The Merlin kimberlites are located in the Batten region of the Norther
n 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 an
d 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 kimber
lites. Well developed pelletal textures and the presence of xenolithic
material show that some of the rocks are diatreme-facies but a few sa
mples 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 Ki/MgO and FeOt/MgO are with
in the normal range of kimberlites and olivine lamproites. Nb/Zr is ex
ceptionally high. Macrocrysts include chrome-spinel, periodotitic garn
ets 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 populati
ons. Exceptionally high levels of hydrogen are indicated by infra-red
spectra for some of the diamonds.