Physics
Scientific paper
Nov 2011
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2011e%26psl.311..172s&link_type=abstract
Earth and Planetary Science Letters, Volume 311, Issue 1, p. 172-181.
Physics
Peridotite Xenolith, Kilbourne Hole Maar, Rio Grande Rift, Mantle Flow, Crystallographic Preferred Orientation (Cpo), Seismic Anisotropy
Scientific paper
Peridotite xenoliths from the Kilbourne Hole maar, New Mexico, consist of spinel lherzolite, harzburgite, and dunite. Because Kilbourne Hole erupted at approximately 10 ka, these xenoliths represent essentially current conditions beneath the Rio Grande rift. In this study, we present detailed petrofabric data and seismic properties obtained from peridotite xenoliths from Kilbourne Hole to illuminate the origin and significance of shear-wave splitting in the uppermost mantle beneath this active rift. Using phase relations and the temperature of equilibration, we infer that these xenoliths were derived from the uppermost mantle, from depths of 35-60 km. Their crystallographic preferred orientations indicate the preservation of olivine b-axis fiber fabrics with a strong concentration of [010] with girdles of [100] and [001]. We consider three geodynamic models for the source region of these xenoliths: horizontal extension, lateral shear, and upwelling. After calculating seismic properties using a volume fraction of olivine, orthopyroxene and clinopyroxene appropriate to each model, we conclude that these xenoliths are derived from a lateral shear zone (vertical foliation (XY plane) and horizontal lineation within the plane of the foliation (X-axis)). However, the degree of seismic anisotropy generated by peridotite xenoliths alone is limited, so that the existence of melt in thin cracks or dikes could be required to cause a significant increase; the orientation of such melt pockets parallel to the XY plane in either model would result in an increase in anisotropy. These results indicate that the shear-wave splitting observed in the Rio Grande rift is a reflection of the lithospheric fabric and the presence of melts as thin cracks or dikes.
Anthony Elizabeth Y.
Gao Stephen S.
Liu Kelly H.
Michibayashi Katsuyoshi
Satsukawa Takako
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