New Insights into Crustal Evolution: The Role of Hf Isotopes and Detrital Zircons Paul Mueller Darrell Henry Joseph Wooden George Kamenov Louisiana State University Frank Mazdab University 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, 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 176 Lu/ 177 Hf ratios of Consequently Initial  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 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 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 177 Hf/ 176 Hf (regardless of exact values assigned to decay and BSE constants), it is necessary to accurately determine initial 177 Hf/ 176 Hf. Extending This Approach to Earlier Times This requires accurate assessment of all measured values used to calculate initial 177 Hf/ 176 Hf, i.e., Lu/Hf, 177 Hf/ 176 Hf, 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 resolved techniques such as ion probes (U-Pb) and laser ablation (Lu-Hf) BSETransmitted 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

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 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 176 Lu/ 177 Hf Ratio and the Subsequent Age Correction Because A Change in the 176 Lu/ 177 Hf Ratio Will Produce an ~ 3  Unit Change in Initial Hf in 4 Ga, Compared to Typical Measurement Errors of Better than 1  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 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% = Ga Multi-grain Lu-Hf  Hf = +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