Phase equilibria modelling of trace element evolution in arc magmas: implications for petrogenesis and copper...

Trace element ratios in arc magmas are widely used to infer petrogenetic conditions, particularly those associated with porphyry-forming versus barren systems. However, quantitatively linking whole-rock signatures to pressure, water content, or redox conditions remains challenging, as trace elements are sensitive to mineral assemblage and compositions, which both evolve during fractional crystallisation. Here, we integrate a recently updated thermodynamic model suite appropriate for arc systems with dynamic (composition-, temperature-dependent) mineral-melt partitioning to track trace element evolution. Benchmarking the model against published experiments, including apatite saturation, shows that the methodology successfully reproduces phase assemblages and compositions. We simulate fractional crystallisation of an average primitive arc magma across mid- to lower-crustal pressures (4-10 kbar), 2-4 wt% initial H[Formula: see text]O, and a range of redox conditions ([Formula: see text]2 log units [Formula: see text]FMQ). We demonstrate how mineral-specific vectors evolve in trace element ratio (e.g. Sr/Y, Dy/Dy*) and rare-earth element shape coefficient ([Formula: see text]) space. Results highlight that amphibole and garnet may produce overlapping [Formula: see text]-space vectors under deep, hydrous conditions-contrary to orthogonal vectors inferred using static (composition-, temperature-independent) partitioning. Multiple petrogenetic paths can yield similar whole-rock trace element outcomes, particularly with poorly constrained primitive melt compositions. High Sr/Y, for example, commonly associated with porphyry systems, can form without particularly deep, hydrous, or oxidised conditions, when dynamic partitioning behaviour is considered. Overall, our modelling framework enables evaluation of arc magma petrogenesis and trace element evolution, with implications for porphyry indicators.

The online version contains supplementary material available at 10.1007/s00410-026-02297-x.