Reintegrating nanogranitoid inclusion composition to reconstruct the prograde history of melt-depleted rocks

A recent fascinating development in the study of high-grade metamorphic basements is represented by the finding of tiny inclusions of crystallized melt (nanogranitoid inclusions) hosted in peritectic phases of migmatites and granulites. These inclusions have the potential to provide the primary composition of crustal melts at the source. A novel use of the recently-published nanogranitoid compositional database is presented here. Using granulites from the world-renowned Ivrea Zone (NW Italy) on which the original melt-reintegration approach has been previously applied, it is shown that reintegrating melt inclusion compositions from the published database into residual rock compositions can be a further useful method to reconstruct a plausible prograde history of melt-depleted rocks. This reconstruction is fundamental to investigate the tectonothermal history of geological terranes. Keywords: Nanogranitoids, Melt-reintegration, Granulite, High-temperature metamorphis

phase equilibria modelling of residual migmatites and granulites an evaluation of the melt reintegration approach

Suprasolidus continental crust is prone to loss and redistribution of anatectic melt to shallow crustal levels. These processes ultimately lead to differentiation of the continental crust. The majority of granulite facies rocks worldwide has experienced melt loss and the reintegration of melt is becoming an increasingly popular approach to reconstruct the prograde history of melt-depleted rocks by means of phase equilibria modelling. It involves the stepwise down-temperature reintegration of a certain amount of melt into the residual bulk composition along an inferred P–T path, and various ways of calculating and reintegrating melt compositions have been developed and applied. Here different melt-reintegration approaches are tested using El Hoyazo granulitic enclaves (SE Spain), and Mt. Stafford residual migmatites (central Australia). Various sets of P–T pseudosections were constructed progressing step by step, to lower temperatures along the inferred P–T paths. Melt-reintegration was done following one-step and multi-step procedures proposed in the literature. For El Hoyazo granulites, modelling was also performed reintegrating the measured melt inclusions and matrix glass compositions and considering the melt amounts inferred by mass-balance calculations. The overall topology of phase diagrams is pretty similar, suggesting that, in spite of the different methods adopted, reintegrating a certain amount of melt can be sufficient to reconstruct a plausible prograde history (i.e., melting conditions and reactions, and melt productivity) of residual migmatites and granulites. However, significant underestimations of melt productivity may occur and have to be taken into account when a melt-reintegration approach is applied to highly residual (SiO2 < 55 wt.%) rocks, or to rocks for which H2O retention from subsolidus conditions is high (such as in the case of rapid crustal melting triggered by mafic magma underplating). This article is protected by copyright. All rights reserved

What can we learn from melt inclusions in migmatites and granulites?

With less than two decades of activity, research on melt inclusions (MI) in crystals from rocks that have undergone crustal anatexis \u2013 migmatites and granulites \u2013 is a recent addition to crustal petrology and geochemistry. Studies on this subject started with glassy inclusions in anatectic crustal enclaves in lavas, and then progressed to regionally metamorphosed and partially melted crustal rocks, where melt inclusions are normally crystallized into a cryptocrystalline aggregate (nanogranitoid). Since the first paper on melt inclusions in the granulites of the Kerala Khondalite Belt in 2009, reported and studied occurrences are already a few tens. Melt inclusions in migmatites and granulites show many analogieswith theirmore common and long studied counterparts in igneous rocks, but also display very important differences and peculiarities,which are the subject of this review. Microstructurally, melt inclusions in anatectic rocks are small, commonly 10 \u3bcm in diameter, and their main mineral host is peritectic garnet, although several other hosts have been observed. Inclusion contents vary from glass in enclaves that were cooled very rapidly from supersolidus temperatures, to completely crystallized material in slowly cooled regional migmatites. The chemical composition of the inclusions can be analyzed combining several techniques (SEM, EMP, NanoSIMS, LA\u2013ICP\u2013MS), but in the case of crystallized inclusions the experimental remelting under confining pressure in a piston cylinder is a prerequisite. The melt is generally granitic and peraluminous, although granodioritic to trondhjemitic compositions have also been found. Being mostly primary in origin, inclusions attest for the growth of their peritectic host in the presence of melt. As a consequence, the inclusions have the unique ability of preserving information on the composition of primary anatectic crustal melts, before they undergo any of the common following changes in their way to produce crustal magmas. For these peculiar features, melt inclusions in migmatites and granulites, largely overlooked so far, have the potential to become a fundamental tool for the study of crustal melting, crustal differentiation, and even the generation of the continental crust

