Helium and lead isotope geochemistry of oceanic volcanic rocks from the East Pacific and South Atlantic

The isotopic evolution of helium and lead in the Earth is coupled by virtue of their common radioactive parents uranium and thorium. The isotopic signatures in oceanic volcanic rocks provide constraints on the temporal evolution of mantle source regions and volcanic magmas. He and Pb isotopes were measured in glassy basalts from young seamounts in the East Pacific, and in phenocrysts and corresponding whole rocks, respectively, from the island of St. Helena. He isotopes were also measured in glassy mid-ocean ridge basalts from the South Atlantic, previously studied for Pb isotopes by Hanan et al. (1986). A precise reconstruction of He-Pb isotope relationships in volcanic source regions is complicated by post-eruptive radiogenic ingrowth of ⁴He in non-zero age basalts, by pre-eruptive radiogenic ingrowth of ⁴He in magmas with elevated (U+Th)/He, by multi-stage fractionation processes involving (U+Th)/He, U/Pb and Th/Pb and by convective mixing in the Earth's interior. Aspects of each of these problems are addressed. (U+Th)/He ages are estimated from the isotope disequilibrium of ³He/ ⁴He between He trapped in vesicles and that dissolved in the glass phase of young alkali basalts at seamount 6 in the East Pacific. ³He/ ⁴He in the glass phase of these alkali basalts is subatmospheric, while in the vesicles it ranges between 1.2-2.5 RA (RA = atmospheric ratio). ³He/ ⁴He in vesicles (extracted by crushing in vacuo) allows a correction to be made in the dissolved phase He (by fusion of the remaining powder) for the inherited component in order to compute the radiogenic [He]. The method is applicable to rocks containing phases with different (U+Th)/He, and the results have implications for dating lavas in the age range of 0l to 106 years, and for reconstructing the temporal evolution of young volcanic systems.

Department dc:contributor.department

Joint Program in Oceanography

Grantor dc:publisher

Massachusetts Institute of Technology

Year dc:date.issued

1987

Author dc:creator

• Graham, David W. (David William)

Advisor dc:contributor.advisor

• William J. Jenkins.

• Joint Program in Oceanography.
• Earth, Atmospheric, and Planetary Sciences.
• Woods Hole Oceanographic Institution.

Statement dc:rights

• M.I.T. theses are protected by copyright. They may be viewed from this source for any purpose, but reproduction or distribution in any format is prohibited without written permission. See provided URL for inquiries about permission.

Licence dc:rights.uri

• http://dspace.mit.edu/handle/1721.1/7582

Language dc:language.iso

eng

Handle dc:identifier.uri

http://hdl.handle.net/1721.1/53529

OAI identifier oai:identifier

oai:dspace.mit.edu:1721.1/53529