Boudewijn Ph. van Milligen

New experiment to model self-organized critical transport and accumulation of melt and hydrocarbons from their source rocks

A new, simple, and easily reproducible experiment was designed to simulate the production, accumulation, and transport of melt within rock. The transport was found to be of the self-organized critical type. The emergence of self-organized criticality is explained by the availability of hydrofracture propagation as a rapid or ballistic transport mechanism. This mechanism also serves as a mechanism for stepwise accumulation. These findings are confirmed by a numerical model, which shows the emergence of self-organized critical behavior when Darcian transport cannot accommodate transport and the dormant transport mechanism of hydrofracture propagation is activated. Ballistic and self-organized critical transport may play a significant role in the transport and accumulation of geological fluids, such as melt and hydrocarbons. This conclusion has a profound impact on the modeling of many transport processes in geology (e.g., accumulation of melt, oil, and gas).

Layered intrusions and traffic jams

A wide range of explanations has been proposed for the origin of repetitive layering in mafic-ultramafic and in (per)alkaline intrusions. Here we propose that the interaction of mineral grains that sink and float in the crystallizing magma is an alternative mechanism that can explain many of the features of layered intrusions, without the need to invoke extrinsic factors. Similar to traffic jams on a motorway, small perturbations in crystal density develop that impede further ascent or descent of buoyant or heavy minerals, respectively. These “traffic jams” separate layers of magma from the rest of the magma chamber. The magma in the individual layers further evolves as a largely independent subsystem, with gravitational sorting organizing the mineral distribution within each layer. Layering can develop in the intermediate range between full mineral separation in low-viscosity or slowly cooling magma chambers and homogeneous crystallization in high-viscosity or fast-cooling chambers. This self-organization mechanism provides a novel explanation for the formation of rhythmic layering in low-viscosity magmas, for example in the Ilimaussaq igneous complex in southwest Greenland.

Magnetic well scan and confinement in the TJ-II stellarator

The magnetic well is the main stabilising mechanism in the TJ-II stellarator, since this is an almost shearless device. TJ-II has the capability of varying its magnetic configuration by changing the currents of its coils, allowing one to change the magnetic well while keeping the vacuum rotational transform profile almost constant, in particular. This characteristic makes this stellarator a suitable device to study the impact of unfavourable magnetic well conditions on plasma performance and stability. The here reported experiments explored a family of ten magnetic configurations with a similar rotational transform and varying magnetic well, from positive to negative values, hence exploring Mercier-unstable configurations. We succeeded in developing reproducible NBI-heated plasmas, even when the most Mercier-unstable configurations were performed, although the turbulence level was higher in the latter configurations. The position of the plasma boundary agreed with the equilibrium calculations in all cases.

Out of Africa by spontaneous migration waves

Hominin evolution is characterized by progressive regional differentiation, as well as migration waves, leading to anatomically modern humans that are assumed to have emerged in Africa and spread over the whole world. Why or whether Africa was the source region of modern humans and what caused their spread remains subject of ongoing debate. We present a spatially explicit, stochastic numerical model that includes ongoing mutations, demic diffusion, assortative mating and migration waves. Diffusion and assortative mating alone result in a structured population with relatively homogeneous regions bound by sharp clines. The addition of migration waves results in a power-law distribution of wave areas: for every large wave, many more small waves are expected to occur. This suggests that one or more out-of-Africa migrations would probably have been accompanied by numerous smaller migration waves across the world. The migration waves are considered “spontaneous”, as the current model excludes environmental or other factors. Large waves preferentially emanate from the central areas of large, compact inhabited areas. During the Pleistocene, Africa was the largest such area most of the time, making Africa the statistically most likely origin of anatomically modern humans, without a need to invoke additional environmental or ecological drivers.