Spinel–olivine–pyroxene equilibrium iron isotopic fractionation and applications to natural peridotites

Abstract

Eight spinel-group minerals were synthesized by a flux-growth method producing spinels with varying composition and Fe 3+ /Fetot ratios. The mean force constants of iron bonds in these minerals were determined by synchrotron Nuclear Resonant Inelastic X-ray Scattering (NRIXS) in order to determine the reduced isotopic partition function ratios (β β β β-factors) of these spinels. The mean force constants are strongly dependent on the Fe 3+ /Fetot of the spinel but are independent, or weakly dependent on other structural and compositional parameters. From our spectroscopic data, it is found that a single redoxdependent calibration line accounts for the effects of Fe 3+ /Fetot on the β β β β-factors of spinels. This calibration successfully describes the equilibrium Fe isotopes fractionation factors between spinels and silicates (olivine and pyroxenes). Our predictions are in excellent agreement with independent determinations for the equilibrium Fe isotopic fractionations for the magnetite-fayalite and the magnetite-hedenbergite couples. Our calibration applies to the entire range of Fe 3+ /Fetot ratios found in natural spinels and provides a basis for interpreting iron isotopic variations documented in mantle peridotites. Except for a few exceptions, most of the samples measured so far are in isotopic disequilibrium, reflecting metasomatism and partial melting processes.

Iron isotopic evolution during fractional crystallization of the uppermost Bushveld Complex layered mafic intrusion

“…They also require an assumption of equilibrium. While there are published beta factors for some of the phases (which agree with the magnetite fractionation factor in our model) [ Roskosz et al ., ], the paucity of these data at relevant conditions for every considered phase (ol, opx, cpx, mgt, ilm, melt , and plag) hindered the attempt to use these more recent data in the model. This may also be the case for the set isotopic value of plagioclase; however, the value does not significantly influence the model.…”

“…[] and is consistent with the study of Roskosz et al . []. The experimental approach utilized the three‐isotope method in which a “spike” of one isotope of Fe is added to one experimental phase and time‐series runs are performed.…”

“…These equations are the only mineral‐melt fractionation factors available in the literature, to our knowledge, which are applicable for the T range and composition of the UUMZ. Clinopyroxene and orthopyroxene compositions were calculated by using the same fractionation factor since Fe isotope fractionation is not expected to differ [ Roskosz et al ., ], while ilmenite was treated like fayalite, consistent with the findings of Sossi and O'Neill []. Magnetite and olivine were not included in the model until the height at which they appear as a cumulus phase (Figures , and Table ), even though small amounts of intercumulus magnetite and olivine occur in the rocks of the UUMZ below these levels.…”

Iron and magnesium isotopic constraints on the origin of chemical heterogeneity in podiform chromitite from the Luobusa ophiolite, Tibet

“…Olivines in the harzburgites have Fe isotopic composition similar to that of orthopyroxene but slightly lighter than the δ 56 Fe values of spinel, consistent with previous studies of mantle peridotites [ Beard and Johnson , ; Williams et al ., ; Weyer and Ionov , ; Zhao et al ., ; Huang et al ., ; Macris et al ., ]. These isotopic differences generally agree with ionic model prediction of intermineral equilibrium fractionation factor and calculated equilibrium Fe isotopic fractionation by synchrotron Nuclear Resonant Inelastic X‐ray Scattering (NRIXS) (Figures a–c) [ Macris et al ., ; Roskosz et al ., ], indicating that Fe isotopes are likely in equilibrium with minerals in the harzburgites.…”

“…[] and Roskosz et al . [], respectively. The grey squares in the figure 4c represent spinel data in mantle peridotite in Figure 4c from Beard and Johnson [], Williams et al .…”