ON BIMODAL DIFFERENTIATION BY SOLIDIFICATION FRONT INSTABILITY IN BASALTIC MAGMAS, PART 1: BASIC MECHANICS

Abstract

Extensive, coarse-grained silicic lenses are common in the upper portions of large diabase sills, lava lakes, and gabbroic intrusions. These lenses, which often contain up to 10 wt% more silica than their host rocks, record a clear process of bimodal differentiation that may play a fundamental role in generating significant quantities of silicic magma in predominantly basaltic systems. The spatial, compositional, textural, and contact relations suggest that these silicic segregations form by infilling of tears formed during gravitational instability of the upper solidification front. This process is investigated in some detail by exploring the thermal and mechanical conditions leading to internal failure of the front and segregation formation. As the upper solidification front grows inward and thickens, the local strength of the developing crystalline network must increase upward through the front sufficiently fast to offset the increasing weight of the front itself. This necessary local variation in strength dictates how crystallinity must vary to prevent failure. But the actual variation in crystallinity is determined by phase equilibria, which in general varies in a fashion that is noncovariant with that required for mechanical stability. It is this mismatch that apparently leads to failure and formation of silicic segregations. The rate of filling of the tears by porous flow of interstitial melt from below regulates the rate of tear opening, and the overall size of the tears is regulated by the local rate of advance of the solidification front. Segregations therefore thicken downward as the growth of the front slows and then die out altogether with approach of the lower solidification front. This process irreversibly introduces siliceous noise into basaltic systems. The silicic signal can be enhanced through subsequent reprocessing of the host rock to form volumetrically important masses of silicic magma. This process may be fundamental to achieving the strong compositional diversity of the igneous rocks, to yielding continental masses, and to the differentiation of Earth itself.