HR: 1340h
AN: V13E-0593    [Abstracts]
TI: Development of Modal Layering by Chemical Segregation
AU: * Hogmalm, K
EM: Johanh@gvc.gu.se
AF: Department of Geology, Earth Sciences Centre, Göteborg University, Box 460, G 405 30 Sweden
AU: Meurer, W P
EM: William.P.Meurer@mail.uh.edu
AF: Department of Geology, Earth Sciences Centre, Göteborg University, Box 460, G 405 30 Sweden
AU: Meurer, W P
EM: William.P.Meurer@mail.uh.edu
AF: Department of Geosciences, University of Houston, 312 Science and Research Bldg. 1, Houston, TX 772045007 United States
AU: Larson, S
EM: Svenake@gvc.gu.se
AF: Department of Geology, Earth Sciences Centre, Göteborg University, Box 460, G 405 30 Sweden
AU: Claeson, D T
EM: Dick@gvc.gu.se
AF: Department of Geology, Earth Sciences Centre, Göteborg University, Box 460, G 405 30 Sweden
AB: The origin of igneous layering remains controversial and the competing models are; 1 mechanical sorting, 2 oscillatory nucleation and growth in boundary layers and 3 post-cumulus chemical segregation. Theoretically, these processes would all leave characteristic signatures in the chemistry of cumulate minerals. We present data from a systematic study of differences in plagioclase and olivine major- and trace element compositions between felsic and mafic layers in the Norra Ulvön Gabbro, Sweden. The central part of this ~300m thick intrusion shows well-developed rhythmic layering ranging in thickness from 1cm to 1m. The boundaries between adjacent layers are sharp, but weak modal grading, both normal and reverse, can often be seen within individual layers. Plagioclase core compositions show no systematic difference in An.%, Ti, V, Sr, Ba, REE, and Pb between adjacent layers. Plagioclase in mafic layers is partially resorbed and lacks evolved overgrowths as found in felsic layers. Olivine in mafic layers has higher Fo.%, Ni and lower Mn, a difference that is positively correlated with the difference in colour index between felsic and mafic layers. However, high concentrations of REE and Zr and the lack of zoning in the olivine suggest that a majority of the crystals have been recrystallized in the presence of evolved interstitial liquids or fluids. Of the three mechanisms proposed chemical segregation best fits our observations. The uniform plagioclase compositions exclude layer-forming processes that require strong compositional variations in the magma (e.g oscillatory nucleation). While we cannot rule out initial heterogeneities induced by mechanical sorting, the resorption of plagioclase and recrystallization of olivine demonstrate the importance of postcumulus processes. Pressure solution and segregation due to the affinity of like crystals are two processes that would act to enhance initial modal heterogeneities. However, we have found modal segregations into pure olivine and pure plagioclase layers in otherwise homogenous gabbro. These segregations are associated with evolved pegmatoidal material, suggesting that this layering formed purely by interaction with fluids. The exact driving force for this mechanism is not clear, but resorption, segregation, and recrystallization of cumulate minerals in response to fluid interaction could potentially explain all rhythmic layering in this intrusion.
DE: 1036 Magma chamber processes (3618)
DE: 3618 Magma chamber processes (1036)
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005