Omar Bartoli

Recovering the composition of melt and the fluid regime at the onset of crustal anatexis and S-type granite formation

Using a metatexite from the Spanish Betic Cordillera as an example, we show that in situ and otherwise impossible to retrieve compositional information on natural anatectic melts can be reliably gained from experimentally rehomogenized melt inclusions in peritectic garnets. Experiments were conducted on single garnet crystals in a piston cylinder apparatus until the complete homogenization of crystal-bearing melt inclusions at the conditions inferred for the anatexis. The compositions of quenched glasses, representative of the early anatectic melts, are leucogranitic and peraluminous, and differ from those of leucosomes in the host rock. The H 2 O contents in the glasses suggest that melts formed at low temperature (∼700 °C) may not be as hydrous and mobile as thought. Providing for the first time the precise melt composition (including the volatile components) in the specific anatectic rock under study, our approach improves our understanding of crustal melting and generation of S-type granites.

Application of the Linkam TS1400XY heating stage to melt inclusion studies

Melt inclusions (MI) trapped in igneous phenocrysts provide one of the best tools available for characterizing magmatic processes. Some MI experience post-entrapment modifications, including crystallization of material on the walls, formation of a vapor bubble containing volatiles originally dissolved in the melt, or partial to complete crystallization of the melt. In these cases, laboratory heating may be necessary to return the MI to its original homogeneous melt state, followed by rapid quenching of the melt to produce a homogeneous glass phase, before microanalyses can be undertaken.Here we describe a series of heating experiments that have been performed on crystallized MI hosted in olivine, clinopyroxene and quartz phenocrysts, using the Linkam TS1400XY microscope heating stage. During the experiments, we have recorded the melting behaviors of the MI up to a maximum temperature of 1360°C. In most of the experiments, the MI were homogenized completely (without crystals or bubbles) and remained homogeneous during quenching to room temperature. The resulting single phase MI contained a homogeneous glass phase. These tests demonstrate the applicability of the Linkam TS1400XY microscope heating stage to homogenize and quench MI to produce homogeneous glasses that can be analyzed with various techniques such as Electron Microprobe (EMP), Secondary Ion Mass Spectrometry (SIMS), Laser ablation Inductively Coupled Plasma Mass Spectrometry (LA ICP-MS), Raman spectroscopy, FTIR spectroscopy, etc.During heating experiments, the optical quality varied greatly between samples and was a function of not only the temperature of observation, but also on the amount of matrix glass attached to the phenocryst, the presence of other MI in the sample which are connected to the outside of the crystal, and the existence of mineral inclusions in the host.

Granitoid magmas preserved as melt inclusions in high-grade metamorphic rock

Abstract 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 P-T-aH2O 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.

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.

Amphiboles and clinopyroxenes from Euganean (NE Italy) cumulus enclaves: evidence of subduction-related melts below Adria microplate

Euganean Hills are a magmatic district belonging to the Veneto Volcanic Province, whose magmatism developed during an extensional tectonic regime within the Alpine orogenesis in an intra-plate setting (the Adria microplate). Mafic and ultramafic cumulus enclaves occur within the Euganean trachytes. We estimated the trace element composition of liquids in equilibrium with cumulus minerals, employing a set of partition coefficients. Parental melts of cumulus clinopyroxenes are characterized by a marked enrichment in LILE, Th and U relative to N-MORB. Conversely, HREE and HFSE concentrations resemble N-MORB contents. These geochemical signatures are typical of subduction-related magmas, and also characterized the parental melts of Adamello cumulates. Conversely, Veneto Volcanic Province mafic lavas show geochemical patterns typical of anorogenic magmas. Therefore, those rocks are not cogenetic with Euganean cumulates, which are interpreted as crystallized from Alpine subduction-related basaltic magmas. These cumulates were subsequently dismembered and transported to shallower levels by ascending lavas related to the Veneto Volcanic Province magmatism. Therefore, magmatic products related to Alpine subduction are more widespread beneath the Adria microplate than previously known.