Temporal Evolution of Cooling by Natural Convection in an Enclosed Magma Chamber

This research numerically studies the transient cooling of partially liquid magma by natural convection in an enclosed magma chamber. The mathematical model is based on the conservation laws for momentum, energy and mass for a non-Newtonian and incompressible fluid that may be modeled by the power law and the Oberbeck–Boussinesq equations (for basaltic magma) and solved with the finite volume method (FVM). The results of the programmed algorithm are compared with those in the literature for a non-Newtonian fluid with high apparent viscosity (10–200 Pa s) and Prandtl (Pr = 4 × 104) and Rayleigh (Ra = 1 × 106) numbers yielding a low relative error of 0.11. The times for cooling the center of the chamber from 1498 to 1448 K are 40 ky (kilo years), 37 and 28 ky for rectangular, hybrid and quasi-elliptical shapes, respectively. Results show that for the cases studied, natural convection moved the magma but had no influence on the isotherms; therefore the main mechanism of cooling is conduction. When a basaltic magma intrudes a chamber with rhyolitic magma in our model, natural convection is not sufficient to effectively mix the two magmas to produce an intermediate SiO2 composition.

From Magma Ocean turbulent convection to lithosphere formation and mantle convection: insights from laboratory experiments

<p>A clear understanding of the transition from a liquid magma ocean (MO) to a convective solid mantle is still lacking. Part of the problem is that there is still no clear view of all the physical phenomena at play during this crucial stage. As the MO cools down, the formation of a solid and therefore very viscous lithosphere at its surface has often been considered to trigger a new pattern of motion where convection occurs below the lithosphere which remains stagnant. However, when the liquid thermal boundary layer at the top of the MO cools down, it first becomes a mushy lithosphere through which melt and exsolved gas bubbles can still percolate to the surface. Using laboratory experiments of thermal convection in colloidal suspensions, we study the formation of this mushy lithosphere and its different regimes of deformation and coupling to mantle convection. We observe that deformation of the lithosphere can include « heat pipe » formation at high heat, melt and volatile flux. On the other hand, rapid thermal contraction of the lithosphere can cause buckling, leading to subduction. Transition from MO to solid-state convection could involve both processes in succession , or in competition, depending on the temperature and volatiles conditions. </p>

Impact of tides on the transit-timing fits to the TRAPPIST-1 system

Transit timing variations (TTVs) can be a very efficient way of constraining masses and eccentricities of multi-planet systems. Recent measurements of the TTVs of TRAPPIST-1 have led to an estimate of the masses of the planets, enabling an estimate of their densities and their water content. A recent TTV analysis using data obtained in the past two years yields a 34 and 13% increase in mass for TRAPPIST-1b and c, respectively. In most studies to date, a Newtonian N-body model is used to fit the masses of the planets, while sometimes general relativity is accounted for. Using the Posidonius N-body code, in this paper we show that in the case of the TRAPPIST-1 system, non-Newtonian effects might also be relevant to correctly model the dynamics of the system and the resulting TTVs. In particular, using standard values of the tidal Love number k2 (accounting for the tidal deformation) and the fluid Love number k2f (accounting for the rotational flattening) leads to differences in the TTVs of TRAPPIST-1b and c that are similar to the differences caused by general relativity. We also show that relaxing the values of tidal Love number k2 and the fluid Love number k2f can lead to TTVs which differ by as much as a few 10 s on a 3−4-yr timescale, which is a potentially observable level. The high values of the Love numbers needed to reach observable levels for the TTVs could be achieved for planets with a liquid ocean, which if detected might then be interpreted as a sign that TRAPPIST-1b and TRAPPIST-1c could have a liquid magma ocean. For TRAPPIST-1 and similar systems the models to fit the TTVs should potentially account for general relativity, for the tidal deformation of the planets, for the rotational deformation of the planets, and to a lesser extent for the rotational deformation of the star, which would add up to 7 × 2 + 1 = 15 additional free parameters in the case of TRAPPIST-1.

Orbicular rhythmic layering in the palabora carbonatite, South Africa

The earliest stage of magmatic activity within the Palabora carbonatite was marked by the intrusion of phosphate-bearing pyroxenite. In good exposures in the north, large-scale (2m diameter) orbicular structures are found. These consist of regularly-spaced alternating dark layers (phlogopite-rich) and light layers (diopside-plus apatite-rich) which, in hand specimen, are very similar to the ‘inch-scale’ planar layering which has been described in layered mafic instrusions. One of the purposes of this paper is to describe these unique features which are currently being destroyed by mining, as they form economic phosphate concentrations.The resemblance of the Palabora orbicules to Liesegang rings has led to the development of a qualitative model whereby the orbicules are regarded as having been formed by concentric periodic precipitation around central cores within dynamically quiet, isolated pockets of largely liquid magma. The controlling parameters are interpreted as being the rates of growth of the constituent minerals and the rates of diffusion of the elements crucial to their growth, i.e. K+, Al3+ and (OH)- for dark layers, and Ca2+ and P5+ for light layers.The presence of these spectacular structures with their delicate layering indicates that at the time of crystallization of the pyroxenites relatively non-turbulent conditions prevailed, and diopside, phlogopite and apatite crystallized essentially coevally. Hence, the vertical large-scale banding in the pyroxenite may also be a function of diffusion controlled processes rather than being caused by separate magma pulses.

Archean variolites—quenched immiscible liquids

Variolitic lavas from Archean tholeiitic series north and south of Rouyn–Noranda (Abitibi Metavolcanic belt, Canada) contain large, sharply defined, spheroidal to subspheroidal felsic varioles (up to 5 cm in diameter) set in a ferruginous matrix of more mafic composition. Quench texture and flow differentiation studies indicate that the variolites were produced by rapid cooling of a two-liquid magma, and that these liquids were in contact and chemically discrete prior to extrusion. Physical mixing models do not adequately account for these contiguous magmas, yet a liquid immiscible model demonstrably satisfies almost all variolite field, microscopic, microprobe, and chemical data. We conclude Archean variolites are formed by immiscible splitting of a magma of tholeiitic composition.