Marko Repac

University of Lausanne

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S. Pilet

University of Lausanne

K. S. Panter

Bowling Green State University

Yury Podladchikov

University of Lausanne

Stefan M. Schmalholz

University of Lausanne

Lyudmila Khakimova

University of Lausanne

Annelore Bessat

University of Lausanne

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University of Lausanne

École Polytechnique Fédérale de Lausanne

Lomonosov Moscow State University

Moscow State University

Institut des Sciences de la Terre

ETH Zurich

California Institute of Technology

National Taiwan University

University Hospital of Lausanne

Skolkovo Institute of Science and Technology

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Reactive Melt Transport Using Porosity Waves Across the Thermal Boundary Layer.

Marko Repac Annelore Bessat Stefan M. Schmalholz Yury Podladchikov K. S. Panter

The lithosphere and the asthenosphere are characterized by different heat transport mechanisms, conductive for the lithosphere, convective for the asthenosphere. The zone associated with the transition between these two distinct mechanisms is known as the "Thermal Boundary Layer" (TBL). How the melt is transported across this zone is an important question regarding intraplate magmatism and for the nature of the seismic Low-Velocity Zone. Numerous studies and models suggest that primary magmas from intraplate volcanos are the product of low degree partial melting in the asthenosphere, while the differentiation process takes place in the crust or shallow lithospheric mantle. The question is how low degree melt ascends through the TBL and the lithospheric mantle. The thermal structure of the lithosphere is characterized by a high geothermal gradient, which questions the ability of melt to cross the lithospheric mantle without cooling and crystallizing. Since the base of the lithosphere is ductile, the possible modes of magma transport are porous flow or porosity waves. For these reasons, we would like to understand how melt is transported and what are the implications on the evolution of primitive melt, going from the convective part of the geotherm to the conductive part of the geotherm and further across the lithosphere.We present the results of a thermo-hydro-mechanical-chemical (THMC) model1 for reactive melt transport using the finite difference method. This model considers melt migration by porosity waves and a chemical system of forsterite-fayalite-silica. Variables, such as solid and melt densities or MgO and SiO2 mass concentrations, are functions of pressure, temperature, and total silica mass fraction (CtSiO2). These variables are pre-computed with Gibbs energy minimization and their variations with evolving P, T, and CtSiO2 are implemented in the THMC model. We consider P and T conditions relevant across the TBL. With input parameters characteristic for alkaline melt and conditions at the base of the lithosphere, we obtain velocities between 1 to 150 m yr-1,which is a velocity similar to melt rising at mid-ocean ridges2. This implies the inability of primary melts to cross the lithosphere. However, melt addition to the base of the lithosphere is important to understand mantle metasomatism, and could, to some extent, contribute to physical properties of the Lithosphere-Asthenosphere Boundary and Mid Lithosphere Discontinuity observed with geophysical methods. We suggest that the appearance of alkaline magmas at the surface requires multiple stage processes as melts rising in the lithosphere progressively modify the geotherm allowing new melts to propagate to the surface. Our earlier modeling results1 demonstrated that a single porosity wave has a minor impact on chemical evolution. In this study, we search for a mechanism responsible for stabilizing porosity wave motion to some lateral location forcing consecutive waves to follow the same ascent path. The passage of a large number of quickly rising porosity waves over a long time through the same path would accumulate large melt to rock ratios and cause significant chemical evolution.&#160;<ul><li>Bessat et at., 2022, G3, in press</li> <li>Connolly et al. 2009, Nature 462, 209-212.</li> </ul>

Asthenosphere

Thermal subsidence

Metasomatism

10.5194/egusphere-egu22-10445

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Multistep Numerical THMC Reactive Porosity Waves Transport Model to Explain Intraplate Volcanism and Mantle Metasomatism.

Marko Repac Lyudmila Khakimova Yury Podladchikov K. S. Panter Stefan M. Schmalholz

