Séminaire Site Buffon - Lundi 31 mars 2014 à 11 h

IMPMC - Site Buffon - 61, rue Buffon, Paris 5e

Salle des élèves - 2e étage

Melt inclusions in migmatites and granulites

Bernardo Cesare - Department of Geosciences, University of Padova, Italy

 

 

Abstract

The study of melt inclusions is a recent, small-scale approach to a better understanding of melting in the continental crust. It builds on the discovery of glassy inclusions and their crystallized counterparts (nanogranites) in garnet and other host minerals from anatectic crustal enclaves in lavas and from regional migmatites and granulites.

Produced by an incongruent melting reaction, e.g., Bt+Sil+Pl+Qtz=Grt+liquid±Kfs, a growing peritectic phase (i.e. garnet) can trap droplets of the anatectic melt that is being formed by the same reaction. Hence, anatectic melt inclusions are trapped during the prograde history of magma formation and, therefore, they differ from the “classic” inclusions found in minerals from igneous rocks, that are trapped during magma crystallization upon cooling.

As the composition of anatectic inclusions is representative of that of the bulk melt in the system during crustal melting, these tiny objects (rarely exceeding 15 mm) represent embryos of anatectic granites. With an appropriate characterization and analytical strategy they can provide the missing information on the primary composition of natural crustal melts before they undergo modification processes such as cumulus, fractional crystallization, mixing or entraiment of exotic material.

In slowly cooled, regional migmatites, melt inclusions are generally completely crystallized as nanogranite: a cryptocrystalline aggregate of quartz, feldspars and micas. After the first discovery in the granulites of the Kerala Khondalite Belt (India), in the last decade we have studied inclusions from a dozen of occurrences worldwide: while glassy inclusions can be directly analysed with minimal sample preparation, nanogranites need to be remelted to a homogeneous liquid and then quenched. Such remelting is achieved in a piston cylinder apparatus, to prevent the decrepitation of inclusions and loss of volatiles.

We have analysed inclusions for major and trace elements, and also for H2O: the melts are all leucogranitic and peraluminous, but show important variations in normative Qtz-Ab-Or proportions, as well as in H2O contents, and generally differ from minimum melts.

Since H2O is one of the main parameters determining melt viscosities and, in turn, the strength of partially melted rocks, the characterization of the fluid contents of nanogranites provides more realistic constraints to the rheological behaviour of the deep crust, and to the timescales of melt extraction from it, and allows better constrained numerical modelling.

Melt inclusions have opened new perspectives both for the microstructural approach to partially melted rocks and for the chemical characterization of natural crustal melts. These perspectives can now be transferred to different systems (e.g. mafic protoliths) and differerent settings (e.g. subduction zones and melting at HP conditions).