Take my place

The chemistry of igneous rocks is much more complicated than described above. But it is the complications that fascinate geologists, because they allow us to become detectives. Tiny and progressive changes in the mineral and chemical compositions of the rocks are the clues to their history. Over the years, this type of forensic geology has led to an ever increasing understanding of how Earth, and the other planets, formed and continue to evolve.

Geologists measure more than the abundant (major) elements, such as magnesium, iron, and calcium. Essential to understanding how minerals have moved back and forth between rock and magma as the cycles of melting and cooling continue are elements that are present in small amounts--mere traces. About 40 of these trace elements are measured regularly. Because they are present in small amounts, the abundance of trace elements can be very sensitive to pressure and temperature, to the composition of the parent rock and to the resulting magma. Often clear differences in the type and amount of the trace elements provide critical clues to the evolution of the magmas, even when the major elements, such as magnesium, iron, and calcium don't change.

Consider substitution of elements into mineral structure. We saw that magnesium, iron, and calcium substitute into mineral structures. Trace elements can also have preferences for particular mineral structures. Strontium (Sr), for example, is a trace element. It's similar to calcium in size and ionic charge--so much so that tiny amounts of strontium substitute for calcium in our bones and teeth. Strontium also substitutes for calcium in minerals and will readily substitute into plagioclase feldspar, rather than stay in the magma. Therefore, as plagioclase feldspar is formed, strontium is extracted from the magma, leaving a magma that is almost completely depleted in strontium. Any later-forming minerals from this magma cannot contain much strontium because there's no longer any available. On the other hand, strontium avoids olivine. If olivine is the primary mineral forming, the magma will be enriched in strontium and may later form strontium-rich minerals. Many minerals and elements have similar relationships. Deciphering these relationships as peridotite melts to form magma, and magma cools to become basalt and gabbro, is essential to determining ocean crust evolution.