碱性碱性火成岩和上地幔火成岩中的富钛和富钡辉石云母;重新评估和合理解释化学计量、稳定性和铁价估计

IF 4.5 1区 地球科学 Q1 GEOCHEMISTRY & GEOPHYSICS Geochimica et Cosmochimica Acta Pub Date : 2024-10-19 DOI:10.1016/j.gca.2024.10.011
C. Michael B. Henderson
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引用次数: 0

摘要

富含钛和钡的三八面体云母出现在碎屑基性火成岩、变质地幔橄榄岩、变质辉长岩/碳酸盐岩以及热液蚀变矿床中。电子微探针分析(EMP)将所有的铁都报告为 FeO,在 1970/80 年代被广泛用于解释 Ti 和 Ba 的置换机制,其依据是 22 O2- 单胞计算,这意味着阳离子空位出现在八面体和/或片间位点。1996 年,通过对铁和总铁、H2O、(OH)的化学和物理分析,以及特定元素铁的 X 射线吸收光谱(K 边和 L 边),EMP 确定了铁和钛的价态以及阳离子位点占有率,确定了 ∼ 50 % 的 O 取代(OH)分子,并确定了 24 个阴离子晶胞公式显示不存在阳离子空位。这里改进了辉石云母的晶胞公式计算规程,并根据无空位辉石的化学计量公式 XIIK2VIMg6IV[Si6Al2]O20(OH)4 对结果进行了测试。 根据与特定混合价元素置换相关的固定空位含量计算出的假想硅酸盐片状成分证实,天然云母固体的可靠晶胞公式要求必须考虑到每个化学计量空位。如果将可靠的 "过量 O"(表示为 WO2-)估计值分配给 EMP 分析,就可以确定云母固体溶液中氧云母成分的比例。我们使用已发表的分析结果对这种方法进行了测试,这些分析结果来自分馏的碱性和超基性火山火成岩(氧云母范围为 2.5-45%;TiO2 高达 14%;BaO 高达 23%)、上地幔橄榄岩(等值为 7-18%;6%;0.7%)和变质上地幔(2-37%;9%;23%)中富含钛和钡的生物岩。云母中Ti和Ba的富集显然与中和取代K+和Mg2+的高电荷Ba2+和Ti4+所需的额外氧电荷有关。
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Ti- and Ba-rich phlogopitic micas in alkaline basic and upper mantle igneous rocks; stoichiometry, stability, and Fe valence estimation reassessed and rationalised
Ti- and Ba-rich tri-octahedral micas occur in fractionated basic igneous rocks, metasomatized mantle peridotites, metamorphosed pelites/carbonates, and hydrothermally altered mineral deposits. Electron microprobe analyses (EMP), with all iron reported as FeO, were widely used in the 1970/80s to interpret Ti and Ba substitution mechanisms, based on 22 O2– unit cell calculations, implying that cation vacancies occur in octahedral and/or intersheet sites. In 1996 EMP with chemical and physical analyses for ferric and total Fe, H2O, (OH), and element-specific Fe X-ray Absorption Spectroscopy (both K and L-edges) established valence states for Fe and Ti and cation site occupancies, that ∼50 % O replaces (OH) molecules, and that 24 anion cell formulae show the absence of cation vacancies. Cell formula calculation protocol for phlogopitic micas is refined here and results tested against the stoichiometric formula for vacancy-free phlogopite, XIIK2VIMg6IV[Si6Al2]O20(OH)4. Hypothetical sheet silicate compositions, calculated with fixed contents of vacancies linked to particular mixed-valence element substitutions, confirm that reliable unit cell formulae for natural mica solids require that each stoichiometric vacancy must be accounted for. If reliable estimates for ‘excess O’ (denoted WO2−) are assigned to EMP analyses, the proportion of the oxy-mica component in a mica solid solution can be defined. This approach is tested using published analyses for Ti- and Ba-rich biotites from fractionated basic and ultramafic volcanic igneous rocks (oxymica range 2.5–45 %; TiO2 up to 14 %; BaO up to 23 %), upper mantle peridotites (equivalent values 7–18 %; 6 %; 0.7 %), and metasomatised upper mantle (2–37 %; 9 %; 23 %). Enrichments of Ti and Ba in micas are clearly linked to the extra oxygen charge required to neutralise the more highly charged Ba2+ and Ti4+ replacing K+ and Mg2+.
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来源期刊
Geochimica et Cosmochimica Acta
Geochimica et Cosmochimica Acta 地学-地球化学与地球物理
CiteScore
9.60
自引率
14.00%
发文量
437
审稿时长
6 months
期刊介绍: Geochimica et Cosmochimica Acta publishes research papers in a wide range of subjects in terrestrial geochemistry, meteoritics, and planetary geochemistry. The scope of the journal includes: 1). Physical chemistry of gases, aqueous solutions, glasses, and crystalline solids 2). Igneous and metamorphic petrology 3). Chemical processes in the atmosphere, hydrosphere, biosphere, and lithosphere of the Earth 4). Organic geochemistry 5). Isotope geochemistry 6). Meteoritics and meteorite impacts 7). Lunar science; and 8). Planetary geochemistry.
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