Studio delle emissioni gassose di CO2 e Radon al suolo e modellazione numerica del reservoir geotermico dell'area del Torrente Milia, Monterotondo Marittimo (GR)

RIASSUNTO. Negli ultimi 20 anni è stato riscontrato un crescente interesse nello studio del processo di degassamento di CO2 dalla geosfera in vari ambiti delle Scienze della Terra, con l’obiettivo di definire le relazioni tra i flussi di gas al suolo e le strutture tettoniche, di quantificare la CO2 di derivazione profonda rilasciata in atmosfera e di esaminare i processi di degassamento sia di natura vulcanica che non vulcanica (Cardellini et al., 2003). Mentre è ormai riconosciuto che il degassamento di origine vulcanica sia in grado di immettere in atmosfera enormi quantitativi di CO2, solo ricerche recenti (Kerrick et al.,1995; Seward & Kerrick, 1996) hanno dimostrato che il processo di degassamento non vulcanico può costituire un input di CO2 in atmosfera globalmente significante e che molte aree caratterizzate da rilevanti emissioni di CO2 non vulcanica sono in aggiunta contraddistinte da elevati valori di heat flow, presenza di magmi sub superficiali e regimi estensionali simicamente attivi (Kerrick et al., 1995). Inoltre la tecnica appare estremamente interessante nell’ottica dell’esplorazione geotermica, ai fini del riconoscimento delle principali strutture tettoniche presenti nel sottosuolo, dell’identificazione delle zone di risalita di fluidi e, in alcuni casi, dei reservoirs geotermici. La prima parte del presente lavoro di tesi ha riguardato l’esecuzione di un survey di misure al suolo di CO2 all’interno della zona geotermica di Larderello e più precisamente nel comune di Monterotondo Marittimo (GR). Tale lavoro si inquadra all’interno di un progetto più grande di valutazione del potenziale geotermico della zona in prossimità del Torrente Milia e ha costituito una delle metodologie utilizzate ai fini dell’ubicazione dei futuri pozzi geotermici. Le misure di CO2 al suolo sono state eseguite avvalendosi di un misuratore di flusso del tipo LICOR 8100 dotato di un analizzatore di gas all’infrarosso, che ha consentito la misura dei flussi di CO2 al suolo. La campagna, realizzata nel mese di Gennaio 2011, ha riguardato l’acquisizione di 63 punti nell’area del Torrente Milia, seguendo come riferimento una griglia di campionamento rettangolare con una maglia di lato pari a 250 m, per una lunghezza e una larghezza complessiva rispettivamente di 3000 m e 1250 m, con una superficie totale di 3,75 Km2. Al di fuori di quest’area sono inoltre state realizzate 20 misure di flusso con ubicazione nelle zone limitrofe ad alcuni pozzi ENEL (Carboli C bis, Poggio Travi e Lumiera 1bis), nelle aree considerate classicamente come ricarica del campo geotermico e presso le fumarole (Le Biancane). Dall’analisi dei valori di flusso di CO2 al suolo eseguita mediante il metodo grafico di analisi statistica (GSA) descritto da Chiodini et al. (1998) è emersa l’esistenza di quattro diverse popolazioni, ognuna contrassegnata da differenti caratteristiche e da diversa derivazione, mediante la cui identificazione è stato possibile eseguire una stima dell’output di CO2 giornaliero per l’intera area campionata del Torrente Milia, che ha fornito il valore di 145,45 t/d su una superficie di 3,75 Km2. Tali dati sono molto simili a quelli riscontrati da campagne eseguite in altre aree della Toscana termicamente anomale o contrassegnate dallo sviluppo di rilevanti discontinuità tettoniche ed evidenziano il problema legato all’immissione in atmosfera di gas serra. L’energia geotermica infatti rappresenta una fonte di energia a basso impatto ambientale, tuttavia il rilascio in atmosfera di CO2 dalle centrali è comunemente additato come uno degli effetti negativi derivanti dall’utilizzo della risorsa. La campagna condotta nell’area del Milia dimostra come anche aree di estensione limitata siano in grado di immettere naturalmente grandi quantitativi di CO2 in atmosfera. In effetti dati provenienti dalla letteratura scientifica attestano come il degassamento di CO2 di origine naturale ecceda, per molti campi geotermici del mondo tra cui Larderello, quello legato allo sfruttamento della risorsa. Inoltre dai dati raccolti è stato verificato come le strutture tettoniche presenti, la geologia dell’area, le differenti caratteristiche del litotipo o i processi attivi nel sottosuolo potessero influenzare significativamente i valori di flusso registrati al suolo, sia per l’area in prossimità del Torrente Milia sia per la zona esterna. Questo ha consentito di poter determinare delle buone correlazioni tra i vari parametri. In aggiunta sono state condotte le repliche di 6 punti di misura alla fine del mese di Giugno, per verificare se esistesse una variazione dei flussi di CO2 legata alla stagionalità. Le misure di CO2 al suolo sono inoltre state affiancate da 7 misure di Radon (222Rn), effettuate mediante un detector per il Radon da campagna del tipo RTM 2100 della SARAD, ubicate in parte nell’area del Torrente Milia ed in parte al di fuori, nelle vicinanze dei pozzi Carboli C bis e Lumiera 1bis e nell’area di ricarica del campo geotermico. Questi dati hanno consentito di avanzare ulteriori interpretazioni per l’area oggetto di studio e di comprovare le ipotesi già avanzate con i soli dati inerenti le misure dei flussi di CO2. La seconda ed ultima parte del lavoro ha invece riguardato la modellazione del comportamento idrologico e delle caratteristiche termiche del serbatoio geotermico, costituito dalla formazione del Calcare Cavernoso, nella zona dove la campagna di misure al suolo era stata effettuata mediante una griglia di campionamento regolare. La simulazione numerica è stata eseguita su un modello tridimensionale avvalendosi del software PetraSim 5.1, utilizzando come simulatore di calcolo TOUGH2 e un’equazione di stato EOS1. Il modello concettuale è stato ideato avvalendosi dei dati fisici, geologico-strutturali, idrogeologici e geochimici a disposizione, ha dimensioni di 2,7 1 0,8 km ed è stato schematizzato come comprendente tre corpi fondamentali: copertura-serbatoio-basamento. Su questo è stata costruita una griglia di calcolo regolare