Forrest Horton
Associate Scientist
Department of Geology & Geophysics
Woods Hole Oceanographic Institution
Hotspot Lavas
Lavas erupted above hotspots that form Iceland, Hawaii, Samoa, Galapagos, and other ocean islands often have higher 3He/4He ratios than regular seafloor basalts. Because of their isotopically light helium (they have more primordial 3He), these lavas are thought to derive from mantle reservoirs that are very old and perhaps very deep. They provide unique glimpses of mantle processes and help us answer questions about long-term mantle evolution: How well mixed is the mantle? And to what extent has it been degassed?
Baffin Island
Lavas erupted on Baffin Island in Arctic Canada have the highest 3He/4He found in any terrestrial igneous rocks and arguably contain the most pristine primordial mantle material exposed on Earth's surface. I conducted fieldwork on Baffin Island in 2018 to investigate these unique rocks. Noble gases, lithophile element isotopes, and trace elements will help us interpret the origins of these rocks.
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The figure to the right shows a sequence of Baffin Island lava flows. Submarine hyaloclastite and lava flows were overlain by an orange pillow basalt sequence as the accumulating lava flows climbed out of the ocean. The upper flows are subaerial and contain abundant large olivine phenocrysts.
Helium Behavior
Despite the importance of helium for studying Earth's mantle, the behavior of helium under magmatic conditions is not well understood. My research employs high resolution 3-D imaging (X-ray computed tomography) of olivine crystals from ocean island lavas to understand the distribution of helium and other volatiles in rocks and minerals. 3-D maps help us to target individual olivines and even individual helium-bearing inclusions in olivine. I am developing new extraction techniques that allow us to extract gases from minerals at smaller and smaller scales.
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My research indicates that most magmatic helium resides in fluid inclusions (at least for olivines from American Samoa). Helium becomes decoupled from CO2 (and probably other gases) during ascent in a magma, probably due to some combination of diffusion and escape through cracks.
For more information, see our paper:
Horton, F., Farley, K., and Jackson, M., 2019, Helium distributions in ocean island basalt olivines revealed by X-ray computed tomography and single-grain crushing experiments: Geochimica et Cosmochimica Acta, v. 244, p. 467–477.