The origin of tholeiitic (TH) versus calc-alkaline (CA) magmatic trends has long been debated. Part of the problem stems from the lack of a quantitative measure for the way in which a magma evolves. Recognizing that the salient feature in manyTH^CA discrimination diagrams is enrichment in Fe during magma evolution, we have developed a quantitative index of Fe enrichment, the Tholeiitic Index (THI):THIFe4·0/Fe8·0, where Fe4·0 is the average FeO* concentration of samples with 41wt % MgO, and Fe8·0 is the average FeO * at 81wt % MgO. Magmas withTHI41 have enriched in FeO * during differentiation from basalts to andesites and are tholeiitic; magmas with THI51 are calc-alkaline. Most subduction zone volcanism is CA, but to varying extents; the THI expresses the continuum of Fe enrichment observed in magmatic suites in all tectonic settings.To test various controls on the develop-ment of CA trends, we present new magmatic water measurements
No takes yet. Share an insight, caveat, or question.
Zimmer et al. (2010) studied this question.
Synapse has enriched 2 closely related papers on similar clinical questions. Consider them for comparative context: