ASCENT MECHANISM OF FELSIC MAGMAS - NEWS AND VIEWS

Diapirism has been discredited as a transport mechanism for magmas par tly because diapirs seem to be unable to bring magmas to shallow crust al levels (<10 km) and partly because recent developments in the theor y of dyke propagation have shown that sufficiently wide dykes are able to efficiently transport felsic magmas through the crust. However, it is still unclear how felsic dykes grow to widths that allow them to p ropagate faster than they close by magma freezing. Ultimately, it may be the ability of felsic dykes to grow within the source that controls which mechanism dominates ascent. The ability of dykes to propagate f rom the top of rising diapirs depends among other factors on the chang ing temperature gradient of the wall rocks. The steep gradient around rapidly rising diapirs in the low viscosity lower crust will cause dyk es to freeze. As diapirs rise to colder stiffer crust and decelerate, heat diffuses further from the diapir, resulting in shallower temperat ure gradients that favour dyke propagation. The mechanism may thus swa p, during ascent, from diapirism to dyking. Calculations of the therma l evolution of diapirs and their surroundings show that basaltic diapi rs may never form because they would be drained by dykes at a very ear ly stage; felsic diapirs may be unable to give rise to successful dyke s, whereas diapirs of intermediate magmas may propagate dykes during a scent.