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

Significant Zr isotope variations in single zircon grains recording magma evolution history

“…Our finding is consistent with the first-order theoretical prediction where higher coordination-number lattices will be enriched in lighter isotopes, since in zircon, Zr has eightfold coordination (36), while in the melt it is generally found with sixfold coordination (37,38). This is also consistent with the α of 0.9995 inferred from bulk analyses of the Hekla volcanic rocks, where rhyolites (SiO 2 >65 wt%) show elevated δ 94 Zr values with reduced Zr contents (21). It is, however, different from the α of 1.00106 suggested from FC1 zircons in the Duluth anorthosite (15,22).…”

“…The average α values for all six samples range from 0.99955 to 0.99988 ( Fig. 2 and SI Appendix, Table S6), closer to 0.9995 inferred from Hekla volcanic rocks (21). The obtained values are all below 1, confirming the preference of light Zr isotopes in zircon.…”

“…In order to directly compare our zircon data with previous bulk analyses, the 94 Zr/ 90 Zr ratios were further expressed as the permil deviation from solution standard IPGP-Zr (Plasma-Cal standard solution; lot 5131203028). This standard has been used as a reference standard in most studies reporting Zr isotopic compositions and was recently cross-calibrated with different geological reference materials (15,20,21,23). The within-run and long-term external precision for δ 94 Zr in this study are better than 0.10‰ (2SE) and 0.15‰ (2SD), respectively.…”

“…2). Most δ 94 Zr values of the zircon cores (as well as the average value of all analyses) are lower than that of the primitive mantle (0.048 ± 0.032‰) (21). Such zoning patterns indicate the preferential incorporation of light Zr isotopes into zircon (i.e., α < 1).…”

“…They show the great potential of Zr isotope in tracing magmatic processes. However, available data have been rather limited and resulted in very controversial interpretation (21,22), such as for fractionation factor α (whether greater or smaller than 1). The α is defined as ( 94 Zr/ 90 Zr) zircon /( 94 Zr/ 90 Zr) melt , which depicts how Zr isotopes partition into the crystallizing zircon and surrounding melt and is thus fundamental to understanding magmatic Zr isotope evolution.…”