Geochemistry of Mauritius and the origin of rejuvenescent volcanism on oceanic island volcanoes

Abstract

[1] We report new trace element concentrations and Sr, Nd, Hf, and Pb isotopic ratios for basalts from the three lava series of Mauritius. Older Series lavas, which represent the shield-building phase of Mauritius, are isotopically similar to other lavas produced by the Réunion mantle plume. The Intermediate and Younger Series lavas, erupted after a hiatus of millions of years, have more depleted isotopic signatures than the Older Series. Incompatible element abundances and major element compositions suggest that the extent of melting was greatest for the Older Series, smallest for the Intermediate Series, and intermediate for the Younger Series. Volcanic evolution on Mauritius is thus broadly similar to the Hawaiian pattern, with lavas of the rejuvenescent phase being produced by small degrees of melting of a more depleted source than the shield phase. We propose that both shield and rejuvenescent phase magmas are derived from a lithologically heterogeneous or ''plum pudding'' mantle plume that consists of pods or veins of low-solidus-temperature material such as eclogite or pyroxenite embedded in peridotite matrix. The plums have a less depleted isotopic signature than the matrix. In the vertical stem of the plume, melting is zoned: at greatest depth, only the plums melt; at intermediate depth, both plums and matrix melt, and at shallowest depth, only the matrix melts. Shield stage magmas are hybrids of melts produced in all three zones. As the plume flattens out against the lithosphere and is dragged downstream of the stem, some melting continues due to buoyant lateral spreading of the plume. In this region, although both plums and matrix are melting, melts produced carry the isotopic signature of the matrix because incompatible elements in the plums were previously stripped during melting in the vertical stem of the plume. The relatively small degree melts produced in the melting tail, which give rise to rejuvenescent stage volcanism, thus carry the isotopic signature of the matrix. From a consideration of thermal and isotopic equilibrium, we judge the scale of heterogeneity to be about 10 2 m.

Helium isotope variations between Réunion Island and the Central Indian Ridge (17°–21°S): New evidence for ridge–hot spot interaction

“…Furthermore, Intermediate series and Younger Series lavas from Mauritius and Rodrigues show 3 He/ 4 He ratios of 6.5 to 8.4 R A [ Hanyu et al , 2001]. These helium isotope signatures suggest that the late‐stage lavas are derived from a mantle source that is dominated by a MORB‐like component, and they rule out melting of a heterogeneous “plum pudding” Réunion mantle component alone as the source of lavas in Mauritius [ Paul et al , 2005, 2007]. Consequently, we propose that the variations in 3 He/ 4 He ratios of glasses recovered along the off‐axis volcanic structures can only be explained by mixing of the (residual) hot spot component with partial melts derived from a MORB‐like mantle reservoir.…”

“…The island is composed of two volcanoes: Piton des Neiges, which was active between ∼2 Ma and 43 ka, and the still active Piton de la Fournaise, where volcanic activity began ∼530 ka ago [ Gillot and Nativel , 1989]. Mauritius is considered to be located on the former site of the Réunion hot spot [ Morgan , 1981; Paul et al , 2005]. Volcanic activity on Mauritius started with a shield‐building stage at 8 to 7 Ma (Older Series), with later stages at 3 to 2 Ma (Intermediate Series) and 0.7 to 0.2 Ma (Younger Series) [ McDougall and Chamalaun , 1969].…”

An Investigation of the Reliability of HF Acid Mixtures in the Bomb Digestion of Silicate Rocks for the Determination of Trace Elements by ICP‐MS

“…) after analysing 50 mg. Paul et al . () obtained the highest Mo mass fraction (0.42 μg g −1 ) from a 25‐mg test portion. This phenomenon is perhaps due to a specific chemical/mineral form of Mo in the basalt BIR‐1 and could also occur in other rock samples.…”

Probing the Earth’s Deep Interior Through Geochemistry

“…There are important differences, however. Rejuvenescent lavas are only slightly less silica saturated and, in contrast to Hawaii, are slightly less incompatible element-enriched than shield stage lavas (e.g., Paul et al, 2005;Moore et al, 2011). Furthermore, the rejuvenescent phase has been extraordinarily long (~3.5 million years) and is interrupted by a hiatus of ~600,000 years separating the so-called Intermediate and Younger Series.…”

“…Further downstream, a bulge or arch develops in response to this loading, relieve horizontal compressive stresses and allowing magma to reach the surface. In combination, a long melting tail and lithospheric flexure could explain both the volcanism and the hiatus (e.g., Paul et al, 2005;Garcia et al, 2010). Bianco et al (2005) extended the flexure model, arguing that sufficient upwelling occurs in the asthenosphere as the lithosphere arches upward to generate the small melt fractions that characterise the rejuvenescent magmas.…”