formata da 3672 celle di forma rettangolare. La prima fase della simulazione numerica ha mirato a ricostruire lo stato imperturbato del modello di partenza, che ha costituito la base di partenza per una seconda fase inerente la simulazione di differenti scenari di sfruttamento dell’area a fini geotermici per un arco di tempo pari a 50 anni. ABSTRACT. In the last 20 years a growing interest is noticed in studying the process of carbon dioxide degassing from the geosphere in various field of Earth Sciences, with the aim of defining relationship between gas flux on the ground and tectonic structures, quantifying deeply derived CO2 released in to the atmosphere and studying volcanic degassing and nonvolcanic degassing processes (Cardellini et al., 2003). While it is already recognized that volcanic degassing can introduce huge quantities of carbon dioxide in to the atmosphere, only recent studies (Kerrick et al.,1995; Seward & Kerrick, 1996) have shown that nonvolcanic degassing may be a globally significant input of CO2 in to the atmosphere and that many areas where large emissions of non-volcanic carbon dioxide take place are also characterized by high heat flow values, by the presence of shallow magmas and seismically-active extensional tectonic regimes (Kerrick et al., 1995). Besides the technique is interesting for geothermal exploring, for recognizing the main tectonic structures in the subsurface, for identifying zones of ascent of fluids and, in certain cases, for locating geothermal reservoirs. In first part of this thesis a survey of measurements of carbon dioxide to the ground was performed in the geothermal area of Larderello and, more exactly in the town of Monterotondo Marittimo (GR). This work is framed within a larger project whose purpose is to assess the geothermal potential of the area near the torrent named Milia and was one of the methodologies used for location of future geothermal wells. The measurements of CO2 were performed using a flow meter of the type LICOR 8100 equipped with a gas analyzer, which allowed the measure of CO2 fluxes to the ground. During the survey, conducted in January 2011, 63 measurements of CO2 flux were taken in the area near the torrent Milia, using as a reference a rectangular sampling grid with a mesh side equal to 250 m, with an overall length and width of 3000 m and 1250 m respectively, with a total area of about 3,75 km2. Outside this area 20 measurements of carbon dioxide flux were also done with location in the zone near certain wells ENEL (Carboli C bis, Poggio Travi e Lumiera 1bis), in areas classically considered as recharge of the geothermal field and in the area of fumaroles (“Le Biancane”). Analysis of the flux values made by graphical statistical analysis method (GSA) described by Chiodini et al. (1998) revealed the existence of four different populations, each of them marked by different characteristics and origin; the identification of these geochemical families allowed to estimate for the sampled area of Milia the daily carbon dioxide output which provided the value of 145,45 t/d over an area of 3,75 km2. Those data are very similar to those obtained by other CO2 flux survey carried out in other Tuscan thermally anomalous areas or in zone characterised by the development of major tectonic discontinuities and also highlight the problem of putting greenhouse gases in to the atmosphere. Geothermal energy is, as a matter of fact, a source of power with low environmental impact, however the release of carbon dioxide in to the atmosphere by power plants is commonly pointed out as one of the negative effects arising from the use of resource. The survey performed in the area near the Milia torrent proves that also area with small extension are able to enter naturally large amounts of CO2 in to the atmosphere. Indeed data from scientific literature certify that carbon dioxide degassing of natural origin exceeds the one produced with the exploitation of geothermal resource, for many geothermal fields included Larderello. From collected data was furthermore verified as the tectonic structures, the geology of zone, the different characteristics of rocks or the processes active in the subsurface environment could substantially influence the CO2 flux measured to the ground, both for the area near the Milia torrent but also for the outer zone. This has allowed to determine good relationship between various parameters. In addition 6 replicas of measured points were carried out at the end of June to verify if there was a variation in CO2 fluxes due to seasonality. Measurements of carbon dioxide flux were also accompanied by 7 Radon (222Rn) measurements, conducted using a portable detector type RTM 2100 produced by SARAD, placed in the area near Milia torrent and partly in the outer zone near the wells Carboli C bis and Lumiera 1bis and in the recharge area of the geothermal field. These data have allowed to advance further interpretations for the studied area and to prove the hypothesis already made by using data from CO2 flux measurements. The second and last part of the work concerned the modelling of the hydrological behaviour and the thermal characteristics of the geothermal reservoir, consisting of the formation of Calcare Cavernoso, in the zone where the survey of CO2 measurements was carried out using a regular sampling grid. Numerical simulation was performed on a three-dimensional model employing the software PetraSim 5.1 and using TOUGH2 as a simulator and a equation of state EOS1. The conceptual model was created with physical, geological-structural, hydrogeological and geochemical available data, it has a length of 2,7 km, a width of 1 km and a thickness of 0,8 km and has been modelled as comprising three fundamental bodies: cap-reservoirbasement. On this model a computational grid consisted of 3672 regular rectangular cells was built. The first phase of numerical simulation aimed to model the current natural state of the initial model, which constituted the basis for a second phase concerning the simulation of different scenarios of exploitation for geothermal purposes for a defined period of time equal to 50 years

