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美国地质调查所的科学家最近对火山岩进行的研究揭示了在喷发、冷却、水化和蚀变过程中铀的释放效应。在可与附近已蚀变的岩石作比较的相对未蚀变玻质火山岩地区进行野外研究是特别有价值的。已报道了下列情况:①采自火山灰流或熔岩流的黑曜岩与珍珠岩的对比,或黑曜岩与霏细岩的对比;②玻质空降火山灰与来自凝灰质沉积物中蚀变产物的蒙脱石、高岭石或斜发沸石的对比;③来自活火山的干燥而新鲜的火山灰与被水漂洗的火山灰的对比。实验室方法包括淋滤(浸出)研究,含铀再生二氧化硅的铀铅年代测定法,铀衰变系列测定以及放射性照相。其结果表明,新喷发的火山灰吸附的水溶铀和玻璃水化期间迁移的铀量并不多。霏细岩一般比共存的黑曜岩贫铀,表明部分铀在高温脱玻期间迁移了,但是增加一些年轻黑曜岩-霏细岩组对评价长期不均匀淋滤的可能叠加是需要的。空降火山灰蚀变成粘土和沸石使玻质主岩释放出大量铀,但是铀的最终迁移很大程度上取决于水文条件、岩石的透水性、溶液的化学性质和吸附剂的丰度等。在合适的环境中,90％的原生铀可能迁移。浸出实验表明,温度对铀从玻璃释放出的速度影响最大。实验室研究应包括高温下的富铀玻璃的脱玻作用和蚀变实验,同时,还需做更多的野外研究。
A recent study by the US Geological Survey scientists on volcanic rocks revealed the release of uranium during eruptions, cooling, hydration and alteration. Field studies in relatively un-altered vitroce volcanic rocks that can be compared to nearby altered rocks are particularly valuable. The following has been reported: (1) the comparison of obsidian with perlite taken from volcanic ash or lava flows, or the contrast between obsidian and feldspathic rock; (2) the correlation between glassy airborne volcanic ash and alteration products from tuffaceous sediments Of montmorillonite, kaolinite or clinoptilolite contrast; ③ from the volcano’s dry and fresh volcanic ash and water rinse pozzolana contrast. Laboratory methods include leaching (leaching) studies, uranium-lead dating with uranium-regenerated silica, uranium decay series and radiography. The results show that the amount of uranium migrated during newly hydrated uranium adsorbed by volcanic ash and hydrated during the hydration process is small. In general, feldspar is coexistent with obsidian depleted uranium, indicating that some uranium migrated during the high temperature demineralisation, but adding some young obsidian-felite to the possible assessment of long-term inhomogeneous leaching is needed. Airborne volcanic ash alteration to clay and zeolite causes large amounts of uranium to be released from the host rocks, but the ultimate migration of uranium depends largely on hydrological conditions, the water permeability of the rock, the chemical nature of the solution and the abundance of the adsorbent. In a suitable environment, 90% of the native uranium may migrate. Leaching experiments show that temperature has the greatest effect on the rate of uranium release from the glass. Laboratory studies should include demineralization and alteration experiments of uranium-rich glass at high temperatures, as well as more field studies.