Role of Syn-eruptive Cooling and Degassing on Textures of Lavas from the AD 1783-1784 Laki Eruption, South Iceland
The Laki eruption involved 10 fissure-opening episodes that produced 15·1 km3 of homogeneous quartz-tholeiite magma. This study focuses on the texture and chemistry of samples from the first five episodes, the most productive period of the eruption. The samples comprise pumiceous tephra clasts from...
| Published in: | Journal of Petrology |
|---|---|
| Main Authors: | , , , |
| Format: | Text |
| Language: | English |
| Published: |
Oxford University Press
2007
|
| Subjects: | |
| Online Access: | http://petrology.oxfordjournals.org/cgi/content/short/48/7/1265 https://doi.org/10.1093/petrology/egm017 |
| Summary: | The Laki eruption involved 10 fissure-opening episodes that produced 15·1 km3 of homogeneous quartz-tholeiite magma. This study focuses on the texture and chemistry of samples from the first five episodes, the most productive period of the eruption. The samples comprise pumiceous tephra clasts from early fallout deposits and lava surface samples from fire-fountaining and cone-building activity. The fluid lava core was periodically exposed at the surface upon lobe breakout, and its characteristics are preserved in glassy selvages from the lava surface. In all samples, plagioclase is the dominant mineral phase, followed by clinopyroxene and then olivine. Samples contain <7 vol. % of euhedral phenocrysts (>100 μm) with primitive cores [An* = 100 × Ca/(Ca + Na) >70; Fo > 75; En* = 100 × Mg/(Mg + Fe) >78] and more evolved rims, and >10 vol. % of skeletal, densely distributed groundmass crystals (<100 μm), which are similar in composition to phenocryst rims (tephra: An* 58–67 , Fo 72–78 , En* 72–81 lava: An* 49–70 , Fo 63–78 , En 57–78 ). Tephra and lava have distinct vesicularity (tephra: >40 vol. %; lava: <40 vol. %), groundmass crystal content (tephra: <10 vol. %; lava: 20–30 vol. %), and matrix glass composition (tephra: 5·4–5·6 wt % MgO; lava: 4·3–5·0 wt % MgO). Whole-rock and matrix glass compositions define a trend consistent with liquid evolution during in situ crystallization of groundmass phases. Plagioclase–glass and olivine–glass thermometers place the formation of phenocryst cores at ∼10 km depth in a melt with ∼1 wt % H 2 O, at near-liquidus temperatures (∼1150°C). Phenocryst rims and groundmass crystals formed close to the surface, at 10–40°C melt undercooling and in an ∼10−20°C cooler drier magma (0–0·1 wt % H 2 O), causing an ∼10 mol % drop in An content in plagioclase. The shape, internal zoning and number density of groundmass crystals indicate that they formed under supersaturated conditions. Based on this information, we propose that degassing during ascent had a ... |
|---|