Etude géochimique et isotopique (Sr, Nd, Pb) de l'ophiolite de Nouvelle Calédonie

L’ophiolite de Nouvelle Calédonie présente une des plus grandes sections de manteau océanique obductées au monde, offrant une perspective unique pour l’étude des processus du manteau supérieur. Les roches du manteau appartiennent à une séquence ophiolitique “atypique”, dominée par des harzburgites réfractaires avec quelques lherzolites à spinelle et à plagioclase. À l'exception de quelques cumulats mafiques-ultramafiques, la partie crustale de l'ophiolite est totalement absente. Cette ophiolite a été étudiée depuis plusieurs décennies, toutefois sa nature ultra-appauvrie a rendu très difficile une caractérisation géochimique détaillée. La littérature scientifique ne regroupe que quelques données sur les éléments en trace et les données isotopiques sont totalement inexistantes. Dans ce travail de thèse, une étude géochimique exhaustive (éléments majeurs, en trace et isotopes Sr-Nd-Pb) des péridotites et des roches mafiques associées à l’ophiolite a été réalisée. Les péridotites sont des tectonites avec des textures porphyroclastiques. Les lherzolites à spinelle ont 7-8 vol.% de clinopyroxène riche en Na2O et Al2O3 (jusqu’à Na2O 0.5 wt.%; 6.5 wt.% Al2O3), teneur en Fo de l’olivine de 88.5 à 90.0 mol.%, bas valeurs du Cr# du spinelle (13-17), attestant la nature fertiles de ces roches. A l’inverse les harzburgites costituent des roches très réfractaires : ils ne contiennent pas de clinopyroxène primaire et les teneurs en Fo de l’olivine (90.9-92.9 mol.%), le Mg# de l’orthopyroxène et le Cr# du spinelle (39-71) sont élevés. Les spectres de concentrations en REE présentent des caractéristiques typiques de formation dans un environnement abyssal pour les lherzolites à spinelle, alors que les harzburgites ont des spectres en U typiques d’environnement d’avant-arc. Les compositions en REE des lherzolites à spinelle sont compatibles avec un bas degré de fusion fractionnée (8-9%) d'une source DMM, commençant dans le domaine de stabilité du grenat. Au contraire les concentrations en REE des harzburgites indiquent un haut degré de fusion d’une source DMM, en accord avec une fusion hydratée en environnement d’avant-arc. Les lherzolites à plagioclase présentent des microtextures d’imprégnation, des spinelles riches en Cr2O3 et TiO2 et un enrichissement progressif en REE, Ti, Y, Zr. Les modèles des éléments en trace indiquent que les lherzolites à plagioclase résultent des lherzolites à spinelle par séquestration des liquides MORB très appauvris au sein de la lithosphère océanique.Les roches intrusives sont des gabbronorites à olivine avec des compositions très appauvries (87.3≤Fo ol≤88.9 mol.%, 87.7≤Mg# Cpx≤92.2, An Pl=90-96 mol.%). Le haut Mg#, le bas teneur de TiO2 des pyroxènes, la composition en anorthite du plagioclase et le modèle des éléments en trace montrent que les magmas parents des gabbronorites sont des magmas primitifs, très appauvries, formés dans un environnement de subduction.The New Caledonia ophiolite hosts one of the largest obducted mantle section in the world, hence providing a unique insight for the study of upper mantle processes. These mantle rocks belong to an “atypical” ophiolitic sequence, which is dominated by refractory harzburgites but it also includes minor spinel and plagioclase lherzolites. Upper crust is notably absent in the ophiolite, with the exception of some mafic-ultramafic