HR: 1340h
AN: DI33A-1125    [Abstracts]
TI: Carbonated Peridotite Xenoliths From the Western Canadian Cordillera
AU: * Peterson, N D
EM: npeterson@eos.ubc.ca
AF: Volcanology and Petrology Laboratory, Earth and Ocean Sciences, University of British Columbia, 6339 Stores Rd., Vancouver, BC V6T 1Z4, Canada
AU: Russell, J K
EM: krussell@eos.ubc.ca
AF: Volcanology and Petrology Laboratory, Earth and Ocean Sciences, University of British Columbia, 6339 Stores Rd., Vancouver, BC V6T 1Z4, Canada
AB: Carbonated mantle is inferred as the source of numerous alkaline magmas. However, carbonate is rarely expressed modally in mantle-derived peridotitic xenoliths. Most carbonate interpreted as primary (i.e. not an alteration product) occurs as interstitial patches associated with second generation crystallization, or as inclusions within crystals. Here we report on a unique occurrence of primary mantle-derived carbonate preserved in spinel peridotite xenoliths within a 19 Ma basanite dike. The dike intrudes Jurassic volcaniclastic rocks near the Intermontane - Coast Belt boundary in western British Columbia. The peridotite xenoliths are concentrated in a 4 meter interval where the dike is narrowest (1 m width); they are ≤40 cm in diameter, and are dominated by lherzolite, with less abundant harzburgite, dunite, and websterite. Thermometry (Brey & Köhler) produces xenolith equilibrium temperatures of 850 to 1000 °C, corresponding to approximately 35 to 50 km depth on an average geotherm for warm, thin Cordilleran-style lithosphere. The basanite also contains primary magmatic calcite occurring as a late stage groundmass crystallization product and as crystalline aggregates lining or filling vesicles. Both habits have isotopic compositions indicating a mantle origin (δ¹⁸O = 14 to 15, δ¹³C = -4 to -5, ⁸⁷Sr/⁸⁶Sr = 0.7040) and are isotopically distinct from secondary carbonate in the adjacent country rocks (δ¹⁸O = -5, δ¹³C = -5, ⁸⁷Sr/⁸⁶Sr = 0.7047).
Calcite occurs in all but three of the suite of 50 xenoliths examined. The calcite appears both as grains that appear to have been in textural equilibrium with neighboring minerals (granoblastic texture), and as intergranular or fracture-filling veins. Calcite grains are ≤0.3 mm in size. The calcite comprises up to 2 % of xenoliths; calcium content (Ca / Ca + Mg + Fe) is ~0.90, contrasting with ~0.98 for the basanite's calcite. Carbonate C and O isotopes (whole rock leachate) indicate a range of δ¹⁸O = 10 to 12, δ¹³C = -3 to -6 for the suite. Ongoing studies are establishing the conditions required for preservation of this unique xenolith-hosted mantle-derived carbonate and the implications for source region and transport processes.
DE: 1025 Composition of the mantle
DE: 1038 Mantle processes (3621)
DE: 3621 Mantle processes (1038)
DE: 8124 Earth's interior: composition and state (1212, 7207, 7208, 8105)
SC: Study of the Earth's Deep Interior [DI]
MN: 2007 Fall Meeting