Noble gases in the Cameroon line and the He, Ne, and Ar isotopic compositions of high μ (HIMU) mantle

• Published in: Journal of Geophysical Research 1999-12, Vol.104 (B12), p.29509-29527
• Main Authors: Barfod, Dan N., Ballentine, Chris J., Halliday, Alex N., Fitton, J. Godfrey
• Format: Article
• Language: English
• Subjects: Crystalline rocks; Earth sciences; Earth, ocean, space; Exact sciences and technology; Igneous and metamorphic rocks petrology, volcanic processes, magmas; Isotope geochemistry; Isotope geochemistry. Geochronology
• ISSN: 0148-0227; 2156-2202
• DOI: 10.1029/1999JB900280

Ultramafic xenoliths, basaltic lavas, and CO2 gases from the Cameroon line volcanic chain provide the first characterization of combined He, Ne, and Ar isotopes in a high‐time‐integrated 238U/204Pb = μ (HIMU) magmatic system. Helium isotopic compositions typically range from 5.0 to 6.7 Ra, with an average of 6.3. These values are indistinguishable from the 3He/4He 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 20Ne/22Ne of 11.87 and 21Ne/22Ne of 0.0508. Argon isotopic compositions in silicates range from atmospheric to 40Ar/36Ar = 4910±430 (crushing) and up to 16,300±1000 (single grain, laser step heating). The correlation between 20Ne/22Ne and 40Ar/36Ar in CO2 gases suggests a minimum 40Ar/36Ar = 1650±30 for the mantle‐derived component. Uniform 3He/4He in silicates and in CO2 fluids across both the continental and oceanic sectors of the Cameroon line argues strongly for a negligible lithospheric contribution to noble gas isotopic compositions. This inference is supported by high 238U/3He in lherzolites, indicating that noble gases in these samples must have been recently introduced (