Multistage melt–rock interaction and redox evolution during chromitite formation in the Bulqiza ophiolite (Albania): Constraints from Mӧssbauer spectroscopy, FeMg isotopes and chromitite geochemistry

IF 3.6 2区地球科学 Q1 GEOCHEMISTRY & GEOPHYSICS

Chemical Geology Pub Date : 2025-06-28 DOI:10.1016/j.chemgeo.2025.122948

Fahui Xiong , Basem Zoheir , Joseph Meert , Xiangzhen Xu , Tian Qiu , Xuxuan Ma , Ibrahim Milushi , Jingsui Yang

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Abstract

FeMg isotopes integrated with Fe³⁺/∑Fe data provide robust constraints on the redox state of the oceanic lithospheric mantle, as both are sensitive to oxygen fugacity and mineral–melt equilibria. Fe isotopes track valence changes, while Mg isotopes reflect melt–rock interaction during partial melting and metasomatism. Applied to ophiolites, these proxies offer detailed insights into mantle melting, melt transport, and redox evolution. The Bulqiza ophiolite massif in Albania, a major chromite-bearing ultramafic complex in the Balkans, presents an ideal natural laboratory to investigate mantle processes during the evolution of the Mesozoic Tethys Ocean. This study integrates mineral chemistry, LA-ICP-MS trace element data, Mössbauer spectroscopy, and Fe and Mg isotope analyses to reconstruct mantle compositional changes and the genesis of ultramafic assemblages.

Harzburgites and dunites from Bulqiza show high forsterite (Fo_91–97) and Ni contents indicative of subsolidus equilibration between olivine and magnesiochromite. Two partial melting stages are identified: an initial ∼25 % melting forming harzburgite, followed by ∼30 % melting producing dunite and chromitite. Variations in spinel Cr# [100Cr/(Cr + Al + Fe³⁺)] and oxygen fugacity suggest that dunite-chromitite formation resulted from focused melt–rock interaction. Chromitites are enriched in Cr₂O₃ (53–61 wt%) and display variable FeO (12–21 wt%), reflecting mantle source heterogeneity and dynamic melt extraction. Isotopic trends, including a negative correlation between δ²⁶Mg and Cr# and a positive δ⁵⁶Fe-δ²⁶Mg relationship, indicate progressive melt–rock interaction and oxidation. Mössbauer spectroscopy reveals Fe³⁺/∑Fe ratios of 0.075–0.253 in magnesiochromite, documenting variable redox conditions during formation. Together, these data support a multistage genesis involving high-degree melting, melt percolation, and redox evolution within an upwelling asthenospheric mantle, consistent with localized mantle flow beneath a slow-spreading ridge or transform margin rather than a subduction-related setting.

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阿尔巴尼亚Bulqiza蛇绿岩中铬铁矿形成过程中的多阶段熔体-岩石相互作用和氧化还原演化：Mӧssbauer光谱、FeMg同位素和铬铁矿地球化学的约束

FeMg同位素与Fe3+/∑Fe数据相结合，对海洋岩石圈地幔的氧化还原状态提供了强有力的约束，因为两者都对氧逸度和矿物-熔体平衡敏感。Fe同位素跟踪价态变化，Mg同位素反映部分熔融和交代过程中熔融-岩石相互作用。将这些指标应用于蛇绿岩，可以详细了解地幔熔融、熔体运输和氧化还原演化。阿尔巴尼亚Bulqiza蛇绿岩地块是巴尔干地区一个主要的含铬铁矿超镁铁质杂岩，为研究中生代特提斯洋地幔演化过程提供了一个理想的天然实验室。本研究结合矿物化学、LA-ICP-MS微量元素数据、Mössbauer光谱和Fe和Mg同位素分析，重建了地幔成分变化和超镁质组合的成因。Bulqiza的哈尔茨伯尔岩和泥质岩显示出高的橄榄石（Fo91-97）和镍含量，表明橄榄石和镁铬铁矿之间存在亚固体平衡。确定了两个部分熔化阶段：最初的~ 25%熔化形成辉锌矿，然后是~ 30%熔化产生坚锌矿和铬铁矿。尖晶石Cr# [100Cr/(Cr + Al + Fe3+)]和氧逸度的变化表明，这是熔体与岩石集中相互作用的结果。铬铁矿富含Cr2O3 (53 ~ 61 wt%)， FeO含量变化较大（12 ~ 21 wt%），反映了地幔源非均质性和动态熔融萃取。δ26Mg与Cr#呈负相关关系，δ56Fe与δ26Mg呈正相关关系，表明熔体与岩石的相互作用和氧化作用逐渐发生。Mössbauer光谱分析显示镁铬铁矿中Fe3+/∑Fe比值为0.075 ~ 0.253，记录了地层中不同的氧化还原条件。总之，这些数据支持一个多阶段的成因，包括在一个上升流软流圈地幔内的高度熔融、熔融渗透和氧化还原演化，与缓慢扩张的脊或转换边缘下的局部地幔流动相一致，而不是与俯冲相关的环境。

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来源期刊

Chemical Geology 地学-地球化学与地球物理

CiteScore

7.20

自引率

10.30%

发文量

374

审稿时长

3.6 months

期刊介绍： Chemical Geology is an international journal that publishes original research papers on isotopic and elemental geochemistry, geochronology and cosmochemistry. The Journal focuses on chemical processes in igneous, metamorphic, and sedimentary petrology, low- and high-temperature aqueous solutions, biogeochemistry, the environment and cosmochemistry. Papers that are field, experimentally, or computationally based are appropriate if they are of broad international interest. The Journal generally does not publish papers that are primarily of regional or local interest, or which are primarily focused on remediation and applied geochemistry. The Journal also welcomes innovative papers dealing with significant analytical advances that are of wide interest in the community and extend significantly beyond the scope of what would be included in the methods section of a standard research paper.