Giovanni B. Piccardo

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.

Subsolidus reactions monitored by trace element partitioning: the spinel- to plagioclase-facies transition in mantle peridotites

Mantle peridotites of the External Liguride (EL) Units (Northern Apennines) mainly consist of fertile spinel-lherzolites partially recrystallized to plagioclase-facies assemblages, and are consequently appropriate to investigate the interphase element partitioning related to the transition from spinel- to plagioclase-facies stability field. Evidence for the development of the plagioclase-facies assemblage is mainly given by: (1) large exsolution lamellae of orthopyroxene and plagioclase within spinel-facies clinopyroxene; (2) plagioclase rims around spinel; (3) granoblastic domains made up of olivine+plagioclase±clino-and orthopyroxene. In situ major and trace [REE (rare-earth elements), Ti, Sc, V, Cr, Sr, Y, Zr and Ba] element mineral analyses have been performed, by electron and ion probe, on selected samples which show the progressive development of the plagioclase-bearing assemblage. The main compositional variations observed during the change from spinel- to plagioclase-facies minerals are as follows: (1) clinopyroxenes decrease in Al, Na, Sr, Eu/Eu* and increase in Y, V, Sc, Cr, Zr and Ti; (2) amphiboles decrease in Eu/Eu*, Sr, Ba and increase in Zr and V; (3) spinels decrease in Al and increase in Cr and Ti. The most striking feature is the decoupling in the behaviour of similarly incompatible elements (D about 0.1) in clinopyroxene, e.g. Sr decrease is mirrored by Zr increase. Massbalance calculations indicate that the trace element interphase redistribution documented in the EL peridotites occurred in a closed system and in response to the metamorphic reaction governing the transition from the spinel- to the plagioclase-facies stability field. The observed element partitioning reveals, moreover, that subsolidus re-equilibration processes in the upper mantle produce HFSE (high-field-strength element)/REE fractionation in minerals, which must be evaluated for a reliable determination of mineral-melt distribution coefficients. The results of this study furnish evidence for subsolidus metamorphic evolution during decompression, without concomitant partial melting processes. This is consistent with the interpretation that the EL peridotites represent subcontinental lithospheric mantle emplaced at the surface in response to lithospheric thinning and tectonic denudation mechanisms related to the Triassic-Jurassic rifting of the Ligure-Piedmontese basin.

Ophiolitic Peridotites of the Alpine-Apennine System: Mantle Processes and Geodynamic Relevance

Ophiolites exposed in the Alpine-Apennine mountain range represent the oceanic lithosphere of the Ligurian Tethys, a small oceanic basin separating the Europe and Adria plates during Mesozoic time. Most of the peridotites represent former subcontinental mantle which was: (a) isolated from the convective mantle at different times (from Proterozoic to Permian); and (b) accreted to the thermal lithosphere, where it cooled along a conductive geothermal gradient under spinel-peridotite facies conditions. These peridotites record two magmatic cycles: (1) early diffuse porous flow percolation and impregnation by single-melt increments, focused percolation in dunite channels, and intrusion of MORB-type melts; and (2) late intrusion and extrusion of magmas deriving from aggregated MORB liquids. The early lithosphere/asthenosphere interaction by melt percolation induced significant depletion/refertilization and heating of mantle peridotites, leading to the thermochemical erosion of lithospheric mantle. Plagioclase-bearing peridotites of the Alpine-Apennine ophiolites were derived from melt impregnation, whereas part of the depleted spinel peridotites resulted from reactive percolation of depleted melts, rather than being refractory residua after near-fractional melting. The presence of large areas of impregnated peridotites indicates that significant volumes of melts were trapped in the lithospheric mantle; subsequently, asthenospheric melts reached the surface, both intruding as MORB gabbroic bodies or extruding as MORB lava flows. Our results provide a mechanism to explain nonvolcanic 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 (nonvolcanic) stages.

The ophiolite-oceanic lithosphere analogue: New insights from the Northern Apennines (Italy)

In this paper, we discuss the results of recent petrologic and isotopic studies on the Northern Apennine ophiolites, which are remnants of the oceanic lithosphere of the Jurassic Ligurian Tethys Ocean. In the Northern Apennines, ophiolites crop out in two distinct geologic units, the External and Internal Ligurides, which have been related, respectively, to pericontinental and intraoceanic settings of the Ligurian Sea sector of the Tethys. In the External Ligurides, subcontinental-lithosphere peridotites (presumably Proterozoic) are associated with Hercynian continental crust. Both mantle and crustal rocks display a composite subsolidus retrograde evolution, related to their uplift toward the seafloor, where they are associated with Jurassic mid-oceanic-ridge basalts (MORBs). The Internal Liguride ophiolites comprise the spatial association of depleted peridotites (of Permian age) and younger (mostly Jurassic) MORB-type magmatism, not linked to the peridotites by a genetic melt and residua relationship. Structural and petrologic features recorded by the Northern Apennine ophiolites thus indicate that the Jurassic Ligurian Sea sector of the Tethys was mostly underlain by older (Proterozoic and Permian), subcontinental lithospheric mantle and younger, unrelated, MORB seafloor. This peculiar lithologic association cannot be produced at mid-ocean ridges of mature oceans; rather, it most likely originated after the breakup of the continental crust in response to passive extension of the continental lithosphere. The exposure of subcontinental mantle peridotites on the ocean floor is a unique feature testifying to the early stages of inception of an oceanic basin by means of passive lithospheric extension. Modern oceanic analogues of the Ligurian Sea sector of the Tethys represented by the Northern Apennine ophiolites are currently found in embryonic oceans (e.g., the Red Sea) and at the transition zone between continental and oceanic lithosphere of mature oceanic basins (e.g., the Galicia margin, western Spain).

