Origin of the Alkaline Post-Erosional Volcanism on the Island of Mauritius

Mauritius is the penultimate island of the Reunion mantle plume. Three episodes of eruptive activity has been recognized on this island: the Older Series, the Intermediate Series, and the Younger Series. The Older Series represent solidified lavas that form the shield volcano. The Intermediate Series and the Younger Series are categorized as post-erosional volcanism. Our new 40Ar/39Ar ages show that the construction of the Mauritius shield was well underway by 8.9 Ma. The shield-stage ended about 4.75 Ma, with the intrusion of trachytes (McDougall and Chamalaun, 1969). The Intermediate Series lavas subsequently erupted between 3.5 Ma and 1.66 Ma. This was followed by a hiatus of more than 0.6 million years. The hiatus ended with eruption of the Younger Series lavas, which continued until nearly the present. We found that the hiatus between the Intermediate and Younger Series was shorter than was previously believed, but appears to be real. While outcrops of the Intermediate Series are restricted to the southwestern area of the island, we found that the Intermediate Series lavas are present beneath Younger Series lava flows in drill cores throughout the rest of the island. The overall evolution of Mauritius resembles that of Hawaii, but there are some significant differences between them. The Older Series lavas on Mauritius are transitional between alkali basalt and tholeiite, different from the tholeiitic composition of shield lavas on Hawaii. Like Hawaii, the post-erosional volcanics have more 'depleted' isotopic signatures than shield-stage lavas. Unlike Hawaii, the post-erosional volcanism was interrupted by a long hiatus and the post-erosional lavas do not show strong silica undersaturation or strong enrichment in incompatible elements. Instead, the post-erosional lavas are only slightly less silica-saturated than the shield-building lavas and are less incompatible-element enriched. Our new isotope data show that the post-erosional lavas could be a mixture of the Reunion mantle plume and the depleted mantle source. We propose the during the early shield stage, melts from the Reunion plume rise into the cold oceanic lithosphere where they react with depleted mantle peridotite to form pyroxenite or eclogite veins. The post-erosional volcanism magmas form when the pyroxenite or eclogite veins are later melted by heat conducted into the lithosphere from the plume below. Mauritius is built on a 65 Ma old and 75 km thick oceanic lithosphere. Our simple calculations show that the conductive heat from the plume will raise the temperature of the lowermost 12 km of the lithosphere up to the pyroxenite solidus within 9 Ma, remelting the veins. Lithospheric flexure due to the construction of the next volcano in the volcanic chain (Reunion Island in the case of Mauritius) may dictate whether melts of the veins can reach the surface. Differences in post-erosional magma compositions between Mauritius and Hawaii may reflect the significantly thicker lithosphere beneath Hawaii and the greater heat flux provided by the Hawaiian plume.