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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.

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Presentation on theme: "What can xenoliths tell us? Roberta L. Rudnick Geochemistry Laboratory Department of Geology University of Maryland Roberta L. Rudnick Geochemistry Laboratory."— Presentation transcript:

1 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

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

3 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”

4 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)

5 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)

6 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

7 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

8 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

9 Mantle Xenoliths Lithologies:PeridotitePyroxeniteEclogiteOthers * Temperature: 2 px or Ca-in- opx thermometry Temperature: 2 px or Ca-in-opx thermometry Pressure: Gt-Opx barometry

10 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

11 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

12 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

13 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

14 143 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

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 presentLithologies present Age: igneous & metamorphicAge: igneous & metamorphic Thermal historyThermal history AnisotropyAnisotropy

18 Mg# Mg# What’s the lower crust made of? Rudnick & Presper, 1990

19 Age: Igneous and Metamorphic Gao et al., 2004

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

21 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

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 Xenoliths studies provide important complements to geophysical & geological studies Caveats: Caveats:  representativeness  post-entrainment alteration

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