Reviews of Geophysics - 2009 - Rasbury - Directly dating geologic events U‐Pb dating of carbonates

Directly Dating Geologic Events

U-Pb Dating of Carbonates
- Researchers: E. Troy Rasbury and Jennifer M. Cole
- Received: September 2007; Accepted: January 2009; Published: July 2009
- Carbonates form in diverse settings: caves, soils, oceans, during burial from hydrothermal fluids, and along faults.
- U-Pb dating of carbonates is effective across the geologic time scale.
- Pleistocene speleothems show potential for dating events < 1 million years old.

Carbonates create in various environments, from soils to deep burial due to hydrothermal fluids.
Direct dating of carbonates can pinpoint conditions responsible for their precipitation, especially for Pleistocene age samples constrained by U-Pb dating.
Marine carbonates (aragonite fossils) have provided U-Pb ages that constrain tectonic events (e.g., Caribbean uplift).
Marine carbonates may alter to more stable minerals (low Mg calcite or dolomite) over time; careful sampling can reveal ages of original deposits and subsequent alteration timing.

Meteoric carbonate materials (e.g., speleothems, tufas) are particularly promising for U-Pb dating due to their stability and favorable U/Pb signatures when free from detritus.
Direct dating shows potential for revealing geological events such as the creation of the Grand Canyon and dating vertebrate fossils in caves.
Direct dating of meteoric carbonates provides insights into unconformities significant for sequence stratigraphy.

Carbonates can also form from deeper fluid flows associated with faulting and ore formation.
Fault zones can reveal multiple generations of calcite and preserved fault motion structures.
U-Pb dating still largely unutilized for fault-related carbonates presents vast potential for dating fluid flows and associated geological activity.

Key questions involve what exactly is being dated in complex carbonate systems, such as the time of formation or alteration of deposits.
U-Pb dating relies on initial homogeneity of daughter isotopes, the half-life of parent isotopes, and closed system behavior.
Employing two decay schemes (238U-206Pb and 235U-207Pb) allows cross-verification of age data from carbonate samples.

The formula for U-Pb dating includes both uranium isotopes' decay principles, affecting determinations based on relative ratios of parent to daughter isotopes.
Precise analytical protocols described, including challenges posed by the decay constants used and discrepancies noted in age calculations.

Initial Pb isotopic composition may not be uniform across environments; variations can yield misleading ages.
External environmental factors strongly influence concentrations and isotopic behavior, complicating the dating of carbonates.

Systems that experience significant diagenetic alteration can produce ages that reflect either younger or older apparent ages based on fluid interaction.
Presentation techniques include 3-D concordia diagram plotting for derivation of consolidated ages without making strong assumptions about initial conditions.

Marine fossils like corals present promising opportunities for U-Pb dating due to their relation to their times of deposition.
Stability concerns are key; fossils often convert from aragonite to calcite, complicating U-Pb dating efforts.

Meteoric carbonate deposits from land environments provide precise U-Pb ages for understanding Earth's climate and geological events.

Field sampling should target carbonates tied closely to geological events for accurate dating.
Thin section petrographic analysis assists in recognizing diagenetic alteration and potential for precise dating.
High variable U-Pb ratios are essential; various techniques can identify U concentrations effectively.

U-Pb dating shows significant promise across different geological contexts, encompassing events from the Quaternary to ancient formations.
Synthesis of methods, improved analytical techniques, and tighter selection criteria could enhance the understanding of carbonate formation and its timing.