Magma transport beneath mid-ocean ridges

<p>Melt transport beneath the lithosphere is elusive. With a distinct viscosity and density from the surrounding mantle, magmatic melt moves on a different time scale as the surrounding mantle. To resolve the temporal scale necessary to accurately capture melt transport in the mantle, the model simulations become numerically expensive quickly. Recent computational advances make possible two-phase numerical explorations to understand magma transport in the mantle. We review results from a suite of two-phase models applied to the mid-ocean ridges, where we varied half-spreading rate and intrinsic mantle permeability using new openly available models, with the goal of understanding melt focusing beneath mid-ocean ridges and its relevance to the lithosphere-asthenosphere boundary (LAB). Here, we highlight the importance of viscosities for the melt focusing mechanisms. In addition, magmatic porosity waves that are a natural consequence of these two-phase flow formulations. We show that these waves could explain long-period temporal variations in the seafloor bathymetry at the Southeast Indian Ridge.</p>

Felsic melt migration via porous flow – a numerical modeling approach

<p>Melting of the continental crust and subsequent melt transport has been most thoroughly described in the case of metasedimentary rocks. In these rocks segregation and migration of melt occur either through an interconnected network of veins and melt-rich layers (leucosome) or in form of diapirs. For these rocks, porous flow of melt at grain scale is mostly regarded only as a transient stage of separation of melt from the solid rock.</p><p>An entirely different style of melting and melt transport occurs in the case of felsic metaigneous rocks. We use the example from the Bohemian Massif, the eastern European Variscan belt, where metaigneous migmatites were studied in large detail. Here, melt did not segregate from the solid rock but migrated pervasively along most of the grain boundaries and equilibrated with the host rock. This equilibration resulted in formation of a continuous sequence of texturally, geochemically and compositionally different migmatites.</p><p>The question arises, what are the conditions and driving forces for this unusual behavior. We attempt to address this question by means of numerical modeling of two-phase flow (i.e. flow of porous solid matrix and melt), using the open-source finite-element ASPECT code (aspect.geodynamics.org). Most previous numerical studies of this process were either purely generic or focused on the melting of the mantle. In order to study this process in crustal conditions, we set up a 2D crustal-scale thermo-mechanical model that includes melting and freezing. We investigate the role of material properties (viscosity, solidus and liquidus temperatures, solid matrix permeability, melt composition) and thermal and velocity boundary conditions, as well as the effect of grid resolution. The results are discussed in terms of realistic parameter values and possible styles of melt migration and deformation of the matrix.</p>

Melting and melt transport mechanisms in the dynamic earth: from melt segregation, extraction to the formation of crustal magmatic systems

<p><strong>Introduction</strong></p><p>At various regions within the dynamic earth melts are generated due to decompressional melting, reduction of the solidus temperature due to volatiles or due to elevated temperatures. They segregate from these partially molten regions, rise by various transport mechanisms and may form crustal magmatic systems where they are emplaced or erupt. The physics of various aspects of this magmatic cycle will be addressed.</p><p><strong>Melt transport mechanisms</strong></p><p>Starting from a partially molten region by which mechanism(s) does the melt segregate out of the melt source region and rise through the mantle or crust? The basic mechanism is two-phase flow, i.e. a liquid phase percolates through a solid, viscously deforming matrix. The corresponding equations and related issues such as compaction or effective matrix rheology are addressed. Beside simple Darcy flow, special solutions of the equations are addressed such as solitary porosity waves. Depending on the bulk to shear viscosity ratio of the matrix and the non-dimensional size of these waves, they show a variety of features: they may transport melt over large distances, or they show transitions from rising porosity waves to diapiric rise or to fingering. Other solutions of the equations lead to channeling, either mechanically or chemically driven. One open question is how do such channels transform into dykes which have the potential of rising through sub-solidus overburden. A recent hypothesis addresses the possibility that rapid melt percolation may reach the thermal non-equilibrium regime, i.e. the local temperature of matrix and melt may evolve differently.  Once dykes have been formed they may propagate upwards driven by melt buoyancy and controlled by the ambient stress field. Often in dynamic models the complexities of melt transport are simplified by parameterized melt extraction. The limitations of such simplifications will be addressed.</p><p><strong>Modelling magmatic systems in thickened continental crust </strong></p><p>Once basaltic melts rise from the mantle, they may underplate continental crust and generate silicic melts. Early dynamic models (Bittner and Schmeling, 1995, Geophys. J. Int.) showed that such silicic magma bodies may rise to mid-crustal depth by diapirism. More recent approaches (e.g. Blundy and Annan, 2016, Elements) emplace sill intrusions into the crust at various levels and calculate the thermal and melting effects responsible for the formation of mush zones. Recently Schmeling et al. (2019, Geophys. J. Int.) self-consistently modelled the formation of crustal magmatic systems, mush zones and magma bodies by including two-phase flow, melting/solidification and effective power-law rheology. In these models melt is found to rise to mid-crustal depths by a combination of compaction/decompaction assisted two-phase flow, sometimes including solitary porosity waves, diapirism or fingering. An open question in these models is whether or how dykes may self-consistently form to transport the melts to shallower depth. First models which combine elastic dyke-propagation (Maccaferri et al., 2019, G-cubed) with the two-phase flow crustal models are promising.</p>

