A Disequilibrium Reactive Transport Model for Mantle Magmatism

Abstract

Besides standard thermo-mechanical conservation laws, a general description of mantle magmatism requires the simultaneous consideration of phase changes (e.g. from solid to liquid), chemical reactions (i.e. exchange of chemical components) and multiple dynamic phases (e.g. liquid percolating through a deforming matrix). Typically, these processes evolve at different rates, over multiple spatial scales and exhibit complex feedback loops and disequilibrium features. Partially due to these complexities, integrated descriptions of the thermal, mechanical and chemical evolution of mantle magmatism have been challenging for numerical models. Here we present a conceptual and numerical model that provides a versatile platform to study the dynamics and nonlinear feedbacks inherent to mantle magmatism and to make quantitative comparisons between petrological and geochemical datasets. Our model is based on the combination of three main modules: i) a Two-Phase, Multi-Component, Reactive Transport module that describes how liquids and solids evolve in space and time, ii) a melting formalism, called Dynamic Disequilibirum Melting, based on thermodynamic grounds and capable of describing the chemical exchange of major elements between phases in disequilibrium, and iii) a grain-scale model for diffusion-controlled trace-element mass transfer. We illustrate some of the benefits of the model by analyzing both major and trace elements during mantle magmatism in a mid-ocean ridge-like context. We systematically explore the effects of mantle potential temperature, upwelling velocity, degree of equilibrium and hetererogeneous sources on the compositional variability of melts and residual peridotites. Our model not only reproduces the main thermo-chemical features of decompression melting but also predicts counter-intuitive differentiation trends as a consequence of phase changes and transport occurring in disequilibrium. These include a negative correlation between and in melts generated at the same and the continued increase of the melt’s after Cpx exhaustion. Our model results also emphasize the role of disequilibrium arising from diffusion for the interpretation of trace-element signatures. The latter is shown to be able to reconcile the major- and trace-element compositions of abyssal peridotites with field evidence indicating extensive reaction between peridotites and melts. The combination of chemical disequilibrium of major elements and slugglish diffusion of trace elements may also result in weakened MREE-to-HREE depletion comparable to the effect of residual garnet in MORB, despite its absence in the modelled melts source. We also find that the crystallization of basalts ascending in disequilibrium through the asthenospheric mantle could be responsible for the formation of olivine gabbros and wehrlites that are observed in the deep sections of ophiolites. The presented framework is general and readily extendable to accommodate additional processes of geological relevance (e.g. melting in the presence of volatiles and/or of complex heterogeneous sources, refertilization of the lithospheric mantle, magma channelization and shallow processes) and the implementation of other geochemical and isotopic proxies. Here we illustrate the effect of heterogeneous sources on the thermo-mechanical-chemical evolution of melts and residues using a mixed peridotite-pyroxenite source.

Derivation of Hawaiian rejuvenated magmas from deep carbonated mantle sources: A review of experimental and natural constraints

“…polybaric, open-system heterogeneous source) scenario is not only challenging (e.g. Oliveira et al, 2020), but it would also require making assumptions that are beyond the scope of this work. Similarly, radiogenic-isotope compositions are not considered here but discussed qualitatively in a separate section; they have been addressed in more detail by Béguelin et al (2019) and Harrison et al (2020).…”

Lithospheric mantle refertilization by DMM-derived melts beneath the Cameroon Volcanic Line—a case study of the Befang xenolith suite (Oku Volcanic Group, Cameroon)

“…Both observations can be explained considering that decompression melting during asthenospheric upwelling was limited due to the presence of a lithospheric lid (85-90 km at present; Goussi Ngalamo et al 2018), whereas N-MORB forms by melting to ~ ≤ 1.2 GPa (Hirschmann and Stolper 1996). At 85-90 km depth (2.6-2.8 GPa) and for the inferred supersolidus temperature (minimum 1200-1275 °C, as discussed above), depleted MORB mantle would contain a small amount of garnet (Ziberna et al 2013), although the signature may also be produced by small amounts of pyroxenite in the source (Hirschmann and Stolper 1996) or by disequilibrium coupled with slow diffusion of trace elements (Oliveira et al 2020).…”

Melting conditions and mantle source composition from probabilistic joint inversion of major and rare earth element concentrations

“…Simulations assume T p = 1300 • C, k 0 = 10 −8 m 2 , W 0 = 5 cm/s, and R j = 10 −12 s −1 as reference values. Initial compositions are given in Table 1 and partition coefficients are from Oliveira et al (2020). Coloured lines refer to instantaneous melt compositions at variable depth, ranging from blue -onset of melting, to red -end of melting.…”

“…where k(φ l ) = k 0 (φ l ) n is the Kozeny-Carman-type permeability law relating permeability and porosity k 0 (φ l ); n is a constant exponent. Here we take To account for disequilibrium processes, we adapt the disequilibrium melting model by Oliveira et al (2020), and assume that i) the solid does not interact chemically with all the melt that passes through it, and that ii) the chemical interaction between solid and liquid phases is controlled by diffusion of chemical species in the solid. Conceptually this corresponds to having two different melt reservoirs flowing through the solid: one in chemical isolation, and the other with diffusion-controlled chemical exchange with the surrounding solid.…”