When the continental crust melts: a combined study of melt inclusions and classical petrology on the Ronda migmatites

Melt inclusions hosted in peritectic minerals of migmatites represent a novel and powerful small-scale tool to investigate the anatexis of the continental crust. In this thesis, taking advantage of a new experimental approach developed during this work, a combined study of classical petrology and melt inclusions in migmatites was performed to characterize in detail the composition and the physical properties of anatectic melts, the fluid regimes, and the melting mechanisms and conditions during the anatexis of the metasedimentary crust located below the Ronda peridotite (Betic Cordillera, S Spain). Here, the tectonic emplacement of mantle rocks within the continental crust produced high-temperature metamorphism and partial melting in the underlying metasedimentary rocks. The investigated migmatites are quartzo-feldspathic metatexites located towards the base of the crustal sequence and quartzo-feldspathic mylonitic migmatites found close to the contact with the peridotite. The petrologic study was made by petrographic observations, compositional characterization of minerals and bulk rocks, conventional thermobarometry and pseudosection calculations. The quartzo-feldspathic migmatites are mainly composed of Qtz + Pl + Kfs + Bt + Sil + Grt and probably derived from a greywacke protolith. Muscovite, very rare in the metatexites, is absent in the mylonites. Graphite is present in all migmatites. The former presence of melt is recorded by melt inclusions and melt pseudomorphs at the microscale, and by peraluminous leucogranitic leucosomes at the mesoscale. The changes in phase compositions and modal contents from the metatexites to the mylonites, along with thermobarometric calculations and pseudosection modelling, are consistent with an increase of melting conditions and degree towards the contact with the peridotite. Melt inclusions have been found in peritectic garnets of the investigated quartzo-feldspathic migmatites. A detailed study was carried out to characterize the microstructural features of melt inclusions and their chemical composition by microscope observation, FESEM imaging, piston cylinder experimental remelting, Raman and EMP analyses. The inclusions are primary in origin and were trapped within peritectic garnet during crystal growth. They are very small in size, mostly ≤ 10 µm, and typically show a well-developed negative crystal shape. Three types of inclusions were identified: totally crystallized (nanogranites), partially crystallized and preserved glassy inclusions. Crystallized melt inclusions contain a granitic phase assemblage with quartz, feldspars and micas. In this study, a new approach for the experimental re-homogenization of melt inclusions was developed, performing remelting experiments at 5 kbar pressure using a piston cylinder apparatus. At the minimum re-homogenization temperature of 700 °C, most melt inclusions are completely re-homogenized. Conversely, melt inclusions display decrepitation cracks and CO2 bubbles when remelted at higher experimental temperatures. For the first time, a pseudosection was constructed using the bulk composition of the fully re-homogenized melt inclusions from the metatexites. This approach constrained the melt entrapment at temperature close to the minimum re-homogenization temperature (T ~700 °C). The composition of those melt inclusions re-homogenized at 700 °C is comparable to that of preserved glassy inclusions in the same rock. In general all the studied melt inclusions in the migmatites and mylonites have peraluminous leucogranitic compositions. However, inclusions in the mylonites (mostly glassy) display quite variable Na2O/K2O and have lower primary H2O contents (1.0-2.6 wt%) than melt inclusions in the metatexites (3.1-7.6 wt%). Combining information from the melt inclusions and from the classical petrology allowed a better understanding of the melting processes occurred in the crustal sequence below the Ronda peridotite. The data collected in this study suggest that the crustal melting at Ronda mainly occurred under H2O-undersaturated conditions by the continuous Bt dehydration melting reaction and at temperatures that did not exceed ~800 °C. In the metatexites, towards the base of the crustal sequence, crustal anatexis likely started at the solidus (T ~700 °C) in presence of H2O-rich intergranular fluids. The growing peritectic garnets entrapped melt droplets produced at slightly different temperatures, whose composition was mainly diffusion-controlled. Melt inclusions seem to record the evolution of melt composition during prograde anatexis. Conversely, most of the peraluminous leucogranitic leucosomes are primary melts produced at, or close to, the metamorphic peak. The anatectic melts at Ronda have viscosity values greater than those commonly considered for granitic melts formed at the same P-T conditions, implying much longer timescales for melt extraction and ascent through the metasedimentary crust at Ronda, as well as much greater strength of the migmatites. The consistency of the collected compositional data along with the careful multidisciplinary approach adopted in this research, indicate that melt inclusions in peritectic minerals from migmatites represent a unique microstructure where anatexis is recorded and can be characterized in situ, in its earliest stage.Le inclusioni di fuso silicatico in minerali peritettici di migamtiti rappresentano un nuovo strumento a disposizione dei petrologi per studiare il processo di anatessi della crosta continentale. Nel presente lavoro di tesi, grazie ad un nuovo approccio