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Hydrothermal alteration patterns below the ocean floor. Low-temperature alteration occurs in broad downflow zones leading to elevation of d 18 O values and enrichment in Mg and depletion in Ca. By contrast, higher-temperature alteration in the relatively narrow upflow zones produce rocks which are Ca-and Mn rich, and Mg-and Na poor, with relatively low d 18 O values. Sulphide mineralisation may occur on the ocean floor in blacksmoker-type deposits
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The subduction of hydrated oceanic lithosphere potentially transports large volumes of water into the upper mantle; however,
despite its potential importance, fluid–rock interaction during high-pressure metamorphism is relatively poorly understood.
The stable isotope and major element geochemistry of Pennine ophiolite rocks from Italy and Switzerl...
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... below the ocean floor produ- ces a characteristic modification of d 18 O values from their original igneous values (F5.7‰). In the higher levels of the oceanic crust, interaction with sea water at low temperatures results in an elevation of d 18 O values, whereas higher-temperature alteration at deeper levels causes d 18 O values to be lowered (Fig. 1). Alteration is generally widespread within the sheeted dykes and pillow basalts but more localised (often around frac- tures) in the deeper gabbros and ultramafic rocks. The pattern of isotopic resetting is also dependent on whether the rocks are situated in the downflow parts of the system (where temperatures at any given depth are ...
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... within the sheeted dykes and pillow basalts but more localised (often around frac- tures) in the deeper gabbros and ultramafic rocks. The pattern of isotopic resetting is also dependent on whether the rocks are situated in the downflow parts of the system (where temperatures at any given depth are relatively cool) or in the warmer upflow zones (Fig. 1). Fluid flow also alters the major element chemistry of the rocks ( Alt et al. 1986;Gillis et al. 1993;Gillis 1995). Basalts from the downflow zones are commonly enriched in Mg and depleted in Ca due to the forma- tion of chlorite and the breakdown of plagioclase. By contrast, Ca-and Mn-rich as well as Mg-and Na-poor epidote-bearing ...
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... are less common than the blueschist-facies rocks, which accords with observa- tions in unmetamorphosed ophiolites that fluid down- flow occurs over broad areas, whereas upflow was much more focussed. The samples that have undergone high-temperature alteration generally have lower d 18 O values, whereas those that underwent low-temperature Fig. 1, this is the pattern that is expected to develop during sub-ocean-floor hydrothermal alteration, suggesting that the major element and isotope geochemistry of these rocks has remained unaltered through high-pres- sure metamorphism. Tables 1 and 2. We analysed mainly minerals from unretrogressed to moderately retrogressed rocks where ...
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... predicted suggests that the veins approach isotopic equilibrium with their hosts, and were locally derived by diffusion or small-scale fluid flow. Further evidence for local vein derivation comes from observations that wallrocks within 20 cm of one of the kyanite-bearing veins is depleted in Al 2 O 3 and SiO 2 (the major components of the veins; Fig. 10; Table 3). Overall, we interpret the data to indicate that the high-pressure veins were formed from fluids that were derived from, or which had equilibrated with, the surrounding rocks. One plausible source of internally generated fluids during high-pressure metamorphism are those liberated by the breakdown of lawsonite ( Fig. 3; e.g. ...
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The Xingzi Group, exposed in Lushan, southeast China, is the oldest basement in the Late Proterozoic Jiangnan orogen and composed mainly of metamorphosed psammites and pelitic schists with the grade of amphibolite facies. Eight quartz and felsic veins from two locations in this group have been chosen to investigate trace element behaviour during me...
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... In fully hydrated parts, eclogitization went to completion. When this initial hydration occurred has remained unclear, however, with some authors suggesting hydration near the sea floor (Barnicoat and Cartwright, 1997;Cartwright and Barnicoat, 1999) and others proposing subduction zone hydration (Bucher and Grapes, 2009;Meyer, 1983;Wayte et al., 1989). ...
... In the "continental gabbro" scenario, schematically drawn in green in Fig. 14, the Allalin gabbro represents a mafic underplate at the base of the continental crust (Bucher and Grapes, 2009), which experienced a high-temperature granulite facies stage followed by subduction during which hydration occurred (Bucher and Grapes, 2009;Meyer, 1983;Wayte et al., 1989). In contrast, the "oceanic gabbro" scenario implies that the Allalin gabbro forms as part of the ZSO (Bearth, 1967;Meyer, 1983) and was partly hydrated near the sea floor (Barnicoat and Cartwright, 1997;Cartwright and Barnicoat, 1999). This P-T evolution scenario is schematically drawn in blue in Fig. 14. ...
