The minerals are homogeneous in terms of major elements and show no variation across the sample, but clinopyroxene shows variable degrees of alteration.
Pure, primary mineral separates that are in O isotope equilibrium yield Triassic Sm–Nd isochron ages of 219±11 and 229±13 Ma that match previously published zircon U–Pb ages for peak metamorphism.
These two independent and agreeing dating methods for of the age of two primary members of the solar system formed a strong case for the correctness of his answer within the scientific community.
This just goes to show that just because independent estimates of age seem to agree with each other doesn't mean that they're correct - despite the fact that this particular argument is the very same one used to support the validity of radiometric dating today.
Both the physical geologists and paleontologists could point to evidence that much more time was needed to produce what they saw in the stratigraphic and fossil records.
As one answer to his critics, Kelvin produced a completely independent estimate -- this time for the age of the Sun.
Analysis of detrital sediments has moved towards the analysis of fine-grained, almost authigenic, minerals such as illite, in order to escape the effects of the detrital component.
However, because this is a small number and counting atoms isn’t easy, it is more useful to use the ratio of one isotope to another.
Accurate dates depend on thorough re-setting of isotopic clocks.
In the case of Rb Sr dating of sediments, this rests on the assumption that Sr isotope systematics in the rock were homogenized during deposition or early diagenesis, and thereafter remained as a closed system until the present day.
Allogenic (detrital) minerals are moderately resistant to open-system behaviour during burial metamorphism, but problems arise from inherited isotopic signatures.
Authigenic minerals are deposited directly from seawater and hence display good initial Sr isotope homogeneity.