N–C–Ar–He Isotopic Systematics of Quenched Tholeiitic Glasses from the Bouvet Triple Junction Area

Resumo

The paper presents pioneering data on the isotopic composition and elemental ratios of nitrogen, carbon (carbon dioxide), helium, and argon in the fluid phase of quenched tholeiitic glasses from different segments of the Bouvet Triple Junction area (BTJ). The data reflect a complicated geodynamic and tectonic history of the area evolution and indicate that the variations in the elemental ratios of the volatile components of the fluid–gas phase were controlled by a number of various factors: elemental fractionation during melt degassing, mixing of gases from different sources, postmagmatic diffusion-controlled helium loss. The nitrogen–argon and noble gas isotope systematics suggest a significant contribution of the atmospheric component to the mantle source of fluids for the samples from the Spiess Ridge and the segment of the Southwest Indian Ridge (SWIR) and a smaller contribution for the Mid-Atlantic Ridge (MAR) samples. For the Spiess Ridge and SWIR, the most probable contaminating agent was water fluid with dissolved gases of atmospheric composition. This fluid may have been brought to the mantle with ancient crustal rocks involved in magma generation. These crustal rocks may represent small fragments of the Gondwana continent with which sedimentary organic matter could be brought into the magma source.