Martian Volcanism: Gateway to the Deep Interior

Professor David Green

Research School of Earth Sciences, Australian National University

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Abstract: Volcanism on planets and their satellites is the expression of melting in the planetary interior and the character of volcanic materials tells us a great deal about the composition and mineralogy of their sources.

This approach continues to provide much information about the Earth’s interior and the processes which continue to cause Earth’s volcanic activity which are otherwise inaccessible. It was not until the first lunar samples were returned that we were able to interpret the lunar surface in terms of primitive ‘Highland’ crust and later flooding by ‘Mare’ basalts of several compositional types. It was also very important to examine the actual samples to determine which samples represented liquid compositions and which were the most primitive or parental liquids. Having made those inferences, the primitive basalts were then crystallised in the laboratory under variable conditions of pressure and temperature, matching those reached in the lunar interior. In this it has been shown that beneath the early-formed lunar crust, the basalt source regions of the lunar mantle are more iron-rich, less silica-rich and more reduced than the Earth’s mantle. Carbon + hydrogen (water and carbon fluid species) apparently play no role in lunar basalt genesis whereas they are of fundamental importance in the Earth’s volcanic processes.

Martian meteorites, many of them collected in Antarctica, have already given some insights. Samples include basalts and also rocks rich in olivine and pyroxene (ultramafic rocks), which suggest comparisons with Earth basalts and Upper Mantle peridotite. Martian basalts are more oxidised than lunar basalts but resemble lunar rather than primitive terrestrial basalts in their relative iron to magnesian ratios. Some samples have provided evidence for small water contents, which would have had profound effects on melting relations in the Martian interior.

With collection of samples from Mars; much greater insight into relationships among volcanics and cumulate or source rocks will be possible. The identification and sampling of explosive volcanic centres (if they occur) will be particularly important as these potentially provide samples of deeper materials, including Martian mantle samples (if the terrestrial analogy is valid). The techniques of experimental petrology at high pressures and temperatures will be used to constrain the conditions and depths of melting in Mars, and particularly the roles of water and carbon dioxide (C-H-O fluids) in controlling melt compositions.

PROFILE: Professor David Green graduated from the University of Tasmania with a Bachelor of Science and went on to complete his PhD in geology at Cambridge University in 1962. He then returned to the University of Tasmania to lecture as Professor of Geology. He has held numerous positions and public pots including Chief Science Adviser, Commonwealth Department Arts, Sport, the Environment and Territories; Chairman of National Committees of the Australian Academy of Science (covering areas such as Antarctic Research, Solid Earth Sciences, Environment and the International Geosphere Biosphere Program); Director, Research School of Earth Sciences, ANU and member of the Australian Space Council. Professor David Green has published hundreds of papers in scientific journals and has been awarded the prestigious Murchison Medal by The Geological Society, UK. He has also been elected as a Fellow of the Royal Society.