Abstract
[Graphic presents]
Zirconium isotopes have the potential to trace both magmatic differentiation and crustal evolution, as well as deep Earth processes. Zirconium is compatible in bridgmanite where it has a higher coordination number than in silicate melt, implying that Zr isotopes could be fractionated during magma ocean crystallisation. We report the Zr isotopic composition of 31 komatiites from around the globe, ranging in age from 2.41 to 3.55 Ga. The δ94/90Zr (per mille deviation of 94Zr/90Zr from IPGP-Zr standard) values for the komatiites are homogeneous at 0.030 ± 0.049 ‰ (all errors are 2 s.d.), and consistent with associated basaltic rocks (0.048 ± 0.032 ‰). These results, identical within uncertainty of the bulk silicate Earth estimate from modern basalts, suggest that the mantle Zr isotope composition has been constant since at least 3.55 Ga. Combining the isotopic compositions of komatiites and basalts of all ages we suggest a conservative δ94/90Zr = 0.040 ± 0.044 ‰ (n = 72) for the mantle composition. Several komatiite systems that we analysed in this study, including Schapenburg, Komati, and Weltevreden, have been previously argued to have isotope signatures consistent with magma ocean crystallisation processes. However, their Zr isotope compositions are indistinguishable from other komatiites, implying that bridgmanite crystallisation did not fractionate Zr isotopes to any measurable extent.
Zirconium isotopes have the potential to trace both magmatic differentiation and crustal evolution, as well as deep Earth processes. Zirconium is compatible in bridgmanite where it has a higher coordination number than in silicate melt, implying that Zr isotopes could be fractionated during magma ocean crystallisation. We report the Zr isotopic composition of 31 komatiites from around the globe, ranging in age from 2.41 to 3.55 Ga. The δ94/90Zr (per mille deviation of 94Zr/90Zr from IPGP-Zr standard) values for the komatiites are homogeneous at 0.030 ± 0.049 ‰ (all errors are 2 s.d.), and consistent with associated basaltic rocks (0.048 ± 0.032 ‰). These results, identical within uncertainty of the bulk silicate Earth estimate from modern basalts, suggest that the mantle Zr isotope composition has been constant since at least 3.55 Ga. Combining the isotopic compositions of komatiites and basalts of all ages we suggest a conservative δ94/90Zr = 0.040 ± 0.044 ‰ (n = 72) for the mantle composition. Several komatiite systems that we analysed in this study, including Schapenburg, Komati, and Weltevreden, have been previously argued to have isotope signatures consistent with magma ocean crystallisation processes. However, their Zr isotope compositions are indistinguishable from other komatiites, implying that bridgmanite crystallisation did not fractionate Zr isotopes to any measurable extent.
| Original language | English |
|---|---|
| Pages (from-to) | 40-43 |
| Number of pages | 4 |
| Journal | Geochemical Perspectives Letters |
| Volume | 15 |
| DOIs | |
| Publication status | Published - 7 Oct 2020 |