Crystallization and Vesiculation of the 1984 Eruption of Mauna Loa

Abstract

Calculations based on existing thermodynamic models for silicate melts are used to model the physical and chemical evolution of the 1984 Mauna Loa magma. Specifically, the calculations simulate the vesiculation-induced crystallization of the ascending magma. The calculations provide a physiochemical model to explain the increase in crystallization under isothermal conditions. The evolution of the Mauna Loa magma is modeled in three separate stages corresponding to (1) the subsurface crystallization of the phenocryst assemblage, (2) the crystallization and vesiculation of the magma during its ascent from depth, and (3) the crystallization and vesiculation of the lavas at the surface. The preeruptive crystallization of olivine and orthopyroxene phenocrysts began at _ 1155øC and 0.2 GPa. Olivine and plagioclase alone crystallized during the ascent of the magma. The bulk of the crystallization and vesiculation of the Mauna Loa magma occurred at pressures less than 20 MPa. Under lithostatic pressures this corresponds to depths as shallow as 600-700 m. Ancillary calculations have established the net heat effect of the prescribed ascent path and demonstrate that the heat associated with vesiculation easily compensates for the heats of crystallization. The calculations suggest that the near-surface processes can be quite endothermic depending on the initial H20 content of the magma. For a magma with I wt % dissolved H20 the heat balance demands approximately 5-10øC cooling. Further calculations predict the liquid line of descent for this ascent path and document the corresponding variations in the physical properties of the melt phase. els that describe (1) the solubility of H•O in silicate melts [Nicholls, 1980] and (2) solid-melt equilibria for silicate melts [Ghiorso et al., 1983]. The calculations allow the PT ascent path of the 1984 Mauna Loa magma to be determined from the observed phenocryst and microphenocryst assemblage. An acceptable calculated ascent path must reproduce the increase in crystallinity described by Lipman et al. [1985]. If the model can reproduce the field observations, then the corresponding assumptions are justified. FIELD OBSERVATIONS The field monitoring of the 1984 Mauna Loa eruption began early in the history of the eruption and continued to its conclusion. The observations of Lockwood et al. [1985], course of the eruption implies subsurface crystallization and Lipman et al. [1985], and Lipman and Banks [1987] provide represents significant variations in magmatic conditions. Lip-a continuous record of eruption rate, eruption temperature, man et al. [1985], for example, postulated that the subsur-lava chemistry, and degree of crystallization. Observations face crystallization resulted from the loss of volatiles during the ascent of the magma. In this case it is inferred that the crystallization and vesiculation attended Mauna Loa magma's ascent and eruption. The crystal content, the degree of vesicularity, the phase assemblage, the phase chemistry, and the temperature relate to these two pro...

H 2 O–CO 2 solubility in low SiO 2 -melts and the unique mode of kimberlite degassing and emplacement

“…The sudden loss of volatile will strongly increase the melt liquidus temperature and decrease the temperature of the system since H 2 O-CO 2 volatilization and decompression are endothermic processes (Russell, 1987). This instant degassing must therefore lead to sudden crystallisation of the melt and stalling of the kimberlite ascent at depth.…”