T65 Frontiers in Mineralogy, Petrology, and Geochronology: A Session in Honor of Dana Medalist Jörg Hermann

Sunday, 4 November 2018
1:30 PM - 5:30 PM
Sagamore Ballroom 6

COMPARISON OF SM-ND AND LU-HF GARNET AGES I: GRANULITE METAMORPHISM, FIORDLAND NEW ZEALAND

Paper number: T65-11
Program: Technical Sessions
Time: 4:05 PM – 4:20 PM

Authors

Abstract

Garnet Sm-Nd & Lu-Hf ages constrain the timing of garnet growth; however, the 2 systems seldom yield the same results. In low T rocks, age discrepancies may result from zoning of parent atoms in garnet and age bias toward initial growth. In high T rocks, age discrepancies are generally attributed to diffusion which resets isotopic systems. The effects of diffusion will depend on T, diffusion constants, cooling rate, grain size/diffusion length scales, and relative diffusion of parent and daughter. The Western Fiordland Orthogneiss is ideal for evaluating these processes because it contains a wide range of garnet sizes and experienced a relatively simple thermal history which peaked at >900°C & 14 kbar.

Samples with garnet ranging from 1 to 0.1 cm diameter were selected to compare ages for ca. 1 cm grains using both systems and to compare Sm-Nd ages for 1.0 to 0.1 cm grains. All garnet grains display weak major element zoning with broad unzoned cores and <1 mm zoned rims. In ca. 1 cm garnet grains, zoning includes high-magnitude, sharp, oscillations in Lu and Hf. Sm is weakly zoned with broad low-amplitude bell-shaped profiles in Sm. We conclude that Lu, Hf, and likely Sm did not diffuse significantly in large garnet. In the 1 to 5 mm garnet, trace element data are not available, but major element data are compatible with diffusional re-equilibration.

Lu-Hf ages for ca. 1 cm garnets in 2 samples (garnet only isochrons) are 114.8±3.5 (N=7; MSWD=0.2) & 116.8±1.8 Ma (N=7; MSWD=1.3) and Sm-Nd ages from the same samples are 113.1±2.3 (N=10; MSWD=2.1) & 114.9±3.5 Ma (N=8; MSWD=2.1). Small garnets in 3 samples have Sm-Nd ages of 103.4±2.3 (1.25 mm; N=5; MSWD=2.1), 108.1±1.8 (2.5 mm; N=5; MSWD=1.0), and 104.4±2.0 Ma (1.5 mm; N=5; MSWD=1.5). Estimated diffusion coefficients of Lu, Hf, Sm, and Nd in garnet have significant ranges and partitioning with adjacent minerals is likely important. However, closure T for the ca. 1 cm garnet grains are >>1,100°C for Lu-Hf and 860-1060°C for Sm-Nd. The Sm-Nd garnet ages decline with grain size compatible with diffusion-controlled ages during cooling from peak conditions. Using the Carlson (2012) Sm and Nd diffusion data, closure T are 865-1065°C with cooling at -15 C/m.y. We conclude that Sm-Nd data for 1.0-0.1 cm garnet grains track the timing of metamorphism from the peak of granulite T to ca. 850°C.

Geological Society of America Abstracts with Programs. Vol. 50, No. 6

doi: 10.1130/abs/2018AM-318884

© Copyright 2018 The Geological Society of America (GSA), all rights reserved. Permission is hereby granted to the author(s) of this abstract to reproduce and distribute it freely, for noncommercial purposes. Permission is hereby granted to any individual scientist to download a single copy of this electronic file and reproduce up to 20 paper copies for noncommercial purposes advancing science and education, including classroom use, providing all reproductions include the complete content shown here, including the author information. All other forms of reproduction and/or transmittal are prohibited without written permission from GSA Copyright Permissions.