• journal_title：Geology
• Contributor：L. Jane. Applegarth ; Hugh Tuffen ; Mike R. James ; Harry Pinkerton ; Katharine V. Cashman
• Publisher：Geological Society of America
• Date：2013-02-01
• Format：text/html
• Language：en
• Identifier：10.1130/G33641.1
• journal_abbrev：Geology
• issn：0091-7613
• volume：41
• issue：2
• firstpage：243
• section：Articles

Magma degassing can trigger crystal growth by increasing the magma liquidus temperature. As crystallization greatly increases magma viscosity, this process can strongly influence eruptive dynamics. We use a microscope and heated stage to obtain the first direct observations of degassing-driven crystal growth in natural basaltic melts at magmatic temperatures. Samples from Mount Etna, Italy (0.39 wt% H<sub>2sub>O), and Kilauea volcano, Hawaii (0.18 wt% H<sub>2sub>O) were heated in air at 1 bar, and held isothermally for 0.5–17 h between 1190 °C and 1270 °C, before cooling to solidus temperatures. On heating, bubble growth at >900 °C indicated volatile exsolution. In the hydrous Etna sample, isothermal conditions produced numerous new plagioclase crystals that grew to ≤160 μm at maximum rates of 5.2–18 × 10<sup>−6sup> cm s<sup>−1sup>. Growth rates and crystal morphologies (tabular to spherulitic) depended on dwell temperature. Growth slowed dramatically after 20 min as equilibrium was approached. In the H<sub>2sub>O-poor Kilauea sample, few new crystals appeared; they grew at maximum rates of 1.7–6.5 × 10<sup>−6sup> cm s<sup>−1sup>. On cooling, crystal nucleation and growth were strongly influenced by preexisting crystal textures, highlighting the importance of studying natural samples. Our results document rapid crystal growth triggered by melt devolatilization when the H<sub>2sub>O content of the glass is sufficiently high. Resultant swift, substantial changes in magma texture are a key control on lava rheology at Etna and elsewhere.