When hearing that the isotopic ratio for uranium has been measured to a more accurate value of 137.818, from a previous value of 137.88, one might not think it is such a big deal. However, a recent study indicates that this small bit of change calls for an age reduction of understood geological processes for up to 700,000 years. The study was conducted at the British Geological Survey and MIT - the same research group that determined the new value.
Radioactive dating is used to determine the age of rocks that were formed on massive time scales. In Uranium-Lead dating, the type discussed in this study, scientists measure the relative amount of U238 and U235 isotopes present in a sample, and then calculate the age.
"Minerals typically grow from a magma, which contains a small amount of uranium," Joe Hiess, a Marie Curie ITN Experienced Researcher at the British Geological Survey, wrote in an email interview to The News-Letter.
Using zircon as an example: as the mineral cools, it sequesters certain amounts of different elements within its crystal lattice, including trace amounts of uranium. "The minerals are initially dissolved into solutions using strong acids and then chemically purified in clean laboratories using resins and various reagents before being introduced into the mass spectrometer for analysis" Hiess explained.
The spectrometer allows scientists to determine what type and how many atoms are present in the mineral sample. Subsequently, the relative amount of U238 and U235 detected through the spectrometer is used to determine the ratio. Hiess's study, for example, uses this technique.
For the past 35 years, the value 137.88 has been used to calculate the age of rocks using the Uranium-Lead dating method. However, Hiess and his team of scientists discovered many flaws.
"Firstly, the consensus value of 137.88 couldn't be traced back to international standard units like the kilogram. Secondly, the old consensus ratio had no uncertainty assigned to it, [and] thirdly, the previous measurements were made on materials like uranium ores - and not on naturally occurring minerals that are routinely used for U-Pb dating studies," Hiess wrote.
The result of these issues was an alarming mixture of a number that was difficult to calibrate, contained an inaccurate assurance of its invariance, and an additional amount of uncertainty based on questionable sampling methods.
Hiess' new number 137.818, with an uncertainty of ? 0.045, promises to provide more accurate dates. "Several research areas will specifically benefit from the increased confidence to U-Pb age dating, such as timescale calibration, the dating of geologically 'instantaneous' events like meteorite impacts, and perhaps most importantly, in refinements to the uranium decay constants using closed system minerals" Hiess wrote.
Though more accurate dates are a plus, geologists must now greatly reconsider many geological processes that we thought were fixed, including the age of planet.
