HR: 0800h
AN: V11B-0585 [Abstracts]
TI: Mineralogy and melt-mineral reactions during transport of xenocrysts in the Thjorsa lava, Iceland
AU: * Halldorsson, S
EM: saemundu@hi.is
AF: Nordic Volcanological Center,
University of Iceland, Askja-Sturlugata 7, Reykjavik, IS-101, Iceland
AU: Gronvold, K
EM: karlgr@hi.is
AF: Nordic Volcanological Center,
University of Iceland, Askja-Sturlugata 7, Reykjavik, IS-101, Iceland
AU: Oskarsson, N
EM: nielso@hi.is
AF: Nordic Volcanological Center,
University of Iceland, Askja-Sturlugata 7, Reykjavik, IS-101, Iceland
AB:
The Thjorsa lava is a voluminous tholeiite fissure lava flow originating within the Bardarbunga volcanic system of
the Eastern Rift Zone, Iceland. The lava contains a high content of xenocrysts of plagioclase and minor olivine.
Detailed analytical work using electron microprobe reveals a very thin reaction rim surrounding the xenocryst.
Individual plagioclase crystals have high Anorthite uniform core with a thin rim of plagioclase with a significantly
lower An composition, identical with the groundmass plagioclase. The olivine also has a high Fo cores but have
normal zoning. Short residence time for the xenocryst in the host magma is inferred from diffusion modeling of
olivine xenocryst rims, indicating that the xenocrysts originate by disintegration of plutonic assemblages by the
ascending host magmas. The mineral analyses and results from an isotope study of seperated groundmass
and xenocrysts can be used to construct a time scale for the disintegration and remobilization of earlier crustal
cumulates, most likely analogous to the layered gabbros in ophiolites. The primitive mineralogy of the xenocrysts
(An83-90 and Fo80-86) formed from highly primitive liquids deep in the crust as gabbro consisting of
oli+pl+cpx(+sp). The disintegration of the gabbro caused assimilation of cpx, which left a chemical signature
reflected in the concentration of cpx-compatible elements in the groundmass and minor but unusually Cr-rich,
partially melted cpx can be found throughout the lava flow. The normal zoning pattern of the olivine xenocrysts can
be used to estimate the time the olivine was in contact with the host magma, assuming the zoning is due to
diffusion resulting in estimates of only a few months. The final fissure eruption is, therefore, a mixture of liquid
(groundmass) with a minor amount of equilibrium microphenocrysts and xenocrysts. The high plagioclase could
reflect density differences of the gabbro minerals. The Thjorsa lava is a remarkable example of a large-scale
remobilization of crustal cumulates only shortly before, or during eruption. The occurrence of the large plutonic
assemblages hosted by the Thjorsa lava (1.25-2.5 km3) shows rapid material transport processes taking place
within the Icelandic rift system. The length of the lava flow (aprox. 120 km) and the apparently rapid emplacement
show that the incorporation of the cumulates did not adversely affect the fluidity of the lava.
DE: 8410 Geochemical modeling (1009, 3610)
DE: 8416 Mid-oceanic ridge processes (1032, 3614)
DE: 8425 Effusive volcanism
DE: 8439 Physics and chemistry of magma bodies
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting