Iron isotope fractionation during fenitization:a case study of carbonatite dykes from Bayan Obo,Inner Mongolia,China

As a powerful tracer in high-temperature geochemistry,Fe isotopes have been studied for their behaviour during fl uid exsolution and evolution related to felsic magma system,but that for carbonatite magma system remains unknown.Here we study the Fe isotope fractionation behaviour during fenitization–processes that widely occur associated with carbonatite or alkaline intrusions.Nine fenite/carbonatite samples from carbonatite dykes at Bayan Obo area are analyzed for their Fe isotope compositions as well as elemental compositions.Combined with previous reported carbonatite δ^(56)Fe data,the results show that carbonatites range from-0.35‰to 0.28‰,with an average of-0.10‰in δ^(56)Fe values,while fenites range from-0.17‰to 0.30‰,with an average of 0.11‰in δ^(56)Fe values.This indicates that fenitizing fl uids exsolved from carbonatite melts are enriched in heavier Fe isotopes.Such a Fe isotope fractionation trend is diff erent from that for fl uid exsolution from felsic magmatism.δ^(56)Fe values in fenites are negatively correlated with indicators of fenitization intensity such as(Na+K),Ti,Ba,Th,Nb,U or Pb abundances,likely refl ecting that Fe isotopes fractionate during the evolution of the fenitizing fl uids.Thus,Fe isotopes are a valuable tool for tracing fl uid exsolution and evolution relevant to carbonatite magmatism and related metal mineralization.

Iron Isotope Compositions of Podiform Chromitites from Dazhuqu and Luobusha Ophiolites, Southern Tibet

Podiform chromitites crop out in ophiolitic harzburgites as pod-like bodies associated with dunite envelopes with various thickness. It is widely accepted that the change of melt compositions caused by melt-rock reaction, especially an increase in silica content, plays a crucial role in the generation of podiform chromitite(e.g., Arai and Yurimoto, 1994;Zhou et al., 1994). Due to the presence of ultrahigh pressure and highly reduced minerals, the genesis of some podiform chromitites was attributed to some deep processes(e.g., Arai, 2013;Yang et al., 2007). Although much progress has been achieved, the formation mechanism of podiform chromitites are still in dispute. Iron isotope may be a potential tool to give further insight to the issue, given that some high temperature processes, such as partial melting, metasomatism, magma differentiation and redox change, can result in measurable iron isotopic fractionation to different extent(e.g. Chen et al., 2014;Weyer and Ionov, 2007;Zhao et al., 2009). This study investigates the Fe isotope compositions of chromitites and chromite dunites from Dazhuqu and Luobusha ophiolites. For Dazhuqu chromite dunites, δ56 Fe(relative to the standard, IRMM-014) values range from-0.02‰ to 0.11‰ in olivines and from 0.03‰ to 0.08‰ in chromites. Chromites in Dazhuqu chromitites show δ56 Fe values varying from-0.03‰ to 0.02‰. In nodular and densely disseminated chromitites from Luobusha, olivines have δ56 Fe values of olivines and chromites are 0.09–0.35‰ and-0.15–0.08 ‰, respectively. Chromites from Luobusha massive chromitites have δ56 Fe values of 0.07–0.12 ‰. Based on theorical calculations, chromites should be heavier than olivines in Fe isotope compositions ?56 FeOl-Chr ≈-0.08‰ at 1300 ℃ according to the ionic model(e.g., Macris et al., 2015;Sossi and O’Neill, 2017). However, most of our samples, except for two samples, have ?56 FeOl-Chr values that are greater than zero, indicating a disequilibrium inter-mineral Fe isotopic fractionation. There is a positive correlation between Fo and δ56 Fe(or ?56 FeOl-Chr) of olivines but no positive correlation between Mg# and δ56 Fe(or ?56 FeOl-Chr) of chromites. This phenomenon suggests that the Fe isotopic dis-equilibration may be caused by migrating melts in dunitic channels rather than by the sub-solidus Fe-Mg exchange(Xiao et al., 2016;Zhang et al., 2019). Additionally, the wide δ56 Fe range of chromites is similar to those of the subduction-related basalts and boninites, inferring that their parental magmas form in the suprasubduction zone.

