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U-Pb on garnet, monazite, rutile and sphene, Ar-Ar on hornblende and biotite. blocking temperature for the Glen Dessary syenite from Scotland.The mineral ages define a cooling curve for the pluton. 23THE Ar-Ar METHODWe would expect the outer portions of a crystal to lose Ar more readily, but the centers retain it longer.21Temperature-time plot showing the cooling history of the Quottoon Pluton, B. 22Temperature-time history for the Valhalla Complex, B. Thus, the rim will give a younger age than the core, and the average age of the crystal will be erroneously young.2740Ar-39Ar age versus cumulative 39Ar released for mica and amphibole samples from a blueschist.Heating started at 550C and continued in 50C steps. 28Schematic 40Ar-39Ar age spectra (A) an undisturbed sample yielding a plateau across the entire gas release (B) a sample showing slight disturbance (C) a disturbed sample yielding no plateau (D) a reset sample yielding a plateau corresponding to the age of overprinting (E) a saddle-shaped spectrum reflecting the presence of excess 40Ar.
If the entire sample was closed to Ar loss throughout its history, the age calculated at each increment would be the same. 29THE Rb-Sr METHOD 31Rb-Sr isochron diagram for a series of meteorites formed at the same time 32Schematic Rb-Sr isochron diagram illustrating how the isochron evolves as a function of time.This plot shows Ar loss from the rim, but a plateau at 185 M. M1 and M2 are cogenetic minerals and R1 and R2 are cogenetic rocks, all with different initial Rb/Sr ratios.33Sr ISOTOPES AS TRACERS OF ROCK ORIGIN 34The evolution of 87Sr/86Sr with time in the continental crust and mantle.35Sr ISOTOPES AS TRACERS OF ROCK ORIGIN 40CALCULATING T-CHURThis model age is very sensitive to the difference in the denominator.Only rocks with Sm/Nd ratios significantly different from CHUR will yield precise ages.41The evolution of Nd isotopes with time in the mantle, the continental crust and the bulk Earth (CHUR).