Dating zircon grains
For example, DZ geochronology cannot differentiate different sources with the same crystallization age, the youngest DZ age population may be older than depositional age, large orogenic or metamorphic episodes may not produce much primary zircon, and recycling of zircon grains can lead to incorrect interpretation of syn-depositional versus primary sediment source.Detrital LT thermochronology has additional problems such as syn-depositional age volcanic grains leading to erroneous lag-time interpretation.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.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.Dating of single detrital mineral grains (typically zircon and apatite) using geochronology and low-temperature (LT) thermochronology is now an established tool in the investigation of a range of geologic processes and problems.
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.
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.
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.
Radioisotopic dating of detrital minerals in sedimentary rocks can constrain sediment sources (provenance), elucidate episodes and rates of ancient orogenesis, and give information on paleogeography and sediment-dispersal patterns.