In the modern Earth, plate tectonics allows for the reworking of surface material (i.e. erosion/weathering – burial – partial melting) in active convergent settings. This process mobilises and drags a substantial volume of mineral-bound volatiles (i.e. CO₂, H₂O) to lower crustal and mantle depths. Most of these volatiles, chief among them carbon, rapidly find their way back to the surface through volcanic activity in subduction zones(Horton, 2021), thus maintaining surface habitability. However, due to its relative stability and longevity^, the partially melted metamorphic continental crust can isolate a significant amount of carbon from the atmosphere for protracted periods of time, modulating the carbon cycle over hundred million of years (Nicoli & Ferrero, 2021).

Over the last decade, our understanding of crustal differentiation has markedly improved thanks to the discovery of melt and fluid inclusions in deep crustal lithologies (Bartoli & Cesare, 2020). The landmark study by Cesare et al. (2009) demonstrated that during partial melting of the lower crust, pristine melt can be trapped and preserved as inclusions within newly formed minerals (e.g. garnet) in regionally metamorphosed granulite and high-pressure rocks. Such melt inclusions have commonly crystallised and are composed of an aggregate of micrometric crystals (i.e. nanorocks or nanogranitoids), although glassy inclusions may also be present, and might occur along with fluid inclusions (CO₂, CH₄, N₂, H₂O) (Carvalho et al., 2019; 2020). Unlike volcanic glass inclusions in magmatic bodies, they preserve the original melt composition (major, trace element and volatile content) at the time of entrapment at peak metamorphic conditions (Bartoli et al., 2014). Analytical developments, such as the advent of nanoscale and time-of-flight secondary ion mass spectrometry, now permit us to retrieve precise three-dimensional microstructural, chemical and isotopic information at the scale < 100 nm (Ferrero et al., 2021; Parisatto et al., 2018). Hence, these “messages in the bottle” (Clemens, 2018) are invaluable archives, which contain clues for the evolution of the solid Earth.

Our recent studies (Nicoli & Ferrero, 2021; Nicoli et al., 2022; Borghini et al., 20223) have shown that melt inclusions in metamorphic rocks can be used to retrieve information on the carbon content of the supracrustal protolith and constrain the volatile budget of mountain belts. Our new flux estimates for the burial of continental material in Phanerozoic continental collision and continental subduction settings rival those of active volcanoes (Lee et al., 2019), making the continental crust a key player in the evolution of the carbon cycle.

Carbon budget of the deep and ultra-deep crust references

Borghini, A., Nicoli, G., Ferrero, S., O’Brien, P.J., Laurent, O., Remusat, L., Borghini, G. and Milani, S. (2023). The role of continental subduction in mantle metasomatism and carbon recycling revealed by melt inclusions in UHP eclogites. Science Advances, 9(6).

Nicoli, G., Borghini, A., & Ferrero, S. (2022). The carbon budget of crustal reworking during continental collision: Clues from nanorocks and fluid inclusions. Chemical Geology, 608, 121025.

Nicoli, G., and Ferrero, S. (2021). Nanorocks, volatiles and plate tectonics. Geoscience Frontiers, 101188.

Other references

Bartoli, O., & Cesare, B. (2020). Nanorocks: a 10-year-old story. Rendiconti Lincei. Scienze Fisiche e Naturali, 31, 249-257. Bartoli, O., Cesare, B., Remusat, L., Acosta-Vigil, A., & Poli, S. (2014). The H2O content of granite embryos. Earth and Planetary Science Letters, 395, 281-290.

Carvalho, B. B., Bartoli, O., Cesare, B., Tacchetto, T., Gianola, O., Ferri, F., … & Szabó, C. (2020). Primary CO2-bearing fluid inclusions in granulitic garnet usually do not survive. Earth and Planetary Science Letters, 536, 116170.

Carvalho, B. B., Bartoli, O., Ferri, F., Cesare, B., Ferrero, S., Remusat, L., … & Poli, S. (2019). Anatexis and fluid regime of the deep continental crust: new clues from melt and fluid inclusions in metapelitic migmatites from Ivrea Zone (NW Italy). Journal of Metamorphic Geology, 37(7), 951-975.

Cesare, B., Ferrero, S., Salvioli-Mariani, E., Pedron, D., & Cavallo, A. (2009). “Nanogranite” and glassy inclusions: The anatectic melt in migmatites and granulites. Geology, 37(7), 627-630.

Clemens, J. D. (2009). The message in the bottle:“Melt” inclusions in migmatitic garnets. Geology, 37(7), 671-672.

Ferrero, S., Ague, J. J., O’Brien, P. J., Wunder, B., Remusat, L., Ziemann, M. A., & Axler, J. (2021). High-pressure, halogen-bearing melt preserved in ultrahigh-temperature felsic granulites of the Central Maine Terrane, Connecticut (USA). American Mineralogist: Journal of Earth and Planetary Materials, 106(8), 1225-1236.

