Contrib Mineral Petrol (1985) 91:24-36 Contributions to Mineralogy and Petrology 9 Springer-Verlag 1985 Rare earth element partitioning between clinopyroxene and silicate liquid at moderate to high pressure T.H. Green and N.J. Pearson School of Earth Sciences, Macquarie University, North Ryde NSW 2113, Australia Abstract. Experimental determination of over seventy sets of clinopyroxene/silicate liquid (glass) partitition coeffi- cients (D) for four rare earth elements (REE - La, Sm, Ho, Lu) in a range of REE-enriched natural rock composi- tions (basalt, basaltic andesite, andesite and rhyodacite) demonstrate a convex upward pattern, favouring the heavy REE (Ho, Lu) and markedly discriminating against the light REE (La). These patterns are consistent with pre- viously documented clinopyroxene D values reported from natural phenocryst/matrix pairs and from experimental work using either REE-enriched compositions and electron microprobe analytical techniques (as in the present study) or natural or synthetic undoped compositions and mass spectrometric, ion probe or X-ray autoradiographic analyti- cal techniques. However, the large data base in the present study allows evaluation of the effect of compositional and physical parameters on REE partitioning relationships. Considering Dno, it is shown that (1) D increases 6-fold with increasing SiO2 content of the coexisting liquid from 50 to ~ 70 wt% SiO2 (2) D increases 4-fold with decreas- ing temperature from 1,120~ C to 900~ (3) D increases 2-fold with increasing pressure from ~2.5 to 20 kb. (4) D increases ~2-fold as fO 2 increases from approximately that of the MW buffer to the HM buffer (5) D remains unchanged within experimental error as the water content of the melt changes from ~0.3 to > 10% by weight H20. The absolute REE content of the clinopyroxene shows no consistent trend with temperature, but decreases slightly with increasing pressure, paralleling an increase in the ja- deite component of the pyroxene. Thus the increase in D with increasing pressure is attributed to changes in the sili- cate liquid structure, which discriminate against accommo- dation of REE with increasing pressure. The clinopyroxene REE content increases with increasingfOz, and in this case the increase in D with increasing fO2 may be attributed mainly to this change in the clinopyroxene composition. Application of the present results to geochemical modelling allows a more appropriate choice of D values, according to the liquid composition and physical conditions appli- cable in the modelled system. They may also be used to evaluate cognate or xenocrystic relationships between clino- pyroxene megacrysts and their host matrix. Introduction In recent years application of trace element geochemical modelling of partial melting and fractional crystallization Offprint requests to: T.H. Green processes has become a powerful tool in evaluating models for the origin of major magma types (Gast 1968; Ewart et al. 1977; Frey et al. 1978; Cameron and Hanson 1982; Pearce 1982; Reid 1983). In particular, the rare earth ele- ments (REE) have proved to be very useful petrogenetic indicators. Simple geochemical modelling - using, for exam- ple, equations for bulk or surface equilibrium processes de- vised by Shaw (1970), Greenland (1970) or Hertogen and Gijbels (1976) and Apted and Roy (1981) requires knowl- edge of at least: (a) equilibrium trace element partition coef- ficients between crystallizing or residual phases and coex- isting liquid; and (b) phase equilibrium data for the condi- tions of melting or fractional crystallization to determine which crystalline phases are involved in the magma deriva- tion process. High pressure experimental petrology and analysis of natural phenocrysts formed from both deep- level and near-surface magma crystallization, have largely satisfied the second requirement. However, our overall knowledge of equilibrium trace element partition coeffi- cients for common magmas and the phases crystallizing from them is relatively restricted, and certainly does not take into account systematically the possible effects of chan- ges in liquid composition, temperature and pressure during melting or crystallization processes (Arth 1976; Irving 1978). Clinopyroxene is well documented as an important re- sidual phase in mantle melting models producing basalts at high pressure (Green and Ringwood 1967), or in deep crustal melting producing more silicic magmas such as an- desites (Holloway and Burnham 1972; Green 1982). It is also recognized as a phase crystallizing from magmas vary- ing in composition from basalt to rhyolite (Ewart et al. 1977) and at depths ranging from the upper mantle (Frey et al. 1978; Green and Ringwood 1967) through the lower crust (Dostal et al. 1983) to the upper crust (Cameron and Hanson 1982). Thus if geochemical modelling based on the REE and incorporating clinopyroxene as a participating phase is to be conducted, knowledge of clinopyroxene/liq- uid partition coefficients for the REE (~cpx/liqh is required \~REE ) for: (a) a liquid compositional range of basalt to rhyolite, (b) temperature as high as 1,300 1,400 ~ C (basalts at mantle depths) or as low as 700-800 ~ C (relatively hydrous rhyo- lites in the crust), and (c) mantle pressures of 20-30 kb extending through lower crust (7.5-12 kb) to near-surface pressure conditions corresponding to high-level, subvol- canic magma chambers. Until recently, commonly used Flepx/liq values were those derived from natural pheno- ~REE cryst/matrix pairs resulting from near-surface crystalliza- tion (summary of data given in Arth 1976) or from experi-