TY - JOUR

T1 - Coupled lithium- and iron isotope fractionation during magmatic differentiation

AU - Weyer, S.

AU - Seitz, H. M.

PY - 2012/2/10

Y1 - 2012/2/10

N2 - In this study, we investigated Fe and Li isotope fractionation between mineral separates of olivine pheno- and xenocrysts (including one clinopyroxyene phenocryst) and their basaltic hosts. Samples were collected from the Canary Islands (Teneriffa, La Palma) and some German volcanic regions (Vogelsberg, Westerwald and Hegau). All investigated bulk samples fall in a tight range of Li and Fe isotope compositions (δ 56Fe wr=0.06-0.17‰ and δ 7Li ma=2.5-5.2‰, assuming δ 7Li of the olivine-free matrix is virtually identical to that of the bulk sample for mass balance reasons). In contrast, olivine phenocrysts display highly variable, but generally light Fe and mostly light Li isotope compositions compared to their respective olivine-free basaltic matrix, which was considered to represent the melt (with δ 56Fe ol=-0.24 to 0.14‰ and δ 7Li ol=-10.5 to +6.5‰, respectively). Single olivine crystals from one sample display even a larger range of δ 56Fe ol between -0.7 and +0.1‰. One single clinopyroxene phenocryst displays the lightest Li isotope composition (δ 7Li cpx=-17.7‰), but no Fe isotope fractionation relative to melt. The olivine phenocrysts show variable Mg# and Ni (correlated in most cases) that range between 0.89 and 0.74 and between 300 and 3000μg/g, respectively. These olivines likely grew by fractional crystallization in an evolving magma. One sample from the Vogelsberg volcano contained olivine xenocrysts (Mg#>0.89 and Ni>3000μg/g), in addition to olivine phenocrysts. This sample displays the highest Li- and the second highest Fe-isotope fractionation between olivine and melt (δ 7Li ol-melt=-13; δ 56Fe ol-melt=-0.29).Our data, i.e. the variable olivine- at constant whole rock and matrix isotope compositions, strongly indicate disequilibrium, i.e. kinetic Fe and Li isotope fractionation between olivine and melt (for Li also between cpx and melt) during fractional crystallization. δ 7Li ol-melt is correlated with the Li partitioning between olivine and melt (i.e. with Li ol/Li melt), indicating Li isotope fractionation due to preferential (faster) diffusion of 6Li into olivine during fractional crystallization. Olivine with low δ 7Li ol-melt, also have low δ 56Fe ol-melt, indicating that Fe isotope fractionation is also driven by diffusion of isotopically light Fe into olivine, potentially, as Fe-Mg inter-diffusion. The lowest δ 56Fe ol-melt (-0.40) was observed in a sample from Westerwald (Germany) with abundant magnetite, indicating relatively oxidizing conditions during magma differentiation. This may have enhanced equilibrium Fe isotope fractionation between olivine and melt or fine dispersed magnetite in the basalt matrix may have shifted its Fe isotope composition towards higher δ 56Fe. The decoupling of Li- and Fe isotope fractionation in cpx is likely due to faster diffusion of Li relative to Fe in cpx, implying that the large investigated cpx phenocryst resided in the magma for only a short period of time which was sufficient for Li- but not for Fe diffusion. The absence of any equilibrium Fe isotope fractionation between the investigated cpx phenocryst and its basaltic host may be related to the similar Fe 3+/Fe 2+ of cpx and melt. In contrast to cpx, the generally light Fe isotope composition of all investigated olivine separates implies the existence of equilibrium- (in addition to diffusion-driven) isotope fractionation between olivine and melt, on the order of 0.1‰.

AB - In this study, we investigated Fe and Li isotope fractionation between mineral separates of olivine pheno- and xenocrysts (including one clinopyroxyene phenocryst) and their basaltic hosts. Samples were collected from the Canary Islands (Teneriffa, La Palma) and some German volcanic regions (Vogelsberg, Westerwald and Hegau). All investigated bulk samples fall in a tight range of Li and Fe isotope compositions (δ 56Fe wr=0.06-0.17‰ and δ 7Li ma=2.5-5.2‰, assuming δ 7Li of the olivine-free matrix is virtually identical to that of the bulk sample for mass balance reasons). In contrast, olivine phenocrysts display highly variable, but generally light Fe and mostly light Li isotope compositions compared to their respective olivine-free basaltic matrix, which was considered to represent the melt (with δ 56Fe ol=-0.24 to 0.14‰ and δ 7Li ol=-10.5 to +6.5‰, respectively). Single olivine crystals from one sample display even a larger range of δ 56Fe ol between -0.7 and +0.1‰. One single clinopyroxene phenocryst displays the lightest Li isotope composition (δ 7Li cpx=-17.7‰), but no Fe isotope fractionation relative to melt. The olivine phenocrysts show variable Mg# and Ni (correlated in most cases) that range between 0.89 and 0.74 and between 300 and 3000μg/g, respectively. These olivines likely grew by fractional crystallization in an evolving magma. One sample from the Vogelsberg volcano contained olivine xenocrysts (Mg#>0.89 and Ni>3000μg/g), in addition to olivine phenocrysts. This sample displays the highest Li- and the second highest Fe-isotope fractionation between olivine and melt (δ 7Li ol-melt=-13; δ 56Fe ol-melt=-0.29).Our data, i.e. the variable olivine- at constant whole rock and matrix isotope compositions, strongly indicate disequilibrium, i.e. kinetic Fe and Li isotope fractionation between olivine and melt (for Li also between cpx and melt) during fractional crystallization. δ 7Li ol-melt is correlated with the Li partitioning between olivine and melt (i.e. with Li ol/Li melt), indicating Li isotope fractionation due to preferential (faster) diffusion of 6Li into olivine during fractional crystallization. Olivine with low δ 7Li ol-melt, also have low δ 56Fe ol-melt, indicating that Fe isotope fractionation is also driven by diffusion of isotopically light Fe into olivine, potentially, as Fe-Mg inter-diffusion. The lowest δ 56Fe ol-melt (-0.40) was observed in a sample from Westerwald (Germany) with abundant magnetite, indicating relatively oxidizing conditions during magma differentiation. This may have enhanced equilibrium Fe isotope fractionation between olivine and melt or fine dispersed magnetite in the basalt matrix may have shifted its Fe isotope composition towards higher δ 56Fe. The decoupling of Li- and Fe isotope fractionation in cpx is likely due to faster diffusion of Li relative to Fe in cpx, implying that the large investigated cpx phenocryst resided in the magma for only a short period of time which was sufficient for Li- but not for Fe diffusion. The absence of any equilibrium Fe isotope fractionation between the investigated cpx phenocryst and its basaltic host may be related to the similar Fe 3+/Fe 2+ of cpx and melt. In contrast to cpx, the generally light Fe isotope composition of all investigated olivine separates implies the existence of equilibrium- (in addition to diffusion-driven) isotope fractionation between olivine and melt, on the order of 0.1‰.

KW - Chemical diffusion

KW - Fractional crystallization

KW - Iron isotopes

KW - Lithium isotopes

KW - Magma differentiation

KW - Redox

UR - http://www.scopus.com/inward/record.url?scp=84155183135&partnerID=8YFLogxK

U2 - 10.1016/j.chemgeo.2011.11.020

DO - 10.1016/j.chemgeo.2011.11.020

M3 - Article

AN - SCOPUS:84155183135

VL - 294-295

SP - 42

EP - 50

JO - Chemical geology

JF - Chemical geology

SN - 0009-2541

ER -