PETROGRAPHIC EVIDENCE OF AN INSTANT FREEZE OF KIMBERLITE DIATREME

Carbonate filled extensional fractures penetrated through fresh kimberlite samples taken from the Jurassic Tanoma Kimberlite Dikes, Indiana County, Pennsylvania. The parallel aligned fractures, spaced from less than 1 mm to more than 1 cm apart, not only passed though the matrix material made of carbonates and 0.1 mm or smaller ilmenite, but also extended through large and small xenocrystals. Elongated xenolith and xenocrystals indicated the tensional fracture is parallel to the direction of kimberlite movemen. The features of tensional strain in xenocrystals include structures observed in olivine, phlogopite and ilmenite. Tension fracture and conjugated shear fractures exist in 90% of euhedral and subhedral olivine grains range in size from 0.2 mm to 10 mm in diameter. The fractures are shown by the orientation of replacing fibrous serpentine. Shear and tension fractures are shown in phlogopite as kink band and slice of carbonate filling along cleavage of the host mica. In more brittle ilmenite, the tensional fracture extended into the mineral and deviated into shear fractures and also commonly created opening of short but wide secondary tensional fractures in the mineral. While the shear fractures in ilmenite are commonly filled with carbonate, the secondary tensional fractures are usually not filled. Overall, shear and tension fractures penetrated minerals and matrix of kimberlite in a consistent orientation.

Kurszlaukis et al (1998) considered a model of phreatomagmatic explosion to explain the lack of vesicles in brecciated kimberlite started from the beginning of the diatreme. With evidences of penetrative fracture system observed in this study, the only possibility to fit these observations into Kurszlaukis’s model would be a concomitant solidification of the kimberlite magma with the phreatomagmatic explosion.