On the thermodynamic boundary conditions of a solidifying mushy layer with outflow

AbstractThe free-boundary problem between a liquid region and a mushy layer (a reactive porous medium) must respect both thermodynamic and fluid dynamical considerations. We develop a steady two-dimensional forced-flow configuration to investigate the thermodynamic condition of marginal equilibrium that applies to a solidifying mushy layer with outflow and requires that streamlines are tangent to isotherms at the interface. We show that a ‘two-domain’ approach in which the mushy layer and liquid region are distinct domains is consistent with marginal equilibrium by extending the Stokes equations in a narrow transition region within the mushy layer. We show that the tangential fluid velocity changes rapidly in the transition region to satisfy marginal equilibrium. In convecting mushy layers with liquid channels, a buoyancy gradient can drive this tangential flow. We use asymptotic analysis in the limit of small Darcy number to derive a regime diagram for the existence of steady solutions. Thus we show that marginal equilibrium is a robust boundary condition and can be used without precise knowledge of the fluid flow near the interface.This research began as a project between D. Conroy and M.G.W. at the Geophysical Fluid Dynamics Program: Woods Hole Oceanographic Institution (2006). We gratefully acknowledge helpful discussions with T. Schulze.This is the accepted manuscript for a paper Journal of Fluid Mechanics, Volume 762, January 2015, R1 (12 pages) © 2014 Cambridge University Press, DOI: 10.1017/jfm.2014.65

Consequences of glacial cycles for magmatism and carbon transport at mid-ocean ridges

Submarine and subaerial volcanism transfers carbon from the solid Earth into the atmosphere. Volcanic activity may be modulated by glacial/interglacial cycling of water between continental ice sheets and oceans. It has been argued that the consequent fluctuations in volcanic carbon emission could have paced Pleistocene glacial cycles. This depends on the amplitude and lag of the mid-ocean ridge response to sea-level changes. Here we develop and analyse a new model for that response, eliminating the questionable assumptions made in previous work. Our model accounts for the thermodynamic effect of mantle carbon: reduction of the solidus temperature and a deeper onset of melting. We discuss models forced by idealised, periodic sea level to conclude that fluctuations in melting rate are the prime control on magma and carbon flux (in contrast to previous work). Our models predict lags of less than 10 kyr between peaks in the rate of sea-level change and peaks in mid-ocean ridge emissions. We also discuss a model forced by a reconstruction of eustatic sea level over the past 800 kyr. It indicates that peak-to-trough variations of magma and carbon flux are up to about 20% and 10% of the mean flux, respectively

Physics of melt extraction from the mantle: speed and style

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. </jats:p><jats:p> 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

Secretor Status, Candidal Carriage and Candidal Infection in Patients with Diabetes-Mellitus

One hundred and nine patients with diabetes mellitus and 100 age-, sex- and denture-status-matched, non-diabetic individuals were investigated prospectively. Comparison was made of oral candidal carriage, clinical infection and inherited ability to secrete blood group antigens in saliva. Diabetic patients had a significantly higher prevalence of oral candidal carriage and infection (P less than 0.001) than non-diabetic individuals, but the candidal load between the 2 groups, was not significant. A comparable proportion of insulin-dependent, non-insulin dependent and control groups were secretors of blood group antigens, and there was no difference in the oral candidal carriage and infection rates between secretors and non-secretors.link_to_subscribed_fulltex