New Constraints on the Origin of the EM‐1 Component Revealed by the Measurement of the La‐Ce Isotope Systematics in Gough Island Lavas

Abstract

Hot spot lavas show a large diversity of isotope compositions resulting from the recycling of surface material into the convective mantle. Amongst the mantle end‐members, EM‐1 (enriched mantle) is widely debated and scenarios involving either old pelagic sediments subducted into the deep mantle or subcontinental lithospheric material incorporated at shallow depths are commonly evoked. We selected 12 lavas from Gough Island (south Atlantic Ocean) for the measurement of 138La‐138Ce, 147Sm‐143Nd, and 176Lu‐177Hf isotope systems. Results show limited ranges for ε143Nd, ε138Ce, and ε176Hf values, and Ce/Ce* do not correlate with measured isotope ratios. Cerium isotope compositions allow us to exclude the contribution of old sedimentary material carrying a negative, elemental cerium anomaly in the mantle source. Pelagic sediments are indeed characterized by strongly negative, elemental cerium anomalies and also high La/Ce ratios. Modeling a primitive mantle source contaminated by 0.4% to 2.2% of different 2.5 Ga‐old pelagic components is able to reproduce the lowest Ce/Ce* values. However, the cerium isotope measurements show ε138Ce values between −0.39 and 0.15, too low to give support to the incorporation of recycled pelagic sediments in the mantle source of the lavas. Our results suggest that the incorporation of subcontinental lithospheric material at shallow depths during the plume ascent is a more suitable model to explain the formation of the EM‐1 component. Hafnium and Nd isotopes also support this scenario. Subcontinental lithosphere sampled via kimberlites and lamproites has isotopic compositions that plot generally below the mantle array, a signature that is also seen in Gough lavas.

Formation of the Ce-Nd mantle array: Crustal extraction vs. recycling by subduction

“…Thus, the incorporation of ROC into the mantle tends to shift the isotopic composition of OIBs toward slightly less radiogenic Ce isotopic ratios than MORBs. Second, as previously shown by , the mantle array can be reproduced by mixing either a DMM-like or a FOZO-like component (FOZO is assumed to have εCe = -0.6 and εNd = 6.24; see Boyet et al, 2019) and recycled surface material (ROC and sediments have the same recycling ages). On the sole basis of Ce and Nd isotopes, it is still unclear whether the different plume sources contain surface material recycled at various time or pre-GOE material in various proportions (the two propositions being not mutually exclusive).…”

“…εNd vs. εCe of mantle-derived samples and loesses from this study (coloured symbols). Nd isotopic compositions of loess and Kerguelen samples are from Chauvel et al (2014) and B. Moine (unpublished data), and Ce and Nd isotopic compositions of Gough samples are from Boyet et al (2019).…”

“…Corresponding samples are classified either as EM1 (samples from Tristan Da Cunha, Gough, Heard, and Kerguelen Islands) or EM2 (Tahaa Island in the Society archipelago). The Ce-Nd isotopic composition of Gough Island lavas was explained by the incorporation of subcontinental lithospheric material at shallow depths into the mantle (Boyet et al, 2019). The lack of correlation between the cerium anomalies and the Ce isotope compositions measured in Gough Island lavas proves that ancient sediments carrier of negative element cerium anomaly were not incorporated in the Gough mantle source.…”

New constraints from Central Chile on the origins of enriched continental compositions in thick-crusted arc magmas

“…Rather than attempt to estimate the full trace element composition of very low-degree metasomatising mantle melts, a proxy for the composition of mantle domains which has been enriched by M-SCLM melts was estimated by inverting for the mantle source composition of Gough Island EM1-type OIB volcanics (Turner et al, 2017). The archetypal EM1-type enrichment at Gough has been specifically linked to the incorporation of subcontinental lithospheric mantle with compositions similar to that sampled by magmas in South America (Gibson et al, 2005;Boyet et al, 2019). Trace element and isotopic ratios of the inverted Gough source composition were subsequently adjusted within the limits of EM1-like ocean island basalts (Stracke et al 2003;Willbold and Stracke, 2010) to produce a generic EM1 source (see Appendix A).…”

“…This is apparent from the similarly high La/Sm ratios of basalts from the Gough Island chain. Like Andean rear-arc lavas, Gough lavas are alkali basalts produced by low-F melting of garnet lherzolite (Søager et al 2015a;Boyet et al, 2019). In Gough, the very low degree melting occurs within a region of enriched ambient mantle, leading to lower 143 Nd/ 144 Nd and even higher La/Sm ratios than the SVZ rear-arc volcanics.…”