HR: 1340h
AN: V43A-1110    [Abstracts]
TI: Infiltration Metasomatism in Stillwater Anorthosite
AU: * Berglund, H
EM: heather.berglund@gmail.com
AF: University of Houston, S&R 1, Houston, TX 77204, United States
AU: Snow, J E
EM: jesnow@uh.edu
AF: University of Houston, S&R 1, Houston, TX 77204, United States
AU: Meurer, W P
EM: william.p.meurer@exxonmobile.com
AF: University of Houston, S&R 1, Houston, TX 77204, United States
AU: Meurer, W P
EM: william.p.meurer@exxonmobile.com
AF: Exxon-Mobil, Hydrocarbon systems division, Houston, tx 77252-2819, United States
AB: The Stillwater Complex is a layered gabbroic intrusion that has been extensively studied because it hosts the bulk of North America’s known reserves of PGE’s (Platinum-Group Elements). Infiltration metasomatism is a process involving the movement of magma and/or fluids into partially molten rock creating a disequilibrium condition. It has been proposed to be the fundamental process responsible for the concentration of PGE’s in the Stillwater Complex and in other layered intrusions. Immediately below the contact between Anorthosite zone II (AN-II) and the overlying troctolites of Olivine-Bearing zone V (OB-V) in the Stillwater Complex is a 15-m-thick section in which the anorthosite differs both texturally and compositionally from that which composes the bulk of AN-II. The rocks in the upper anorthosite section are nearly monomineralic and have plagioclase grains that are 2-3 times smaller than those found in the lower anorthosites, which also contain up to 10% interstitial pyroxene. To further document this discontinuity and to attempt to determine whether it was formed by infiltration metasomatism or as a result of a cumulus replenishment event, samples were collected along a vertical traverse of this transition zone at Picket Pin Mountain and analyzed by electron microprobe and LA-ICP-MS. Analysis of plagioclase shows that the medium-grained anorthosites at the top of AN-II are chemically more similar to the overlying troctolites than they are to the coarse-grained anorthosites below. Plagioclase texture and zoning profiles also show that the zone at the top of AN-II is more favorably comparable with the overlying troctolites than the anorthosites forming the bulk of AN-II. The close chemical and textural resemblance of the upper anorthosites to the troctolites, in conjunction with evidence of compaction and fluid involvement, suggest that the infiltration metasomatism model better elucidates the nature of this discontinuity.
DE: 3618 Magma chamber processes (1036)
DE: 3620 Mineral and crystal chemistry (1042)
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting