4.4 billion years of maturation of the continental crust?

The Earth is divided structurally into several different shells, an iron rich core, and silicate (rocky) mantle and crust. The continental crust is generally topographically high, and the oceanic crust is low lying, a consequence of their distinct density and composition. This bi-model crustal structure is unique within the solar system. Also unique is the free water at the Earth's surface, (the hydrosphere), and a vital part for the creation of life on Earth, constantly shaping and reacting with the rocks at the its surface. Geophysical methods, such as the transmission of seismic waves have mainly revealed the broad structure of the Earth, but geochemistry (the chemical study of geological samples) has revealed much about the processes by which the crust formed. The bulk Earths composition is similar to that of parent meteorite bodies, but different parts of Earth have very different compositions. The so called 'enriched' continental crust has a complementary composition to the so called 'depleted' mantle, consistent with the crust having been extracted from the mantle as a melt product early in Earths history. There is increasing evidence for the formation of the continental crust to be old, with a large portion of it having formed in the first half of Earth's History. However, the details of the processes by which the crust formed by partial melting of the mantle remain highly controversial. One difficulty with establishing and testing models for the extraction of the crust from the mantle, is that the crust is old, and its composition may have changed over Earth's history. Because much of the crust is old and continually reworked by sedimentary and plate tectonic processes, much of the crust has been altered or weathered. The soil that mantles much of the present surface of the Earth is the modern manifestation of such weathering. During continental weathering, many soluble elements (such as those found in a mineral water for example) get transferred to seawater. Some elements get cycled from seawater back into rocks. Calcium is precipitated as calcite shells by many marine organisms which eventually becomes limestone. Other elements, such as magnesium, are returned to the oceanic crust during water-rock interaction in the fluid convection cells that cools the hot oceanic crust after magma generation at mid-ocean-ridges. Over long time-scales, magnesium is transferred from the continents to the oceanic crust. Magnesium (a soluble element) is particularly depleted in the continental crust compared to mantle rocks, and it is important to constrain the extent to which this reflects weathering of the crust, or melt processes. This project proposes to evaluate the degree of alteration of the continental crust by exploiting very small differences (as low as 1 part per 10000 parts) in the isotope ratios of elements such as magnesium, (the fifth most abundant element in the continental crust). Such differences become imparted to the rock record during chemical reactions. In particular, low temperature reactions (such as weathering) create a distinct and larger signature compared to high temperature reactions (melting). Recent advances in mass spectrometry have made such minute differences in isotope ratios detectable. Detection of distinct Mg isotope ratios in the continental crust compared to the mantle will enable the quantification of the loss of Mg by weathering from the continental crust. This will enable refinement of models for the formation of the continental crust, because the initial composition will be better constrained.