EXPERIMENTAL DETERMINATION OF RADIUM PARTITIONING BETWEEN ANORTHITE AND CMAS MELT

We present experimental radium partitioning data for anorthite and a CMAS melt at atmospheric pressure. Partitioning behavior of radium between feldspars and coexisting melts are of particular importance because the large M site of the feldspar structure is one of the few sites in common crustal minerals able to accommodate a measurable amount of radium. The presence of feldspar also has potential to influence (²²⁶Ra/²³⁰Th) disequilibria in magmas generated within the plagioclase lherzolite stability field of the upper mantle, possibly by source melting or diffusive reaction of rising melts with gabbroic cumulates (Van Orman et al., 2006). Ion probe measurement of coexisting anorthite and glass phases produces a molar D_Ra = 0.040(6) at 1400°C.The addition of radium partitioning data to a plagioclase Onuma curve (D vs. ionic radius) for other large alkaline earth cations (Ca, Sr, and Ba) provides a direct test of lattice strain model (Blundy and Wood, 1994) with applications to predictions of geologically short-lived radioactive isotope partitioning. Lattice strain model parameters for this data when fitted in natural log space are D_o = 0.93(5), r_o = 1.207(6) Å, and E = 107(9) GPa. These results indicate that lattice strain partitioning models fit the partition coefficient data of this study well, supporting previous work on crustal melting and magma chamber dynamics that has relied on such models to approximate radium partitioning behavior in the absence of experimentally determined values. We measure D_Ra/D_Ba = 0.23(5) for this composition, which demonstrates that Ba does not behave as an identical chemical analogue of Ra. Given that Ra²⁺ has been shown here to be a well-behaved ion whose partitioning behavior between plagioclase and melt is closely approximated by a lattice strain partitioning model, this study lends further confidence to such modeling of D_Ra between melts and clinopyroxene, which likely dominates radium fractionation deeper in the mantle.

References:

Blundy, J.D. and Wood, B.J. (1994) Nature, 372, 452-454. Van Orman, J.A., Saal, A.E., Bourdon, B., and Hauri, E.H. (2006) Geochimica et Cosmochimica Acta, 70, 4797-4812.