Temperatures and H2O contents of low-MgO high-alumina basalts

Experimental evidence is used to estimate H₂O contents in low-MgO high-alumina basalts (HABs) (<6 wt.% MgO) and basaltic andesites (BAs) (<5 wt.% MgO) that occur worldwide in magmatic arcs. Wholerock compositions of low-MgO HABs and BAs, phenocryst assemblages, and mineral chemistry match the compositions of liquids, phase assemblages, and mineral-compositions produced in H₂O-saturated melting experiments on HABs at moderate pressure (1–2 kb). Low-MgO HABs and BAs therefore could have existed as H₂O-rich multiply-saturated liquids within the crust. Results are presented for melting experiments on two HABs and an andesite at 1 kb pressure, H₂O-saturated, with fO₂ at the NNO buffer. These data and other experimental results on HABs are used to develop a method to estimate the temperature and H₂O content of HAB or BA liquids saturated with olivine, plagioclase, and either high-Ca pyroxene or hornblende. Estimated H₂O contents of HAB liquids are variable and range from 1 to 8 wt.%. High-MgO HABs (>8wt.% MgO) could have H₂O contents reaching no more than 1–2wt.%. The more common low-MgO HABs could have existed as liquids within the crust with H₂O contents of 4 wt.% or higher at temperatures<1100°C. Magmas with these high H₂O contents will saturate with and exsolve aqueous fluid upon approaching the surface. They cannot erupt as liquids and must grow crystals at shallow depths, thus accounting for the abundant phenocrysts in low-MgO HABs and BAs.