Petrology and Trace Element Budgets of High-pressure Peridotites Indicate Subduction Dehydration of Serpentinized Mantle (Cima di Gagnone, Central Alps, Switzerland)

At Cima di Gagnone, garnet peridotite and chlorite harzburgite lenses within pelitic schists and gneisses correspond to eclogite-facies breakdown products of hydrated peridotites and are suitable for studying dehydration of serpentinized mantle. Thermobarometry and pseudosection modelling yield peak temperatures of 750-850°C and pressures <3 GPa. The minimum temperature recorded by the garnet peridotite corresponds to the maximum conditions experienced by the chlorite harzburgite, suggesting that these rocks recrystallized cofacially at ∼800°C. Alternatively, they might have decoupled during subduction, as achieved in tectonically active plate interface boundaries. The major and rare earth element (REE) variability of the peridotites was mostly acquired during pre-subduction mantle evolution as a result of partial melting and reactive melt flow. The ultramafic suite is also characterized by fluid-mobile element enrichments (B, Pb, As, Sb, Cs, Li, U, Be), which confirm derivation from variably serpentinized protoliths. Similarity in the U, Pb, B, Li and Sr contents of the Gagnone peridotites to present-day oceanic serpentinites suggests that these elements were partly taken up during initial serpentinization by seawater-derived fluids. Positive Be, As and Sb anomalies suggest involvement of fluids equilibrated with crustal (metasedimentary) reservoirs during subsequent subduction metamorphism and peridotite entrainment in (meta)sediments. Fluid-mobile element enrichment characterizes all peak eclogitic minerals, implying that multiple hydration events and element influx pre-dated the eclogite-facies dehydration. Peak anhydrous minerals retain B, Li, As and Sb concentrations exceeding primitive mantle values and may introduce geochemical anomalies into the Earth's mantle. The relatively low contents of large ion lithophile elements and light REE in the Gagnone peridotites with respect to much higher enrichments shown by metasomatized garnet peridotite pods hosted in migmatites (Ulten Zone, Eastern Alps) suggest that the crustal rocks at Gagnone did not experience partial melting. The Gagnone garnet peridotite, despite showing evidence for chlorite dehydration, retains significant amounts of fluid-mobile elements documenting that no partial melting occurred upon chlorite breakdown. We propose that the Gagnone ultramafic rocks represent a prime example of multi-stage peridotite hydration and subsequent dehydration in a plate interface settin

Multi-stage reactive formation of troctolites in slow-spreading oceanic lithosphere (Erro-Tobbio, Italy): a combined field and petrochemical study

partially_open5Many recent studies have investigated the replacive formation of troctolites from mantle protoliths and the compositional evolution of the percolating melt during melt-rock interaction processes. However, strong structural and geochemical constraints for a replacive origin have not yet been established. The Erro-Tobbio impregnated mantle peridotites are primarily associated with a hectometre-size troctolitic body and crosscutting gabbroic dikes, providing a good field control on melt-rock interaction processes and subsequent magmatic intrusions. The troctolitic body exhibits high inner complexity, with a host troctolite (Troctolite A) crosscut by a second generation of troctolitic metre-size pseudo-tabular bodies (Troctolite B). The host Troctolite A is characterized by two different textural types of olivine, corroded deformed millimetre- to centimetre-size olivine and fine-grained rounded undeformed olivine, both embedded in interstitial to poikilitic plagioclase and clinopyroxene. Troctolite A shows melt-rock reaction microstructures indicative of replacive formation after percolation and impregnation of mantle dunites by a reactive melt. The evolution of the texture and Crystallographic Preferred Orientation (CPO) of olivine are correlated and depend on the melt/rock ratio involved in the impregnation process. A low melt/rock ratio allows the preservation of the protolith structure, whereas a high melt/rock ratio leads to the disaggregation of the pre-existing matrix. The mineral compositions in Troctolite A define reactive trends, indicative of the buffering of the melt composition by assimilation of olivine during impregnation. The magmatic Troctolite B bodies are intruded within the pre-existing Troctolite A and are characterized by extreme textural variations of olivine, from decimetre-size dendritic to fine-grained euhedral crystals embedded in poikilitic plagioclase. This textural variability is the result of olivine assimilation during melt-rock reaction and the correlated increase in the degree of undercooling of the percolating melt. In the late gabbroic intrusions, mineral compositions are consistent with the fractional crystallization of melts modified after the reactive crystallization of Troctolites A and B. The Erro-Tobbio troctolitic body has a multi-stage origin, marked by the transition from reactive to fractional crystallization and diffuse to focused melt percolation and intrusion, related to progressive exhumation. During the formation of the troctolitic body, the melt composition was modified and controlled by assimilation and concomitant crystallization reactions occurring at low melt supply. Similar processes have been described in ultra-slow spreading oceanic settings characterized by scarce magmatic activity.openBasch, Valentin; Rampone, Elisabetta; Crispini, Laura; Ferrando, Carlotta; Ildefonse, Benoit; Godard, MargueriteBasch, Valentin; Rampone, Elisabetta; Crispini, Laura; Ferrando, Carlotta; Ildefonse, Benoit; Godard, Marguerit

