Environment Counts | Reevaluation of a fundamental constant for estimating geological chronology
Author: Geoff Zeiss – Published At: 2012-06-26 11:06 – (14800 Reads)
The uranium-lead (U-Pb) system is widely used as an isotopic chronometer for geological and meteoritic materials that range between one million to greater than 4.5 billion years old. The U238-Pb206 and U235-Pb207 decay sequences provide a built-in crosscheck that allows the accurate determination of the age of geological materials. Daughter isotope determinations from the two decay systems may also be combined to calculate a Pb207-Pb206 date using an assumed or measured present-day U238/U235 ratio. Recent cosmochronology studies have identified the need for coupled U238-U235 and Pb207-Pb206 data sets in order to determine accurate Pb207-Pb206 dates. This study reports U238/U235 determinations on 58 samples of U-bearing minerals that are used for U-Pb geochronology. Adoption of a new average U238/U235 zircon value decreases Pb207-Pb206, Pb207-U235, and Pb206-U238 dates relative to those calculated using the conventional U238/U235 value. For Pb207-Pb206 dates the difference is largest and is about 1 million years. Science 30 Mar 2012
Radioactive isotope dating (radiometric dating) was invented in 1905 by Ernest Rutherford. It is based on a comparison between the observed abundance of a naturally occurring radioactive isotope and its decay products. It is the principal source of information about the absolute age of rocks, including the age of the Earth itself. Together with stratigraphic dating, radiometric dating methods are used in geochronology to establish the geological time scale. Among the best-known techniques are radiocarbon dating, potassium-argon dating and uranium-lead dating.
In a material containing a radioactive isotope, the proportion of the original isotope to its decay product changes in a predictable way as the original nuclide decays over time and this allows the relative abundances of related nuclides to be used as a clock to measure the time from the incorporation of the original nuclides into a material to the present.
A collection of atoms of a radioactive isotope decays exponentially at a rate determined by its half-life in years. After one half-life has elapsed, half of the atoms of the parent isotope will have decayed into a daughter nuclide.
The mathematical expression that relates radioactive decay to geologic time is
D = D0 + N(t) (e**Î»t – 1)
|t||age of the sample|
|D||number of atoms of the daughter isotope in the sample|
|D0||number of atoms of the daughter isotope in the original composition|
|N||number of atoms of the parent isotope in the sample at the present time t|
|N(t)||= No * e**-Î»t|
|Î»||decay constant of the parent isotope,|
|Î»||= ln 2 / half-life of parent|
Naturally occurring uranium is composed primarily of two major isotopes, uranium-238 (99.27% natural abundance), uranium-235 (0.73%).
The uranium-lead (U-Pb) system is widely used as an isotopic chronometer for geological and meteoritic materials that are less than 1 million to greater than 4.5 billion years old. This system is particularly useful because the two long-lived isotopes, U238 and U235, decays at different rates to Pb206 and Pb207, respectively.
|U238 -> Pb206||4.5 billion years|
|U235 -> Pb207||710 million years|
Together these uranium isotope decay sequences provide a built-in crosscheck that allows the accurate determination of the age of the sample. Daughter isotope determinations from the two decay systems may also be combined to calculate a Pb207-Pb206 date using an assumed or measured present-day U238/U235 ratio.
The uranium-lead radiometric dating scheme has been refined to the point that the error margin in dates of rocks can can exceed 0.1%, or less than two million years in two-and-a-half billion years.
Uranium-lead dating is often performed on the mineral zircon (ZrSiO4). Zircon incorporates uranium atoms into its crystalline structure as substitutes for zirconium, but strongly rejects lead. It has a very high closure temperature, is resistant to mechanical weathering and is very chemically inert. Zircon also forms multiple crystal layers during metamorphic events, which each may record an isotopic age of the event.
Recent cosmochronology studies have highlighted the need for coupled U238-U235 and Pb207-Pb206 data sets in order to determine accurate Pb207-Pb206 dates. Thus, it is crucial to reevaluate the range of natural variation of U238/U235 ratios in U bearing minerals commonly analyzed for U-Pb age determinations.
The present-day U238/U235 ratio has fundamental implications for uranium-lead geochronology and cosmochronology. Until recently, the present-day U238/U235 ratio of all natural materials was considered invariant. In geo- and cosmochronology, a U238/U235 value equal to 137.88 has been used almost exclusively for the past 35 years and is based on studies of magmatic and sedimentary uranium ore deposits.
The authors performed 141 U238/U235 determinations on 58 samples of U-bearing minerals that are used for U-Pb geochronology (zircon, monazite, apatite, titanite, uraninite, xenotime, and baddeleyite), spanning the Quaternary to the Eoarchean and covering a diverse range of igneous and metamorphic petrogenetic settings and geographic locations. The study determined that a mean U238/U235 value of 137.818 +/-0.045 (2Ïƒ) in zircon samples reflects the average uranium isotopic composition and variability of terrestrial zircon. This distribution is broadly representative of the average crustal and â€œbulk Earthâ€ U238/U235 composition. The authors propose that this average zircon value is applicable for the majority of U-Pb determinations and, in the absence of an independently determined U238/U235 value, should be adopted for future use in U-Pb geochronology of zircon.
Adoption of the average U238/U235 zircon value of 137.818 for use in zircon geochronology will decrease Pb207-Pb206, Pb207-U235, and Pb206-U238 dates relative to those calculated using the conventional U238/U235 value of 137.88. For Pb207-Pb206 dates, the U238/U235 ratio is implicit in the age equation and the magnitude of the difference is largest, changing gradually from ~1 million years for samples dated 100 million years ago (Ma) to ~700,000 years for samples dated 4 billion years ago (Ga).