Cr-Saturation Arrays in Concentrate Garnet Compositions from Kimberlite and their Use in Mantle Barometry

The spinel-garnet transition in Cr/Al-enriched peridotitic bulk compositions is known from experimental investigations to occur at 20-70 kbar, within the pressure range sampled by kimberlites. We show that the Cr 2 O 3 -CaO compositions of concentrate garnets from kimberlite have maximum Cr/Ca arrays characterized by Cr 2 O 3 /CaO $096-081, and interpret the arrays as primary evidence of chromite-garnet coexistence in Cr-rich harzburgitic or lherzolitic bulk compositions derived from depth within the lithosphere. Under Cr-saturated conditions on a known geotherm, each Cr/Ca array implicitly delineates an isobar inside a garnet Cr 2 O 3 -CaO diagram. This simplification invites a graphical approach to calibrate an empirical Cr/Ca-in-pyrope barometer. Carbonaceous chromite-garnet harzburgite xenoliths from the Roberts Victor kimberlite tightly bracket a graphite-diamond constraint (GDC) located at Cr 2 O 3 094CaO 50 (wt %), representing a pivotal calibration corresponding to 43 kbar on a 38 mW/m 2 conductive geotherm. Additional calibration points are established at 14, 174 and 591 kbar by judiciously projecting garnet compositions from simple-system experiments onto the same geotherm. The garnet Cr/Ca barometer is then simply formulated as follows (in wt %):