Physics of Melt Extraction from the Mantle: Speed and Style

Abstract

Melt extraction from the partially molten mantle is among the fundamental processes shaping the solid Earth today and over geological time. A diversity of properties and mechanisms contribute to the physics of melt extraction. We review progress of the past ∼25 years of research in this area, with a focus on understanding the speed and style of buoyancy-driven melt extraction. Observations of U-series disequilibria in young lavas and the surge of deglacial volcanism in Iceland suggest this speed is rapid compared to that predicted by the null hypothesis of diffuse porous flow. The discrepancy indicates that the style of extraction is channelized. We discuss how channelization is sensitive to mechanical and thermochemical properties and feedbacks, and to asthenospheric heterogeneity. We review the grain-scale physics that underpins these properties and hence determines the physical behavior at much larger scales. We then discuss how the speed of melt extraction is crucial to predicting the magmatic response to glacial and sea-level variations. Finally, we assess the frontier of current research and identify areas where significant advances are expected over the next 25 years. In particular, we highlight the coupling of melt extraction with more realistic models of mantle thermochemistry and rheological properties. This coupling will be crucial in understanding complex settings such as subduction zones. ▪ Mantle melt extraction shapes Earth today and over geological time. ▪ Observations, lab experiments, and theory indicate that melt ascends through the mantle at speeds ∼30 m/year by reactively channelized porous flow. ▪ Variations in sea level and glacial ice loading can cause significant changes in melt supply to submarine and subaerial volcanoes. ▪ Fluid-driven fracture is important in the lithosphere and, perhaps, in the mantle wedge of subduction zones, but remains a challenge to model. Expected final online publication date for the Annual Review of Earth and Planetary Sciences, Volume 50 is May 2022. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Persistent Magma‐Rich Waves Beneath Mid‐Ocean Ridges Explain Long Periodicity on Ocean Floor Fabric

“…If the observed long period fluctuations in ocean bathymetry (Goff, 2020;Shinevar et al, 2019) are indeed attributable to melt-rich porosity waves, this agrees with the larger mantle permeability that has been suggested (Katz et al, 2022). For sufficiently large mantle permeabilities, the models presented here suggest that porosity waves produce time-varying crustal thicknesses regardless of spreading rates (Figure 4); previous modeling studies show porosity waves persistent only at intermediate and slower spreading rates (Parnell-Turner et al, 2020;Sim et al, 2020).…”

Influence of late Pleistocene sea-level variations on midocean ridge spacing in faulting simulations and a global analysis of bathymetry

“…Onset of 41-ky scaling may also reflect the admittance of the ridge-magmatic system to different frequencies of sea-level forcing. Admittance involves a trade-off between more rapid changes in sea level causing larger fluctuations in melt generation and an anomaly needing to be of longer duration to rise through the melting regime and be expressed at the ridge axis ( 12 , 28 ). Melt reservoirs at fast-spreading ridges could further damp fluctuations of melt supply ( 24 ).…”

“…This representation is an upper bound on magma-supply variability and is substantially larger than the order 10% variations predicted by foregoing models of melt generation ( 12 , 14 ). These models, however, do not account for phenomena associated with melt-channelization dynamics that could lead to greater-amplitude variations in melt supply ( 27 , 28 ). Although the FLAC model is inevitably incomplete, it nonetheless gives specific predictions regarding the relationship between spreading rate and fault spacing that can be compared to observations.…”

Mantle mineralogy limits to rocky planet water inventories

“…If this melt is less dense than its associated solid residue, it will quickly rise upwards, and its water might eventually degas to the surface through volcanism-this is the scenario, for example, in the mantle beneath mid-ocean ridges on Earth. Where melt is denser than the solid, water transport can only occur via solid-state convection, which is orders of magnitude slower (not more than metres per year; Ricard 2015) than melt migration (∼30 m yr −1 ; Katz et al 2022). In general, melt is more compressible than solid mantle, meaning that the melt formed during partial melting may become negatively buoyant (i.e., denser than the solid residue) at some pressure in a planet.…”