EQUILIBRIUM THERMODYNAMIC MODELLING OF ARCLOGITES: ARE WE THERE YET? AN EXAMPLE FROM THE ANDEAN NORTHERN VOLCANIC ZONE (NVZ)

The NVZ in the Andes is a continental magmatic arc emplaced in >50 km-thick continental crust. In S Colombia, a Pleistocene eruption exhumed a variety of (ultra)mafic xenoliths derived from lower-crust and mantle, providing a unique opportunity to study petrologic processes operating in the roots of the Andean arc. Here, we focus on garnet-clinopyroxenite (aka ‘arclogite’) xenoliths, which are lithologies expected to drive density instabilities in arcs. These rocks consist predominantly of garnet (30-57%) and clinopyroxene (20-66%), with amphibole as an additional primary phase (10-47%) in some cases.

Phase equilibria modeling of pyroxenites using free-energy minimization has been shown to be troublesome, but more recent optimizations of thermodynamic databases are yet to be tested. We used Perple_X and recent thermodynamic databases to model pyroxenite phase equilibria and compare the results with traditional thermobarometry. For amphibole-free xenoliths, we constructed P-T diagrams with garnet compositional isopleths to obtain P-T conditions. For amphibole-bearing xenoliths, we created isobaric T-X(H₂O) diagrams using previous P estimates, to evaluate the predicted stability of amphibole as a function of water content. For all samples, diagrams were first created assuming all iron as ferrous (FeO^T) from XRF data. Ferric iron was gravimetrically determined in the samples using potentiometric titration, and phase diagrams were recalculated using appropriate Fe₂O₃/FeO for each, to evaluate the effects of Fe speciation in the models.

For anhydrous compositions, our results indicate better agreement between recent thermodynamic databases and experimental constraints than in the past, indicating that recent optimizations are more suitable for the study of dry pyroxenites. For amphibole-free NVZ arclogites, we found P-T conditions of low Fe³⁺/Fe²⁺ samples agree well with previous P-T estimates when Fe₂O₃ is taken into account. However, the agreement degrades for samples with high Fe³⁺/Fe²⁺, indicating Fe speciation is an important parameter to consider in more oxidized arclogites. For amphibole-bearing xenoliths, observed phase assemblages could not be reproduced by the models, indicating that modeling of amphibole-bearing arclogitic cumulates remains a challenge.