The Allalin gabbro of the Zermatt-Saas meta-ophiolite consists of variably metamorphosed Mg- to Fe-Ti-gabbros, troctolites, and anorthosites, which are crosscut by basaltic dykes. Field relationships of the various rock types and petrographic studies together with bulk rock and mineral chemical composition data allow the reconstruction of the complete geological history of the Allalin gabbro. With increasing magmatic differentiation, the incompatible element content in clinopyroxene increases (e.g., REEs and Zr by a factor of 5), whereas the Mg# decreases (from 86.4 to 74.6) as do the compatible element contents (e.g., Cr and Ni by factors of 3.5 and 5, respectively). Exhumation to shallower depths led to subsolidus ductile deformation and cooling of the gabbro followed by the intrusion of fine-grained basaltic dykes, which display chilled margins. Bulk rock data of these dykes reveal strong similarities in fluid-immobile trace element patterns to tholeiitic pillow basalts of the Zermatt-Saas and nearby meta-ophiolites. The recalculated REE patterns of the melt in equilibrium with igneous clinopyroxene is very similar to the REE patterns of the mafic dykes, indicating a cogenetic origin of pillow basalts, dykes, and gabbros. Together with the previously determined Jurassic intrusion age of the gabbro, our observations demonstrate that the Allalin gabbro intruded as a tholeiitic magma in a slow spreading MOR environment of the Piemonte-Ligurian ocean of the Alpine Tethys. Subduction of the Allalin gabbro resulted in different eclogitization extents of the Mg-gabbros as a function of variable hydration degrees. Metagabbros with low extents of hydration record incomplete eclogitization; the magmatic mineralogy (olivine + clinopyroxene + plagioclase) is preserved together with disequilibrium textures in the form of reaction coronae surrounding mineral boundaries. Metagabbros with high extents of hydration are completely eclogitized and display pseudomorphic replacement textures of magmatic minerals by eclogite-facies mineral assemblages, which required significant major to trace element transport across mineral domains. The locally variable extents of hydration took place near the sea floor, as recorded by the presence of Cl-apatite (6.28 wt% Cl), and an increase in B concentrations of minerals pseudomorphically replacing olivine (e.g., chlorite with 0.20–0.31 µg/g B and omphacite with 0.22–0.25 µg/g B) compared to magmatic olivine (0.12–0.16 µg/g B). Moreover, the chemical zonation pattern of metamorphic garnet coronae is different in completely eclogitized gabbros and gabbros with relic igneous minerals, in agreement with a main hydration event prior to subduction. The Allalin gabbro therefore represents a classical example of an oceanic gabbro formed in a slow spreading setting in the mid Jurassic that experienced heterogeneous hydration near the sea floor. Paleogene subduction of the gabbro to 70–80 km depth produced variably equilibrated gabbroic eclogites. In eclogite-facies Mg-gabbros, the water-rich minerals chlorite, talc, and chloritoid pseudomorphing magmatic olivine remained stable to these depths, revealing the potential relevance of hydrated Mg-gabbros as a fluid source at subarc depths in subduction zones.
Supplementary Information
The online version contains supplementary material available at 10.1186/s00015-024-00461-8.
... +0.9‰ shown by sample FL4 may be related to a partial contribution of retrograde glaucophane due to re-hydration event(s) during initial stage(s) of exhumation. Although the B isotope compositions of the Vara Fe-Ti eclogites can be attributed to prograde metamorphism, their δ 18 O composition averaging at +5.8 ± 0.5‰ appears to reflect the initial oceanic imprint (Barnicoat and Cartwright, 1997;Cartwright and Barnicoat, 1999;Putlitz et al., 2000) (Figs. 4B, C). ...
Trace element and isotopic compositions of exhumed high-pressure (P) mafic rocks are an important archive to investigate chemical processes in subduction zones. Here we report the B isotope (δ11B) composition of eclogitic mafic rocks enclosed in high-P serpentinite from the Voltri Massif, Ligurian Alps (Italy). Combined with bulk δ18O values, 87Sr/86Sr ratios and trace element data, the δ11B of eclogitic mafic rocks were investigated to test oceanic inheritance vs. subduction-related processes and to provide inferences on the timing of B uptake in eclogitic metagabbros.
Petrographic observations along with major and trace element data indicate that the eclogite facies mafic rocks derived from primitive Mg-Al-bearing gabbros (Erro-Tobbio eclogites) and differentiated Fe-Ti-bearing oxide gabbros (Vara eclogites). Metarodingite from Vara shares similar REE pattern to that of the Erro-Tobbio eclogites, suggesting a plagioclase-rich gabbroic protolith. The studied rocks show variable enrichments in fluids-mobile elements (e.g., Cs, Ba, Li, Sr; high Li/Y, B/Nb). Boron concentrations range between 2.3 and 7.6 ppm, with no significant differences among the different samples. Oxygen isotope compositions (1σ ± 0.1‰) for the Vara and the Erro-Tobbio eclogites range from +5.4 to +6.4‰ and from +3.1 to +5.3‰, respectively, whereas the 87Sr/86Sr ratios range between 0.7036 and 0.7042 for the Vara eclogites and from 0.7030 to 0.7034 for Erro-Tobbio metagabbros. The B isotope compositions for the Vara eclogites range between −3.2 to +0.9‰, which are in striking contrast with the positive δ11B signatures of the Erro-Tobbio eclogitic metagabbros ranging from +4.3 to +8.9‰, the highest positive δ11B signatures observed in eclogitic metagabbros so far. Metarodingite from Vara has a δ18O of +3.7‰ and 87Sr/86Sr of 0.7046 and a δ11B value of +11.5‰.
