If Rocks Could Talk ... the Ion Microprobe Extracts Hidden Clues about Our Planet's History and Evolution

Article excerpt

Every rock on Earth contains a clock, a thermometer, and a barometer. Inside all rocks are elements, or isotopes of elements, called "natural tracers" By examining the presence, proportion, and distribution of natural tracers within rocks, we can reveal the conditions under which the rocks formed. They can tell us when the rocks formed (clocks), how fast they cooled and how they crystallized (thermometers), and the temperatures and pressures they experienced at their creation (barometers).

Just as radioactive tracers are used to understand the dynamics of chemical reactions, natural tracers in rocks can be used to help decipher the whens, wheres, and hows of the complex geological processes that create and maintain our planet. With the right tools, we can extract long-dormant, hidden information about Earth's inner workings from rocks.

The Ion Microprobe Facility at Woods Hole Oceanographic Institution is just such a tool. With it, we can peer far back in time and deep into the Earth. To understand processes that form new oceanic crust, for example, we have used the WHOI ion microprobe to study ancient rocks from the crust and underlying mantle, which have been thrust up and exposed on land (in formations known as ophiolite massifs). And we have compared those with rocks from active mid-ocean ridges on the seafloor.

We can also peer into rocks from the surface to incredible depths--almost 450 kilometers down--by probing mineral inclusions in some diamonds formed under pressures at great depth. (Inclusions are minute foreign bodies enclosed within the mass of another mineral.)

Extracting information from rocks

The ion microprobe offers great advantages over previous methods to glean natural tracer information out of rocks. Before, scientists had to break apart sample rocks and extract minerals containing specific tracers. The purified minerals were then chemically processed, and the amounts or types of tracers were determined using various instruments.

It is a painstaking and time-consuming process, and something important is destroyed in the process of mineral extraction and purification: the textural relationships in which mineral crystals occur in the rock. Rock texture is significant because it reflects the dynamic conditions under which minerals crystallized, and it presents a geologic framework within which to interpret the tracer information.

For example, if a rock forms while conditions around it are changing, the minerals in the rock will show different textures or grain sizes depending on the conditions. This information is lost in traditional processing, but retained with ion microprobe analysis because the rock is not broken up. With the ion microprobe, we can look at the composition of very small samples and identify components in situ, even over distances only micrometers apart.

From electron beams to ion beams

The first tools that allowed analysis of a sample's composition without chemically processing it were earlier electron-beam microprobes. These machines generated electron beams and focused and directed them at a rock sample. The electrons hitting a sample caused the production of X-rays, and measuring the X-ray spectra allowed us to determine the chemical composition of the samples.

In contrast, ion microprobes use focused beams of ions (charged atoms) to bombard a sample. Ions are much heavier than electrons, and the ion beam causes the sample to eject atoms and ions, rather than just emit X-rays. When the ion beam strikes the sample, atoms and ions are "sputtered" (sprayed out) from the sample.

The ion microprobe has two basic parts: the ion-beam source, which focuses and directs microbeams of ions onto the sample; and the mass spectrometer, which measures the signal intensities (abundances) of ions ejected from the sample. Sputtered ions are accelerated into the mass spectrometer, where the ions are detected and distinguished based on their different masses and charges. …