Transcript Hf-isotopes and crustal growth - The University of Texas at Dallas

New Insights into Crustal Evolution:
The Role of Hf Isotopes and Detrital Zircons
Paul Mueller
George Kamenov
University of Florida
Darrell Henry
Louisiana State University
Joseph Wooden
Frank Mazdab
U. S. G. S.
Hellroaring Plateau, Beartooth Mountains, Montana
Detrital Zircons, Hf, and Crustal Growth
Attributes
Representatives of periods when/where rock record not available
Hf and U-Pb age measured on same grain, or parts of grains, by
solution, ablation(LA-ICP-MS), and/or ion probe
Most robust record of initial Hf because zircons have low Lu and
high Hf (~1%), yielding typical 176Lu/177Hf ratios of 0.001
Consequently
Initial eHf can be determined more confidently
than common whole-rock parameters such as eNd or eSr
Relation to Plate Tectonics
Q: What can detrital zircon systematics tell us about
plate tectonics?
A: A single grain captures the initial Hf isotopic composition
of the rock in which it formed, so, barring subsequent exchange,
it can tell us something about reservoirs, such as existence of
crust and depleted mantle.
A: More significantly, populations of grains can tell us about
orogenic processes, some of which we would associate with plate
tectonics. Unfortunately, this becomes progressively more
difficult as we examine older grains, i.e. when we need it most.
Crustal Growth: Age vs. Initial Ratio
Magmatic and tectonic accretion yield distinct patterns
0.2835
DM_T
Dupont
Ordovician
0.2832
176Lu/177Hf=0.007
176Hf/177Hf (Lu corrected)
176Lu/177Hf=0.018
176Lu/177Hf=0.007 model
0.2829
176Lu/177Hf=0.018 model
Granite-Rhyolite
176/177=0.018 Gran/Rhyolite
0.2826
0.2823
0.2820
G-R
0.2817
0.2814
0
200
400
600
800 1000 1200 1400 1600 1800 2000
Age (Ma)
LA-ICPMS U-Pb and Hf in zircons from Florida heavy mineral mine
Extending This Approach to Earlier Times
1. Because Lu/Hf fractionation in Early Archean and
Hadean time is expected to produce limited variations
in 177Hf/176Hf (regardless of exact values assigned to
decay and BSE constants), it is necessary to accurately
determine initial177Hf/176Hf.
DM
BSE
Depending on Model,
Expected Range of eHf
at 4.0 Ga = 4-6
eHf
~Lower Crust
Time (Ga)
This requires accurate assessment of all measured values used
to calculate initial 177Hf/176Hf, i.e., Lu/Hf, 177Hf/176Hf, and age.
Extending This Approach to Earlier Times
2. Assure that Hf, U, and Pb isotopes are measured on a
homogeneous volume of material. Particularly critical
for older zircons that commonly have distinct volumes
with distinct ages; good imaging necessary.
BSE
Transmitted Light
Best opportunity is to sequentially apply spatially resolved
techniques such as ion probes (U-Pb) and laser ablation (Lu-Hf)
Assessing Integrity of Measured Values
Maximum Confidence Associated With
Concordance of U-Pb ages
Low common Pb content
“Igneous” Th/U (e.g., > 0.1)
Low contents of alteration-related trace elements (e.g., F, Fe, Al)
Consistency of measured Lu-Hf system isotopes during ablation
Low O-isotope ratios
Consistent trace element patterns
Low Lu/Hf corroborated by SIMS
Consistency of Trace Element Patterns
Important for Populations
FFC
10000
1000
Ce
Chondritic
100
Series12
Series11
Series10
Series9
Series8
Series7
Series6
Series5
Series4
Series3
Series2
Series1
10
1
Eu
0.1
0.01
56
58
60
62
64
66
68
70
72
REE
Pattern Suggests All Zircons Formed in Equilibrium with Plagioclase
Y vs. Lu/Hf for Standard Zircon FC-1
From Forest City Site, Duluth Gabbro
0.020
0.016
Lu/Hf wt
0.012
0.008
0.004
0.000
0
1000
2000
3000
Y ppm
35 Analyses of 27 Grains Using the SHRIMP RG
Significance of Link Between Y and Lu/Hf
The Most Challenging Link in Correcting Observed eHf
to Initial eHf is an Accurate Determination of the
176Lu/177Hf Ratio and the Subsequent Age Correction
Because
A 0.001 Change in the 176Lu/177Hf Ratio Will Produce an
~ 3 e Unit Change in Initial Hf in 4 Ga, Compared to
Typical Measurement Errors of Better than 1 e by LA
So
Measuring Y Content (by EMP) Is a Particularly Useful
Pre-ablation Step Because of the Strong Positive Correlation
Between Y and Lu/Hf
Trace Elements As Guides
Y & Ce Along Cross-section of Complex Late
Archean Igneous Zircon (Metamict Zones in Gray)
Transect
Application: Northern Wyoming Province
MMP
BBMZ
Detrital Zircon Ages
Northern Wyoming Province
Eastern Beartooth Mountains
~20% = 3.4 - 4.0 Ga
Multi-grain Lu-Hf
eHf = +3 to -3
Conclusion
Accurate Initial Hf Isotopic Compositions
Can Be Obtained From Detrital and Other Zircons,
However
Care Must be Taken to Assure
That All Measured Parameters Are Co-genetic
(easier said than done!)
Plane Light
BSE
X-Nicols