Garnet: Tree Rings of Crustal ProcessesEthan F. Baxter Boston University

Mineralogical Society of AmericaDistinguished Lecturer 2011-2012

Garnet: Tree Rings of Crustal ProcessesEthan F. Baxter Boston UniversityJason 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 BarbaraFawna Korhonen Univ. of MarylandMatt Gatewood Univ. of Alabama

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.

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

• 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)

Garnet Geochronology

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…

init

t

Nd

Nd

Nd

Sme

Nd

Nd

144

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144

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1

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

• PROS– Generally high 147Sm/144Nd ratios in pure garnet

provide opportunity for high precision age… <1Ma– Generally uniform Sm and Nd conc 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)

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

Sm/Nd Garnet Geochronology

100 microns

Ca map showing “dirty” garnet

Three key advances integrated together1.Improved TIMS precision for small samples2.Improved protocols for cleansing inclusions3.Improved microsampling of growth zones

Sm/Nd Zoned Garnet Geochronology

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

Mn FormulaUnit Contours

Chemically contouredMicrodrill trough

1 mm

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

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)

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)

0

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6 8 10 12 14 16 18 20 22

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143/144Nd 2sigma RSE

Internal Precision on 4ng Nd Loads

10 ppm averageSince 2008

10ppm RSD (2σ) In-Barrel External Precision on 4ng Nd Loads

.5121302 ± .0000049 2σn=20

Pollington & Baxter 2010 EPSL

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

<±1Ma precisionfor garnet with147Sm/144Nd>1.0…possible with 4ng Nd

Age Precision vs. Age (for garnet with 147Sm/144Nd=4.0 and yielding 10ppm on 143Nd/144Nd)

0.1% 147 Sm/1

44 Nd analytical precision

0.01% 147Sm/144Nd analytical precision

<±1Ma precisionpossible for garnetof any age

Baxter & Inglis 2010 AGU Fall Meeting

• Contamination from inclusions can lead to poor precision and inaccuracy in ages

2. Improved Protocols for Cleansing of Inclusions

Pollington & Baxter 2011 Baxter et al. 200242

8 ±

46 M

a

• Many workers have developed and employed partial dissolution acid cleansing …for example:Zhou & Hensen 1995DeWolf et al. 1996Amato et al. 1999Scherer et al. 2000Baxter et al. 2002Thöni 2002Anczkiewicz et al. 2003Pollington & Baxter 2010, 2011

2. Improved Protocols for Cleansing of Inclusions

1. Crush & seive 100-200 mesh fraction†*

2. HF acid bath, 30-120 minutes*

3. Perchloric acid bath, ~12 hours

4. Nitric acid bath, 1-3 hrs† Fine powders don’t clean up well!* Significant sample loss is possible!

EVERY GARNET IS DIFFERENT

Preliminary experimentation is always needed to find best

recipe.

1.Acids used2.Temperatures

3.Durations4.Grain size

5.Freq. of Ultrasonicating

• How do you know your garnet is clean?1. Clean garnet has low Nd concentration (<1ppm)

2. Improved Protocols for Cleansing of Inclusions

• How do you know your garnet is clean?1. Clean garnet has low Nd concentration (<1ppm)

2. Improved Protocols for Cleansing of Inclusions

0

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0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1

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Nd (ppm) in "cleansed" garnet

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

2. Improved Protocols for Cleansing of Inclusions

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147/Sm/144Nd of "cleansed" garnet

• How do you know your garnet is clean?1. Clean garnet has low Nd concentration (<1ppm)2. Clean garnet has high 147Sm/144Nd (>1.0)3. Carry out multiple tests to find cleansing recipe

that produces highest 147Sm/144Nd and lowest Nd concentration (and consistent ages)

4. Laser analysis of “clean” spot on garnet can help confirm actual garnet Sm/Nd

…(but not 143Nd/144Nd isotope ratio!)

2. Improved Protocols for Cleansing of Inclusions

It Works! Example – Sifnos, Greece

(Dragovic et al. 2012 Chem Geol)

dirtygarnet!

Cleansed garnets in redMatrix in blueMulti-point garnet-matrix isochron (MSWD=0.78)

Real Published Data

Lancaster et al.2008 JMG

Peterman et al.2009 Eur J Min

Pollington & Baxter 2010EPSL

Korhonen et al.2012 JMG

Dragovic et al.2012 Chem Geol

Note: many samples yielded <4ng Nd

• 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

3. Improved microsampling of growth zones(Pollington & Baxter 2011, Chem Geol)

Example: Tauern Window, Austria

AnthonyPollington

Garnet Microsampling

Pollington & Baxter EPSL 2010

Pollington & Baxter EPSL 2010

Core to rim ageprogression

8 million year growth history

Growth pulsesresolved

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

6 microdrilled zones from 1cm garnet (with 0.4ppm Nd and 50% loss during preparation)

500-800 microndrill bit width

Each annulus (white) contains ~20mg of raw garnet

2mm

Pollington & Baxter 2011 Chem Geol

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