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Experimental determination of equilibrium magnesium isotope fractionation between spinel, forsterite, and magnesite from 600 to 800 ºC

Macris, Catherine A. and Young, Edward D. and Manning, Craig E. (2013) Experimental determination of equilibrium magnesium isotope fractionation between spinel, forsterite, and magnesite from 600 to 800 ºC. Geochimica et Cosmochimica Acta, 118 . pp. 18-32. ISSN 0016-7037. https://resolver.caltech.edu/CaltechAUTHORS:20131003-115228027

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Abstract

Magnesium isotopes are potentially powerful tracers for high-temperature geochemical processes if relevant fractionation factors are known. However, experimental data for Mg isotope fractionation are lacking at high temperatures. We performed experiments at 600, 700, and 800 °C and 1 GPa to establish the equilibrium magnesium isotope partitioning between forsterite (Mg_2SiO_4) and magnesite (MgCO_3) and between spinel (MgAl_2O_4) and magnesite, making use of the carbonate as an isotope exchange medium to overcome sluggish diffusion-limited magnesium isotope exchange between spinel and forsterite. Using the three-isotope method, the magnitudes of exchange between forsterite and magnesite, and between spinel and magnesite, were determined at the three temperatures for varying lengths of time, allowing equilibrium isotope partitioning to be established. Results are as follows: Δ^(26)Mg_(Fo–Mgs) = 0.04 ± 0.04‰ at 800 °C, 0.11 ± 0.10‰ at 700 °C, and 0.44 ± 0.10‰ at 600 °C; and Δ^(26)Mg_(Spl–Mgs) = 0.90 ± 0.28‰ at 800 °C, 1.10 ± 0.27‰ at 700 °C, and 1.73 ± 0.38‰ at 600 °C. From these experimentally determined equilibrium fractionation values, we derive the temperature-dependent equilibrium fractionation between spinel and forsterite by difference, yielding Δ^(26)Mg_(Spl–Fo) values of 0.86 ± 0.29‰ at 800 °C, 0.99 ± 0.29‰ at 700 °C, and 1.29 ± 0.39‰ at 600 °C. These data agree well with first-principles estimates of equilibrium magnesium isotope fractionation between spinel and forsterite at high temperatures. The data allow the calculation of an experimentally determined equation for the temperature dependence of ^(26)Mg/^(24)Mg fractionation between spinel and olivine: Δ^(26)Mg_(Spl–Fo)=0.96(±0.21)×10^6/T^2. This first high-T experimental calibration of Mg isotope fractionation of mantle minerals is consistent with expectations based on the crystal chemical environment of Mg in these phases.


Item Type:Article
Related URLs:
URLURL TypeDescription
http://dx.doi.org/10.1016/j.gca.2013.05.008DOIArticle
http://www.sciencedirect.com/science/article/pii/S001670371300272XPublisherArticle
ORCID:
AuthorORCID
Macris, Catherine A.0000-0001-6444-1524
Additional Information:© 2013 Published by Elsevier Ltd. Received 30 January 2013; accepted in revised form 5 May 2013; available online 14 May 2013. We express thanks to Edwin Schauble for helpful discussions on equilibrium stable isotope fractionation theory, useful experimental ideas, and insightful comments on the manuscript. Adam Makhluf and Michael Huh are thanked for their assistance in the laboratory. Constructive comments from Fangzhen Teng, Ralph Dohmen, and an anonymous reviewer are greatly appreciated. We acknowledge support from National Science Foundation Grant EAR0711411 to E.D.Y., C.E.M., and E.S. Associate editor: Stefan Weyer.
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Funding AgencyGrant Number
NSFEAR0711411
Record Number:CaltechAUTHORS:20131003-115228027
Persistent URL:https://resolver.caltech.edu/CaltechAUTHORS:20131003-115228027
Official Citation:Catherine A. Macris, Edward D. Young, Craig E. Manning, Experimental determination of equilibrium magnesium isotope fractionation between spinel, forsterite, and magnesite from 600 to 800°C, Geochimica et Cosmochimica Acta, Volume 118, 1 October 2013, Pages 18-32, ISSN 0016-7037, http://dx.doi.org/10.1016/j.gca.2013.05.008. (http://www.sciencedirect.com/science/article/pii/S001670371300272X)
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:41658
Collection:CaltechAUTHORS
Deposited By: Ruth Sustaita
Deposited On:03 Oct 2013 20:43
Last Modified:09 Mar 2020 13:19

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