Compositional diversity among primitive lavas of Mauritius, Indian Ocean: Implications for mantle sources

Abstract

Three distinct phases of basaltic volcanism (Older Series, 7.8-5.4 Ma; Intermediate Series, 3.5-1.9 Ma; Younger Series, 0.7-0.03 Ma), related to the Deccan-Reunion mantle-plume, occurred over a time span of nearly 8 million years on the island of Mauritius, Indian Ocean. Bulk rock compositions of 52 analyzed samples (including samples from 32 recent borehole sections), as well as the compositions of olivine and spinel phenocrysts, reveal significant temporal variations in the composition of the Mauritius magmas. Two distinct compositional groups are identified based on their major and trace element compositions. The Group 1 basalts comprise the Younger Series, Intermediate Series and the earlier products of the Older Series rocks, and have higher MgO (7.6-13.6 wt.%) and lower K2O (0.2-0.5 wt.%) and La/Sm (2.1-3.5) compared to Group 2 basalts (MgO 6.2-9.2 wt.%; K2O 1.0-1.5 wt.%; La/Sm 3.6-4.1), which exclusively encompass the later differentiation products erupted during the end of the Older Series volcanism, denoted as Older Differentiated (OD) Series. Petrographic and mineralogic studies further confirm the presence of two distinct groups. Relatively primitive olivine compositions (Fo: 87-80) in Group 1 rocks are characterized by increasing CaO (0.2 to 0.3 wt.%) with decreasing Fo content, whereas in Group 2 rocks olivine (Fo: 86-80) show decreasing CaO (0.4 to 0.3 wt.%) with decreasing Fo content. Olivine-hosted spinel inclusions also show strong compositional differences between groups 1 and 2. Unusually Al-rich spinels (compared to that usually found in plume derived magma) and the positive Sr anomalies in Group I rocks suggest assimilation of crustal gabbros by the plume magmas. We propose a laterally heterogeneous plume and the contributions from variable plume components to account for the compositional differences observed in the two Mauritian lava groups. The early-Shield building Older Series lavas (oldest Group 1 lavas), characterized with a relatively less enriched trace element abundances, most likely derive from extensive melting of a depleted peridotite matrix in the core of the plume. Melting of this already depleted source produced the later Intermediate and Younger Series eruptions. Towards the late-shield stage, melts presumably derive from the enriched pyroxenite/eclogite components in the outer part of the plume.