Augite-olivine equilibria in the Kiglapait Intrusion, Labrador, Canada

The first appearance of abundant cumulus augite in the Kiglapait layered mafic body, on the coast of Labrador, defines the base of the Upper Zone at 84 PCS (percent solidified). The appearance of this phase is gradual, culminating in an impressive overproduction in the region around 90 PCS. This behavior can be rectified to yield an "errorless equivalent" cumulus arrival of augite at 81 PCS. The observed modal behavior was interpreted as evidence for a poorly stirred magma in which augite components were sequestered in the core of a convection cell. The compositional relations of augite and olivine offer some support for this hypothesis. These relations are best viewed in the context of linear partitioning in binary solutions extended to coexisting pairs of mineral species that share the same exchange reaction, here FeMg (sub -1) . Hence, rather than a melting loop, the partitioning of Fe and Mg involves the two crystalline solutions augite and olivine. Here, the partition coefficient D is formulated as X (sub Fe) (super Aug) /X (sub Fe) (super Ol) (hence D < or =1.0). A test of linear partitioning is applied: D = X (sub Mg) (super Aug*) K (sub D) + X (sub Fe) (super Aug) , where K (sub D) has its usual Roeder-Emslie meaning. For the subsolidus Kiglapait equilibria, a strong linear correlation is found (r (super 2) = 0.995) for PCS>91.4, running from fictive D = K (sub D) = 0.32 at the pure Mg end of the augite series to D = 1.0 at the pure Fe end of the augite series. For the stratigraphic interval 84-91 PCS, D is clustered well above the line at D = 0.63-0.68. This higher value of D suggests a metastable equilibrium on a narrower loop driven by an overstepping reaction. At high temperature modeled by QUILF, the same general relations hold, but all values of D are higher, and the linear array leads back to a significantly higher K (sub D) = 0.44, as would be expected for higher-temperature interphase equilibria. Hence, linear partitioning can be used successfully to describe both liquidus and subsolidus interphase equilibria. With calibration, the fictive K (sub D) can be correlated with T, from liquidus equilibria at the higher limit to intracrystalline cation diffusion blocking at the lower limit.