Diffusion-reaction in a thermal gradient: Implications for the genesis of anorthitic plagioclase, high alumina basalt and igneous mineral layering

Textures and mineralogical changes in the gabbro indicate that chemical transport occurs throughout the experiments despite temperatures at the cold end of the experimental capsule approaching 500 °C. For instance, apatite, FeNiS, olivine and almost pure albite occur at distinct, specific horizons in the gabbro within the 26-day experiment. Because the bulk element profiles indicating chemical transport reflect analyses of almost completely solid gabbro, equilibration between minerals and fluids/melts must be rapid. The overall effect of the diffusion-reaction process is to make an ascending magma more primitive in composition (and in this case, produce anorthitic plagioclase) while making surrounding crustal wall rocks more evolved. Several observations within igneous rocks support the occurrence of this process, suggesting that the genesis of porphyritic high alumina basalt, ubiquitously observed at convergent margins, could reflect a diffusion-reaction process in the crust.