Compositional convection caused by olivine crystallization in a synthetic basalt melt

Abstract

Compositional convection in magma chambers is thought to be an important process in the fractionation of liquid from crystals during the differentiation of magmas. It has been tested for in this study by undertaking isothermal crystal growth experiments in a silicate melt at atmospheric pressure in air. The melt used is a synthetic basalt in which iron is replaced by cobalt to minimise redox problems. Co-Mg olivine rims were overgrown on forsteritic olivine seeds cemented to the floor of a 2.4 cm deep alumina crucible. Following quenching and sectioning, glasses were examined optically for colour variations and by EPMA for compositional variations. It had been expected that the colour intensity of the blue glass would diminish in the Co-depleted zone that develops around crystal overgrowths, whereas in fact little difference is normally found, except for a slight fading of colour in glass above the apex of a seed in a few experiments. By contrast EPMA revealed zones up to 50 gm wide around seeds that are depleted in Co and Mg by up to 25 % at the crystalglass interface and in patches above some crystals. Contour maps of X-ray count-rate data obtained in grids of analytical points show Co-and Mg-depleted glass around the overgrowths and in patches above the highest point of each seed, demonstrating that convection in the melt does occur during growth of individual crystals. As the experiments were carried out in a stable temperature gradient and the crystal seeds had no contact with the melt meniscus, thermal and surface-tensional convection are both eliminated, and the convection is inferred to be caused by a density difference resulting from compositional variation across the chemical boundary layer around a growing crystal. The density difference between the inside and outside of a boundary layer is calculated to be approximately -1%.

Magmatic karst reveals dynamics of crystallization and differentiation in basaltic magma chambers

“…We can envisage only one mechanism of magma differentiation that is able to operate in such extremely confined spaces. This is compositional convection governed by a gravitational instability of a thin liquid boundary layer around in situ growing magnetite crystals 7 , 8 , 35 , 65 . During magnetite crystallization, such a boundary layer gradually increases in thickness and decreases in density until it obtains sufficient buoyancy to be released upwards into the main magma body, either as a constant stream of melt or as a series of plumes 35 .…”

Magma–Carbonate Interaction Processes and Associated CO2 Release at Merapi Volcano, Indonesia: Insights from Experimental Petrology

“…Minor amounts of compositional convection (e.g. Seedhouse & Donaldson, 1996) may also give rise to the mingled textures and may be an additional mechanism of mass transport. The major interactive process between the melts, however, is chemical mixing by interdiffusion.…”

In situ crystallization of non-cotectic and foliated igneous rocks on a magma chamber floor

“…As magnetite components are extracted from the overlying liquid, it results in a boundary layer that becomes increasingly richer in plagioclase components until it is light enough to convect. The boundary layer may break away as a constant stream of melt or as a series of plumes [61][62][63][64] . While it is not clear what convective style will dominate, for the model presented here to work, we must assume that the boundary layer breaks away periodically as a series of plumes.…”