, J.G., Leeman, W.P., Bebout, A.E. (1999): Earth Planet. Sci. Lett, 171, 63–81; [2] Becker, H., Jochum, K.P., Carlson, R.W. (2000): Geology, 163, 65–99; [3] Scambelluri, M., Philippot, P. (2001): Lithos, 55, 213–227; [4] Spandler, C.J., Hermann, J., Arculus, R.J., Mavrogenes, J.A. (2003): Contrib. Mineral. Petrol., 146, 205–222; [5] Malaspina, N., Hermann, J., Scambelluri, M., Compagnoni, R. (2006): Lithos, 90, 19–42; [6] Yang, J.J. (2006): J. Petrol., 47, 965–990; [7] Scambelluri, M., Fiebig, J., Malaspina, N., Müntener, O., Pettke, T. (2004): Int. Geol. Rev., 46, 595–613
Subduction fluids and their interaction with the mantle wedge: a perspective from the study of high-pressure ultramafic rocks
No full textWe review three case studies emphasizing the role of ultramafic rocks in the recycling of volatiles and trace elements at convergent plate margins. Serpentinites are major water carriers in subduction zones and their breakdown liberates large quantities of water at sub-arc depths. The incompatible elements incorporated during oceanic serpentinization are released into the fluid phase produced once antigorite dehydrates to olivine + orthopyroxene. Importantly, the antigorite breakdown can trigger either wet melting or production of supercritical fluids in altered basalts and sediments. The produced fluid phases incorporate substantial amounts of incompatible element, initially residing in the crustal reservoirs. The fluid phase which exits the slab is highly reactive with respect to the overlying, silica undersaturated, mantle rocks. This leads to formation of reactive (ortho)pyroxenite layers, which filter the uprising hydrous melt/supercritical fluid to produce aqueous, solute-rich solutions. This fluid has equilibrated with peridotites and is mobile in the mantle.
A consequence of these subduction fluid/mantle reactions is that the mantle wedge domains overlying the slabs can be heterogeneous in composition and layered, due to the presence of reactive pyroxenite bodies. Another aspect regards the debate whether supercritical fluids or hydrous melts are effective media for trace element transport. Since both agents are saturated in silica, they will react with the silica-undersaturated mantle wedge peridotites to produce aqueous, incompatible trace element-rich residual fluids. Hence, while hydrous melt and/or supercritical fluids are important for scavenging incompatible elements from the slab, they may not be the agents that transfer the metasomatic subduction signature to the inner parts of the mantle wedges
Fluid/mineral interaction in UHP garnet peridotite
No full textWe present two case studies of metasomatised garnet peridotite from the Sulu (Zhimafang) and of garnet orthopyroxenite from the Dabie Shan (Maowu) ultrahigh-pressure terranes (Eastern China). The mantle-derived peridotite from Zhimafang shows two ultrahigh-pressure (UHP) mineral assemblages. The older one is made of porphyroclastic garnet rich in inclusions (Grt1), coarse exsolved clinopyroxene (Cpx1) and coarse phlogopite flakes (Phl1). The younger paragenesis consists of fine-grained olivine + clinopyroxene (Cpx2) + orthopyroxene ± magnesite ± Phl2 equilibrated with neoblastic garnet (Grt2). The inclusions inside porphyroclastic Grt1 are polyphase secondary inclusions related to microfractures cutting the garnet core. They display irregular shapes and contain microcrystals of calcic-amphibole, chlorite, phlogopite and rare talc, associated with pyrite and/or spinel. The low Al2O3 content ( 5.0 GPa). The microtextural identification of pseudosecondary inclusions in the porphyroclastic garnet core and their geochemical characterisation indicate that an incompatible element- and silicate-rich fluid subsequently metasomatised the garnet peridotite and equilibrated with the newly formed Cpx2 probably during Triassic UHP metamorphism.
