Melt inclusions from the deep continental crust: recovering the starting composition of crustal melts
Abstract
Progress in the investigation of melting processes in the deep continental crust has been made in the last decade by studying melt inclusions in migmatites and granulites. Here, we investigate the inclusions hosted in peritectic garnet from metapelitic migmatites in the Ivrea Zone (NW Italy) to recover the starting composition (major elements and volatiles) of the anatectic melts throughout the upper amphibolite to granulite-facies zones. Inclusions have negative crystal shapes, size from 2 to10 μm and are regularly distributed in the core of the garnet. Micro-Raman investigation has shown the presence of two types of inclusions: crystallized silicate melt inclusions (i.e. nanogranitoids, hereafter NI) and fluid inclusions (FI).
A piston-cylinder apparatus was used to re-homogenize NI from the different zones at 820°C, 850°C and 900°C, and 1.2 GPa. Our data show that all the melts are leucogranitic but have slightly different compositions. In samples from the upper amphibolite facies, melts have lower Na2O/K2O(0.4-0.9), are relatively less peraluminous (ASI 1.1-1.5), less mafic (FeO+MgO 2-3.4 wt. %), contain 860-1700 ppm CO2 andreach the highest H2O contents (6.5-10 wt. %). In the transition zone melts are more peraluminous (ASI 1.1-1.8), reach higher Na2O/K2O (0.1-1.2), and have intermediate H2O (4.8-8.5 wt. %), CO2(457-1534 ppm) and maficity (FeO+MgO 2.3- 3.9 wt. %). The melts at granulite facies are peraluminous (ASI 1.2-1.3), reach highest Na2O/K2O (1.2-1.9), CaO, FeO+MgO (3.2-4.7 wt. %) and CO2(up to 2400 ppm), with H2O contents (5.4-8.3 wt. %) comparable with the other two zones. Our data suggests that anatexis of these metapelites occurred through muscovite and biotite melting but always in the presence of a COH fluid, in agreement with the presence of graphite. Changes in the composition of the melts are related to higher temperature of melting and changes of melting reactions, i.e. complete consumption of biotite and greater contribution of anhydrous reactants (plagioclase?). The H2O contents of the melts throughout the three zones are comparable, and higher than values assumed for such P-T conditions, while the CO2contents are highest at granulite facies, but similar to values expected at these P-T conditions. As expected, the aH2Oof the fluid dissolved in granitic melts decreases with increasing metamorphic grade.- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2018
- Bibcode:
- 2018AGUFM.V43D0145C
- Keywords:
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- 1036 Magma chamber processes;
- GEOCHEMISTRYDE: 1043 Fluid and melt inclusion geochemistry;
- GEOCHEMISTRYDE: 8430 Volcanic gases;
- VOLCANOLOGY