INITIAL RATIOS, INDIVIDUAL MINERALS AND INCONSISTENT AGES

Holmes’ recognition of the potential of radioactivity for geochronology is rightly celebrated. A corollary of dating methods is the recovery of initial isotope ratios (I₀), which can be used to fingerprint sources in magmatic processes. I₀ is dictated by the time-integrated parent/daughter of the source. The development of high precision mass spectrometry from the 1970s resulted in the definition of a plethora of reservoirs in Pb-Sr-Nd-Hf-He I₀ space.

By the 1980s mantle isotope heterogeneity was well established and isotopic fingerprinting added to the toolbox of petrologists working on magma differentiation. Open system differentiation, in which I₀ variation is produced by crustal contamination accompanied by crystallisation, was widely documented. In principle, isotopic fingerprinting is an ideal method for highlighting contamination, since continental crust is typically old and evolved and, consequently, isotopically distinct from the mantle. In practice blurring may occur if sediment is added to mantle sources. Distinguishing I₀ variations inherited from mantle source(s) from those resulting from open system processes subsequent to separation from the mantle can then be difficult.

A potential way to resolve this conundrum is to integrate petrographic and isotopic constraints through in situ isotopic microsampling. We take advantage of the fact that magmas crystallise during differentiation, with crystals acting as recorders of the evolving magmatic environment. Changes in I₀ from core-to-rim of a crystal then indicate that the magma composition changed with time. The nature of I₀ variation and how it correlates with textural features, allow us to infer that processes such as contamination and recharge occurred through time. Investigations of this type have confirmed and quantified the widespread occurrence of open system differentiation.

Ironically perhaps, the mineral-scale isotopic variations that can be used to tease out and quantify differentiation processes raise a potential problem with isochron dating. Interpretation of a mineral isochron requires that minerals at t₀ share the same I₀, which is clearly not always the case. Conversely though, independent age determination allows initial ratio variations to be recovered from old rocks and used for petrogenetic interpretation.