Using titanomagnetite textures to elucidate volcanic eruption histories

Mineral assemblages in volcanic rocks record both pre-eruptive conditions and changes experienced by magma as it rises. Titanomagnetite in andesitic magmas is especially sensitive to changes in temperature and oxygen fugacity immediately prior to and during eruptions. Two end-member eruption states can be distinguished by examining titanomagnetite textures in erupted rocks. Slow-ascent eruptions-characterized by near-stagnant magma bodies and slow effusion of lava domes-show solid-state exsolution of titano-hematite/ilmenite lamellae within titanomagnetite hosts. By contrast, fast-ascent eruptions-characterized by rapid chilling of magma in sub-Plinian eruptions-contain titanomagnetites; without such exsolution features. This mineralogical distinction is particularly useful in examining very fine-grained distal tephra layers where other characteristic properties of the two eruptions types are not present. Such tephra records in lake deposits typically provide the most precise long-term eruption records from andesitic volcanoes. Using an example from Mount Taranaki, New Zealand, we show that by classifying eruption styles within such sequences, the underlying magmatic system processes at a volcano can be elucidated and separated from other environmental factors such as vent/crater configuration.