HR: 1330h
AN: T32B-0921    [PDF]
TI: Geophysical Signatures of Chemical Depletion in Slave Cratonic Peridotite.
AU: * Kopylova, M G
EM: mkopylov@eos.ubc.ca
AF: University of British Columbia, Dept. of Earth and Ocean Sci. 6339 Stores Rd.,, Vancouver, BC V6T 1Z4 Canada
AU: Jones, A
EM: ajones@nrcan.gc.ca
AF: Geological Survey of Canada, Natural Resiurces Canada 615 Booth St, Ottawa, ON K1A OE9 Canada
AU: McCammon, C
EM: catherine.mccammon@uni-bayreuth.de
AF: Bayerisches Geoinstitut, Universitat Bayreuth, Bayreuth, D-95440 Germany
AB: Variously depleted peridotites of the Slave craton (Northwest Canada) provide an excellent natural laboratory that allows us to investigate effects of depletion on the chemical and physical characteristics of rocks. We computed seismic velocities for the variously depleted peridotites of the N and SE Slave based on single-crystal elastic moduli and volume fractions of constituent minerals. The depleted peridotites enriched in MgO have lower Vp and higher Vs, where lower Poisson's ratios are due to orthopyroxene enrichment. The predicted effect on seismic wave speeds would be up to 0.05 km/s, or 0.6 rel.% of Vp. The correlation observed on the Slave craton contradicts the established view that peridotite depleted in basaltic magmaphile elements has higher seismic wave velocities. However, evidence amassed in the past 15 years suggests that cratonic mantle peridotite is chemically distinct from off-cratonic peridotite. In cratonic peridotite, the Mg-number of olivine is negatively correlated with its mode, and Ni in olivine is positively correlated with orthopyroxene mode and olivine Mg-number. These patterns hint that depletion in the cratonic mantle may have a distinct seismic signature compared to the off-cratonic mantle. Our data suggest that chemical depletion of peridotites should also affect their redox state. The shallow, more depleted spinel peridotite of the N Slave shows a distinctly lower bulk Fe2O3 abundance and lower Fe3+ concentration in spinel, consistent with a lower oxygen fugacity. Moreover, different types of peridotite that coexist at a given depth have different redox states, and the redox state of peridotitic mantle changes sharply between the layers with different bulk compositions and oxide mineralogies. Such a pattern of oxygen fugacity is expected if oxygen is controlled intrinsically by Fe equilibria. In an Fe-buffered peridotitic mantle, domains depleted in Fe3+ following partial melting should be more reduced and therefore should contain more elemental carbon. A larger abundance of graphite may explain the higher EM conductivity of the ultra-depleted layer mapped in the Central Slave mantle.
DE: 0600 ELECTROMAGNETICS
DE: 0935 Seismic methods (3025)
DE: 1025 Composition of the mantle
DE: 3640 Igneous petrology
DE: 3929 NMR, Mossbauer spectroscopy, and other magnetic techniques
SC: Tectonophysics [T]
MN: 2003 Fall Meeting