Petrological-thermomechanical Model Research Articles

Cold plumes trigger contamination of oceanic mantle wedges with continental crust-derived sediments: Evidence from chromitite zircon grains of eastern Cuban ophiolites

The origin of zircon grains, and other exotic minerals of typical crustal origin, in mantle-hosted ophiolitic chromitites are hotly debated. We report a population of zircon grains with ages ranging from Cretaceous (99 Ma) to Neoarchean (2750 Ma), separated from massive chromitite bodies hosted in the mantle section of the supra-subduction (SSZ)-type Mayarí-Baracoa Ophiolitic Belt in eastern Cuba. Most analyzed zircon grains (n = 20, 287 ± 3 Ma to 2750 ± 60 Ma) are older than the early Cretaceous age of the ophiolite body, show negative εHf(t) (−26 to −0.6) and occasional inclusions of quartz, K-feldspar, biotite, and apatite that indicate derivation from a granitic continental crust. In contrast, 5 mainly rounded zircon grains (297 ± 5 Ma to 2126 ± 27 Ma) show positive εHf(t) (+0.7 to +13.5) and occasional apatite inclusions, suggesting their possible crystallization from melts derived from juvenile (mantle) sources. Interestingly, younger zircon grains are mainly euhedral to subhedral crystals, whereas older zircon grains are predominantly rounded grains. A comparison of the ages and Hf isotopic compositions of the zircon grains with those of nearby exposed crustal terranes suggest that chromitite zircon grains are similar to those reported from terranes of Mexico and northern South America. Hence, chromitite zircon grains are interpreted as sedimentary-derived xenocrystic grains that were delivered into the mantle wedge beneath the Greater Antilles intra-oceanic volcanic arc by metasomatic fluids/melts during subduction processes. Thus, continental crust recycling by subduction could explain all populations of old xenocrystic zircon in Cretaceous mantle-hosted chromitites from eastern Cuba ophiolite. We integrate the results of this study with petrological-thermomechanical modeling and existing geodynamic models to propose that ancient zircon xenocrysts, with a wide spectrum of ages and Hf isotopic compositions, can be transferred to the mantle wedge above subducting slabs by cold plumes.

Decarbonation of subducting slabs: Insight from petrological–thermomechanical modeling

Subduction of heterogeneous lithologies (sediments and altered basalts) carries a mixture of volatile components (H2O±CO2) into the mantle, which are later mobilized during episodes of devolatilization and flux melting. Several petrologic and thermodynamic studies investigated CO2 decarbonation to better understand carbon cycling at convergent margins. A paradox arose when investigations showed little to no decarbonation along present day subduction geotherms at subarc depths despite field based observations. Sediment diapirism is invoked as one of several methods for carbon transfer from the subducting slab. We employ high-resolution 2D petrological–thermomechanical modeling to elucidate the role subduction dynamics has with respect to slab decarbonation and the sediment diapirism hypothesis. Our thermodynamic database is modified to account for H2O–CO2 binary fluids via the following lithologies: GLOSS average sediments (H2O: 7.29wt.% & CO2: 3.01wt.%), carbonated altered basalts (H2O: 2.63wt.% & CO2: 2.90wt.%), and carbonated peridotites (H2O: 1.98wt.% & CO2: 1.50wt.%). We include a CO2 solubility P–x[H2O wt.%] parameterization for sediment melts. We parameterize our model by varying two components: slab age (20, 40, 60, 80Ma) and convergence velocity (1, 2, 3, 4, 5, 6cmyear−1). 59 numerical models were run and show excellent agreement with the original code base. Three geodynamic regimes showed significant decarbonation. 1) Sedimentary diapirism acts as an efficient physical mechanism for CO2 removal from the slab as it advects into the hotter mantle wedge. 2) If subduction rates are slow, frictional coupling between the subducting and overriding plate occurs. Mafic crust is mechanically incorporated into a section of the lower crust and undergoes decarbonation. 3) During extension and slab rollback, interaction between hot asthenosphere and sediments at shallow depths result in a small window (~12.5Ma) of high integrated CO2 fluxes (205kgm−3Ma−1).

Tectonics and melting in intra-continental settings

Most of the geodynamic theories of deformation as well as metamorphism and melting of continental lithosphere are concentrated on plate boundaries and are dominated by the effects of subduction upon deformation of the margins of continental lithospheric blocks. However, it is becoming increasingly apparent that suture zones, or mobile belts, presumably representing fossil subduction zones, or other types of pre-deformation which occur far from present plate boundaries, play a key role in intra-continental deformation. In such zones, the crust is strongly sheared and the mantle lithosphere is metasomatized. Reworking of such settings reveals a surprisingly large range of instabilities that develop in compressed lithosphere with lateral heterogeneities inherited from previous deformational processes. Structural complexity arises, which is sensitive to lithospheric age and tectonic setting. This complexity influences localization of deformation, topographic evolution, melt generation, and melt generation, ascent and emplacement. In this paper, using fully coupled petrological–thermomechanical modeling, various tectonic responses are correlated with magmatic events in intra-continental settings and are compared to observed intra-cratonic orogenies and magmatic events.