Ultramafic metasomatic layers at Maowu Ultramafic Complex (Dabie Shan) consist of layered websterite and orthopyroxenite which preserve an old olivine + orthopyroxene (Opx1) + garnet (Grt1) ± Ti-clinohumite paragenesis, overgrown by poikilitic Opx2. Grt2 is associated with Opx2 + phlogopite along the foliation, and fine-grained idiomorphic clinopyroxene also occurs. Grt2 cores contain disseminated primary polyphase inclusions. The textural and geochemical analyses of the primary polyphase inclusions indicate that they derive from a homogeneous fluid characterised by high LILE concentrations with spikes in Cs, Ba, Pb and high U/Th. These inclusions are interpreted as remnants of the LILE- and LREE-enriched residual fluid produced when a crust-derived Si-rich metasomatic agent reacted with a previous harzburgite to form garnet orthopyroxenite. The in-situ trace element analyses of the major phases garnet, clinopyroxene and phlogopite that formed at the same time as the polyphase inclusions at Maowu, permit the determination of empirical mineral/fluid partitioning at pressures relevant for element recycling in subduction zones. Our estimated DCpx/fluid suggests that all LILE are highly incompatible, Th and U are moderately incompatible, Pb is close to unity and Sr is moderately compatible. Phlogopite preferentially incorporates Rb and K with respect to Ba and Cs, and Th with respect to U.
The similarity between the residual Maowu fluid with the secondary inclusions in the UHP wedge-type garnet peridotite from Sulu, indicates that the fluids produced from reactions at the slab–mantle interface may be effective metasomatic agents in the mantle wedge. Such reactions may produce phlogopite, which plays an important role in controlling the LILE characteristics of the slab-derived fluid in subduction zones
The “W-type” LILE signature of deep subduction zone fluids
No full textSubduction zones are the Earth’s environments where fluids or melts released by the
slab recycle elements into the mantle wedge, triggering partial melting and arc volcan-
ism. Despite the recent advancements in understanding the nature and composition of
the fluid phase released from the subducted slab, the interaction of this fluid with the
overlying mantle wedge remains poorly constrained. Information on the geochemical
exchange processes between suduction fluids and sub-arc mantle can be gained by
the study of metasomatised UHP ultramafic rocks in continental basements. A rele-
vant case study is represented by garnet orthopyroxenites from the Maowu Ultramafic
Complex, Dabie Shan, China. Such pyroxenites are locally bounded by phlogopite-
rich layers and are hosted by garnet-coesite gneisses. They contain orthopyroxene
(Opx2 ) + garnet (Grt2 )
± clinopyroxene which form at the expense of a previous
ultramafic olivine + garnet bearing paragenesis. Grt2 includes core clusters of primary
polyphase inclusions corresponding to a solute-rich aqueous fluid enriched in LILE
and LREE (Malaspina et al., 2006a). Textural and geochemical data demonstrate that
the Maowu Ultramafic Complex consists of metasomatic layers produced after the
reaction of mantle peridotites with a hydrous granitic melt (or a solute-rich supercrit-
ical liquid) sourced by the associated crustal rocks at UHP conditions (4.0-6.0 GPa,
700-750 ̊C). This hydrous metasomatic fluid phase likely loses elements such as SiO2 ,
Al2 O3 and K2 O during the reactive flow in the mantle peridotites, forming phlogopite-
rich layers and garnet orthopyroxenites. On the other hand, the H2 O component of the
hydrous solute-rich agent evolves into a residual aqueous fluid which concentrates the
incompatible LILE and LREE.
