FRACTIONATION OF REES BY OLIVINE AND THE ORIGIN OF KAMBALDA KOMATIITES, WESTERN-AUSTRALIA

Analysis of trace elements in spinifex and cumulate samples from a single komatiite lava flow from Kambalda (Western Australia) reveals significant variations in the relative proportions of REE and other elements normally considered insensitive to olivine fractionation. Variations of LREE result from a combination of mobility during alteration and variable degrees of wall-rock assimilation. Relative concentrations of HREE also vary: in spinifex-textured lavas, the heavier elements are depleted [(Gd/Yb)N approximately 1.08]; in olivine cumulates these elements are enriched [(Gd/Yb)N approximately 0.921. Although the influence of alteration during metamorphism cannot be ruled out completely, we prefer to attribute these effects to olivine crystallization and accumulation, a process that can effectively fractionate REE only if: (a) heavier REE are much more compatible in olivine than lighter REE; and (b) olivine partition coefficients for komatiitic liquids are approximately ten times higher than published values for basaltic liquids. This interpretation is supported by analyses of REE and other trace elements (by IDMS, INAA, and ion microprobe) in olivine separated from the Kambalda komatiite. These analyses reveal extreme fractionation of HREE [(Gd/ Yb)N approximately 0.28 ] and unusually high concentrations of REE (Sm = 0.0 1 3 ppm, Yb = 0.06 ppm) in the olivine. Partition coefficients calculated by relating the olivine to an appropriate spinifex lava using Mg-Fe partitioning are: La to Sm less-than-or-equal-to 0.02, Gd = 0.02, and Yb = 0.09. A possible role of chromite is eliminated by analyses of a chromite separate, which yields partition coefficients for all REE < 0.01. These partitioning data provide a basis for discussing the origin of komatiites. Munro-type komatiites (chondritic Al2O3/TiO2 and HREE) result from relatively high degrees of mantle melting, 30% or more. The melt segregates at pressures around 8 GPa, the pressure at which melt becomes less dense than the ol +/- opx matrix. Barberton-type komatiites (low Al2O3/TiO2 and depleted HREE) form by lower degrees of melting at depths greater than 15 GPa, and their segregation is driven by the density difference between melt and a majorite-rich matrix.