Numerical modelling of spontaneous slab breakoff dynamics during continental collision

Abstract Slab detachment or breakoff is directly associated with phenomena like morphological orogenesis, occurrence of earthquakes and magmatism. At depth the detachment process is slow and characterized by viscous rheolgy, whereas closer to the surface the process is relatively fast and plastic. Using a 2D mantle model 1500 km deep and 4000 km wide we investigated, with finite-difference and marker-in-cell numerical techniques, the impact of slab age, convergence rate and phase transitions on the viscous mode of slab detachment. In contrast to previous studies exploring simplified breakoff models in which the blockage responsible for inducing breakoff is kinematically prescribed, we constructed a fully dynamic coupled petrological–thermomechanical model of viscous slab breakoff. In this model, forced subduction of a 700 km-long oceanic plate was followed by collision of two continental plates and spontaneous slab blocking resulting from the buoyancy of the continental crust once it had been subducted to a depth of 100–124 km. Typically, five phases of model development can be distinguished: (a) oceanic slab subduction and bending; (b) continental collision initiation followed by the spontaneous slab blocking, thermal relaxation and unbending – in experiments with old oceanic plates in this phase slab roll-back occurs; (c) slab stretching and necking; (d) slab breakoff and accelerated sinking; and (e) post-breakoff relaxation. Our experiments confirm a correlation between slab age and the time of spontaneous viscous breakoff as previously identified in simplified breakoff models. The results also demonstrate a non-linear dependence of the duration of the breakoff event on slab age: a positive correlation being characteristic of young (<50 Ma) slabs while for older slabs the correlation is negative. The increasing duration of the breakoff with slab age in young slabs is attributed to the slab thermal thickness, which increases both the slab thermal relaxation time and duration of the necking process. In older slabs this tendency is counteracted by negative slab buoyancy, which generate higher stresses that facilitate slab necking and breakoff. A prediction from our breakoff models is that the olivine–wadsleyite transition plays an important role in localizing viscous slab breakoff at depths of 410–510 km due to the buoyancy effects of the transition.

Physical controls of magmatic productivity at Pacific-type convergent margins: Numerical modelling

We use a coupled petrological–thermomechanical model of subduction with spontaneous slab bending to investigate magmatic productivity at active continental margins. The model is designed to simulate fossil Pacific-type margins that have a broad well-developed fore-arc accretionary wedge system. The degree of plate coupling strongly depends on the dimensionless ratio ( R H 2 O ) between the plate convergence rate and the water propagation velocity. Delamination of the slab from the overriding plate followed by trench retreat is common for models with relatively slow convergence rates ( R H 2 O < 4 ). In contrast, higher convergence rates ( R H 2 O > 4 ) result in continuous plate coupling. The slab bending curvature increases with the length of the subducted plate. Periodic variations of the slab angle with time are observed at the later stages of subduction and become more conspicuous with depth. These variations are favoured by slower subduction rates and stronger oceanic crust. Two fundamentally different regimes of melt productivity are observed in numerical experiments and correlated with natural observations: (1) during continuous convergence with coupled plates (as in the Late Paleozoic margin of central Chile) the largest melt productivity occurs at the onset of subduction due to temporary steepening of the slab, productivity then decays rapidly with time due to flattening of the slab inclination angle, thereby precluding further formation of partially molten mantle wedge plumes. (2) Highest melt productivity is obtained in simulations associated with slab delamination and trench retreat. As a result an extension occurs with decoupling of plates that finally leads to the formation of a pronounced back-arc basin (similar to the Mesozoic margin of southernmost Chile). In this case melt production increases with time due to stabilization of slab inclination associated with upward asthenospheric mantle flow toward the spreading centre, thereby favouring the generation of hydrous partially molten plumes from the slab.

Seismic implications of mantle wedge plumes

We use a coupled petrological–thermomechanical model to investigate the dynamical effects of metamorphic reactions and melting on the seismic structure of thermal–chemical plumes beneath volcanic arcs. Plume generation is driven by the subduction of buoyant crustal rocks and expulsion of aqueous slab fluids that causes hydration and partial melting of the mantle wedge. The model demonstrates two chemically distinct types of plumes. Unmixed plumes initiate from the melting front within the mantle that arises as a consequence of infiltration of slab-derived water-rich fluids, whereas mixed plumes initiate from the slab itself and entrain both slab- and mantle-derived magmas. Mixed plumes explain magmas such as adakites with crustal signatures, while primitive arc tholeiites are attributed to unmixed plumes. As a consequence of the interplay between water content and temperature both positive and negative seismic velocity anomalies are associated with the plumes. Positive anomalies are prevalent close to the slab due to the lowered temperatures associated with regions of cold plume initiation. Negative seismic anomalies develop at shallower depth due to the partially molten rocks that form plume heads. Flat lying partially molten regions that form beneath volcanic arcs as a consequence of cold wet plumes are manifest by >20% variations in the local Poisson ratio, as compared to variations of ∼2% expected as a consequence of temperature variation within the mantle wedge. In contrast to models that attribute a purely thermal origin for mantle wedge seismic anomalies, the petrological–thermomechanical model is able to explain the strong seismic velocity variations existing beneath volcanic arcs.