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Understanding Our Planet Through Chemistry

Application of Instrumental Neutron Activation Analysis

Some of our understanding of the source of molten magma has been obtained by analyzing rocks for a group of 15 elements called the rare-earth elements (REE). In a type of rock called basalt, the total amount of all the REEs is often less than 100 parts per million (ppm).

One well proven analytical technique used to determine the concentrations of REE in rocks and minerals is instrumental neutron activation analysis (INAA). In this technique, a rock or a single mineral that the rock contains is irradiated using a nuclear reactor. This causes the elements to become radioactive and to emit gamma rays with distinct energies. The sample is then placed on a detector that measures how many gamma-rays of these energies are emitted. The number of distinct gamma rays emitted is proportional to the abundance of that particular element.

Photo of reactor. To get better sensitivity necessary to measure rare-earth elements in specific rocks, samples can be irradiated in a low-power reactor. It turns some of the element into an unstable isotope whose decay can then be detected and counted to determine the quantity of the element in the sample.[49k] [171k]

To understand what the REE can tell us about how magmas are formed, scientists have developed mathematical formulas. These formulas suggest that when certain minerals interact with molten rock, there can be appreciable effects on the rock's REE contents. In a process called partial melting, for example, if a source rock contains minerals (such as garnet) that can hold high concentrations of certain REE, then these elements tend to be prevented from entering the molten rock. Because Hawaiian basalts have low concentrations of the heavier REE, and garnet has high concentrations of heavy REE, some Earth scientists conclude that the magmas have formed by partial melting of a source rock that contains garnet, and the garnet held back the heavy REE.

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