HR: 1340h
AN: V23B-1430    [Abstracts]
TI: The relationship between garnet growth and MSWDs in garnet Lu-Hf dating
AU: * Kohn, M J
EM: mattkohn@boisestate.edu
AF: Boise State University, Department of Geosciences 1910 University Dr; MS1535, Boise, ID 83725, United States
AU: Corrie, S L
EM: staceycorrie@mail.boisestate.edu
AF: Boise State University, Department of Geosciences 1910 University Dr; MS1535, Boise, ID 83725, United States
AU: Vervoort, J D
EM: vervoort@wsu.edu
AF: Washington State University, School of Earth and Environmental Sciences, Pullman, WA 99164, United States
AB: The advent of high-precision Lu-Hf analysis by MC-ICP-MS has led to unprecedented age resolution for garnet. Typical analytical uncertainties of ±0.005% in ¹⁷⁶Hf/¹⁷⁷Hf combined with a bulk ¹⁷⁶Lu/¹⁷⁷Hf ratio in metapelitic garnets of ~10 (±0.5% uncertainty) result in theoretical age uncertainties of only ±2 Myr and ±200 kyr for 400 and 30 Ma garnets respectively. Yet some garnet Lu-Hf isochrons have MSWD values higher than desired; traditionally these would be viewed as indicating "bad data" due to such factors as inclusions in the garnet or problems in analytical methods. Although several considerations are important in evaluating Lu-Hf garnet data, here we show that high MSWD's may readily result from typical (c. 10 Myr duration) garnet growth.

We modeled growth of 5-10 wt% garnet, assuming a linear increase in garnet volume with time and simple Rayleigh distillation. Growth durations of 5-15 Myr were based on previous publications and on combined theoretical thermal and thermodynamic models. Garnet-matrix partition coefficients were determined from natural garnet core vs. whole rock compositions from metapelites in Great Smoky Mountains, and were 25-100 for Lu and 0.04-0.2 for Hf.

The models show the following: (1) Garnets exhibit bell-shaped profiles in Lu, but nearly constant Hf contents. (2) Plots of ¹⁷⁶Hf/¹⁷⁷Hf vs. ¹⁷⁶Lu/¹⁷⁷Hf for different garnet splits are concave upward. (3) In metapelites (high Lu/Hf) and amphibolites (low Lu/Hf) modeled MSWD values are commonly >10 and >2 respectively. (4) Higher MSWD's result from better separation of different stages of garnet growth (i.e., the best mineral separation actually results in the worst statistics) and from longer durations of garnet growth. (5) The matrix Lu/Hf ratio evolves because of garnet growth, but the effects on isochrons are minor. (6) The most radiogenic split provides a minimum estimate of the age of garnet nucleation; the least radiogenic split provides a maximum estimate of the termination of garnet growth. Predictions 1-3 are directly corroborated by natural data, supporting the theoretical models and interpretations based on previously collected Lu-Hf data. Prediction 6 implies that high MSWD's are not detrimental, rather the complexities of the Lu-Hf system permit inference of garnet growth rates independently of other isotopic systems, allowing improved interpretation of metamorphic processes.
DE: 1040 Radiogenic isotope geochemistry
DE: 1065 Major and trace element geochemistry
DE: 1115 Radioisotope geochronology
DE: 3610 Geochemical modeling (1009, 8410)
DE: 3660 Metamorphic petrology
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting