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Garnet: Tree Rings of Crustal Processes

Garnet: Tree Rings of Crustal Processes. Ethan F. Baxter Boston University Mineralogical Society of America Distinguished Lecturer 2011-2012. Garnet: Tree Rings of Crustal Processes. Ethan F. Baxter Boston University Jason Harvey (TIMS Lab Manager) Jeremy Inglis (TIMS Lab Manager)

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Garnet: Tree Rings of Crustal Processes

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  1. Garnet: Tree Rings of Crustal Processes Ethan F. Baxter Boston University Mineralogical Society of America Distinguished Lecturer 2011-2012

  2. Garnet: Tree Rings of Crustal Processes Ethan F. Baxter Boston University Jason Harvey (TIMS Lab Manager) Jeremy Inglis (TIMS Lab Manager) Denise Honn (TIMS Lab Manager) Leah Samanta (graduate student) Anthony Pollington (graduate student) Besim Dragovic (graduate student) Nora Sullivan (graduate student) Katie Eccles (graduate student) Penelope Lancaster (undergraduate) Julie Barkman (undergraduate) Michelle Jordan (undergraduate) Claire Ostwald (undergraduate) Emily Peterman UC Santa Barbara Fawna Korhonen Univ. of Maryland Matt Gatewood Univ. of Alabama

  3. When using the content of this presentation, please refer to and cite the following papers (and other references contained therein) • Dragovic, B., Samanta, L.M., Baxter E.F., Selverstone, J. 2012. Using garnet to constrain the duration and rate of water-releasing metamorphic reactions during subduction: An example from Sifnos, Greece. Chemical Geology (in press) doi:10.1016/j.chemgeo.2012.04.016 • Pollington, A.D. and Baxter, E.F. 2011. High precision microsampling and preparation of zoned garnet porphyroblasts for Sm-Nd geochronology. Chemical Geology, 281, p. 270-282. • Pollington, A.D. and Baxter, E.F., 2010. High resolution Sm/Nd garnet geochronology reveals the uneven pace of tectonometamorphic processes. Earth and Planetary Science Letters, 293, p. 63-71 • Harvey J. and Baxter E.F., 2009. An improved method for TIMS high precision neodymium isotope analysis of very small aliquots (1 – 10 ng). Chemical Geology, 258, p. 251-257 • Baxter, E.F., DePaolo, D.J., Ague, J.J., 2002. Prograde Temperature-Time Evolution in the Barrovian Type-Locality Constrained by Precise Sm/Nd Garnet Ages from Glen Clova, Scotland. Journal of the Geological Society, London, 159, p. 71-82.

  4. Garnet Geochronology • Potentially provides longest continuous record of tectonic and metamorphic processes • P-T-t-d • Volatile fluxes, fluid flow • Exhumation, erosion • Crustal transformation • Subduction zone processes • Orogenic processes … if only we could date the growth history from core to rim

  5. Garnet Geochronology • Several isotopic systems have been used to date garnet • Rb-Sr (e.g. Christensen et al. 1989) • Sm-Nd (e.g. Vance & O’Nions 1991) • Lu-Hf (e.g. Duchene et al. 1997)

  6. The Isochron Method • Start with basic “age equation” for decay system of interest: 147Sm 143Nd + α • Form of a line: y=mx+b • Slope of the line is proportional to the AGE (t) • Need (at least) two points to make a line…

  7. The Isochron Method • Garnet has high Sm/Nd ratio • If we sample the rock matrix from which the garnet grew (in Nd isotopic equilibrium), we have two points to make a line: an “isochron” Baxter et al. 2002 J Soc Lond

  8. Sm/Nd Garnet Geochronology • PROS • Generally high 147Sm/144Nd ratios in pure garnet provide opportunity for high precision age… <1Ma • Generally uniform Sm and Ndconc throughout garnet (e.g. Lapen et al. 2003; Kohn 2009) provides good opportunity for core-rim age zonation study • Sm/Nd system is very resistant to open system mobilization or diffusional resetting except at T>700 C (e.g. Tirone et al. 2005; Pollington & Baxter 2011) • Concerns over possible initial garnet-matrix disequilibrium for Sm/Nd are insignificant in most systems (e.g. Thoni 2002; Xiao & Romer 2005; Pollington & Baxter 2011)

  9. Sm/Nd Garnet Geochronology • CONS • Nd concentrations in pure garnet are often very low (<1ppm) making precise 143Nd/144Nd isotopic analysis challenging for such small samples… thus compromising age precision • Contaminating micro-inclusions (e.g. monazite, epidote, other silicates) reduce 147Sm/144Nd if not removed… thus compromising age precision and, in some cases, accuracy Ca map showing “dirty” garnet

  10. Sm/Nd Zoned Garnet Geochronology Three key advances integrated together • Improved TIMS precision for small samples • Improved protocols for cleansing inclusions • Improved microsampling of growth zones

  11. Need for High Precision on Smaller Samples • Sm/Nd geochronology • (zoned garnets) • Earth Evolution • (opx, zircons) • Solar System Evolution • (chondrules, CAIs) • Earth Surface Processes & Provenance • (dust, aerosol, natural waters and their precipitates) Low Nd concentrations &/or Limited sample size

  12. High Precision Nd Isotope Analysis • Recent technological advances yield 10ppm (2σ RSD) precision [Triton TIMS specification) • Some studies demonstrate precision approaching 2ppm (e.g. Boyet & Carlson 2005; Caro et al. 2006) • But this requires 100s of nanograms of Nd to be run as the metal

  13. NdO+ Analysis Yields Improved Ionization • Oxygen Bleed: • DePaolo & Wasserburg (1976); • Lugmair et al. (1976), • Sharma et al. 1995 (50-130ppm 2σ RSE, 3-11ng) • Silica gel: • Thirlwall 1991 (14ppm 2σ RSD, 30ng) • Amelin 2004 (21ppm 2σ RSD, 10-15ng) • “TaCl emitter”: • Griselin et al. 2001 (25-40ppm 2σ RSD, 1-5ng)

  14. Ta2O5 Phosphoric Acid Slurry(Harvey & Baxter, Chem. Geol. 2009) • Add Tantalum-oxide powder (~50mg) in a slurry with 5% phosphoric acid (~3mL) • Single Re filaments (Re Alloys commercial & H-Cross zone refined)

  15. Internal Precision on 4ng Nd Loads 10 ppm average Since 2008

  16. 10ppm RSD (2σ) In-Barrel External Precision on 4ng Nd Loads .5121302 ± .0000049 2σ n=20 Pollington & Baxter 2010 EPSL

  17. Column Chemistry • 3 columns needed (for garnet) • Fe “clean-up” column • TRU-spec • MLA column (needed for clean Pr separation) • 3 column blank after distillation of MLA: • 5-10pg

  18. <±1Ma precision for garnet with 147Sm/144Nd>1.0 …possible with 4ng Nd

  19. Age Precision vs. Age (for garnet with 147Sm/144Nd=4.0 and yielding 10ppm on 143Nd/144Nd) <±1Ma precision possible for garnet of any age 0.1% 147Sm/144Nd analytical precision 0.01% 147Sm/144Nd analytical precision Baxter & Inglis 2010 AGU Fall Meeting

  20. 2. Improved Protocols for Cleansing of Inclusions • Contamination from inclusions can lead to poor precision and inaccuracy in ages X 428 ± 46 Ma Pollington & Baxter 2011 Baxter et al. 2002

  21. 2. Improved Protocols for Cleansing of Inclusions • Many workers have developed and employed partial dissolution acid cleansing …for example: Zhou & Hensen 1995 DeWolf et al. 1996 Amato et al. 1999 Scherer et al. 2000 Baxter et al. 2002 Thöni 2002 Anczkiewicz et al. 2003 Pollington & Baxter 2010, 2011 • EVERY GARNET IS DIFFERENT • Preliminary experimentation is always needed to find best recipe. • Acids used • Temperatures • Durations • Grain size • Freq. of Ultrasonicating • Crush & seive 100-200 mesh fraction†* • HF acid bath, 30-120 minutes* • Perchloric acid bath, ~12 hours • Nitric acid bath, 1-3 hrs • † Fine powders don’t clean up well! • * Significant sample loss is possible!

  22. 2. Improved Protocols for Cleansing of Inclusions • How do you know your garnet is clean? • Clean garnet has low Nd concentration (<1ppm)

  23. 2. Improved Protocols for Cleansing of Inclusions • How do you know your garnet is clean? • Clean garnet has low Nd concentration (<1ppm)

  24. 2. Improved Protocols for Cleansing of Inclusions • How do you know your garnet is clean? • Clean garnet has low Nd concentration (<1ppm) • Clean garnet has high 147Sm/144Nd (>1.0)

  25. 2. Improved Protocols for Cleansing of Inclusions • How do you know your garnet is clean? • Clean garnet has low Nd concentration (<1ppm) • Clean garnet has high 147Sm/144Nd (>1.0) • Carry out multiple tests to find cleansing recipe that produces highest 147Sm/144Nd and lowest Nd concentration (and consistent ages) • Laser analysis of “clean” spot on garnet can help confirm actual garnet Sm/Nd …(but not 143Nd/144Nd isotope ratio!)

  26. It Works! Example – Sifnos, Greece(Dragovic et al. 2012 Chem Geol) Cleansed garnets in red Matrix in blue Multi-point garnet-matrix isochron (MSWD=0.78) dirty garnet!

  27. Real Published Data Lancaster et al. 2008 JMG Peterman et al. 2009 Eur J Min Pollington & Baxter 2010 EPSL Korhonen et al. 2012 JMG Dragovic et al. 2012 Chem Geol Note: many samples yielded <4ng Nd

  28. 3. Improved microsampling of growth zones(Pollington & Baxter 2011, Chem Geol) • Chemically contoured microdrilling • Make Mn map of large garnet • Use microdrill to cut trenches following contours • Throw out powder from trench • Collect solid annulus left between trenches for geochronology

  29. Example: Tauern Window, Austria Anthony Pollington

  30. Garnet Microsampling Pollington & Baxter EPSL 2010

  31. Core to rim age progression 8 million year growth history Growth pulses resolved Pollington & Baxter EPSL 2010

  32. How Much Garnet Do you Need? • To get 4 ng of Nd needed for optimal 10-20ppm isotopic analysis… • Best case: 1ppm Nd and 30% loss • 6mg raw garnet needed • Worse case: <0.1ppm Nd and >70% loss • >130mg raw garnet needed Garnet mass (mg) X Garnet Nd conc. (ppm) - loss during prep. = mass of Nd (ng) Pollington & Baxter 2011 Chem Geol

  33. 6 microdrilled zones from 1cm garnet (with 0.4ppm Nd and 50% loss during preparation) 500-800 micron drill bit width 2mm Each annulus (white) contains ~20mg of raw garnet Pollington & Baxter 2011 Chem Geol

  34. Conclusions • Partial dissolution successfully removes inclusions, revealing garnet 147Sm/144Nd > 1.0; Nd (ppm) < 1 • Garnet-matrix age precision of <1 Myrs on 10’s of milligrams of garnet of any age is possible • Chemically contoured microdrilling of large (>5mm diameter) garnet porphyroblasts permits age zoning analysis • Detailed methods in Pollington & Baxter (2011, Chemical Geology)… or come visit the lab www.bu.edu/TIMS

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