TY - JOUR

T1 - Degassing-induced crystallization in basalts

AU - Applegarth, Louisa

AU - Tuffen, Hugh

AU - James, Michael

AU - Pinkerton, Harry

PY - 2013/1/1

Y1 - 2013/1/1

N2 - Syn-eruptive crystallisation can drastically increase magma viscosity, with profound implications for conduit dynamics, lava emplacement and volcanic hazards. There is growing evidence that crystallisation is not only cooling-driven, but can also occur almost isothermally during decompression-induced degassing on ascent from depth. Here we review field and experimental evidence for degassing-driven crystallisation in a range of magma compositions. We then present new results showing, for the first time, experimental evidence for this process in basaltic magma.Our experiments use simultaneous thermogravimetric analysis and differential scanning calorimetry coupled with mass spectrometry (TGA-DSC-MS) to monitor degassing patterns and thermal events during heating and cooling of porphyritic basaltic samples from Mt. Etna, Italy. The partly degassed samples, which contained 0.39–0.81 wt.% total volatiles in the glass fraction, were subjected to two cycles of heating from ambient to 1250 °C. On the first heating, TGA data show that 30–60% of the total volatiles degassed slowly at < 1050 °C, and that the degassing rate increased rapidly above this temperature. DSC data indicate that this rapid increase in the degassing rate was closely followed (≤ 3.4 min) by a strongly exothermic event, which is interpreted as crystallisation. Enthalpies measured for this event suggest that up to 35% of the sample crystallises, a value supported by petrographic observations of samples quenched after the event. As neither degassing nor crystallisation was observed at high temperature during the second heating cycle we infer that the events on first heating constitute degassing-driven crystallisation. The rapidity and magnitude of the crystallisation response to degassing indicates that this process may strongly affect the rheology of basaltic magma in shallow conduits and lava flows, and thus influence the hazards posed by basaltic volcanism.

AB - Syn-eruptive crystallisation can drastically increase magma viscosity, with profound implications for conduit dynamics, lava emplacement and volcanic hazards. There is growing evidence that crystallisation is not only cooling-driven, but can also occur almost isothermally during decompression-induced degassing on ascent from depth. Here we review field and experimental evidence for degassing-driven crystallisation in a range of magma compositions. We then present new results showing, for the first time, experimental evidence for this process in basaltic magma.Our experiments use simultaneous thermogravimetric analysis and differential scanning calorimetry coupled with mass spectrometry (TGA-DSC-MS) to monitor degassing patterns and thermal events during heating and cooling of porphyritic basaltic samples from Mt. Etna, Italy. The partly degassed samples, which contained 0.39–0.81 wt.% total volatiles in the glass fraction, were subjected to two cycles of heating from ambient to 1250 °C. On the first heating, TGA data show that 30–60% of the total volatiles degassed slowly at < 1050 °C, and that the degassing rate increased rapidly above this temperature. DSC data indicate that this rapid increase in the degassing rate was closely followed (≤ 3.4 min) by a strongly exothermic event, which is interpreted as crystallisation. Enthalpies measured for this event suggest that up to 35% of the sample crystallises, a value supported by petrographic observations of samples quenched after the event. As neither degassing nor crystallisation was observed at high temperature during the second heating cycle we infer that the events on first heating constitute degassing-driven crystallisation. The rapidity and magnitude of the crystallisation response to degassing indicates that this process may strongly affect the rheology of basaltic magma in shallow conduits and lava flows, and thus influence the hazards posed by basaltic volcanism.

KW - basalt

KW - crystallisation

KW - degassing

KW - lava

KW - Etna

KW - Kilauea

KW - crystal growth

KW - magma degassing

KW - thermal analysis

KW - Thermogravimetric

KW - differential scanning calorimetry

U2 - 10.1016/j.earscirev.2012.10.007

DO - 10.1016/j.earscirev.2012.10.007

M3 - Journal article

VL - 116

SP - 1

EP - 16

JO - Earth-Science Reviews

JF - Earth-Science Reviews

SN - 0012-8252

ER -