摘要
辉玻无球粒陨石的氧逸度是示踪其母体岩浆氧化还原环境演化过程的关键指标。铁钛氧化物温度-氧逸度计因铁钛氧化物的广泛分布、高灵敏度和高精度等特点,成为玄武质辉玻无球粒陨石氧逸度研究中应用最广泛的测定体系。NWA 13106陨石是一块新发现的玄武质辉玻无球粒火星陨石(Shergottite),具有次辉绿结构,并发育有不同产状的铁钛氧化物。其中,钛磁铁矿固溶体普遍具有典型的出溶结构,根据出溶片晶的分布,可以划分为夹层型、网格型和复合型。基于扫描电镜的岩相学观察,铁钛氧化物的产状包括两种:1)出溶片晶宽度<1μm的钛磁铁矿固溶体,普遍与原生钛铁矿共生;2)出溶钛铁矿片晶宽度>2μm的钛磁铁矿固溶体,且未见与原生钛铁矿共生。本文利用电子探针对NWA 13106开展矿物成分分析,对通过Mg/Mn平衡检验的铁钛氧化物矿物对(钛磁铁矿固溶体-钛铁矿和钛铁矿-磁铁矿)采用Sauerzapf模型估算其温度和氧逸度。研究表明,NWA 13106陨石中的铁钛氧化物大致经历了三个演化阶段:1)当温度处于1016~1167℃,氧逸度ΔQFM值为–0.44~–0.18时,钛磁铁矿结晶,并与钛铁矿共生;2)当温度降至868~909℃,氧逸度ΔQFM值为+0.43~+0.66,钛磁铁矿出溶形成宽度>2μm的钛铁矿,这一阶段为高温亚固相氧化-出溶再平衡;3)当温度持续下降至约650~868℃时,钛磁铁矿出溶形成亚微米级(<1μm)的钛铁矿片晶,这属于低温扩散主导下的再平衡。经研究发现,铁钛氧化物成分的不均一性是影响其温度-氧逸度计使用的关键因素,会导致温度和氧逸度估算出现偏差。基于上述研究,本文还进一步明确了铁钛氧化物温度-氧逸度计使用的适用性条件,并针对玄武质辉玻无球粒陨石中铁钛氧化物温度-氧逸度计的使用,提出了标准化操作流程的建议。
The oxygen fugacity(ƒO_(2))of basaltic shergottites serves as a key indicator for tracing the redox evolution of their parental magma.The Fe-Ti oxide thermometer-ƒO_(2) barometer has become the predominant approach for determining the oxygen fugacity of basaltic shergottites,due to its exceptional sensitivity,measurement precision,and the widespread occurrence of Fe-Ti oxides.NWA 13106,a newly discovered basaltic shergottite,displays a sub-augite structure with Fe-Ti oxides in diverse textural features.Titanomagnetite solid solutions in this sample generally display exsolution textures,including sandwich-,trellis-,and composite-type morphologies.Petrographically,two types of Fe-Ti oxide assemblages are identified:(1)titanomagnetite solid solutions with fine exsolution lamellae(<1μm),commonly coexisting with primary ilmenite;and(2)titanomagnetite solid solutions with coarse exsolved ilmenite lamellae(>2μm),without associated primary ilmenite.Through application of the Sauerzapf model with Mg/Mn equilibrium criteria,we reconstruct three evolutionary stages of Fe-Ti oxides in NWA 13106:(1)crystallization of titanomagnetite at temperatures of 1016–1167°C under relatively reduced condition(ΔQFM=–0.44 to–0.18),coexisting with ilmenite;(2)subsequent cooling to 868–909°C accompanied by oxygen fugacity increased toΔQFM=+0.43 to+0.53,leading to exsolution of ilmenite lamellae from titanomagnetite;(3)further cooling to 650–868°C resulted in exsolution of titanomagnetite forming sub-micron(<1μm)ilmenite lamellae in a diffusion-dominated process.This study highlights that compositional heterogeneity in Fe-Ti oxides can significantly affect the reliability of temperature-ƒO_(2) determinatios,causing deviations in both values.Based on these findings,the study further establishes applicability criteria for the Fe-Ti oxide thermometer-ƒO_(2) barometer and provides specific procedural recommendations for its application to basaltic shergottites.
作者
陈渟怡
徐于晨
刘洋
CHEN Tingyi;XU Yuchen;LIU Yang(National Space Science Center,Chinese Academy of Sciences,Beijing 100190,China;University of the Chinese Academy of Sciences College of Earth and Planetary Sciences,Beijing 100049,China)
出处
《矿物学报》
北大核心
2025年第6期1197-1211,共15页
Acta Mineralogica Sinica
基金
国家自然科学基金项目(批准号:42430210)
中国科学院青年创新促进会(编号:2022147)。