We argue that the δ18O signatures and the 87Sr/86Sr ratios of metamafic rocks from the Voltri Unit mainly reflect inherited signatures from the seafloor, whereas subduction-related processes are mainly traced through δ11B variations. Progressive B isotope fractionation due to dehydration processes during subduction is responsible for the dominantly negative δ11B signatures of eclogites from the Vara area and elsewhere, whereas the positive δ11B of the high-P metarodingite is compatible with an inherited oceanic signature, although interaction with fluids released by the surrounding serpentinites at peak conditions cannot be ruled out. We discuss three scenarios to explain the positive δ11B imprint reported for the Erro-Tobbio eclogites: (i) inheritance of oceanic signatures, (ii) records of B isotope fractionation from an extreme 11B-rich protolith, or (iii) interaction with 11B-rich slab fluids derived from serpentinite hydration at high-P. Based on these scenarios, two potential implications of global relevance are presented: (1) the transfer of largely unmodified oceanic signatures up to eclogite facies conditions, and (2) the formation of newly (subduction-related) 11B-rich reservoirs. We propose that the protolith composition of the mafic crust controls the development of hydrous phases at high-P thus enabling the preservation of high δ11B signatures. This mechanism of chemical transfer may generate a 11B-rich reservoir that, together with high-P serpentinites, may form positive δ11B domains in the mantle.
... Another probable source in the geologic context of the Monte Rosa nappe is the Bündnerschiefer from the Zermatt-Saas zone (ZSZ), which can be found at a few hundred meters distance from the whiteschist outcrop discussed here (Fig. 1). Nevertheless, the Bündnerschiefer, and the mafic rocks in immediate contact with the nappe, all have higher δ 18 O values relative to the whiteschist (Cartwright and Barnicoat 1999;Dessimoz 2005). A more plausible source are the Zermatt-Saas serpentinites, which are abundant in the ZSZ (Steck et al. 2015). ...
... Serpentinites from the Zermatt-Saas unit have δ 18 O values between 0 to 5‰ in the Upper Ayas Valley (Dessimoz 2005). Fluids equilibrated or produced by these serpentinites would be a suitable source for the low δ 18 O, in agreement with serpentinite data from other locations in the Zermatt-Saas unit (Cartwright and Barnicoat 1999). Serpentinites can potentially release a substantial amount of fluid during dehydration reactions, as indicated by the positive slope of the antigorite breakdown reaction in a P-T diagram (Kerrick and Connolly 2001;Ulmer and Trommsdorff 1995). ...
In situ oxygen isotope compositions of white mica and quartz have been used to characterize the interplay of metamorphism and fluid events between a metasomatic whiteschist and its granite protolith in the Monte Rosa nappe, Western Alps. New natural muscovite and phengite reference materials were calibrated for in situ Secondary Ion Mass Spectrometry (SIMS) oxygen isotope measurement. White mica and quartz oxygen isotope compositions were measured in situ in one whiteschist and two metagranites. Based on microtextural observation, phengite composition of white mica, and phase petrology modeling, it is possible to identify two events of fluid infiltration and one event of fluid expulsion, all of which were responsible for forming this unique whiteschist occurrence and for tracing its metamorphic evolution from late Permian intrusion to Alpine subduction and finally to the present day, exhumed whiteschist.
Metagranite samples contain three generations of white mica: igneous, high-P metamorphic, and late Alpine, retrograde compositions. In the whiteschist samples, we distinguish two distinct Alpine white mica generations: (1) prograde to peak generation and (2) retrograde generation.
The δ18OVSMOW values of white mica and quartz from a whiteschist of 5.3 to 7.3‰ and 9.1 to 10.6‰ are significantly lower than in the metagranites, with 9.1 to 10.8‰ and 13.2 to 14.6‰, respectively. This indicates a complete recrystallization of the whiteschist protolith during intense fluid-rock interaction. Subsequent Alpine metamorphism transformed the protolith into the whiteschist. The isotopic composition of the whiteschist, fine-grained, retrograde white mica (5.3 to 6‰) is lower than that of the high-pressure phengite (6.2 and 7.5‰). The low δ18O values could be explained by infiltration of external fluids with δ18O values of 2 to 6‰. Such fluids would carry the isotopic signature of the serpentinites of the Piemonte-Liguria Ocean by either equilibration of fluids with or dehydration of serpentinites. Another, more simple explanation would be the infiltration of very small quantities of fluids leading to the breakdown of chloritoid. Local inheritance of the oxygen composition would then hide the origin of the fluids.
