Extended Data Fig. 2: Whole-rock compositions.
From: Carbon concentration increases with depth of melting in Earth’s upper mantle
(a) Total Alkali Silica (TAS) diagram for the 12 volcanoes. The most primitive magmas erupted from Kilauea, Piton de la Fournaise, Erta Ale, Ardoukoba, Surtsey and Holuhraun (Bardabunga) are subalkaline, with Na2O + K2O contents not exceeding 3.5 wt%, at SiO2 comprised between 43 and 50 wt%. They all plot in the basaltic field, except for the one belonging to Piton de la Fournaise, which is a picrite. The most primitive magmas erupted from Etna and Nyamuragira are mildly alkaline (alkali basalts; Na2O + K2O around 4.0 wt%; SiO2 = 45-46 wt%), whereas those belonging to Erebus, Pico do Fogo, Nyiragongo and Ol Doinyo Lengai are alkaline to highly alkaline, with alkali and silica contents up to 7.4 wt% and down to 35 wt%, respectively. They range in composition from tephrites/basanites (Erebus and Pico do Fogo) to melilitites/nephelinites (Nyiragongo and Ol Doinyo Lengai). (b) Chondrite-normalised spider diagram84. The chondrite-normalized incompatible element patterns demonstrate the discrimination between subalkaline and alkaline rocks. The Bardarbunga basalt has the most depleted pattern, with a flat REE profile [(La/Yb)N = 1.7]. Primitive basalts/picrites from Kilauea, Surtsey, Piton de la Fournaise, and Ardoukoba are progressively enriched in LILE and Nb-Ta and are all characterized by Ti negative anomalies, except for Surtsey. They are characterized by variable HREE abundances at comparable LREE contents, resulting in moderately to significantly steep REE profiles. (La/Yb)N vary from 3.6 (Ardoukoba) to 5.0-6.3 (Kilauea and Piton de la Fournaise), testifying for the increasingly important role played by garnet in their mantle sources. With respect to the other subalkaline rocks, Erta Ale basalt is enriched in Th-U, Nb-Ta, and is typified by steeper REE profiles [(La/Yb)N = 6.5]. Etna and Nyamuragira alkali basalts are enriched in Rb, Ba, Th, U and have analogous REE profiles [(La/Yb)N = 15.3-22.1], at HREE abundances comparable to those of Piton de la Fournaise picrite. With respect to Nyamuragira, however, Etna has lower Nb-Ta and Ti concentrations. Erebus basanite and Pico do Fogo tephrites have Rb-Ba enrichments, Nb-Ta positive anomalies and steep REE profiles, at HREE concentrations comparable to Etna, Nyamuragira and Piton de la Fournaise basalts/alkali basalts [(La/Yb)N = 10.1-19.1]. The Nyiragongo and Ol Doinyo Lengai melilitites/nephelinites exhibit the most significant enrichments in incompatible elements, accompanied by Ti negative anomalies and very steep REE patterns, with (La/Yb)N up to 41.9-50.9 at YbN values comprised between 17 and 23.
