The role of clinopyroxene in generating U-series disequilibrium during mantle melting

We develop recent models of crystal-liquid partitioning (Blundy and Wood, 1 994; Wood and Blundy, 1997) to predict the effects of crystal chemistry on the partitioning of U-series elements between clinopyroxene and coexisting silicate melt. With increasing pressure along the mantle solidus, clinopyro xene becomes richer in Na2O and Al2O3 and these components are predicted to reduce the size of the large M2 site. This means that, because U4+ is Smal ler than Th4+, clinopyroxene should progressively discriminate in favour of U4+ with increasing pressure. We find that the effect is sufficiently grea t that D-U/D-Th should change from <1.0 at pressures up to 1.0 GPa to >1.0 above 1.5 Cpa. Thus, melting in the stability field of clinopyroxene should produce excess U-238 relative to Th-230 at low pressure and excess Th-230 above 1.5 GPa. Observed excesses of Th-230 in MORB may not, therefore, requ ire the presence of garnet in the source region. We performed experiments a t 1.5-1.9 Cpa on compositions doped with U and Th to test the calculations. Crystals and quenched melts were analysed by ion microprobe. On the mantle solidus at 1.5 GPa, D-U/D-Th for clinopyroxene was found to be 1.07 +/- 0. 08, while a value of 1.19 +/- 0.04 was obtained at 1.9 GPa. These results c onfirm the prediction that mantle clinopyroxenes have D-U/D-TH > 1.0 at hig h pressure. Absolute values of D-U for clinopyroxene are 0.029 at 1.5 GPa a nd 0.023 at 1.9 GPa in good agreement with earlier measurements on aluminou s clinopyroxenes. Corresponding values for orthopyroxene at 1.5 GPa are 1.7 x 10(-3) for D-U and 8 x 10(-4) for D-Th while olivine gave D-U of 1.8 x 1 0(-5) and D-Th of 1.2 x 10(-5) under similar conditions. When the new data are used in dynamic melting calculations we find that Th-230/U-238 activity ratios of up to about 1.23 in the liquid are consistent with melting in th e spinel Iherzolite field provided porosity is approximately 10(-3) and mel ting rate less than or equal to 3 x 10(-5) kg. m(-3) y(-1). This confirms o ur model prediction that residual garnet is not required to generate excess Th-230 in mantle melts. Solid-liquid partition coefficients for other memb ers of the U decay series, Ra and Ac may be calculated from the model. For clinopyroxene we obtain D-RA of about 10(-6) and D-Ac of 5 x 10(-4). The va lue for Ac is large enough that Ac-227 activities may provide information o n melt extraction processes. The M2 site of orthopyroxene is found to be el astically similar to the corresponding clinopyroxene site, which enables us to calculate for this phase, D-Ra of 4 x 10(-10) and D-Ac of 2 x 10(-6). C opyright (C) 1999 Elsevier Science Ltd.