cumulates cropping out in the southern part of the island. Although the New Caledonia ophiolite has been under investigation for decades, its ultra-depleted nature has made its characterization an analytical challenge, so that few trace element data are available, while isotopic data are completely missing. In this thesis a comprehensive geochemical study (major, trace element and Sr-Nd-Pb isotopes) of the peridotites and the associated intrusive mafic rocks from the New Caledonia ophiolite has been carried out. The peridotites are low-strain tectonites showing porphyroclastic textures. Spinel lherzolites are undepleted lithotypes, as attested by the presence of 7-8 vol.% of Na2O and Al2O3-rich clinopyroxene (up to 0.5 wt.% Na2O; 6.5 wt.% Al2O3), Fo content of olivine (88.5-90.0 mol.%) and low Cr# of spinel (13-17). Conversely, harzburgites display a refractory nature, proven by the remarkable absence of primary clinopyroxene, very high Fo content in olivine (90.9-92.9 mol.%), high Mg# in orthopyroxene (89.8-94.2) and Cr# in spinel (39-71). REE contents show abyssal-type patterns for spinel lherzolites, while harzburgites display U-shaped patterns, typical of fore-arc settings.Spinel lherzolites REE compositions are consistent with relatively low degree (8-9%) of fractional melting of a DMM source, starting in the garnet stability field. Conversely, REE models for harzburgites indicate high melting degrees (20-25%) of a DMM mantle source under spinel faies conditions, consistent with hydrous melting in forearc setting. Plagioclase lherzolites exhibit melt impregnation microtextures, Cr- and TiO2-enriched spinels and REE, Ti, Y, Zr progressive increase with respect to spinel lherzolites. Impregnation models indicate that plagioclase lherzolites may derive from spinel lherzolites by entrapment of highly depleted MORB melts in the shallow oceanic lithosphere. Mafic intrusives are olivine gabbronorites with a very refractory composition, as attested by high Fo content of olivine (87.3-88.9 mol.%), very high Mg# of clinopyroxene (87.7-92.2) and extreme anorthitic content of plagioclase (An = 90-96 mol.%). The high Mg#, low TiO2 concentrations in pyroxenes and the anorthitic composition of plagioclase point out an origin from ultra-depleted primitive magmas in a convergent setting. Geochemical trace element models show that the parental melts of gabbronorites are primitive magmas with striking depleted compositions, bearing only in part similarities with the primitive boninitic melts of Bonin Islands. The first Sr, Nd and Pb isotope data obtained for the New Caledonia ophiolite highlight the presence of DM mantle source variably modified by different processes.Nd-Sr-Pb isotopic ratios for the lherzolites (+6.98≤ƐNdi≤+10.97) indicate a DM source that suffered low-temperature hydrothermal reactions. Harzburgites are characterized by a wide variation of Sr, Nd and Pb isotopic values, extending from DM-type to EMII compositions (-0.82≤ƐNdi≤+17.55), suggesting that harzburgite source was strongly affected by subduction-related processes. Conversely, combined trace element and Sr-Nd-Pb isotopic data for gabbronorites indicate a derivation from a source with composition similar to Indian-type mantle, but affected by fluid input in subduction environment

Temperatures and Cooling Rates Recorded by the New Caledonia Ophiolite: Implications for Cooling Mechanisms in Young Forearc Sequences

