Nanogranite inclusions in migmatitic garnet: behavior during piston‐cylinder remelting experiments

Abstract

Nanogranites represent totally crystallized inclusions of anatectic melt trapped within peritectic minerals of\ud migmatites and granulites. They have recently been discovered in several locations. This discovery opens new\ud possibilities for investigating crustal melting processes, provided that an appropriate method for retrieving the\ud information contained within nanogranite inclusions is available. Here, we describe a series of remelting experiments\ud that have been performed at different temperatures and under dry, and H2O-added, conditions on nanogranite\ud inclusions hosted in migmatitic garnet, using a piston-cylinder apparatus. The glasses obtained by\ud quenching the sample from temperature that approaches the trapping temperature have compositions very similar\ud to those of preserved glassy inclusions coexisting with nanogranites in the same cluster. No significant differences\ud in H2O contents were observed for nanogranites rehomogenized under dry and wet conditions. Higher\ud (50–100°C) experimental temperatures resulted in dissolution of the host into the melt and inclusion decrepitation\ud with the loss of volatiles. Therefore, piston-cylinder remelting experiments may eliminate inclusion decrepitation,\ud maintaining the primary fluid contents in the originally trapped melt. These volatiles would otherwise be lost during\ud remelting experiments at ambient pressure. By preventing volatile loss, the inclusion does not have to be\ud overheated to achieve homogenization, and the compositions of quenched glasses so obtained can be assumed\ud to be those of melts produced (and trapped as inclusions) during crustal anatexis. The experimental approach\ud described here represents a promising technique for the successful rehomogenization of crystallized melt inclusions\ud from high-pressure environments, such as the mafic continental crust

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

“…Because anatexis takes place in rocks of the middle to lower continental crust, at the bottom of thickened continental crust, and even in continental materials subducted to mantle depths, re-melting experiments of nanogranitoids must be performed under high confining pressure, using a piston-cylinder apparatus, to prevent the inclusion from decrepitating, volatile loss, and melt contamination. Indeed, as a consequence of the H 2 O loss, crystallized inclusions can be completely re-melted only at temperatures higher than the trappingi.e., H 2 O loss results in an increased solidus temperature-leading to an inevitable melt-host interaction (see details in Bartoli et al 2013c). The first attempts to remelt nanogranites using the routine technique in igneous petrology, namely the high-temperature heating stage at ambient pressure (Esposito et al 2012), produced the decrepitation of the nanogranitoid inclusions, making them poorly suited for a geochemical study (Cesare et al 2009).…”

“…The P-T conditions to be used during piston-cylinder remelting should be initially established based on phase-equilibria modeling or classic thermobarometry, which can provide a fair estimate of the (presumed) P-T conditions of MI entrapment (Bartoli et al 2013a(Bartoli et al , 2013cFerrero et al 2015). Conditions of further experiments are then fine-tuned based on the results of the first runs.…”

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

“…Remelting experiments of MI contained within fragments of Grt followed the methodology described by Bartoli et al (2013b), and were conducted using a single-stage, piston-cylinder apparatus at the Laboratory of Experimental Petrology, Dipartimento di Scienze della Terra (Università di Milano, Italy). Chips and fragments of Grt were loaded into Au capsules with external diameter of 3 mm and 5 mm, respectively, together with powdered silica to isolate Grt pieces from each other.…”

“…Sawyer 1996Sawyer , 2008; and references therein), (ii) glass analyses from the relatively few reported studies of glassy MI in metasedimentary/metagranitic anatectic enclaves and rehomogenized nanogranitoids in migmatites and granulites (Cesare et al 1997;2009;Acosta-Vigil et al 2007;Ferrero et al 2012Ferrero et al , 2014Ferrero et al , 2015Bartoli et al 2013aBartoli et al , 2013b press), and (iii) Helz 1976;Rutter and Wyllie 1988;Hacker 1990;Rushmer 1991;Rapp et al 1991;Wolf and Wyllie 1994;Rapp and Watson 1995;Skjerlie and Patiño Douce 2002;Schmidt et al 2004;Laurie and Stevens 2012;Quian and Hermann 2013).…”

“…Most of the MI in crystalline rocks, however, appear today crystallized to a granitic assemblage due to slow cooling at depth, with grain size commonly <1 µm; Cesare et al (2009) named them as nanogranites. 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.…”