Mechanisms of magma degassing at mid-oceanic ridges and the local volatile composition (~4He-~(40)Ar~*-CO_2) of the mantle by laser ablation analysis of individual MORB vesicles

摘要

Individual vesicles in <1cm~3 samples of MORB glasses commonly preserve significantly different volatile (He, Ar, CO_2) compositions. Five separate MORB glass samples from different ridges were investigated, selectively opening targeted vesicles using a 193nm excimer laser. One sample shows evidence of syn-emplacement selective helium loss from the vesicles in the glass rim close to the crystallised zone of the pillow lava, proving that He contents and low He/Ar ratios do not always reflect mantle and/or magmatic processes. However the composition of the different vesicles of three of these samples covers large ranges in He/Ar and Ar/CO_2 with linear variations in plots of ln(He/Ar) vs. ln(Ar/CO_2) which are consistent with a Rayleigh distillation at equilibrium and allow the relative abundances of the volatiles in the MORB mantle source to be estimated by correcting for degassing processes on a sample-by-sample basis. This technique presents a new tool for characterising and correcting for volatile fractionation processes that have modified the initial mantle source composition. The results confirm a heterogeneous CO_2/~3He ratio in the MORB source regions with the Azores mantle source enriched in C/~3He compared to that of N-MORBs. The considerable heterogeneity in vesicle compositions within such a small volume (<1cm~3) requires injection of less degassed magma at shallow level before eruption. The maximum time interval between magma mixing and quenching on the seafloor is of the order of a few hours.