Fluid assisted mineral replacement processes

流体存在下的矿物置换反应在许多岩石中是一种广泛存在的现象。我们选择了金红石(TiO2)+流体(Ca~(2+)AQ+Si4+AQ)=钛铁矿(CaTiSiO_5)的反应，既有自然的例子，也有实验的方法。我们的目标是解释在这种条件下生长的钛铁矿的微量元素特征和分带模式，以及它在地质温压测量和同位素测定中的应用。在高压岩石的抬升和水化过程中发现了典型的自然例子。尽管该反应是溶解-沉淀反应，但它并不是一个体积守恒反应，形成的是假晶态，而是钛的化学成分基本相同。所研究的钛铁矿反应圈样品显示不同大小的金红石残留物和强烈的富集物，具有典型的HFS微量元素向金红石扩散的轮廓。钛铁矿在铝和锶中呈不规则的带状分布。我们的工作假设是不同的元素迁移率是造成主量元素和微量元素变化的原因。该反应是在400 Mpa/600℃的实验条件下模拟的，其中钛源在穿孔的内囊中物理分离，而Ca和Si(+Al，+微量元素)源在外囊中物理分离。元素在NaC l(±F)溶液中发生迁移。Tisource是天然金红石，水含量很高。反应立即开始，在一天的运行时间后，大量的钛铁矿晶体已经覆盖在金红石表面；反应主要是反应边缘的粗化(运行长达107天)。我们将继续用熔融生长的人工合成金红石(纯的和掺杂HFSE、Cr和其他元素)进行实验，并探索名义上无水的金红石中H对反应的影响。

Mineral replacement reactions in the presence of a fluid phase are a wide spread phenomenon in many rocks. We choose to study the reaction rutile (TiO2) + fluid (Ca2+ aq + Si4+ aq) = titantite (CaTiSiO5) both, in natural examples and by experimental methods. Our goal is to explain the trace element signature and zoning pattern of titanite grown under such conditions, with its applications to geothermobarometry and isotope determination. Typical natural examples are found in high-pressure rocks during uplift and hydration. Although the reaction is a dissolution-precipitation reaction, it is not volume-conserving with the formation of pseudomorphs, but largely isochemical for Ti. The investigated samples of titanite reaction rims show rutile relicts of different size and strong enrichment with a typical diffusion profile of HFS trace elements into rutile. Titanite is irregularly zoned in Al and Sr. Our working hypothesis is that different element mobilities are responsible for the major and trace element variation. The reaction is simulated experimentally at 400 MPa/600°C in a setup, where the Ti-source is physically separated in a perforated inner capsule from the Ca and Si (+Al, +trace elements) source in an outer capsule. Element transport takes place in a NaCl (±F) solution. The Tisource is natural rutile with high ‘water’ contents. The reactions starts immediately with the formation of numerous titanite crystals which cover the rutile surface already after one day run time; reactions proceeds mainly with coarsening of the reaction rim (runs up to 107 days). We will continue our experiments with synthetic (pure and doped with HFSE, Cr and other elements) melt grown rutile and will explore the influence of H in the nominally anhydrous rutile on the reaction.