Abstract

The average andesitic composition of the continental crust leaves a number of outstanding questions concerning its formation. Contemporary melting of the mantle generally forms melts of basaltic composition, and so more exotic models have been proposed to generate crust of an andesitic composition. One such model suggests that the crust has lost Ca and Mg, two of the major components of the crust through chemical weathering. These two elements are highly soluble, and two of the most concentrated cations in many low temperature aqueous fluids at the Earth’s surface, making it potentially plausible that the crust that we observe today, could differ from its pristine protolith. Such “far-fetched” models have naturally received little attention because there has been no method to begin to test them. The stable isotope ratios of elements such as Mg (which shows one of the most important depletions in the crust compared to the mantle) could provide an opportunity to test such hypotheses through mass balance, provided that data can be generated with sufficient precision and accuracy. This talk will consider crustal recycling and review existing geochemical evidence for crustal alteration, but in particular will show how stable Mg isotope ratios could be well suited to tracking crustal evolution. Initial Mg isotope data from crustal materials will be compared that that of the mantle and any discrepancies explored.