Degassing and contamination of noble gases in Mid‐Atlantic Ridge basalts

Abstract

New He, Ne, Ar and CO 2 stepped‐crushing data from the Mid‐Atlantic Ridge show that contamination of basalts by atmospheric noble gases involves three or more components: unfractionated air, fractionated air with high 36 Ar/ 22 Ne (≥45) and fractionated air with low 36 Ar/ 22 Ne (≤5). In addition, the magmatic noble gases trapped in these basaltic glasses are variably fractionated such that 4 He/ 40 Ar* (where the asterisk indicates corrected for atmospheric contamination based on all 36 Ar being atmospheric in origin) is in the range 3–12. Single samples have a range in 4 He/ 40 Ar* with the highest ratios in the final crush steps, consistent with the most fractionated (highest 4 He/ 40 Ar*) volatiles trapped in the smallest vesicles. It is not possible to distinguish between batch and Rayleigh degassing mechanisms. The complexities of the contamination and magmatic fractionation processes means that it is not possible to estimate 40 Ar/ 36 Ar of the mantle source to these basalts other than it must be higher than the highest ratio measured (26,200 ± 5200). Noble gas/CO 2 ratios are also variable. While some CO 2 adsorption during crushing exaggerates the variations in He/CO 2 and Ar/CO 2 , we show that it is not possible to account for the entire variation as an analytical artefact: some of the variation is present in the vesicles. Variations in He/CO 2 cannot be attributed to solubility controlled degassing because of the broadly similar solubilities of He and CO 2 in tholeiitic magmas. The large range in He/CO 2 in these glasses (factor of 10) is not accompanied by indications of major changes in melting regime or source region chemistry, therefore is thought to reflect late‐stage (magmatic) fractionation of CO 2 from the noble gases. It is not possible to identify an explicit mechanism, although both CO 2 reduction (e.g., to hydrocarbons or graphite) and kinetic CO 2 ‐noble gas fractionation could account for the variations.