Isotope exchange temperatures calculated from high-P phengite-quartz pairs in whiteschist give an average temperature of 440 ± 50 °C. These are lower than the best T-estimates from phase petrology of 570 °C, at 2.2 GPa. Igneous muscovite-quartz pairs in the metagranite yield 400 ± 40 °C. Only one high-P phengite-quartz pair was analyzed, resulting in 350 ± 40 °C. Greenschist facies, low silica phengites give an average temperature of 310 ± 10 °C. Propagation of analytical uncertainty suggests large errors of 60 to 120 °C, due to the relatively small T-dependence of the quartz-white mica fractionation factor for oxygen isotopes.
... The presence and movement of fluids and melts at a variety of scales and levels facilitates and governs geochemical cycles and fluid-enhanced deformation within subduction zones [5][6][7][8]. Geochemical studies [6,9,10] and numerical simulations [11] suggest that the fluid flow in subduction zones may be strongly channelized but not pervasive, under the control of the low permeability in subducting slabs, the high dihedral angles between fluid and minerals, and the negative rock volume change during prograde metamorphism [12,13]. Investigation of fluid channelization is therefore critical for the understanding of fluid fluxes and flow paths in the subduction zones [12], and sequentially, for the understanding of the material cycles of substances such as water, carbon, water-soluble salts, and abiogenic hydrocarbons. ...
... (Table 2). For such C-O isotopic characteristics, a fluid source of devolatilized metasedimentary rocks is unlikely, given that hydrothermal fluid from this source generally forms carbonates with much heavier oxygen isotopes (δ 18 O V-SMOW around 17-27‰) [6,10,97]. A fluid source of devolatilized metabasite is more likely, as fluid derived from such a source precipitates carbonates with a large δ 18 O V-SMOW range of about 8-22‰ [5,96,98], which covers the measured dolomite δ 18 O V-SMOW values. ...
... To confirm the origin of the fluid further, we compare our data with C-O isotopes of high-pressure veins from published literature, which are thought to represent the conduits of fluid that internally derives from the host rocks during metamorphic devolatilization [10,57,[98][99][100][101][102]. The C-O isotopes of vein carbonates in metabasites were compiled in Figure 7 in order to trace the fluids sourced from different facies of metabasite. ...
Greenschist facies metabasite (chlorite schist) and metasediments (banded iron formation (BIF)) in the Wutai Complex, North China Craton recorded extensive fluid activities during subduction-related metamorphism. The pervasive dolomitization in the chlorite schist and significant dolomite enrichment at the BIF–chlorite schist interface support the existence of highly channelized updip transportation of CO2-rich hydrothermal fluids. Xenotime from the chlorite schist has U concentrations of 39–254 ppm and Th concentrations of 121–2367 ppm, with U/Th ratios of 0.11–0.62, which is typical of xenotime precipitated from circulating hydrothermal fluids. SHRIMP U–Th–Pb dating of xenotime determines a fluid activity age of 1.85 ± 0.07 Ga. The metasomatic dolomite has δ13CV-PDB from −4.17‰ to −3.10‰, which is significantly lower than that of carbonates from greenschists, but similar to the fluid originated from Rayleigh fractionating decarbonation at amphibolite facies metamorphism along the regional geotherm (~15 °C/km) of the Wutai Complex. The δ18OV-SMOW values of the dolomite (12.08–13.85‰) can also correspond to this process, considering the contribution of dehydration. Based on phase equilibrium modelling, we ascertained that the hydrothermal fluid was rich in CO2, alkalis, and silica, with X(CO2) in the range of 0.24–0.28. All of these constraints suggest a channelized CO2-rich fluid activity along the sediment–basite interface in a warm Paleoproterozoic subduction zone, which allowed extensive migration and sequestration of volatiles (especially carbon species) beneath the forearc.
... An original bulk δ 18 O composition of ~ 2‰ is calculated for the mafic fels in equilibrium with the measured garnet δ 18 O of 1‰. Such low bulk δ 18 O is below the MORB range (Taylor 1968;Muehlenbachs and Clayton 1972;Alt et al. 1986) but is reported for hydrothermally altered mafic oceanic crust (Cartwright and Barnicoat 1999;Miller et al. 2001;McCaig et al. 2007). Therefore, the relatively low bulk δ 18 O values of the mafic fels could be ascribed to high-temperature seafloor alteration prior to Alpine metamorphism. ...
