A new microscopy tool in the geologist's kit

In the late 1990s, researchers at Pacific Northwest National Laboratory published an article containing an image of three cervical cancer cells, in which each cell's mitochondria appear as bright yellow splotches in a sea of red cytoplasm. The researchers used no extrinsic labels or staining to generate the image. Rather, they used a technique known as coherent anti-Stokes Raman scattering microscopy. In CARS microscopy, chemical contrast is generated by a pair of lasers whose frequencies, f and f − v, differ by the frequency v of the molecular vibration of interest. A nonlinear interaction then generates a blueshifted signal with frequency f + v. By mapping the signal's intensity as a function of location in the sample, one can rapidly construct molecule-specific, three-dimensional images. The technique has stirred the interest of biologists, and now work by Robert Burruss (US Geological Survey), Albert Stolow (National Research Council Canada), Aaron Slepkov (Trent University, Ontario), and coworkers suggests it could also prove useful in geology, at least for translucent samples. Applied to sedimentary quartz, calcite, and hornblende, CARS produced richly informative maps of the minerals' interiors. The CARS image at right, for instance, shows how tiny pockets of methane are distributed throughout the outlined region of the quartz sample pictured at left. Such information could help elucidate how Earth produces and distributes fossil fuels. (R. C. Burruss et al., Geology, in press, doi: 10.1130/G33321.1.)—Ashley G. Smart