Environment Counts  Reevaluation of a fundamental constant for estimating geological chronology
Author: Geoff Zeiss – Published At: 20120626 11:06 – (14800 Reads)
The uraniumlead (UPb) 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 U238Pb206 and U235Pb207 decay sequences provide a builtin 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 Pb207Pb206 date using an assumed or measured presentday U238/U235 ratio. Recent cosmochronology studies have identified the need for coupled U238U235 and Pb207Pb206 data sets in order to determine accurate Pb207Pb206 dates. This study reports U238/U235 determinations on 58 samples of Ubearing minerals that are used for UPb geochronology. Adoption of a new average U238/U235 zircon value decreases Pb207Pb206, Pb207U235, and Pb206U238 dates relative to those calculated using the conventional U238/U235 value. For Pb207Pb206 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 bestknown techniques are radiocarbon dating, potassiumargon dating and uraniumlead 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 halflife in years. After one halflife 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)
where
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 / halflife of parent 
Uraniumlead dating
Naturally occurring uranium is composed primarily of two major isotopes, uranium238 (99.27% natural abundance), uranium235 (0.73%).
The uraniumlead (UPb) 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 longlived isotopes, U238 and U235, decays at different rates to Pb206 and Pb207, respectively.
Isotopes  Halflife 
U238 > Pb206  4.5 billion years 
U235 > Pb207  710 million years 
Together these uranium isotope decay sequences provide a builtin 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 Pb207Pb206 date using an assumed or measured presentday U238/U235 ratio.
The uraniumlead 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 twoandahalf billion years.
Uraniumlead 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 U238U235 and Pb207Pb206 data sets in order to determine accurate Pb207Pb206 dates. Thus, it is crucial to reevaluate the range of natural variation of U238/U235 ratios in U bearing minerals commonly analyzed for UPb age determinations.
The presentday U238/U235 ratio has fundamental implications for uraniumlead geochronology and cosmochronology. Until recently, the presentday 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 Ubearing minerals that are used for UPb 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 UPb determinations and, in the absence of an independently determined U238/U235 value, should be adopted for future use in UPb geochronology of zircon.
Adoption of the average U238/U235 zircon value of 137.818 for use in zircon geochronology will decrease Pb207Pb206, Pb207U235, and Pb206U238 dates relative to those calculated using the conventional U238/U235 value of 137.88. For Pb207Pb206 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).