Abstract
Kimberlite magmatism occurs as a result of volatile fluxed melting of the convecting upper mantle underlying cratonic lithospheric mantle regions. During passage to the Earth's surface, proto-kimberlite magma can interact with, and assimilate, variably enriched cratonic mantle producing hybrid melts consisting of asthenospheric and cratonic mantle components including contributions from metasomatic domains. The halogen elements (F, Cl, Br, I) and chlorine isotope ratios (37Cl/35Cl) are increasingly used as tracers of recycled crustal materials within the Earth's mantle yet are only rarely reported in analyses of kimberlites. As a result, the origin and distribution of halogens in kimberlite magmas is poorly constrained. Here, we present novel, combined elemental (F, Cl, Br) and isotopic (δ37Cl) halogen data for 14 fresh kimberlite samples from the North Atlantic Craton (NAC) of West and South-West Greenland.The F composition of kimberlites from the NAC appears to be controlled by melting in the convecting upper mantle with minimal effect from interaction with metasomatized lithospheric mantle or volatile loss during or after emplacement. By contrast, Cl and Br in the studied samples have undergone significant devolatilization during kimberlite dyke emplacement and post-emplacement processes, whereby up to 99% of the original halogen budget was removed.Whilst all the studied kimberlites broadly follow the same geochemical pattern, there exists some regional variability in their halogen systematics. The northern NAC kimberlite samples from Majuagaa have mantle-like δ37Cl values of −0.2 to −0.5‰ [versus SMOC (standard mean ocean chloride)]. In contrast, kimberlite dykes from Nigerdlikasik and Pyramidefjeld near the southern craton margin display positive δ37Cl values of +0.4‰ to +1.3‰, in addition to a relative Cl and Br enrichment, which is consistent with the assimilation of recycled crust-derived halogens by the kimberlite magmas. The data support a scenario in which recycled halogens were sampled either from within an OIB-type reservoir in the convecting mantle or through interaction with subduction-modified lithospheric mantle reservoir during eruption. We prefer a scenario in which the ascending kimberlite magmas assimilated Cl-rich, metasomatized regions within cratonic mantle lithosphere.
Highlights
Relatively minor constituents of continental magmatism, kimberlites garner significant interest due to their hosting of diamonds and mantle xenoliths scavengedJournal Pre-proof from great depths during magma ascent
Our main findings are as follows: 1. Fluorine in kimberlite appears to be controlled by melting in the convecting upper mantle evidenced by F/Nd ratios that overlap those of MORB and OIB
Unlike F, Cl and Br undergo significant loss of up to 99% during kimberlite magma emplacement in the crust and postemplacement processes, as evidenced by depleted Cl/Nb and Br/Nb ratios
Summary
Relatively minor constituents of continental magmatism, kimberlites garner significant interest due to their hosting of diamonds and mantle xenoliths scavengedJournal Pre-proof from great depths during magma ascent. The high MgO, NiO, and incompatible trace element contents of kimberlites are best explained as a result of low-degree partial melting of carbonated peridotite within the convecting ambient upper mantle le Roex, 1986; Tachibana et al, 2006) These varieties of magmatism display extremely high concentrations of incompatible trace elements in addition to extreme concentrations of volatiles such as H2O and CO2 (Kjarsgaard et al, 2009). The origins and distributions of halogens in kimberlite magmas are only poorly constrained
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