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

Abstract

<p>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 plate<sup>1</sup>. 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, Tharimena<sup>1</sup> 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.</p> <p> </p> <p>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<sup> </sup>for reactive melt transport using the finite difference method. Our first model <sup> </sup>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 Thermolab<sup>2</sup>, 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.<br />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.</p> <ul> <li>Tharimena et al., 2017, J.Geophys.Res.Solid Earth, 122</li> <li>Vrijmoed & Podladchikov, 2022, <em>G<sup>3</sup></em><em>, <strong>23</strong>,</em> <em>e2021GC010303</em></li> </ul>