Mass change calculations in altered rock series

A technique is presented whereby precursors of altered rocks in a fractionated rock series can be identified, and changes in chemical components, mass and volume accurately calculated. The procedure utilizes elements with high degrees of immobility in hydrothermal and associated interactive water-rock systems. Igneous rock series, and sedimentary and metamorphic rocks with primary continuity of chemical composition, are amenable to this treatment. In the procedure for igneous rocks, best-fit fractionation lines are established for an immobile igneous compatible-incompatible element pair, the latter element functioning as the fractionation monitor. Alteration paths are linear and pass through the origin. Intersections of the fractionation and alteration lines yield the immobile element concentrations in the precursors to the altered rocks; mobile components are determined from fractionation lines and monitor values. Total mass changes are proportional to the displacement of altered samples from the fractionation line. Masses of other elements in an altered sample (reconstructed values) are recalculated to the concentration of the monitor element in the precursor. Chemical changes are the differences between the calculated precursor compositions and the reconstructed altered rock compositions. Errors are mainly a function of the positioning of the fractionation lines.