Generation of CO2-rich melts during basalt magma ascent and degassing

To testmechanisms of basalticmagma degassing,
continuous decompressions of volatile-bearing (2.7–3.8 wt%
H2O, 600–1,300 ppm CO2) 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 bubblefree
to bubble-rich, the latter with bubble number densities
from 104 to 106 cm-3, similar to Stromboli pumices. Vesicularities
range from 0 to *20 vol%. Final melt H2O concentrations
are homogeneous and always close to solubilities.
In contrast, the rate of vesiculation controls the finalmelt CO2
concentration. High vesicularity charges have glass CO2
concentrations that follow theoretical equilibrium degassing
paths, whereas glasses from low vesicularity charges show
marked deviations from equilibrium, with CO2 concentrations
up to one order of magnitude higher than solubilities.
FTIR profiles and maps reveal glass CO2 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 CO2 and H2O 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 gas–
melt interface and is kinetically limited by the diffusivities of
volatiles in the melt. Our experiments show that CO2-oversaturated
melts can be generated as a result of magma
decompression. They provide a new explanation for the
occurrence of CO2-rich natural basaltic glasses and open new
perspectives for understanding explosive basaltic volcanism