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
Jason Harvey
Jeremy Inglis
Denise Honn
Leah Samanta
Anthony Pollington
Besim Dragovic
Nora Sullivan
Katie Eccles
Penelope Lancaster
Julie Barkman
Michelle Jordan
Claire Ostwald
Emily Peterman
Fawna Korhonen
Matt Gatewood
Boston University
(TIMS Lab Manager)
(TIMS Lab Manager)
(TIMS Lab Manager)
(graduate student)
(graduate student)
(graduate student)
(graduate student)
(graduate student)
(undergraduate)
(undergraduate)
(undergraduate)
(undergraduate)
UC Santa Barbara
Univ. of Maryland
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
Garnet Geochronology
• Several isotopic systems have been used to
date garnet
– Rb-Sr
– Sm-Nd
– Lu-Hf
(e.g. Christensen et al. 1989)
(e.g. Vance & O’Nions 1991)
(e.g. Duchene et al. 1997)
The Isochron Method
• Start with basic “age equation” for decay
143Nd + α
system of interest: 147Sm
143
144
Nd
Nd
 e

 1   144
 
Nd 
147
 t
Sm
Nd 

144

Nd  init
143
• 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…
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
Sm/Nd Garnet Geochronology
• 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
10 0 m icr o n s
Ca map showing “dirty” garnet
Sm/Nd Zoned Garnet
Geochronology
Three key advances integrated together
1. Improved TIMS precision for small samples
2. Improved protocols for cleansing inclusions
3. Improved microsampling of growth zones
Need for
High Precision on Smaller Samples
• Sm/Nd geochronology
– (zoned garnets)
• Earth Evolution
Chemically contoured
Microdrill trough
Low Nd concentrations
&/or
Limited sample size
– (opx, zircons)
• Solar System Evolution
– (chondrules, CAIs)
• Earth Surface Processes & Provenance
1 mm
Mn Formula
– (dust, aerosol, natural
waters and their
Unit Contours
precipitates)
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 & HCross zone refined)
Internal Precision on 4ng Nd Loads
60
10 ppm average
Since 2008
50
Count
40
30
20
10
0
6
8
10
12
14
16
18
20
1 4 3 /1 4 4 N d 2 sig m a R S E
22
10ppm RSD (2σ) In-Barrel External
Precision on 4ng Nd Loads
.5121302 ± .0000049 2σ
n=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 precision
for garnet with
147Sm/144Nd>1.0
…possible with 4ng Nd
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
Baxter & Inglis 2010 AGU Fall Meeting
2. Improved Protocols for Cleansing of Inclusions
• Contamination from inclusions can lead to
poor precision and inaccuracy in ages
Pollington & Baxter 2011
Baxter et al. 2002
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
1. Crush
& seive 100-200
DIFFERENT
mesh fraction†*
experimentation
2.Preliminary
HF acid bath,
30-120 is
always needed to find best
minutes*
recipe.
3. Perchloric acid bath,
1. Acids used
~12 hours
2. Temperatures
4. Nitric
acid bath, 1-3 hrs
3. Durations
† Fine
powders
don’t clean
4. Grain
size up well!
* Significant
loss is possible!
5. Freq.sample
of Ultrasonicating
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
• How do you know your garnet is clean?
1. Clean garnet has low Nd concentration (<1ppm)
25
C ount
20
15
10
5
0
0 .0 0 .1 0 .2 0 .3 0 .4 0 .5 0 .6 0 .7 0 .8 0 .9 1 .0 1 .1
N d (p p m ) in "c le a n s e d " g a rn e t
2. Improved Protocols for Cleansing of Inclusions
• 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)
20
C ount
15
10
5
0
0
1
2
3
4
5
6
7
8
1 4 7 /S m /1 4 4 N d o f "c le a n s e d " g a rn e t
9
10
2. Improved Protocols for Cleansing of Inclusions
• 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!)
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!
Real Published Data
Lancaster et al.
2008 JMG
Peterman et al.
2009 Eur J Min
Pollington
& Baxter 2010
EPSL
Note: many samples yielded <4ng Nd
Korhonen et al.
2012 JMG
Dragovic et al.
2012 Chem Geol
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
Example: Tauern Window, Austria
Anthony
Pollington
Garnet Microsampling
Pollington & Baxter EPSL 2010
Core to rim age
progression
8 million year
growth history
Growth pulses
resolved
Pollington & Baxter EPSL 2010
How Much Garnet Do you Need?
• To get 4 ng of Nd needed for optimal 1020ppm isotopic analysis…
Garnet mass (mg) X Garnet Nd conc. (ppm) - loss during prep. = mass of Nd (ng)
• Best case: 1ppm Nd and 30% loss
– 6mg raw garnet needed
• Worse case: <0.1ppm Nd and >70% loss
– >130mg raw garnet needed
Pollington & Baxter 2011 Chem Geol
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
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