Calcium isotopes track volatile components in the mantle sources of alkaline rocks and associated carbonatites

Abstract

The volatile components CO2 and H2O induce mantle melting and thus exert major controls on mantle heterogeneity. Primitive intraplate alkaline magmatic rocks are the closest analogues for incipient mantle melts and provide the most direct method to assess such mantle heterogeneity. Given the considerable Ca isotope differences among carbonate, clinopyroxene, garnet, and orthopyroxene in the mantle (up to 1 ‰ for δ44/40Ca), δ44/40Ca of alkaline rocks is a promising tracer of lithological heterogeneity. We present stable Ca isotope data for ca. 1.4 Ga lamproites, 590–555 Ma ultramafic lamprophyres and carbonatites, and 142 Ma nephelinites from Aillik Bay in Labrador, eastern Canada. These primitive alkaline rock suites are the products of three stages of magmatism that accompanied lithospheric thinning and rifting of the North Atlantic craton. The three discrete magmatic events formed by melting of different lithologies in a metasomatized lithospheric mantle column at various depths: (1) MARID-like components (mica-amphibole-rutile-ilmenite-diopside) in the source of the lamproites; (2) phlogopite-carbonate veins were an additional source component for ultramafic lamprophyres during the second event; and (3) wehrlites at shallower depths were an important source component for nephelinites during the final event.

The Mesoproterozoic lamproites show lower δ44/40Ca values (0.58 to 0.66 ‰) than MORBs (0.84 ± 0.03 ‰, 2se). This cannot be explained by fractional crystallization or melting of the clinopyroxene-dominated source but can be attributed to a source enriched in the alkali amphibole K-richterite, which has characteristically low δ44/40Ca. The δ44/40Ca values of the ultramafic lamprophyre suite during the second rifting stage are remarkably uniform, with overlapping ranges for primary carbonated silicate melts (aillikite: 0.67 to 0.75 ‰), conjugate carbonatitic liquids (0.71 to 0.82 ‰) and silicate-dominated damtjernite liquid (primary damtjernite: 0.68 to 0.72 ‰). This suggests negligible Ca isotope fractionation during liquid immiscibility of carbonate-bearing magmas. Combined with previously reported δ44/40Ca values for carbonatites and kimberlites, our data suggest that carbonated silicate melts in Earth's mantle have δ44/40Ca compositions resolvably lower than those for MORBs (0.74 ± 0.02 ‰ versus 0.84 ± 0.03 ‰, 2se). The δ44/40Ca values of the Cretaceous nephelinites (0.72 to 0.78 ‰) are homogenous and similar to those of the 590–555 Ma ultramafic lamprophyres, suggesting that the wehrlitic source component for the nephelinites formed by mantle metasomatism during interaction with rising aillikite magmas during the second rifting stage. Our results highlight that both K-richterite and carbonate components in mantle sources can result in the systematically low δ44/40Ca values of alkaline magmas, which may explain previously reported low δ44/40Ca values of alkaline rocks and some carbonatites. Our study indicates that Ca isotopes are a robust tracer of lithological variation caused by volatiles in the Earth's upper mantle