Floating Lids on Large Mid-Ocean Spreading Center Magma Chambers?: A Discussion

Abstract

Rosencrantz and Nelson (1982) have concisely presented arguments in favor of the hypothesis that lids overlying ocean ridge magma chambers are supported by flotation of the lid on magma of equal or higher density. I concur that, given the highly fractured nature of oceanic crust, the lids to oceanic magma chambers can only be supported by flotation, regardless of the width of the magma chamber. In their quantitative analysis Rosencrantz and Nelson implicitly made two assumptions: on the shape of the magma chamber and therefore on the nature and density of the lid; and on the nature, and therefore the density, of the contents of the magma chamber. It is important to realize that in a situation in which either one of these assumptions is thought not to hold, the acceptance of the flotation hypothesis places severe constraints on other properties of the situation. The problem is raised in the light of the Archean ocean ridge model of Sleep and Windley (1982, fig. 3) in the same issue of Journal of Geology, but is of general relevance given our ignorance concerning magma chambers. There have been several suggestions for the shape of ocean ridge magma chambers in situations where permanent magma chambers are thought to exist. Sleep (1975), Rosendahl (1976), and Dewey and Kidd (1977) envisaged a magma chamber narrowing upwards, with a triangular cross-section and a relatively flat base. Hereafter this shape is referred to as an inverted funnel. From their work on the Samail ophiolite in Oman, Pallis