International audienceTo unveil how forearc lithosphere cools and re-equilibrates, we carried out a comprehensive geothermometric investigation of the New Caledonia ophiolite, which represents a rare example of proto-arc section generated during subduction infancy. A large data set, including more than 80 samples (peridotites and mafic-ultramafic intrusives), was considered. Closure temperatures calculated for the lherzolites using slow (TREE-Y) and fast diffusing (TCa-in-Opx, TBKN, TCa-in-Ol, TOl-Sp) geothermometers provide some of the highest values ever documented for ophiolitic peridotites, akin to modern sub-oceanic mantle. Cooling rates deduced from TREE-Y and TBKN yield values of ≈10−3°C/y, similar to those obtained with TCa-in-Ol. These features are consistent with a post-melting history of emplacement, possibly along a transform fault, and thermal re-equilibration via conduction. Cpx-free harzburgites register a high-T evolution, followed by quenching and obduction. The relatively high TCa-in-Ol, TOl-Sp and cooling rates computed from TCa-in-Ol (≈10−3°C/y) are atypical for this geodynamic setting, mirroring the development of an ephemeral subduction system, uplift and emplacement of the Peridotite Nappe. Temperature profiles across the crust-mantle transect point to high closure temperatures, with limited variations with depth. These results are indicative of injection and crystallization of non-cogenetic magma batches in the forearc lithosphere, followed by thermal re-equilibration at rates of ≈10−4–10−3°C/y. Our study shows that the thermal conditions recorded by forearc sequences are intimately related to specific areal processes and previous lithospheric evolution. Thus, detailed sampling and exhaustive knowledge of the geological background are critical to unravel the cooling mechanisms in this geodynamic settin

Multi-stage evolution of peridotites of New Caledonia: preliminary results

The presumably Late Cretaceous-Early Eocene Peridotite Nappe of New Caledonia, is one of the largest and best exposed ophiolitic complex in the world. It is largely dominated by harzburgite and dunite but it also includes lherzolites in northern massifs and mafic/ultramafic cumulates in the south of the island. Although the mantle rocks have been studied for almost 30 years, their history still remains controversial, partly because of the scarcity of geochemical data and the total lack of isotopic data. Recent studies (Marchesi et al., 2009; Ulrich et al., 2010; Pirard et al., 2013) proved that these mantle rocks have experienced a complex evolution including different phases of melting, melt-rock interaction and re-melting that led to an overall ultra-depleted composition. Here, we present the preliminary results of a petrological and geochemical study on a new set of peridotite samples from New Caledonia. Harzburgites, consisting of variable proportions of olivine (from 70-75% up to > 80 vol.%), orthopyroxene (from 12%-15% to 25 vol.%) and Cr-rich spinel (≈1% or less), are highly refractory rocks, as attested by the absence of primary clinopyroxene, very high Fo content in olivine (90.7-92.9 mol%), high Mg# in orthopyroxene ([Mg/(Mg+Fe)]= 91.0-92.7) and Cr# in spinel ([100 • Cr/(Cr+Al)]= 40-71). In contrast, lherzolites (clinopyroxene = 5-10 vol.%), display a fairly fertile nature, with lower Fo in olivine (88.5-91.8 mol%) and Mg# in orthopyroxene (89.0-91.3), low Cr# in spinel (0.132-0.167) and relatively high Na 2 O (up to 0.80 wt%) and Al 2 O 3 (3.1-6.7 wt%) contents in clinopyroxene. Secondary, interstitial and undeformed clino-and orthopyroxenes have also been observed in harzburgites. These phases testify melt percolation after partial melting and re-equilibration at lithospheric conditions. Their chemical compositions, i.e. low Al 2 O 3 and CaO contents in orthopyroxene and very low or negligible Na 2 O and TiO 2 in clinopyroxene, may suggest a metasomatic origin by SiO 2-rich fluids and/or depleted melts in a subduction-related setting. Mineral compositional variations (e.g. Mg# (Ol) vs Cr# (Spl) and Cr# (Spl) vs Mg# (Spl), show that most investigated harzburgites plot in the field of SSZ (forearc) peridotites, whereas some cpxpoor lherzolites are more akin to abyssal peridotites, or, even, to passive margin peridotites for the most fertile types. The peridotites are low strain tectonites with porphyroclastic textures partially overprinted by mosaic equigranular textures, probably recording an asthenospheric HT origin followed by subsolidus re-equilibration. Geothermometric estimates provide temperatures of 930-1145°C and 870-1080°C for the porphyroclastic assemblages of harzburgites and lherzolites, respectively; lower temperatures are recorded for the spinel facies recrystallization (≈ 830°C-980°C for both lithotypes). These preliminary results are consistent with a multi-stage history of melting, deformation, recrystallization and melt-rock interaction. Geochemical and radiogenic isotope analyses (in progress) are expected to decipher the depletion vs. refertilization evolution of the different peridotite types and put constrain on their geodynamic significance

