Experimental constraints on shallow differentiation of high-Mg andesite at Whakaari, New Zealand

Phase equilibrium experiments were used to determine conditions of melt evolution and phenocryst growth in high-Mg andesite magmas that were erupted at Whakaari (White Island) in New Zealand between 1976 and 2000. The high-Mg andesites are both mafic (7.21–10.3 wt% MgO) and silica-rich (55.3–58.6 wt% SiO₂) with phenocrysts of plagioclase, orthopyroxene, clinopyroxene, olivine, Cr-spinel and Fe–Ti oxides contained in a glassy to fine-grained matrix of mostly dacitic composition. Experiments were conducted on one of the most primitive samples available (the high-Mg andesite TRW34) at conditions ranged from 1 atm to 500 MPa at temperatures of 950 to 1200 °C with total water concentrations of 0 to 10 wt%. Except for the 500 MPa experiments, ƒO₂ was buffered at 1 or 2 log units above Ni–NiO. Consistent with earlier thermodynamic modelling, our results demonstrate that residual Whakaari melts (now represented by matrix glasses) evolved along a plagioclase + two-pyroxene cotectic (± magnetite ± ilmenite) under comparatively low-pressure, shallow conditions (< 200 MPa or < 6 km) and were relatively hot (> 950 °C) and dry (≤ 3 wt% melt-H₂O), with oxygen fugacities either at, or slightly above Ni–NiO + 1 log unit. Although the bulk-rock trends of Whakaari volcanic rocks are clearly calc-alkaline, those of the residual matrix glasses are only weakly so. A likely explanation for this contrast is that the primary magmas were relatively hydrous, but became dehydrated when intruded at shallow depths. The effectiveness of water in this role, combined with the demonstrable presence of primitive calc-alkaline magmas in the upper-crust, highlights the importance of magmatic water, in place of deep crustal fractionation, for shaping the calc-alkaline evolutionary trend.