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.

Introduction to Carbonates

• 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 Versus Marine Carbonates

• 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.

Formation with Hydrothermal Fluids

• 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.

Theoretical Basis of U-Pb Geochronology

• 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.

Age Calculation and Decay Constants

• 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.

Challenges of Initial Conditions

• 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.

Disturbed Systems and U-Pb Data Presentation

• 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.

U-Pb Data from Various Realms

1. Marine Realm

• 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.

2. Meteoric Realm

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

Recommendations for U-Pb Carbonate Dating Protocols

• 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.

Conclusion

• 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.