Uranium lead dating meteorites
Uranium lead dating meteorites - online dating italian black
Second, zircons are durable and chemically inert, able to resist chemical weathering and even high-grade metamorphism up to about 900 °C.
The problem is that sediment is made up of clasts of some parent rock, and when we date these clasts, we are in effect dating the parent rock rather than the the sediment as such.The zircon will only start acquiring its xenotime crust after weathering and erosion have freed it from its parent rock and it becomes sediment.So by dating the xenotime crust, we can find out when the zircon it's growing on became sediment; dating the zircon itself would tell you the age of the parent rock.First of all, uranium will readily substitute for the zirconium (Zr) in the mineral, whereas lead is strongly rejected.For this reason we expect zircons, when formed, to contain some uranium, but virtually no lead.Now lead and uranium are particularly susceptible to such shuffling in the event of even mild metamorphism.
The other problem is that uranium is particularly susceptible to weathering.
Uranium can and often does substitute for the element yttrium, whereas lead cannot, making xenotime suitable for radiometric dating.
The key fact about xenotime is that since it has the same crystal structure as zircon, it can grow on zircon crystals, forming a crust; and this process, of course, cannot begin to take place while the zircon crystal is still locked inside its parent rock.
Although it is not abundant in igneous rocks, it is sufficiently common to be used for the purposes of radiometric dating.
It has two properties which make it useful for this purpose.
Planetary scientists maintain that they should, for reasons which are somewhat beyond the scope of this textbook.