... For the mafic schist and Cld-schist, the calculated bulk δ 18 O is ~ 11‰ based on the δ 18 O of 9.5‰ of the garnet core. The bulk rock δ 18 O composition of the schists corresponds to the lower side of the wide δ 18 O range usually observed for metasediments and more specifically pelites (Rumble et al. 1982;Bebout and Barton 1989;Cartwright and Barnicoat 1999). On the other hand, a bulk rock with δ 18 O of ~ 3‰ would be in equilibrium with the low δ 18 O garnet rim in the schists (Online Resource 1: Fig. S7, S8). ...
... (1b) Lawsonite-out dehydration reaction, in ZSZ. Lawsonite relicts have been described in the ZSZ metabasalts (Angiboust and Agard 2010) that have bulk rock δ 18 O values from 4.4 to 9.2‰ (Barnicoat and Cartwright 1995;Cartwright and Barnicoat 1999). Such bulk rock compositions would produce fluids with even higher δ 18 O values. ...
Dehydration reactions in the subducting slab liberate fluids causing major changes in rock density, volume and permeability. Although it is well known that the fluids can migrate and interact with the surrounding rocks, fluid pathways remain challenging to track and the consequences of fluid-rock interaction processes are often overlooked. In this study, we investigate pervasive fluid-rock interaction in a sequence of schists and mafic felses exposed in the Theodul Glacier Unit (TGU), Western Alps. This unit is embedded within metaophiolites of the Zermatt-Saas Zone and reached eclogite-facies conditions during Alpine convergence. Chemical mapping and in situ oxygen isotope analyses of garnet from the schists reveal a sharp chemical zoning between a xenomorphic core and a euhedral rim, associated to a drop of ~ 8‰ in δ ¹⁸ O. Thermodynamic and δ ¹⁸ O models show that the large amount of low δ ¹⁸ O H 2 O required to change the reactive bulk δ ¹⁸ O composition cannot be produced by dehydration of the mafic fels from the TGU only, and requires a large contribution of the surrounding serpentinites. The calculated time-integrated fluid flux across the TGU rocks is 1.1 × 10 ⁵ cm ³ /cm ² , which is above the open-system behaviour threshold and argues for pervasive fluid flow at kilometre-scale under high-pressure conditions. The transient rock volume variations caused by lawsonite breakdown is identified as a possible trigger for the pervasive fluid influx. The calculated schist permeability at eclogite-facies conditions (~ 2 × 10 –20 m ² ) is comparable to the permeability determined experimentally for blueschist and serpentinites.
... The crustal contamination for the protoliths (Section 5.1) can only explain a < 0.5‰ increase in δ 18 O, as inferred from the δ 18 O values of the paragneisses. In ophiolitic crust, however, δ 18 O values are typically raised by LT alteration in the extrusive section and lowered by high-temperature alteration in the gabbro section (e.g., Gregory and Taylor Jr, 1981;Cartwright and Barnicoat, 1999). Fluid-rock interactions can also modify δ 18 O values during subduction when rocks react with externally-derived fluids, but this effect is thought to be modest unless the rocks were located close to a fluid flow pathway and/ or strongly retrogressed (e.g., Cartwright and Barnicoat, 1999;Miller and Cartwright, 2000). ...
... In ophiolitic crust, however, δ 18 O values are typically raised by LT alteration in the extrusive section and lowered by high-temperature alteration in the gabbro section (e.g., Gregory and Taylor Jr, 1981;Cartwright and Barnicoat, 1999). Fluid-rock interactions can also modify δ 18 O values during subduction when rocks react with externally-derived fluids, but this effect is thought to be modest unless the rocks were located close to a fluid flow pathway and/ or strongly retrogressed (e.g., Cartwright and Barnicoat, 1999;Miller and Cartwright, 2000). For our samples, widespread subduction metasomatism is not supported by field and petrographic evidence, and retrogression is mostly modest or absent. ...
Eclogites, metagabbros, and paragneisses from the Variscan Münchberg Massif record a complex succession of igneous, hydrothermal and metamorphic processes. The geodynamic setting related to the protolith formation and the impact of different types of fluid-rock interactions have been uncertain up to now. We use major and trace element chemistry as well as oxygen isotopes to disentangle the geochemical signatures related to the different stages of the rocks' history.
In the Münchberg Massif, dark eclogites (kyanite-free; Fe-Ti-MORB signature) are distinguished from light eclogites (kyanite-bearing; higher Mg#, Al2O3, and Cr; lower incompatible element contents; positive Eu anomalies; MORB to arc basalt signature). The δ18O values for both types (+5.0 to +10.8‰) are equal to, or higher than those of MORB. Amphibolite facies metagabbros have a more enriched, almost OIB-like trace element signature and high δ18O values (+9.4 to +10.3‰).
