P ‐ T Uncertainty in Subsolidus Metamorphic Systems: Sources, Patterns and Controls

Phase equilibrium modelling of metamorphic rocks is subject to multiple, superimposed sources of uncertainty that affect pressure–temperature (‐) estimates, making it difficult to independently distinguish and evaluate the contribution of each source. Here, we summarise the principal sources of uncertainty (geological, analytical, parameter and model) and identify those that can be robustly quantified within current modelling frameworks, focusing on uncertainty arising from natural petrological variability and measurement uncertainty in the bulk rock composition and mineral data (compositions and modal proportions). To explore these effects on ‐ estimates under an equilibrium scenario we construct a synthetic system based on the median worldwide pelite composition of Forshaw and Pattison (2023), in which individual sources of uncertainty are isolated and perturbed independently. We perturb either the bulk composition, from which forward models and predicted mineral data are generated through phase equilibrium modelling, the predicted mineral data, or both simultaneously. The grid‐search inversion workflow of Mackay‐Champion and Cawood (2026) is used to quantify the effects of the perturbed datasets on ‐ estimates across contact‐ and regionally metamorphosed subsolidus conditions. Lower‐grade conditions yield nearly twice the ‐ uncertainty of higher‐grade conditions. Uncertainty, presented at 1 , decreases with increasing metamorphic grade: from 25°C to 30°C and 1.2–1.8 kbar at lower‐temperature conditions (400°C–550°C) to 16°C and 0.8 kbar at higher‐temperature conditions (550°C–700°C). Temperature uncertainty is largest (29°C) at lower pressure conditions (1–3.5 kbar) and smallest (16°C) at higher pressure conditions ( 6.5 kbar), whereas pressure uncertainty remains broadly constant (0.9–1.1 kbar) across all pressure conditions (1–12 kbar). The source of uncertainty and choice of input variables strongly influence ‐ estimates, with uncertainty in the bulk composition exerting the dominant control, and inversions using only mineral modal proportion variables typically yielding approximately double the uncertainty of those using only mineral composition variables. These results provide an equilibrium‐defined baseline for ‐ uncertainty arising from uncertainties in mineral data and bulk composition, although tectonometamorphic and kinetic controls may amplify or reduce this uncertainty.