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

Horton, Forrest and Farley, Kenneth A. and Jackson, Matthew (2019) Helium distributions in ocean island basalt olivines revealed by X-ray computed tomography and single-grain crushing experiments. Geochimica et Cosmochimica Acta, 244 . pp. 467-477. ISSN 0016-7037. https://resolver.caltech.edu/CaltechAUTHORS:20181029-094838605

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Abstract

X-ray computed tomography of individual olivine crystals in basalts from Ofu and Olosega islands, American Samoa, reveals that a small fraction of the olivines contain the vast majority of the fluid inclusions. Single-grain crushing experiments demonstrate that He and CO_2 reside primarily in these inclusions. Low CO_2 pressures in most grains, corresponding to depths of less than 1 km, provide evidence of ubiquitous decrepitation and associated pressure reduction in the fluid inclusions. Even so, the olivines with the highest inclusion volumes yielded sufficient He to obtain precise He concentrations and isotopic compositions. Within analytical uncertainty, ^3He/^4He ratios are homogeneous among the olivines from each basalt, but among basalts, the ratios range from 21 to 35 Ra. The total range in C/^3He ratio within the analyzed olivines is from 3.6 × 10^7 to 1.5 × 10^(10), and varies by nearly an order of magnitude within the olivines from each basalt. We postulate that this wide range of C/^3He ratios is caused by grain-scale decoupling of C and ^3He due to extensive He diffusion out of fluid inclusions through the olivine lattice during magma ascent and cooling. If so, primary Ofu-Olosega magmas probably had C/^3He ratios less than 4 × 10^8, which is lower than previous estimates for hotspot magmas.


Item Type:Article
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URLURL TypeDescription
https://doi.org/10.1016/j.gca.2018.10.013DOIArticle
ORCID:
AuthorORCID
Farley, Kenneth A.0000-0002-7846-7546
Additional Information:© 2018 Elsevier. Received 30 April 2018, Revised 1 October 2018, Accepted 12 October 2018, Available online 28 October 2018. This research was supported by NSF award number 1650308. We thank the members of the X-ray CT Facility at University of Texas Austin; William Carlson and Jessica Maisano, in particular, provided crucial assistance and advice. A thorough review by Jonathan Tucker improved this manuscript, as did two anonymous reviews.
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Funding AgencyGrant Number
NSFEAR-1650308
Record Number:CaltechAUTHORS:20181029-094838605
Persistent URL:https://resolver.caltech.edu/CaltechAUTHORS:20181029-094838605
Official Citation:Forrest Horton, Kenneth Farley, Matthew Jackson, Helium distributions in ocean island basalt olivines revealed by X-ray computed tomography and single-grain crushing experiments, Geochimica et Cosmochimica Acta, Volume 244, 2019, Pages 467-477, ISSN 0016-7037, https://doi.org/10.1016/j.gca.2018.10.013. (http://www.sciencedirect.com/science/article/pii/S0016703718305994)
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:90456
Collection:CaltechAUTHORS
Deposited By: George Porter
Deposited On:29 Oct 2018 17:41
Last Modified:03 Oct 2019 20:25

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