Evidence from alkaline magmatism for fluorine- and bromine-enriched cratonic mantle roots

The base of cratonic lithospheric mantle (cratonic root) is a key volatile-rich reservoir (e.g., carbon and halogens), formed through long-term metasomatism by deep-derived incipient melts, whose reactivation has been proposed to drive climatic change over the course of Earth history. However, halogen compositions and budgets of cratonic roots remain poorly constrained. We present halogen (F, Cl, Br, I) data of intraplate alkaline rocks from Aillik Bay, Labrador, eastern Canada, that originate from melting of an evolving cratonic root during three distinct time periods: Mesoproterozoic lamproites, Neoproterozoic ultramafic lamprophyres including aillikites and carbonatites, and Cretaceous nephelinites. Compared to mid-ocean ridge basalts (MORBs), lamproites and lamprophyres show higher F (1200−6040 ppm) and Br/Cl ratios (3.3−30.0 × 10−3), which are inconsistent with a degassing signature and instead reflect enrichment of F and Br in their mantle sources. In contrast, nephelinites have high F (1470−2430 ppm) but MORB-like Br/Cl ratios (1.3−2.4 × 10−3), consistent with Br loss during fluid exsolution. These results agree with published halogen data for carbonatites, kimberlites, inclusions in diamonds and cratonic peridotites, showing that cratonic roots worldwide are enriched in F and Br. Our estimates indicate that the cratonic root hosts ∼3.7 ± 0.8 × 1017 kg F (1sd), 1.3 ± 0.4 × 1016 kg Cl (1sd), and 1.9 ± 0.9 × 1014 kg Br (1sd), whose rapid release (generally <5 m.y.) through large igneous province magmatism during mantle plume-cratonic root interaction may have triggered severe ozone depletion and environmental crises.