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

Abstract

To testmechanisms of basalticmagma degassing,\ud continuous decompressions of volatile-bearing (2.7–3.8 wt%\ud H2O, 600–1,300 ppm CO2) Stromboli melts were performed\ud from 250–200 to 50–25 MPa at 1,180–1,140 C.Ascent rates\ud were varied from 0.25 to *1.5 m/s. Glasses after decompression\ud show a wide range of textures, from totally bubblefree\ud to bubble-rich, the latter with bubble number densities\ud from 104 to 106 cm-3, similar to Stromboli pumices. Vesicularities\ud range from 0 to *20 vol%. Final melt H2O concentrations\ud are homogeneous and always close to solubilities.\ud In contrast, the rate of vesiculation controls the finalmelt CO2\ud concentration. High vesicularity charges have glass CO2\ud concentrations that follow theoretical equilibrium degassing\ud paths, whereas glasses from low vesicularity charges show\ud marked deviations from equilibrium, with CO2 concentrations\ud up to one order of magnitude higher than solubilities.\ud FTIR profiles and maps reveal glass CO2 concentration gradients\ud near the gas–melt interface. Our results stress the\ud importance of bubble nucleation and growth, and of volatile\ud diffusivities, for basaltic melt degassing. Two characteristic\ud distances, the gas interface distance (distance either between\ud bubbles or to gas–melt interfaces) and the volatile diffusion\ud distance, control the degassing process. Melts containing\ud numerous and large bubbles have gas interface distances\ud shorter than volatile diffusion distances, and degassing proceeds\ud by equilibrium partitioning of CO2 and H2O between\ud melt and gas bubbles. For melts where either bubble nucleation\ud is inhibited or bubble growth is limited, gas interface\ud distances are longer than volatile diffusion distances.\ud Degassing proceeds by diffusive volatile transfer at the gas–\ud melt interface and is kinetically limited by the diffusivities of\ud volatiles in the melt. Our experiments show that CO2-oversaturated\ud melts can be generated as a result of magma\ud decompression. They provide a new explanation for the\ud occurrence of CO2-rich natural basaltic glasses and open new\ud perspectives for understanding explosive basaltic volcanism

Melt inclusion constraints on petrogenesis of the 2014–2015 Holuhraun eruption, Iceland

“…However, the rate of 0.75 m s −1 is based on the outgassing of volatile radionuclides upon early CO 2 loss at depth within the magmatic system, and thus represents a different stage of magma ascent than has been considered in our calculation. Our calculated ascent rates are slower than the 0.05-0.45 MPa s −1 (~ 1.7-15 m s −1 ) obtained for various eruptions at Kilauea, Hawaii (Ferguson et al 2016), but our higher estimates are comparable with ascent rates of ~ 0.2-2.0 m s −1 modelled for Stromboli (e.g., Pichavant et al 2013). …”

Magma decompression rates during explosive eruptions of Kīlauea volcano, Hawaii, recorded by melt embayments

“…For embayments with bubbles attached to the embayment mouth, this diffusion, while a consequence of the drop in solubility during decompression, is driven by the concentration of dissolved volatiles at the melt-bubble interface, a direct function of vapor pressure inside the bubble. The diffusivities of H 2 O, CO 2 , and S in silicate melts are sufficiently small to prevent the attainment of equilibrium concentrations within embayments that have lengths on the order of 100 microns for a wide range of decompression rates (e.g., Lensky et al, 2004;Gonnerman and Manga, 2005;Pichavant et al, 2013;Zhang et al, 2007Zhang et al, , 2010. Consequently, distinct diffusion-controlled concentration profiles may be preserved within an embayment, allowing the estimation of decompression rates via diffusion modeling (Liu et al, 2007).…”

NanoSIMS results from olivine-hosted melt embayments: Magma ascent rate during explosive basaltic eruptions

“…On the other hand, syneruptive processes such as CO 2 gas fluxing (Blundy et al, 2010), nonequilibrium CO 2 degassing during rapid ascent (Pichavant et al, 2013), or non-equilibrium sulfur degassing (Fiege et al, 2104) can be invoked to explain the deviations from a closed-system…”

“…3), the extent of supersaturation and disequilibrium degassing of H 2 O during decompression may be limited in the Fuego magmatic system. Although H 2 O may be undergoing equilibrium degassing, CO 2 -over-saturated basaltic melts have been generated experimentally for ascent rates great than ~1.5 m/s (Pichavant et al, 2013). However, Fuego MI show no obvious signs of disequilibrium degassing, which would manifest as CO 2 in excess of closed system degassing.…”