The Merlin kimberlites: (Northern Territory, Australia)

Abstract

The Merlin kimberlites are located in the Batten region of the Northern Territory of Australia, 800 km southeast of Darwin. The kimberlites are in a 10 km by 5 km field on the eastern side of the Batten Trough and occur in four clusters. None outcrop but eleven have been found and it is probable that more remain to be discovered. Two generations of pseudomorphed olivine are present and the nature of the olivines, the matrix and the complex texture of the rocks are all typical of kimberlites. Well developed pelletai textures and the presence of xenolithic material show that some of the rocks are diatreme-facies but a few samples have uniform groundmasses and appear to be hypabyssal kimberlite. Extensive alteration and the presence of xenoliths has affected the geochemistry of most samples but the rocks are ultrapotassic (molar K/Na >2) with a low TiO2/K2O ratio, similar to micaceous kimberlites and some phlogopite lamprophyres. The ratios Ni/MgO and FeOt/MgO are within the normal range of kimberlites and olivine lamproites. Nb/Zr is exceptionally high. Macrocrysts include chrome-spinel, periodotitc garnets and chrome diopside. Megacrysts are absent. Diamonds occur in all eleven kimberlites. Infra-red spectra of microdiamonds indicates that there are three main time-temperature populations present and the size distribution and range of morphologies also suggests several populations. Exceptionally high levels of hydrogen are indicated by infra-red spectra for some of the diamonds.