Oxide mineral petrogenesis in the evolution of the Fayette County kimberlite, Pennsylvania

A narrow dike (1-6 m wide) of micaceous kimberlite is intruded into the sedimentary sequences of the Appalachian Plateau in Fayette County of southwestern Pennsylvania. This relatively unaltered rock consists of megacrysts of olivine >> garnet > ilmenite > phlogopite with a finer-grained porphyritic groundmass containing phenocrysts of phlogopite, olivine, Cr-, Fe-, and Ti-rich spinel, ilmenite, and perovskite. These are set in a matrix consisting of calcite and serpentine. Mantle xenoliths consist of nodular garnet Iherzolite, harzburgite, and dunite. Crustal xenoliths include a nodular gabbroic and a nodular chert fragment as well as abundant country-rock fragments. Flow textures are common including an unusual autolith-like texture of phlogopite laths tangentially oriented around the larger grains.

Mineral chemistry and inclusion relationships indicate the presence of two populations of megacrysts. The "evolved" assemblage consists of olivine (Fo 84-86), ilmenite (16-37 mol% MgTiO₃), garnet (Cr-poor), and phlogopite (0.1-0.2% Cr₂O₃). The "primitive" assemblage consists of olivine (Fo 91-92), and garnet (>1% CrO₃). The minerals in this suite are of the same composition as those in the garnet Iherzolite assemblage which give equilibration temperatures of 1300-1340°C at 55-58 kb.

The mixing of these assemblages, perhaps occurring as the kimberlite passed through the low-velocity zone, was responsible for the chemical zonation that is present in many megacrysts. Ilmenite megacrysts have reverse Mg zonation (up to 51 mol% MgTiO₃) in the outer 200-300 μm of the grains. Olivine from both populations have narrow rims that equilibrated at Fo 89.

Groundmass crystallization began, just prior to or during the mixing of the two assemblages, with the crystallization of the olivine (Fo 91-89) and phlogopite (1.5% Cr₂O₃) phenocrysts. Inclusions of Cr-spinel [17.7-63.7 mol% (Fe,Mg)Cr₂O₄] and ilmenite (50-55 mol% MgTiO₃) in these phenocrysts indicate the penecontemporaneous crystallization of these phases in the groundmass.

Reaction rims of perovskite + Ti-magnetite are ubiquitous on the ilmenite megacrysts. These indicate the breakdown of ilmenite in the presence of a Ca-enriched fluid. The change from groundmass Cr-spinel and ilmenite crystallization to the crystallization of Ti-Magnetite occurred at this time. The formation of the reaction rims appears to have taken place under reducing conditions. Factors leading to this conclusion are: (1) the observed decrease in Fe³⁺ in the ilmenite megacryst rims; (2) the presence of reduction exsolution lamellae of Ti-magnetite in the ilmenite megacryst rims; (3) the association of sulfides with the reaction rims; and (4) the low Fe³⁺ content of the perovskite. The mechanism leading to the formation of the reaction rims could be the removal of CO₂ from the system by pressure release. This would cause an increased a_Ca decrease in fO₂, and could create a fluidizing agent responsible for final emplacement of the kimberlite.