Mid- to late-stage kimberlitic melt evolution: phlogopites and oxides from the Fayette County kimberlite, Pennsylvania

Hypabyssal-facies kimberlite from Fayette County, Pennsylvania, comprises megacrysts/inclusions, peridotite xenoliths, and crustal fragments set in a matrix of phlogopite, spinel, ilmenite, perovskite, rutile, carbonate and minor serpentine. Two chemically distinct populations of megacrysts/inclusions are present: a Cr-rich suite (olivine(Fo 90–93), Cr-pyrope, Cr-diopside, enstatite, Cr-spinel, and immiscible sulfide melt products); and a Cr-poor suite (olivine (Fo 81–85), pyrope, diopside, and picroilmenite (16–37 mol% MgTiO₃)). These minerals record the evolution of two chemically distinct kimberlitic melts within the low velocity zone (LVZ). Olivines of both populations have rims of Fo 88–89, and ilmenite megacrysts possess reverse-zoned rims (34–51 mol% MgTiO₃). These zonations reflect mixing of the two populations of megacrysts and their host melts within the LVZ.

The chemical evolution of this hybrid melt during mid- to late-stages of the kimberlite’s history is recorded by groundmass phlogopites and oxides. Phlogopites document increases in activities of Fe*, Ca, and Na and a decrease in Cr-activity. Groundmass ilmenites are compositionally similar to ilmenite megacryst rims. Spinels show zonation from titanian Mg-Al-chromite (TMAC) through chromian Mg-Al-titanomagnetite (CMAT) to magnesian Al-titanomagnetite (MAT), reflecting increase in Ti and Fe³⁺ activities and decrease in activities of Cr and Mg during the evolution of the hybrid melt. Late-stage rutile indicates high activities of Ti and near-solidus temperatures. Ilmenite instability is a function of a late-stage increase in Ca activity with stabilization of Nb-, REE-bearing perovskite. Spinels reaction rims of ilmenite megacrysts are compositionally similar to CMAT in the groundmass; adjacent to the matrix, they are zoned to MAT. Inhomogeneous distribution of sulphides and perovskite in the groundmass and variable modal proportions of spinel and perovskite in the ilmenite megacryst reaction rims indicate local variations in fugacités of O₂, S₂, and CO₂.