The Role of Water in Generating the Calc-alkaline Trend: New Volatile Data for Aleutian Magmas and a New Tholeiitic Index

Abstract

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):THI1⁄4Fe4·0/Fe8·0, where Fe4·0 is the average FeO* concentration of samples with 4 1wt % MgO, and Fe8·0 is the average FeO* at 8 1wt % 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 development of CA trends, we present new magmatic water measurements in melt inclusions from eight volcanoes from the Aleutian volcanic arc (Augustine, Emmons, Shishaldin, Akutan, Unalaska, Okmok, Seguam, and Korovin). Least degassed H2O contents vary from 2 wt % (Shishaldin) to47 wt % (Augustine), spanning the global range in arc mafic magmas.Within the Aleutian data, H2O correlates negatively with THI, from strongly calc-alkaline (Augustine, THI1⁄4 0·65) to moderately tholeiitic (Shishaldin, THI1⁄41·16). 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