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“Lithium Isotope Geochemistry: applications to high-temperature processes”

“Lithium Isotope Geochemistry: applications to high-temperature processes”. Dr. Ian Buick Research School of Earth Science The Australian National University 4th Feb 2008. Overview. 2 isotopes: 6 Li (7.5 %) and 7 Li (92.5%)

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“Lithium Isotope Geochemistry: applications to high-temperature processes”

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  1. “Lithium Isotope Geochemistry: applications to high-temperature processes” Dr. Ian Buick Research School of Earth Science The Australian National University 4th Feb 2008

  2. Overview • 2 isotopes: 6Li (7.5 %) and 7Li (92.5%) • Data as 7Li (‰ )= 1000*((7Li/6LiUnk - 7Li/6LiL-SVEC)-1), where 7Li/6LiL-SVEC = 12.0192 (also see 6Li used) • L-SVEC = manufactured Li2CO3 standard (NST SRM-8545) • Large mass difference between 6Li and 7Li (~16%) : large fractionations due to (low T) geological processes large range of 7Li in earth materials (> 60 ‰)

  3. from Tomascak RIMG v55 (2004)

  4. Uses for Lithium isotopes • Li-isotopes may provide information about: • Subduction-related addition and crustal recycling at active continental margins • Fluid transfer processes in subduction zones • Material cycling between mantle and oceanic crust • Isotopic fractionation during metamorphism, melting and fractional crystallisation

  5. Lithium isotope cycle: MOR

  6. Lithium isotope cycle: Subduction Zones Elliot et al. (2004) EPSL

  7. Methods of 7Li data collection • Bulk rock (TIMS or MC-ICP-MS) v. Ionprobe (SIMS) • Potentially extreme Li-isotopic fractionations during wet chemistry need for 100% extraction of Li • TIMS: significant 7Li/6Li mass fractionation on surface of filament. Problems overcome by std-bracketed MC-ICP-MS analysis • SIMS: Required due to common occurrence mineral inclusions (metamorphic rocks); documented extreme 7Li heterogeneity on a 10’s-100’s m-scale in some minerals

  8. Li abundances (ppm) • Li distribution coefficients are commonly poorly known as a function of P-T, mineral composition

  9. SIMS analytical set-up • Analyses undertaken on Cameca 3f (Nancy); 4f (Edinburgh); 3f & 6f (Arizona); 1270 (Misasa, Japan; Edinburgh) • Edinburgh 4f method (Kasemann et al., 2005): • secondary 6Li+ & 7Li+ produced by a 5-40nA, 15kV, 16O primary beam focused to ~15-40m spot size. • energy window of 52 eV, at a mass resolution of 1200 (to resolve closest interference, 6Li1H+). • counted by peak jumping on an ETP electron multiplier. • internal uncertainty of <1‰ (1mean) 7Li/6Li ratio measured over 50-120 cycles (depending on Li ppm), each cycle consisting of 5-s and 2-s integrations of 6Li+ and 7Li+ 10-50 minutes/analysis.

  10. SIMS Standards • Glass stds: widely available • BCR-2G (USGS): 7Li = +4.08 ±1.0 ‰, 9±2 ppm Li • GSD-1g (NIST): 7Li = +31.14±0.8 ‰, ~30-40 ppm Li both from Kasemann et al. (2005, Anal. Chem.): • MPI-DING glasses ( 5 stds with 7Li = +2.0 to +17.1 ‰; Jochum et al. 2006; G3) • Minerals: typically in house; developed using TIMS/MC-ICP-MS cross calibration, not widely available • Ol (Arizona, Nancy, Misasa) • Cpx (Misasa, Nancy, Japan) • Crd (Edinburgh)

  11. Matrix Effects: • Instr. mass fractionation (inst = 7Li/6Litrue/7Li/6Limeas; >1) typically corrected for by use of glass stds • Currently lack of consensus and/or knowledge of extent of matrix effects • Decritre et al. (2002; G3) : no matrix effects on inst for range of low-SiO2 minerals (Ol, Opx, Cpx, Amph, Bt etc) using low -SiO2 (basaltic) glass std • Kasemann et al.(2005): ~4-5 ‰ matrix effect for high-Si glass using basaltic glass std. Unpublished large matrix effect for Crd as a fn of XMg. • Bell et al. (2007): ~26 ‰ matrix effect in Ol as a function of XMg (0.94-0.74)

  12. Processes that may potentially change 7Li values • Rayleigh-type fractionation e.g. metamorphic devolatilisation, fractional crystallisation, fluid exsolution: 6Li partitions into solids phases over fluids or melts. • Kinetic diffusion driven by mineral-scale concentration gradients: a potentially major problem for mantle samples and mantle-derived magmas; possibly also during crustal metamorphism and magmatism..