The trace element pattern of this fluid shows a peculiar “W-type” LILE signature char-
acterised by positive spikes of Cs, Ba, and Pb relative to Rb and K, thereby suggesting
selective enrichment in some LILE. Previous works on K-Rb-Cs partitioning between
phlogopite and fluid at 2.0 and 4.0 GPa (Melzer and Wunder, 2001) indicate that phl-
ogopite preferentially retains Rb and K with respect to Cs. They also demonstrate
that with increasing phlogopite crystallisation, the Cs/Rb ratio in the coexisting fluid
continuously increases. Formation of phlogopite layers bounding the Maowu orthopy-
roxenites may play an important role on the partitioning of incompatible elements,
resulting in a selective LILE enrichment of the residual crustal metasomatic fluid. The
mantle phases pyroxene and olivine do not incorporate large amounts of LILE (Ay-
ers et al., 1997). Moreover, absence of amphibole at P>3.0 GPa enables the residual
aqueous fluid to transfer the W-shaped signature (positive spikes of Cs, Ba, Pb, and
negative anomalies of Rb, K) to the locus of sub-arc partial melting, once it escapes the
metasomatic slab-mantle interface. An important aspect of this filtering process is that
the trace element fingerpint of other slab lithologies in Dabie Shan (Malaspina et al.,
2006b), as well as of wedge-type Alpine amphibole + garnet peridotites (Ulten Zone,
Italian Alps) is similar to the one of such residual fluid (Scambelluri et al., 2006). This
indicates that the “W-type” slab fluids are able to transfer their trace element signature
within the slab and to the locus of fluid-assisted mantle melting.
Malaspina et al. 2006a, EPSL 249, 173–187; Melzer and Wunder 2001, Lithos 59, 69-
90; Ayers et al. 1997, EPSL 150, 371-398; Malaspina et al. 2006b, Lithos 90, 19-42;
Scambelluri et al. 2006, CMP 151, 372-394
The oxidation state of metasomatised mantle wedge : insights from C-H-bearing garnet peridotite
No full textIn subduction environments the fluid phases released by the subducting plates are
vehicles for the slab-to-mantle element transfer, leading to the metasomatism, refertilisation
and partial melting of the mantle. Occurrences of hydrous minerals
coexisting with carbonates and C polypmorphs (e.g. phlogopite + magnesite +
graphite/diamond) in mantle wedge peridotites evidence that such fluids are represented
by C-O-H solutions, derived by dehydration reactions and decarbonation of the
slab. The equilibria involving the volatile elements play an important role in controlling
the iron oxidation state of mantle silicates and oxides by redox reactions. Alternatively,
Fe3+/Fe2+ equilibria between mantle minerals may buffer the fluid speciation,
and therefore oxygen fugacities (fO2). Despite a number of studies have been devoted
to determine the redox state of the upper mantle, the fO2of supra-subduction mantle
wedge, used as monitor of its oxidation state, is still poorly investigated.
An essential input for fO2 estimates is therefore represented by an accurate determination
of the ferric-ferrous iron content of key mantle minerals such as garnet. As case
study, we selected samples of a mantle wedge garnet peridotite from the UHP Sulu belt
(Eastern China), where magnesite + phlogopite occur in equilibrium with olivine + orthopyroxene
+ garnet ±clinopyroxene (Malaspina et al., in press). For the olivine + orthopyroxene
+ Fe3+-garnet assemblage, fO2 can be calculated from the reaction (1):
2 Fe2+
3 Fe3+
2 Si3O12 (skiagite) = 4 Fe2+
2 SiO4 (fayalite) + 2 Fe2+
2 Si2O6 (ferrosilite) +
O2 (Gudmundsson andWood, 1995). Fe2+/Fe3+ ratio in garnet has been measured by
the "flank method" electron microprobe analyses (Höfer et al., 1994). These measurements
reveal that the Sulu peridotite garnet contains considerable amounts of Fe3+,
showing a zonation in Fe3+/Fetot ratios, which vary from 0.06 up to 0.21. The “flank
method” has been calibrated on almandine, andradite and skiagite end-members with
fixed Fe3+/Fetot (0, 1 and 0.4 respectively). We have synthesised the end-member
skiagite and garnet along the almandine-skiagite join, where Al and Fe3+ substitute
on the octahedral sites. The experiments were performed with a glass and slag with a
fixed ratio of Fe3+/Fetot in a multianvil apparatus using tungsten carbide cubes with
a 14-mm truncated edge and Au capsule, at P=10 GPa. The correct Fe3+/Fetot ratio
of the starting material was achieved by controlling the fO2of the furnace atmosphere
using CO-CO2 gas mixes.
