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

Abstract

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

The composition of nanogranitoids in migmatites overlying the Ronda peridotites (Betic Cordillera, S Spain): the anatectic history of a polymetamorphic basement

“…Recent studies have demonstrated that melt inclusions (MI) constitute a new and powerful tool to investigate crustal anatexis (Cesare 2008;Cesare et al 2009Cesare et al , 2011Cesare et al , 2015. Studies of MI in crustal anatectic enclaves found within peraluminous dacites of SE Spain have shown that these trapped droplets of melt, now solidified to glass due to rapid ascent and extrusion, can provide precise information on the composition of primary anatectic melts during, and on the mechanisms of, crustal anatexis (Cesare et al 1997(Cesare et al , 2003Acosta-Vigil et al 2007, 2012a.…”

“…A previous and fundamental step for the detailed studies on these crystallized MI has been the development and further refinement of appropriate methodologies to rehomogenize them successfully under high P, in order to prevent MI decrepitation and interaction with the host mineral and matrix rock (Bartoli et al 2013b; see also Malaspina et al 2006;Perchuk et al 2008). After application of this experimental methodology and subsequent analysis of several occurrences of nanogranites, Cesare et al (2015) have observed that the studied MI have bulk compositions varying from granitic to granodioritic, trondhjemitic and tonalitic, and hence they have renamed them as nanogranitoids. Barich et al (2014) have documented the presence of crystallized MI in Grt from strongly deformed, former high P granulitic migmatites overlying the Ronda peridotite slab, in the Betic Cordillera of S Spain (hereafter mineral abbreviations after Kretz 1983).…”

Modelagem termodinâmica de fusão parcial e metamorfismo em condições de fácies granulito: exemplo do complexo Itatins, SP

“…Melt compositions trends in mesonorm diagrams plot over the anticipated areas, when compared with leucosome compositions from metasedimentary protoliths from Sawyer (2008). It also shows that the melt produced, after accumulation or extraction of 15 mol %, with garnet or cordierite stable in residual assemblage, has similar compositions to melt inclusions trapped in garnet or cordierite reported by Cesare et al (2015). This indicates that melt inclusions reflect not the composition of the "initial" melt, but the composition of the melt generated with a particular peritectic phase, as was proposed by Cesare et al, (2015).…”

“…The melt compositions plotted in fig.11 show a superposition with the field from the compositions of the melt inclusions given by Cesare et al, (2015). This correlation indicates that melt produced with peritectic phases, either garnet or cordierite, from hydrate-breakdown melting, displays compositions close to those analysed from natural samples.…”

“…Methods used for this task are based on experimental petrology (Vielzeuf & Holloway, 1988;Vielzeuf & Montel, 1994;Montel & Vielzeuf, 1997;Stevens, Clemens, Droop, 1997;Patiño-Douce & Johnston, 1997;Patiño-Douce & Harris, 1998), melt inclusions (Cesare et al, 2007;Cesare et al, 2009;Ferrero et al, 2012;Bartolli et al, 2013;Bartolli et al, 2014;Cesare et al, 2015) mass-balance calculations based on stoichiometry of melt-forming reactions (Hasalová et al, 2008), chemical analysis of leucosome and using thermodynamic calculations (White et al, 2011 and2016). Nonetheless, results provided by the methods above may not represent the "true" composition of anatectic melt, as melt produced by experiments involves very small volumes of material, including natural starting materials, in a closed-system and a very short time-scale for melting.…”