Abstract

In 1964 Paul Gast discovered that basalts from oceanic islands had quite different radiogenic isotope ratios from those of basalts from spreading ridges, and that these differences required their two source regions to have been separate for Ga. At the time he made this discovery almost all earth scientists believed that the continents had never moved. This belief changed completely over the following five years, to be replaced by a general acceptance that the Earth's mantle is a vigorously convecting fluid with a Rayleigh number of . Gast's discovery, which was quickly confirmed by many other measurements, then slowly became recognised as more and more unexpected. How can the source regions of ocean island basalts remain intact for long periods of time in a vigorously convecting fluid, and, if they are drawn out into thin sheets, as seems almost inevitable, how can melt be extracted preferentially from such a well stirred inhomogeneous material? Attempts to answer these questions have concerned igneous petrologists and geochemists ever since Gast's discovery, and are of central concern to this workshop. Though the resulting work has lead to considerable progress in understanding the process of melt generation, and some progress in understanding how it moves, it has not yet thrown much light on the original question.