Applications of geochemistry to igneous petrology

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Questions of interest and importance to us all are how did the oceans, continents, atmosphere and interior of the Earth form, and how do these Earth systems operate and interact today. Answers affect our understanding of present-day climate, human impacts on the atmosphere and ocean, and where to look for resources that sustain society. A problem is that the largest part of the Earth, the mantle (interior of Earth), is inaccessible and our understanding of it is poor. My research uses basalt lava flows to study the interior of the Earth. Basalts are the most common rocks on Earth and they form due to melting of the mantle. Studies of basalts from the ocean basins tell us that there are different types of mantle. For example, one type apparently had elements such as potassium and silicon extracted from it to form the continents billions of years ago. Other types may contain "recycled" rocks that were once at the Earth's surface. Mantle rocks beneath the continents appear to have unusual compositions that make them the source of most diamonds. An important question is how the types of mantle formed because they will contain a record of how the planet's oceans, continents and atmosphere evolved. By comparing the concentrations of elements in different "basalts" from the ocean basins and continents, it will be possible to answer these questions. However, the comparisons are not easy. For example, when molten basalt crystallizes to form minerals, different elements enter different minerals. Some minerals form early and some form late. The result is that the composition of a basalt changes as elements get used up. This process is called "differentiation". Although this complicates studying the interior of the Earth, these processes can lead to ore deposits because some minerals that form are rich in nickel, chromium, platinum and other valuable elements. In summary, the research is of substantial human interest because basalts are important to understanding the evolution of the Earth. However, the work also has economic implications. It may help explain why some ore deposits (e.g. diamonds and platinum) occur where they do, or point to chemical tests that assist in discovering new deposits.