Conditions of diamond growth: a proton microprobe study of inclusions in West Australian diamonds

Crystalline primary inclusions in diamonds from the Argyle and Ellendale lamproites have been analyzed for Mn, Ni, Cu, Zn, Ga, Pb, Rb, Sr, Y, Zr, Nb, Ta, Ba and Mo by proton microprobe. Eclogite-suite inclusions dominate at Argyle and occur in equal proportions with peridotite-suite inclusions at Ellendale. Eclogitic phases present include garnet, omphacitic clinopyroxene, coesite, rutile, kyanite and sulfide. Eclogitic clinopyroxenes are commonly rich in K and contain 300-1060 ppm Sr and 3-70 ppm Zr: K/Rb increases with K content up to 1400 at 0.7-1.1% K. Rutiles have high Zr and Nb contents with Zr/Nb=1.5-4 and Nb/Ta ∼16. Of the peridotite-suite inclusions, olivine commonly contains > 10 ppm Sr and Mo; Cr-pyropes are depleted in Sr, Y and Zr, and enriched in Ni, relative to eclogitic garnets. Eclogite-suite diamonds grew in host rocks that were depleted in Mn, Ni and Cr, and enriched in Sr, Zn, Cu, Ga and Ti, relative to Type I eclogite xenoliths from the Roberts Victor Mine. Crystallization temperatures of the eclogite-suite diamonds, as determined by coexisting garnet and clinopyroxene from single diamonds, range from ∼1085 to ∼1575° C. Log K_D (C_i^cpx/C_i^gnt) varies linearly with 1/T for Zr, Sr and Ga in most of the same samples. This supports the validity of the temperature estimates; Argyle eclogite-suite diamonds have grown over a T range ≥400° C. Comparison with data from eclogite xenoliths in kimberlites suggests that K_D^Srand K_D^Zrare mainly T-dependent, while K_D^Gamay be both temperature-and pressuredependent. K_D^Ni, K_D^Cuand K_D^Znshow no T dependence in these samples. In several cases, significant major-and/or trace-element disequilibrium is observed between different grains of the same mineral, or between pyroxene and garnet, within single diamonds. This implies that these diamonds grew in an open system; inclusions trapped at different stages of growth record changes in major and trace-element composition occurring in the host rock. Diamond growth may have been controlled by a fluid flux which introduced or liberated carbon and modified the composition of the rock. The wide range of equilibration temperatures and the range of composition recorded in the inclusions of single diamonds suggest that a significant time interval was involved in diamond growth.