Major volatiles from a North Atlantic MORB glass and calibration to He: A size fraction analysis

Abstract

The analysis of major volatiles in a North Atlantic mid-ocean ridge basalt (MORB) glass, has been performed using a new procedure based on the analysis of several size fractions of one single sample. It enables us to separate the contributions of volatiles from the glass phase, from the vesicle gas phase and eventually gas adsorbed on the grain surface. The vesicle composition (mole fractions) obtained upon crushing is 0.94 CO 2, 0.002 H 2, 0.06 H 2O. Volatiles extracted from the glass at 1150°C are 9.7 · 10 −6 mol g −1 CO 2, 156 · 10 −6 mol g −1 H 2O. The gases released at 500°C are mostly adsorbed contaminants but contribute marginally to the total amount except for numerous organic compounds which are destroyed at higher temperature. When previous noble-gas data for the same sample are considered, one obtains the evidence that: (1) the He CO 2 ratio does not fractionate between glass (4.1 · 10 −5) and vesicles (4.6 · 10 −5); and (2) water is not quantitatively degassed from MORB glasses. The ratio He : CO 2 : H 2O of 10 −4 : 2.4 : 13 for primitive magma has to be confirmed by further results to be considered as a typical value for a MORB source.