Petrology of a chromitite micropod from Hess Deep, equatorial Pacific: a comparison between abyssal and Alpine-type podiform chromitites

This is the first report of podiform chromitite from the ocean floor. A micropod of chromitite, about 2 cm across and more than 10 cm long, was found in dunite drilled from Hess Deep, equatorial Pacific, during ODP Leg 147. The pod is composed of coarse, up to 5 mm across, chromian spinel with the Cr# (= Cr/[Cr + Al] atomic ratio) of 0.5 to 0.6, often full of minute mineral inclusions, such as hydrous minerals relatively enriched with TiO2 (< 2 wt.%). Troctolites associated with dunite from Hess Deep sometimes have spinel disseminations. The chromitite micropod from Hess Deep, although very small in size, is very similar to alpine-type chromitite pods, in that (1) chromitite is set within harzburgite with a dunite envelope, and (2) chromian spinel sometimes includes hydrous and other silicates enriched with incompatible elements, e.g., phlogopites and pargasites. This similarity indicates that the mechanism of formation is the same for both chromitites. The spinel concentrations from Hess Deep were due to interaction between MORE and harzburgite at the uppermost mantle, which formed the plutonic rock suite (harzburgite, dunite, troctolite and gabbros). The podiform chromitite, therefore, can be formed within the abyssal harzburgite, i.e., at fast-spreading ridge environments, although it has been hardly found because of extremely incomplete sampling. Most of the podiform chromitites, as well as enclosing dunites, however, have much higher-Cr# (0.7 to 0.8) spinel, than the ocean floor chromitite and other magmatic rocks, possibly indicating their derivation from supra-subduction zone mantle. (C) 1998 Elsevier Science B.V.