Journal Article

Generation of CO.sub.2-rich melts during basalt magma ascent and degassing

Rotolo, Silvio G, Pichavant, Michel, Scaillet, Bruno, Burgisser, Alain, Gall, Nolwenn Le, Martel, Caroline, Carlo, Ida Di

2013

Overview

To test mechanisms of basaltic magma degassing, continuous decompressions of volatile-bearing (2.7-3.8 wt% [H.sub.2]O, 600-1,300 ppm C[O.sub.2]) Stromboli melts were performed from 250-200 to 50-25 MPa at 1,180-1,140 °C. Ascent rates were varied from 0.25 to ~1.5 m/s. Glasses after decompression show a wide range of textures, from totally bubble-free to bubble-rich, the latter with bubble number densities from [10.sup.4] to [10.sup.6] [cm.sup.-3] , similar to Stromboli pumices. Vesicularities range from 0 to ~20 vol%. Final melt [H.sub.2]O concentrations are homogeneous and always close to solubilities. In contrast, the rate of vesiculation controls the final melt C[O.sub.2] concentration. High vesicularity charges have glass C[O.sub.2] concentrations that follow theoretical equilibrium degassing paths, whereas glasses from low vesicularity charges show marked deviations from equilibrium, with C[O.sub.2] concentrations up to one order of magnitude higher than solubilities. FTIR profiles and maps reveal glass C[O.sub.2] concentration gradients near the gas-melt interface. Our results stress the importance of bubble nucleation and growth, and of volatile diffusivities, for basaltic melt degassing. Two characteristic distances, the gas interface distance (distance either between bubbles or to gas-melt interfaces) and the volatile diffusion distance, control the degassing process. Melts containing numerous and large bubbles have gas interface distances shorter than volatile diffusion distances, and degassing proceeds by equilibrium partitioning of C[O.sub.2] and [H.sub.2]O between melt and gas bubbles. For melts where either bubble nucleation is inhibited or bubble growth is limited, gas interface distances are longer than volatile diffusion distances. Degassing proceeds by diffusive volatile transfer at the gasmelt interface and is kinetically limited by the diffusivities of volatiles in the melt. Our experiments show that C[O.sub.2]-oversaturated melts can be generated as a result of magma decompression. They provide a new explanation for the occurrence of C[O.sub.2]-rich natural basaltic glasses and open new perspectives for understanding explosive basaltic volcanism.

• Publisher: Springer
• Subject: Basalt