Mona Kabesh

Podiform chromitites are found in the mantle sections of ophiolites in many places and times in the geologic record. Leading theories for their origin invoke melt-rock reaction processes involving hydrous melts in supra-subduction zone settings. Here we develop a case study of a Neoproterozoic example and a quantitative thermodynamic model of the multistage process leading to nearly monomineralic chromitite pods. We demonstrate that the Hagar Dungash ophiolite of the Eastern Desert of Egypt — despite being dismembered, incomplete, and affected by variable degrees of alteration and metamorphism — is nevertheless plainly a supra-subduction zone ophiolite containing podiform chromitite deposits. Textural, mineralogical, and chemical evidence from the serpentinized ultramafic rocks point to mantle protoliths residual to high degrees of partial melt extraction, characteristic of fore-arc ophiolites. The chromitite pods are distinguished into massive and nodular types; Cr-spinel in the massive type is almost fresh, with intermediate to high Cr# (0.60–0.85) and extremely low Fe3+# (nil-0.04). Cr-chlorite with up to ∼ 15 wt% Cr2O3 is a common interstitial mineral in the chromitites. The observations are consistent with the conceptual multistage model wherein boninitic melt develops as the end-product of melt-harzburgite reaction in the shallow upper mantle. Mixing between this boninitic melt and ascending peridotite-derived basaltic melt drives the liquid deep into the Cr-spinel stability field and precipitates nearly monomineralic chromitite. Our thermodynamic model of this mixing and precipitation process reproduces the mineral chemistry trends of the fresh chromite samples and yields an estimate of the efficiency of chromite production relative to melt flux through the system, which can be applied to development of podiform chromitites in supra-subduction systems worldwide and across geologic eras.