Dan McKenzie

The Structure of Planetary Lithospheres and the Generation of Melt

Current research projects:

• The gravity field of the Earth and other rocky planets and satellites contains a great deal of information about their internal structure. On Earth at wavelengths of less than 200-500 km the signal mostly comes from topography, and can be used to find the elastic thickness of the lithosphere. There has been a major controversy about the elastic thickness of the old parts of the Earth's continents: whether it is greater than 100 km, as many people have believed for more than twenty years, or everywhere less than 40 km. I think this issue is now settled, in favour of the smaller value, which is also consistent with the depths of earthquakes. Together with Keith Priestley, James Jackson and our students and postdocs, I am carrying out a number of regional studies of the temperature and velocity structure of the continental and oceanic lithosphere, with the aim of understanding its rheology.
• The gravity field of Venus, Mars and the Moon controls the orbits of spacecraft, and has been intensively studied by NASA for this reason. Several recent Ph.D. theses have been concerned with using the raw tracking data to estimate the elastic thickness of the lithospheres of these bodies at the time the loads were imposed. On Venus the loads are relatively young (200-500 Ma), and the elastic thickness is everywhere about 30 km. On Mars the elastic thickness varies from about 15 km for loads imposed very early in its history to more than 80 km now. This behaviour results from the cooling of the planet as the radioactivity of potassium decays. The Moon shows a very rapid change in elastic thickness between the time the Highlands formed and the major impacts that generated the mare basins. The cause of this change is uncertain, partly because these events are not yet well dated.
• Ten years ago I started a major field program in NE Iceland. The principal aim of the project was to use all available geochemical and petrological techniques to understand the nature of the source of the melt, and how it evolved as it moved to the surface. The project has generated a large number of publications and has involved a number of geochemistry laboratories in the U.S., France and Germany. The two most important results to come from these studies is that melt is initially generated in small volumes, of about 0.01 km3, and moves upwards in channels. Surprisingly, percolation is unimportant. These small volumes separate from the melting rock at depths between 120 km and the Moho, at 20 km, and are partially mixed in a relatively small magma chamber before being erupted. However, the composition of even single flows is variable, because the melt still retains some of the original variability. The dominant process that controls melt composition is mixing, both in the mantle as the melt moves upwards with little fractionation, and in the crust where it mixes with evolved lavas.
• The other major result concerns the origin of isotopic heterogeneities. In Iceland these result from the evolution of mantle material formed by the extraction of between 0.5% and 1% melt from a mantle source, and not from subducted ridge basalts or sediments. The source composition required is the same as that of Suiko, one of the Emperor seamounts of the Hawaiian Chain. It now seems likely that subduction of such seamounts is the principal process that generates isotopic heterogeneity in the mantle.