Melt migration in ophiolitic peridotites: the message from Alpine-Apennine peridotites and implications for embryonic ocean basins

Abstract Results of a field study as well as petrological and geochemical data demonstrate that substantial portions of the lithospheric mantle, exhumed during opening of the Jurassic Piedmont Ligurian ocean, were infiltrated by and reacted with migrating melts. Intergranular flow of ascending liquids produced by the underlying hot asthenosphere dissolved clinopyroxene ± spinel and precipitated orthopyroxene + plagioclase ± olivine, forming orthopyroxene + plagioclase-rich perioditite. Migrating liquids became progressively saturated in clinopyroxene, and then precipitated microgranular aggregates of clinopyroxene-bearing gabbronorite. Later, diffuse porous melt flow was replaced by focused porous flow, producing a system of discordant dunite bodies. Upon cooling, liquids migrating in dunite channels became progressively saturated in clinopyroxene and plagioclase, forming interstitial clinopyroxene at olivine triple points followed by clinopyroxene ± plagioclase megacrysts and gabbro veinlets within the dunite, and gabbro dykelets within plagioclase peridotites. Subsequent cooling during continued exhumation was accompanied by intrusion of kilometre-scale gabbroic dykes evolving from troctolite to Mg-Al and Fe-Ti gabbros. Migrating liquids, which infiltrated peridotite and formed gabbroic rocks, span a wide range of compositions from silica-rich single melt fractions to T- and N-MORB (mid-ocean ridge basalt), characteristic of the melting column beneath midocean ridges. Explanations for the progressive evolution of an igneous system from diffuse to focused porous flow and finally dyking include the competing effects of heating of the lithospheric mantle by ascending magmas from the underlying hot asthenosphere and conductive cooling by exhumation. Whether or not rift-related melt infiltration and heating is recorded by exhumed subcontinental lithospheric mantle along ocean-continent transitions and/or oceanic lithospheric mantle along slow-spreading ridges depends on the relative position to the underlying upwelling asthenosphere.

Fossil crust‐to‐mantle transition, Val Malenco (Italian Alps)

An exhumed, undisturbed fossil lower crust to upper mantle section is preserved in Val Malenco, Italian Alps, and is now exposed along the boundary between Penninic and Austroalpine nappes. Lower-crustal metapelitic rocks are welded to upper-mantle ultramafic rocks by a mid-Permian gabbro intrusion. The underplating of gabbro caused granulite metamorphism and partial melting of the metapelites. In the crust-to-mantle transition zone of at least 1 km thickness, gabbros, large xenoliths of restitic metapelites and ultramafic rocks occur, with densities of 2.95-3.14, 3.25 and 3.27 g/cm 3 , respectively. The seismic Moho therefore did not coincide with the boundary between peridotites and crustal rocks but was situated above the upper limit of the peridotitic mantle. The whole complex underwent cooling with only moderate decompression within the kyanite field. This process started at 1 GPa and ∼800°C and ended at 0.85 GPa and 600°C and is interpreted as thermal relaxation after the gabbro intrusion. Later, during Jurassic rifting, the crust-to-mantle section was exhumed at the Adria margin of the Tethys ocean.

Evolution of the lithospheric mantle in an extensional setting: Insights from ophiolitic peridotites

We present a model of mantle lithosphere evolution in an extensional setting based on the structure and petrology of the Alpine-Apennine ophiolitic peridotites from the Jurassic Ligurian Tethys Ocean. Continental extension and rifting in the Ligurian domain were induced by far-field tectonic forces and were characterized by the interplay of tectonic and magmatic processes. Extension and thinning of the lithosphere induced adiabatic upwelling and decompression melting of the asthenosphere. Mid-ocean-ridge basalt (MORB) melts percolated by porous flow through the extending lithospheric mantle and were trapped therein by interstitial crystallization. Therefore, melt emplacement at the seafloor during continental rifting and breakup was prevented, and the resulting rifted margins of the basin were nonvolcanic. The asthenosphere-lithosphere interaction induced significant rheological modifications of the percolated mantle. Weakening and softening of the mantle lithosphere significantly lowered the total lithospheric strength. Accordingly, thermo-mechanical erosion of the mantle lithosphere promoted lithosphere stretching and thinning and enhanced the transition from distributed continental deformation to localized oceanic spreading.

