Ti-Al zoning of experimentally grown titanite in the system CaO-Al2O3-TiO2-SiO2-NaCl-H2O-(F): Evidence for small-scale fluid heterogeneity
Ti-Al zoning of experimentally grown titanite in the system CaO-Al2O3-TiO2-SiO2-NaCl-H2O-(F): Evidence for small-scale fluid heterogeneity
复制标题
CaO-Al2O3-TiO2-SiO2-NaCl-H2O-(F) 系统中实验生长钛矿的 Ti-Al 分带：小尺度流体非均质性的证据
DOI：
10.2138/am.2010.3518
复制
发表时间：
2010
影响因子：
3.1
通讯作者：
R. Wirth
中科院分区：
文献类型：
--
作者：
F. Lucassen;G. Franz;D. Rhede;R. Wirth
根据Al +（OH，F）= Ti + O置换式的化学分带是天然钛铁矿中普遍存在的现象，可用来重建钛铁矿的生长条件。我们合成了钛铁矿，它具有类似于天然钛铁矿中发现的以金红石为代价生长的分区模式，在元素分布图中可以看到不规则的不规则图案。在CaO-Al_2 O_3-TiO_2-SiO_2-NaCl-H_2 O（-CaF_2）体系中的运行条件为600 ℃，0.4GPa，1至107天持续时间。将天然金红石晶体置于多孔Pt管中，置于含有硅灰石、γ-Al 2 O3、任选粉末状CaF 2和NaCl盐水的外部Au胶囊内，以模拟其中金红石通过含水Ca-Si-Al（-F）流体转化为钛铁矿的条件，所述含水Ca-Si-Al（-F）流体由内部胶囊和外部胶囊之间的化学势梯度驱动。钛酸盐的自发成核仅限于内囊中金红石表面上的少量晶体。在长时间运行中，生长从1天后的稀疏孤立的钛酸盐晶体到反应边缘中的几个100 μm大晶体的集合体。钛铁矿在Al-Ti中强烈分区，在含F的实验中显示高达约0.5 Al/分子式单位（pfu）;不含F，取代限于约0.25 Al pfu。Ti-Al交换的范围在短时间内已经很大，并且在长时间运行中观察到相同的异质性。Al-Ti分布（含F和不含F）在成分域（高达几十微米）是不规则的和不规则的。没有观察到分区与时间（核心边缘）或优选的生长方向的关系。在流体介导的生长的情况下，矿物生长分区表示在过饱和条件下的不平衡。考虑到与Al、Ca和Si相比的低溶解度，流体中Ti的浓度可能是钛酸盐生长的限制因素。钛没有被大量运输到外囊中，表明与被运输到内囊中的Al、Ca和Si相反，流动性低。假设缓冲的Al浓度从溶解的Al源，我们推测，在沉淀现场的流体的Ti/Al由本地Ti浓度控制，这在从金红石表面的距离上的生长的钛酸盐晶体，并产生不规则的斑片状分区。
Abstract Chemical zoning according to the substitution Al + (OH,F) = Ti + O is a common phenomenon in natural titanite and can be used to reconstruct its growth conditions. We synthesized titanite, which has a zoning pattern similar to that found in natural titanite that grew at the expense of rutile, visible in element distribution maps as a patchy irregular pattern. Run conditions in the system CaO-Al2O3- TiO2-SiO2-NaCl-H2O(-CaF2) were 600 °C, 0.4 GPa, 1 to 107 days duration. Natural rutile crystals were placed in a perforated Pt-tube within an outer Au-capsule containing wollastonite, γ-Al2O3, optionally powdered CaF2, and a NaCl brine, to simulate conditions, where rutile is transformed into titanite by a hydrous Ca-Si-Al(-F) fluid, driven by a chemical potential gradient between inner and outer capsule. Spontaneous nucleation of titanite is restricted to a small number of crystals on the rutile surface in the inner capsule. Growth proceeds from sparse isolated titanite crystals after 1 day to assemblages of several 100 μm large crystals in a reaction rim in the long-time runs. Titanite is strongly zoned in Al-Ti and shows up to ~0.5 Al per formula unit (pfu) in experiments containing F; without F, substitution is limited to ~0.25 Al pfu. The range of the Ti-Al exchange is already large in short run times and the same heterogeneity is observed in long-time runs. The Al-Ti distribution (with and without F) in compositional domains (up to several tens of micrometers) is patchy and irregular. No relation of the zoning with time (core-rim) or preferred growth directions was observed. In the case of fluid-mediated growth, mineral growth zoning represents a disequilibrium at supersaturated conditions. The concentration of Ti in the fluid is likely the limiting factor for titanite growth considering the low solubility compared to that of Al, Ca, and Si. Titanium is not transported in significant amounts into the outer capsule, indicating low mobility, in contrast to Al, Ca, and Si, which are transported into the inner capsule. Assuming buffered Al concentration from dissolution of the Al-source, we speculate that the Ti/Al of the fluid at the precipitation site was controlled by the local Ti concentration, which varies over the distance from the rutile surface to the growing titanite crystal and produces the irregular patchy zoning.
巴基斯坦喜马拉雅山不同榴辉岩类型的多阶段反应历史及其对折返过程的影响
影响因子：
3.5
作者：
O'Brien;Altenberger;Konrad-Schmolke;Ahmed Khan
通讯作者：
Ahmed Khan