The Highly Compatible Trace Element Paradox--Fractional Crystallization Revisited

Field relations in dissected volcanic terrains and the internalevidence of persistent low-pressure cotectic character in eruptedbasalts point to the frequent and substantial modification ofliquid compositions by some form of partial crystallizationwithin the crust In contrast, the highly compatible trace elementsdo not display the marked variations and extreme depletionswhich are predicted to result from perfect fractional crystallization(PFC). Imperfect fractional crystallization, refilling of magmachambers during fractionation and in situ crystallization areimportant factors which can help to explain this apparent paradox.This paper explores another effect, the integration of residualliquids from differing extents of partial crystallization, whichcan help to resolve this paradox, even while still permittingperfect fractional crystallization at all points in the magmachamber. Integration of such residual liquids through the thicknessof 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 aprocess for purposes of mathematical modelling from the basicconcept of partial crystallization of small packets of magmawith remixing of the residual liquids into the main body ofmagma. Integration of melts from differing extents of partialcrystallization might in principle also be applied to the caseof lateral variations in the mass fraction crystallized withposition in the magma chamber. Integrated PFC itself can developresidual liquids which differ little from products of equilibrium(batch) crystallization (FTC) at the same average mass fractionof liquid remaining in both incompatible and compatible traceelement concentrations. For one specific combination of parametersthese integrated liquids are identical in composition at allvalues of the distribution coefficient to the EPC liquid. Atother values of the parameters the integrated liquids may even—anew paradox— have higher relative concentrations of highlycompatible elements than the EPC products. Any integration ofresidual liquids from different mass fractions of PFC rapidlyeliminates what have in the past been taken to be the diagnosticdifferences between PFC and EPC Integration of EPC liquids (towardswhich the products of imperfect fractional crystallization processeswill tend) produces even more pronounced effects, with highlycompatible elements less depleted even than in EPC and far lessdepleted than would be predicted by simple models. When interpretedaccording to oversimplified models, sequences of residual liquidsproduced in such processes might appear to be inconsistent withproducts of a partial crystallization process and to requirea process of progressively smaller mass fractions of meltingof inhomogeneous and progressively more refractory (higher mg-number)source regions. KEY WORDS: highly compatible elements; in situ crystallization; boundary layer; integrated crystallization ^*e-mail: oharamj{at}cardiff.ac.uk