What Prevents the Crystallization of Magma from the Roof Downwards in Large Basaltic Magma Chambers?

Abstract

A common feature of layered intrusions is the absence of the Upper Border Series (UBS)—a rock sequence that is expected to grow from the roof downwards in large basaltic magma chambers. This is surprising because magma cooling occurs predominantly through the roof of magma chambers. The lack of the UBS is thus indicative of some fundamental reason that prevents basaltic magmas from crystallizing at the chamber roof. Here, we propose that this could be attributed to a layer of anatectic granitic melt that develops on the top of evolving basaltic chambers. The layer may be formed by partial melting of silica-rich crustal rocks in response to the upward transfer of heat from the crystallizing magma. The heat transfer takes place through the initial UBS, resulting in this unit becoming suspended between two buoyant liquids: granitic melt above and basaltic melt below. This unstable position leads to the disruption of the UBS and its collapse onto the temporary chamber floor. A two-layer magma system thus forms, with a light granitic melt floating atop a denser basaltic magma. The granitic melt is expected to be superheated with respect to its liquidus temperature via heat transfer from the underlying basaltic magma. In the absence of a solid surface at the top of the basaltic magma layer, the growth of the UBS cannot resume due to the lack of a solid place for new crystals to attach. Further crystallization of basaltic magma may therefore only continue from the base upwards, producing a layered intrusion that consists of only floor rock sequences.