The role of water in generating the calc-alkaline trend: New volatile data for aleutian magmas and a new tholeiitic index

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 many TH–CA discrimination diagrams is enrichment in Fe during magma evolution, we have developed a quantitative index of Fe enrichment, the Tholeiitic Index (THI): THI = Fe_4·0/Fe_8·0, where Fe_4·0 is the average FeO* concentration of samples with 4 ± 1 wt % MgO, and Fe_8·0 is the average FeO* at 8 ± 1 wt % MgO. Magmas with THI > 1 have enriched in FeO* during differentiation from basalts to andesites and are tholeiitic; magmas with THI 7 wt % (Augustine), spanning the global range in arc mafic magmas. Within the Aleutian data, H₂O correlates negatively with THI, from strongly calc-alkaline (Augustine, THI = 0·65) to moderately tholeiitic (Shishaldin, THI = 1·16). The relationship between THI and magmatic water is maintained when data are included from additional arc volcanoes, back-arc basins, ocean islands, and mid-ocean ridge basalts (MORBs), supporting a dominant role of magmatic water in generating CA trends. An effective break between TH and CA trends occurs at ∼2 wt % H₂O. Both pMELTs calculations and laboratory experiments demonstrate that the observed co-variation of H₂O and THI in arcs can be generated by the effect of H₂O on the suppression of plagioclase and the relative enhancement of Fe-oxides on the liquid line of descent. The full THI–H₂O array requires an increase in fO₂ with H₂O, from ≤FMQ (where FMQ is the fayalite–magnetite–quartz buffer) in MORB to ∼ΔFMQ +0·5 to +2 in arcs, consistent with inferences from measured Fe and S species in glasses and melt inclusions. A curve fit to the data, H₂O (wt % ± 1·2) = exp[(1·26 – THI)/0·32], may provide a useful tool for estimating the H₂O content of magmas that are inaccessible to melt inclusion study.