Jasper Gerard Konter

The return of subducted continental crust in Samoan lavas

Substantial quantities of terrigenous sediments are known to enter the mantle at subduction zones, but little is known about their fate in the mantle. Subducted sediment may be entrained in buoyantly upwelling plumes and returned to the Earth’s surface at hotspots, but the proportion of recycled sediment in the mantle is small, and clear examples of recycled sediment in hotspot lavas are rare. Here we report remarkably enriched 87Sr/86Sr and 143Nd/144Nd isotope signatures in Samoan lavas from three dredge locations on the underwater flanks of Savai’i island, Western Samoa. The submarine Savai’i lavas represent the most extreme 87Sr/86Sr isotope compositions reported for ocean island basalts to date. The data are consistent with the presence of a recycled sediment component (with a composition similar to the upper continental crust) in the Samoan mantle. Trace-element data show affinities similar to those of the upper continental crust—including exceptionally low Ce/Pb and Nb/U ratios—that complement the enriched 87Sr/86Sr and 143Nd/144Nd isotope signatures. The geochemical evidence from these Samoan lavas significantly redefines the composition of the EM2 (enriched mantle 2; ref. 9) mantle endmember, and points to the presence of an ancient recycled upper continental crust component in the Samoan mantle plume.

Samoan hot spot track on a “hot spot highway”: Implications for mantle plumes and a deep Samoan mantle source

We report new geochemical data for submarine lavas from the Samoan region that greatly enhance the geochemical data set for volcanoes from the hot spot. Additionally, two volcanoes dredged in the northern Lau Basin, Futuna Island and Manatu seamount, are young (<5 Ma), appear to be genetically related, and may have been generated by melting a component of Samoan mantle that has been advected into the region. We also find evidence for three seamounts and one atoll along the Samoan hot spot track that are not geochemically related to Samoa. We use a plate motion model to show that three non-Samoan hot spots, currently active in the Cook-Austral Islands, provided volcanism to the Pacific Plate now in the Samoan region approximately 10–40 Ma. The four interloping volcanoes in the Samoan region exhibit geochemical affinities with the three hot spots. All three hot spots would have left a depleted, viscous, refractory keel that is coupled to the base of the Pacific lithosphere that has been “rafted” to the Samoan region. Therefore, the new data also have implications for the origin of the Samoan hot spot as its origin has been suggested to be a result of either a deep-seated mantle plume or a consequence of lithospheric cracking. Without major modification of the current “propagating lithospheric cracks” model, it is not clear how such cracks could yield melts from the refractory keel present under the Samoan lithosphere. Instead, a region of buoyantly upwelling mantle, or plume, is suggested to generate the shield stage volcanism in the Samoan region.

Samoa reinstated as a primary hotspot trail

The classical model for the generation of hotspot tracks maintains that stationary and deep-seated mantle plumes impinge on overriding tectonic plates, thereby generating age-progressive trails of volcanic islands and seamounts. Samoa has played a key role in discrediting this model and the very existence of mantle plumes, because early geochronological work failed to demonstrate a linear age progression along this chain of islands. Specifically on Savai9i Island, the bulk of the subaerial volcanics is younger than 0.39 Ma, much younger than the 5.1 Ma age predicted from the classical hotspot model and a constant 7.1 cm/yr Pacific plate motion. This discrepancy led to alternative magma-producing mechanisms that involve the cracking of the lithosphere beneath the Samoan islands, as a result of the extensional regime generated by the nearby Tonga Trench. Here we report 40 Ar/ 39 Ar ages from the submarine flanks of Savai9i Island showing that its volcanic construction began as early as 5.0 Ma and in a true intraplate setting. This reinstates Samoa as a primary hotspot trail associated with a deep mantle plume and a linear age progression.

Helium and lead isotopes reveal the geochemical geometry of the Samoan plume.

Hotspot lavas erupted at ocean islands exhibit tremendous isotopic variability, indicating that there are numerous mantle components hosted in upwelling mantle plumes that generate volcanism at hotspots like Hawaii and Samoa. However, it is not known how the surface expression of the various geochemical components observed in hotspot volcanoes relates to their spatial distribution within the plume. Here we present a relationship between He and Pb isotopes in Samoan lavas that places severe constraints on the distribution of geochemical species within the plume. The Pb-isotopic compositions of the Samoan lavas reveal several distinct geochemical groups, each corresponding to a different geographic lineament of volcanoes. Each group has a signature associated with one of four mantle endmembers with low 3He/4He: EMII (enriched mantle 2), EMI (enriched mantle 1), HIMU (high µ = 238U/204Pb) and DM (depleted mantle). Critically, these four geochemical groups trend towards a common region of Pb-isotopic space with high 3He/4He. This observation is consistent with several low-3He/4He components in the plume mixing with a common high-3He/4He component, but not mixing much with each other. The mixing relationships inferred from the new He and Pb isotopic data provide the clearest picture yet of the geochemical geometry of a mantle plume, and are best explained by a high-3He/4He plume matrix that hosts, and mixes with, several distinct low-3He/4He components.

Primordial helium entrained by the hottest mantle plumes

Helium isotopes provide an important tool for tracing early-Earth, primordial reservoirs that have survived in the planet’s interior. Volcanic hotspot lavas, like those erupted at Hawaii and Iceland, can host rare, high 3He/4He isotopic ratios (up to 50 times the present atmospheric ratio, Ra) compared to the lower 3He/4He ratios identified in mid-ocean-ridge basalts that form by melting the upper mantle (about 8Ra; ref. 5). A long-standing hypothesis maintains that the high-3He/4He domain resides in the deep mantle, beneath the upper mantle sampled by mid-ocean-ridge basalts, and that buoyantly upwelling plumes from the deep mantle transport high-3He/4He material to the shallow mantle beneath plume-fed hotspots. One problem with this hypothesis is that, while some hotspots have 3He/4He values ranging from low to high, other hotspots exhibit only low 3He/4He ratios. Here we show that, among hotspots suggested to overlie mantle plumes, those with the highest maximum 3He/4He ratios have high hotspot buoyancy fluxes and overlie regions with seismic low-velocity anomalies in the upper mantle, unlike plume-fed hotspots with only low maximum 3He/4He ratios. We interpret the relationships between 3He/4He values, hotspot buoyancy flux, and upper-mantle shear wave velocity to mean that hot plumes—which exhibit seismic low-velocity anomalies at depths of 200 kilometres—are more buoyant and entrain both high-3He/4He and low-3He/4He material. In contrast, cooler, less buoyant plumes do not entrain this high-3He/4He material. This can be explained if the high-3He/4He domain is denser than low-3He/4He mantle components hosted in plumes, and if high-3He/4He material is entrained from the deep mantle only by the hottest, most buoyant plumes. Such a dense, deep-mantle high-3He/4He domain could remain isolated from the convecting mantle, which may help to explain the preservation of early Hadean (>4.5 billion years ago) geochemical anomalies in lavas sampling this reservoir.