This means that a mineral (of age T ∘) has lost a certain percentage of its radiogenic Pb at a time T 1 after its formation (e.g., during metamorphism), after which the system closed again and further accumulation of radiogenic Pb proceeds normally until present. The Concordia diagram is a useful tool for investigating and interpreting disruptions of the U-Pb system caused by ‘episodic lead loss’. įigure 1: ‘Wetherill’ Concordia diagram showing concordant (filled symbols) and discordant (empty symbols) analyses affected by different degrees of Pb (or U) loss. This can be visualised by plotting the 206Pb/ 238U-ratio measurements against the 207Pb/ 235U-measurements on a ‘Wetherill Concordia’ diagram depicted in Figure 1. The simultaneous decay of two isotopes of the same radioactive parent (U) to two isotopes of the same stable daughter (Pb) of the U-Pb method, provides for the U-Pb clock with an internal consistency check that is absent from most other geochronometers. The U-Pb method also yields ages which are accurate. The U-Pb method is the oldest of all the isotopic dating methods, with a natural cross-check built into it that shows when nature has tampered with the evidence. The abundance of uranium minerals in most rock types as well as the resistance of many of these minerals to chemical and physical weathering, contributes to the popularity and prolificacy (richness) of the U-Pb system. Uranium–lead (U-Pb) dating is a geochronological method that uses final decay products in the 238U and 235U radioactive decay chains to determine the length of time required to accumulate present amounts of stable daughter isotopes 206Pb and 207Pb respectively. The measured age is referred to as the “model” age as it relies on two primary assumptions: 1) complete separation of the progeny isotope from the parent at time zero (if the separation is incomplete, the calculated age or “model” age will be older than the true age) and 2) that the system is “closed”, i.e., no loss or gain of the parent or progeny other than through radioactive decay of the parent. Īge determination is an extremely useful nuclear forensic signature as it is non comparative or “predictive”, meaning, it does not need to be compared to other materials in the database. It is used to identify or eliminate possible contributors in the life of the material. Ī nuclear material age is defined as the age of material at the time since the last separation of the progeny isotopes from the radioactive parent (uranium or plutonium), referred to as the production or sometimes separation date. The term “signatures” describes material characteristics such as isotopic abundances, elemental concentrations, physical and chemical forms and physical dimensions that may be used to link a material, either nuclear or other radioactive (non-nuclear, such as those used for medical imaging), to individuals, locations, or processes, date of production and on the intended use. Nuclear forensics requires the ability to determine characteristics or ‘ signatures’ of nuclear and other radioactive materials. Nuclear forensics is defined as the examination of nuclear or other radioactive material, or of evidence that is contaminated with radionuclides, in the context of legal proceedings under international or national law related to nuclear security. Key words: Lead isotopic ratios, uranium ore mineralization, nuclear forensics, geochronology Introduction The studied showed that lead isotopic ratios can provide an estimate of the age of the uranium ore mineralisation and aid as a fool in nuclear forensics. The estimated age of the uranium ore mineralisation was in the range documented for the Namibian Damara Orogen. 206Pb was abundant in the ore samples and was determined to be geogenic and not anthropogenic. Inductively Coupled Plasma Mass Spectrometer was employed for sample analysis. The objective of the study was to determine signatures of Pb from uranium ore mineralisation, identify as to the Pb present is geogenic and/or antrogenic and provided an estimate of the age of the uranium ore mineralisation from measured Pb isotope.Ī total of 24 samples of uranium ore were collected from three mines and the lead isotopic ratios and age estimation signatures of the uranium ore mineralisation determined. The study investigated the use of Pb isotope signatures as an indicative of origin assessment of uranium bearing ore mineralisation from the uranium province in Namibia.
0 Comments
Leave a Reply. |
AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |