Experimental study of metamorphic reactions and dehydration processes at the blueschist–eclogite transition during warm subduction

Abstract

The transition between blueschist and eclogite plays an important role in subduction zones via dehydration and densification processes in descending oceanic slabs. There are a number of previous petrological studies describing potential mineral reactions taking place at the transition. An experimental determination of such reactions could help constrain the pressure–temperature conditions of the transition as well as the processes of dehydration. However, previous experimental contributions have focused on the stability of spontaneously formed hydrous minerals in basaltic compositions rather than on reactions among already formed blueschist facies minerals. Therefore, this study conducted three groups of experiments to explore the metamorphic reactions among blueschist facies minerals at conditions corresponding to warm subduction, where faster reaction rates are possible on the time scale of laboratory experiments. The first group of experiments was to establish experimental reversals of the reaction glaucophane+paragonite to jadeite+pyrope+quartz+H2O over the range of 2.2–3.5 GPa and 650–820°C. This reaction has long been treated as key to the blueschist–eclogite transition. However, only the growth of glaucophane+paragonite was observed at the intersectional stability field of both paragonite and jadeite+quartz, confirming thermodynamic calculations that the reaction is not stable in the system Na2O–MgO–Al2O3–SiO2–H2O. The second set of experiments involved unreversed experiments using glaucophane+zoisite ±quartz in low‐Fe and Ca‐rich systems and were run at 1.8–2.4 GPa and 600–780°C. These produced omphacite+paragonite/kyanite+H2O accompanied by compositional shifts in the sodium amphibole, glaucophane, towards sodium–calcium amphiboles such as winchite (☐(CaNa)(Mg4Al)Si8O22(OH)2) and barroisite (☐(CaNa)(Mg3Al2)(AlSi7)O22(OH)2). This suggests that a two‐step dehydration occurs, first involving the breakdown of glaucophane+zoisite towards a paragonite‐bearing assemblage, then the breakdown of paragonite to release H2O. It also indicates that sodium–calcium amphibole can coexist with eclogite phases, thereby extending the thermal stability of amphibole to greater subduction zone depths. The third set of experiments was an experimental investigation at 2.0–2.4 GPa and 630–850°C involving a high‐Fe (Fe#=Fetotal/(Fetotal+Mg)≈0.36) natural glaucophane, synthetic paragonite and their eclogite‐forming reaction products. The results indicated that garnet and omphacite grew over most of these pressure–temperature conditions, which demonstrates the importance of Fe‐rich glaucophane in forming the key eclogite assemblage of garnet+omphacite, even under warm subduction zone conditions. Based on the experiments of this study, reaction between glaucophane+zoisite is instrumental in controlling dehydration processes at the blueschist–eclogite transition during warm subduction.

Dehydration‐driven deformation of eclogite: Interplay between fluid discharge and rheology

“…In contrast, matrix rutile in all samples show a wider temperature range with two peaks. The uneven distribution of Zr observed in matrix rutile within each eclogite sample may indicate (i) incomplete equilibration of prograde rutile to peak conditions (Penniston‐Dorland et al, 2018), or (ii) various local equilibration conditions hindered by slow reactions at low‐temperature conditions (e.g., Cheng & Jenkins, 2021). Although the current dataset does not allow us to pinpoint the exact reason for the lower Zr peaks, the higher Zr peaks contain more significant information.…”

“…Inclusions of Omp 1 , Lws-pseudomorphs, Chl 1 and (Na-Ca)-Amp 1 and (Na)-Amp 1 hosted within the garnet core (Grt 1 ), suggest coeval stability during the entrapment in garnet. Although the growth of omphacite at the expense of the (Na)-and (Na-Ca)-amphibole is indicative of the blueschist to eclogite facies transition (Evans, 1990;Reinsch, 1979;Ridley & Dixon, 1984), recent experiments by Cheng and Jenkins (2021) show that partial decomposition of (Na)amphibole can lead to the prograde (or peak) formation of (Na-Ca)-amphibole coexisting with eclogite phases. Garnet preserves a prograde zoning expressed by decreasing X Alm , X Grs and X Sps within the core (Grt 1 ) overgrown by high-X Prp rim (Grt 2 ; Figure S2).…”

Tectono‐Magmatic Evolution of Asymmetric Coronae on Venus: Topographic Classification and 3D Thermo‐Mechanical Modeling

“…Along with the slight negative buoyancy of the cold lithosphere, it provides the necessary driving force for the sinking of slab/drips. While the parametrization of eclogitization in our numerical code (described in Text S2.3 in Supporting Information ) is fairly simple, the transformation of basaltic rocks (e.g., basalt, gabbro, blueschist, amphibolite) into eclogite is a complex process that involves several net‐transfer and Fe‐Mg exchange mineral reactions studied experimentally (e.g., Cheng & Jenkins, 2021; Griffin, 1972; Zhang & Liou, 1997), which is complex to translate to geodynamical software. While the eclogite phase change has been studied numerically in relation to (early) Earth geodynamics (e.g., Perchuk et al., 2023; Van Hunen et al., 2004), it remains a future target to shed light on its relationship to Venus geodynamics.…”