Anatexis and fluid regime of the deep continental crust: New clues from melt and fluid inclusions in metapelitic migmatites from Ivrea Zone (NW Italy)

Abstract

We investigate the inclusions hosted in peritectic garnet from metapelitic migmatites of the Kinzigite Formation (Ivrea Zone, NW Italy) to evaluate the starting composition of the anatectic melt and fluid regime during anatexis throughout the upper amphibolite facies, transition, and granulite facies zones. Inclusions have negative crystal shapes, sizes from 2 to 10 μm and are regularly distributed in the core of the garnet. Microstructural and micro‐Raman investigations indicate the presence of two types of inclusions: crystallized silicate melt inclusions (i.e., nanogranitoids, NI), and fluid inclusions (FI). Microstructural evidence suggests that FI and NI coexist in the same cluster and are primary (i.e., were trapped simultaneously during garnet growth). FI have similar compositions in the three zones and comprise variable proportions of CO2, CH4, and N2, commonly with siderite, pyrophyllite, and kaolinite, suggesting a COHN composition of the trapped fluid. The mineral assemblage in the NI contains K‐feldspar, plagioclase, quartz, biotite, muscovite, chlorite, graphite and, rarely, calcite. Polymorphs such as kumdykolite, cristobalite, tridymite, and less commonly kokchetavite, were also found. Rehomogenized NI from the different zones show that all the melts are leucogranitic but have slightly different compositions. In samples from the upper amphibolite facies, melts are less mafic (FeO + MgO = 2.0–3.4 wt%), contain 860–1700 ppm CO2 and reach the highest H2O contents (6.5–10 wt%). In the transition zone melts have intermediate H2O (4.8–8.5 wt%), CO2 (457–1534 ppm) and maficity (FeO + MgO = 2.3–3.9 wt%). In contrast, melts at granulite facies reach highest CaO, FeO + MgO (3.2–4.7 wt%), and CO2 (up to 2,400 ppm), with H2O contents comparable (5.4–8.3 wt%) to the other two zones. Our results represent the first clear evidence for carbonic fluid‐present melting in the Ivrea Zone. Anatexis of metapelites occurred through muscovite and biotite breakdown melting in the presence of a COH fluid, in a situation of fluid–melt immiscibility. The fluid is assumed to have been internally derived, produced initially by devolatilization of hydrous silicates in the graphitic protolith, then as a result of oxidation of carbon by consumption of Fe3+‐bearing biotite during melting. Variations in the compositions of the melts are interpreted to result from higher T of melting. The H2O contents of the melts throughout the three zones are higher than usually assumed for initial H2O contents of anatectic melts. The CO2 contents are highest at granulite facies, and show that carbon‐contents of crustal magmas are not negligible at high T. The activity of H2O of the fluid dissolved in granitic melts decreases with increasing metamorphic grade. Carbonic fluid‐present melting of the deep continental crust represents, together with hydrate‐breakdown melting reactions, an important process in the origin of crustal anatectic granitoids.

High-pressure, halogen-bearing melt preserved in ultrahigh-temperature felsic granulites of the Central Maine Terrane, Connecticut (U.S.A.)

“…DOI: https://doi.org/10.2138/am-2021-7690. http://www.minsocam.org/ that CMT nanogranitoids likely formed at significantly greater depth than IVZ nanogranitoids, 1.7-2.0 GPa (this study) vs. 0.8 GPa (Carvalho et al, 2019).…”

“…This CO 2 may be internally produced as result of the devolatilization of hydrous silicates in the presence of graphite (Connolly and Cesare, 1993;Cesare et al, 2005) visible in the CMT felsic granulites as trapped minerals in inclusions and/or as mineral inclusions in garnet and the rock matrix (Axler and Ague 2015). The CO 2 content of the CMT nanogranitoids is higher, on average, than in the nanogranitoids measured in the graphite-bearing metasediments of the Ivrea-Verbano Zone (IVZ; Carvalho et al, 2019), the only other case study so far where CO 2 was measured in situ in re-homogenized nanogranitoids. It is likely that the different depth at which melting occurred played a major role in shaping the CO 2 content of the melt.…”

“…NanoSIMS analyses on re-homogenized nanogranitoids is a routine approach in nanogranitoid studies (e.g. Acosta-Vigil et al 2016), and it has shown that nanogranitoids generally preserve their original composition in terms of H 2 O (Bartoli et al, 2014) and CO 2 contents (Carvalho et al, 2019). In particular, the loss of H 2 O (even as OH) from the This is the peer-reviewed, final accepted version for American Mineralogist, published by the Mineralogical Society of America.…”

“…Piston cylinder re-homogenization followed by in situ characterization is a common and wellestablished approach for nanogranitoids studies to follow in order to (1) prove that they originally were indeed droplets of melt, and (2) characterize geochemistry and fluid content and speciation content of anatectic melt (e.g. Acosta-Vigil et al, 2016;Bartoli et al, 2016;Carvalho et al, 2019;Bartoli et al, 2019). Our detailed experimental and microchemical work confirmed that the CMT multiphase inclusions were originally a hydrous and halogen-rich granitic melt formed at T in excess of 1000°C, the hottest preserved melt so far found in natural metasedimentary protolith.…”

Dmisteinbergite, CaAl2Si2O8, a Metastable Polymorph of Anorthite: Crystal-Structure and Raman Spectroscopic Study of the Holotype Specimen

“…Feldspars are the most common rock-forming minerals of the Earth's crust, and constitute more than 50% of the continental crust [1]. Recent investigations revealed the existence of a number of metastable feldspar polymorphs in relevant melt inclusions [2][3][4][5], meteorites [6][7][8][9], high-pressure and ultra-high-pressure rocks [10][11][12][13][14][15][16], pyrometamorphic formations [17], and pseudotachylytes [18][19][20]. Similar to rock-forming feldspars, these metastable phases are based upon tetrahedrally coordinated Al 3+ and Si 4+ cations, but with drastically different structural topologies from the feldspar framework topology.…”