THE HIGHLY COMPATIBLE TRACE ELEMENT PARADOX–FRACTIONAL CRYSTALLIZATION REVISITED

Abstract

Field relations in dissected volcanic terrains and the internal evidence of persistent low-pressure cotectic character in erupted basalts point to the frequent and substantial modification of liquid compositions by some form of partial crystallization within the crust In contrast, the highly compatible trace elements do not display the marked variations and extreme depletions which are predicted to result from perfect fractional crystallization (PFC). Imperfect fractional crystallization, refilling of magma chambers during fractionation and in situ crystallization are important factors which can help to explain this apparent paradox. This paper explores another effect, the integration of residual liquids from differing extents of partial crystallization, which can help to resolve this paradox, even while still permitting perfect fractional crystallization at all points in the magma chamber. Integration of such residual liquids through the thickness of the crystallization zone is explicit, although not implemented, in the model of in situ crystallization proposed by Langmuir (Nature 340, 199–205, 1989). It may be separated as a process for purposes of mathematical modelling from the basic concept of partial crystallization of small packets of magma with remixing of the residual liquids into the main body of magma. Integration of melts from differing extents of partial crystallization might in principle also be applied to the case of lateral variations in the mass fraction crystallized with position in the magma chamber. Integrated PFC itself can develop residual liquids which differ little from products of equilibrium (batch) crystallization (FTC) at the same average mass fraction of liquid remaining in both incompatible and compatible trace element concentrations. For one specific combination of parameters these integrated liquids are identical in composition at all values of the distribution coefficient to the EPC liquid. At other values of the parameters the integrated liquids may even—a new paradox— have higher relative concentrations of highly compatible elements than the EPC products. Any integration of residual liquids from different mass fractions of PFC rapidly eliminates what have in the past been taken to be the diagnostic differences between PFC and EPC Integration of EPC liquids (towards which the products of imperfect fractional crystallization processes will tend) produces even more pronounced effects, with highly compatible elements less depleted even than in EPC and far less depleted than would be predicted by simple models. When interpreted according to oversimplified models, sequences of residual liquids produced in such processes might appear to be inconsistent with products of a partial crystallization process and to require a process of progressively smaller mass fractions of melting of inhomogeneous and progressively more refractory (higher mg-number) source regions.