Methods

Apatite and zircon based on LA-ICP-MS

The use of Laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) is a modern alternative to the traditional external detector method (EDM) for determining uranium concentrations in apatite fission-track (AFT) dating

The general procedure involves:

Sample preparation: Apatite grains are polished and etched reveal spontaneous fission tracks that can be counted using a microscope.
Track counting: The density of spontaneous fission tracks is determined on the polished surface of mineral grains. The location of each grain is mapped using a computer-controlled microscope stage.
Laser ablation: The same individual grains are placed into the LA-ICP-MS. A laser ablates a microscopic pit, and the aerosolized material is carried into an ICP-MS, which measures the concentration of U-238.
Data processing: The spontaneous track counts and the laser-measured U-238 concentrations are used to calculate the fission-track age for each grain.

Advantages of LA-ICP-MS for FT analysis

Eliminates reactor access: The method avoids the logistical and safety issues associated with neutron irradiation in a nuclear reactor.
Faster turnaround time: Skipping the irradiation step significantly reduces the time required for sample processing.
Enables “double dating” as well as the determination: LA-ICP-MS allows for simultaneous measurement of trace elements, including Pb isotopes for apatite/zircon U-Pb dating on the same mineral grains. This provides complementary thermochronological data.
Multi-element analysis: It provides trace-elemental data (e.g., Rare Earth Elements, Cl, Mn) from the same apatite grain. This information is valuable for provenance studies and for understanding the annealing behavior of the apatite, which is influenced by its chemical composition.

Zircon U-Pb

Interpreting U-Pb zircon data LA-ICP-MS involves analyzing isotopic ratios to determine geological ages. The process requires careful data reduction, plotting on concordia diagrams, and considering potential sources of error like lead loss.

Overview of the U-Pb zircon dating system

The U-Pb dating method is a radiometric technique based on the decay of two uranium isotopes into lead isotopes (²³⁸U/²⁰⁶Pb; ²³⁵U/²⁰⁷Pb and ²³²Th/²⁰⁸Pb). Zircon is an ideal mineral for this dating because it incorporates uranium into its crystal structure during formation, allowing it to calculate the time since the crystal formed.

Data reduction and correction

Raw isotopic signals collected by the ICP-MS must undergo several corrections before age calculation can begin:

Standard bracketing: The raw data from unknown samples is corrected using values from a known-age zircon standard, like FC-1, BB or Plešovice, analyzed during the same session. This corrects for instrumental drift and mass bias.
Common lead correction: Zircons can contain small amounts of non-radiogenic, or “common,” lead. Since the LA-ICP-MS signal for common lead’s marker isotope, can be compromised by isobaric interference from corrections often assume a model of common lead evolution or exclude data with high common lead contamination.
Elemental fractionation correction: The LA-ICP-MS process can cause different elements to be transported from the sample to the mass spectrometer at different rates, leading to time-dependent changes in U/Pb ratios. These effects are corrected for using the known-age standard.
Software analysis: Specialized software is used to apply these corrections and calculate the final isotopic ratios and ages.

Concordant data: Data points that fall on the concordia curve are “concordant,” meaning the two U-Pb decay systems yield the same age. This represents a robust, primary crystallization age.

Discordant data: Data points that plot off the concordia curve are “discordant.” This usually indicates that the U-Pb system was disturbed sometime after crystallization, most commonly by lead loss. A line connecting discordant data points is called a discordia line.

Upper intercept age: The intersection of a discordia line with the concordia curve at an older age provides an estimate for the original crystallization age of the zircon.
Lower intercept age: The intersection of the discordia line with the concordia curve at a younger age may represent the time of a later thermal event (e.g., metamorphism) that caused lead loss.

Interpreting age populations (for detrital samples)

For sediments (detrital zircons), the age data represents a population of zircons from various source rocks.

Age populations: Plots of many zircon analyses (e.g., as histograms or probability density plots) will reveal distinct age populations. Each population corresponds to a different source rock or region from which the zircons were eroded.
Maximum depositional age: The age of the youngest coherent population of detrital zircons provides the maximum possible age for the deposition of the sedimentary rock.

