Elsevier

Chemical Geology

Volume 524, 5 October 2019, Pages 67-76
Chemical Geology

A trace of recycled continental crust in the Réunion hotspot

https://doi.org/10.1016/j.chemgeo.2019.06.009Get rights and content

Abstract

Réunion Island is the present surface expression of a major mantle plume whose homogeneity and isotopic signature, near the convergence point of many hotspot isotopic arrays, have long puzzled geochemists. This signature could, in part, reflect oversampling of the most recent (<0.53 Ma) Piton de la Fournaise volcano. To resolve this issue, we studied the older Piton des Neiges volcano and made a synthesis of the Sr-Nd-Pb isotope compositions of lavas produced during the early stage of La Réunion and contemporaneously at Mauritius, the second youngest island of the hotspot track.

New samples from Piton des Neiges have 87Sr/86Sr from 0.70429 to 0.70441, 143Nd/144Nd from 0.51282 to 0.51290, 206Pb/204Pb from 18.765 to 19.004, 207Pb/204Pb from 15.558 to 15.605 and 208Pb/204Pb from 38.850 to 39.082. On a Srsingle bondNd isotope plot, the extended isotope field defined by the Réunion and Mauritius volcanoes overlaps with the OIB field of the Society islands, where an EM-2 signature has been recognized. Réunion also has higher 207Pb/204Pb and 208Pb/204Pb ratios for a given 206Pb/204Pb than expected from a binary mixing between depleted and enriched mantle components. Thus, the overall Sr-Nd-Pb isotope variations shown by Réunion and Mauritius volcanoes require a third source component. In addition to the already known depleted and enriched mantle components, the third component involved in the mixing is interpreted as reflecting a small contribution (≤ 8%) of Seychelles/Madagascar-like continental crust, which is incorporated into the plume before it rises into the depleted mantle. This new model challenges the proposed existence of a hidden continent below the island of Mauritius, by suggesting that the continental component is located in the asthenosphere.

Introduction

Oceanic volcanic islands are of great interest to geologists because they provide indirect access to the structure and composition of mantle plumes, to their temporal variations, and hence to the dynamics of the Earth's mantle (Steinberger, 2000). Seismic tomography and isotope geochemistry have revealed that mantle plumes are highly variable in structure, composition and temporal evolution despite their common characteristics (thermal anomaly, buoyancy of deep mantle material, relatively fixed position through time, mixing of enriched lower mantle and depleted upper mantle) (Zhao, 2007).

Several suites of ocean island basalts (OIBs) from individual hotspots generally display large, correlated Sr-Nd-Pb variations (Hart et al., 1992). These arrays constrain the nature and distribution of components within mantle plumes (Abouchami et al., 2005; Bryce et al., 2005; Shorttle et al., 2013) and how they are sampled during the different stages of ocean island volcanism (Paul et al., 2005; Garcia et al., 2010).

One of these hotspots, the Réunion Island in the Indian Ocean, stands out because its basalts show limited isotopic range (Fisk et al., 1988, Graham et al., 1990, Albarède et al., 1997, Hanyu et al., 2001, Fretzdorff and Haase, 2002) and plot near the convergence point of many OIB isotopic arrays. This, together with the occurrence of incompatible trace element ratios similar to primitive mantle (Valer et al., 2017), unusually high Pd/Ir and Ru/Ir ratios (Peters et al., 2016), and 142Nd isotope anomalies (Peters et al., 2018), supports the idea, initially proposed by Vlastélic et al. (2006), that the Réunion mantle source has been isolated from the convecting mantle through much of Earth's history. Nevertheless, the increasing number and precision of isotope analyses has revealed small isotopic heterogeneities between the different volcanoes of Réunion Island (Bosch et al., 2008) and even among historical eruptions of the Piton de la Fournaise volcano (Vlastélic et al., 2009). However, these heterogeneities remain much smaller than those found in the three lava series of Mauritius, the second youngest island along the Réunion hotspot track (Nohda et al., 2005; Paul et al., 2005; Moore et al., 2011). The compositional variations of Réunion and Mauritius lavas have been interpreted as reflecting a heterogeneous plume structure (Paul et al., 2005; Bosch et al., 2008) with major contributions from the C-FOZO and EM-1 (Bosch et al., 2008) or EM-2 like plume components (Smietana, 2011), mixed with depleted mantle from the Central Indian Ridge (Nohda et al., 2005; Füri et al., 2011; Moore et al., 2011). Small variations in the historical volcanic activity of the Réunion hotspot have been interpreted as being due to interaction of Réunion magmas with the oceanic crust or the volcanic edifice (Pietruszka et al., 2009).

