RE-EVALUATION OF THE PROCESS PRODUCING EARTH'S GRANITES AND CONTINENTAL CRUST

Earth is unique among the planets in our solar system, having two types of crust: oceanic crust, consisting of dense rock with a composition similar to basalt; and continental crust, consisting of a variety of rocks with a less-dense composition similar to granite. By comparison, other terrestrial planets (e.g., Mars and Venus) have crusts composed only of basalt and do not have contrasting continents and oceans. Our bimodal crust governs many geological phenomena and thus, ultimately, the evolution of complex life.

Yet a key question remains unanswered. What geological processes led to formation of the distinctive granitic rocks in our continental crust? In the first half of the twentieth century there was great debate between (a) an igneous magma origin, i.e., formed by intrusion and then cooling of magma, and (b) a metamorphic origin, i.e., formed by changing the composition of preexisting rock without an igneous component.

Now Professor Lundstrom is proposing a wholly different theory of origin. Recent experiments in his laboratory have shown that rocks of granitic composition can be produced by a mechanism he terms diffusion-reaction. Basically, under high pressure and at high temperature, the transport properties of H2O can diffuse atoms through the rock, resulting in chemical reactions that change the rock’s mineral phases. In the natural world, this process could take place where hot ascending magmas pool beneath and “steam cook” overlying volcanic rocks, converting them over million-year time-scales into granite.

During his Center appointment Professor Lundstrom will focus on a combined experimental, observational, and numerical modeling approach to test his hypo-thesis. Using high pressure-temperature facilities in the laboratory, his group will seek to determine the key parameters related to isotopic fractionation in partially molten rocks. They will also use a state-of-the-art mass spectrometer to measure isotope ratios in both experimental products and samples from natural rock formations in California and Minnesota. The resulting data should confirm whether water--one of the most extraordinary elements in the universe--played a critical role in forming Earth’s distinctive continental crust.