  13. Fractional Crystallisation • eqm Li-isotope partitioning (7LiMin-Min or 7LiMin-Melt) a fn of T and Li coordination • v. small little shifts in 7LiWR in differentiation series small 7Li Min-Minor -Melt at T of mafic magmatism

  14. Fractional Crystallisation • Li in 4-fold co-ordn in high-Si melts,but Li in minerals may be 8-fold (Grt) 6-fold (e.g.Crd, Micas, Px), 4-fold (Sta). 6Li favours larger co-ordn number • significant 7Li(Min-Melt) and • 7LiMelt • inc. for extreme fraction at low T • little change in • 7LiMelt due to fluid exsolution from Teng et al. (2006, Am. Mineral)

  15. Metamorphic devolatilisation • some contradictory results: • Alpine eclogites may have very low 7Li compared to MORB ie 7Li(Omph) as low as -11‰ (Zack et al., 2003 EPSL). Used as a basic input into recent subduction zone Li cycling models • Zack et al. model recently challenged by coupled 7Li - metamorphic modelling-mass balance approach (Marshall et al., 2007, EPSL) : devolatilisation cannot account for such low 7Li(WR). Therefore what is the cause?

  16. Metamorphic devolatilisation • studies of metamorphism in contact aureoles and regional LP/HT metamorphism imply little or no change in 7Li(WR) from Chl Zone to beginning of anatexis e.g Teng et al. (2007, Chem Geol.) from Teng et al (2007, Chem. Geol.)

  17. Kinetically-driven fractionation Li-doped • Exp diffusion couples at 1350-1450 ˚C for 0.1-15hours, 12-13 kbar • Diffusivity Li 102-103 x > all other major and TE • Diffusivity 6Li >7Li by 2-3% RB5: 6 mins @1350˚C RB4: 1 hr @1350˚C 6 mins @1350˚C from Richter et al (2003, GCA)

  18. Isotope modelling • model assumes 3% relative difference in diffusivity of 6Li v. 7Li; 7Li initially homogeneous; 10x Li concentration gradient. • trough of low 7Li migrates towards the low-Li phase as initial Li-concentration step function is removed due to diffusion

  19. Kinetically-driven fractionation • Diffusion due to gradient between low-Li phenocrysts (Cpx/Ol) and high-Li matrix Li-isotope fractionation. • Developed in lavas with recrystallised matrix v. glassy matrix cooling history 7Li of mantle minerals (and Melt Incs) may not give source information after Beck et al. (2006), GCA

  20. Kinetically-driven fractionation: potential processes • changing D(Li)Min-Min or D(Li)Min-Melt during cooling • dissolution of xenocrysts in melt • infiltration of out of equilibrium melts; magma mixing • fluid exsolution into country rocks adjacent to plutons • homogenisation of Li-growth zoning during prograde metamorphism

  21. Is kinetic fractionation always a problem ? • may depend on the T range and minerals of interest • there are few experimental studies of Li diffusivity for different materials (Ol (?) < Cpx <<Plg ≤ Glass) • can look at empirical studies of natural Li diffusion couples at T> 700 ˚C. • Expect [Li]Crd, Bt>> [Li]Pl, Kfs; [Li]Grt,Opx> [Li]Pl,Kfs

  22. Granulite-facies metased, S. India • highly restitic Li-poor, metapelitic lenses (Crd-Grt-Sil-Hrc-Bt) within relatively Li-rich leucogranite. • Feldspar thermometry in leucogranite: T ~ 800˚C

  23. Granulite-facies metapelite, S. India • Preliminary data indicate Grt (also Crd±Bt) are little or unaffected by Li diffusion on a 100m-mm scale at T = 750-900˚C. Ian Buick, unpublished data

  24. 7Li Mineral-Fluid factors • Limited experimental data for staurolite, Li white mica and Li-clinopyroxene • Different behavior Sta due to tetrahedral Li co-ordn. • Potential use for estimating 7Lifluid liberated during devolatilisation e.g in subduction settings

  25. 7Li in Crd: LP/HT melting • Crd is product of ~3-3.5kbar, ~680-810˚C melting of metapelites at Mt. Stafford, central Oz.

  26. grain-scale ~uniformity in 7Li(Crd) and values are low. • very little change in 7Li(Crd) with inc. grade (T ~100˚C) : most Crd is produced at ~675-700˚C (3.5 kbar) from Bt+Sil dehydration melting in which Bt is rapidly exhausted Ian Buick, unpublished data

  27. Restitic B-rich granulite-facies metapelites have much higher 7Li(Crd) than in B-poor restitic metapelites at the same grade • preservation of distinctly different 7Li(Crd) in different bulk comps at T~800˚C suggests potential for isotopic tracing Ian Buick, unpublished data

  28. 7Li in pegmatites • Core to Rim dec in [Li], [HREE], Y, Ti : magmatic zoning • high 7Li(Grt) consistent with advanced fractionation; • little variation in 7Li(Grt) [Li] gradient doesn’t drive diffusive fractionation in Grt, at least at ~650˚C Ian Buick, unpublished data

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