Modelización del proceso de episienitización (decuarcificación-albitización): formulación cinética y transporte advectivo en medio continuo

Abstract

The development of episyenites from Sierra de Guadarrama has been modelized. The process mainly consists in a coupled process of dequartztification-albitization. The model is based on pure advective mass transport in a continuum medium, where the reactive process is formulated from a kinetic point of view.
It is demonstrated the possibility of its formation through the action of low-salinity fluids (2,32 wt % NaCl equiv.), partially equilibrated with a biotite monzogranite, linked to isobaric reaction paths across negative geothermal gradients, intersecting the quartz retrograde solubility field, and associated to a quick fluid-resident mobilisation due to the environment tectonic opening.
The model explains the albitic nature of the alteration against former models. These predict microcline formation from the gradient relations ∂logK_mic/∂T > ∂logK_ab/∂T (0.05739 and 0.05735) and ∂logK_KCl/∂T < ∂logK_NaCl/∂T (-0.038 and -0.036). Albite formation arises from the anortite component presence, because ∂logK_an/∂T =0,1293 ≻ ∂logK_mic/∂T.
Non-equilibrium relations are very small, which implies that alteration intensity arises from the circulation of huge fluid volumes. This process feature together the kinetic rate constants result in a small change, spatial and temporal, of the fluid composition, being practically null for its main components (Na, K and, subordinately to these, the pH).
The model also shows the necessity of handling geochemical systems as complex as possible. In this way, the proposed aqueous speciation proves that pH evolution along decreasing temperature is not necessarily acid, but inlet fluid pH dependent due to the calculated concentrations show that its behaviour mainly depends as much from Hel as from the NaOH base dissociation.