sperimentale sviluppato durante questa ricerca, è stato condotto uno studio combinato di petrologia classica ed inclusioni di fuso silicatico in migmatiti, per caratterizzare in dettaglio la composizione e le proprietà fisiche dei fusi anatettici, i regimi fluidi e le condizioni e i meccanismi di fusione durante l’anatessi della crosta meta sedimentaria, che si trova al di sotto delle peridotiti di Ronda (Cordigliera Betica, Spagna meridionale). In questa zona, l’impilamento tettonico di porzioni di mantello litosferico al di sopra di successioni di crosta continentale ha prodotto un metamorfismo di alta temperatura e innescato processi di fusione parziale nelle sottostanti rocce metasedimentarie. Le migmatiti studiate sono costituite da metatessiti quarzo feldspatiche, che affiorano verso la base delle sequenza crostale e da migmatiti milonitiche quarzo feldspatiche presenti in prossimità del contatto con le peridotiti. Lo studio petrologico è stato condotto attraverso osservazioni petrografiche, caratterizzazione composizionale dei minerali e delle rocce, termobarometria convenzionale e pseudosezioni. Le migmatiti sono principalmente composte da Qtz + Pl + Kfs + Bt + Sil + Grt e probabilmente derivano da grovacche. La muscovite è rara nelle metatessiti e scompare nelle miloniti. La grafite è presente in tutte le rocce. La presenza di fuso anatettico nelle rocce studiate è testimoniata alla microscala da inclusioni di fuso silicatico e dalle pseudomorfosi di minerali su originari film di fuso e alla mesoscala da leucosomi leucogranitici peralluminosi. Le variazioni delle composizioni dei minerali e dei contenuti modali, i risultati dei calcoli termobarometrici e delle pseudosezioni indicano un aumento delle condizioni e del gradi di fusione dalla base della sequenza crostale verso il contatto con le peridotiti. Le inclusioni di fuso silicatico sono state rinvenute nei granati peritettici delle migmatiti. La petrografia, le microstrutture e la composizione chimica delle inclusioni sono state caratterizzate attraverso l’uso del microscopio ottico e del microscopio elettronico a scansione con sorgente ad emissione di campo (FESEM), la rifusione sperimentale con piston cylinder e le analisi alla microsonda elettronica (EMP) e in spettroscopia Raman. Le inclusioni di fuso silicatico hanno un’origine primaria e sono state intrappolate durante la crescita del granato. Esse hanno dimensioni ≤ 10 µm e spesso mostrano un’evidente forma a cristallo negativo. All’interno dello stesso granato, sono state individuate tre tipologie di inclusioni: completamente cristallizzate (nanograniti), parzialmente cristallizzate ed inclusioni vetrose preservate. Le inclusioni cristallizzate contengono una paragenesi mineralogica granitica caratterizzata da quarzo, feldspati e miche. In questo lavoro di tesi è stato sviluppato un nuovo approccio per la riomogeneizzazione sperimentale delle inclusioni di fuso silicatico, effettuando esperimenti di rifusione a 5 kbar di pressione con il piston cylinder. Alla temperatura minima di rifusione di 700 °C, molte delle inclusioni nei granati sono completamente riomogeneizzate. Le inclusioni invece sono decrepitate e contengono bolle di CO2 quando vengono rifuse a più alta temperatura. Per la prima volta è stata costruita una pseudosezione utilizzando la composizione delle inclusioni completamente riomogeneizzate presenti nella metatessite. La pseudosezione indica temperature di intrappolamento del fuso silicatico molto vicine a quella minima di rifusione delle inclusioni (T ~700 °C). La composizione delle inclusioni riomogeneizzate a 700 °C è simile a quella delle inclusioni vetrose preservate nella stessa roccia. In generale, tutte le inclusioni di fuso silicatico nelle metatessiti e nelle miloniti hanno composizioni leucogranitche peralluminose. Tuttavia, molte delle inclusioni vetrose nella milonite mostrano un rapporto Na2O/K2O variabile e hanno contenuti primari di H2O (1.0-2.6 wt%) più bassi delle inclusioni nella metatessite (3.1-7.6 wt%). Una miglior comprensione dei processi di fusione si può avere combinando le informazioni ottenute dalle inclusioni di fuso silicatico e quelle dallo studio petrologico. I dati ottenuti in questo studio indicano che la fusione crostale a Ronda è avvenuta principalmente attraverso le reazione di fusione continua di deidratazione della biotite in condizioni di sottosaturazione di H2O, a temperature più basse di 800 °C. Nelle metatessiti, verso la base della successione crostale, la fusione probabilmente è iniziata al solidus (T ~700 °C) in presenza di una fase intergranulare ricca in H2O. Il granato peritettico che stava crescendo ha intrappolato delle piccole porzioni di fuso anatettico prodotto a temperature leggermente diverse, le cui composizioni erano controllate principalmente dalla diffusione. Le inclusioni sembrano registrare l’evoluzione della composizione del fuso durante la storia prograda. Molti leucosomi leucogranitici peralluminosi, invece, rappresentano dei fusi primari prodotti in condizioni prossime a quelle del picco metamorfico. I fusi anatettici a Ronda hanno una viscosità più alta di quella comunemente considerata per fusi granitici prodotti alle stesse condizioni di pressione e temperatura. Questa caratteristica implica tempi molto più lunghi per l’estrazione di questi fusi anatettici e la loro ascesa attraverso la crosta metasedimentaria. La consistenza dei dati composizionali ottenuti e l’accurato approccio multidisciplinare adottato in questa ricerca indicano che le inclusioni di fuso silicatico in minerali peritettici da migmatiti rappresentano l’unico strumento con cui studiare e caratterizzare le fasi più precoci dell’anatessi crostale

