Petrological and thermodynamic constraints on carbonate stability in altered oceanic crust subducted to subarc depths: Insights from carbonate-bearing eclogites in the Western Alps

Altered carbonates within the oceanic crust, which constitute a major carbon reservoir in subducting slabs, remain poorly constrained with respect to the factors controlling their deep recycling processes. In this study, we investigate carbonate-bearing eclogites from the Zermatt–Saas ophiolite by integrating petrological analysis, fluid inclusion studies and multi-scenario thermodynamic modelling to constrain decarbonation processes in subducted altered oceanic crust. Both studied magnesite- and dolomite-bearing eclogites experienced high-pressure metamorphism under low subduction zone thermal gradients (< 240 °C/GPa). The magnesite-bearing sample records peak conditions of ∼2.80 GPa at 575 °C and higher oxygen fugacity (ΔFMQ = +1.3 ± 0.86) during subduction and exhumation, in contrast to the dolomite-bearing sample (∼2.45 GPa at 575 °C; ΔFMQ = +0.4 ± 0.3). Decarbonation modelling indicates carbon losses of approximately 30 % and 43 %, respectively, driven by carbonate dissolution and fluid-induced metamorphic decarbonation during subduction and exhumation. CO₂-bearing fluid inclusions in host magnesite were primarily trapped during recrystallization linked to carbonate phase transitions under high-pressure conditions. The survival of subducted carbon is governed by multiple factors, including the carbon phase (graphite vs. carbonate), bulk-rock carbonate content, subduction zone thermal gradient and external fluid infiltration. Quantitative modelling of carbon survival across a range of subduction zone thermal gradients indicates that in cold subduction zones (< 320 °C/GPa), decarbonation is primarily controlled by the dehydration depths of lawsonite and epidote. In contrast, in warmer subduction zones, decarbonation is predominantly driven by metamorphic reactions, either occurring under dehydration-suppressed conditions or associated with epidote–hornblende breakdown. Open-system modelling reveals that complete decarbonation at lower temperatures (e.g., ∼500 °C) requires extensive fluid infiltration, whereas at subarc depths with higher P–T conditions, even minimal infiltration can significantly enhance decarbonation efficiency. Lawsonite dehydration appears to dominate the decarbonation process during subduction along thermal gradients below 270 °C/GPa, resulting in enhanced carbon release in colder subduction zones before reaching subarc depths. In contrast, thermal gradients exceeding ∼330–360 °C/GPa lead to nearly complete decarbonation prior to subarc depths. These results provide theoretical constraints on the initiation and episodic nature of deep carbonate cycling during the secular thermal evolution of subduction zones. Our findings elucidate the pivotal role of subducted carbonates, hosted within altered oceanic crust, in mediating deep mantle oxidation over Earth’s geological history.