Trapped magma droplets leave clues to the make-up of partially melted Earth’s crust
The crust is the absolute outermost layer of the Earth and constitutes less than 1 % of the Earth's volume. Composed of a variety of igneous, metamorphic and sedimentary rocks, it is arranged in a series of tectonic plates. Migmatites are partially melted metamorphic rocks where granite (igneous) rock is originated. Partial melting is what happens when only some parts of a rock melt and it takes place because rocks are heated to temperatures exceeding 700 °C deep in the crust. Important as is a key process in the evolution, partial melting is responsible for the compositional differentiation of the Earth’s crust. Further knowledge about these processes is hampered by the absence of precise information on the composition of natural crustal melts.
A unique approach integrating observations and experimental work
Researchers working on the EU-funded NANOGRANITES project had previously pioneered a cutting-edge approach to studying partially melted rocks (migmatites) to determine the composition of natural crustal melts. “Studies on nanogranites represent a paradigm shift in crustal melting, adding new information to the experimental and theoretical approaches used for over 50 years. We can now determine (rather than infer or model), precisely and in situ, the composition of the magma produced in a specific rock under certain geological site conditions,” notes project coordinator Bernardo Cesare. Nanogranites derive their name from their tiny grain size: when they crystallise in an aggregate of minerals, their size ranges on the sub-micron scale. Observations over time indicated the presence of trapped materials other than granite in these melt inclusions so nanogranites are called nanogranitoids since 2015. “Sheltered in the host rock, nanogranites’ presence is often the sole way to infer that a rock had partially melted. Importantly, they constitute the only tool to probe the processes of magma formation in the crust and to analyse the composition of natural crustal melts,” notes Cesare.
Investigating the volatile content of nanogranites
A major challenge was to find out more about the amount of different volatile elements that nanogranites contain. Volatile composition can strongly influence the temperature range in which magma forms, the amount of magma produced as well as its viscosity and explosivity. In the lab, researchers successfully reproduced the pressures and temperatures that typically prevail in the deep crust and re-melted nanogranites into tiny droplets. Analysis results revealed the entire chemical composition of specific rocks in terms of major, trace and volatile element concentrations. This cutting-edge re-melting approach, developed at the University of Padova, has so far only been adopted in three laboratories in the world. The team conducted 60 experiments of re-melting nanogranites. Partial melting of rocks was achieved by using a piston cylinder apparatus which generates high pressures and temperatures. Samples under study stemmed from well-characterised geological site conditions. One specimen came from diamond-bearing ultra-high pressure rocks that are typically found in deep crustal subduction zones. Analysis of the experimentally produced crustal melts enabled researchers to refine a geochemical database of natural crustal melts that is widely used by the international geological community. Project results were disseminated through seminars, conferences and two publications in international peer-reviewed journals. “Nanogranites can open new windows of understanding of processes occuring in the Earth’s crust using experimental methods such as the re-melting method devised originally by our project,” concludes Cesare.
Keywords
NANOGRANITES, magma, crustal melt, partial melting, migmatite, nanogranitoids