HR: 17:15h
AN: V14C-06    [Abstracts]
TI: Crystallization and Degassing in the Ferrar Sills, Antartica
AU: * Boudreau, A E
EM: boudreau@duke.edu
AF: Duke University, Earth and Ocean Sciences Box 90227, Durham, NC 27708 United States
AU: Simon, A
EM: Adam.Simon@ccmail.nevada.edu
AF: Univeristy of Nevada, Dept. Geosciences, , Las Vegas, NV 89154-4010 United States
AB: Halogen variations in apatite from a suite of samples from the orthopyroxene-rich "tongue" West Bull Pass and Dais area of the basal Ferrar sill were examined for evidence of degassing in a shallow sill. The ~350 M thick sill is characterized by a phenocrysts-free lower and upper margin and an orthopyroxene-rich central section about 350 m thick. Apatite grains from any given sample are unzoned in regard to Cl and F concentrations, but may vary by 20-30 mole % between grains. Overall average Cl/F ratios increase with height until the center of the sill, and then decreases to near zero towards the top where the rocks are relatively oxide-rich. The Cl/F trend parallels those of bulk MgO and grain size. Apatite in pegmatoids in the upper part of the sill have lower Cl/F ratio as compared with host rocks, although REE show no measurable difference. Modeling suggests that a cooling and crystallizing sill initially develops two separate vapor-saturated zones at the lower and upper margins. Vapor separating from the lower zone migrates upward into hotter rocks, where it both enhances the Cl/F ratio and allows apatite to precipitate earlier. It can also aid compaction and grain growth in the central part of the sill. In contrast, the relatively iron-rich, Cl-poor nature of the upper zone rocks also suggests that these rocks crystallized from more evolved, degassed liquids, possibly compacted out of the underlying crystal mush. In addition, vapor separated from the lower and central parts of the sill eventually moves into cooler upper zone rocks, where it is channelized (along with late interstitial liquids) to form pegmatoids and at temperatures where Cl is less favored in apatite and can be leached from existing apatite.
DE: 3616 Hydrothermal systems (0450, 1034, 3017, 4832, 8135, 8424)
DE: 3618 Magma chamber processes (1036)
DE: 3643 Layered magma chambers
DE: 5418 Heat flow
DE: 5480 Volcanism (6063, 8148, 8450)
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005