A geochemical link between plume head and tail volcanism

doi: 10.7185/geochemlet.1742 Geodynamical models of mantle plumes often invoke initial, high volume plume ‘head’ magmatism, followed by lower volume plume ‘tails’. However, geochemical links between plume heads, represented by flood basalts such as the Deccan Traps, and plume tails, represented by ocean islands such as La Réunion, are ambiguous, challenging this classical view of mantle plume theory. Using Sr-Nd-Os isotope data, we demonstrate a geochemical link between archetypal plume head and tail volcanism in the Réunion hotspot. Similar plume head-tail relationships have not been definitively shown in previous geochemical studies for Réunion or other global hotspots. Such a link is enabled by use of compatible elements, such as Os, which can circumvent complexities introduced by magmatic assimilation of crust or lithosphere because these elements are scarce in crust compared to primary mantle melts. We calculate Sr-Nd-Os isotopic compositions for the Réunion primary magma and find these are identical to predictions for the Deccan primary magma. Our result provides geochemical evidence for a temporally stable mantle plume that samples a primitive reservoir associated with the African large low-shear-velocity province and with a heritage beginning at the Cretaceous-Palaeogene boundary. Received 5 April 2017 | Accepted 28 September 2017 | Published 3 November 2017 1. Geosciences Research Division, Scripps Institution of Oceanography, University of California San Diego, La Jolla, CA 92093, USA # Now at Department of Terrestrial Magnetism, Carnegie Institution for Science, Washington, DC 20015, USA * Corresponding author (email: bpeters@carnegiescience.edu) Letter The origins of terrestrial intraplate “hotspot” volcanism remain controversial. One common model suggests that hotspots are derived from thermo-chemically buoyant mantle plumes that rise from the deep mantle, initiating short-lived, high volume ‘flood basalt’ volcanism that likely had collateral tectonic and environmental consequences (Richards et al., 1989; Hill, 1991; Self et al., 2008). Other models posit that hotspot magmatism is initiated by shallow melting in response to tectonic processes (e.g., Foulger et al., 2015). Three arguments currently favour a plume origin for intraplate magmatism. First, the formation of a continuous, age-progressive and roughly linear volcanic track independent of plate tectonic activity likely requires an active mantle source, such as a plume. Second, seismic tomography suggests deep, low-shear-velocity upwelling beneath some intraplate volcanic centres (French and Romanowicz, 2015). Third, the isotopic signatures of many modern hotspots are distinct from those of the mantle underlying continents and mid-ocean ridges and remain relatively consistent through time, suggesting that they have isolated, deep mantle source reservoirs that continually replenish shallow magma reservoirs (Hofmann, 1997). One of the best studied examples of plume head and tail relationships along a linear, age-progressive volcanic track is the Deccan-Réunion hotspot. There, a volumetrically massive continental flood basalt (CFB) province, the Deccan Traps, is linked by aseismic, submarine ridges to ocean island basalts (OIB) that actively erupt on the island of La Réunion (Fig. 1). A genetic link between Deccan CFB and Réunion OIB, and for other similar hotspot tracks, has often been implicitly assumed by mantle plume theory and is demanded by recent geodynamical models (Glišović and Forte, 2017). Such assumptions, however, are at odds with expected physical consequences of ancient mantle plumes; for example, modern heat flow measurements do not record increased thermal output beneath the Deccan Traps, where an ancient mantle plume might be expected to have thinned the continental crust and lithosphere and increased heat transfer from the mantle to the surface (Roy and Rao, 2000). In cases like these, geochemistry is a potentially diagnostic tool for linking CFB and OIB magmatism. If flood basalts and later ‘plume tail’ magmas are found to originate from compositionally distinct sources, it would provide no definitive evidence for a long lived mantle plume; however, if they share a common geochemical source that is distinct from the modern accessible mantle, then the persistence of such signatures in the mantle would strongly implicate the existence of a mantle plume. Decades of geochemical research utilising lithophile isotope systems have resulted in dubious associations between plume heads and tails. For example, Sr-Nd isotopic compositions of Deccan basalts deflect away from a Réunion end member (e.g., Fig. 2 of Peng et al., 1994), which may be a consequence of extensive assimilation of lithospheric or crustal material into CFB magmas. This effect has also been observed for other hotspot systems (Gibson et al., 1995; Peate, 1997; Geochemical Perspectives Letters Letter Geochem. Persp. Let. (2017) 5, 29-34 | doi: 10.7185/geochemlet.1742 30 Figure 1 Satellite bathymetry map of the western Indian Ocean basin. Approximate aerial extent of Deccan Traps lava flows are shown by the gray fields on the Indian subcontinent. Numbers in the shaded region correspond to sampling regions: 1 – Kutch (samples 1-5), 2 – Saurashtra (samples 6-46), 3 – Pavagadh, Kalsubai, Amba Dongar and surrounds (samples 48-54, 63-78), 4 – Dhule and surrounds (samples 55-62), 5 – Mumbai, Western Ghats and coastal Maharashtra (samples 79-115, MMF7). Approximate trace of the Réunion hotspot is shown by the transparent black arrow, approximate plate motion vectors are shown by solid black arrows over land areas and are proportional to plate motions. Base map reproduced from the GEBCO world map 2014, www.gebco. net. Dale et al. 2009; Day, 2016), meaning that it may be an inevitable consequence of magma ascent through lithosphere and crust, effectively masking primary magmatic signatures. The siderophile 187Re-187Os isotope system (t1/2 ≈ 42 Gyr) (Shirey and Walker, 1998) provides an opportunity to resolve a geochemical link between plume head and tail volcanism. Rhenium is relatively incompatible while Os is highly compatible, leading to high Re/Os and long term radiogenic 187Os/188Os signatures in crust (>1; Peucker-Ehrenbrink and Jahn, 2001), while the primitive mantle preserves lower Re/ Os and less radiogenic 187Os/188Os signatures (~0.1296) relative to chondrites (0.126–0.128; Meisel et al., 1996; Day et al., 2016). In contrast, long lived lithophile isotope systems (e.g., Rb-Sr, Sm-Nd, Lu-Hf, U/Th-Pb) involve exclusively incompatible elements that are enriched in lithospheric melts and crustal materials relative to mantle-derived magmas. This explains why the Sr-Nd-Pb isotopic characteristics of flood basalts can be dominated by contributions from lithospheric melt and partially melted crust (e.g., Peng et al., 1994; Gibson et al., 1995). Mantle Os isotopic signatures, however, may persist through variable extents of differentiation or assimilation of lithosphere or crust, making 187Os/188Os a diagnostic tool for examining geochemical links between plume head and tail magmatism. Notwithstanding, previous examination of the