Good linear correlations between fluid-immobile elements throughout the eclogite types confirm their derivation from a common, N-MORB to E-MORB-like parental magma. We interpret the light eclogites as former plagioclase-rich cumulates and the dark eclogites as their complementary differentiates. This relationship is partly obscured by variable degrees of magma contamination by sediments, which also affected the metagabbros. However, the metagabbros originated from a more enriched mantle source than the eclogites. Following intrusion, the eclogites were subjected to hydrothermal alteration under the influence of seawater, as indicated by positive correlations between Li, B, Sb, and δ18O. Metamorphic fluid-rock interactions appear to be mostly of limited extent, probably due to the lack of lawsonite dehydration as a fluid source. Nevertheless, the contents at least of some fluid-mobile elements, such as LILE, Li, and Pb, were probably modified during the subduction-exhumation cycle of the eclogites.
The crustal contamination of the protolith magmas argues against derivation of the eclogites and metagabbros from typical oceanic crust. Instead, a rift-drift transition setting related to the opening of the Rheic or Saxothuringian Ocean seems most likely. The eclogites and metagabbros, alongside with similar rocks in the Mariánské Lázně complex and other resembling high-pressure massifs, may record different stages of this rift-drift transition.
... This is, in fact, consistent with the systematic variation in bulk δ 18 O values with depth observed in a complete section of oceanic crust in the Oman ophiolite, where altered gabbros have δ 18 O values as low as ~ 3‰ due to interaction with heated hydrothermal fluids, in contrast to pillow lavas characterised by δ 18 O values as high as ~ 10‰ due to low-temperature alteration at shallower depths (Gregory and Taylor, 1981;Yamaoka et al., 2012). Similar low δ 18 O values are also measured in serpentinites from mid-ocean ridge environments (e.g., Mével, 2003;Rouméjon et al., 2018), as well as from ophiolitic terrains that were either affected by or escaped high-pressure metamorphism (e.g., Cartwright and Barnicoat, 1999). ...
... Studies of serpentinised mantle that experienced renewed olivine growth in a convergent plate boundary UHP metamorphic environment, show that the metamorphism does not significantly change the dehydrating serpentinite's major element, REE and HFSE signatures (Padrón-Navarta et al., 2013;Scambelluri et al., 2014). Hence this compositional congruence, regardless of their contrasting mineralogy and olivine δ 18 O values, demonstrates that much of their geochemical signatures has been retained from their serpentinised mantle protoliths, and have been preserved through UHP metamorphism (e.g., Cartwright and Barnicoat, 1999). ...
The ~ 3700 Ma Inner Arc Group of the Isua supracrustal belt (Greenland) contains a 10 km long strip of ultramafic schists with two ≤ 1 km long meta-dunite lenses, preserving relict olivine + antigorite + titano-clinohumite and titano-chondrodite ~ 2.6 GPa ultra-high-pressure (UHP) assemblages. There are two distinct relict meta-peridotite variants: The southern lens ‘A’ variant is dominated by an aggregate of Fo91-92 olivine with δ¹⁸OVSMOW of + 5.4 ± 0.1‰ (95% confidence) in which are rare small clinopyroxene inclusions, plus rare interstitial accessory chrome spinel partly altered to magnetite. The olivine grains are bounded by serpentine domains, which are intergrown with the olivine margins with a marginally more iron-rich composition of Fo90.5. The northern lens ‘B’ contains coarser-grained Fo96-98 olivine which encloses magnetite as clusters and trails, some arranged with 60°-120° conjugate intersections. The Fo96-98 olivine has δ¹⁸OVSMOW of + 3.2 ± 0.2‰ and is in equilibrium with high-Al antigorite and the accessory UHP Ti-rich minerals.
We interpret that the Fo90-92 versus the Fo96-98 olivine assemblage variants are caused by a varying degree of fluid influx during ca. 3700 Ma serpentinisation, prior to peak UHP metamorphism. The Fo90-92 variant was a rock-dominated system with a postulated early serpentine + ferroan brucite alteration assemblage, with abundant relict mantle-like δ¹⁸OVSMOW + 5.4‰ olivine. The Fo96-98 variant was a fluid-dominated system, where obliteration of the protolith olivine formed serpentine + magnetite + brucite ± magnesite. During prograde UHP metamorphism consumption of brucite by reaction with serpentine gave rise to a lower δ¹⁸OVSMOW Fo96-98 olivine + serpentine assemblage. Both varieties have similar bulk chemistry, interpreted as depleted mantle that was enriched in LILE and LREE by a melt or fluid, prior to varying degrees of serpentinisation and superimposed UHP metamorphism.
In Phanerozoic abyssal peridotites similar textural, chemical and isotopic variations are the result of rock versus fluid-dominated serpentinisation and variable alteration temperatures. Analogously, the Fo96-98 Isua peridotites are interpreted as mantle peridotite entirely serpentinised by high temperature seawater in a fluid-dominated environment and then metamorphosed under UHP conditions at a convergent plate boundary. This indicates lateral lithosphere motions facilitating hydrosphere - mantle communication early in Earth’s history.
