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What can xenoliths tell us?

What can xenoliths tell us?. Roberta L. Rudnick Geochemistry Laboratory Department of Geology University of Maryland. Outline. Promises and pitfalls Mantle samples Crustal samples Xenoliths in Northern Rockies. The beauty of xenoliths. Direct sampling of deep lithosphere: composition

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What can xenoliths tell us?

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  1. What can xenoliths tell us? Roberta L. Rudnick Geochemistry Laboratory Department of Geology University of Maryland

  2. Outline • Promises and pitfalls • Mantle samples • Crustal samples • Xenoliths in Northern Rockies

  3. The beauty of xenoliths • Direct sampling of deep lithosphere: • composition • age • temperature • thickness • deformation • fluids “The poor man’s drill hole”

  4. The beauty of xenoliths • Discern temporal evolution, if host magmas span significant time frame. Examples: • Sierra Nevada (Ducea and colleagues) • North China craton (Menzies, Griffin)

  5. Potential Pitfalls • Sample may not be random: difficult to determine “representativeness” Need to relate to geophysical data: • seismic velocities • heat flow (infer heat production)

  6. Paleozoic Orogen Rifted Margin Rift Arc Contractional Shield & Platform Extensional Forearc 0 20 40 Vp 60 Km 6.4 6.6 6.8 7.0 7.2 Rudnick & Fountain, 1995

  7. 8.5 8.0 7.5 Metapelite - Granulite facies 7.0 6.5 Metapelite - Amphbolite facies 6.0 6.0 6.5 7.0 7.5 8.0 8.5 Average Vp for lower crustal rock types (0oC, 600 MPa) Eclogite Rudnick & Fountain (1995) Mafic granulite Mafic gt granulite Anorthosite Amphibolite Felsic granulite Felsic amphibolite Christensen & Mooney (1995)

  8. Potential Pitfalls • Post-entrainment modifications • Decompression (e.g., kelyphite on garnet) • Chemical changes (e.g., K-enrichment) Such irreversible changes compromise ultrasonic measurements, whole rock geochemistry

  9. Temperature: 2 px or Ca-in-opx thermometry Temperature: 2 px or Ca-in-opx thermometry Pressure: Gt-Opx barometry Mantle Xenoliths Lithologies: Peridotite Pyroxenite Eclogite Others *

  10. Mantle xenolith studies may elucidate: • Temperature & thickness of lithosphere(mantle lid) • Age of deep lithosphere • Magmatic history • Anisotropy

  11. Jericho Lac de Gras Torrie Grizzly 0 200 400 600 800 1000 1200 1400 1600 200 400 600 800 1000 1200 1400 1600 Temperature & thickness oflithosphere: Garnet peridotites 0 Kalihari Slave 50 2 100 4 Pressure (GPa) 150 Depth (km) 6 Lesotho 200 Kimberley Best Fit Kalihari Letlhakane 8 250 300 10 Temperature (oC) Temperature (oC) From Rudnick & Nyblade, 1999

  12. Re Os 187 187 Re Os 187 187  Æ T = 42 Ga T = 42 Ga 1/2 1/2 Age oflithosphere: Osmium model ages Re-Os Systematics Komatiite Komatiite 0.13 Basalt 0.13 (28) Basalt (28) Primitive Mantle Primitive Mantle (1500) (1500) (3.3) (3.3) T T RD RD 0.12 0.12 Os/ Os 187 188 Os/ Os 187 188 Basalt Residue Basalt Residue (2.1) (2.1) 0.11 0.11 T Komatiite Residue Komatiite Residue MA (Re/Os = 0) (Re/Os = 0) 0.10 0.10 1.0 2.0 3.0 4.0 1.0 2.0 3.0 4.0 Time (Ga) Time (Ga) After Walker et al., 1989

  13. Magmatic history: • Radiogenic isotope systems based on incompatible elements (e.g., Rb-Sr, Sm-Nd, Lu-Hf) may record metasomatic interactions • U-Pb of (rare) metasomatic zircons Composite xenolith Liu et al., 2004

  14. Host lavas Mixing with host Mixing with host Peridotite xenolithsfrom Great Falls Tectonic Zone record 1.8 Ga and ~50 Ma magmatism 0.5120 0.5115 0.5110 0.5105 143Nd/ 144Nd Highwood peridotites Eagle Buttes peridotites Glim., Web., gabbro Eagle Buttes cpx Highwood Mt. Dunite Age =1.8 Ga eNd(0) = -9.5 0.06 0.08 0.10 0.12 0.14 0.16 0.18 147Sm/ 144Nd From Carlson & Irving, 1994

  15. Metasomatic zircon in mantle xenolith Great Falls Tectonic Zone From Rudnick et al., 1999

  16. Anisotropy: Studies of microstructure, texture and olivine preferred lattice orientation provides direct information on seismic anisotropy

  17. Lower crustal xenolith studies may elucidate: • Lithologies present • Age: igneous & metamorphic • Thermal history • Anisotropy

  18. Granulite Facies Terranes 90 80 70 60 50 40 30 20 10 30 40 50 60 70 80 90 Lower crustal xenoliths 90 80 70 60 50 40 30 20 10 30 40 50 60 70 80 90 SiO2 (wt. %) What’s the lower crust made of? Mg# Mg# Rudnick & Presper, 1990

  19. 0.6 Xu-Huai Garnet clinopyroxenite 603-2-1 2600 0.5 2400 0.4 U 2000 238 Pb/ 0.3 1600 1743 ± 23Ma (2s) MSWD =0.24 4 analyses 206 1200 0.2 800 0.1 129 ± 11Ma (2s) MSWD ±=5.7 3 analyses (b) 0.0 0 2 4 6 8 10 12 14 207 235 U Pb/ Age: Igneous and Metamorphic Gao et al., 2004

  20. Thermal history: U-Pb dating of accessory phases From Schmitz & Bowring (2003)

  21. Xenolith Localities in Montana Crust Mantle & Lower crust Mantle GFTZ Sweetgrass Bear Paw Eagle Buttes Williams Highwood Homestead Porcupine Dome Modified from Carlson & Irving, 1994; Carlson et al., 2004; Hearn, 2004

  22. Summary of Montana Studies • At 50 Ma: • Wyoming craton underlain by thick (~170 km) cratonic root

  23. 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

  24. 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!

  25. Conclusions • Xenoliths studies provide important complements to geophysical & geological studies • Caveats: • representativeness • post-entrainment alteration

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