Petrology of ultramafic xenoliths, Rayfield River and Big Tomothy mountain areas, British Columbia
Date
2012
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Abstract
Ultramafic xenoliths from Rayfield River and Big Timothy Mountain, British Columbia are interpreted to represent the underlying upper mantle. Both localities are Tertiary volcanic cones consisting of alkali basalt flows. The xenoliths are spine! lherzolites and less commonly harzburgites and pyroxenites. Mineral assemblages include olivine, clinopyroxene, 01thopyroxene, and spinel. Microprobe analysis of the individual grains determined that all phases were generally compositionally homogeneous. A number of geothermometers have been assessed to determine the thermal conditions under which the peridotite samples equilibrated. These include twopyroxene thermometry, spinel-orthopyroxene, and spinel-olivine equilibration. Temperatures calculated using the Brey and Kohler calibration of two-pyroxene thermometry are interpreted to represent the temperature of equilibration. The Rayfield River xenoliths, average two-pyroxene temperatures of 912 ± 42° C. Orthopyroxenespinel and olivine-spine! results are 889 ± 60° C and 825 ± 88° C, respectively. Big Timothy Mountain xenoliths have an average two-pyroxene temperature of 959 ± 62° C. Orthopyroxene-spinel and olivine-spine! temperatures are 844 ± 63° C and 896 ± 232°C, respectively. The pressure range in which the peridotite equilibrated within the upper mantle must be estimated based on the stability of the minerals. Isochemical phase diagrams (pseudosections) constrain the P-T field in which spine! is stable and is the sole aluminous phase (i.e. garnet and plagioclase remain unstable). With the use of these diagrams it can be concluded that this assemblage equilibrated in pressures ranging from approximately 9.6 to 14 kbar and at depths of approximately 33 to 48 km. The effect of pressure on the temperature calculations is negligible; they change only ~2°C/kbar. Overall, there is very good agreement between the isochemical phase diagrams, temperatures, and the observed mineral assemblage. Oxygen fugacity (??logfo2) calculations were done on both of the peridotite suites to constrain the oxidation state of the upper mantle. The results are typical of a moderately reduced and depleted upper mantle, having an average of~ l.3 log units below FMQ (fayalite-magnetite-quartz). In addition, the peridotites represent residua from the partial melting of the upper mantle. Compositionally, the peridotites from Big Timothy Mountain are fairly fertile with a MgO weight percent ranging from ~35-43% and Ab03 around 3%; these correspond to 8-
10% partial melting of primitive mantle. This relatively fertile lherzolite represents upper mantle underlying the Canadian Cordillera. A result of this study, determining the thermal conditions of the upper mantle, is to build a geothermal gradient for the southern Intermontane belt by combining thermal data determined for the area and the calculated pressure/temperature conditions.
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Bibliography: p. 127-131
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Citation
Greenfield, A. (2012). Petrology of ultramafic xenoliths, Rayfield River and Big Tomothy mountain areas, British Columbia (Master's thesis, University of Calgary, Calgary, Canada). Retrieved from https://prism.ucalgary.ca. doi:10.11575/PRISM/4760