Rock dating

If you know the half-lives of uranium isotopes and the percentage of lead isotopes in some uranium-bearing rock, you can calculate the date the rock was formed. Rocks dated in this way have been found to be as much as 3.7 billion years old. Samples from the moon have been dated at 4.2 billion years, which is close to the estimated age of our solar system 4.6 billion years. 

Amber can be found in rocks dating back to the Mesozoic era, about 80 million years ago. There undoubtedly were deposits of plant resin before that time, but they have failed to survive to this day. This may be due to the gradual breakdown of the hydrocarbons that constitute most amber, as well as its dehydration after burial. 

Invertebrates are common as fossils, and are found in rocks dating back almost to the beginning of life on Earth. Most animals that survive the fossiliza-tion process do so because their bodies include some hard part(s), such as a shell, exoskeleton, endoskeleton, or teeth. Some completely soft invertebrates, however, have been fossilized as impressions. These include jellyfish, insects, eggs, and larvae. 

Table 7. Argon (40Ar/36Ar)tr ratios taken from a summary of Heymann et al.60) Rock datings from Wasserburg et al.62)...

Each option has attractions and problems. The choice of a round number goes sharply against long-held stratigraphic logic, which firmly maintains any definition should be in the rock. Dating calibrations shift when decay constants are remeasured and can be made more precise such... 

There are few terrestrial materials older than about 3.8 Ga and so our knowledge of the Hadean, the first 750 Ma of Earth history, depends upon a small number of terrestrial samples, supplemented with inferences from the study of meteorites and planetary materials and deductions based upon the distribution of some radiogenic isotopes in the Earth s mantle. In contrast Archaean rocks abound and regions with rocks dated between 2.5 and 3.8 Ga are found in most major continents. A map showing their distribution is given as Fig. 1.4. 

Inferring The oldest rocks dated so far on Earth are about 3.8 billion years old. Do you... 

Current evidence indicates that the excess of oxygenic photosynthesis over respiration and decomposition produced the major fraction of the oxygen of the Earth s atmosphere. [The overall reaction for aerobic respiration and decomposition is the reverse of reaction (3).] Oxygenic photosynthesis has been dated to before 2Ga ago by some distinctive geological formations. One of these formations is the extensive banded iron formations, our chief economic source of iron, found in rocks dated to 2-... 

Niedermann S (2002) Cosmic-ray-produced noble gases in terrestrial rocks Dating tools for surface processes. Rev Mineral Geochem 47 731-784... 

Ar. With its half-life of 269 yr, Ar covers a very important age range between the ranges accessible by H- He, Kr, CFCs and Sp6 (younger than 50 yr) and by (older than about 1 kyr). It also has quite ideal properties, similar to those of Kr, with the exception that subsurface production cannot a priori be neglected and may in some cases lead to deviations of the evolution of the Ar concentration from the pure radioactive decay. In crystalline rocks, dating may even be impossible, but Ar may then be used to study water-rock interactions (Loosli et al. 1989 Loosli et al. 1992). As for Kr, the reason why Ar has not been applied more widely lies in the difficulty of its analysis. 

Terrestrial Rocks Dating Tools for Surface Processes... 

Volcanic rocks dated by the C and Ar- Ar methods have thus provided quite an extensive data set for the He production rate (predominantly determined in olivine separates), along with a few Ne data. In addition, the Cl production rate was determined in basalt lavas from the western USA (Zreda et al. 1991 Stone et al. 1996), in dependence of the K and Ca content. However, to my knowledge there have not been any studies of the °Be and Al production rates in basalt, probably because of experimental difficulties to detect these radionuclides in such material. 

The ages of the youngest volcanic rocks dated by the Ar-Ar technique have now reached the historic realm. For example, Renne et al. (1997) have measured the age of the Vesuvius eruption observed and recorded by Pliny in 79 AD. Other work has focussed upon dating human evolution (Deino et al. 1998 Ludwig and Renne 2000 Renne et al. 

Rocks dating from Early Devonian are made up of sandstones with plant remains resting discordantly on the Silurian deposits. Throughout the Devonian, sedimentation is characterized by a repetition of transgressive/regressive cycles with an alternation of mudstones and sandstones. The reservoirs in Devonian sandstones represent important productive levels in the Illizi Basin. Uplift occurred during the upper Devonian and even part of the upper Silurian in the southern part of the Eastern Erg Basin. 

Graham IJ, Palmer K (1987) New precise Rb-Sr mineral and whole-rock dates for I-type granitoids from Granite Harbor, south Victoria Land, Antarctica. New Zealand Antarctic Record 8(l) 72-80... 

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