Xenolith in alkali basalt from Takashima, Saga Prefecture. Dunite, a kind of peridotite, is representative of the uppermost part of the Earth's upper mantle layer. It occurs frequently as xenoliths in volcanics, e.g., in Cenozoic alkali basalts from Southwest Japan. It is almost completely composed of olivine, the main mantle mineral, and can be formed by deep-seated magmatic processes, such as crystal accumulation from magma or a reaction between pyroxene-bearing peridotite and magma. Although it has a simple mineral assemblage, its genesis is difficult for us to specify. Some olivine grains are kinked to show kink bands (e.g., upper right of the center), due to deformation after magmatic formation. If we observe a thin section, ca. 0.03 mm in thickness, of peridotites under crossed-polarized light with a polarizing microscope, beautiful stained-glass patterns made by olivine grains are visible. The colors observed under crossed-polarized light with a microscope are called interference colors. The vivid interference colors from the 0.03-millimeter thin section in this photo are characteristic of olivine because of its optical properties.
(Photograph & Explanation: Shoji ARAI)
