Deciphering magma storage and ascent processes beneath Taranaki volcano, New Zealand, from the complexity of amphibole breakdown textures

Abstract:

Lava flows that constitute the current edifice of Taranaki volcano, New Zealand, contain a complex crystal cargo including plagioclase, pyroxene, amphibole, oxides and rarely olivine. Amphibole textures and mineral chemistry indicate that crystals are pargasitic antecrysts entrained at various depths by ascending magmas. The crystals record a complex growth history within a range of temperature and pressure conditions (c. 950-1000°C and c. 9 - 4 kbar). Most amphiboles show the development of distinct reaction rims where the mineral is in contact with the ambient melt. Texturally, the rims are identified as detached, symplectic, granular or coarse. True rim thicknesses vary little (±20%, 1σ, on average) within individual thin sections but show a large variation between samples from different lava flows, from < 5 µm to > 50 µm. Reaction rim formation on Taranaki amphiboles is attributed to degassing during magma ascent. Associated with the formation of rims, the amphiboles also show two types of volumetric decomposition, one indicative of the role of local kinetic controls on amphibole breakdown during decompression-induced degassing, the other indicative of heating-induced breakdown of the amphibole triggered by their uptake into hot magmas prior to the onset of eruption. The combined evidence indicates that despite the complex and variable growth history of the remobilized crystal cargo, such antecrysts can be used to constrain pre- and syn-eruptive processes and their rates. At Taranaki volcano, magma ascent times calculated from reaction rim thicknesses are estimated to be typically shorter than 10 days.