N–Ar–He–CO2 Systematics Combined with H2O, Cl, K Abundances in MORB Glasses Demonstrate Interaction of Magmatic and Hydrothermal Systems: a Case for MAR at 16°07′–17°11′ N

Here we present data on nitrogen and argon isotopic compositions and He–Ar–N–C(CO 2 ) elemental ratios obtained during stepwise crushing of fresh basaltic glasses of the N-MORB family from Mid-Atlantic ridge rift valley at 16°07′–17°11′ N. The bulk nitrogen isotopic composition in the samples varies from δ 15 N (total) = –5.2 ± 0.2‰ (i.e., typical for MORB glasses) to δ 15 N (total) = +4.6 ± 0.3‰ (pointing to the presence of organic nitrogen). The δ 15 N variations in the crushing steps are wider and range from –13.8 to +8.3‰. The 40 Ar/ 36 Ar ratios in the crushing steps vary from the value close to that in the atmosphere (~296) up to 11 100 ± 590 (the bulk values cover a range from 355 ± 11 to 2799 ± 159). Correlations between argon and nitrogen isotopic and elemental ratios imply mixing between an N-MORB type mantle component and a surface-derived component enriched in 15 N. Carbon (CO 2 )—nitrogen systematic suggests that the most plausible source for isotopically heavy nitrogen is the organic matter brought into the fluid source. Strong relationships between Ar and N isotopic compositions and Cl, H 2 O, and K, as well as Ar–N 2 , N 2 –CO 2 and Ar–He–CO 2 systematics, indicate that melt degassing and contamination with atmospheric Аr and organic nitrogen are the two dominant processes responsible for elemental and isotopic variation. The contamination of magmatic melts with surface related noble gases and organic nitrogen occurred through their interaction with high salinity hydrothermal brines. We propose that this contamination mechanism may be universal and largely responsible for the observed variations in the isotopic composition for a number of volatile elements in MORB glasses. However larger set of samples from the hydrothermal fields’ related areas has to be studied for better understanding how common is the established contamination mechanism.