SOME GRAINS REMEMBER. OTHERS WERE CHANGED.
Find the age the rock kept.
Eight minerals were measured. Two were altered after the rock formed. Exclude the two grains that break the shared line, give a geological reason for each exclusion, retain at least six, and report the rock’s crystallization age.
THE EVIDENCE TRAY
Eight mineral separates
The isochron method · science & source
We plot ⁸⁷Rb/⁸⁶Sr horizontally and ⁸⁷Sr/⁸⁶Sr vertically. For coeval closed-system minerals, a shared initial ratio gives a line whose slope is exp(λt) − 1. Here λ = 0.0142 per billion years. The displayed age is ln(1 + slope)/λ. Least-squares fits are computed by a port of the ML.js regress routine, MIT.
These are synthetic teaching cases with exactly two altered grains and equal simplified measurement precision. Real geochronology requires uncertainties, geological evidence, appropriate error-weighted regression, and tests of closed-system assumptions. Excluding points solely to get a better line is not sufficient in real research. The petrographic notes here are contextual clues, not real laboratory evidence.
Source: mljs/regression-simple-linear, commit 1fb3f811 (October 2024), 31 observed stars. Original case generator, diagnostics and game; GPT-generated mineral specimens. License.