Lake Turkana has a geologic history that favored the preservation of fossils. Scientists suggest that the lake as it appears today has only been around for the past , years. The current environment around Lake Turkana is very dry. Over the course of time, though, the area has seen many changes. Over time the sediment solidified into rock.
The Cretaceous-Tertiary boundary in the geological age scale was associated with an iridium-rich layer which suggested that the layer was caused by an impact with an extraterrestrial object.
Because that time period, commonly referred to as the K-T boundary, was associated with the extinction of vast numbers of animals in the fossil record, much effort was devoted to dating it with potassium-argon and other methods of geochronology.
The time of 65 million years was associated with the K-T boundary from these studies.
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Other large impact craters such as the Manson crater in Iowa dated to 74 My were examined carefully as candidates for the cause of the extinction, but none were close to the critical time. Chicxulub was not so obvious as a candidate because much of the evidence for it was under the sea.
More attention was directed to the Yucatan location after published work by Alan Hildebrand in demonstrated the chemical similarity of Chicxulub core samples with material found distributed in the K-T boundary layer.
Carl Swisher organized a team to produce three independent measurements of the age of intact glass beads from the C-1 core drill site in the Chicxulub impact area.
The measurements were done by the argon-argon method. Even this extraordinary matching with the age of the K-T boundary was insufficient to convince many geologists.
The team proceeded to date spherules of glass found in Haiti to provide another bit of evidence. Many pieces of glass ejecta had been found on Haiti, which is over a thousand miles from the impact point currently.
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But geologists project a much smaller distance between the points at the time of the impact because of measured sea floor expansion. The Haitian spherules were measured to have age to melting of A third piece of evidence came from age measurements of shocked zircon crystals which were found in the K-T layer as far away as Colorado and Saskatchewan.
Zircon has sometimes produced puzzles in radiometric dating because its melting temperature is so high that the crystals sometimes survive in hot melted minerals, giving different melt dates than the other minerals surrounding them. But in this case the nature of zircon was an advantage. The shocked crystals were partially melted, and when measured by the uranium-lead method method gave two ages, 65 My and My.
Potassium-Argon datinghas the advantage that the argon is an inert gas that does not react chemically and would not be expected to be included in the solidification of a rock, so any found inside a rock is very likely the result of radioactive decay of potassium.
Since the crustal basement in the Yucatan area was known to have an age in the neighborhood of the older age, this gave some confirmation to the Chicxulub crater as the origin of the K-T boundary layer. According to Frankel, this was the step that had most geologists convinced by that this impact was the source of the iridium-rich K-T boundary deposit and the extinction of the dinosaurs.
The common potassium-argon dating process makes use of the decay of 40 K to 40 Ar, even though much more of the 40 K decays to 40 Ca. The reason is that 40 Ca is common in minerals, and sorting out what fraction of that calcium came from potassium decay is not practical.
Following the standard approach for decays by multiple pathwaysthe expression for the age from the radiogenic 40 Ca can be written Using non-radiogenic 42 Ca for comparison, the equation for an isochron can be developed.
The slope of the isochron line gives a measure of the radiometric age. Potassium-Argon Dating Potassium-Argon dating has the advantage that the argon is an inert gas that does not react chemically and would not be expected to be included in the solidification of a rock, so any found inside a rock is very likely the result of radioactive decay of potassium.
Argon-Argon Dating and the Chicxulub Impact In the early s there was an intense controversy about the association of the Chicxulub Crater of the Mexican Yucatan Peninsula with the extinction of the dinosaurs in the period about 65 million years ago.
Clocks in the Rocks. Index Frankel, "The End of the Dinosaurs".
Potassium-Calcium Isochrons The common potassium-argon dating process makes use of the decay of 40 K to 40 Ar, even though much more of the 40 K decays to 40 Ca.
Heating causes the crystal structure of the mineral or minerals to degrade, and, as the sample melts, trapped gases are released. The gas may include atmospheric gases, such as carbon dioxide, water, nitrogen, and argon, and radiogenic gases, like argon and helium, generated from regular radioactive decay over geologic time.
This method is commonly called "argon-argon dating." The physical procedure for 40ArAr dating is the same except for three differences: Before the mineral sample is put in the vacuum oven, it is irradiated along with samples of standard materials by a neutron source. There is no 38Ar spike needed. The field of archeology often uses carbon isotopes, which are much more common, but the field of paleontology often uses a potassium- argon dating technique because it can be used to date much older rock material. Over time, the unstable potassium isotope (40 K) from the rocks decays into a stable isotope of argon (40 Ar). Potassium-argon dating, method of determining the time of origin of rocks by measuring the ratio of radioactive argon to radioactive potassium in the rock. This dating method is based upon the decay of radioactive potassium to radioactive argon in minerals and rocks; potassium also decays to .
The J factor relates to the fluence of the neutron bombardment during the irradiation process; a denser flow of neutron particles will convert more atoms of 39 K to 39 Ar than a less dense one.
However, in a metamorphic rock that has not exceeded its closure temperature the age likely dates the crystallization of the mineral. Thus, a granite containing all three minerals will record three different "ages" of emplacement as it cools down through these closure temperatures.
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Thus, although a crystallization age is not recorded, the information is still useful in constructing the thermal history of the rock. Dating minerals may provide age information on a rock, but assumptions must be made.
Minerals usually only record the last time they cooled down below the closure temperature, and this may not represent all of the events which the rock has undergone, and may not match the age of intrusion.
Thus, discretion and interpretation of age dating is essential. This technique allows the errors involved in K-Ar dating to be checked.
Argon dating of rocks
Argon-argon dating has the advantage of not requiring determinations of potassium. Modern methods of analysis allow individual regions of crystals to be investigated.
This method is important as it allows crystals forming and cooling during different events to be identified.