The viscosity of magmatic silicate liquids: A model for calculation

A simple model has been designed to allow reasonably accurate calculations of viscosity as a function of temperature and composition. The problem of predicting viscosities of anhydrous silicate liquids has been investigated since such viscosity numbers are applicable to many extrusive melts and to nearly dry magmatic liquids in general. The fluidizing action of water dissolved in silicate melts is well recognized and it is now possible to predict the effect of water content on viscosity in a semiquantitative way. Water was not incorporated directly into the model. Viscosities of anhydrous compositions were calculated, and, where necessary, the effect of added water and estimated. The model can be easily modified to incorporate the effect of water whenever sufficient additional data are accumulated

Effects of water and fluorine on the viscosity of albite melt at high pressure: a preliminary investigation

The viscosities of fluorine- and water-bearing melts based on albite composition have been determined at 7.5, 15 and 22.5 kbar by the falling-sphere method. All melt viscosities decrease isothermally with increasing pressure. At 1200°C the viscosity of the fluorine-bearing melt (albite + 5.8 wt.% fluorine substituted for oxygen, denoted AbF2O−1) decreases from5000 ± 750P at7.5kbar to1600 ± 240P at22.5kbar. At 1400°C the viscosity of this melt decreases from1300 ± 200P at7.5kbar to430 ± 65P at22.5kbar. At 1400°C the viscosity of albite + 2.79 wt.% water (denoted AbH2O) decreases from650 ± 100P at7.5kbar to400 ± 60P at22.5kbar. Fluorine (as F2O−1) and water strongly decrease the viscosity of albite melt over the entire range of investigated pressures. The ratio of the effects of 5.8 wt.% fluorine [F/(F + O)molar = 0.10] and 2.79 wt.% water [OH/(OH + O)molar = 0.10] on the log of melt viscosity [Δ log η(AbF2O−1)/Δ log η(AbH2O)] equals0.90 ± 0.05, 0.84 ± 0.05and0.97 ± 0.05at7.5, 15and22.5kbar, respectively. Comparison with available data on the high-pressure viscosity of albite melt indicates that both F2O−1 and H2O maintain their viscosity-reducing roles to lower crustal pressures. The difference between the viscosities of melts of albite, AbF2O−1 and AbH2O, may be explained in terms of the relatively depolymerized structures of AbF2O−1 and AbH2O melts. The depolymerization of albite melt by the addition of water results from the formation of SiOH bonds. The depolymerization of albite melt by F2O−1 substitution results from the formation of non-bridging oxygens associated with network-modifying aluminum cations that are formed upon fluorine solution. The strong viscosity-reducing effects of water and fluorine in albite melt at pressures corresponding to the mid- to lower continental crust indicate that these two components will strongly influence the dynamic behavior of anatectic melts during initial magma coalescence and restite-melt segregation

Lower crustal crystallization and melt evolution at mid-ocean ridges

Author Posting. © The Author(s), 2012. This is the author's version of the work. It is posted here by permission of Nature Publishing Group for personal use, not for redistribution. The definitive version was published in Nature Geoscience 5 (2012): 651–655, doi:10.1038/ngeo1552.Mid-ocean ridge magma is produced when Earth’s mantle rises beneath the ridge axis and melts as a result of the decrease in pressure. This magma subsequently undergoes cooling and crystallization to form the oceanic crust. However, there is no consensus on where within the crust or upper mantle crystallization occurs1-5. Here we provide direct geochemical evidence for the depths of crystallization beneath ridge axes of two spreading centres located in the Pacific Ocean: the fast-spreading-rate East Pacific Rise and intermediate-spreading-rate Juan de Fuca Ridge. Specifically, we measure volatile concentrations in olivine-hosted melt inclusions to derive vapour-saturation pressures and to calculate crystallisation depth. We also analyse the melt inclusions for major and trace element concentrations, allowing us to compare the distributions of crystallisation and to track the evolution of the melt during ascent through the oceanic crust. We find that most crystallisation occurs within a seismically-imaged melt lens located in the shallow crust at both ridges, but over 25% of the melt inclusions have crystallisation pressures consistent with formation in the lower oceanic crust. Furthermore, our results suggest that melts formed beneath the ridge axis can be efficiently mixed and undergo olivine crystallisation in the mantle, prior to ascent into the ocean crust.This research was supported by the National Science Foundation (EAR-0646694) and the WHOI Deep Ocean Exploration Institute/Ocean Ridge Initiative.2013-02-1

Effect of Re-impacting Debris on the Solidification of the Lunar Magma Ocean

Anorthosites that comprise the bulk of the lunar crust are believed to have formed during solidification of a Lunar Magma Ocean (LMO) in which these rocks would have floated to the surface. This early flotation crust would have formed a thermal blanket over the remaining LMO, prolonging solidification. Geochronology of lunar anorthosites indicates a long timescale of LMO cooling, or re-melting and re-crystallization in one or more late events. To better interpret this geochronology, we model LMO solidification in a scenario where the Moon is being continuously bombarded by returning projectiles released from the Moon-forming giant impact. More than one lunar mass of material escaped the Earth-Moon system onto heliocentric orbits following the giant impact, much of it to come back on returning orbits for a period of 100 Myr. If large enough, these projectiles would have punctured holes in the nascent floatation crust of the Moon, exposing the LMO to space and causing more rapid cooling. We model these scenarios using a thermal evolution model of the Moon that allows for production (by cratering) and evolution (solidification and infill) of holes in the flotation crust that insulates the LMO. For effective hole production, solidification of the magma ocean can be significantly expedited, decreasing the cooling time by more than a factor of 5. If hole production is inefficient, but shock conversion of projectile kinetic energy to thermal energy is efficient, then LMO solidification can be somewhat prolonged, lengthening the cooling time by 50% or more

Oxygen isotope composition of a section of lower oceanic crust, ODP Hole 735B

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/95244/1/ggge964.pd

Hydrothermal alteration and microbial sulfate reduction in peridotite and gabbro exposed by detachment faulting at the Mid‐Atlantic Ridge, 15°20′N (ODP Leg 209): A sulfur and oxygen isotope study

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/95304/1/ggge1087.pd

Fluidal pyroclasts reveal the intensity of peralkaline rhyolite pumice cone eruptions

This work is a contribution to the Natural Environment Research Council (NERC) funded RiftVolc project (NE/L013932/1, Rift volcanism: past, present and future) through which several of the authors are supported. In addition, Clarke was funded by a NERC doctoral training partnership grant (NE/L002558/1).Peralkaline rhyolites are medium to low viscosity, volatile-rich magmas typically associated with rift zones and extensional settings. The dynamics of peralkaline rhyolite eruptions remain elusive with no direct observations recorded, significantly hindering the assessment of hazard and risk. Here we describe uniquely-preserved, fluidal-shaped pyroclasts found within pumice cone deposits at Aluto, a peralkaline rhyolite caldera in the Main Ethiopian Rift. We use a combination of field-observations, geochemistry, X-ray computed microtomography (XCT) and thermal-modelling to investigate how these pyroclasts are formed. We find that they deform during flight and, depending on size, quench prior to deposition or continue to inflate then quench in-situ. These findings reveal important characteristics of the eruptions that gave rise to them: that despite the relatively low viscosity of these magmas, and similarities to basaltic scoria-cone deposits, moderate to intense, unstable, eruption columns are developed; meaning that such eruptions can generate extensive tephra-fall and pyroclastic density currents.Publisher PDFPeer reviewe