Tristan da Cunha, South Atlantic: Geochemistry and Petrogenesis of a Basanite-Phonolite Lava Series

The island of Tristan da Cunha, located at 37°S, 12°W in the South Atlantic, is the largest of a group of three islands, the others being Nightingale and Inaccessible. Tristan da Cunha comprises a continuous series of alkaline lavas ranging in composition from ankaramitic basanite through phonotephnte and tephnphonolite to phonolite. Moderately porphyritic basanite is the dominant rock type (˜on the island. Major and trace element variations in the lavas describe well-defined trends with increasing differentiation which are generally consistent with control by fractional crystallization of phenocryst phases. None of the lavas can be considered to be primary in composition, mg-numbers range from 40 to 62, and covariation of certain minor and trace elements (e.g., Sr, Ba, P) suggests the presence of at least two distinct fractionation trends. Sr, Nd, and Pb isotopic analyses of a subset of the lavas confirm previously published data for the island, but show a slightly greater range: 87Sr/86Sr = 0˙70495–0˙70517; 143Nd/144Nd = 0˙51259–0˙51247; 206Pb/204Pb = 18˙47–18-˙74. Quantitative modelling of the compositional variations suggests that the ankaramitic basanites are partial olivine + clinopyroxene + titanomagnetite (±minor plagioclase) cumulates, with ˜40% crystal accumulation being required to account for the most porphyritic varieties. The range in composition from basanite to phonotephrite can be accounted for by up to 50% fractional crystallization of clinopyroxene, olivine, titanomagnetite, and plagioclase, with minor apatite and, in some models, amphibole. Average proportions of these phases in the fractionate are Cpx 40, TiMgt 20, Plag 30, Oliv 10. As much as 20% amphibole fractionation is required in models involving the phonotephrites. The compositions of the evolved tephriphonolites and phonolites are consistent with extensive (up to 80%) fractional crystallization of an initial basanitic magma with clinopyroxenc (1–4%), amphibole (17–23%), plagioclase (6–20%) alkali feldspar (0–13%), and titanomagnetite (4–6%)±minor apatite and sphene being the dominant fractionating phases. Inferred trace element and isotopic characteristics of the source regions of the Tristan lavas are distinct from those giving rise to the Walvis Ridge, Gough Island, or Discovery Seamount basalts. Normalized trace element abundances of the Tristan lavas are more similar to those of Marion Island, whereas Sr-, Nd-, dnd Pb-isotopic ratios are most similar to nearby Inaccessible