U th dating on calcite
In the present study, 18 aragonite and calcite corals were collected from several elevated coral reef terraces off the coast, south of the city of Aqaba.While aragonite corals were dated with the conventional closed system age equation (assuming zero initial Th), the dating of the calcite corals required the development of adequate equations to allow the calculation of both the initial formation age of the aragonite corals and the time of recrystallization to calcite.The two age calculations were based on the assumptions that each reef terrace went through a single and rapid recrystallization event and that the pristine aragonite corals were characterized by a rather uniform initial U concentration, typical for pristine modern corals.Two recrystallization events were identified at 104 ± 6 ka and 124 ± 8 ka.During the alpha decay steps, the zircon crystal experiences radiation damage, associated with each alpha decay.This damage is most concentrated around the parent isotope (U and Th), expelling the daughter isotope (Pb) from its original position in the zircon lattice.Apparently, sea level of the GOA did not fluctuate a lot during the period between ∼130 ka and ∼104 ka and remained close to the Marine Isotopic stage (MIS) 5e highstand.The availability of freshwater (during the sapropel periods) and limited sea level fluctuations facilitated the recrystallization of the GOA reef corals to calcite.
and most refined of the radiometric dating schemes.
Zircon is very chemically inert and resistant to mechanical weathering—a mixed blessing for geochronologists, as zones or even whole crystals can survive melting of their parent rock with their original uranium-lead age intact.
Zircon crystals with prolonged and complex histories can thus contain zones of dramatically different ages (usually, with the oldest and youngest zones forming the core and rim, respectively, of the crystal), and thus are said to demonstrate inherited characteristics.
Unraveling such complications (which, depending on their maximum lead-retention temperature, can also exist within other minerals) generally requires in situ micro-beam analysis via, say, ion microprobe (SIMS) or laser ICP-MS.
The question of when the first humans arrived in the New World has been a bone of contention for several decades.