The correlation of these six points to the 1500-million-year isochron is almost as good as the six points forming the 1090-million-year isochron for the Cardenas Lavas (Figure 1), which overturned five K-Ar "model dates." Geologists, however, must reject the alleged "isochron" for the Western Grand Canyon lava flows (Figure 2) because these most recent flows would otherwise appear to be the oldest rocks of the Grand Canyon.
The Rb-Sr ratios in various rock samples from the same lava flow might differ, but all should plot as a straight line on the graph if the lava was isotopically homogeneous (each rock sample had the same initial Sr ratio) and each rock remained closed to gain or loss of rubidium and strontium since crystallization.
In the ideal case, multiple samples from the same lava flow should plot perfectly along a line with slope e Ar give lines whose slopes supposedly reflect the ages of lava flows.
Hart, "Ancient Lithosphere: Its Role in Young Continental Volcanism," Science, 193 (17 Sept.
There were other estimates but the calculations were hotly disputed because they all were obviously flawed by uncertainties in both the initial assumptions and the data.
Popular Dating Methods Both the K-Ar and Rb-Sr methods make use of radioactive decay of a parent isotope to a stable daughter isotope. Leeman, "Late Cenozoic Alkali-Rich Basalt from the Western Grand Canyon Area, Utah and Arizona: Isotopic Composition of Strontium," Geological Society of America Bulletin, 85 (Nov. Six analyses of Uinkaret Plateau basalts (U-33, U-35, U-59, U-24, U-43, U-58) were used.
Potassium-40 (Rb has occurred since the rock cooled from the lava flow. All five analyses by the K-Ar method would appear to limit the age of the Cardenas Lavas to somewhere between 760 and 860 million years, except for the fact that a discordant Rb-Sr isochron was obtained by Mc Kee and Noble of 1090 ± 70 million years. Figure 1 shows the analyses of six samples which plot nicely along a line and seem persuasively to describe an isochron. Such ages defined by these fines are called "isochrons." An "isochron date," which relies on multiple rock samples, is considered to be much more reliable than a "model date" determined from a single rock sample. Isochron confidence is well founded, for the assumptions of the dating method appear to be internally verified. These flows came from volcanoes on the Ulnkaret Plateau north of the Colorado River.