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Titolo:
Noble gases in the Cameroon line and the He, Ne, and Ar isotopic compositions of high mu (HIMU) mantle
Autore:
Barfod, DN; Ballentine, CJ; Halliday, AN; Fitton, JG;
Indirizzi:
Univ Michigan, Dept Geol Sci, Ann Arbor, MI 48109 USA Univ Michigan Ann Arbor MI USA 48109 pt Geol Sci, Ann Arbor, MI 48109 USA ETH Zurich, Dept Earth Sci, Inst Isotope Geol & Mineral Resources, CH-8092Zurich, Switzerland ETH Zurich Zurich Switzerland CH-8092 ources, CH-8092Zurich, Switzerland Univ Edinburgh, Grant Inst Geol, Edinburgh EH9 3JW, Midlothian, Scotland Univ Edinburgh Edinburgh Midlothian Scotland EH9 3JW Midlothian, Scotland
Titolo Testata:
JOURNAL OF GEOPHYSICAL RESEARCH-SOLID EARTH
fascicolo: B12, volume: 104, anno: 1999,
pagine: 29509 - 29527
SICI:
0148-0227(199912)104:B12<29509:NGITCL>2.0.ZU;2-J
Fonte:
ISI
Lingua:
ENG
Soggetto:
EAST PACIFIC RISE; VOLCANIC-ROCKS; LOIHI SEAMOUNT; TRACE-ELEMENT; INDIAN-OCEAN; ULTRAMAFIC XENOLITHS; MAGMATIC PROCESSES; SUBMARINE BASALTS; HELIUM-ISOTOPES; DEPLETED MANTLE;
Tipo documento:
Review
Natura:
Periodico
Settore Disciplinare:
Physical, Chemical & Earth Sciences
Citazioni:
103
Recensione:
Indirizzi per estratti:
Indirizzo: Barfod, DN Scottish Univ Res & Reactor Ctr, Isotope Geosci Unit, E Kilbride G75 0QF, Lanark, Scotland Scottish Univ Res & Reactor Ctr E Kilbride Lanark Scotland G75 0QF
Citazione:
D.N. Barfod et al., "Noble gases in the Cameroon line and the He, Ne, and Ar isotopic compositions of high mu (HIMU) mantle", J GEO R-SOL, 104(B12), 1999, pp. 29509-29527

Abstract

Ultramafic xenoliths, basaltic lavas, and CO2 gases from the Cameroon linevolcanic chain provide the first characterization of combined He, Ne, and Ar isotopes in a high-time-integrated U-238/Pb-204 = mu (HIMU) magmatic system. Helium isotopic compositions typically range from 5.0 to 6.7R(a), withan average of 6.3. These values are indistinguishable from the He-3/He-4 of other HIMU locales (Austral Islands, St. Helena). Neon isotopic compositions for xenoliths and CO2 gases are mid-ocean ridge basalt (MORB)-like, with a maximum Ne-20/Ne-22 of 11.87 and Ne-21/Ne-22 of 0.0508. Argon isotopic compositions in silicates range from atmospheric to Ar-40/Ar-36 = 4910 +/- 430 (crushing) and up to 16,300 +/- 1000 (single grain, laser step heating)The correlation between Ne-20/Ne-22 and Ar-40/Ar-36 in CO2 gases suggests a minimum Ar-40/Ar-36 = 1650 +/- 30 for the mantle-derived component. Uniform He-3/He-4 in silicates and in CO2 fluids across both the continental andoceanic sectors of the Cameroon line argues strongly for a negligible lithospheric contribution to noble gas isotopic compositions. This inference issupported by high U-238/He-3 in lherzolites, indicating that noble gases in these samples must have been recently introduced (<50,000 years ago) to the sample, most likely from the host magma. Ocean crust recycling models ofmixing between MORE source regions and highly radiogenic slabs cannot produce the observed He and Ne isotopic compositions. Isolation and aging of MORE source mantle can generate the isotopic compositions but require extremeHe-3/Ne-22 fractionation. Involvement of plume-derived gases, consistent with the lithophile element isotopic compositions, alleviates the need for strong He-3/Ne-22 fractionation. Closed-system aging of plume-derived heterogeneities can reproduce the data with minimum He-3/Ne-22 fractionation at reasonable U-238/He-3 ratios. However, diffusive exchange of He and to a lesser extent Ne between aged MORE source and aged plume veins could explain the occurrence of low He-3/He-4 compositions in all HIMU centers and the apparent low time-integrated He-3/Ne-22 of the Cameroon line.

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Documento generato il 27/11/20 alle ore 10:39:20