Session 1: Introduction to Isotope Geochemistry

By the end of this session, participants should be able to:

Distinguish between stable and radiogenic isotopes and explain their geochemical significance.
Describe the principles of mass-dependent fractionation and express isotopic compositions using δ and ε notation.
Identify common mass interferences encountered during isotope analysis (e.g., isobars, oxides, hydrides).
Understand the concept of interference corrections and their role in accurate isotope ratio determination.
Appreciate how isotopic data provide insights into geological, biological, and environmental processes, from magma evolution to paleoclimate reconstruction.

Definition of isotopes: same element, different mass.
Overview of nuclear stability and decay pathways.
Stable isotopes: fractionation in natural systems (e.g., H, C, O, N, S, B, Ca, Sr).
Radiogenic isotopes: isotopic evolution through radioactive decay (U-Pb, Rb-Sr, Sm-Nd, Lu-Hf systems).
Examples:
- δ¹⁸O in carbonates → paleotemperature proxies.
- ⁸⁷Sr/⁸⁶Sr → tracing continental weathering and seawater evolution.
- U-Pb → absolute dating of minerals.

Mass-dependent fractionation: lighter isotopes react faster and diffuse more readily.
Rayleigh fractionation and equilibrium vs kinetic effects.
Isotope notation:
- δ = [(R_sample / R_standard) – 1] × 1000 ‰
- Common reference materials (VSMOW, VPDB, AIR, NBS 987, etc.)
ε notation and its relevance to high-precision radiogenic systems (e.g., εNd, εHf).
Examples:
- δ¹³C variations in organic vs inorganic carbon.
- εHf–εNd correlation in mantle and crustal sources.
Simple exercises: interpreting δ and ε values from real datasets.

Definition: overlapping ion masses from polyatomic or isobaric species.
Examples:
- ⁸⁷Rb on ⁸⁷Sr, ⁴⁰Ar²⁺ on ⁸⁰Se, ¹⁴N¹⁶O⁺ on ³⁰Si.
- ¹⁷⁶Yb + ¹⁷⁶Lu on ¹⁷⁶Hf.
Strategies for mitigation:
- High-resolution sector field separation (e.g., Element 2).
- Chemical separation (e.g., Sr, Nd, Pb columns).
- On-line MS/MS interference removal (e.g., Agilent 8800).
- Mathematical corrections using interference equations.
Discussion: implications for accuracy in isotope ratio measurements.

Tracing sources and processes:
- Mantle vs crustal signatures (Sr-Nd-Hf isotopes).
- Hydrothermal and diagenetic overprinting in carbonates.
Quantifying timescales:
- U-Pb dating for crystallisation and diagenesis.
- Rb-Sr and Sm-Nd systems for metamorphic and magmatic processes.
Environmental and biological applications:
- Stable isotopes in shells, bones, and fossils as records of paleoclimate and seawater chemistry.
Integration with trace elements: linking isotope ratios to elemental behaviour during melting, crystallisation, or alteration.
Case study examples from:
- Orthoceras fossils (Silurian seawater signatures).
- Pre-salt carbonates (South Atlantic rift timing).
- Pegmatite genesis and isotopic fingerprinting.