Extreme fractionation in felsic magma chambers: a product of liquid-state diffusion or fractional crystallization?

Extreme fractionation of minor and trace elements commonly accompanies very modest changes in major element concentrations in highly felsic igneous sequences. In such sequences, Si increases by only a few percent while, for example, Sr, Ba, Mg, and light rare earth elements decrease drastically, commonly by a factor of 10 or more. It has been argued, most notably by Hildreth (e.g. [1]), that such trends observed in tuffs were not induced by fractional crystallization (FC), but rather are a manifestation of compositional gradients in parental magma chambers which form via liquid-state thermogravitational diffusion (LSTD). The strongest arguments against FC are that (1) crystal settling is not a viable mechanism for crystal-liquid separation, and (2) extensive recrystallization is required to produce the observed trends, yet the tuffs are relatively crystal-poor. Many workers have noted trends in plutonic as well as volcanic rocks which are strikingly similar to those for which LSTD has been proposed, and some have concluded that LSTD was the fractionating mechanism.Several lines of evidence lead us to the conclusion that FC is the dominant differentiating process in high-silica magmas: (1) elemental trends are strikingly consistent with those predicted for FC; it would be a remarkable coincidence if diffusion-induced trends mimicked FC so closely; (2) large phenocryst assemblages in high-silica tuffs indicate low-variance liquid compositions that would be improbable if crystal-liquid equilibria were not controlling differentiation; (3) highly evolved plutonic rocks in many cases do not form the caps expected for LSTD, but rather occur in dikes and pods where they apparently segregated as late liquids; (4) recent experimental studies suggest that trends induced by diffusion differ drastically from observed felsic igneous trends.We do not believe that the principal arguments against FC in high-silica systems (unlikelihood of crystal settling; crystal-poor nature of tuffs) refute the reality of the chemical process, but rather emphasize the need for a better understanding of the physical mechanisms of crystal-liquid fractionation and eruption.     

Magma–ice–sediment interactions and the origin of lava/hyaloclastite sequences in the Síða formation, South Iceland

Products of subglacial volcanism can illuminate reconstructions of paleo-environmental conditions on both local and regional scales. Competing interpretations of Pleistocene conditions in south Iceland have been proposed based on an extensive sequence of repeating lava-and-hyaloclastite deposits in the Síða district. We propose here a new eruptive model and refine the glacial environment during eruption based on field research and analytical data for the Síða district lava/hyaloclastite units. Field observations from this and previous studies reveal a repeating sequence of cogenetic lava and hyaloclastite deposits extending many kilometers from their presumed eruptive source. Glasses from lava selvages and unaltered hyaloclastites have very low H₂O, S, and CO₂ concentrations, indicating significant degassing at or close to atmospheric pressure prior to quenching. We also present a scenario that demonstrates virtual co-emplacement of the two eruptive products. Our data and model results suggest repeated eruptions under thin ice or partially subaerial conditions, rather than eruption under a thick ice sheet or subglacial conditions as previously proposed.     

RCP4.5: a pathway for stabilization of radiative forcing by 2100

Representative Concentration Pathway (RCP) 4.5 is a scenario that stabilizes radiative forcing at 4.5?W?m^?2 in the year 2100 without ever exceeding that value. Simulated with the Global Change Assessment Model (GCAM), RCP4.5 includes long-term, global emissions of greenhouse gases, short-lived species, and land-use-land-cover in a global economic framework. RCP4.5 was updated from earlier GCAM scenarios to incorporate historical emissions and land cover information common to the RCP process and follows a cost-minimizing pathway to reach the target radiative forcing. The imperative to limit emissions in order to reach this target drives changes in the energy system, including shifts to electricity, to lower emissions energy technologies and to the deployment of carbon capture and geologic storage technology. In addition, the RCP4.5 emissions price also applies to land use emissions; as a result, forest lands expand from their present day extent. The simulated future emissions and land use were downscaled from the regional simulation to a grid to facilitate transfer to climate models. While there are many alternative pathways to achieve a radiative forcing level of 4.5?W?m^?2, the application of the RCP4.5 provides a common platform for climate models to explore the climate system response to stabilizing the anthropogenic components of radiative forcing.