Rare earth element partitioning between clinopyroxene and silicate liquid at moderate to high pressure

Experimental determination of over seventy sets of clinopyroxene/silicate liquid (glass) partitition coefficients (D) for four rare earth elements (REE - La, Sm, Ho, Lu) in a range of REE-enriched natural rock compositions (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 previously 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 analytical 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 D_Ho, 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 decreasing temperature from 1,120°C to 900° C (3) D increases 2-fold with increasing pressure from ∼ 2.5 to 20 kb. (4) D increases ∼ 2-fold fO₂ 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 H₂O. The absolute REE content of the clinopyroxene shows no consistent trend with temperature, but decreases slightly with increasing pressure, paralleling an increase in the jadeite component of the pyroxene. Thus the increase in D with increasing pressure is attributed to changes in the silicate liquid structure, which discriminate against accommodation of REE with increasing pressure. The clinopyroxene REE content increases with increasing fO₂, 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 applicable in the modelled system. They may also be used to evaluate cognate or xenocrystic relationships between clinopyroxene megacrysts and their host matrix.