Tracking magma movement using olivine crystal “clocks”

Images of olivine from Hawaiian volcanoes. In olivine the abundance of magnesium (Mg) is expressed as the forsterite content (Fo)—which is a ratio of how much Mg there is compared to the iron (Fe). Left: Green olivine from Mauna Loa’s 1852 eruption, viewed under a microscope. USGS photo by K. Lynn. Middle: Zoomed in electron image of the inside of an olivine from Kīlauea’s December 2020 eruption, where grayscale indicates the relative abundance of iron (Fe). The darker core (black inside) of the olivine is higher in Mg (and a higher Fo content) than the lighter rim (gray outside). This crystal is approximately 800 microns (0.3 inches) across. Right: Another electron image of olivine from 29 September 2021 that also has changes in Fo content between the core and rim. This crystal is smaller, only 400 microns (0.15 inches) across. Images from the University of Hawai‘i at Mānoa electron microprobe.