U pb age dating
A literature survey using the keywords ‘detrital’, ‘zircon’, and ‘provenance’ indicates that the proliferation of detrital zircon studies has followed an exponential trend, with the number of publications doubling roughly every five years over the past two decades.At present, nearly a thousand detrital zircon publications appear each year.
They indicate that liquid water was present on the surface of our planet early on in its history.Zircon U-Pb dating is the most commonly used method for isotopic geochronology.However, it has been a difficult issue when relating zircon U-Pb ages to metamorphic conditions in complex metamorphic rocks.In those cases, the inherited component (aka ‘common Pb’) needs to be quantified, which is done by normalising to non-radiogenic U) based on different isotopes of the same parent-daughter pair (i.e. This built-in redundancy provides a powerful internal quality check which makes the method arguably the most robust and reliable dating technique in the geological toolbox.The initial Pb composition can either be determined by analysing the Pb composition of a U-poor mineral (e.g., galena or feldspar) or by applying the isochron method to samples with different U and Th concentrations.Such losses cause the U-ratios which yield the same ages (t) against one another, they form a so-called ‘concordia’ curve.
The concordia diagram is a very useful tool for investigating and interpreting disruptions of the U-Pb system caused by ‘episodic lead loss’.
It is an effective way to relate zircon growth to certain P-T conditions by studying the trace element partitioning between coexisting metamorphic zircon and garnet in high-grade metamorphic rocks containing both zircon and garnet.
Primary mineral inclusions in zircon can also provide unambiguous constraints on its formation conditions.
Using modern microprobe technology (SIMS and LA-ICP-MS, see Chapter 3.1), it is quite easy to date, say, a hundred grains of zircon in a matter of just a few hours.
Due to the robustness of zircons as a tracer of sedimentary provenance, and the relative ease of dating them, the use of detrital zircon U-Pb geochronology has truly exploded in recent years.
For those minerals, the non-radiogenic Pb can be safely neglected (at least for relatively young ages), so that we can assume that .