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

## TL;DR

This paper models how trace elements in volcanic magmas evolve, showing that traditional assumptions about their origins may be misleading.

## Contribution

The study introduces a dynamic thermodynamic model to track trace element evolution in arc magmas, revealing new insights into petrogenesis and porphyry indicators.

## Key findings

- Amphibole and garnet can produce overlapping trace element patterns under deep, hydrous conditions.
- High Sr/Y ratios in magmas can form without deep, hydrous, or oxidized conditions when dynamic partitioning is considered.
- Multiple petrogenetic paths can lead to similar whole-rock trace element signatures, especially with uncertain primitive melt compositions.

## Abstract

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\documentclass[12pt]{minimal}
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\begin{document}$$\lambda $$\end{document}-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.