|Sidebar - March 2011|
Here’s why evolutionists believe that Lucy lived about 3.5 million years ago.
The Laetoli footprints and Lucy’s skeleton were both found in the same layer of sedimentary rock. Evolutionists determined the age of this sedimentary layer from the ages of the layers of volcanic ash above and below it using potassium-argon dating.
It was originally believed that all argon escapes from volcanic ash and lava at the time of eruption. Therefore, any argon gas found in the ash must have come from radioactive decay of potassium. The longer the time since the eruption, the more argon gas there would be trapped in the solid ash.
The assumption that all the argon gas escapes at the time of eruption was shown to be false by measuring the amount of argon gas present in ash and lava from modern volcanic eruptions. So, evolutionists attempt to figure out how much argon was in the ash originally, and how much has been produced by radioactive decay since the eruption. The United Stated Geological Survey (USGS) has an excellent description of how this is done.
The conventional K-Ar dating method depends on the assumption that the rocks contained no argon at the time of formation and that all the subsequent radiogenic argon (i.e., 40Ar) was quantitatively retained.
Under some circumstances the requirements for successful K-Ar dating may be violated. For example, if 40Ar is lost by diffusion while the rock cooled, the age-dates represent the time elapsed since the rock cooled sufficiently for diffusive losses to be insignificant. Or if excess 40Ar is present in the rock, the calculated age-dates are too old. The 40Ar/39Ar dating method can overcome these limitations of conventional K-Ar dating, and has the added advantage that potassium and argon are determined on the same sample and that only measurements of the isotopic ratios of argon are required. The method is suitable for use with small and precious samples, such as extraterrestrial materials. 1
Since the assumption that all the argon escaped at the time of eruption is clearly invalid, they have to estimate how much of the argon gas was created by decay of potassium since the eruption, and how much was there before the eruption and did not escape. They do this by comparing isotopes of argon gas. You should read the USGS web page that explains in detail how this comparison of isotopes is done. Here’s the key point.
The main isotopes of argon in terrestrial systems are 40Ar (99.6%), 36Ar (0.337%), and 38Ar (0.063%). Naturally occurring 40K decays to stable 40Ar (11.2%) by electron capture and by positron emission … 2
All argon gas atoms have 18 protons. 36Ar has 18 neutrons for a total mass of 36. 40Ar has 22 neutrons for a total mass of 40. These different isotopes of argon gas have the same electrical charge, but slightly different weights. They can be separated using an atomic mass spectrometer, allowing scientists to determine the percentage of each isotope.
Naturally occurring argon gas in the atmosphere is 99.6% 40Ar and 0.337% 36Ar, which is a ratio of 295.55 to 1. Therefore, one might be led to believe that if the ratio of 40Ar to 36Ar is more than 295.55, the extra 40Ar came from decay of potassium since the eruption.
The obvious flaw in this reasoning is that there is no reason to believe that the ratio of isotopes of argon gas inside a volcano has anything to do with the ratio of isotopes of argon gas in the atmosphere. If one believes that the Earth is billions of years old, there have been billions of years for lots of 40K to decay to 40Ar. The ratio of 40Ar to 36Ar inside the Earth could be much more than 295.55 to 1.
Isotopic ratios aren’t as consistent as one might believe.
Just as the weight listed on your driver’s license doesn’t necessarily reflect your actual poundage, the official atomic weights of most chemical elements are actually more like ballpark estimates than precise constants. In acknowledgment of this natural variation, the official weights of 10 chemical elements will no longer be expressed as single numbers, but as ranges. The adjustments, published online December 12  in Pure and Applied Chemistry, are the first in an overhaul of the atomic weight of almost every element on the periodic table.
Instead of being described by a single fuzzy number, the atomic weights of oxygen, hydrogen, lithium, boron, carbon, nitrogen, silicon, sulfur, chlorine and thallium will now be expressed as intervals. The change, long overdue, explicitly acknowledges the fact that most of the 118 elements come in multiple forms of varying weight.
Previously, a given element’s official atomic weight was actually an average of this variation. But as the number of discovered isotopes grew — there are more than 2,000, but only 118 elements — weights kept needing adjustment. These numerical tweaks implied that the numbers couldn’t be pinned down with precision, when in fact such measurements are more precise than ever, says Coplen, who headed the international task force charged with surveying various isotope abundances in nature so that the numbers could be revised. 3
Since one can’t know for certain the ratio of the various isotopes of argon gas inside the Earth, one can’t legitimately compare the ratio of isotopes to determine how much argon gas has been produced by decay of potassium since the eruption, making the potassium-argon method of determining age invalid.
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3 Ehrenberg, Science News, January 29, 2011, “Periodic table gets some flex”, page 5, http://www.sciencenews.org/view/generic/id/67938/title/Periodic_table_gets_some_flex