Equilibrium olivine-melt Mg isotopic fractionation explains high δ26Mg values in arc lavas

Abstract

We determined equilibrium Mg isotope fractionation between olivine and melt (Δ 26/24 Mg Ol/melt ) in five, naturally quenched, olivine-glass pairs that were selected to show clear textural and chemical evidence of equilibration. We employed a high-precision, critical mixture double-spiking approach to obtain a weighted mean of Δ 26/24 Mg Ol/melt = −0.071 ± 0.010 ‰, for values corrected to a common olivineglass temperature of 1438 K. As function of temperature, the fractionation can be expressed as Δ 26/24 Mg Ol/melt = (−1.46 ± 0.26) × 10 5 /T 2 . The samples analysed have variable H 2 O content from 0.1 to ∼1.2 wt. %, yet no discernible difference in Δ 26/24 Mg Ol/melt was evident. We have used this Δ 26/24 Mg Ol/melt to revisit the puzzling issue of elevated Mg isotope ratios in arc lavas. In new Mg isotope data on sample suites from the Lesser Antilles and Mariana arcs, we show that primitive samples have MORB-like Mg isotope ratios while the evolved samples tend to have isotopically heavier compositions. The magnitude of this variability is well explained by olivine fractionation during magmatic differentiation as calculated with our new equilibrium Δ 26/24 Mg Ol/melt .

Heavy magnesium isotopic signatures in arc lavas may be attributed to dehydration of subducting hydrated mantle

“…In particular, several subduction zones exhibit elevated δ 26 Mg values in their arc lavas. For instance, Lesser Antilles shows δ 26 Mg values of −0.18 ± 0.07‰ 20 , Mariana displays δ 26 Mg values of −0.15 ± 0.05‰ 31 , and Philippines exhibits values of −0.01 ± 0.18‰ 16 . The elevated δ 26 Mg values in the arc lavas of these subduction zones might stem from fluids released during the dehydration of hydrated mantle peridotites in subducting slabs.…”

“…The Mg isotope composition of arc lavas has been investigated in a variety of subduction zones 16,17,20,31 . In contrast to the uniform δ 26 Mg values in global peridotites (−0.25 ± 0.04‰) 32 , arc lavas exhibit large variations in δ 26 Mg values among various subduction zones.…”

“…While the lighter Mg isotope signatures in arc lavas from subduction zones like Costa Rica and Kamchatka could be linked to recycled carbonates from the subducting slabs 16 , the sources contributing to the elevated δ 26 Mg values observed in arc lavas from other subduction zones remain a perplexing enigma. Crust assimilation or fractional crystallization may drive the evolving magmas toward higher δ 26 Mg values 31,[33][34][35][36] . Their impacts are particularly pronounced in samples with MgO content below 3 wt%, whereas samples with MgO content exceeding 3 wt% are anticipated to show minimal influence from these processes 33,36 .…”

“…Their impacts are particularly pronounced in samples with MgO content below 3 wt%, whereas samples with MgO content exceeding 3 wt% are anticipated to show minimal influence from these processes 33,36 . Notably, elevated δ 26 Mg values have been widely observed in arc lava samples with MgO content higher than 3 wt% 16,31 . Moreover, assimilation is commonly associated with elevated 87 Sr/ 86 Sr and Ba/Th ratios.…”

The effect of alkalinity on Ni–O bond length in silicate glasses: Implications for Ni isotope geochemistry

“…Beta factors for silicate glasses and melts can be measured using nuclear resonant inelastic X-ray scattering (NRIXS) for a few elements such as Fe and Sn (e.g., Dauphas et al, 2014;Prissel et al, 2018;Roskosz et al, 2020;Roskosz et al, 2022). For most other elements, mineral-melt isotopic fractionation factors have to be derived through the measurement of experimental or natural phenocryst-melt pairs (e.g., Prissel et al, 2018;Rzehak et al, 2021;Hoare et al, 2022, Liu et al, 2022, or through the inversion of natural magmatic differentiation suites (e.g., Teng et al, 2007;Zhang et al, 2018). Given this complexity, it is challenging to explicitly consider the potentially significant influence of the silicate melt composition on the magnitude of mineral-melt isotopic fractionation factors.…”