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A new understanding of kimberlite magmas from deep Earth to diamond mines

Applicant Giuliani Andrea
Number 180126
Funding scheme Ambizione
Research institution Institut für Geochemie und Petrologie ETH Zürich
Institution of higher education ETH Zurich - ETHZ
Main discipline Geochemistry
Start/End 01.06.2019 - 31.05.2023
Approved amount 952'334.00
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All Disciplines (2)

Discipline
Geochemistry
Geochronology

Keywords (5)

isotope geochemistry; cycling of volatiles; diamond; kimberlite; mantle evolution

Lay Summary (Italian)

Lead
In sintesiLe kimberliti sono rare rocce vulcaniche prodotte dai magmi più profondi che esistano sulla superficie della Terra (probabilmente oltre 500 kilometri di profondità). Per questo motivo le kimberliti possono fornire informazioni uniche sulla composizione delle viscere del nostro pianeta. Queste rocce hanno un ruolo unico nelle Science della Terra anche perché sono la principale risorsa di diamanti per utilizzo industriale e nella gioielleria. Nonostante questo, la nostra comprensione della composizione e origine delle kimberliti rimane limitata. Inoltre, la relazione tra magmi kimberlitici e i diamanti che le kimberliti contengono non è mai stata studiata in dettaglio, e potrebbe rivelarsi importante per migliorare le tecniche di stima delle risorse diamantifere.
Lay summary

Soggetto e obiettivo

 

Questo progetto ha tre obiettivi: i) capire qual è la composizione dei magmi kimberlitici studiando kimberliti da una collezione unica di rocce non alterate provenienti da tutto il mondo, incluse miniere di diamanti attive; ii) esaminare una potenziale correlazione tra la composizione chimica delle kimberliti e il contenuto e la qualità dei diamanti che le kimberliti contengono; iii) migliorare la nostra comprensione della composizione ed evoluzione della Terra profonda utilizzando rocce kimberlitiche di età variabile (da 2 miliardi a 50 milioni di anni fa) e provenienti da tutti i continenti.

 

 

Contesto socio-scientifico

 

Questo progetto contribuirà a migliorare la limitata conoscenza che abbiamo dei processi di fusione parziale delle rocce che avvengono nelle profondità della Terra, ma che sono estremamente importanti nel controllare l’evoluzione dell’atmosfera e, quindi, l’abitabilità del nostro pianeta. Il mio lavoro fornirà nuove e importanti informazioni sulla composizione della Terra profonda da utilizzare nei modelli che simulano la formazione ed evoluzione della Terra. Infine, è possibile che i risultati di questo progetto siano di aiuto all’industria dei diamanti per capire più facilmente se una kimberlite contenga diamanti in abbondanza e di buona qualità, quindi limitando i costi di valutazione di potenziali depositi diamantiferi.

Direct link to Lay Summary Last update: 29.08.2018

Responsible applicant and co-applicants

Employees

Abstract

Kimberlites are volatile-rich (CO2 and H2O) volcanic rocks, which derive from the deepest melts (>150 and, perhaps, >500 km) erupted at Earth’s surface, and are the primary host rocks to diamonds. Kimberlites occur on every continent and were emplaced in the upper crust since 2.7 Ga, and potentially earlier. These rocks can therefore provide unique insights into the composition and evolution of the deep Earth in space and time. However, the composition of primary kimberlite melts is a hotly debated issue with proposals ranging from H2O-rich ultramafic (i.e. silicate) melts to carbonate-dominated compositions. This is due to contamination of kimberlite melts by mantle and crustal rocks during magma ascent and emplacement, and near surface alteration of primary kimberlite mineralogy. The makeup of deep kimberlite sources and the role of recycled crustal material in kimberlite petrogenesis are equally not well understood. Worldwide kimberlites older than >250 Ma show Nd-Hf isotope compositions consistent with derivation from a relatively homogeneous convecting mantle; conversely, recycled components may have contributed to the deep mantle sources of <250 Ma-old kimberlites from Africa, Canada and, potentially, other regions.Lastly, due to early recognition that diamonds are mantle-derived xenocrysts (i.e. foreign crystals) in kimberlites, few attempts have been made to relate the geochemical features of kimberlites to the grade and/or quality of diamonds they host. Recent observations have inferred that percolation of early CO2-rich kimberlite-related fluids/melts in the mantle causes diamond resorption. Early kimberlitic metasomatism (i.e. fluid-mediated enrichment) also affects the composition of kimberlite magmas erupted at surface due to assimilation of previously metasomatised mantle material. The geochemical composition of kimberlites might therefore provide unexplored clues into the grade/quality of hosted diamonds, with potential revolutionary implications for the diamond exploration industry.In this project, I will tackle these questions with a multi-technique approach, which combines detailed petrological studies with examinations of stable and radiogenic isotope compositions of worldwide kimberlites. The petrogenesis of 16 kimberlites, which have been selected for their freshness and to cover different continents, settings (i.e. on- and off-craton), ages (Neoproterozoic to Eocene) and diamond contents (high grade to uneconomic), will be studied in detail by coupling petrography, mineral chemistry and isotope geochemistry. A recently developed method to reconstruct the composition of parental kimberlite melts, including their evolution during ascent, will be applied to provide unrivalled constraints on the compositions of kimberlite melts at depths and their variations in space and time. The S, N and B isotope compositions of these and other kimberlites will be explored to constrain the potential occurrence of recycled crustal material in kimberlite sources. Finally, additional insights into the evolution of the deep mantle will be gained by enriching the global kimberlite dataset with new Sr, Nd, Hf isotope and age determinations for samples from little explored kimberlite provinces in South and Western Australia, Finland, north-western Russia and Angola, together with additional data for north American kimberlites.Understanding how these enigmatic magmas form and unravelling the composition of their deep sources will provide a fundamental step forward in the Earth Sciences. This project will provide new input into modelling of partial melting processes in the deep Earth, which control the long-term evolution of the atmosphere and, ultimately, the habitability of our planet. It will deliver new insights into the global cycling of volatiles elements (C, N, S, B) between surface and deep reservoirs, thus helping to address the origin of the mantle budget of these elements. It will provide superior constraints on the evolution of the convecting mantle and the potential impact of crustal recycling in modifying mantle compositions. The results of this project, combined with information on diamonds from the studied localities, have the potential to transform current approaches towards diamond exploration with significant economic benefit for the diamond industry.
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