isochron

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isochron

[′ī·sə‚krän]
(geochemistry)
A line on a graph defined by data for rocks of the same age with the same initial lead isotopic composition, the slope of which is proportional to the age. Also known as geochron.
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Our data from various layers of Bilong Co oil shale do not define isochrons, indicating one or some of the four conditions determining accurate and precise depositional ages for shales with the [sup.
North Dakota, yielded a Re-Os isochron age of 354 [+ or -] 49 Ma ([lambda] = 1.
The Re-Os isochron calculated by ISOPLOT [16] is shown in Fig.
The RATE group shows large discordances in isochron estimates of the age of rocks and minerals to be normative and as large as factors of two or three in some cases, much larger than the 15% Isaac stated in his review.
The isochron methodology and abundant data are reported, for example, by Dalrymple,(9) who cites more than 250 measurements of terrestrial, lunar, and meteoric rocks with excellent concordance.
1984) reported a 934 [+ or -] 82 Rb/Sr isochron from granitic gneiss.
The claim that the earth is approximately 6,000 years old is supported from biblical interpretation and from four areas of scientific studies: helium diffusion in zircons, radiohalos in granites, isochron discordances, and the presence of trace amounts of carbon-14 (C-14) in pre-Cambrian material.
Sm-Nd isochrons have an advantage over Rb-Sr isochrons in that rare-earth elements (REE) like Sm and Nd are relatively immobile during geologic heating events, and, therefore, are unlikely to diffuse into or out of rocks.
To obtain crystallization ages, geochronologists also commonly employ the graphical isochron and concordia methods.
The straight line that is generally obtained is termed a whole-rock isochron.
To understand why the whole-rock isochron method works and why we might obtain a straight line that yields age information, let's try to visualize the major geologic process of magmatic crystallization.
If we could analyze the isotope ratios of several chunks of rock as soon as the magma crystallized and plot the results on an isochron diagram, we would normally obtain a virtually horizontal line (Figure 2).