... Overall, the measured O isotopic signature does not reflect that of marine carbonates and, in the case of the host calcschist and marbles, it cannot be considered pristine since fresh marine carbonates cluster in the range of +25 to +32‰ (see the field in Fig. 10A). In the Alps, a number of authors have demonstrated that carbonate-bearing metasediments have been affected by regional-scale δ 18 O equilibration lowering its value from c. +29‰ to values as low as +12‰ while their original δ 13 C remained relatively unaffected (− 2 to +2‰; Cartwright and Barnicoat, 1999;Miller and Cartwright, 2000;Cook-Kollars et al., 2014;Jaeckel et al., 2018;Epstein et al., 2020). Jaeckel et al. (2018) demonstrated that, along Dent Blanche paleosubduction interfaces in the Italian and Swiss Alps, non-pristine δ 18 O ratios between +18 to +22‰, observed regionally in the Schistes Lustres, were overprinted by metasomatism resulting in values as low as +12‰. ...
... Thus, Sr-Nd systematics do not indicate clear evidence for seafloor alteration in the studied blueschists and carbonate-bearing veins, supporting the inferred scenario where most veins and host rock features were produced and evolved during subduction. A first, straightforward explanation, is that the related fluids are locally-derived (e.g., Nadeau et al., 1993;Cartwright and Barnicoat, 1999;Spandler and Hermann, 2006;Hermann et al., 2006;Taetz et al., 2016). This hypothesis, which involves mass-transfer at the local scale either by dissolution-precipitation or decarbonation (Putnis and Austrheim, 2012) is at odds with petrological, structural and geochemical evidences, including (i) the occurrence of these veins as hydraulic breccias and hydrofractures, implying vast amounts of advected fluids, (ii) crosscutting relationships where the hydraulic breccias sharply transect the strongly foliated fabrics of the hosts, (iii) the systematic absence of carbonates in the matrix of Seghin complex lithologies -except in the metasediments-and (iv) virtually identical O -C isotopic signatures in veins and metasediments. ...
We investigate the late Cretaceous blueschist-facies (480 °C-1.8 GPa) segment of the Zagros suture zone, a well-preserved block-in-matrix paleo-subduction channel. We aim to determine the relative chronology, conditions of deformation, and potential fluid sources and processes associated with the widespread occurrence of lawsonite + clinopyroxene + glaucophane veins and aragonite-bearing hydraulic breccias. We use a multi-scale approach methodology to provide new insights into deep fluid flow mechanisms as well as to constrain possible sinks of CO2-bearing fluids in the subducting slab. Petrological analyses suggest that silicate-rich vein systems began precipitating during early burial and evolved with ongoing burial and shearing-related deformation in the blueschist-facies, while most carbonate-rich veins and hydrofractures formed at near-peak P-T conditions. In situ LA-ICP-MS trace element analyses reveal that: (i) individual silicate host-vein pairs have similar REE signatures, reflecting local-scale fluid-mediated element redistribution, (ii) carbonate-bearing veins and metasediments also have similar trace element signatures and (iii) lawsonite in blueschist-hosted veins exhibit REE enrichments along their rims, suggesting an increasing contribution of metasedimentary-derived fluids upon approaching peak P-T. Carbonate OC isotope compositions of the veins and metasedimentary rocks range from +13.6 to +17.9‰ (δ¹⁸OVSMOW) and − 1.0 to +3.1‰ (δ¹³CVPDB), demonstrating metasedimentary-derived fluid sources related to large-scale H2O homogenization with far-traveled mafic- ultramafic-derived fluids. SrNd isotopic ratios in carbonate veins and the adjacent host resemble their host composition indicating that host rock-buffered isotopic homogenization occurred between the infiltrating fluids and the rock matrix, possibly during episodic porous flow. Thermodynamic modeling predicts that decarbonation via fluid-assisted reactions is inefficient at blueschist-facies and that carbon release likely occurs deeper along the subduction interface (i.e., at eclogite-facies). We propose that deeply produced H2O-rich fluids interacted with the carbonate-bearing lithologies along the subduction interface facilitating fluid-mediated decarbonation and further fluid transport as hydraulic pulses (e.g., porosity waves) that traveled at the kilometer-scale parallel to the subduction interface, (i) contributing to the isotopic homogenization herein observed and (ii) triggering episodic hydrofracturing in the lawsonite-blueschist-facies (≈50-60 km depth). Veinsets in exhumed subducted rocks hence provide a unique opportunity to understand fluid-rock interaction processes in the region at which episodic tremor and slow slip events phenomena occur.
... The recognition of structural relics as the D 1 clastic texture and the S 1 foliation in the MH suggests that the D 2 layering derives from tectonic transposition of a previous (already metasomatized?) rock consisting of carbonate-rich and mafic/ultramafic assemblages. In addition to this transposition effect, the D 2 deformation (Collins et al., 2015) and calcschists (Cartwright and Barnicoat, 1999;Cook-Kollars et al., 2014) are reported. The Zermatt-Saas serpentinites whole rock δ 18 O VSMOW values are taken from Cartwright and Barnicoat (1999). ...
