Introduction to Radioisotope Geochronology/Part 4 - Sample selection and Characterization
This section deals with the start of the process through to the start of the analytical part of geochronology. Sample selection, characterization and processing are some of the most important stages in the process of 'dating a rock', for the derived dates to be meaningful the samples themselves need to be understood...
Get informed - what is possible given current knowledge about techniques and useful minerals. Talk to us about our experiences and what we’d suggest. Maximize on-outcrop documentation at sampling site. Collect samples responsibly. Take measures to ensure cross-contamination is reduced to nil. Sample at a scale appropriate for the job. Use your geological imagination.
Consider lithological context, use everything in your geological toolbox. Igneous? Single vs multiphase - paragenetic sequence, crosscutting and contact relationships, alteration state? Major element chemistry: peraluminous vs. calc-alkaline = zircon - monazite - xenotime vs. zircon - titanite - allanite? Tectonic setting? Carbonatites?? Highly alkaline rocks?? Pegmatites??? Issues with high U??? Mafic? - Baddeleyite/zircon - late mafic pegmatite phases, melt pods. Contacts - incorporation of country rocks, xenocrysts, beneficial reactions at contacts e.g. titanite? Volcanic? - magma mixing, zircon saturation issues, proximal vs distal facies, crystal lithic tuffs vs. air-fall tuffs, alteration/clays?
Metamorphic? Grade and implications on minerals - reaction rims, primary vs metamorphic minerals, internal zonation, recrystallisation. Para- vs. orthogneisses, or indeterminate? What about melt pods and leucosomes, e.g. Himalayan leucogranites. Sedimentary? Heavy mineral bands, representative (or not) sampling of source regions, geochron +/- tracers? Ore deposits: typical minerals +/- opportunities provided by unusual mineralogy? Structures: Primary vs deformation-related minerals, effects of fluid flow on minerals, etc, cross-cut lithologies, melt rocks.
Assessing scale of heterogeneity