A Geochemical Link between Plume Head and Tail Volcanism - 2017

Rare Earth Deposits & Mining

Peters and Day reported in 2017 that 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, the authors demonstrated that a geochemical link is apparent between the 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. The authors calculated the Sr-Nd-Os isotopic compositions for the Réunion primary magma and find these are identical to predictions for the Deccan primary magma. The authors' 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-Paleogene boundary.

The authors concluded that the negative correlation between age-corrected ⁸⁷Sr/⁸⁶Sr and ¹⁴³Nd/¹⁴⁴Nd (Fig. 3e,f) matches a two stage assimilation scenario, in which a Réunion-like parental magma first assimilates a relatively large proportion of lower continental crust (estimated as 50–90 % of the Sr-Nd mass) followed by smaller amounts of upper continental crust (0-50 % by mass). Alternatively, assimilation may be single stage and involve crust with a composition intermediate to our chosen end members. For all combinations of Sr-Nd-Os isotopes, data arrays of Deccan lavas can be explained by a Réunion-like mantle source that has assimilated lower and/or upper continental crust with no required role from lithosphere or a primitive mantle-like source (Meisel et al., 1996).

The authors also found that measured HSE variations in Deccan basalts can be quantitatively related to a Réunion parental magma (Supplementary Information). Accounting for localized ‘nugget’ effects, differentiation of a Réunion-like parental magma that has undergone sulphide saturation can produce HSE abundances similar to those measured in the most evolved lavas of the Maharashtra “main sequence” and Saurashtra low Ti lavas. These findings do not require contributions from a source similar to the modern Comoros hotspot, as suggested by Glišović and Forte (2017), however they cannot definitively exclude such contributions. Additional geochemical data for the ancient Comoros hotspot may be needed to evaluate this hypothesis.

A geochemical link between canonical plume head and plume tail volcanism demonstrates that mantle plumes and their mantle sources can remain physically resilient through tens of millions of years of Earth’s history. Similar geochemical links may also exist for intraplate volcanism displaying physiographic ties between ‘head’ and ‘tail’ provinces, such as the Iceland or Tristan da Cunha hotspots. These hotspots, as well as the Réunion hotspot, lie above the edge of the African large low-shear-velocity province (LLSVP), which may serve as the lower mantle source to many plumes (Zhao et al., 2015).

If this is the case, the African LLSVP must actively transport material with primordial compositions to Earth’s surface (Peters et al., 2016). If the LLSVP serves as the source for Réunion primary magmas, this is consistent with the notion that LLSVP’s are not only presently stable (Zhao et al., 2015), but that they have remained stable through at least the Cenozoic. The temporal geochemical homogeneity in the Réunion plume source strongly implies that there are regions of the mantle capable of preserving signatures of their provenance since very early in Earth’s history, remaining exempt from processes that otherwise act to keep the mantle well-mixed.

Original working URL: https://www.geochemicalperspectivesletters.org/documents/GPL1742_SI.pdf