2014 GSA Annual Meeting in Vancouver, British Columbia (19–22 October 2014)

Paper No. 118-8
Presentation Time: 10:45 AM

ZN ISOTOPE FRACTIONATION IN THE FERTILE MANTLE


DOUCET, Luc S., Laboratoire G-Time, Département des Sciences de la Terre et de l'Environnement, Université Libre de Bruxelles, CP 160/02, Av. FD Roosevelt 50, Brussels, 1050, Belgium, MATTIELLI, Nadine, Laboratoire G-Time, Département des Sciences de la Terre et de l'Environnement, Université Libre de Bruxelles, CP 160/02, Avenue F.D. Roosevelt 50, Brussels, 1050, Belgium and IONOV, Dmitri A., Geoscience Montpellier, Université Montpellier 2 & UMR-CNRS 5243, Montpellier, 34095, France

We present Zn isotope data on fresh garnet and spinel peridotite xenoliths from Vitim and Tariat (central Asia) to explore potential heterogeneities of fertile mantle, which is currently assumed to be unaffected by Zn isotope fractionation, i.e. homogeneous, as inferred from studies of MORB, OIB and serpentinized orogenic peridotites [1].

The 13 peridotites in this study are un-serpentinized (LOI = 0.66% to 0.36%) and have modal, major and trace element compositions close to the primitive mantle (av. ~55% olivine, ~23% orthopyroxene, ~15% clinopyroxene, ~12% garnet, ~3% spinel; Mg#WR ~0.89, REEN ~1) [2-4]. They experienced no or little partial melting or metasomatism and are considered as some of the most pristine fertile peridotites available.

The Zn isotope data are expressed as δ66Zn and were obtained by high precision measurements on a Nu Plasma MC-ICPMS. The δ66Zn values range from +0.29±0.09‰ (2SD) to +0.58±0.09‰ (av. 0.38‰) for the spinel peridotites, and from +0.37±0.03‰ to +0.71±0.01‰ (av. 0.50‰) for the garnet peridotites.

These new data overlap the narrow Zn isotope variation range of terrestrial basalts (δ66Zn = +0.2 to +0.4‰) [5, 6], but the fertile mantle tends to be heavier than the basalts, with garnet peridotites commonly heavier than the spinel peridotites. The δ66Zn values show robust negative correlations with Mg#WR (R2=0.8) and modal orthopyroxene (R2=0.5) and poor correlations with modal olivine (R2=0.4). δ66Zn data do not correlate with the Zn or other trace element concentrations.

Our data show that (1) the fertile mantle has more heterogeneous Zn isotope compositions than presumed, (2) the inter-mineral fractionation appears to be a key factor controlling Zn isotopes in the mantle, and (3) the observed δ66Zn co-variation trends suggest a mantle process that significantly fractionates Zn isotopes (Δ66Zn~0.4‰), but is not detected by traditional geochemical proxies.

This study shows that Zn isotopes may be used as tracers for mantle processes, but further work on residual and metasomatized peridotites as well as on mineral separates is needed.

[1] Othman et al (2006) GCA, 70; [2] Ionov et al (2005) CG 217; [3] Ionov (2007) CMP 154; [4] Ionov and Hofmann (2007) EPSL 261; [5] Herzog et al (2009) GCA 73; [6] Pons et al (2013) Geobio 11.

Session No. 118

T117. Magmas and Their Sources: A Tribute to the Distinguished Career of Fred Frey (Posters)
Monday, 20 October 2014: 9:00 AM-6:30 PM
Exhibition Hall C (Vancouver Convention Centre-West)
Geological Society of America Abstracts with Programs. Vol. 46, No. 6, p.299
© Copyright 2014 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.
Back to: T117. Magmas and Their Sources: A Tribute to the Distinguished Career of Fred Frey (Posters)
<< Previous Abstract | Next Abstract >>