HR: 08:50h
AN: T21E-04 INVITED     [Abstracts]
TI: Archean Ophiolite or Oceanic Plateau? Lithologic and Geochemical Ambiguities
AU: * Condie, K C
EM: kcondie@nmt.edu
AF: Earth & Environmental Science, New Mexico Tech, Socorro, NM 87111 United States
AB: Although young oceanic plateaus (? 20 km thick) are considerably thicker than oceanic crust (5 - 7 km), only the upper few km (? 7 km) are usually preserved during tectonic emplacement on the continents. From about 3.5 Ga onwards, typically ? 8 km of section are preserved in single greenstones with oceanic affinities and there is no relationship of age to thickness. Hence, if Archean oceanic crust had Penrose ophiolite stratigraphy, only the upper few kilometers of the pillow basalt unit would be preserved. This unit may be lithologically indistinguishable from that scraped off the top of Archean oceanic plateaus during emplacement. Thus only after about 2 Ga, when oceanic crust decreased in thickness to ? 10 km would deeper ophiolite stratigraphy be preserved during emplacement. Although the presence of komatiites in greenstones is often equated with a mantle plume source, komatiites also occur to some "arc-type" greenstones, where they may represent hydrous magmas related to convergent plate margins. Hence, without other evidence, komatiites may not be sufficient to identify mantle plume sources. Based on the distribution of high field strength elements (Th, Nb, Ta, Zr, Y), the first appearance of oceanic basalts derived from shallow depleted mantle (DM) is between 2.0 and 2.7 Ga. Before this time, oceanic basalts (MORB and OPB [oceanic plateau basalts]) appear to have been produced in a relatively uniform mantle reservoir similar to primitive mantle in composition, but with variable contributions from enriched sources (continental crust, subcontinental lithosphere) and/or deep depleted mantle sources (DEP). The first evidence for recycled sources (HIMU, EM1 and 2) in the mantle appears in basalts at 2.7 Ga. Only after this time are trace element distributions helpful in distinguishing ocean ridge from oceanic plateau basalts. Rapid growth of continental crust and DM and the onset of isolation of distinct mantle plume reservoirs at 2.7 Ga, may reflect a global mantle plume event at this time.
DE: 1025 Composition of the mantle
DE: 8125 Evolution of the Earth (0325)
DE: 8137 Hotspots, large igneous provinces, and flood basalt volcanism
DE: 8140 Ophiolites (3042)
DE: 8178 Tectonics and magmatism
SC: Tectonophysics [T]
MN: Fall Meeting 2005