HR: 16:45h
AN: V34A-04    [Abstracts]
TI: Continental mantle in the Rio Grande Rift: Chemistry and Fabric Studies of Xenoliths from Kilbourne Hole, New Mexico
AU: * Anthony, E Y
EM: eanthony@utep.edu
AF: University of Texas at El Paso, Department of Geological Sciences, El Paso, TX 79968, United States
AU: Ren, M
EM: ren@geo.utep.edu
AF: University of Texas at El Paso, Department of Geological Sciences, El Paso, TX 79968, United States
AU: Cardon, K P
EM: kpcardon@miners.utep.edu
AF: University of Texas at El Paso, Department of Geological Sciences, El Paso, TX 79968, United States
AU: Stern, R J
EM: rjstern@utdallas.edu
AF: University of Texas at Dallas, Department of Geosciences, Dallas, TX 75204, United States
AU: Kimura, J
EM: jkimura@riko.shimane-u.ac.jp
AF: Shimane University, Department of Geoscience, Matsue, 690-8504, Japan
AU: Michibayashi, K
EM: sekmich@ipc.shizuoka.ac.jp
AF: Skizuoka University, Institute of Geosciences, Shizuoka, 422-8529, Japan
AU: Satsukawa, T
EM: sekmich@ipc.shizuoka.ac.jp
AF: Skizuoka University, Institute of Geosciences, Shizuoka, 422-8529, Japan
AU: Ducea, M
EM: ducea@email.arizona.edu
AF: University of Arizona, Department of Geosciences, Tucson, AZ 85721, United States
AB: Kilbourne Hole maar, Potrillo volcanic field (PVF), erupted basanite approximately 10 ka ago and thus the melt composition represents essentially the present-day thermal and compositional character of this segment of the continental mantle. Mantle xenoliths are lherzolite (cpx mode 10-25 percent; Cr number of spinel approx. 0.1) and more depleted harzburgite and dunite (cpx mode 1-8 percent; Cr number of spinel 0.2-0.5). Clinopyroxenite (cpx mode 90-95 percent; Mg number olivine 0.8-0.85; Cr number spinel less than 0.01) occurs as veins crosscutting lherzolite. Bulk rock ICPMS analyses reveal that lherzolite is moderately melt-depleted. Harzburgite and dunite have lower abundances of moderately incompatible elements and higher abundances of strongly incompatible elements. Harzburgite and dunite compositions are successfully modeled by infiltration of melts, which have similar composition to PVF basanites. Pyroxenites are modeled as high pressure cumulate of the same basanitic melts. Lherzolite includes both fine-grained equigranular and porphyroclastic textures. Harzburgite and dunite demonstrate coarse-grained porphyroclastic texture. Fine-grained lherzolite has the lowest temperatures (approx. 980 C), porphyroclastic lherzolite intermediate temperature (1020-1044 C), and harzburgite and dunite the highest temperature (1050-1135 C). No petrographic evidence exists for remnant garnet, implying all samples originated in the spinel stability field. Thus, the Moho, known from current geophysical studies to vary from 30 - 35 km, currently experiences temperatures of approx. 1000 C. This temperature, which is shared by other North American Cenozoic xenolith localities, is a substantial deviation from a steady state (i.e. thermal equilibrium) geotherm and requires either advective or convective heat transfer into this portion of the continental mantle. This conclusion is consistent with tomographic images that show western NA, and particularly areas with Cenozoic volcanism, have slow seismic velocities. J-index shows a positive correlation with temperature and varies from 5 to 10 for the low temperature lherzolite to 20 for the harzburgite and dunite. J-index of 20 is an unusually high value for peridotite. Shear wave-splitting studies from LA RISTRA (a 950 km line from west Texas to Colorado) show fast polarization directions subparallel to NA absolute plate motion and no deviation in direction or delay time as the line crosses the Rio Grande rift. We conclude that the high J-index values for the deepest and hottest xenoliths, which also have experienced basanite melt infiltration, represent deformation at the lithosphere-asthenosphere boundary.
DE: 1025 Composition of the mantle
DE: 3621 Mantle processes (1038)
DE: 8120 Dynamics of lithosphere and mantle: general (1213)
DE: 8130 Heat generation and transport
DE: 9350 North America
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting