Isotopic fractionation of zirconium during magmatic differentiation and the stable isotope composition of the silicate Earth

摘要

High-precision double-spike Zr stable isotope measurements (expressed as delta Zr-94/90(IPGP-Zr), the permil deviation of the Zr-94/Zr-90 ratio from the IPGP-Zr standard) are presented for a range of ocean island basalts (OIB) and mid-ocean ridge basalts (MORB) to examine mass-dependent isotopic variations of zirconium in Earth. Ocean island basalt samples, spanning a range of radiogenic isotopic flavours (HIMU, EM) show a limited range in delta Zr-94/90(IPGP-Zr) (0.046 +/- 0.037 parts per thousand; 2sd, n = 13). Similarly, MORB samples with chondrite-normalized La/Sm of 0.7 show a limited range in delta Zr-94/90(IPGP-Zr )(0.053 +/- 0.040 parts per thousand; 2sd, n = 8). In contrast, basaltic lavas from mantle sources that have undergone significant melt depletion, such as depleted normal MORB (N-MORB) show resolvable variations in delta Zr-94/90(IPGP-Zr), from -0.045 +/- 0.018 to 0.074 +/- 0.023 parts per thousand. Highly evolved igneous differentiates (65 wt% SiO2) from Hekla volcano in Iceland are isotopically heavier than less evolved igneous rocks, up to 0.53 parts per thousand. These results suggest that both mantle melt depletion and extreme magmatic differentiation leads to resolvable mass-dependent Zr isotope fractionation. We find that this isotopic fractionation is most likely driven by incorporation of light isotopes of Zr within the 8-fold coordinated sites of zircons, driving residual melts, with a lower coordination chemistry, towards heavier values. Using a Rayleigh fractionation model, we suggest a alpha(zircon-melt) of 0.9995 based on the whole rock delta Zr-94/90(IPGP-Zr) values of the samples from Hekla volcano (Iceland). Zirconium isotopic fractionation during melt-depletion of the mantle is less well-constrained, but may result from incongruent melting and incorporation of isotopically light Zr in the 8-fold coordinated M2 site of orthopyroxene. Based on these observations lavas originating from the effect of melt extraction from a depl