granitoid magmas preserved as melt inclusions in high grade metamorphic rock

This review presents a compositional database of primary anatectic granitoid magmas, entirely based on melt inclusions (MI) in high-grade metamorphic rocks. Although MI are well known to igneous petrologists and have been extensively studied in intrusive and extrusive rocks, MI in crustal rocks that have undergone anatexis (migmatites and granulites) are a novel subject of research. They are generally trapped along the heating path by peritectic phases produced by incongruent melting reactions. Primary MI in high-grade metamorphic rocks are small, commonly 5–10 μm in diameter, and their most common mineral host is peritectic garnet. In most cases inclusions have crystallized into a cryptocrystalline aggregate and contain a granitoid phase assemblage (nanogranitoid inclusions) with quartz, K-feldspar, plagioclase, and one or two mica depending on the particular circumstances. After their experimental remelting under high-confining pressure, nanogranitoid MI can be analyzed combining several techniques (EMP, LA-ICP-MS, NanoSIMS, Raman). The trapped melt is granitic and metaluminous to peraluminous, and sometimes granodioritic, tonalitic, and trondhjemitic in composition, in agreement with the different ![Formula][1] conditions of melting and protolith composition, and overlap the composition of experimental glasses produced at similar conditions. Being trapped along the up-temperature trajectory—as opposed to classic MI in igneous rocks formed during down-temperature magma crystallization—fundamental information provided by nanogranitoid MI is the pristine composition of the natural primary anatectic melt for the specific rock under investigation. So far ~600 nanogranitoid MI, coming from several occurrences from different geologic and geodynamic settings and ages, have been characterized. Although the compiled MI database should be expanded to other potential sources of crustal magmas, MI data collected so far can be already used as natural "starting-point" compositions to track the processes involved in formation and evolution of granitoid magmas. [1]: /embed/mml-math-1.gi

