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

摘要: New He, Ne, Ar and CO2 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 36Ar/22Ne (45) low (5). In addition, magmatic trapped in these basaltic glasses are variably such 4He/40Ar* (where asterisk indicates corrected for based on all 36Ar being origin) is range 3–12. Single samples have a highest ratios final crush steps, consistent most (highest 4He/40Ar*) volatiles smallest vesicles. It not possible to distinguish between batch Rayleigh degassing mechanisms. The complexities fractionation processes means it estimate 40Ar/36Ar mantle source other than must be higher ratio measured (26,200 ± 5200). Noble gas/CO2 also variable. While some adsorption during crushing exaggerates variations He/CO2 Ar/CO2, we account entire variation as an analytical artefact: present Variations cannot attributed solubility controlled because broadly similar solubilities He tholeiitic magmas. large (factor 10) accompanied indications major changes melting regime region chemistry, therefore thought reflect late-stage (magmatic) gases. identify explicit mechanism, although both reduction (e.g., hydrocarbons graphite) kinetic CO2-noble gas could variations.