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

• Published in: Earth and Planetary Science Letters
• Main Authors: Chen, Chunfei, Foley, Stephen F., Tappe, Sebastian, Ren, Huange, Feng, Lanping, Liu, Yongsheng
• Format: Article in Journal/Newspaper
• Language: English
• Published: 2024
• Subjects: Alkaline rocks; Ca isotopes; Carbonate; Carbonatites; K-richterite; Volatiles; Canada; North Atlantic
• Online Access: https://researchers.mq.edu.au/en/publications/0d17c25a-1382-49d4-aa73-a9183a042bbd; https://doi.org/10.1016/j.epsl.2023.118489; https://research-management.mq.edu.au/ws/files/320464052/320189483.pdf; http://www.scopus.com/inward/record.url?scp=85181718389&partnerID=8YFLogxK; http://purl.org/au-research/grants/arc/FL180100134

The volatile components CO 2 and H 2 O 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 δ 44/40 Ca), δ 44/40 Ca 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 δ 44/40 Ca 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 δ 44/40 Ca. The δ 44/40 Ca 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 ...