Quantification of Crustal Contamination in Open Magmatic Systems

• Published in: Journal of petrology 1994-04, Vol.35 (2), p.461-488
• Main Authors: AITCHESON, S. J., FORREST, A. H.
• Format: Article
• Language: English
• Subjects: Crystalline rocks; Earth sciences; Earth, ocean, space; Exact sciences and technology; Igneous and metamorphic rocks petrology, volcanic processes, magmas
• ISSN: 0022-3530; 1460-2415
• DOI: 10.1093/petrology/35.2.461

The DePaolo (1981a, Earth Planet. Sci. Lett. 53, 189–202) equations for assimilation with fractional crystallization (AFC) are extended and solved to yield directly the fraction of crust assimilated during AFC with or without magma replenishment or recharge. Errors in the computed crust/magma ratio are also presented. Using graphical methods and a sufficient number of elements no assumptions need be made about the value of the ratios r (= rate of assimilation/rate of fractional crystallization) and β (= rate of recharge/rate of assimilation); in fact, the method yields independent estimates of these ratios in addition to the crust/magma ratio. Thus the average relative sizes of the fluxes of wall-rock melt, replenishing magma, and fractionating phases are known and can be incorporated in a budget for the whole magma-crust system. The method for calculating the crust/magma ratio is extended to encompass variable values of r, β, and bulk distribution coefficient (D). Simple (bulk) mixing is effectively a special case of AFC where β or r → ∞ and/or D = 0. Only for highly incompatible elements (D