PARTITIONING AND MASS-BALANCE RELATIONS IN IHERZOLITES

The dependence of mineral composition on modal amounts in lherzolites is examined using mass-balance equations coupled with inter-phase partition coefficients. The composition of clinopyroxene changes dramatically as its modal abundance approaches zero because clinopyroxene strongly concentrates incompatible elements compared with orthopyroxene and olivine. Compositional variables for clinopyroxene such as element concentrations (La), element ratios (La/Yb) and interpolated concentrations (Nb/Nb*) are not representative of bulk-rock composition when clinopyroxene weight fraction < 0.2. In the absence of other minerals that concentrate incompatible elements, clinopyroxene always has higher La/Yb and lower HFSE/HFSE* than the host lherzolite. Claims of bulk-rock REE enrichment or HFSE depletion in lherzolites based solely on clinopyroxene analyses may be in error.