Uraninite [UO] is an increasingly recognized accessory mineral for geochronological studies of the mid to upper crust. Similar to what is seen for zircon and monazite, the U-Pb system of uraninite can become reset under relatively low temperatures in certain domains via the action of fluids through the process of coupled dissolution-reprecipitation. Whether or not the uraninite geochronometer is reset will be dependent on the chemistry of the fluid it interacts with as opposed to being purely dependent on P-T. This makes uraninite a mineral of interest for the dating of low- to mid-temperature, fluid-controlled geological processes. In order to better understand which factors cause the recrystallization and/or metasomatic alteration of uraninite, a set of 5 metasomatism experiments have been performed in cold seal autoclaves on a hydrothermal line involving a natural uraninite from Příbram, Czech Republic and a series of Na-, Ca-, OH-, and F-bearing fluids at 600 °C and 200 MPa for 21 days. A second set of the same 5 experiments, to which elemental sulfur was added, were subsequently run at 450 °C and 200 MPa for 66 days. Generally, little textural alteration of the starting material was observed in any of the experiments, which was independent of the fluid chemistry and temperature, except for an increase in the apparent porosity of the reacted grains. In the second set of experiments galena formed as small grains in four of the runs, indicating that Pb had migrated out from the uraninite into the solution and reacted with the sulfur to form galena. The excessive depletion of Pb in the metasomatized uraninite to negligible amounts in some of these fluids is especially evident if the solutions were NaF + HO and 2 M NaOH. This suggests that interaction of uraninite with F- or high pH Na-bearing fluids can metasomatically reset the uraninite geochronometer at 450 °C and mid to upper crustal pressures and by analogy to even lower temperatures given sufficient time.

中文翻译：

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交代重置铀矿地质计时仪：含 Na-Ca-F-OH 流体在 450 和 600 °C 和 200 MPa 下的作用

铀矿 [UO] 是一种日益被认可的用于中上地壳地质年代学研究的辅助矿物。与锆石和独居石类似，铀铀矿的 U-Pb 系统可以在某些区域的相对较低温度下通过耦合溶解-再沉淀过程中的流体作用而重置。铀矿地质计时仪是否重置将取决于与其相互作用的流体的化学性质，而不是纯粹取决于 PT。这使得铀矿成为对中低温、流体控制的地质过程进行测年的重要矿物。为了更好地了解哪些因素导致铀铀矿重结晶和/或交代蚀变，在冷封高压釜中的热液线上进行了一组 5 次交代实验，涉及来自捷克共和国普日布拉姆的天然铀铀矿和一系列 Na- 、Ca-、OH- 和 F- 含流体在 600 °C 和 200 MPa 下持续 21 天。第二组同样的 5 次实验，其中添加了元素硫，随后在 450 °C 和 200 MPa 下运行 66 天。一般来说，除了反应颗粒的表观孔隙率增加之外，在任何实验中都观察到起始材料几乎没有结构改变，这与流体化学性质和温度无关。在第二组实验中，方铅矿在四次试验中形成小颗粒，表明铅已从铀矿中迁移到溶液中，并与硫反应形成方铅矿。如果溶液是 NaF + H2O 和 2 M NaOH，则某些流体中交代化铀矿中的 Pb 过度消耗至可忽略不计的量尤其明显。这表明铀矿与含氟或高 pH 值钠流体的相互作用可以在 450 °C 和中上地壳压力下交代重置铀矿地质计时仪，并类比到在足够时间的情况下甚至更低的温度。