COG3 Seminar: Forrest Horton (WHOI)

Friday, April 20, 2018 at 10:00am to 11:00am

Building E25, 119
45 CARLETON ST, Cambridge, MA 02142

Helium and CO2 distributions in ocean island basalt olivine phenocrysts revealed by X-ray computed tomography and single-grain crushing experiments

The isotopic composition of helium carried in basaltic magmas is diagnostic of mantle origins. Nevertheless, the scarcity of helium in geologic samples complicates efforts to understand helium behavior in magmatic systems and limits the utility of helium as a geochemical tracer. X-ray computed tomography of olivine phenocrysts from ocean island basalt (OIB) and single-grain crushing experiments can reveal helium distributions at finer scales than previously possible in natural samples. In OIB phenocrysts, helium primarily resides in fluid inclusions, and a small fraction of the grains contain the vast majority of the fluid inclusions. Low CO2 pressures in most phenocrysts and ubiquitous evidence of fluid inclusion decrepitation indicate that volatiles trapped in OIB olivine generally do not correspond to the conditions of olivine growth. Intriguingly, CO2/3He ratios vary by nearly an order of magnitude among the olivines from each basalt. This can be attributed to extensive helium diffusion out of fluid inclusions through the olivine lattice during magma ascent and cooling. These observations highlight the fact that complex subgrain mineral mechanics and intragrain diffusion control the volatile contents of OIB olivines.

About the Speaker

Forrest Horton studies how Earth's crust and mantle have evolved over time using a variety of quantitative geochemical and geochronologic techniques.

About this Seminar

The Chemical Oceanography, Geology, Geochemistry, and Geobiology Seminar [COG3] is a student-run seminar series. Topics include chemical oceanography, geology, geochemistry, and geobiology. The seminars take place on Fridays from 10-11am in Building E25, Room 119, unless otherwise noted (term-time only).

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Department of Earth, Atmospheric and Planetary Sciences
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