Thus all K-Ar and Ar-Ar "dates" of crustal rocks are questionable, as well as fossil "dates" calibrated by them.
Dalrymple, referring to metamorphism and melting of rocks in the crust, has commented: "If the rock is heated or melted at some later time, then some or all the In a recent study 128 Ar isotopic analyses were obtained from ten profiles across biotite grains in high-grade metamorphic rocks, and apparent Ar-Ar "ages" within individual grains ranged from 161Ma-514Ma.
is known to cause grave problems in regional geochronology studies.
Any alteration or fracturing means that the potassium or the argon or both have been disturbed.
The site also must be geologically meaningful, clearly related to fossil-bearing rocks or other features that need a good date to join the big story.
An Ar-Ar "dating" study of high-grade metamorphic rocks in the Broken Hill region (New South Wales) found widely distributed excess will always be retained in those trapping sites in minerals where it is "held" more tightly.
A viable interpretation of these Broken Hill data was only produced because assumptions were made about the age of the rocks and of a presumed subsequent heating event (based on Pb-Pb and Rb-Sr dating), when it is conjectured that accumulated The six domains are physically distinct units which exhibit wide differences in average physical and chemical properties, as well as structure and tectonic behavior.
But micas, plagioclase, hornblende, clays and other minerals can yield good data, as can whole-rock analyses.
Young rocks have low levels of Ar, so as much as several kilograms may be needed.
represents primordial Ar carried from source areas in the earth's mantle by the parent magmas, is inherited by the resultant volcanic rocks, and thus has no age significance.
However, are all other rocks in the earth's crust also susceptible to "contamination" by excess emanating from the mantle?
Developed in the 1950s, it was important in developing the theory of plate tectonics and in calibrating the geologic time scale.