Geochemistry and tectonic significance of lherzolites from New Caledonia Ophiolite

The association of rocks with contrasting geodynamic affinities is a quite common feature in ophiolites and it has been documented in several ophiolitic complexes, such as Pindos (Greece), and Lycian and Antalya ophiolites (Turkey) (Saccani, Photiades 2004; Aldanmaz et al. 2009). New Caledonia hosts one of the World’s largest ultramafic terrane termed “Peridotite Nappe” belonging to an “atypical” ophiolitic sequence of presumed Late Cretaceous- Early Eocene age. It is dominated by mantle lithologies, mostly harzburgite and minor spinel and plagioclase lherzolite, together with some mafic and ultramafic cumulates. Despite their ultra-depleted nature, some efforts were devoted to characterize the mantle rocks from a geochemical point of view. A supra-subduction affinity is generally accepted for harzburgites (Marchesi et al. 2009; Ulrich et al. 2010); in contrast, the origin and evolution of lherzolites still remain a matter of debate. This presentation will focus on the petrological and geochemical characterization of the lherzolitic rocks. Lherzolites are mainly found in northern massifs, where spinel lherzolites extensively crop out in association with minor plagioclase lherzolites. These rocks are low-strain porphyroclastic tectonites, locally grading into protomylonite. They likely record an asthenospheric HT origin followed by sub-solidus re-equilibration, which is also testified by geothermometric estimates (870-1080°C and 830-980°C for porphyroclastic assemblages and recrystallization in the spinel facies, respectively). Spinel lherzolites are relatively undepleted, as attested by the presence of 7-8 vol% of Na and Al-rich clinopyroxene (up to 0.8 wt% Na2O; 3.1-6.7 wt% Al2O3), low Fo in olivine (88.5-90.0 mol%) and Cr# in spinel ([100 • Cr/(Cr+Al)]= 13-17). Major element mineral compositions (e.g. Mg# (Ol) vs Cr# (Spl), Cr# (Spl) vs Mg# (Spl)) coupled with Cpx and whole-rock REE geochemistry show that the lherzolites are akin to abyssal peridotites. In particular, the spinel lherzolites have REE patterns characterized by MREE and HREE enrichment over LREE (NdN/SmN = 0.18-0.27, NdN/YbN = 0.02-0.70) for YbN = 1.0-1.3. The REE patterns of clinopyroxenes have strong LREE depletion (NdN/YbN= 0.001-0.05) and nearly flat HREE segments for YbN = 5.5-5.9. Melting modelling based on REE compositions of Cpx and whole-rock indicate that HREE patterns can be explained by 8-10% fractional melting of a DMM source in the spinel stability field. However, slight MREE/HREE fractionation suggest that melting may have initiated at higher pressure, in the garnet stability field. Whole-rock Nd isotope ratios (Nd = 6.98-11.86) also suggest derivation from a relatively homogenous MORB-type depleted mantle that experienced a recent depletion event, leading to variable 147Sm/144Nd ratios. The aforementioned features point out an origin in a spreading ridge environment, probably related to pre-Early Eocene marginal basin development within the Southeast Gondwana margin

Origin of the spinel-pyroxene symplectites in the harzburgites from the New Caledonia Peridotite

The New Caledonia ophiolite (Peridotite Nappe) hosts one of the largest and best-exposed mantle section worldwide, providing an exceptional insight into upper mantle processes. The Peridotite Nappe is mostly dominated by harzburgites, locally overlain by mafic-ultramafic cumulates, but also includes minor spinel and plagioclase lherzolites, cropping out in the northern part of the island. The New Caledonia harzburgites are low-strain tectonites, showing dominant porphyroclastic textures. The main mantle paragenesis is constituted by olivine (~ 75-85 vol%), orthopyroxene (~ 15-25 vol%) and spinel (< 1 vol%), while primary clinopyroxene is notably absent. An important textural feature of these mantle rocks is represented by the common occurrence of spinel-pyroxene symplectitic aggregates.In this work, we present a petrographical, textural and major element chemical characterization of the spinel-pyroxene symplectitic intergrowths occurring in the New Caledonia harzburgites (Kopeto, Central Massif, and Yaté, Massif du Sud). Based on textures, size and relationships with the other mineral phases, these spinel-pyroxene clusters have been divided into two types, named type-A and type-B.Type-A symplectites occur in the Kopeto harzburgites and are composed of spinel-orthopyroxene (± clinopyroxene) intergrowths. In type-A symplectites, symplectitic spinel (Spl2) occurs as abundant vermicular shaped grains, ranging in size from ~ 0.5 to 2 mm. By contrast, spinel of the porphyroclastic assemblage (Spl1) shows smaller size (in the range of few m) and notably lower abundances (< 1%). Type-A symplectites develop exclusively on porphyroclastic olivine, which in turn displays evidence of chemical disequilibrium and corroded outlines. Bulk major element composition reconstructed for type-A symplectites rule out a derivation from a pre-existing garnet phase, as the model garnet compositions do not satisfy garnet stoichiometry, being characterized by Si deficiency. By contrast, major element chemical variations of the symplectitic phases, coupled with the high abundance of Spl2 and olivine resorption, suggest an origin from reactive percolation of opx-saturated hydrous melts or slab-derived fluids in a subduction zone setting.Type-B symplectites are found in Yaté sample and consist of spinel-orthopyroxene (± clinopyroxene). They are characterized by smaller size (few hundreds of m, i.e. “micro-symplectites”) and different shapes compared to type-A symplectites, growing as vermicular, “myrmekite-like” intergrowths at the rims of porphyroclastic orthopyroxene. Major element chemical compositions of type-B symplectites are consistent with an origin as “cooling textures”. These structures may derive from unmixing of a high-T, Al-Cr rich, orthopyroxene due to the decreased solubility of the Cr-Al component (CrMgTs) during post-melting lithospheric cooling at T < 900°C

