It is easy to imagine diffusion in liquid phase as ink spreading in water. Solid-state diffusion of Pb is the net exchange of Pb in the solid mineral with the external environment, which is usually a fluid.
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In most of the cases, Pb is transported from the mineral to the fluid, resulting in Pb loss and thus age resetting. However, as the mineral cools and the crystal structure becomes more complete, the diffusions of parent and daughter isotopes slows down and finally become insignificant at a certain temperature.
Once the temperature falls below Tc, the system is closed and the clock starts counting. A Reaction ceased due to recrystallisation of precipitating phase dark orange.
B Reaction ceased due to change in reaction system blue. Unlike solid-state diffusion, fluid-assisted dissolution-precipitation occurs below Tc. Interaction between mineral phase and coexisting fluid phase during geological events directly contributes to this process.
It is a chemical reaction driven by the system stabilisation from minimising Gibbs free energy. If a geological process gives a suitable fluid and temperature, monazite dissolves along the contact with the fluid reaction frontand reprecipitates as an altered monazite with a new chemical composition. The rates of the dissolution and reprecipitation are the same, so that the original mineral phase is always in contact with the precipitating phase, separated by only a thin layer of fluid as a reaction medium.
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The reaction front migrates towards the centre of the parent monazite, leaving behind the newly formed monazite, forming a core-rim structure. The composition of the precipitating phase depends on the fluid composition and temperature.
During most of the reactions, Pb is efficiently removed and the precipitating phase is Pb-free. There are basically two factors causing the reaction to cease.
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A Reaction ceases due to the recrystallisation of precipitating phase, removing all the fluid infiltration paths. This results in fluid inclusion in monazite.
B Reaction ceases due to change in system such as composition of fluid and monazite, making this reaction no longer reactive. Yet as reaction proceeds, dissolving phase and the fluid are separated by the solid precipitating phase, blocking the transport of reactants. Therefore, there must be some inter-connected porosity in the precipitating phase, which allows the fluid to infiltrate and fuel the reaction front.
Once they are subject to a temperature higher than Tc, all age information will be reset, losing information of the past geological events.
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In contrast, since monazite has a high Tc, even it experiences younger high-grade metamorphism with high temperatures, it is likely that the previous geological history is preserved. Furthermore, dissolution-precipitation is usually triggered by geological events such as metamorphismdeformation and hydrothermal alternation below Tc. Each of these events icpms fors a new age information by precipitating a new domain without erasing the older information.
Therefore, it is likely that monazite preserves a complete history of generations. However, they behave differently throughout the geological history. Zircon is not as reactive as monazite during metamorphism reaction and better in recording igneous events cooling ages. A single monazite grain can contain domains of distinctively different compositions and ages.
These domains are widely accepted to represent episodes in geological history during monazite growth or recrystallisation. The age of the event is thus represented by the domain age.
The ideal formula of monazite is [LREE PO4 ], the variation in composition is mainly due to the chemical substitutions of light rare earth elements REE in monazite by other elements. Since all three minerals share the same chemical structure, they are the three endmembers in their solid solutionmeaning that they https://imlibuk.info/taxonomy/term/15/6135.php in a same solid phase where substitutions happen.
It is important to note that the composition zonation pattern may not be the same when we are considering different elements. And age zonation may have no relationship with composition zonation at all. In natural monazite, the zonation pattern maybe complex and hard to interpret. Below we describe some simple chemical zonation patterns and the associated interpretations.
Zonation patterns associated with igneous activity are usually easy to interpret. However, those associated with metamorphism are more complicated. Intensity of colour represents concentration of certain element. Edited after Williams,  One of the monazite formations is crystallization of an igneous melt. The concentric zoning pattern reflects the changing composition of the melt which affect the crystallising composition of monazite. However, some elements may have a tendency to crystallise onto a specific crystal face.
It results in uneven growth and composition around monazite. Other zoning patterns[ edit ] Dating approaches[ edit ] Isotopic dating and chemical dating are the two typical dating approaches used in monazite geochronology. Both methods make use of the radioactive nature of.