MODELING OF REE FRACTIONATION IN THE ACID MELT-FLUORIDE-CHLORIDE FLUID SYSTEM

Abstract

High-F silicic (HS) rocks have some characteristic geochemical features. The REE distribution, for exam-ple, is characterized by approximately equal chondrite-normalized abundances of LREE and HREE (equal-arm distribution) and a significant Eu minimum. Strong Eu depletion in HS rocks is related to either feldspar fractionation [3] or selective fluid removal of Eu from the melt [4, 5]. Recent petrological data [6] suggest fluid–magmatic differentiation of REE in HS rocks. This work presents estimates of the effect of REE fractionation between fluid and HS melt, which confirms this viewpoint. The problem of REE fractionation in the acid melt– fluid system has seemingly been solved for a long time, especially after the pioneer experiments of Flynn and Burnham [4]. However, later experiments [7] on REE distribution between fluid and magma were inconsis-tent with the earlier data. In addition, REE transfer was mainly studied for the aqueous–chloride system, whereas the fluoride mode of REE transfer by fluids dominates in HS rocks. Thermodynamic data on solu-ble REE fluorides and chlorides [8] make it possible to simulate the element dissolution in the chloride–fluo-ride solutions, i.e., to reconstruct the HS rock evolution. Fluid and melt compositions for modeling can be taken from the available data on well-studied magmatic systems. The REE variations during differentiation (Fig. 1) are expressed in ratios of element concentra-tions in strongly differentiated rocks to those in weakly differentiated rocks for well-studied HS systems [9]. REE distribution correlates well with F concentration in magma: Ce/Yb and Eu/Eu* ratios are lower in high-F magmas. The LREE/HREE ratio decreases owing to HREE increase or LREE decrease. The analysis of such trends shows that the evolution of HS rocks is not accompanied by a total REE increase; therefore, such a REE distribution is mainly caused by LREE removal from the melt and F accumulation. Unlike F, Cl is not correlated with Ce/Yb and Eu/Eu* ratios, thus indicat-ing its subordinate role in REE redistribution. Such relations suggest that Eu and other REE are mainly redistributed in magma as fluoride compounds. The Ln (i.e., REE) redistribution between the fluid and acid melt may be described by the following reac-tions: