Carbon- rich fluids infiltration into the mantle-wedge: from early carbonate inclusions to late veins in orogenic peridotites from the Ulten Zone
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At subduction zones, a number of geologic processes are caused by influx in the supra-subduction mantle wedge of fluid phases released by the subducting plates. The distribution of fluids in such settings affects the mineralogical, chemical and structural transformation of rocks and, hence, the survival of relict minerals and structures of previous events. These features can be investigated by means of field-based studies of high and ultrahigh-pressure (HP-UHP) orogenic terrains that contain mantle wedge materials tectonically sampled by the subducting plates. Here we review two examples of garnet peridotites hosted in HP-UHP continental crust, which record different P-T stories: (i) shallow spinel-facies lithospheric mantle wedge down-dragged to depth during subduction and recrystallized to garnet + amphibole assemblages due to the infiltration of crust-derived fluids (Ulten Zone garnet peridotites, Eastern Alps, Italy); (ii) transition-zone mantle upwelled and accreted to cratonic roots, and involved in subduction-zone recrystallization at 200 km depth enhanced by crustal fluids (UHP garnet peridotites, Western Gneiss Region, Norway). Our textural and petrologic study shows that the water distribution controls development of the new assemblages and the metasomatic imprints of these rocks, independently on the depth and degree of metamorphism. We conclude that mantle re-fertilization by crust-derived subduction fluids is an effective mechanism working on a 100-200 km depth range.
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The peridotites from the Ulten Zone (UZ) in the Italian Eastern Alps derive from a mantle wedge, were incorporated in a crustal slab during continental subduction in the course of the Variscan orogeny, and subsequently exhumed in a crust-mantle melange. The complex pre-Alpine metamorphic history is documented by a variety of mineral assemblages and microstructures. This PhD work aims to shed light on individual melt- and fluid-mediated metasomatic stages affecting the UZ peridotites and to contribute to the understanding of the element cycle during crust-mantle interaction in a continental collisional setting: The UZ peridotites were subjected to refertilization likely induced by interaction with rising metasomatic liquids from deeper parts of the subduction zone. Inclusions of dolomite in primary spinel formed in the high-temperature spinel-stability field from these liquids. At the slab-mantle interface, the peridotites experienced interaction with aqueous fluids released from the crustal slab. The formation of discrete dolomite grains in fine-grained garnet-amphibole peridotites indicates that the fluids carried a carbon-component. Zircon geochronology yields an age of ca. 333 Ma reflecting the time of peridotite entrapment into the crustal host rocks. Zircon grew from aqueous fluid/s released from crystallizing leucosome during emplacement of the peridotites into the crustal host rocks. During exhumation to crustal levels, the peridotites interacted with fluids sourced from different adjacent lithologies. This is mirrored by highly variable stable-isotope (carbon and oxygen) compositions of carbonates in UZ peridotites. Late-stage serpentinization was accompanied by formation of calcite-brucite intergrowths as a product of fluid-mediated dolomite breakdown according to the reaction CaMg(CO3)2 + H2O → CaCO3 + Mg(OH)2 + CO2. Total carbon concentrations in UZ peridotites suggest that mantle-wedge peridotites can store crust-derived carbon via various processes involving melt/fluid-rock interaction, and that carbon is ultimately mobilized to crustal levels during exhumation of an orogenic crust-peridotite association.
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New field, textural, petrologic and geochemical researches on the Lanzo South ophiolitic peridotite constrain the mantle processes which accompanied the geodynamic evolution during rifting and opening of the Ligurian Tethys ocean basin. They reveal the presence of different types of peridotites with variable structural-geochemical characteristics and mutual relationships: (1) “lithospheric” spinel peridotites preserve records of a long residence in the thermal lithosphere, (2) “reactive” spinel peridotites record effects of melt/rock interaction; (3) “impregnated” plagioclase-rich peridotites show significant enrichment of basaltic components (“refertilization”) in the form of mm-size plagioclase-pyroxene-rich veins and pockets.
Our present results indicate that the Lanzo South peridotites were accreted to the thermal lithosphere, probably from garnet-peridotite-facies depths, where progressively cooled and completely recrystallized under spinel-peridotite-facies conditions. Subsequently, in response to the pre-oceanic rifting related to the opening of the Ligurian Tethys, lithospheric mantle sections of the Lanzo massif were progressively exhumed to shallow lithospheric levels, whereas the underlying asthenosphere rose and underwent near-adiabatic decompression melting. The resulting fractional melts migrated through and reacted with the overlying extending mantle lithosphere.
