Evolution of shield-building and rejuvenescent volcanism of Mauritius

Abstract

We report chemical and isotopic analyses of 68 samples and 40Ar/ 39Ar ages of 47 samples from Mauritius undertaken to understand the compositional evolution of the volcano and its causes through time. New 40Ar/ 39Ar ages show that construction of the Mauritius shield was well underway by 8.9 Ma, 1.1 m.y. earlier than previously thought and that the hiatus between the Intermediate and Younger Series was shorter than previously thought, as eruption of the rejuvenescent Intermediate Series continued through at least 1.66 Ma and Younger Series volcanism began by at least 1.0 Ma. Eruption frequency over the last 50 ka has been rather typical of Younger Series volcanism over the last 400 ka and future eruptions are possible. Although outcrops of the Intermediate Series lavas are confined to the Southwest, Intermediate Series are present beneath Younger Series flows in drill cores throughout the island. We estimate the total volume of rejuvenescent lavas at ~ 35 km 3 or about 0.05% of the volume of the volcano, similar to rejuvenescent volume fractions on Hawaiian volcanoes. As earlier studies found, Older Series lavas, which on average are slightly normatively silica-saturated, are somewhat more incompatible-element enriched than are the Intermediate and Younger Series, which are both slightly silica-undersaturated on average. Mean Sr and Nd isotope ratios of the Intermediate and Younger Series are nearly identical, but mean Pb isotope ratios, La/Sm, Nb/Y, and Nb/Zr of the Intermediate Series are higher than in the Younger Series. New high precision Pb isotope data, which shows considerably less scatter than previously published data, plot between the Older Series and basalts of the Central Indian Ridge, allowing the possibility that the source of the rejuvenescent lavas is a mixture of this plume and depleted mantle. We propose two possible explanations for the composition of rejuvenescent lavas. The first is that plume-derived melts reacted with deep lithosphere to form pyroxenite veins during the early shield-building stage. Later, these veins melted as a consequence of conductive heating of the lithosphere by the plume to produce the rejuvenescent lavas. Alternatively, rejuvenescent lavas may be derived from a sheath of thermally entrained mantle that surrounds the plume and is a mixture of plume material and depleted upper mantle.