Ongoing debates persist regarding the role of the lithospheric mantle in the generation of intraplate volcanoes. Most geochemical models propose that these volcanoes originate from magmas formed in the asthenosphere. Intraplate basalt composition, particularly their high content of trace elements, implies that these magmas are produced at low degree of partial melting. However, the melt migration through the lithospheric mantle remains largely unexplored. As these small quantities of magma traverse the lithosphere, their limited heat transport results in rapid cooling due to the lithosphere's strong geotherm (McKenzie, 1989), casting doubt on their ability to directly reach the surface. In contrast, this process induces a chemical effect characterized by the metasomatic enrichment of the lithospheric mantle, observed across oceanic, continental, and cratonic environments. Here, we developed a numerical finite difference model, incorporating thermo-hydro-chemical-thermal (THMC) processes, to investigate melt migration across the lithosphere. The model includes conservation equations for mass, fluid, and solid momentum, featuring a non-linear porosity-permeability relation for decompaction weakening and reactive porosity waves essential for flow channelization. Thermodynamic calculations employed Thermolab (Vrijmoed & Podladchikov, 2022), a versatile Gibbs energy minimizer. Amphiboles and phlogopites are crucial phases for mantle metasomatism and alkaline magma generation. We successfully model these phases within expected PT ranges. Using both solution models and fixed composition phases. The model progresses through multiple steps, initiating at the asthenosphere-lithosphere boundary. Initial melt, present there if volatile content is sufficient, migrates upward with a decreasing volume but increasing volatile content as the pressure and temperature decrease. At a given point, the freezing effect of volatiles on mantle melting temperature is no longer sufficient to stabilize melt in equilibrium with the surrounding mantle. Melt migration concludes with the formation of hydrous phases like pargasite or phlogopite depending on the pressure. The second step, addressing excess volatiles after hydrous phases crystallization, involves their further upward transport as fluid, metasomatizing the overlying mantle until depletion of fluid. This process explains several aspects of metasomatism, such as hydrated phase formation and cryptic metasomatism associated with fluid migration. On the other hand, our model confirms that magma does not seem capable of crossing the lithosphere without reacting with the surrounding mantle and crystallizing. To take this further, we consider the hypothesis that the process of melt transport in the lithosphere occurs through the repeated migration of several pulses of magma from the asthenosphere. The emplacement of the first porosity wave is fundamental in establishing a pathway through which all successive pulses will traverse. Following this intricate process, a more extensive segment of the lithosphere undergoes metasomatism. Additionally, the recurrent influx of melt/fluid gradually elevates temperatures in the metasomatized area, potentially leading to the subsequent re-melting of hydrous phases, thereby engendering alkaline melts observed at the surface. McKenzie, D. (1989). Some remarks on the movement of small melt fractions in the mantle. Earth and Planetary Science Letters, 53-72. Vrijmoed & Podladchikov. (2022). Thermolab: A Thermodynamics Laboratory for Nonlinear Transport Processes in Open Systems .G3, 23, e2021GC010303

Metasomatism

10.5194/egusphere-egu24-16643

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Using reactive melt transport by porosity waves to understand Lithosphere-asthenosphere boundary and intraplate volcanism

Marko Repac Annelore Bessat Lyudmila Khakimova K. S. Panter Stefan M. Schmalholz

The Lithosphere-Asthenosphere boundary (LAB) is a conceptual zone decoupling cold and rigid lithosphere from the hot and weak asthenosphere. It is marked by the changes in many physical properties which are measured with different geophysical techniques. Seismic anomalies associated with the LAB are observed at 45-80 km depth across the old Pacific plate1. These are interpreted as the presence of melt, but their locations are significantly shallower than the LAB depth predicted by the thermal model. The extraction of MORB at mid-ocean ridges will produce a residual mantle relatively poor in volatiles, which questions the origin of magmas observed under the Pacific plate, Tharimena1 and co-authors recognize this issue and suggest that the observed seismic anomalies are associated with the migration of magma from the asthenosphere which accumulates at the base of the lithosphere. Many studies on intraplate magmatism suggested that primary partial melts are produced in the asthenosphere followed by differentiation in the crust. Melt-rock interactions during melt transport across the LAB and lithosphere are often neglected or overly simplified given that it is likely that melt will react with the surrounding mantle and cool as it passes through this zone. &#160; To understand how to stabilize melt at the P-T conditions of observed anomalies and to understand what is the transport mechanism associated with the migration of magmas into and across LAB as well as the geochemical and geophysical implications of such transport, we have developed a thermo-hydro-mechanical-chemical (THMC) model for reactive melt transport using the finite difference method. Our first model&#160; considered melt migration by reactive porosity waves in 1D within a simplified forsterite-fayalite-silica chemical system. This numerical model is based on solving the differential equations for the conservation of mass, conservation of fluid, and solid momentum, including a nonlinear relation between porosity and permeability. With our initial model, we have shown that the single porosity wave has only a minor impact on the chemical evolution of the lithosphere. With this new ongoing model development, we have greatly increased compositional complexity by using Thermolab2, which is a versatile Gibbs energy minimizer that uses local thermodynamic equilibrium and permits multicomponent thermodynamic calculations. The model confirmed by thermodynamic calculations that at the corresponding P-T conditions we will first start to crystalize anhydrous cumulates followed by hydrous cumulates at lower temperatures and depths. The new model is in 2D and therefore allows us to explore the channeling effect of porosity wave on melt evolution. Additionally, we were able to produce spontaneous initiation of subsequent pulses of porosity which are following the already established channel of the previous pulse. This stabilization of porosity waves could progressively increase the chemical evolution of even highly compatible elements. This leads to the conclusion that the formation of intraplate volcanism is not a simple process driving melt across the lithosphere, but requires percolation, differentiation, and reaction probably occurring in multiple stages. These mechanisms are important to consider when making geophysical interpretations of the asthenosphere-lithosphere boundary. <ul> <li>Tharimena et al., 2017, J.Geophys.Res.Solid Earth, 122</li> <li>Vrijmoed & Podladchikov, 2022, G3, 23, e2021GC010303</li> </ul>

Asthenosphere

10.5194/egusphere-egu23-16432

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