The Isomass Method: Verifying conserved elements in geochemically open geological processes

Abstract

This contribution presents the Isomass method, aimed at verifying the conserved behaviour of elements in geochemically open systems even when the parent rock composition is lacking. The method estimates system size changes for a specific element that is assumed to be conserved by calculating the amount of material transfer for each of the other elements, thus verifying (or rejecting) the initial assumption. By analysing the calculated amount of material transfer (or daughter system size ratios), additional conserved elements (if any) can be identified. The Isomass method is used here to evaluate a set of numerically generated samples whose element concentrations derived after mass additions and losses are assigned to daughter rocks. In addition, it is also applied to three real datasets that examine soil formation, magmatic fractionation in a komatiitic lava, and hydrothermal metasomatism. The method is capable of: (1) determining which elements confirm their conserved behaviour for a variety of geological environments; (2) identifying which other elements are conserved, added or lost; (3) calculating the amount of material transfer; and (4) providing a measure of the extent of the conserved character of elements. The method illustrates that the whole-rock compositions of parent and daughter samples do not represent the actual material transfer that occurred during geochemically open geological processes, as conserved elements may appear enriched or depleted, and non-conserved elements may have concentrations that do not reflect the actual material transfers that took place. The Isomass method is therefore a proper and valuable tool for the verification of conserved elements and the investigation of material transfer in rocks.