TY - JOUR

T1 - Fe-Mg in olivine

T2 - simultaneous experimental determination of the diffusion-driven isotope fractionation along with tracer- and inter-diffusion coefficients

AU - Oeser, Martin

AU - Dohmen, Ralf

AU - Weyer, Stefan

N1 - Publisher Copyright: © 2025 The Author(s). Published by Elsevier Ltd. This is an open access article under the CC BY license. http://creativecommons.org/licenses/by/4.0/

PY - 2026/1/15

Y1 - 2026/1/15

N2 - Olivine is one of the most abundant magmatic minerals and frequently displays chemical and isotopic zoning which has been used to determine timescales of magmatic processes by diffusion modeling. However, the extent of diffusion-driven Fe-Mg isotope fractionation in olivine (characterized by the empirical parameters βFeand βMg), and the parameters that control them, are not yet well constrained. The latter may bear a potential to better determine the boundary conditions of diffusion in order to (1) unravel complex diffusion histories and (2) receive more precise diffusion timescales.In this study, we have performed a series of diffusion experiments at temperatures of 1100–1300 °C and oxygen fugacities between 10-2Pa and 10-7Pa using a25Mg- and57Fe-enriched powder source to produce Fe-Mg chemical and isotopic diffusion profiles in crystallographically oriented San Carlos olivine crystal cuboids. Experimental parameters have been systematically varied in order to investigate the dependence of βFeand βMgon temperature and crystallographic orientation. Chemical and isotopic diffusion profiles were analyzed by electron microprobe and femtosecond-laser ablation-ICP-MS (major and minor element concentrations), and by femtosecond-laser ablation-MC-ICP-MS (Fe-Mg isotopic variations). Additionally, we have applied a multicomponent diffusion model which considers the coupled diffusion of Fe and Mg isotopes in order to simulate the measured chemical and isotopic zoning of our experimental olivine crystals. The selective doping of25Mg and57Fe in combination with the multicomponent diffusion model enabled us to simultaneously determine βFe- and βMg-values as well as the tracer diffusion coefficients of Fe and Mg (D*Fe, D*Mg) and the inter-diffusion coefficient DFe-Mgin olivine.The results of this study show that diffusion anisotropy in olivine has a strong effect on the diffusion-driven Fe-Mg isotope fractionation, i.e. for diffusion parallel to the crystallographic a- or b-axis we determined βFe//a = 0.19 ± 0.07, βFe//b = 0.22 ± 0.06, βMg//a = 0.18 ± 0.06, and βMg//b = 0.17 ± 0.05, while for diffusion //c-axis βFeand βMgare < 0.1 in most cases. We also demonstrate that ignoring the coupled chemical diffusion of the three Mg- and the four Fe isotopes in a diffusion model yields different βFe- and βMg-values that show a clearly disparate dependence on the ratio of the tracer diffusion coefficients (D*Fe/D*Mg). Regarding the diffusivity of Fe and Mg in olivine, at T = 1150 °C we observe a significant change in the anisotropy of D*Fe, D*Mg, and DFe-Mg, i.e. at T ≥ 1150 °C D//c > D//b > D//a, while at T < 1150 °C D//c > D//b = D//a. As this change coincides with relatively high activation energies for D*Fe, D*Mg, and DFe-Mgdeduced from our dataset (i.e. 230–370 kJ/mol) and observed in the study by Tachibana et al.(2013), we suggest that these observations indicate a transition of the diffusion mechanism at temperatures around 1150 °C.

AB - Olivine is one of the most abundant magmatic minerals and frequently displays chemical and isotopic zoning which has been used to determine timescales of magmatic processes by diffusion modeling. However, the extent of diffusion-driven Fe-Mg isotope fractionation in olivine (characterized by the empirical parameters βFeand βMg), and the parameters that control them, are not yet well constrained. The latter may bear a potential to better determine the boundary conditions of diffusion in order to (1) unravel complex diffusion histories and (2) receive more precise diffusion timescales.In this study, we have performed a series of diffusion experiments at temperatures of 1100–1300 °C and oxygen fugacities between 10-2Pa and 10-7Pa using a25Mg- and57Fe-enriched powder source to produce Fe-Mg chemical and isotopic diffusion profiles in crystallographically oriented San Carlos olivine crystal cuboids. Experimental parameters have been systematically varied in order to investigate the dependence of βFeand βMgon temperature and crystallographic orientation. Chemical and isotopic diffusion profiles were analyzed by electron microprobe and femtosecond-laser ablation-ICP-MS (major and minor element concentrations), and by femtosecond-laser ablation-MC-ICP-MS (Fe-Mg isotopic variations). Additionally, we have applied a multicomponent diffusion model which considers the coupled diffusion of Fe and Mg isotopes in order to simulate the measured chemical and isotopic zoning of our experimental olivine crystals. The selective doping of25Mg and57Fe in combination with the multicomponent diffusion model enabled us to simultaneously determine βFe- and βMg-values as well as the tracer diffusion coefficients of Fe and Mg (D*Fe, D*Mg) and the inter-diffusion coefficient DFe-Mgin olivine.The results of this study show that diffusion anisotropy in olivine has a strong effect on the diffusion-driven Fe-Mg isotope fractionation, i.e. for diffusion parallel to the crystallographic a- or b-axis we determined βFe//a = 0.19 ± 0.07, βFe//b = 0.22 ± 0.06, βMg//a = 0.18 ± 0.06, and βMg//b = 0.17 ± 0.05, while for diffusion //c-axis βFeand βMgare < 0.1 in most cases. We also demonstrate that ignoring the coupled chemical diffusion of the three Mg- and the four Fe isotopes in a diffusion model yields different βFe- and βMg-values that show a clearly disparate dependence on the ratio of the tracer diffusion coefficients (D*Fe/D*Mg). Regarding the diffusivity of Fe and Mg in olivine, at T = 1150 °C we observe a significant change in the anisotropy of D*Fe, D*Mg, and DFe-Mg, i.e. at T ≥ 1150 °C D//c > D//b > D//a, while at T < 1150 °C D//c > D//b = D//a. As this change coincides with relatively high activation energies for D*Fe, D*Mg, and DFe-Mgdeduced from our dataset (i.e. 230–370 kJ/mol) and observed in the study by Tachibana et al.(2013), we suggest that these observations indicate a transition of the diffusion mechanism at temperatures around 1150 °C.

KW - Diffusion

KW - Diffusion coefficients

KW - Fe-Mg isotope fractionation

KW - Olivine

UR - http://www.scopus.com/inward/record.url?scp=105020174895&partnerID=8YFLogxK

U2 - 10.1016/j.gca.2025.09.036

DO - 10.1016/j.gca.2025.09.036

M3 - Article

AN - SCOPUS:105020174895

VL - 413

SP - 48

EP - 66

JO - Geochimica et cosmochimica acta

JF - Geochimica et cosmochimica acta

SN - 0016-7037

ER -