HR: 1330h
AN: V13B-07    [Abstracts]
TI: Ti and Al Pyroxene in the Egersund Dikes: Analogue for Martian Meteorites?
AU: * Ziga, J M
EM: ziga.1@osu.edu
AF: The Ohio State University, Department of Geological Sciences 275 Mendenhall Laboratory 125 South Oval Mall, Columbus, OH 43210 United States
AU: Barton, M
EM: barton.2@osu.edu
AF: The Ohio State University, Department of Geological Sciences 275 Mendenhall Laboratory 125 South Oval Mall, Columbus, OH 43210 United States
AB: The Egersund dikes in SW Norway intruded at 650-600 Ma. Most formed by intrusion of olivine tholeiites that are geochemically similar to modern OIB. Olivines range in composition from Fo₈₄ to Fo₇₆ and show a variety of textural features including skeletal crystals, strained crystals, crystals intergrown with plagioclase, and resorbed crystals. Inclusions with high-Ca pyroxene, plagioclase, amphibole, Fe-Ti oxide, and very rare spinel and apatite appear to represent partly crystallized melt trapped in some olivines. The pyroxene is titanaugite with very high Al₂O₃ (up to 18 wt%) and TiO₂ (up to 6.4 wt%), low SiO₂ (40.6 ave wt%), and low Mg&35; (ave 0.58). Coexisting amphibole is kaersutite (Mg&35; 0.25-0.58) with up to 0.77 Ti afu and 2.8-3.5 Al afu. Si contents (5.74-5.34 afu) are the lowest recorded for this amphibole. Inclusions containing pyroxene and kaersutite occur in resorbed olivines. Some of the latter show reversed zoning (cores - Fo₇₆, rims Fo₈₂), and they are interpreted to be xenocrysts that formed in magma chambers at pressures of P=0.8-1.2 GPa. The pressures are constrained from phase equilibrium data, and indicate crystallization in chambers located at or near the crust-mantle boundary (ca 28 km). The inclusions possibly represent alkaline magmas that resided in the magma chamber prior to flushing by olivine tholeiitic magma. This is supported by the Ti-rich composition of the pyroxene by extensive substitution of Al into the pyroxene tetrahedral site, and by the occurrence of kaersutite, which does not form from tholeiitic melts based on experimental studies. Alkaline magmas (trachybasalts) occur in the Egersund dike swarm. The high Al₂O₃ and TiO₂ character of pyroxene and the occurrence of kaersutite in the Egersund dikes are similar to that found in olivine hosted melt inclusions in Martian meteorites (Chassignites and Nakhlites). These similarities allow the Egersund dolerites to be used as a possible analogue for igneous processes on Mars. The results of this study suggest that melt compositions inferred from inclusions in olivine may not provide reliable estimates of the parent magma composition. Further study of these inclusions could provide a better understanding of the Al and Ti enrichment observed in Martian meteorite melt inclusions.
DE: 1099 General or miscellaneous
DE: 3655 Major element composition
DE: 3662 Meteorites
DE: 3699 General or miscellaneous
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2005 Joint Assembly