Primary crustal melt compositions: Insights into the controls, mechanisms and timing of generation from kinetics experiments and melt inclusions

We explore the controls, mechanisms and timing of generation of primary melts and their compositions, and show that the novel studies of melt inclusions in migmatites can provide important insights into the processes of crustal anatexis of a particular rock. Partial melting in the source region of granites is dependent on five main processes: (i) supply of heat; (ii) mineral–melt interface reactions associated with the detachment and supply of mineral components to the melt, (iii) diffusion in the melt, (iv) diffusion in minerals, and (v) recrystallization of minerals. As the kinetics of these several processes vary over several orders of magnitude, it is essential to evaluate in Nature which of these processes control the rate of melting, the composition of melts, and the extent to which residue–melt chemical equilibrium is attained under different circumstances. To shed light on these issues, we combine data from experimental and melt inclusion studies. First, data from an extensive experimental program on the kinetics of melting of crustal protoliths and diffusion in granite melt are used to set up the necessary framework that describes how primary melt compositions are established during crustal anatexis. Then, we use this reference frame and compare compositional trends from experiments with the composition of melt inclusions analyzed in particular migmatites. We show that, for the case of El Hoyazo anatectic enclaves in lavas, the composition of glassy melt inclusions provides important information on the nature and mechanisms of anatexis during the prograde suprasolidus history of these rocks, including melting temperatures and reactions, and extent of melt interconnection, melt homogenization and melt–residue equilibrium. Compositional trends in several of the rehomogenized melt inclusions in garnet from migmatites/granulites in anatectic terranes are consistent with diffusion in melt-controlled melting, though trace element compositions of melt inclusions and coexisting minerals are necessary to provide further clues on the nature of anatexis in these particular rocks.This work was supported by the National Science Foundation [grants EAR-9603199, EAR-9618867, EAR-9625517 and EAR-9404658], the Italian Consiglio Nazionale delle Ricerche, the European Commission (grant 01-LECEMA22F through contract No. ERAS-CT-2003-980409; and a H2020 Marie Skłodowska-Curie Actions under grant agreement No. 654606), the Italian Ministry of Education, University and Research (grants PRIN 2007278A22, 2010TT22SC and SIR RBSI14Y7PF), the Università degli Studi di Padova [Progetto di Ateneo CPDA107188/10 and a Piscopia—Marie Curie Fellowship under grant agreement No. 600376], the Australian Research Council (Australian Professorial Fellowship and Discovery Grants Nos. DP0342473 and DP0556700), and the National Research Foundation (South Africa; Incentives For Rated Researchers Program)

Thermochemical Energy Storage using Ca(OH)2/CaO reversible reaction

This work analyses a thermochemical energy storage process using Ca(OH)2/CaO chemical loop. A fluidized bed reactor is proposed to carry out the dehydration-hydration process (respectively charging and discharging stages) alternating reaction. The processes use superheated steam as the fluidizing agent. The model proposed is analyzed by sensitivity analysies, focused on the effects on the reactor performance when temperature and pressure of the inlet flows are changed. Also, the geometry of the reactor and the solid particle size distribution are investigated. Results obtained, in terms of solids conversion and heat duty required/provided from the reactor during the charging and discharging step respectively, leads to a possible integration whit a Rankine-Hirn cycle for electric production. The model of the study has been implemented by software Aspen Plus