A new insight into metasomatism through mass-density-solid volume variation maps

Abstract

In order to present a new petrological approach to quantify mass transfer redistribution, especially regarding chemical zonation, published data of a centimeter sized aluminium-rich clinopyroxene from the Bergen Arcs in Norway is used. The sample comes from the island of Holsnøy where the dry granulite-facies protolith underwent hydration reaction under amphibolite-facies conditions. This aluminium-rich clinopyroxene developed kinks during deformation along which fluid has been introduced. It reacted in two different ways: to garnet plus a less aluminous pyroxene along kinks and to chlorite along cleavage planes. Compositional maps from electron probe microanalyzer (EPMA) were coupled to the mass balance equation of Gresens (Gresens, 1967) to quantify for each pixel present in a studied surface area, element gains and losses, density changes and solid volume variations. This new method allows to estimate the solid volume variation associated with the reaction by assuming minimal mass transfer, or by considering an immobile element i.e. aluminium immobile or by preserving the mass during the reaction. Results demonstrate that for all three assumptions made, the actual mass behaviour does not change significantly for major elements as opposed to the solid volume variation and the actual sum of mass transfer. In the high strain domain (kink) the element losses and gains in replacing the aluminium-rich clinopyroxene by garnet, are balanced by the opposite gains and losses associated with the less aluminous clinopyroxene. This implies that the local fluid chemical composition required for the formation of one mineral is equal to that in equilibrium with the second one after the reaction. The same observation can be made regarding solid volume variation that is balanced between garnet and the less aluminous clinopyroxene. Considering now the specific reaction in the kink and outside the kink (chlorite), mass transfer and redox conditions (Fe3+ and Fe2+) tend to behave in the same way supposing a temporal relationship between these two parageneses.