Meter-scale Nd isotopic heterogeneity in pyroxenite-bearing Ligurian peridotites encompasses global-scale upper mantle variability

Pyroxenites embedded in peridotite are often invoked as a major cause of short-length scale isotopic heterogeneities in the upper mantle, but there has been little direct evidence. We report spatially controlled chemical and Sr-Nd isotopic compositions of pyroxenites and their host peridotites from an ophiolitic mantle sequence in the Northern Apennines, Italy, with depleted mantle compositions, representing a surface exposure of veined upper mantle, a potential source for mid-oceanic-ridge basalts (MORB). Interaction between pyroxenites and adjacent mantle rocks results in centimeter-scale chemical modifications in the host peridotites, systematically lowering their Sm/Nd ratios. Over time, this interaction causes the host peridotite at greater than 0.1 m scale to acquire an isotopic heterogeneity larger than the range defined by the peridotite and pyroxenite end-members. Moreover, the 143Nd/144Nd variation of a single outcrop covers most of the global Nd isotopic variability documented in abyssal peridotites. Such pyroxenite-peridotite veined mantle domains may represent the enriched component rarely found in abyssal peridotites, but often invoked to account for the low end of 143Nd/144Nd variations in MORB

Meter-scale Nd isotopic heterogeneity in pyroxenite-bearing Ligurian peridotites encompasses global-scale upper mantle variability

Pyroxenites embedded in peridotite are often invoked as a major cause of short-length scale isotopic heterogeneities in the upper mantle, but there has been little direct evidence. We report spatially controlled chemical and Sr-Nd isotopic compositions of pyroxenites and their host peridotites from an ophiolitic mantle sequence in the Northern Apennines, Italy, with depleted mantle compositions, representing a surface exposure of veined upper mantle, a potential source for mid-oceanic-ridge basalts (MORB). Interaction between pyroxenites and adjacent mantle rocks results in centimeter-scale chemical modifications in the host peridotites, systematically lowering their Sm/Nd ratios. Over time, this interaction causes the host peridotite at greater than 0.1 m scale to acquire an isotopic heterogeneity larger than the range defined by the peridotite and pyroxenite end-members. Moreover, the 143Nd/144Nd variation of a single outcrop covers most of the global Nd isotopic variability documented in abyssal peridotites. Such pyroxenite-peridotite veined mantle domains may represent the enriched component rarely found in abyssal peridotites, but often invoked to account for the low end of 143Nd/144Nd variations in MORB

Melt-rock reaction and melt migration in the MORB mantle through combined natural and experimental studies

