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

Abstract

Ultramafic xenoliths, basaltic lavas, and CO 2 gases from the Cameroon line volcanic chain provide the first characterization of combined He, Ne, and Ar isotopes in a high‐time‐integrated 238 U/ 204 Pb = μ (HIMU) magmatic system. Helium isotopic compositions typically range from 5.0 to 6.7 R a , with an average of 6.3. These values are indistinguishable from the 3 He/ 4 He of other HIMU locales (Austral Islands, St. Helena). Neon isotopic compositions for xenoliths and CO 2 gases are mid‐ocean ridge basalt (MORB)‐like, with a maximum 20 Ne/ 22 Ne of 11.87 and 21 Ne/ 22 Ne of 0.0508. Argon isotopic compositions in silicates range from atmospheric to 40 Ar/ 36 Ar = 4910±430 (crushing) and up to 16,300±1000 (single grain, laser step heating). The correlation between 20 Ne/ 22 Ne and 40 Ar/ 36 Ar in CO 2 gases suggests a minimum 40 Ar/ 36 Ar = 1650±30 for the mantle‐derived component. Uniform 3 He/ 4 He in silicates and in CO 2 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 238 U/ 3 He 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 of mixing between MORB source regions and highly radiogenic slabs cannot produce the observed He and Ne isotopic compositions. Isolation and aging of MORB source mantle can generate the isotopic compositions but require extreme 3 He/ 22 Ne fractionation. Involvement of plume‐derived gases, consistent with the lithophile element isotopic compositions, alleviates the need for strong 3 He/ 22 Ne fractionation. Closed‐system aging of plume‐derived heterogeneities can reproduce the data with minimum 3 He/ 22 Ne fractionation at reasonable 238 U/ 3 He ratios. However, diffusive exchange of He and to a lesser extent Ne between aged MORB source and aged plume veins could explain the occurrence of low 3 He/ 4 He compositions in all HIMU centers and the apparent low time‐integrated 3 He/ 22 Ne of the Cameroon line.