The Lachlan Fold Belt is well known for its complex deformational history, with a number of orogenic events and associated structural styles recognised. Recent syntheses of this orogenic evolution have emphasised the difficulty of identifying fold-belt-wide deformational episodes and have inferred the existence of diachronous deformation zones with an overall merging of episodic orogenic events...

The Moon experienced an intense period of impacts about 4 Gyr ago. This cataclysm is thought to have affected the entire inner Solar System and has been constrained by the radiometric dating of lunar samples: 40Ar–39Ar ages reflect the heating and degassing of target rocks by large basin-forming impacts on the Moon. Radiometric dating of meteorites from Vesta and the H-chondrite parent body als...

Neutron irradiation of samples for 40Ar/39Ar dating in a 235U fission reactor requires error-producing corrections for the argon isotopes created from Ca, K, and, to a lesser extent, Cl. The fission spectrum includes neutrons with energies above 2-3 MeV, which are not optimal for the 39K(n,p)39Ar reaction. These higher-energy neutrons are responsible for the largest recoil displacements, which ...

Precise dating of K-rich minerals in volcanic rocks via the 40Ar/39Ar geochronometer has become crucial to resolving many geological problems. In some cases, ash beds containing biotite phenocrysts but lacking sanidine are the only datable horizons at key stratigraphic intervals, necessitating comparison of sanidine and biotite ages within the same chronostratigraphic framework. To assess the i...

Structural and thermochronological data from the Sulu terrane of eastern China document the exhumation of high-pressure and ultrahigh-pressure (UHP) metamorphic rocks to upper-crustal depths by ca. 206 Ma. 40Ar/39Ar ages from K-bearing phases record recrystallization and cooling through amphiboliteand upper greenschist– facies conditions during top-to-the-NW noncoaxial shear and suggest cooling...

Accurate and precise ages of large silicic eruptions are critical to calibrating the geologic timescale and gauging the tempo of changes in climate, biologic evolution, and magmatic processes throughout Earth history. The conventional approach to dating these eruptive products using the 40Ar/39Ar method is to fuse dozens of individual feldspar crystals. However, dispersion of fusion dates is co...

The deposits of Eocene Lake Gosiute that constitute the Green River Formation of Wyoming contain numerous tuff beds that represent isochronous, correlatable stratigraphic markers. Tuff beds selected for 40Ar/39Ar analysis occur within laminated mudstone, are matrix supported, and lack evidence of reworking. These tuffs contain 2%–15% euhedral phenocrysts of quartz, plagioclase, sanidine, biotit...

The metamorphic facies of the subsurface Precambrian of Alberta are charted for the first time as a contribution to the new tectonometamorphic map of the Canadian Shield. Inferences about facies are based primarily on petrographically determined mineral assemblages. A few P–T determinations from results of electron-microprobe analyses are available. The chronology of metamorphic events is based...