Sr, Nd, Pb and trace element systematics of the New Caledonia harzburgites: Tracking source depletion and contamination processes in a SSZ setting

International audienceThe New Caledonia ophiolite (Peridotite Nappe) consists primarily of harzburgites, locally overlain by mafic-ultramafic cumulates, and minor spinel and plagioclase lherzolites. In this study, a comprehensive geochemical data set (major and trace element, Sr-Nd-Pb isotopes) has been obtained on a new set of fresh harzburgites in order to track the processes recorded by this mantle section and its evolution.The studied harzburgites are low-strain tectonites showing porphyroclastic textures, locally grading into protomylonitic textures. They exhibit a refractory nature, as attested by the notable absence of primary clinopyroxene, very high Fo content of olivine (91–93 mol.%), high Mg# of orthopyroxene (0.91–0.93) and high Cr# of spinel (0.44–0.71). The harzburgites are characterised by remarkably low REE concentrations (<0.1 chondritic values) and display "U-shaped" profiles, with steeply sloping HREE (DyN/YbN = 0.07–0.16) and fractionated LREE-MREE segments (LaN/SmN = 2.1–8.3), in the range of modern fore-arc peridotites. Geochemical modelling shows that the HREE composition of the harzburgites can be reproduced by multi-stage melting including a first phase of melt depletion in dry conditions (15% fractional melting), followed by hydrous melting in a subduction zone setting (up to 15%–18%). However, melting models fail to explain the enrichments observed for some FME (i.e. Ba, Sr, Pb), LREE-MREE and Zr–Hf. These enrichments, coupled with the frequent occurrence of thin, undeformed films of Al2O3, and CaO-poor orthopyroxene (Al2O3 = 0.88–1.53 wt.%, CaO = 0.31–0.56 wt.%) and clinopyroxene with low Na2O (0.03–0.16 wt.%), Al2O3 (0.66–1.35 wt.%) and TiO2 (0.04–0.10 wt.%) contents, point to FME addition during fluid-assisted melting followed by late stage metasomatism most likely operated by subduction-related melts with a depleted trace element signature.Nd isotopic ratios range from unradiogenic to radiogenic (−0.80≤εNdi≤+13.32) and negatively correlate with Sr isotopes (0.70257≤87Sr/86Sr ≤ 0.70770). Pb isotopes cover a wide range, trending from DMM toward enriched, sediment-like, compositions. We interpret the geochemical signature displayed by the New Caledonia harzburgites as reflecting the evolution of a highly depleted fore-arc mantle wedge variably modified by different fluid and melt inputs during Eocene subduction

Melt extraction and enrichment processes in the New Caledonia lherzolites: Evidence from geochemical and Sr-Nd isotope data

