Compositions of HIMU, EM1, and EM2 from global trends between radiogenic isotopes and major elements in ocean island basalts

Abstract

Sr and Pb isotopes exhibit global trends with the concentrations of major elements (SiO 2, TiO 2, FeO, Al 2O 3 and K 2O) and major elements ratios (CaO/Al 2O 3 and K 2O/TiO 2) in the shield-stage lavas from 18 oceanic hotspots (including Hawaii, Iceland, Galapagos, Cook-Australs, St. Helena, Cape Verde, Cameroon, Canary, Madeira, Comoros, Azores, Samoa, Society, Marquesas, Mascarene, Kerguelen, Pitcairn, and Selvagen). Based on the relationships between major elements and isotopes in ocean island basalts (OIBs), we find that the lavas derived from the mantle end members, HIMU (or high ‘μ’ = 238U/ 204Pb), EM1 (enriched mantle 1), EM2 (enriched mantle 2), and DMM (depleted MORB [mid-ocean ridge basalt] mantle) exhibit distinct major element characteristics: When compared to oceanic hotspots globally, the hotspots with a HIMU (radiogenic Pb-isotopes and low 87Sr/ 86Sr) component, such as St. Helena and Cook-Australs, exhibit high CaO/Al 2O 3, FeO T, and TiO 2 and low SiO 2 and Al 2O 3. EM1 (enriched mantle 1; intermediate 87Sr/ 86Sr and low 206Pb/ 204Pb; sampled by hotspots like Pitcairn and Kerguelen) and EM2 (enriched mantle 2; high 87Sr/ 86Sr and intermediate 206Pb/ 204Pb; sampled by hotspots like Samoa and Societies) exhibit higher K 2O concentrations and K 2O/TiO 2 weight ratios than HIMU lavas. EM1 lavas exhibit the lowest CaO/Al 2O 3 in the OIB dataset, and this sets EM1 apart from EM2. A plot of CaO/Al 2O 3 vs K 2O/TiO 2 perfectly resolves the four mantle end member lavas. Melting processes (pressure, temperature and degree of melting) fail to provide an explanation for the full spectrum of major element concentrations in OIBs. Such processes also fail to explain the correlations between major elements and radiogenic isotopes. Instead, a long, time integrated history of various parent–daughter elements appears to be coupled to major element and/or volatile heterogeneity in the mantle source. End member lava compositions are compared with experimental partial melt compositions to place constraints on the lithological characteristics of the mantle end members.