Simultaneous crystallization and melting at both the roof and floor of crustal magma chambers: Experimental study using NH4Cl-H2O binary eutectic system

Thermal and compositional evolution of magmas after emplacement of basalt i nto continental crust has been investigated by means of fluid dynamic exper iments using a cold solid mixture with eutectic composition and a hot liqui d with higher salinity in the NH4Cl-H2O binary eutectic system. The experim ents were designed to simulate cases where crystallization of a basalt magm a is accompanied by melting at both the roof and floor of a crustal magma c hamber. The results show that thermal and compositional convection occur si multaneously in the solution; the thermal convection is driven by cooling a t the roof and the compositional convection is driven by melting and crysta llization at the floor. The roof was rapidly melted by the convective heat flux, which resulted in formation of a separate eutectic melt layer (the up per liquid layer) with negligible mixing of the underlying liquid (the lowe r liquid layer). On the other hand, a mushy layer formed at the floor. The compositional convection at the floor carried a low heat flux, so that the heat transfer at the floor was basically explained by simple heat conductio n. The thermal boundary layer in the lower liquid layer at the interface wi th the upper liquid layer became thicker with time and subsequently tempera ture decreased upward throughout the lower liquid layer. Compositional grad ient with NH4Cl content decreasing upward formed by compositional convectio n in the lower liquid layer. The formation of these gradients resulted in f ormation of double-diffusive convecting layers in the lower liquid layer. T he upward hear transfer was suppressed when compared with the case where th e liquid region is homogenized by vigorous convection. These experimental results imply that, when a basalt magma is emplaced in c ontinental crust, floor melting does not always enhance the cooling of the magma, but it may even reduce the total heat loss from the magma to the cru sts due to suppression of convection by formation of a stabilizing composit ional gradient. (C) 2000 Elsevier Science B.V. All rights reserved.