Exploring the effects of a time- and space-dependent eruption efficiency on planetary evolution.

<p>It has been shown that melting and crust production strongly influences the convection regime of terrestrial planets, potentially even more than the vigor of convection. A planet producing and erupting a lot of crust can hardly remain in the stagnant lid regime and produces resurfacings or even reaches some mobile-lid regime. On the other hand, a planet that intrudes its melt in the lithosphere tends to have a larger conductive heat flux and cools efficiently without much lid mobility. Thus, the question of the amount of melts being erupted or intruded might dominate the cooling of terrestrial planets. So far, an "eruption efficiency", which gives the ratio of melt that erupts over the remaining melt fraction, has been imposed in numerical simulations. The eruption efficiency in the convection code StagYY has thus far been treated as a constant in time and space. Here, we explore the effects of a time- and space-dependent eruption efficiency on planetary evolution in the planetary convection code StagYY. An equation was devised that describes how eruptive a system is, based on the main characteristics of lithospheric melt transport: the amount of melt and the local stress state. In a range of systematic simulations, we explore the consequences of this parameter. </p><p>In a first set of simulations this parameter is explored while keeping the eruption efficiency constant. Results show that the most important parameters are the amount of melt, where the stress has smaller local effects. Additionally, changing the yield stress, viscosity or constant eruption efficiency has a large effect on what the eruptivity should be based on this equation. Parameters that govern the global mantle temperature are less important for the eruptivity. </p><p>A second set of simulations was performed with the eruption efficiency behaving in a fully self-consistent manner. These models tend to behave like intrusive systems, except during resurfacing episodes when the models become very extrusive. Models that show mobile behaviour at almost all times in the planetary evolution will have an almost constant spatially averaged eruption efficiency. In these models the eruption efficiency does vary locally however.</p>

Granites and crustal heat budget

The origin of large I-type batholiths remains a disputed topic. One model states that I-type granites form by partial melting of older crustal lithologies (amphibolites or intermediate igneous rocks). In another view, granites are trapped rhyolitic liquids occurring at the end of fractionation trends defining a basalt–andesite–dacite–rhyolite series. This paper explores the thermal implications of both scenarios, using a heat balance model that abstracts the heat production and consumption during crustal melting. Heat is consumed by melting and by losses through the surface (conductive or advective, as a result of eruption). It is supplied as a basal conductive heat flux, as internal heat production or as advective heat carried by an influx of hot basalt into the crust. Using this abstract approach, it is possible to explore the role different parameters play in the balance of granites formed by differentiation of basalts or by crustal melting. Two end-member situations appear equally favourable to generating large volumes of granites: (1) short-lived environments dominated by high basaltic flux, where granites result mostly from basalt differentiation; and (2) long-lived systems with no or minimal basalt flux, with granites resulting chiefly from crustal melting.

Observations and modelling of first-year ice growth and simultaneous second-year ice ablation in the Prydz Bay, East Antarctica

ABSTRACT The seasonal cycle of fast ice thickness in Prydz Bay, East Antarctica, was observed between March and December 2012. In March, we observed a 0.16 m thickness gain of 0.22 m-thick first-year ice (FYI), while 1.16 m-thick second-year ice (SYI) nearby simultaneously ablated by 0.59 m. A 1-D thermodynamic sea-ice model was applied to identify the factors that led to the simultaneous growth of FYI and melt of SYI. The different evolutions were explained by the difference in the conductive heat flux between the FYI and SYI. As the FYI was thin, there was a large temperature gradient between the ice base and the colder ice surface. This generated an upward conductive heat flux, which was larger than the heat flux from the ocean to the ice base, yielding basal growth of ice. In the case of the thicker SYI the temperature gradient and, hence, the conductive heat flux were smaller, and not sufficient to balance the oceanic heat flux at the ice base, yielding basal ablation. Penetration of solar radiation affected the conductive heat flux in both cases, and the model results were sensitive to the initial ice temperature profile and the uncertainty of the oceanic heat flux.