Geochronological techniques measure radioactive isotope systems in specific minerals, dating major tectonic events that affected those minerals and therefore source rocks feeding the sedimentary system. As a result detrital geochronology tends to produce several ‘populations’ of similar-aged minerals and these can be interpreted to provide highly diagnostic provenance information. Formations containing consistent populations can also be correlated across a basin.
Our primary geochronological service is detrital zircon U-Pb geochronology. We also offer bespoke dating of other mineral-isotope systems that may provide solutions to specific problems, including geochronological dating of igneous and metamorphic rocks.
Detrital zircon U-Pb geochronology
Detrital zircon U-Pb geochronology is a powerful technique used for studies of sand and sandstone provenance. It has been successfully employed in siliciclastic sediments for mapping reservoirs in the basins, tracing sedimentary pathways, recording denudation histories and dating volcano-magmatic events. This approach identifies characteristic detrital zircon age spectra, compares them with those from other stratigraphic units in the basin and matches them with potential sediment source areas.
Zircon is ubiquitous in a wide variety of crustal rocks and sediments. Its tendency to incorporate radioactive elements U and Th as well as low levels of Pb enables determinations of its crystallisation age to be determining using the U-Pb radioactive decay systems. As zircon is a resilient mineral that can survive through prolonged weathering, sedimentary transport, metamorphism and in some instances even mantle melting, its crystallisation age can be preserved through multiple sedimentary cycles, making zircon an ideal mineral for provenance studies, particularly in reworked sediments.
Chemostrat employ a laser ablation system coupled with an ICPMS to analyse and date zircon crystals.
Detrital zircon CL imaging
Cathodoluminescence (CL) images can be collected from zircon grains using scanning electron microscopy (SEM) with the attached CL detector. This technique allows for selecting locations of analyses spot in order to ensure that data are collected within a homogeneous zircon region. Diagnostic CL zoning patterns, together with other zircon morphological features (e.g. roundness), will also be used to differentiate between igneous, metamorphic and sedimentary zircons. Different zones of a single zircon grains also often preserve different ages reflecting a complex thermal history that they have undergone (e.g. inherited core and metamorphic overgrowth), adding valuable details to geological interpretations.