Up to date, the lack of thermodynamic data for the Fe3+-garnet component (skiagite),
and of an appropriate solid solution model for this phase, limited the applicability of
equilibrium (1). We therefore modelled non-ideal mixing of Al and Fe3+ on the octahedral
site by a symmetric regular solution model, combining previous experimental
and thermochemical data on skiagite and almandine (Woodland and O’Neill, 1993;
Ottonello et al., 1996). This enabled us to calculate garnet-peridotite fO2, given the
presence of Fe3+ in the garnet from equilibrium (1).
The determination of fO2, and therefore of the oxidation state, of this garnetperidotite
would be a powerful tool to compare the buffering capacity of Fe2+/Fe3+ in
the mantle wedge, relative to the C-O-H fluid speciation. This will permit to unravel
the devolatilisation processes in subduction zones and the transfer of C-O-H components
from the slab to the mantle wedge.
Gudmundsson and Wood (1995) CMP, 119, 56-67; Höfer et al. (1994) EJM, 6, 407-
418; Malaspina et al. (in press) Lithos; Ottonello et al. (1996) AM, 81, 429-447;
Woodland and O’Neill (1993) AM, 78, 1002-1015
The role of C-O-H and oxygen fugacity in subduction-zone garnet peridotites
No full textC-O-H fluids are released by dehydration, partial melting and/or decarbonation of the slab and transferred to the mantle, where they interact with the surrounding rocks, prompting the growth of carbonates, hydrous minerals and C polymorphs. In the pure C-O-H system, C-saturated fluid speciation is a function of the oxygen chemical potential. Therefore, in natural systems, the fluid speciation can be imposed by the redox state of the rock-forming phases. Alternatively, C-O-H fluids may control the bulk oxidation state of the rock system by redox reactions with the mineral phases. We selected three case studies of garnet-bearing ultramafic rocks (Ulten zone, Italy; Sulu, China; Bardane, Norway), which record metasomatic processes driven by C-O-H fluids at the interface between a subducting slab and the overlying mantle wedge. All these rocks contain carbonates (dolomite-only at P 2.4 GPa at 900 degrees C, dolomite + magnesite in between) and hydrous phases (amphibole, phlogopite) equilibrated at some stages in the garnet stability field. The fO(2) values, estimated by analysing the Fe3+ content (skiagite mole fraction) in garnet, indicate that the Ulten and Sulu peridotites record high oxygen fugacities (FMQ to FMQ+2) and a retrograde path with decreasing P and T. The fO(2) values obtained for the Bardane garnet websterites, which record a prograde path with increasing T and P, are up to 2 log units lower than the FMQ. When combined with data for subduction-zone systems (arc lavas and their mantle sources), the studied ultramafic rocks define a trend of decreasing fO(2) with increasing pressure.