International audienceThe New Caledonia ophiolite (Peridotite Nappe) is dominated by mantle lithologies, composed of forearc-related refractory harzburgites and minor lherzolites in both the spinel and plagioclase facies.In this study, a comprehensive geochemical data set (major, trace elements and Sr–Nd isotopes) is used to constrain the mantle evolution of the lherzolites and their relationships with the basalts from the Poya Terrane, which tectonically underlies the mantle rocks. The majority of the lherzolites are low-strain porphyroclastic tectonites. They likely record an asthenospheric origin followed by re-equilibration at lithospheric conditions, as supported by geothermometric estimates (T = 1100–940 °C and 920–890 °C for porphyroclastic and neoblastic spinel-facies assemblages, respectively). Olivine composition (Fo = 88.5–90.0 mol%), spinel Cr# ([molar 100 • Cr/(Cr + Al)] = 13–17) and relatively high amounts (7–8 vol%) of Al2O3- and Na2O-rich clinopyroxene (up to 0.5 and 6.5 wt.%, respectively) indicate a moderately depleted geochemical signature for the spinel lherzolites. Bulk rock and clinopyroxene rare earth elements (REE) patterns display a typical abyssal-type signature, i.e. steeply plunging LREE accompanied by nearly flat HREE to MREE. Clinopyroxene REE compositions of the spinel lherzolites may be reproduced by small amounts of fractional melting of a garnet lherzolite precursor (~ 4%), followed by 4%–5% melting in the spinel peridotite field. The plagioclase lherzolites show melt impregnation microstructures, Cr- and Ti-rich spinels and incompatible trace element enrichments (REE, Ti, Y, and Zr) in bulk rocks and clinopyroxenes. Impregnation modelling for these elements suggests that the plagioclase lherzolites originated from residual spinel lherzolites by entrapment of highly depleted (non-aggregated) MORB melt fractions in the shallow oceanic lithosphere. Nd isotope compositions of the investigated peridotites are consistent with derivation from an asthenospheric mantle source that experienced a recent MORB-producing depletion event. This evolution was most likely accomplished in a spreading ridge. However, geochemical trace element modelling and Nd isotopes do not support a genetic mantle–crust link between the lherzolites and enriched-MOR-type basalts from the Poya Terrane

Subduction-related ultradepleted melts in a nascent arc: geochemical and isotopic evidence from the intrusive sequence of the new caledonia ophiolite

The obducted sections of intraoceanic arc lithosphere represent a favourable setting to investigate the crust generation in nascent arcs. The New Caledonia ophiolite hosts a mafic-ultramafic igneous sequence overlying harzburgitic rocks that underwent very high degrees of partial melting in a forearc setting (Marchesi et al., 2009; Secchiari, 2016). This sequence has been interpreted as a crust-mantle section in a nascent arc (Pirard et al., 2013). The igneous rocks include dunites, wehrlites and mafic rocks (mainly gabbronorites). Dunites and wehrlites derive from melt/peridotite interactions involving primitive arc tholeiites and boninitic liquids, whereas the mafic rocks are the result of cumulus processes (Marchesi et al., 2009; Pirard et al., 2013). The samples of this study are olivine gabbronorites occurring in the upper part of the sequence as metre-sized sills. Subhedral Ca-rich plagioclase (An = 90-96 mol. %) is the dominant phase (50-80 vol %). Mg-rich olivine (5-15 vol %, Fo = 87-89 mol %) occurs as irregularly shaped crystals. Cpx (15-20 vol%) is generally rimmed by interstitial to poikilitic Opx (5-15 vol %). Cpx shows high Mg# (88-92), low Al 2 O 3 (1.5-2.4 wt%) and negligible TiO 2 and Na 2 O contents. Bulk rock compositions are characterized by high Mg# (86-93) and very low incompatible trace element contents (e.g. LREE concentrations mostly below 0.1 times chondrite). They show marked LREE depletion, nearly flat and low HREE (Yb N = 0.2-0.9) and positive Eu anomalies. The highly depleted nature of these rocks is mirrored by Cpx trace element composition. In contrast, fluid-mobile trace elements are relatively enriched and distinct Pb and Sr positive anomalies are observed in both whole rocks and Cpx. The calculated compositions of putative liquids in equilibrium with the most primitive gabbronorites are consistent with melting of a refractory peridotite source in the spinel stability field. The inferred melts have high Mg# of 75-76 and very low REE, Nb, Zr, Hf contents. On the other hand, Ba, Pb and Sr spikes in the calculated liquids argue for the involvment of a subduction-related component. Overall, the liquids show geochemical signatures akin to olivine boninite melts with lower absolute concentrations of LREE and other incompatible trace elements (Ba, Pb, Sr, Zr, Hf). We propose that the New Caledonia gabbronorites reflect injection of primitive, ultradepleted (“boninitelike”) magma batches in the lower forearc crust, possibly before aggregation, during the first phases of arc formation. The investigated gabbronorites provide homogeneous initial Nd isotope compositions (e Nd ranging between +8.2 and +9.4), coupled with high 208Pb/207Pb and 207 Pb /Pb ratios, falling in a domain intermediate between DMM and Pacific sediments. The heterogeneous Pb isotopic signature of the gabbronorites most likely reflect a subarc asthenospheric mantle source variably modified by subduction-related fluids