Magma formation at ocean ridges is the primary process that produces the large scale lithologic and chemical heterogeneity of the Earth. Studies on the mid-ocean ridge basalts (MORBs) have documented that their mantle sources are chemically and isotopically heterogeneous at all spatial scales. Pyroxenite components in the mantle likely have a key role in the major processes of MORB formation, including polybaric melting, melt segregation from the source region and melt migration through the lithosphere. Because direct sampling of deep mantle is not accessible, the origin of pyroxenite heterogeneities is poorly understood leaving their strategic role in the generation of oceanic basalts still unclear. This project wishes to integrate field, petrological and geochemical studies on ophiolitic and oceanic mantle sequences with experiments on solids and melts, to explore the heterogeneity of the MORB mantle. Major goal of the project is to improve our knowledge on: (1) the role of pyroxenites in creating chemical and isotopic heterogeneities in the MORB mantle sources, (2) the consequence of a heterogeneous mantle source on the chemistry of MORBs, and (3) the relationships between mantle heterogeneity and channelized melt migration. To address these issues, the project plans to investigate: (i) the origin and geochemical signature of pyroxenites in fertile and depleted oceanic peridotites, which represent the best available proxies of MORB mantle sources and mantle residues after MORB production, (ii) the chemical/isotopic modifications produced by pyroxenite formation in the host peridotite, (iii) the origin of secondary pyroxenites by melt-peridotite reaction, and (iv) the chemical variability of melts migrating through replacive dunites. The selected sample set encompasses: (i) "aged" pyroxenites from fertile mantle sequences of the Alpine-Apennine ophiolites; (ii) pyroxenites from depleted mantle sequences of the Alpine-Apennine ophiolites and modern oceanic setting (Smoothseafloor region, Southwest Indian Ridge); (iii) abyssal peridotites - MORB suites from Vema Lithosperic Section (Mid-Atlantic Ridge) and Smoothseafloor region; (iv) replacive dunites from the Alpine-Apennine ophiolites. The origin of secondary pyroxenites will be simulated by melt-rock reaction experiments. A set of high-pressure experiments (1.5-2.5 GPa) will be devoted to define the distribution of trace elements during interaction between peridotite and pyroxenite-derived melts. In addition, partial melting experiments (1200-1400 \ub0C; 1.0-1.5 GPa) on pyroxenite lithologies will shed light on the contribution of these components in the genesis of MORBs. Key information will be provided on the petrologic processes controlling the generation and evolution of lithological-chemical-isotopic heterogeneities along the mantle column, thus improving our knowledge on magmatic processes ruling the chemical differentiation of the Earth's mantle and oceanic crust

A global overview of isotopic heterogeneities in the oceanic mantle

Studies on modern oceanic lithosphere and ophiolites have revealed high degrees of chemical and isotopic heterogeneity in the mantle, as well as isotopic contrasts between mantle and crust. These features cannot be explained just by simple extraction of partial melt, but require considerably more complex petrogenetic processes. Here we present an overview of the present knowledge on isotopic heterogeneities of Sr, Nd, Hf and Os in oceanic peridotites (by reviewing data on modern abyssal peridotites and the Alpine\u2013Apennine ophiolites), and discuss their significance in terms of i) length scale and extent of isotopic heterogeneities in the upper mantle and ii) isotopic mantle\u2013crust relations at oceanic settings. Overall results show that mantle peridotites record significant isotopic heterogeneity, detectable on awide range of length scales,much larger than observed in associated MORB. In addition, abyssal peridotites are on averagemore depleted than MORB. The high degree of isotopic heterogeneity is clear evidence for the inefficiency ofmantle convection in homogenizing mantle rocks. It may be caused by i) variably old depletion events (unrelated to recentMORB production), ii) pyroxenite components in the mantle source, iii) recent pre- and/or post melting metasomatism. Some abyssal peridotites have extremely depleted isotopic compositions, not seen in MORBs, and these have been interpreted as the evidence for old (1 to 2 Ga) refractory domains in the asthenospheric mantle or, alternatively, as evidence for recent incorporation of (also old) subcontinental lithospheric mantle, potentially through delamination during continental breakup. The first hypothesis has been corroborated by finding, in a few ridge segments (e.g. Gakkel Ridge) of correlations between chemical fertility indexes and isotopic (Os, Hf) ratios, indicative of recycling of old residual oceanic lithospheric mantle into the MORB source. However, no general consensus exists yet on the two proposed models. The difference in average isotopic depletion between peridotites and basalts has been also ascribed to the presence of pyroxenites, which have \u201cenriched\u201d isotopic signature relative to the peridotite component. The origin and composition of such small-scale lithological heterogeneities remain however still controversial and poorly constrained, due to the difficulty to link petrologic and geochemical studies with direct field observations, and to the scarcity of chemical and isotopic data on pyroxenites in ophiolitic and abyssal peridotites, i.e. the closest available \u201cproxies\u201d of the MORB mantle. Larger isotopic homogeneity observed in MORB relative to peridotites in single ridge segments clearly reflect their origin as aggregated melts which inevitably \u201csmooth\u201d and averagemantle source heterogeneities. Overall, the questions about the origin and spatial distribution of chemical and isotopic heterogeneities are not resolved, and this calls for detailed field-based studies in spatially-controlled settings to shed light on the issue of small-scale mantle heterogeneities and the role of enriched (e.g. pyroxenites) and highly depleted domains in MORB melting