TY - JOUR
T1 - Orbicular volcanic rocks of Cerro Panizos
T2 - their origin and implications for orb formation
AU - Ort, M. H.
PY - 1992
Y1 - 1992
N2 - The orbs consist of two to five crystalline rings surrounding a xenolithic or orthopyroxene core. The rings alternate between bands of large, radially oriented plagioclase and orthopyroxene crystals and bands of small, tangentially or radially oriented biotite and ilmenite crystals. The ignimbrite orbs are associated with two types of pumice: 1) a coarsely porphyritic biotite-quartz-plagioclase dacite with 35-40% crystals found throughout the ignimbrite, and 2) a finely porphyritic biotite-plagioclase quartz dacite with 75-80% crystals found only in association with the orbs. The major- and trace-element and isotopic compositions of the two pumice types are identical. The orbs formed in a water-rich cupola along the roof of the magma body, where the magma was superheated and most crystals were resorbed. Pressure release related to eruption caused exsolution of water, leading to large degrees of undercooling. Orbs formed rapidly around the few available nuclei. As limited mixing with the surrounding coarsely porphyritic magma occurred, heterogeneous nucleation in the supercooled magma began, forming abundant small crystals seen in the finely porphyritic pumice. Eruption of the orbicular dacite occurred when a ring vent conduit tapped the magma in the cupola. -from Author
AB - The orbs consist of two to five crystalline rings surrounding a xenolithic or orthopyroxene core. The rings alternate between bands of large, radially oriented plagioclase and orthopyroxene crystals and bands of small, tangentially or radially oriented biotite and ilmenite crystals. The ignimbrite orbs are associated with two types of pumice: 1) a coarsely porphyritic biotite-quartz-plagioclase dacite with 35-40% crystals found throughout the ignimbrite, and 2) a finely porphyritic biotite-plagioclase quartz dacite with 75-80% crystals found only in association with the orbs. The major- and trace-element and isotopic compositions of the two pumice types are identical. The orbs formed in a water-rich cupola along the roof of the magma body, where the magma was superheated and most crystals were resorbed. Pressure release related to eruption caused exsolution of water, leading to large degrees of undercooling. Orbs formed rapidly around the few available nuclei. As limited mixing with the surrounding coarsely porphyritic magma occurred, heterogeneous nucleation in the supercooled magma began, forming abundant small crystals seen in the finely porphyritic pumice. Eruption of the orbicular dacite occurred when a ring vent conduit tapped the magma in the cupola. -from Author
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U2 - 10.1130/0016-7606(1992)104<1048:OVROCP>2.3.CO;2
DO - 10.1130/0016-7606(1992)104<1048:OVROCP>2.3.CO;2
M3 - Article
AN - SCOPUS:84879880360
SN - 0016-7606
VL - 104
SP - 1048
EP - 1058
JO - Geological Society of America Bulletin
JF - Geological Society of America Bulletin
IS - 8
ER -