Geochemistry of Lavas from Mauritius, Indian Ocean: Mantle Sources and Petrogenesis

Abstract

Trace-element and Nd-Sr-Pb isotopic data for mafic lavas from the Mascarene island of Mauritius reveal that the shield-stage Older Series (with 143 are strongly enriched in highly incompatible elements relative to moderately incompatible ones (e.g., chondrite-normalized La/Lu = 4.03-17.8 vs. 9.38-10.2 for the Older Series), but the Younger Series lavas display much less relative enrichment (e.g., chondrite-normalized La/Lu = 3.86-5.37) and have low concentrations of incompatible elements. Modeling suggests that variable partial melting and mixing between mid-ocean-ridge-basalt (MORB) source mantle and mantle with bulk-Earth-like Nd and Sr isotopic and incompatible-element ratios, and "C"-like Pb isotope ratios, can explain the general relationship between the shield and post-shield lavas. All Mauritius and Réunion lavas can be explained as 2-8% partial melts of a bulk-Earth-like source that had been 1.5-0.25% metasomatized by a 0.1% MORB-source melt. The range in Pb isotopic compositions requires mixing with highly variable Sr/Pb and Nd/Pb ratios in the end members; such variation, however, is not supported by measured Sr/Pb, Nd/Pb, etc. in the lavas. Some isotopic heterogeneity thus appears to be required in the end members and/or much of the range in Pb isotopes represents radiogenic ingrowth since the metasomatic event. Near-constant 207 Pb/ 204 Pb values over the range of 206 Pb/ 204 Pb among the Older Series lavas suggest that the metasomatic event must have occurred within the last few hundred million years.

Compositionally diverse magmas erupted close together in space and time within a Karoo flood basalt crater complex

“…The current petrogenetic model for the Karoo province suggests that the source rocks for magmas at Sterkspruit were a chemically heterogenous mantle, perhaps subjected to varying degrees of partial melting . Further, detailed mineralogical and isotopic studies are necessary to fingerprint mantle components, the depth and degree of partial melting and the time scales involved (e.g., Sheth et al 2003;Ramos et al 2005), but the conclusion that the Sterkspruit chemical types each tap a different, deep source is consistent with available data. To our knowledge, the results of this study are unique in the following ways:…”

A well-sampled phylogenetic analysis of the polystichoid ferns (Dryopteridaceae) suggests a complex biogeographical history involving both boreotropical migrations and recent transoceanic dispersals

“…The alternative hypothesis, involving island-hopping across the Indian Ocean, should also be considered, as recent investigations have highlighted the critical role archipelagos that are currently immersed but were likely above water when the sea-level was low (but see Sheth et al, 2003 andThébaud et al, 2009) in the biogeography of the southwestern Indian Ocean region (Cuenca et al, 2008;Le Péchon et al, 2015;Renner et al, 2010;Strijk et al, 2014). An Asian origin of the Southwestern Indian Ocean Polystichum species is in line with the view that a large part of biota of Madagascar and nearby islands share strong biogeographical affinities with Asia (Warren et al, 2010;Yoder and Nowak, 2006).…”

Diversification of Dombeyoideae (Malvaceae) in the Mascarenes: Old Taxa on Young Islands?

“…These former islands have been submerged, leading to complete extinction of their biota. In the Indian Ocean Region, the stepping-stone hypothesis is corroborated by geological evidence (Sheth et al 2003) in combination with sea-level curves and bathymetry information, which together support the presence of several large islands near the Mascarene archipelago (Thébaud et al 2009;Warren et al 2010). This Fig.…”

“…The Mascarene Archipelago consists of three young oceanic islands , , and Rodrigues [8][9][10]; McDougall et al 1965;McDougall and Chamalaun 1969;Sheth et al 2003) that (with the exception of Rodrigues) originated from the activity of the Madagascar and Indian Ocean islands biodiversity hot spot. The largest island, Réunion, possesses an active volcano and a high mountainous formation, with its highest peak at 3070 m. Although older and lower than Ré-union, Mauritius also presents a high diversity of isolated habitats because of the natural erosion and recent lava flows that have deeply modified the island (McDougall and Chamalaun 1969).…”