The Bardane websterites contain C-polymorphs in polyphase inclusions, which precipitated from entrapped metasomatic fluids at ultrahigh pressures. The calculated C-O-H fluid phase in equilibrium with the solid phases consists of mixtures of H2O and CO2. Semi-quantitative estimates for the Ulten and Sulu peridotites, in which C-polymorphs have not been found, and petrographic constraints for the Ulten peridotites indicate that the C-O-H component of the fluid could consist of H2O+CO2
From oceanic to continental subduction : Implications for the geochemical and redox evolution of the supra-subduction mantle
No full textProcesses taking place in subduction zones are the main controller of the chemical cycle of volatile and incompatible elements in the Earth system. Metamorphic devolatilization reactions occurring during slab burial play a key role in the transfer of elements to the supra-subduction mantle, from forearc to sub-arc depth. Here, we discuss the elements released in fluids and melts from oceanic (i.e., sediments, altered oceanic crust, and hydrated lithospheric mantle) and continental slab materials during prograde subduction and the consequent implications in the chemical evolution of the supra-subduction mantle. We use bulk data and fluid and melt inclusions analyses to show and to constrain the mobility of elements from top to bottom in the subduction zone setting. The development of melange domains at the slab-mantle interface and its influence in the element cycle are also taken into account. Coupled with trace-element mobility, we review the redox evolution of slab materials during subduction and its implication in the redox conditions of the supra-subduction mantle due to fluid and melt infiltration down to sub-arc depth
Redox processes and the role of carbon-bearing volatiles from the slab-mantle interface to the mantle wedge
No full textThe valence of carbon is governed by the oxidation state of the host system. The subducted oceanic lithosphere contains considerable amounts of iron so that Fe3+/Fe2+ equilibria in mineral assemblages are able to buffer the (intensive) fO2 and the valence of carbon. Alternatively, carbon itself can be a carrier of (extensive) ‘excess oxygen’ when transferred from the slab to the mantle, prompting the oxidation of the sub-arc mantle. Therefore, the correct use of intensive and extensive variables to define the slab-to-mantle oxidation by C-bearing fluids is of primary importance when considering different fluid/rock ratios. Fluid-mediated processes at the slab–mantle interface can also be investigated experimentally. The presence of CO2 (or CH4 at highly reduced conditions) in aqueous COH fluids in peridotitic systems affects the positions of carbonation or decarbonation reactions and of the solidus. Some methods to produce and analyse COH fluid-saturated experiments in model systems are introduced, together with the measurement of experimental COH fluids composition in terms of volatiles and dissolved solutes. The role of COH fluids in the stability of hydrous and carbonate minerals is discussed comparing experimental results with thermodynamic models and the message of nature
The role of garnet and pyroxenes in redox processes in the mantle wedge
No full textRedox equilibria in the Earth’s mantle control many chemical and physical processes such as magma genesis, chemical differentiation and fluid-related metasomatism. The whole rock oxidation state is conventionally described in petrology by the oxygen fugacity (fO2). Many studies focussed on the redox state of low pressure assemblages in the mantle system, whereas the fO2 of supra-subduction mantle wedge is still poorly investigated. Orogenic garnet peridotites can be important witnesses of the processes occurring in the deep sub-arc mantle. However, the fO2 determination in most metasomatised garnet peridotites is a demanding task because of the large number of phases that may incorporate both ferric and ferrous iron, and that may show zonations. fO2 in high pressure peridotites is traditionally determined from the Fe2+-Fe3+ content of a garnet in equilibrium with olivine and orthopyroxene, but the Fe3+ partitioning among the peridotite mineral phases is often neglected. Works by [1-3] demonstrate that the increase of Fe3+ in garnet with increasing temperature and pressure is the consequence of the redistribution of Fe3+ from clinopyroxene into the garnet. This implies that the Fe3+ enrichment in garnet is not necessarily indicative of high whole-rock oxygen contents or of the interaction with more oxidised metasomatic agents.
We selected a sample of orogenic peridotite from the ultrahigh pressure Sulu belt (Eastern China) corresponding to a slice of metasomatised mantle wedge equilibrated at 5 Gpa and 900 °C. This shows phlogopite + magnesite in equilibrium with olivine, orthopyroxene, clinopyroxene and garnet. The Fe3+/ΣFe of garnet, clinopyroxene and phlogopite was measured by two combined techniques: the "Flank Method" on wavelength dispersive spectra, acquired on garnets with electron microprobe at the University of Milano, and electron energy loss spectroscopy, at the Bayerisches Geoinstitut, employed to analyse clinopyroxene and phlogopite, and to check possible zonations. The measurements indicate that the pyrope-rich garnets are zoned and contain Fe3+/ΣFe up to 0.12–0.14. These results are consistent with the relatively high oxygen fugacities (FMQ to FMQ+2) previously calculated for this sample [4]. Results on clinopyroxene and phlogopite indicate that only clinopyroxene contains some amounts of Fe3+ (Fe3+/ΣFe=0.48–0.51), while phlogopite is below the detection limits. Interestingly, also orthopyroxene contain some Fe3+/ΣFe, up to 0.10. Garnet/clinopyroxene and orthopyroxene/clinopyroxene qualitative partitioning apparently do not follow the same trend described by [1] for a suite of sub-cratonic garnet peridotite xenoliths. The determination of Fe2+/Fe3+ equilibria in mineral assemblages of metasomatised mantle-wedge peridotites therefore represents only the first step in unravelling the relationships between fO2 and phase assemblages in multi-component mantle systems.