In contrast with this apparently simple geochemical model, recent seismic tomography and SKS splitting results depict the Réunion hotspot as a complex low-velocity structure interacting with the Central Indian Ridge (Mazzullo et al., 2017; Scholz et al., 2018). In addition, the recent discovery of Archean zircons in Mauritius has raised the possibility that plume magmas may also be contaminated by fragments of continental crust left behind when India separated from Madagascar (Torsvik et al., 2013; Ashwal et al., 2017).

Most of what we know about the isotopic signature of the Réunion plume over the past 10 Ma is derived from the study of Piton de la Fournaise (PF), which has received considerable attention due to its intense activity (>500 samples analyzed for at least one of Sr, Nd, Hf and/or Pb isotope systems since 2000). Mauritius Island is also relatively well studied (>140 samples, mostly after 2000). In comparison, Piton des Neiges (PN), the other now dormant volcano of Réunion Island, is less studied (<80 data, half of them prior to 1990). As PN is by far the largest and longest-lived emerged edifice on La Réunion Island, this relatively limited number of studied samples might bias our knowledge of the plume's isotopic signature.

Here, we present a new set of Sr-Nd-Pb isotope data from samples belonging to PN. Our first objective is to increase the number and precision of the isotope data for PN, and second to compare our data with those from PF and Mauritius Island. Based on this comparison, we propose a new plume model taking into account all the geochemical, geophysical and geodynamic studies available for the Indian Ocean. This model adds fuel to the controversy about the existence of hidden continental fragments left behind during the northward migration of Indian plate.

Section snippets

Geological setting

Plate displacement over the Réunion hotspot has resulted in an alignment of intraplate volcanism starting in the Indian plate at the Deccan Traps (66–61 Ma), continuing on to the Maldives (~58 Ma), the Chagos archipelagos (~50 Ma), and then on to the Somalian plate with the Saya de Malha bank (50–40 Ma), the Chargados-Carajos islands (~30 Ma), Mauritius (10–0 Ma) and finally Réunion Island (5–0 Ma) (Fig. 1a). Mauritius is built at the tip of the Mascarene plateau on a ~4000 m-deep ocean floor,

Materials and analytical methods

Great care was taken to constitute a suite of 43 samples spanning the entire lifetime of Piton des Neiges from its emergence, encompassing its shield building and differentiated stages, and including the whole range of intrusive and extrusive rock compositions (location Fig. 1b, Table 1). Two samples from Piton de la Fournaise were also collected to check the consistency of our analyses with previous isotopic studies. Samples were selected by cross-correlated dating and major element studies

Results

Sr-Nd-Pb isotope and major-trace element data are reported in Table 1. Sr and Nd isotope ratios for PN range from 0.70404 to 0.70429 and from 0.51282 to 0.51290, respectively. These data are within the range reported by previous authors (87Sr/86Sr: 0.70397–0.70437 and 143Nd/144Nd: 0.51280–0.51288, (Fisk et al., 1989, Smietana, 2011), Fig. 2). The variations are even more restricted compared to those from PF (87Sr/86Sr from 0.70398 to 0.70432 and 143Nd/144Nd from 0.512786 to 0.512786, (Vlastélic

Building a high-precision isotope database for the Réunion hotspot

The first step to be taken while interpreting the isotopic variations is to compile a reliable database of Réunion hotpot rocks younger than 10 Ma, i.e. including samples from Piton des Neiges, Piton de la Fournaise, Mauritius, and possibly Rodrigues. Regarding Piton des Neiges, seventy-eight Sr, Nd or Pb data are available in the literature (McDougall and Compston, 1965, Cooper and Richards, 1966, Oversby, 1972, Ludden, 1978, Dupré and Allègre, 1983, Fisk et al., 1988, Bosch et al., 2008,

Conclusion

We report a new set of Sr-Nd-Pb isotope compositions from samples collected at Piton des Neiges volcano. These data, together with filtered literature data from both Réunion and Mauritius Islands, are used to create a high-precision isotope database for the Réunion hotspot products younger than 10 Ma. This database is used to discuss the origin of the isotopic variations along the Mauritius-Réunion hotspot track. These variations best explained by a three-component, two-step mixing involving

Acknowledgement

Authors are pleased to thank the French Government Laboratory of Excellence initiative ANR-10-LABX-0006, and the INSU,CNRS for the financial and logistic supports. We would deeply thank B. Peters and B. Hanan for their thoughtful reviews that improved the manuscript and C. Chauvel for the Editorial handling. This is Laboratory of Excellence ClerVolc contribution number 354, and IPGP contribution 4047.

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