Solidification age

The basic idea in radioactive age-dating of rocks (from the Earth, Moon and meteorites) is to find the ratio of daughter to parent in an isolated system. Thus the age inferred is usually the solidification age which is the time since the last occasion when chemical fractionation was halted by solidification. (K-Ar dating gives a gas-retention age which can be slightly shorter.)... 

From the front side we now have exact solidification ages for the rocks of the major maria. The numbers in Table 11 refer to the most recent lava flows detected and represent the time of solidification of the outermost layer. 

Table 11. Solidification ages of the lunar maria as derived from Rb/Sr age determinations (isochron ages) by Wasserburg and coworkers...

Solidification ages establish the time elapsed since the last homogenization of parent and daughter nuclides, normally by rock or mineral crystallization (72, 75). Nuclides used to establish solidification ages are isotopes of non-gaseous elements insensitive to events that might affect gas retention. Some techniques, such as the Pb/Pb method that involves the ultimate decay products of and Th ( ° Pb, Pb and Pb, respectively) depend upon relatively mobile Pb that is more readily redistributed than is the Sm- Nd dating pair (cf, 7, 2,... 

Common techniques found to yield useful solidification ages include the Pb-Pb method Sm (t,/2 = 106 Ga) - Nd "Rb (t,/2 = 48 Ga) - Sr and Re (ti/2 = 41 Ga) Os (cf, 1, 2, II, IS). Generally, methods used to determine solidification ages depend upon data depicted in isochron diagrams, for example, in which enrichment of radiogenic Sr is proportional to the amount of Rb, with Sr being taken for normalization. The slope of such a line yields an "internal isochron" for a meteorite or a single inclusion of a... 

While most meteorites have solidification ages around 4.56 Ga, there is clear evidence of more recent disturbances of chronometric systems -particularly Pb-Pb and Rb-Sr - in many meteorites. For example, Rb-Sr internal isochrons for E chondrites (believed to have experienced open-system thermal metamorphism) were disturbed 4.3-4.45 Ga ago. Of course, chronometers in heavily shocked L chondrites show clear evidence for late disturbance. 

Figure 9. U-Pb ages of phosphates from ordinary chondrites. Numbers on the Concordia line are in Ga. The oldest solidification age (for H chondrites) is --4.563 Ga ago and thermal metamorphism occupied the next 60-70 Ma. (Reproduced with permission from reference 1. Copyright 2006 Elsevier.)...

Das, Chang, Raybould High Performance Magnesium Alloys by Rapid Solidification Processing , Light Metal Age, Dec. 5-8 1986... 

The decay of °Th leads to radioisotopes of other elements, ultimately concluding with the stable isotope lead-206. Happily, some of the oldest rocks on Earth, called zircons, contain no lead when they are formed. This means that the amount of lead they accumulate over time from uranium decay reflects their age. Until the rocks crystallized, uranium atoms could move freely through the molten magma from which they formed, and decayed uranium could be replenished. Solidification of a zircon does for uranium what an organism s death does for radiocarbon it stops the influx of fresh radioactive material, and the decay clock starts ticking. Because of U s long half-life, zircons can be dated back to the Earth s earliest days. 

The aging processes leading to the changes of the properties of samples overtime (such as solidification of liquids, decrease in solubility, increase in evaporation temperature, changes in the composition of the fragments in the mass-spectra, crystallization of amorphous solids) have become wellknown. These processes, very characteristic also for the hydroxides, are caused by aggregation or polymerization and also by condensation of oligomeric molecules. 

For concrete at early ages the most important is the effect of chemical processes on its transport and strength properties, e.g. porosity n=f(Thydr)> intrinsic permeability T=f(Thydr,d), and mass source related to the hydration tn-hydr = f (Thydr) Creep of concrete is modeled by means of the solidification theory [17], where the degree of cement hydration Thy dr is used as the volume fraction of the load-bearing portion of hydrated cement. 

Aging of the gel is called syneresis, which is the process of separating the liquid from the gel to allow further solidification. As the sols become interconnected, the solvent and water from the condensation are pushed outside the pores. The condensation reactions continue as the colloidal sols are brought closer together. Aging determines the average pore size and the resulting density of the... 

Practice Problem 21.13 Calcnlate the age of a sample of rock containing 2.57 X 10 atoms of IgK for every 7.71 X 10 atoms of tsAr, its stable danghter isotope. Assnme that no argon was in the rock when it solidified and that no argon escaped from the rock since its solidification. The half-life of years. ... 

Suppose one holds an igneous rock specimen in which all of the mineral grains crystallized at very nearly the same time, for which the crystallization time was short compared to the period since solidification, and which has not experienced any reheating or alteration since initial crystallization. One could then measure some time-dependent property of the specimen (e.g., the amount of " °Ar accumulated from the decay of " °K) and infer a reasonably well-defined crystallization age for the rock. Unformnately, an analogous set of conditions is quite unlikely to hold rigorously for any sample bottle full of groundwater. As previously emphasized by Davis and Bentley (1982), the dynamic nature of groundwater systems renders such simplistic scenarios unlikely. This review has emphasized that, due to the... 

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