References: [1] Canil D., O’Neill H.S.C. (1996): J Pet 37, 609–635; [2] Woodland A.B., Koch, M. (2003): Earth Planet Sci Lett 214, 295–310; [3] Woodland A.B. (2009): Lithos, in press; [4] Malaspina N., Poli S., Fumagalli P. (2009): J Petrol 50, 1533–1552
Multistage metasomatism in ultrahigh-pressure mafic rocks from the North Dabie Complex (China)
No full textRelease of metamorphic fluids within the slab and/or from the slab to the mantle wedge in subduction environments can produce important metasomatic effects. Ultrahigh-pressure (UHP) metasomatised rocks represent ideal materials to study the element exchange at pressures corresponding to sub-arc depths in subduction zones. We present a petrologic and geochemical study of eclogites (s.l.) from the Dabie Mountains (China). The investigated samples were collected in the North Dabie Complex, where eclogite-facies rocks are significantly overprinted by granulite-facies metamorphism and partial melting. The studied eclogites are included in meta-lherzolitic bodies, which are in turn hosted by leucocratic gneisses. The textural relations among the various rock-forming minerals enabled us to identify several re-crystallisation stages. The peak (UHP) paragenesis consists of
garnet, clinopyroxene and rutile. UHP garnet and clinopyroxene display oriented inclusions of polycrystalline rods of rutile + ilmenite and of albite, K–Ba-feldspar and quartz, respectively. Garnet and clinopyroxene are both rimmed by an inclusion free zone that formed after the peak, still at high-pressure conditions. Such optical zoning does not correspond to a difference in major element concentrations between garnet core and rim. This observation provides evidence that the major element composition of garnet was reset during exhumation, thus preventing thermobarometric determination of peak metamorphic conditions. Further
decompression is documented by the formation of limited ilmenite+amphibole and granulite-facies coronas consisting of
clinopyroxene, orthopyroxene, plagioclase and amphibole around garnet. In order to investigate the stability of observed mineral parageneses, a series of reconnaissance piston cylinder synthesis experiments were carried out in an identical bulk composition.
The experimental study indicates that the peak metamorphic paragenesis is stable at P∼3.5 GPa and T≥750–800 °C. The
petrological study, combined with bulk-rock and mineral trace element analyses, provides evidence of intense metasomatism
affecting these eclogites. The bulk-rock major and trace element compositions indicate that the eclogites derive from basaltic protoliths with MORB and E-MORB affinity. Compared with such basalts, the studied rocks show strong depletion in SiO2 and alkalis and enrichment in MgO and FeO. These features likely derive from element exchange with ultramafic rocks prior to subduction, possibly related with the influx of Si-depleted and Mg-enriched fluids produced during the serpentinisation of the associated lherzolitic rocks. On the other hand, the trace element bulk-rock compositions show strong enrichment in Cs, Ba and Pb. The same characteristic enrichment and fractionation is recorded by peak metamorphic clinopyroxene but not in retrograde amphibole. Therefore, influx of crustal fluids transporting LILE and light elements must have occurred during subduction at UHP conditions. This stage likely records the tectonic coupling of the mafic–ultramafic rocks with the associated crustal rock units and provides evidence of LILE mobility between different slab components
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