Implications of thermal disequilibrium during channelized melt-transport for the evolution of the lithosphere-asthenosphere boundary in intraplate settings

<p>This study explores how thermal disequilibrium during channelized melt-infiltration modifies the continental lithosphere from beneath. For this purpose, a 1D model of thermal disequilibrium between melt-rich channels and surrounding melt-poor material was developed, allowing us to estimate heat exchange across channel walls during melt transport at the lithosphere-asthenosphere boundary (LAB).  For geologically-reasonable values of volume fraction of channels (<em>φ</em>), relative velocity across channel walls (<em>v</em>), channel spacing (<em>d</em>), and timescale of episodic melt-infiltration (<em>τ</em>), disequilibrium heating may contribute >10<sup>-3</sup> W/m<sup>3</sup> to the LAB heat budget. During episodic melt-infiltration, a thermal reworking zone (TRZ) associated with spatio-temporally varying disequilibrium heat exchange forms at the LAB. The TRZ grows by the transient migration of a disequilibrium-heating front at material-dependent velocity, reaching a maximum steady-state width δ∼[<em>φvd<sup>-</sup></em><sup>2</sup><em>τ</em><sup>2</sup>]. The model results have implications for the Cenozoic evolution of the western US, specifically during the time period following the middle-Cenozoic ignimbrite flareup, and can be used to interpret a disparate set of previously published geophysical and geologic observations from the western US. The spatio-temporal scales associated with establishment of the TRZ in the models are found to be comparable with those inferred for the migration of the LAB based on geologic and petrologic observations within the Basin and Range province. More generally, the geochemistry of Cenozoic basalts across the region indicate a process in which melt-infiltration may have hastened the thinning and weakening of the lithosphere during and following the mid-Cenozoic ignimbrite flare-up, prior to Neogene extension.</p>

Evidence for Multi-stage Melt Transport in the Lower Ocean Crust: the Atlantis Bank Gabbroic Massif (IODP Hole U1473A, SW Indian Ridge)

The architecture of lower oceanic crust at slow- and ultraslow-spreading ridge is diverse, yet the mechanisms that produce this diversity are not well understood. Particularly, the 660-km2 gabbroic massif at Atlantis Bank (Southwest Indian Ridge) exhibits significant compositional zonation, representing a high magma supply end member for accretion of the lower ocean crust at slow and ultraslow-spreading ridges. We present the petrographic and geochemical data of olivine gabbros from the 809-metre IODP Hole U1473A at Atlantis Bank gabbroic massif. Structurally, the upper portion of U1473A consists of a ∼600-metre shear zone; below this, the hole is relatively undeformed, with several minor shear zones. Olivine gabbros away from the shear zones have mineral trace element compositions indicative of high-temperature reaction with an oxide-undersaturated melt. By contrast, olivine gabbros within shear zones display petrographic and chemical features indicative of reaction with a relatively low-temperature, oxide-saturated melt. These features indicate an early stage of primitive to moderately evolved melt migration, followed by deformation-driven transport of highly evolved Fe–Ti-rich melts to high levels in this gabbroic massif. The close relationship between shear zones and the reaction with oxide-saturated melts suggests that syn-magmatic shear zones provide a conduit for late-stage, Fe–Ti-rich melt transport through Atlantis Bank lower crust. This process is critical to generate the compositional zonation observed. Thus, the degree of syn-magmatic deformation, which is fundamentally related to magma supply, plays a dominant role in developing the diversity of lower ocean crust observed at slow- and ultraslow-spreading ridges.