New Insights into Crustal Evolution:The Role of Hf Isotopes and Detrital Zircons
Paul Mueller Darrell Henry Joseph WoodenGeorge Kamenov Louisiana State University Frank MazdabUniversity of Florida 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, bysolution, 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
ConsequentlyInitial Hf can be determined more confidently
than common whole-rock parameters such as Nd or Sr
Relation to Plate Tectonics
Q: What can detrital zircon systematics tell us aboutplate tectonics?
A: A single grain captures the initial Hf isotopic compositionof the rock in which it formed, so, barring subsequent exchange,it can tell us something about reservoirs, such as existence ofcrust 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
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0.2835
0 200 400 600 800 1000 1200 1400 1600 1800 2000
Age (Ma)
176Hf/177Hf (Lu corrected)
DM_T
Dupont
Ordovician
176Lu/177Hf=0.007
176Lu/177Hf=0.018
176Lu/177Hf=0.007 model
176Lu/177Hf=0.018 model
Granite-Rhyolite
176/177=0.018 Gran/Rhyolite
LA-ICPMS U-Pb and Hf in zircons from Florida heavy mineral mine
G-R
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.
Extending This Approach to Earlier Times
This requires accurate assessment of all measured values used to calculate initial 177Hf/176Hf, i.e., Lu/Hf, 177Hf/176Hf, and age.
Time (Ga)
Hf
BSE
DM
~Lower Crust
Depending on Model,Expected Range of Hf at 4.0 Ga = 4-6
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.
Best opportunity is to sequentially apply spatially resolvedtechniques such as ion probes (U-Pb) and laser ablation (Lu-Hf)
BSE Transmitted Light
Assessing Integrity of Measured Values
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
Maximum Confidence Associated With
FFC
0.01
0.1
1
10
100
1000
10000
56 58 60 62 64 66 68 70 72
REE
Chondritic
Series12
Series11
Series10
Series9
Series8
Series7
Series6
Series5
Series4
Series3
Series2
Series1
Consistency of Trace Element Patterns Important for Populations
Pattern Suggests All Zircons Formed in Equilibrium with Plagioclase
Eu
Ce
Y vs. Lu/Hf for Standard Zircon FC-1
0.000
0.004
0.008
0.012
0.016
0.020
0 1000 2000 3000
Y ppm
Lu/Hf wt
From Forest City Site, Duluth Gabbro
35 Analyses of 27 Grains Using the SHRIMP RG
Significance of Link Between Y and Lu/Hf
The Most Challenging Link in Correcting Observed Hf to Initial Hf is an Accurate Determination of the
176Lu/177Hf Ratio and the Subsequent Age CorrectionBecause
A 0.001 Change in the 176Lu/177Hf Ratio Will Produce an~ 3 Unit Change in Initial Hf in 4 Ga, Compared to
Typical Measurement Errors of Better than 1 by LASo
Measuring Y Content (by EMP) Is a Particularly Useful Pre-ablation Step Because of the Strong Positive Correlation
Between Y and Lu/Hf
QuickTime™ and aTIFF (LZW) decompressor
are needed to see this picture.
Trace Elements As Guides
Transect
Y & Ce Along Cross-section of Complex Late Archean Igneous Zircon (Metamict Zones in Gray)
Application: Northern Wyoming Province
BBMZ
MMP
Detrital Zircon Ages Northern Wyoming Province
Eastern Beartooth Mountains~20% = 3.4 - 4.0 Ga
Multi-grain Lu-HfHf = +3 to -3
Conclusion
Accurate Initial Hf Isotopic CompositionsCan 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