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

0.2814

0.2817

0.2820

0.2823

0.2826

0.2829

0.2832

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