During initial melting stages of the ascending asthenosphere, fractional melt increments migrated upwards in the lithospheric mantle column via diffuse and reactive porous flow, and caused depletion of the lithospheric mantle by melt/rock interaction (pyroxene dissolution and olivine precipitation), being olivinesaturated but pyroxene-undersaturated. Large areas of pyroxene-depleted, olivine-enriched “reactive” peridotites were thus formed. Subsequently, progressively pyroxene-saturated melts migrated pervasively in the “lithospheric” and “reactive” peridotites; at shallower levels, the competing effects of heating by melt percolation and cooling by ongoing exhumation led to interstitial crystallization of percolating melts, and to progressive clogging of melt channels. This process formed an upper zone of refertilized, “impregnated” plagioclase peridotites, and forced the ascending melts to percolate along focused channels where high melt/peridotite ratios caused the complete dissolution of pyroxenes and the formation of “replacive” spinel dunites. These high-porosity dunite channels allowed “rapid” migration of the first aggregated MORB melts, which were produced in the underlying asthenosphere and escaped melt/rock interaction during upwelling.
The rheology of the lithospheric mantle was modified largely by lithosphere-asthenosphere interaction; the lithospheric mantle attained asthenospheric characteristics during erosion by melt percolation. Following continuous upwelling in the thermal lithosphere and increasing conductive heat loss, the thermochemically modified lithospheric mantle returned to more cold and brittle conditions. Later, the Lanzo South peridotites were intruded along fractures by variably fractionated, Mg-rich to Fe-rich magmas deriving from MORB primary melts, most probably aggregated at asthenospheric levels and differentiated in shallow magma chambers.
The above evidence reveals a composite magmatic stage recorded in the mantle; percolating melts were trapped in the lithospheric mantle and never reached the surface. This magmatic stage preceded later shallow emplacement of MORB magmas, which formed gabbroic and basaltic rocks of the Jurassic oceanic crust.
Records of melt percolation, impregnation and melt/peridotite reaction are common in ophiolitic peridotites and present-day oceanic mantle lithosphere. Relationships between the results obtained for the Lanzo ophiolitic peridotites and those determined for others Alpine-Apennine peridotites, provide a mechanism to explain non-volcanic and volcanic stages during rift evolution of the Ligurian Tethys, and might be equally applicable to modern slow spreading ridges, which are characterized by variable magmatic (volcanic) and amagmatic (non-volcanic) stages.
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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 exhumation-related lithospheric history of the Erro–Tobbio mantle. The tectono-metamorphic 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 ± 16 Ma and 313 ± 16 Ma, 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.
Tectonite
Mylonite
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Greenschist
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Metasomatism
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Mylonite
Eclogitization
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Convergent plate boundaries are sites of sustained chemical exchanges between the Earth’s surface and deep geochemical reservoirs, playing a major role in the global cycle of carbon and sulfur. However, carbon and sulfur recycling processes continue to be hotly debated. A critical gap in the knowledge of the whole subduction factory is given by the limited accessibility to the upper mantle residing above the subducting plate, the so-called mantle wedge.
This thesis investigates the carbonate and sulfide metasomatism taking place during the whole metamorphic evolution of a mantle wedge involved in the Variscan continental collision.
We integrate different detailed geochemical and petrological techniques to orogenic carbonated spinel and garnet peridotites from the Ulten Zone of the Eastern Italian Alps. Our data show that the Ulten Zone peridotite experienced multiple stages of addition and removal of carbon and sulfur throughout its metamorphic evolution, as follows: (1) The Variscan lithospheric mantle was initially depleted and sulfide-poor. It subsequently inherited a sulfur and carbon component during an early metasomatic stage, when hot, H2S-CO2-bearing melts leaving a subduction-modified source percolated the overlying spinel-facies peridotite in the mantle wedge; (2) Under peak eclogite-facies P-T conditions, pervasive carbonation and sulfidation occurred. Heterogeneous melt and fluid sources variably enriched in carbon, isotopically heavy sulfur and radiogenic Sr were involved; (3) Shortly after the attainment of peak-P conditions, peridotite bodies were incorporated in a tectonic melange with the neighboring gneisses. Here, the Ulten Zone peridotite was exposed to channelized infiltration of hybridized C-O-H fluids that promoted the formation of veinlets of carbonates locally associated with sulfide grains. (4) Upon late retrogression, infiltration of serpentinizing fluids promoted C and S remobilization at shallow crustal levels.
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