Abstract
Many studies have shown that the average iron (Fe) isotope compositions of mantle-derived rocks, mantle peridotite and model mantle are close to those of chondrites. Therefore, it is considered that chondrite values represent the bulk Earth Fe isotope composition. However, this is a brave assumption because nearly 90% of Fe of the Earth is in the core, where its Fe isotope composition is unknown, but it is required to construct bulk Earth Fe isotope composition. We approach the problem by assuming that the Earth’s core separation can be approximated in terms of the Sudbury-type Ni-Cu sulfide mineralization, where sulfide-saturated mafic magmas segregate into immiscible sulfide liquid and silicate liquid. Their density/buoyancy controlled stratification and solidification produced net-textured ores above massive ores and below disseminated ores. The coexisting sulfide minerals (pyrrhotite (Po) > pentlandite (Pn) > chalcopyrite (Cp)) and silicate minerals (olivine (Ol) > orthopyroxene (Opx) > clinopyroxene (Cpx)) are expected to hold messages on Fe isotope fractionation between the two liquids before their solidification. We studied the net-textured ores of the Sudbury-type Jinchuan Ni-Cu sulfide deposit. The sulfide minerals show varying δ56Fe values (−1.37–−0.74‰ (Po) < 0.09–0.56‰ (Cp) < 0.53–1.05‰ (Pn)), but silicate minerals (Ol, Opx, and Cpx) have δ56Fe values close to chondrites (δ56Fe = −0.01 ± 0.01‰). The heavy δ56Fe value (0.52–0.60‰) of serpentines may reflect Fe isotopes exchange with the coexisting pyrrhotite with light δ56Fe. We obtained an equilibrium fractionation factor of Δ56Fesilicate-sulfide ≈ 0.51‰ between reconstructed silicate liquid (δ56Fe ≈ 0.21‰) and sulfide liquid (δ56Fe ≈ −0.30‰), or Δ56Fesilicate-sulfide ≈ 0.36‰ between the weighted mean bulk-silicate minerals (δ56Fe[0.70ol,0.25opx,0.05cpx] = 0.06‰) with weighted mean bulk-sulfide minerals (δ56Fe ≈ −0.30‰). Our study indicates that significant Fe isotope fractionation does take place between silicate and sulfide liquids during the Sudbury-type sulfide mineralization. We hypothesize that significant iron isotope fractionation must have taken place during core–mantle segregation, and the bulk Earth may have lighter Fe isotope composition than chondrites although Fe isotope analysis on experimental sulfide-silicate liquids produced under the varying mantle depth conditions is needed to test our results. We advocate the importance of further research on the subject. Given the close Fe-Ni association in the magmatic mineralization and the majority of the Earth’s Ni is also in the core, we infer that Ni isotope fractionation must also have taken place during the core separation that needs attention.
Highlights
The rapid technological development in mass spectrometry has allowed determination of non-traditional stable isotopes of many elements (e.g., Mg, Fe, Cu, Zn, Mo, etc.) in all sorts of Earth materials, which has led to the knowledge that these isotopes do vary on all scales
The net-textured ores are characterized by euhedral silicate minerals surrounded by aggregates of sulfide mineral assemblage as “interstitial” fills (Figures 3 and 4)
We found that the serpentinization process that transformed the silicate minerals in questions (Ol > Opx > Cpx) into serpentines is associated with the losses of light Fe isotopes (e.g., 54 Fe), i.e., δ56 Fe(Ol, Opx, Cpx) < δ56 Fe(Serp) (or gains of heavy Fe isotopes (e.g., 56 Fe))
Summary
The rapid technological development in mass spectrometry has allowed determination of non-traditional stable isotopes of many elements (e.g., Mg, Fe, Cu, Zn, Mo, etc.) in all sorts of Earth materials, which has led to the knowledge that these isotopes do vary on all scales. It is common to read in the literature that isotope variations can be used to constrain Earth processes, but in practice we cannot yet constrain any process before we fully understand the mechanisms of isotope fractionation in response to varying chemical and physical conditions and processes. For this very reason, the iron isotope compositions of the coexisting minerals (e.g., silicates and sulfides) in Sudbury-type ores from the Jinchuan magmatic Ni-Cu sulfide deposit were studied to accumulate observations and to evaluate the possible iron isotope fractionation between the silicate and sulfide liquids prior to the crystallization of these minerals. The data and understanding of this work provide a fundamental first step for discussing possible Fe isotope fractionation between the Earth’s mantle and its core that hosts almost 90% of the Earth’s iron
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