U-Pb Dating of Carbonates

研究表明，碳酸盐岩的238U-206Pb等时线与沉积年龄一致，尽管在寄主碳酸盐岩中有成岩作用的证据。碳酸盐中观测到的同位素比值的分布足以给出古生代+ 1 Ma的潜在年龄分辨率；然而，迄今为止的研究已经达到了20个误差极限，不超过大约+ 10 Ma，这是由于等时线（“误差时线”）的散射超出了可归因于分析误差的范围。本研究的目的是通过研究下侏罗统原始和接近原始的低镁方解石化石以及二叠纪礁杂岩中保存完好的海相和成岩碳酸盐相的U-Pb系统来了解这种分散的原因。成岩作用和可能的微量相（氧化铁、磷矿物、有机物）的重要性将通过使用微采样方法、部分浸出和适当时监测各种同位素和化学示踪剂来评估。更好地了解控制碳酸盐238U/204Pb比值初始变化以及随后改变U-Pb同位素系统的过程，将导致更可靠和精确的碳酸盐238U- 206pb和207Pb-206Pb等时线。碳酸盐岩的直接测年可以直接应用于地球科学中的许多问题，包括显生宙时间尺度的标定、前寒武纪沉积物的测年和对比以及成岩作用的测年。

Studies have demonstrated that 238U-206Pb isochrons derived from carbonate rocks yield dates consistent with depositional ages despite evidence of diagenesis in the host carbonates. The spread of observed isotope ratios in carbonates is sufficient to give a potential age resolution of + 1 Ma for the Paleozoic; however, studies to date have achieved 2o error limits of no better than roughly + 10 Ma due to scatter about the isochron ('errorchron') beyond what is attributable to analytical error. The aim of this study is to gain an understanding of the causes of this scatter by studying U-Pb systematics in pristine and near-pristine low- Mg calcite fossils from the Lower Jurassic and in a number of well-preserved marine and diagenetic carbonate phases from the Permian Reef Complex. The importance of diagenesis and possible trace phases (Fe oxides, phosphate minerals, organic matter) will be assessed through the use of microsampling methods, partial leaching and, as applicable, the monitoring of a variety of isotopic and chemical tracers. A better understanding of the processes that control l the initial variability of carbonate 238U/204Pb ratios and that subsequently alter U-Pb isotopic systems will lead to more reliable and precise carbonate 238U-206Pb and 207Pb-206Pb isochrons. The direct dating of carbonates has immediate application to a number of problems in the earth sciences, including the calibration of the Phanerozoic time scale, the dating and correlation of Precambrian sediments, and the dating of diagenesis.