Petrology of podiform chromitite from the ocean floor] at the 15°20'N FZ in the MAR, Site 1271, ODP Leg 209

摘要

Several podiform chromitite samples from abyssal peridotite have been recovered from Site 1271 at the 15°20' N fracture zone (FZ) in the Mid-Atlantic Ridge (MAR) during an Ocean Drilling Program cruise, ODP Leg 209. It is observed that only chromian spinel is preserved as the primary mantle mineral and that all other primary minerals in the chromitite samples are completely altered. The primary chromian spinel has moderately high Cr#= Cr/(Cr + Al) atomic ratio; approximate value = 0.5, which is comparable with Cr# value of the chromitite "minipod" sampled from Hess Deep (Arai and Matsukage, 1996). Samples associated with chromitites consist of mainly dunite, some amphibole-bearing gabbros and troctolite, and a very small amount of harzbur-gite. Therefore, the chromitites from this site are categorized under the rock series that composes the Moho transition zone, similar to those in ophiolites. This implies that a mass of melt existed, but was consumed by melt-rock interactions in the uppermost mantle beneath this area. This in turn means that the area is not magma-starved, although it is unclear when and where the interactions occurred. The formation of the chromitites at Site 1271 is probably occurs through the series of hybridization processes which suggested by Takazawa et al. (2007). Some grains of the chromian spinel in the samples have thick rims of Cr-magnetite or of magnetites. Increase in the Cr content in the Cr-magnetite rim is accompanied by Fe enrichment. The chemical modification of the chromian spinel suggests that the chromitite from Site 1271 was metamorphosed at a high temperature up to the upper greenschist facies because the depletion of Al in the region extending from spinel cores to the rim takes place at high temperature as a result of equilibration of fluids in equilibrium with chlorite.