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What can xenoliths tell us? Roberta L. Rudnick Geochemistry Laboratory Department of Geology University of Maryland Roberta L. Rudnick Geochemistry Laboratory Department of Geology University of Maryland
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Outline Promises and pitfalls Promises and pitfalls Mantle samples Mantle samples Crustal samples Crustal samples Xenoliths in Northern Rockies Xenoliths in Northern Rockies
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The beauty of xenoliths Direct sampling of deep lithosphere: Direct sampling of deep lithosphere: composition age temperature thickness deformation fluids “The poor man’s drill hole”
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The beauty of xenoliths Discern temporal evolution, if host magmas span significant time frame. Discern temporal evolution, if host magmas span significant time frame.Examples: Sierra Nevada (Ducea and colleagues) North China craton (Menzies, Griffin)
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Potential Pitfalls Sample may not be random: difficult to determine “representativeness” Sample may not be random: difficult to determine “representativeness” Need to relate to geophysical data: seismic velocities heat flow (infer heat production)
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Rifted Margin ContractionalShield & Platform Paleozoic Orogen Rift Extensional Arc Forearc 0 20 40 60 Km VpVp 6.4 6.6 6.8 7.0 7.2 Rudnick & Fountain, 1995
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6.0 6.5 7.0 7.5 8.0 8.5 6.06.57.07.58.08.5 Rudnick & Fountain (1995) Christensen & Mooney (1995) Average Vp for lower crustal rock types (0 o C, 600 MPa) Eclogite Mafic gt granulite Felsic granulite Felsic amphibolite Mafic granulite Anorthosite Amphibolite Metapelite - Amphbolite facies Metapelite - Granulite facies
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Potential Pitfalls Post-entrainment modifications Post-entrainment modifications Decompression (e.g., kelyphite on garnet) Chemical changes (e.g., K-enrichment) Such irreversible changes compromise ultrasonic measurements, whole rock geochemistry
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Mantle Xenoliths Lithologies:PeridotitePyroxeniteEclogiteOthers * Temperature: 2 px or Ca-in- opx thermometry Temperature: 2 px or Ca-in-opx thermometry Pressure: Gt-Opx barometry
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Mantle xenolith studies may elucidate: Temperature & thickness of lithosphere (mantle lid)Temperature & thickness of lithosphere (mantle lid) Age of deep lithosphereAge of deep lithosphere Magmatic historyMagmatic history AnisotropyAnisotropy
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Temperature & thickness oflithosphere: Temperature & thickness of lithosphere: Garnet peridotites KalihariSlave Pressure (GPa) Lesotho Kimberley LetlhakaneJericho Lac de Gras Torrie Grizzly Depth (km) Best Fit Kalihari 50 100 150 200 250 300 0 2 4 6 8 1002004006008001000120014001600 2004006008001000120014001600 Temperature ( o C) From Rudnick & Nyblade, 1999
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Re-Os Systematics Primitive Mantle (3.3) Primitive Mantle (3.3) Basalt (1500) Basalt (1500) Komatiite (28) Komatiite (28) Basalt Residue (2.1) Basalt Residue (2.1) Komatiite Residue (Re/Os = 0) Komatiite Residue (Re/Os = 0) 1.0 2.0 3.0 4.0 Time (Ga) 187 Os/ 188 Os 187 Os/ 188 Os 187 Re Æ 187 Os T 1/2 = 42 Ga 187 Re 187 Os T 1/2 = 42 Ga After Walker et al., 1989 0.12 0.11 0.13 0.10 T RD T Age oflithosphere: Osmium model ages Age of lithosphere: Osmium model ages T MA
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Magmatic history: Radiogenic isotope systems based on incompatible elements (e.g., Rb-Sr, Sm-Nd, Lu-Hf) may record metasomatic interactionsRadiogenic isotope systems based on incompatible elements (e.g., Rb-Sr, Sm-Nd, Lu-Hf) may record metasomatic interactions U-Pb of (rare) metasomatic zirconsU-Pb of (rare) metasomatic zircons Composite xenolith Liu et al., 2004
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143 Nd/ 144 Nd 147 Sm/ 144 Nd Peridotite xenoliths from Great Falls Tectonic Zone record 1.8 Ga and ~50 Ma magmatism Age =1.8 Ga Nd (0) = -9.5 0.5120 0.5115 0.5110 0.5105 0.06 0.08 0.10 0.12 0.14 0.16 0.18 Highwood peridotites Eagle Buttes peridotites Glim., Web., gabbro Eagle Buttes cpx Highwood Mt. Dunite From Carlson & Irving, 1994 Host lavas Mixing with host
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Metasomatic zircon in mantle xenolith Great Falls Tectonic Zone From Rudnick et al., 1999
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Anisotropy: Studies of microstructure, texture and olivine preferred lattice orientation provides direct information on seismic anisotropy
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Lower crustal xenolith studies may elucidate: Lithologies presentLithologies present Age: igneous & metamorphicAge: igneous & metamorphic Thermal historyThermal history AnisotropyAnisotropy
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Mg# Mg# What’s the lower crust made of? Rudnick & Presper, 1990
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Age: Igneous and Metamorphic Gao et al., 2004
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Thermal history: U-Pb dating of accessory phases From Schmitz & Bowring (2003)
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Modified from Carlson & Irving, 1994; Carlson et al., 2004; Hearn, 2004 Xenolith Localities in Montana Williams Crust Mantle & Lower crust Bear Paw Eagle Buttes Sweetgrass Homestead Highwood GFTZ Mantle Porcupine Dome
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Summary of Montana Studies At 50 Ma: Wyoming craton underlain by thick (~170 km) cratonic root
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Summary of Montana Studies At 50 Ma: Wyoming craton underlain by thick (~170 km) cratonic root Great Falls Tectonic Zone underlain by Archean lithosphere, heavily overprinted at 1.8 Ga
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Summary of Montana Studies At 50 Ma: Wyoming craton underlain by thick (~170 km) cratonic root Great Falls Tectonic Zone underlain by Archean lithosphere, heavily overprinted at 1.8 Ga Metasomatic component in GFTZ looks like crust!
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Conclusions Xenoliths studies provide important complements to geophysical & geological studies Xenoliths studies provide important complements to geophysical & geological studies Caveats: Caveats: representativeness post-entrainment alteration
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