Thus the large majority of historic lava flows that have been studied either give correct ages, as expected, or have quantities of excess radiogenic 40Ar that would be insignificant in all but the youngest rocks.
There are cows and other animals grazing nearby." Gonzalez told Live Science that she and her colleagues will reply to Renne's findings formally in a scientific paper, but said that one possible reason for the large discrepancy in dating may be due to the mixed nature of the volcanic ash. The result will still have the same meaning: the time since all of the samples were isotopically homogenized with respect to each other.
However, that result does not have to be the time since each sample formed.
Their measurements on quartz crytals found in the ash using Optically Stimulated Luminescence (OSL) indicated an age of 39 /- 9 thousand years, but subsequent measurements by a team from Berkeley using the Ar-Ar method yielded an age of 1.3 /- 0.03 million years. (abstract The Cardenas basalt at the bottom of the Grand Canyon was dated by the Rubidium-Strontium method to 1,070 million years (Mc Kee and Noble, 1976), whereas lava which flowed into the canyon after it was cut—and therefore should be much younger—was dated with the same method to 1,340 million years (Austin, 1992).
OSL is a non-radioactive technology to determine the time since certain crystals were last exposed to sunlight or last heated above 400 degrees, and is typically applied to sediments thought to be 300 to 100,000 years. Waters, Joaquin Arroyo-Cabrales, Patricia Ochoa-Castillo, Mario Perez-Campa and Kim B. Whole rock samples from (in the first case) six or (in the second case) four different different lava flows were analyzed, a technique which only gives the age of the flows if they occurred at nearly the same time fed by an isotopically well-mixed source.
They engaged the best laboratories in Britain and Australia, and used multiple dating methods on a range of different samples.5,6 Check this: The mammoth molar was dated at approximately 27,000 years old using ESR; while an organic ball in the gravels was dated at 25,000 years old using C14. He found a significant ( and Rb/Sr ratios (equivalent to an isochron) only for the four samples which showed independent signs of contamination from crust material.
These materials were transported and incorporated into the fluvial sequence of the Barranca Caulapan. Mixing of two reservoirs with different compositions is one of the ways a straight line can be produced in an isochron plot that has no relation to the age of the material.
Thus, it is misleading for Austin to pretend that his resulting isochron plot should be expected to represent the age of the flows themselves.
The isotopic and chemical data are compatible with the model proposed by Best and Brimhall (1974) in which the Grand Canyon basalt flows are derived by variable degrees of partial melting of mantle material over a range in depth and modified by polybaric crystal fractionation during ascent to the surface.
The validity of either hypothesis #2 or #3 would provide additional evidence that Austin's application of the K-Ar method is flawed and that he has failed to prove that the K-Ar method is universally invalid.
A British team reported finding markings in a layer of volcanic ash in a Mexican quarry that they interpreted as human footprints. Bennett, Silvia Gonzalez, David Huddart (2010) Techniques for verifying human footprints: reappraisal of pre-Clovis footprints in Central Mexico Quaternary Science Reviews, Volume 29, Issues 19-20, September 2010, Pages 2571-2578.
In fact, as discussed above, the selection of non-cogenetic samples is sometimes used intentionally by isotope geologists.