Melt inclusion analysis of the Unzen 1991–1995 dacite: implications for crystallization processes of dacite magma

Abstract

Dacitic magma, a mixture of high-temperature (T) aphyric magma and low-T crystal-rich magma, was erupted during the 1991-1995 Mount Unzen eruptive cycle. Here, the crystallization processes of the low-T magma were examined on the basis of melt inclusion analysis and phase relationships. Variation in water content of the melt inclusions (5.1-7.2 wt% H 2 O) reflected the degassing history of the low-T magma ascending from deeper levels ($250 MPa) to a shallow magma chamber ($140 MPa). The ascent rate of the low-T magma decreased markedly towards the emplacement level as crystal content increased. Cooling of magma as well as degassing-induced undercooling drove crystallization. With the decreasing ascent rate, degassing-induced undercooling decreased in importance, and cooling became more instrumental in crystallization, causing local and rapid crystallization along the margin of the magma body. Some crystals contain scores of melt inclusions, whereas there are some crystals without any inclusions. This heterogeneous distribution suggests the variation in the crystallization rate within the magma body; it also suggests that cooling was dominant cause for melt entrapment. Numerical calculations of the cooling magma body suggest that cooling caused rapid crystal growth and enhanced melt entrapment once the magma became a crystal-rich mush with evolved interstitial melt. The rhyolitic composition of melt inclusions is consistent with this model.

Spine growth and seismogenic faulting at Mt. Unzen, Japan

“…In order to solve the problem of heat transfer during slip events (prior to melting), we use the 1‐D calculation for temperature change at negligible distance from a heat source (i.e., a slip zone of 0 thickness) derived by Carlslaw and Jaeger []: where heat capacity ( C p ) is estimated at 1000 J kg −1 K −1 at 900°C, thermal diffusivity ( k ) is estimated at 0.7 mm 2 s −1 [ Nishimura et al , ; Cordonnier et al , ], time ( t ) is constrained by dividing the average slip distance by the slip velocity constrained from the seismicity, and the heat flux ( q ) is defined as q = μσ n V e , in which the coefficient of friction ( μ ) is set to 0.85 to constrain heat generated from the onset of a slip event at low normal stresses [cf. Byerlee , ].…”

“…where heat capacity (C p ) is estimated at 1000 J kg À1 K À1 at 900°C, thermal diffusivity (k) is estimated at 0.7 mm 2 s À1 [Nishimura et al, 2005;Cordonnier et al, 2012b], time (t) is constrained by dividing the average slip distance by the slip velocity constrained from the seismicity, and the heat flux (q) is defined as q = μσ n V e , in which the coefficient of friction (μ) is set to 0.85 to constrain heat generated from the onset of a slip event at low normal stresses [cf. Byerlee, 1978].…”

Magma chamber of the Campi Flegrei supervolcano at the time of eruption of the Campanian Ignimbrite

“…The associated melt would have lost some water due to a decrease in water solubility. Under these new pressure-temperaturecomposition conditions, the difference between magma temperature (ϳ980 ЊC) and the liquidus temperature (T liquidus Ͼ 1020 ЊC for Ͻ3 wt% of dissolved H 2 O) represents an undercooling driving force (Nishimura et al, 2005) that induced magma crystallization and a suddenly increased growth rate, enabling the easy trapping of MIs and a slight differentiation in trapped melts. It has also been demonstrated for dacites from Mount Pinatubo, Philippines, that, for a decrease of 70 MPa over the short time interval of three weeks, crystallization began immediately via the growth of existing crystals, resulting in the extensive trapping of MIs (Hammer and Rutherford, 2002).…”

Controls on explosive-effusive volcanic eruption styles

“…Higher dissolved volatile contents (in particular H 2 O) should promote faster ascent through exsolution-driven expansion 24 , as is sometimes the case 18 , 25 – 28 . However, similarly high-water contents can also lead to slow ascent rates and effusive eruptions 17 , 29 – 31 . This issue is compounded by the difficulty in gaining accurate dissolved volatile data for magmas 32 – 34 .…”