Fractionation mechanism of iron isotopes in highly fractionated granites from the Xinxian Pluton,Western Dabie Orogen,Central China

Iron isotopes are important for tracing the magmatic process.The fractionation of iron isotopes in granite is up to 0.55‰.In this study,Wangjiagou(XWJ)granite and Tayueping(XTY)granite in the Xinxian pluton of the Western Dabie orogen were evaluated.Both the XTY and XWJ granite belong to monzogranites,with high SiO2(74.42-76.82 wt.%)contents.The granites are depleted of Nb and Ti but enriched with Pb and K,and they display negative Eu anomalies(Eu/Eu^(*)=0.40-0.52)on REE plots that are normalized by chondrite.Theδ^(56)Fe values of the XTY granites vary from 0.19±0.03‰to0.27±0.04‰,and theδ^(56)Fe values of the XWJ granites are 0.34±0.02‰and 0.36±0.01‰,respectively.Both the XTY and the XWJ granites belong to highly fractionated granites due to their SI(solidification index),DI(differentiation index),and content of CaO.Evidence from the iron isotopes shows that neither fluid exsolution,alteration,weathering,nor partial melting can explain the enrichment of the heavy iron isotopes.The results modeled using the Rayleigh equation showed that fractional crystallization can produceΔ^(56)Femelt-crystalwith the value of0.08-0.15‰.In conclusion,fractional crystallization was the main factor controlling the fractionation of iron isotopes,and the change of melt composition may also lead to the enrichment of heavy iron isotopes in the residual melt.

Iron Isotopic Fractionation and Origin of Chromitites in the Paleo-Moho Transition Zone of the Kop Ophiolite, NE Turkey

The Kop ophiolite in NE Turkey is a fragment of Neo-Tethyan forearc.It can be mainly divided into a paleo-Moho transition zone(MTZ)in the North and a harzburgitic mantle sequence in the South.Dunites are predominant in the MTZ of the Kop ophiolite,and they are locally interlayered with chromitites and enclose minor bodies of harzburgites near the petrological Moho boundary.Large Fe isotopic variations were observed for magnesiochromite(-0.14‰to 0.06‰)and olivine(-0.12‰to 0.14‰)from the MTZ chromitites,dunites and harzburgites.In individual dunite samples,magnesiochromite usually has lighter Fe isotopic compositions than olivine,which was probably caused by subsolidus Mg-Fe exchange between the two mineral phases.Both magnesiochromite and olivine display an increasing trend ofδ56Fe along a profile from chromitite todunite.This trend reflects continuous fractional crystallization in a magma chamber,which resulted in heavier Fe isotopes concentrated in the evolved magmas.In each cumulative cycle of chromitite and dunite,dunite was formed from relatively evolved melts after massive precipitation of magnesiochromite.Mixing of more primitive and evolved melts in the magma chamber was a potential mechanism for triggering the crystallization of magnesiochromite,generating chromitite layers in the cumulate pile.Before mixing happened,the primitive melts had reacted with mantle harzburgites during their ascendance;whereas the evolved melts may lie on the olivine-chromite cotectic near the liquidus field of pyroxene.Variable degrees of magma mixing and differentiation are expected to generate melts with differentδ56Fe values,accounting for the Fe isotopic variations of the Kop MTZ.

Marine Redox Conditions in the Early Cambrian Ocean: Insights from the Lower Cambrian Phosphorite Deposits, South China

It is generally considered that a significant change in oceanic redox conditions occurred during the Ediacaran-Cambrian transition. However, there are currently two major conflicting views on the degree of oxygenation of deep water（oxic vs. ferruginous） during this interval. To date, the oxygenation conditions of the Early Cambrian ocean have not been well constrained. The oxygenation magnitude and mechanism of the Early Cambrian ocean could be critical to the significant biological evolution of the ＂Cambrian Explosion＂. To constrain the Early Cambrian oceanic redox environment, we conducted an integrated study on iron and sulfur isotopes and redox-sensitive elements（Mo, U, and V） of Lower Cambrian phosphorite deposits from two shallow sections（Meishucun and Gezhongwu） and a deeper water section（Zunyi） from the Yangtze Platform, South China. The near zero δ^（56）Fe values from the two shallow sections studied here reflect oxic conditions in the lower phosphorite deposition. An obvious positive shift in δ^（56）Fe and redox-sensitive element content was observed in the middle parts of the two shallow water sections, which might reflect loss of light iron by dissimilatory iron reduction during early diagenesis under suboxic shallow water in the platform. However, the highly positive δ^（56）Fe values in the deep section could reflect a lower oxidation degree of dissolved Fe（II） under anoxic deep water. The data suggest redox-stratified oceanic conditions during the Early Cambrian, in which completely oxygenated shallow water（platform） coexisted with anoxic deep water（slope）. We propose that prolonged upwelling of dissolved organic carbon（DOC）-, Fe（II）- and phosphorus-rich anoxic deep water in a redox-stratified ocean could have increased exchange with the open ocean, resulting in major phosphorite deposition in oxic-suboxic conditions. The progressive oxygenation of the ocean may have facilitated the Early Cambrian biotic diversification.