Direct observations of degassing-induced crystallization in basalts

作者：Applegarth L Jane; Tuffen Hugh^*; James Mike R; Pinkerton Harry; Cashman Katharine V

来源：Geology, 2013, 41(2): 243-246.

DOI：10.1130/G33641.1

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% H2O), and Kilauea volcano, Hawaii (0.18 wt% H2O) were heated in air at 1 bar, and held isothermally for 0.5-17 h between 1190 degrees C and 1270 degrees C, before cooling to solidus temperatures. On heating, bubble growth at %26gt;900 degrees C indicated volatile exsolution. In the hydrous Etna sample, isothermal conditions produced numerous new plagioclase crystals that grew to %26lt;= 160 mu m at maximum rates of 5.2-18 x 10(-6) cm s(-1). Growth rates and crystal morphologies (tabular to spherulitic) depended on dwell temperature. Growth slowed dramatically after 20 min as equilibrium was approached. In the H2O-poor Kilauea sample, few new crystals appeared; they grew at maximum rates of 1.7-6.5 x 10(-6) cm s(-1). 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 H2O 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.

• 出版日期2013-2