Apatite U-Pb

Data reduction and common Pb correction are crucial and complex step that uses specialized software like IsoplotR:

Time-resolved signal: The raw data plots the signal intensity of each isotope over the ablation time. This is used to identify and filter out analyses that hit inclusions.
Common Pb correction: The most significant challenge is correcting for initial common lead. Several methods exist:

Tera-Wasserburg Concordia plot: The uncorrected analyses plot on a discordia line, and the intersection with the concordia curve provides the age and initial Pb isotopic composition. After correction, the software calculates the ages, which are interpreted geologically

Some Interpretation and applications

Intermediate-temperature thermochronology: Due to its closure temperature, apatite U-Pb dating is ideal for understanding the mid-to-upper crustal thermal history, such as uplift and exhumation events.
Thermo-tectonic history: Combining apatite U-Pb data with other thermochronometers, like zircon U-Pb (higher closure temperature) or apatite fission track (lower closure temperature), provides a detailed, multi-stage thermal history of rocks.
Igneous crystallization age: In mafic intrusions that cool rapidly, apatite U-Pb ages can approximate the crystallization age.
Provenance studies: Detrital apatite U-Pb ages can be used to determine the source of sediments.
Fluid-mediated alteration: Apatite’s chemistry can be affected by fluid interaction, so integrating trace element chemistry with U-Pb data can help identify fluid alteration versus simple diffusive loss of Pb

Apatite and Zircon (U-Th)/He

The Apatite and Zircon (U-Th)/He geological dating methods are low-temperature thermochronometers used to determine the time elapsed since a mineral cooled below a specific temperature range, known as its closure temperature (Tc). This method is crucial for studying the cooling and exhumation history of the upper Earth’s crust (typically the upper few kilometers).

The dating method is based on the radioactive decay of Uranium (U) and Thorium (Th) isotopes present as trace elements within the crystal lattice of minerals like apatite and zircon.

Production of Helium (He): The decay series of ²³⁸U, ²³⁵U and ²³²Th involves the emission of alpha particles, which are simply the nuclei of ⁴ Over geological time, these particles capture electrons to form stable He atoms, which accumulate within the crystal. The He production is directly proportional to the amount of parent isotopes (U and Th and the elapsed time.

Helium Loss by Diffusion: At high temperatures, the He atoms can move freely through the crystal lattice and escape (diffuse out). The crystal is an open system for He.

Helium Retention (closure temperature): As the rock is brought closer to the surface (e.g., through uplift and erosion), it cools. When the temperature drops below the closure temperature (Tc), the rate of He diffusion becomes negligible, and He begins to accumulate effectively within the crystal structure. The crystal becomes a closed system for He.

Apatite’s low Tc makes it highly sensitive to the low-temperature thermal history near the surface.

Zircon’s higher Tc records cooling that occurred at greater depths or earlier in the history of the rock.

The general steps for both methods are similar:

Crystal Selection: Single, high-quality, euhedral, and inclusion-free crystals are carefully selected under a microscope. Crystal size and geometry are measured to perform an alpha-ejection correction.
Helium Measurement: The crystal is sealed in a container (often a Pt or Nb capsule) and heated using a laser to a very high to release the accumulated ⁴He. The amount of ⁴He is measured using a noble gas mass spectrometer.
Parent Isotope Measurement: After ⁴He extraction, the degassed crystal is dissolved in strong acids. The concentrations of the parent isotopes (U and Th) and eventually ¹⁴⁷Sm are measured using an ICP-MS.
Age Calculation and Correction: The raw date is calculated using the decay equation, and then a critical alpha-ejection correction is applied.

Some Interpretation and applications

The (U-Th)/He methods are powerful tools in low-temperature thermochronology and are used in various fields:

Tectonics and Exhumation: Quantifying the rate and timing of rock uplift and erosion in mountain belts and rift systems.
Geomorphology: Analyzing landscape evolution, river incision rates, and the history of surface processes.
Sedimentary Basin Analysis: Determining the thermal history of sediments and constraining the provenance (source) of detrital grains.

Mineral Separation

Mineral separation involves a multi-step procedure that exploits the physical and chemical properties of minerals. A typical workflow for isolating heavy minerals like zircon from a granite sample includes:

Crushing and grinding: A whole-rock sample, often weighing several kilograms, is first reduced to smaller pieces using a jaw crusher to liberates individual mineral grains from the host rock.
Sieving (sizing): The crushed material is passed through a series of sieves to sort the mineral grains by size. This is important for preparing the sample for later separation stages and helps focus on grain sizes appropriate for analysis.
Gravity separation: The sieved material is processed using techniques that separate minerals based on their density.

- Sample washing: This procedure allows to sort heavy mineral grains from lighter, less dense ones.

Heavy liquids: This is a definitive method that uses dense, such as bromoformium or lithium metatungstate (LMT) and Diiodomethane to float lighter minerals and sink heavier ones.
Magnetic separation: A Frantz isodynamic separator is used to separate minerals based on their magnetic susceptibility.