NOBLE GAS COMPOSITIONS OF THE LITHOSPHERIC MANTLE BELOW THE CHYULU HILLS VOLCANIC FIELD, KENYA

Abstract

We performed a noble gas isotope study on mantle xenoliths from the Quaternary volcanic field of Chyulu Hills, S Kenya, c. 100 km east of the Kenya rift graben. 4He/3He ratios were in the range of 100 000-120 000 and indistinguishable from other subcontinental lithospheric mantle compositions worldwide and similar to most results of the East African rift system. 21Ne/22Ne ratios corrected for atmospheric contributions displayed less nucleogenic values compared to mid ocean ridge basalts. This requires addition of Ne from a more primitive mantle source, that could be assigned to the postulated Kenyan mantle 'plume' or alternatively might be attributed to the 'high 3He/4He' Afar hotspot c. 2000 km distant. Whereas He and Ne isotopes reflect a rather homogeneous source composition, we observe a variable air corrected Ar isotopic composition. This might mirror additional contributions of atmospheric fluids strongly enriched in Ar. Nevertheless, in the least contaminated xenolith we were able to calculate a maximum 40Ar/36Ar ratio of 34,800 ± 500 assuming a mantle 20Ne/22Ne = 12.5. This is regarded as best approximation of the real 40Ar/36Ar ratio of the investigated mantle source. The same sample showed the highest observed 129Xe/130Xe and 136Xe/130Xe ratios of 7.01 ± 0.12 and 2.37 ± 0.05, respectively. The correlation of 129Xe/130Xe-136Xe/130Xe ratios is in broad agreement with mantle Xe in OIB and MORB sources. Within uncertainties a minor contribution of crustally derived xenon is possible, though not compelling. © 2007 Elsevier B.V. All rights reserved.