Radiometric dating of sedimentary rocks: the application of diagenetic xenotime geochronology
Radiometric dating of sedimentary rocks: the application of diagenetic xenotime geochronology
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DOI：
10.1016/j.earscirev.2004.05.004
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发表时间：
2005
影响因子：
12.1
通讯作者：
B. Rasmussen
中科院分区：
文献类型：
--
作者：
B. Rasmussen
地质年代学领域的最新进展使人们对地质过程的规模和持续时间有了更深入的了解。目前可以通过各种辐射测量方法对火成岩和变质岩进行测定，精确到一百万年之内，但确定沉积岩的沉积年龄仍然极其困难。这个问题对于前寒武纪岩石最为明显，其中生物的多样性和丰度较低，阻碍了建立任何有意义的生物地层框架来关联地层。此外，大多数前寒武纪序列已经发生变质，使得原始矿物和纹理难以解释，并重置成岩矿物。磷钇矿 (YPO4) 是一种同位素稳定的计时器，越来越多地被认为是硅质碎屑沉积岩中的微量成分。它可能在早期成岩作用期间开始生长，通常在碎屑锆石颗粒上形成结构产物。成岩磷钇矿出现在多种岩石类型中，包括砾岩、砂岩、粉砂岩、页岩、磷矿和火山碎屑岩，年龄从早太古代到中生代不等。成岩磷钇矿的形成主要与铁羟基氧化物的氧化还原循环和有机质的微生物分解有关，导致沉积物孔隙水中溶解的磷酸盐和稀土元素（REE）浓度升高。磷钇矿具有理想的 U-Pb 天文计时器的特性，其中含有较高浓度的 U（通常 >1000 ppm）和极低浓度的初始普通 Pb。此外，它具有在后期热事件期间保持接近元素迁移率的特殊能力，并且通常会产生一致且精确的日期。由于成岩磷钇矿晶体尺寸较小且结构复杂，需要空间分辨率<10μm的原位同位素技术才能成功测定磷钇矿的年代；迄今为止，这仅通过离子微探针实现。在变质沉积岩中，成岩磷钇矿保留了其年龄信息，直至砂岩中的下角闪岩相，以及泥质岩中的中上绿片岩相。在许多前寒武纪盆地（例如南非威特沃特斯兰德盆地），成岩磷钇矿中生长着与埋藏成岩作用、接触变质作用、热液蚀变或区域变质作用相关的化学成分不同、结构较年轻的磷钇矿。借助岩相学、地球化学微量分析和具有精细空间分辨率的同位素技术，可以利用磷钇矿来确定早期成岩作用的年代，并可能确定影响沉积盆地的每一个主要流体和热事件。
Recent advances in the field of geochronology have led to a greater understanding of the scale and duration of geological processes. It is currently possible to date igneous and metamorphic rocks by a variety of radiometric methods to within a million years, but establishing the depositional age of sedimentary rocks has remained exceedingly difficult. The problem is most pronounced for Precambrian rocks, where the low diversity and abundance of organisms have prevented the establishment of any meaningful biostratigraphic framework for correlating strata. Also, most Precambrian successions have been metamorphosed, rendering original minerals and textures difficult to interpret, and resetting diagenetic minerals. Xenotime (YPO4) is an isotopically robust chronometer, which is increasingly being recognized as a trace constituent in siliciclastic sedimentary rocks. It may start to grow during early diagenesis, typically forming syntaxial outgrowths on detrital zircon grains. Diagenetic xenotime occurs in a wide variety of rock types, including conglomerate, sandstone, siltstone, shale, phosphorite and volcaniclastic rocks, varying from early Archaean to Mesozoic in age. The formation of diagenetic xenotime is principally related to redox cycling of Fe-oxyhydroxides and microbial decomposition of organic matter, leading to elevated concentrations of dissolved phosphate and rare earth elements (REE) in sediment pore-waters. Xenotime has the properties of an ideal U–Pb chronometer, containing elevated levels of U (generally >1000 ppm) and very low concentrations of initial common Pb. In addition, it has an exceptional ability to remain closed to element mobility during later thermal events, and commonly yields concordant and precise dates. Because of the small size of diagenetic xenotime crystals and common textural complexities, an in situ isotopic technique with a spatial resolution of <10 μm is required to successfully date xenotime; to date, this has only been achieved by ion microprobe. In metamorphosed sedimentary rocks, diagenetic xenotime retains its age information up to lower amphibolite facies in sandstone, and up to mid-upper greenschist facies in pelitic rocks. In many Precambrian basins (e.g., Witwatersrand Basin, South Africa), diagenetic xenotime is overgrown by chemically distinct and texturally younger xenotime related to burial diagenesis, contact metamorphism, hydrothermal alteration or regional metamorphism. With the aid of petrography, geochemical microanalysis and the use of isotopic techniques with fine spatial resolution, it may be possible to use xenotime to date early diagenesis, and potentially every major fluid and thermal event to have affected a depositional basin.