Application of the 138La-138Ce systematics as tracer of recycled sediment in the mantle

Abstract:

The isotopic heterogeneities displayed by mantle-derived magmas provide strong constraints on the Earth’s evolution and its reservoirs through time. Rare earth elements include two radiogenic systems with long-lived parents: ¹³⁸La-¹³⁸Ce (T_1/2= 292.5 Ga) and ¹⁴⁷Sm-¹⁴³Nd (T_1/2= 106 Ga). The inverse relationship in terms of incompatibility of the two parent/daughter systems generates a linear anticorrelation in a Ce-Nd isotope diagram. However, the two systems can also be decoupled because Ce acts differently in supergene environments due to its two valence states: Ce^3+/Ce⁴ [1]. Indeed, sediments formed in equilibrium with seawater (authigenic and hydrothermal sediments) have strong negative Ce_N/Ce* (Ce*=[La_N+Pr_N]/2) anomalies. Their involvement during magma genesis has been mentioned in geodynamical contexts such as subduction zones (Mariana arc system [2]) or the plume source of Gough Island lavas [3]. Here we present a compilation of new Ce and Nd isotope measurements of a large variety of samples: i) sediments drilled in pre-arc domains and arc lavas from two different oceanic arc systems (Lesser Antilles [4] and Mariana); ii) oceanic carbonatites from Cape Verde and Canary archipelagoes [5]; iii) ocean island basalts. The incorporation of material with negative Ce anomalies should produce lava with Ce-Nd signatures that lie off the mantle correlation. Our results provide further constraints on the mixing relationships between sedimentary and mantle components in mantle-derived magmas.

[1] Eldefield, 1988, Phil. Trans. R. Soc. A, 325; [2] Hole et al., 1984, J. Geol. Soc London, 141; [3] Class and le Roex, 2008, EPSL 265; [4] Bellot et al., GCA, in press; [5] Doucelance et al., 2014, EPSL 407.