Petrology and geochemistry of the fine-grained, unbrecciated diogenite Northwest Africa 4215

摘要

We report on the petrology and geochemistry of Northwest Africa (NWA) 4215, an unbrecciated diogenite recovered in the Sahara. This single stone, weighing 46.4 g, displays a well-preserved cumulative texture. It consists of zoned xenomorphic orthopyroxene grains on the order of 500 mu m in size, along with a few large chromite crystals (< 5 vol%, up to 3 mm). Accessory olivine and scarce diopside grains occur within the groundmass, usually around the chromite crystals. Minor phases are cristobalite, troilite, and metal. Unlike other diogenites, orthopyroxenes (En(76.2)Wo(1.1)Fs(22.7) to En(68.6)Wo(5.5)Fs(25.9)), olivines (Fo(76) to Fo(71)), and chromites (Mg# = 14.3 44.0, Cr# = 42.2-86.5) are chemically zoned. The minor element behavior in orthopyroxenes and the intricate chemical profiles obtained in chromites indicate that the zonings do not mirror the evolution of the parental melt. We suggest that they resulted from reaction of the crystals with intercumulus melt. In order to preserve the observed zoning profiles, NWA 4215 clearly cooled significantly faster than other diogenites. Indeed, the cooling rate determined from the diffusion of Cr in olivine abutting chromite is in the order of 1050 degrees C/a, suggesting that NWA 4215 formed within a small, shallow intrusion.