Horton, F. (2021). Rapid recycling of subducted sedimentary carbon revealed by Afghanistan carbonatite volcano. Nature Geoscience, 14(7), 508-512.

Lee, C. T. A., Jiang, H., Dasgupta, R., & Torres, M. (2019). A framework for understanding whole-Earth carbon cycling. In Deep Carbon: Past to Present (pp. 313-357). Cambridge University Press.

Parisatto, M., Turina, A., Cruciani, G., Mancini, L., Peruzzo, L., & Cesare, B. (2018). Three-dimensional distribution of primary melt inclusions in garnets by X-ray microtomography. American Mineralogist: Journal of Earth and Planetary Materials, 103(6), 911-926.

A great triptych of papers by Prof. Marian Holness in Journal of Petrology on convection in tabular intrusions and our work on the Isle of #Skye #Scotland #olivinehttps://t.co/qB4yBXKxaNhttps://t.co/vR4MJRQyIGhttps://t.co/bK4Mw3mBj2@EarthSciCam @GeowPotsdam @UoBEarthScience pic.twitter.com/SP3PNEgZZE

— Gautier Nicoli (@geolegologist) November 22, 2022

A great triptych of papers by Prof. Marian Holness in Journal of Petrology on convection in tabular intrusions and our work on the Isle of Skye, Scotland

Presentation at the 2022 EGU on melt inclusions in the Adirondacks garnets.

ABSTRACT: The Adirondack Mountains, New York State, USA belongs to the Canadian Grenville Province (Darling and Peck, 2016). The rocks exposed in the Adirondacks are interpreted to be the lower plate of a thrust-system at crustal levels during the Ottawan Orogeny (1090-1050 Ma) of the Grenvillian orogenic cycle. Garnet is abundant throughout the Adirondacks, with the greatest occurrence of megacrystic garnets within central Highlands. In the Gore Mountain area, the Hooper Mine is located 5 kilometers northwest of the Barton Mine and consists of partially melted mafic granulite. The mineral assemblage consists of medium grain size plagioclase, green hornblende and garnet in proportion 60:20:20. We separated the garnets of the Hooper Mine in two categories according to their size, chemical zoning and habitus: (1) Large, euhedral garnet porphyroblasts of diameter > 5 cm (LG), and (2) and small, xenoblastic grains (SG). Both types of garnets contain quartz, rutile and melt inclusions, similar to those observed in Barton Mine (Ferrero et al., 2021). In LG, chemical zoning is weak and inclusions are scattered randomly within the mineral. In SG, zoning coincides with the presence of quartz and melt inclusions in domain of low Ca and Y. Ti-in-quartz and Ti-in-amphibole thermometers in SG give equilibrium temperatures of 800-900 °C at 10 kbar.

Major and trace element analyses on rehomogenised melt inclusions in both types of garnet indicate two types of melts are present in the migmatite – granitic melt in SG and trondhjemitic melt in LG. Stable isotope ratios of oxygen and hydrogen in hornblende (δ²H: -62 to -73 ⁰/₀₀ and δ¹⁸O: 4.7 to 6.7 ⁰/₀₀) indicate that partial melting occurs in a closed isotopic system and records the primary magmatic δ²H signature of the protolith. The range of melt chemistries, combined with the information previously collected in the Barton Mine defines a trend characteristic of primitive TTG melts or TTG embryos. These melts, combined with different proportion of peritectic phases (i.e. garnet, plagioclase and quartz), reproduces the full TTG chemistry range (Moyen, 2011). Therefore, the Mesoproterozoic mafic lower crust might be a perfect laboratory to test early granitoids genesis processes and better understand the link between melt inclusions, plate tectonics and the formation of the continental crust (Nicoli & Ferrero, 2021). 

How to cite: Nicoli, G., Ferrero, S., Darling, R., Yakymchuk, C., Wunder, B., and Tollan, P.: Multiple partial melts trapped in garnets from the Adirondacks lower crust: clues for TTG formation, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-2271, https://doi.org/10.5194/egusphere-egu22-2271, 2022.

MSG Video Contest

Have you ever worried being invited to a social gathering and after a series of small talks being asked the fatidic question: “So, which rocks do you like the best?”. Don’t panic, we are here to help. We put together a short video which explains why metamorphic rocks are the best. In less than two minutes, Ms. Garnet will take you on a journey of a lifetime, from the surface of the Earth to the hot core of mountain belts! With this new knowledge, you’ll be able to impress friends and family.

For the past two years I started communicating my research more frequently via social media (Twitter, Instagram) by drawing a series of cartoon to advertise new findings, conferences or just fun facts about geology in general. The MSG video contest has then an excellent opportunity to try to animate some of my drawings.

Beside being a very enjoyable thing to do, the main motivation for this project is to engage people of all ages, gender and background in STEM. The use of cartoons and humour by linking geology with pop culture references is an easy way to reach both expert audiences and the general public.