What can xenoliths tell us Roberta L Rudnick

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: vcomposition vage vtemperature vthickness vdeformation vfluids “The poor man’s drill hole”

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

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

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

8. 5 Average Vp for lower crustal rock types (0 o. C, 600 MPa) Rudnick & Fountain (1995) 8. 0 Eclogite 7. 5 Mafic granulite Mafic gt granulite Anorthosite 7. 0 Metapelite - Granulite facies Amphibolite Felsic granulite 6. 5 Metapelite - Amphbolite facies Felsic amphibolite 6. 0 6. 5 7. 0 7. 5 Christensen & Mooney (1995) 8. 0 8. 5

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

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

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

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

Age of lithosphere: Osmium model ages Re-Os Systematics 0. 13 0. 12 187 Os/188 Os 0. 11 Primitive Mantle (3. 3) T RD Komatiite (28) Basalt Residue (2. 1) Komatiite Residue (Re/Os = 0) Basalt (1500) T MA 0. 10 1. 0 187 Re Æ Os T 1/2 = 42 Ga 2. 0 3. 0 4. 0 Time (Ga) After Walker et al. , 1989

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

Peridotite xenoliths from Great Falls Tectonic Zone record 1. 8 Ga and ~50 Ma magmatism Host lavas Mixing with host 0. 5120 143 Nd/ 0. 5115 Mixing with host 144 Nd 0. 5110 Highwood peridotites Eagle Buttes peridotites 0. 5105 Age =1. 8 Ga e. Nd(0) = -9. 5 0. 06 From Carlson & Irving, 1994 Glim. , Web. , gabbro 0. 08 0. 10 0. 12 Eagle Buttes cpx Highwood Mt. Dunite 0. 14 147 Sm/ 144 Nd 0. 16 0. 18

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

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

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

90 Granulite Facies Terranes 80 What’s the lower crust made of? 70 60 Mg# 50 40 30 20 10 30 40 50 90 60 70 80 90 Lower crustal xenoliths 80 70 60 Mg# 50 40 30 20 10 30 40 Rudnick & Presper, 1990 50 60 70 Si. O 2 (wt. %) 80 90

Age: Igneous and Metamorphic 0. 6 Xu-Huai Garnet clinopyroxenite 2600 603 -2 -1 2400 0. 4 2000 0. 3 1600 206 Pb/238 U 0. 5 1743 ± 23 Ma (2 s) MSWD =0. 24 4 analyses 1200 0. 2 800 0. 1 129 ± 11 Ma (2 s) MSWD ± =5. 7 3 analyses (b) 0. 0 0 2 Gao et al. , 2004 4 6 8 207 Pb/235 U 10 12 14

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

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

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

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

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!

Conclusions • Xenoliths studies provide important complements to geophysical & geological studies • Caveats: vrepresentativeness vpost-entrainment alteration