A simplified minimum norm approach for estimation of volume strain and geochemical mass transfer in metasomatic alterations

Group GSI.IR
Location International Geological Congress,oslo 2008
Author Mukherjee, Pulok۱; Gupta, Preveen۲
Holding Date 08 October 2008

In view of the wide-ranging application of geochemical mass-balance in metasomatic alteration, the Gresens’ volume-composition relationship has been re-examined for a practical solution. The solution to the system of simultaneous equations for all components is not straight forward and it requires a very crucial assumption of conserved components as reference frame. Thus the implementation of geochemical mass-balance in alteration process is controlled by two basic operations, (i) the identification of a set of immobile elements and (ii) optimal estimation of the volume change with respect to the critically selected conserved elements. Both these issues are addressed and a definitive criterion is suggested for identification of likely immobile species and a simple new procedure for obtaining a rational solution to the mass-balance equation. Although all the elements may, depending upon the prevailing physicochemical condition, be potentially mobile in alteration processes, yet the extent of loss or gain may be insignificant for some. Such likely immobile elements (LIEs) serve as the reference frame, with respect to which the volume strain and mass transfer is quantified. Graphical solution by ISOCON method leads to ambiguous results where arbitrary scaling of immobile elements gives illogical weight to different elements. However, it is proposed here that probable immobile elements may be effectively identified through their identical residual enrichment factor or the isochemical volume factor values. Each one of these LIEs, as reference frame, would define its own mass-balance estimates within a limited range. Best estimate for the reference frame would therefore be the one that also quantitatively minimizes the mobility of the LIEs. A simplified minimum norm approach (MINMAS) is proposed here and applied to compute the volume change by minimzing the mass transfer of LIEs. MINMAS offers a consequential basis to the solution of Gresens’ equation that seeks an optimum volume factor consistent with least collective mass transfer of the identified or assumed immobile elements. This method has been successfully applied and demonstrated using published dataset.