Uranium–lead, diagenetic processes

It is often possible to decipher the diagenetic history of a sedimentary rock through petrographic and geochemical investigation. Clastic rocks may have cements that formed early, even during initial deposition, or later through compaction and burial and may have even later crosscutting veins resulting from tectonic processes. Whole-rock analyses of such a rock would give a U/Pb result that is a mix of these processes and would not likely give a meaningful U–Pb age. However, if one could separate out generations of cements based on careful petrographic analyses and if these cement generations had not been altered since formation and have high enough U/Pb ratios to have produced measurable radiogenic Pb, it would be possible to date the diagenetic events. While the application of U/Pb to dating diagenetic phases in clastic sedimentary rocks is rare, numerous studies have investigated other geochemical constraints such as fluid inclusions and oxygen isotopes which can be used to place bounds on the temperature and salinity of the fluids. These phases are often quite small, on the order of tens of microns, but as techniques such as laser ablation MC-ICP-MS (Multi-Collector Inductively Coupled Plasma Mass Spectrometry) and SIMS (Secondary Ion Mass Spectrometry) continue to be developed, it is likely that U–Pb dating of these phases will offer important insights that have otherwise had to be inferred as to the process (es) of clastic diagenesis. Imagine the potential for understanding the geologic processes a rock has experienced if, in addition to temperature and salinity, one could also date the generations of cements that are recognized in a paragenetic study. This is a frontier in the Earth Sciences. The major difference between clastic and chemical sedimentary rocks is that the initial constituents for clastic rocks have an inherited history, while newly formed chemical constituents in chemical sedimentary rocks, if unaltered, will give the age of deposition if they have high enough U/Pb ratios to be dated. After deposition, these chemical constituents also experience diagenesis to eventually become a rock, with unstable phases dissolving or being replaced and various cement phases and crosscutting veins influencing the U–Pb system in a similar manner as we discussed for clastic sedimentary rocks in the preceding paragraph. That is to say, in order to become a rock, we can imagine that all sediments have experienced multiple diagenetic realms with fluids that have different U/Pb which may dissolve some phases and precipitate others perhaps further down the path of fluid-rock interaction. These changes will alter the U–Pb systematics making it impossible to obtain a meaningful age through whole-rock analyses. However, if individual phases can be sampled, and this is increasingly being used particularly in carbonate rocks, a meaningful age of the new phases may be obtained which will allows geologists to date the timing of that event, not the rock. Through geologic time, a rock is likely to have been exposed to multiple diagenetic fluids resulting in dissolution and precipitation of various minerals as well as solid-state exchange with others and thus producing a complex array of isotope and element ratios. It is in this framework that diagenesis of U–Pb must be considered. Sampling for U–Pb dating should target phases that can be recognized petrographically and can be put in a paragenetic framework. In this way, the system becomes even more powerful with the potential not only to date the time of deposition but also to date diagenetic events that postdate deposition.