Fluid assisted mineral replacement processes

流体相存在下的矿物置换反应是许多岩石中广泛存在的现象。我们选择研究反应金红石（TiO 2）+流体（Ca 2 + aq + Si 4 + aq）=钛铁矿（CaTiSiO 5），在自然的例子和实验方法。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，及其在地质温压和同位素测定中的应用。典型的自然例子是在高压岩石中发现的隆起和水合作用。虽然该反应是溶解-沉淀反应，但它不是体积守恒的假晶形成，但很大程度上是Ti的等化学反应。研究结果表明，在钛铁矿反应边的样品中，金红石残留物具有不同的尺寸和强烈的富集作用，并具有典型的HFS微量元素向金红石中的扩散特征。钛铁矿是不规则分区的铝和锶。我们的工作假设是，不同的元素迁移率负责的主要和微量元素的变化。该反应在400 MPa/600°C下在一个装置中进行实验模拟，其中Ti源在穿孔的内囊中与外囊中的Ca和Si（+Al，+微量元素）源物理分离。元素迁移发生在NaCl（±F）溶液中。钛铁矿是天然金红石，具有高“水”含量。反应立即开始，在一天的运行时间后，形成了许多覆盖金红石表面的钛酸盐晶体;反应主要随着反应边缘的粗化而进行（运行长达107天）。我们将继续我们的实验与合成（纯的和掺杂HFSE，铬和其他元素）熔体生长的金红石，并将探讨在名义上无水金红石的反应的影响。

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.