Ophiolitic magmatism in the Ligurian Tethys: an ion microprobe study of basaltic clinopyroxenes

Ion microprobe data (REE, Na, Sc, Ti, V, Cr, Sr, Zr) of unaltered clinopyroxenes in the ophiolitic basalts from the Northern Apennines have been used in a epx-based geochemical modelling of MORB magmatism from both External (EL) and Internal (IL) sectors of the Ligurian Tethys (i.e. Jurassic Ligure-Piemontese basin), alternative to the more common whole-rock approach. Clinopyroxenes from EL basalts display slightly fractionated LREE (CeN/SmN∼0.5) and HREE (GdN/ YbN∼1.5) patterns and large variations in the REE composition (up to 6 times from microphenocryst cores to interstitial clinopyroxenes). Interstitial clinopyroxenes in IL basalts are similar to the microphenocrysts from the most primitive EL basalts. By contrast, IL microphenocrysts are characterized by greater LREE (CeN/SmN ∼0.3) and lesser HREE (GdN/YbN<1.2) fractionation. The comparison of trace element variations in wholerocks and clinopyroxenes clearly shows that the olivine and plagioclase portion of the fractionation sequence is poorly represented by the EL and IL basalts. In fact, ophiolitic basalts mainly consist of a minor interstitial glass (now deeply altered) associated with a prevailing plagioclase-clinopyroxene assemblage crystallized from liquids significantly evolved along the olivine-plagioclase-clinopyroxene saturation boundary. Thus, bulk rock chemistry is largely governed by clinopyroxene composition. This, in addition to alteration, indicates that the bulk rock chemistry does not provide reliable chemical information to constrain the composition and the generation of the parental magmas. Unfortunately, most clinopyroxenes are characterized by complex zoning, probably caused by disequilibrium partitioning during crystal growth as a result of kinetic factors. On this ground, estimation of melt chemistry and inferences about the origins of these basalts are only allowed by the core compositions of microphenocrystic clinopyroxenes. Modelling of (Nd/Yb)N and Ti/Zr in the parental magmas, as deduced from the clinopyroxene compositions, indicates thata EL and IL basalts do not represent products of different mantle source composition. Rather, they were generated by varying degrees of fractional melting in the spinel stability field, lower for the EL (a few percent) relative to IL, totalling no more than 10% of an asthenospheric MORB source, and leaving in the residua clinopyroxene with REE patterns similar to those shown by IL suboceanic type peridotites. Accordingly, these latter are interpreted as refractory residua after MORB-generating fractional melting occurred during rifting and opening of the Ligure-Piemontese basin. By contrast, residual clinopyroxenes from the EL subcontinental type peridotites are not consistent with low degrees of fractional melting in agreement with the current interpretation that EL peridotites are unrelated to the MORB magmatism in the Ligure-Piemontese basin and represent lithospheric mantle material already emplaced towards the surface by a tectonic denudation mechanism during the early stages of oceanic rifting.

Alpine peridotites from the Ligurian Tethys: an updated critical review

We present an updated overview of the petrogenetic evolution of upper mantle peridotites exposed in the Alpine–Apennine ophiolites; the latter represent fragments of Jurassic oceanic lithosphere derived from the Ligurian–Piedmontese basin. This synthesis is based on the results of recent multidisciplinary studies of these peridotite massifs in the Eastern, Central and Western Alps, Ligurian Alps, Northern Apennines and Alpine Corsica, and has been developed within the framework of the geodynamic evolution of the Ligurian–Piedmontese basin between Europe and Adria. The Ligurian–Piedmontese basin began forming through continental rifting during the Triassic. As the continental lithosphere was progressively stretched and thinned, the underlying asthenosphere underwent adiabatic upwelling and decompressional partial melting. The subsequent crustal break-up during the Jurassic led to the development of the non-volcanic passive margins of Europe and Adria, and to the exhumation and exposure of the sub-continental lithospheric mantle on the seafloor. The rifting stage of the basin was characterized by percolation through diffuse porous flow of asthenospheric mid-ocean ridge basalt (MORB) melts via the continental lithospheric mantle. Melt–rock interaction caused depletion/re-fertilization of the lithospheric mantle and generated the extreme compositional heterogeneity of the upper mantle peridotites, which were later exposed on the seafloor at distal settings of the basin. Depleted spinel peridotites, which were previously interpreted as refractory residua after asthenosphere partial melting, are now recognized as reactive peridotites formed by melt–rock interaction. MORB melts migrating through the sub-continental lithospheric mantle along the axial zone of the extending system modified the upper mantle peridotites significantly and were entrapped by interstitial crystallization.