... In addition to this transposition effect, the D 2 deformation (Collins et al., 2015) and calcschists (Cartwright and Barnicoat, 1999;Cook-Kollars et al., 2014) are reported. The Zermatt-Saas serpentinites whole rock δ 18 O VSMOW values are taken from Cartwright and Barnicoat (1999). phase commonly produced boudinage and shearing of rock components as attested by the occurrence, in the MH and in the CCU, of sigmoid-shaped clasts and blocks. ...
... In this scenario, the permeability contrast between serpentinite basement and CCU could have allowed fluid flow along their interface (e.g., Bebout and Penninston-Dorland, 2016) and consequently behaved as a preferential channel for fluid circulation where the metasomatic reaction fronts were probably asymmetric/heterogeneous (see Piccoli et al., 2018 and references therein), in some extent depending on the textural features of the MH protoliths. Then, this kind of channel could have acted also as a major shear zone (e.g., Angiboust et al., 2014), where the ductile deformation was also assisted and imbrication enhanced by the flow of fluids (e.g., Cartwright and Barnicoat, 1999;Gerya et al., 2002;Angiboust et al., 2012;Zheng et al., 2013;Fagereng and den Hartog, 2017;Prigent et al., 2018, Hirauchi et al., 2020. The transitional nature of the MH contact with CCU also records the effects of channelling fluid-driven reactions along the contrasting permeability interface between the two protoliths. ...
A metasomatic horizon (MH) occurs between the metaophiolite (serpentinite and metaophicarbonates) basement and metasedimentary sequence (chaotic rocks and calcschists) of the Lake Miserin Ophiolite, in the high pressure Zermatt-Saas Zone of the Northwestern Alps. Macro- and microstructural analyses combined with petrological and geochemical investigations of the MH and surrounding lithologies unravelled a polyphase blastesis-deformation history, which led to the formation of a complex fabric and minero-chemical alteration of the serpentinite basement-metasediments interface. Dehydration, decarbonation and carbonation interplayed from early Alpine subduction up to HP-LT metamorphic peak (T = 550-630°C, P = 1.8-2.5 GPa), to produce a distinctive, pervasive amphibole (tremolite/actinolite) replacement both in carbonate-rich and serpentinite-rich domains pertaining to the MH protoliths, i.e., serpentinite and carbonate-bearing metabreccia of the chaotic rock unit. This characteristic amphibole metasomatism is more pronounced toward the contact with the metaophicarbonates, and the average δ18OVSMOW and δ13CVPDB values of dolomite within the MH (+14.4‰ and +0.7‰ respectively) lie between those of the metaophicarbonates and of calcschist. These results suggest that Mg-H2O-rich fluids from the dehydrating slab, CO2 released by decarbonation and SiO2-rich fluids evolved in calcschists mixed together and circulated mostly along the metaophiolite basement/metasediments interface, where the MH developed and recorded a preferential channel for mixed metamorphic fluid flow. These findings highlight and confirm that the study of metasomatic rocks in convergent systems is crucial to comprehend the behaviour of different fluids circulating, mixing and interacting with lithologies along slab-parallel discontinuities, which act as major fluid conduits for deep volatile recycling.
... An original bulk δ 18 O composition of ~ 2‰ is calculated for the mafic fels in equilibrium with the measured garnet δ 18 O of 1‰. Such low bulk δ 18 O is below the MORB range (Taylor 1968;Muehlenbachs and Clayton 1972;Alt et al. 1986) but is reported for hydrothermally altered mafic oceanic crust (Cartwright and Barnicoat 1999;Miller et al. 2001;McCaig et al. 2007). Therefore, the relatively low bulk δ 18 O values of the mafic fels could be ascribed to high-temperature seafloor alteration prior to Alpine metamorphism. ...
... For the mafic schist and Cld-schist, the calculated bulk δ 18 O is ~ 11‰ based on the δ 18 O of 9.5‰ of the garnet core. The bulk rock δ 18 O composition of the schists corresponds to the lower side of the wide δ 18 O range usually observed for metasediments and more specifically pelites (Rumble et al. 1982;Bebout and Barton 1989;Cartwright and Barnicoat 1999). On the other hand, a bulk rock with δ 18 O of ~ 3‰ would be in equilibrium with the low δ 18 O garnet rim in the schists (Online Resource 1: Fig. S7, S8). ...
... (1b) Lawsonite-out dehydration reaction, in ZSZ. Lawsonite relicts have been described in the ZSZ metabasalts (Angiboust and Agard 2010) that have bulk rock δ 18 O values from 4.4 to 9.2‰ (Barnicoat and Cartwright 1995;Cartwright and Barnicoat 1999). Such bulk rock compositions would produce fluids with even higher δ 18 O values. ...
