To testmechanisms of basalticmagma degassing,continuous decompressions of volatile-bearing (2.7–3.8 wt%H2O, 600–1,300 ppm CO2) Stromboli melts were performedfrom 250–200 to 50–25 MPa at 1,180–1,140 C.Ascent rateswere varied from 0.25 to *1.5 m/s. Glasses after decompressionshow a wide range of textures, from totally bubblefreeto bubble-rich, the latter with bubble number densitiesfrom 104 to 106 cm-3, similar to Stromboli pumices. Vesicularitiesrange from 0 to *20 vol%. Final melt H2O concentrationsare homogeneous and always close to solubilities.In contrast, the rate of vesiculation controls the finalmelt CO2concentration. High vesicularity charges have glass CO2concentrations that follow theoretical equilibrium degassingpaths, whereas glasses from low vesicularity charges showmarked deviations from equilibrium, with CO2 concentrationsup to one order of magnitude higher than solubilities.FTIR profiles and maps reveal glass CO2 concentration gradientsnear the gas–melt interface. Our results stress theimportance of bubble nucleation and growth, and of volatilediffusivities, for basaltic melt degassing. Two characteristicdistances, the gas interface distance (distance either betweenbubbles or to gas–melt interfaces) and the volatile diffusiondistance, control the degassing process. Melts containingnumerous and large bubbles have gas interface distancesshorter than volatile diffusion distances, and degassing proceedsby equilibrium partitioning of CO2 and H2O betweenmelt and gas bubbles. For melts where either bubble nucleationis inhibited or bubble growth is limited, gas interfacedistances are longer than volatile diffusion distances.Degassing proceeds by diffusive volatile transfer at the gas–melt interface and is kinetically limited by the diffusivities ofvolatiles in the melt. Our experiments show that CO2-oversaturatedmelts can be generated as a result of magmadecompression. They provide a new explanation for theoccurrence of CO2-rich natural basaltic glasses and open newperspectives for understanding explosive basaltic volcanism.
|Numero di pagine||17|
|Rivista||Contributions to Mineralogy and Petrology|
|Stato di pubblicazione||Published - 2013|
All Science Journal Classification (ASJC) codes