UNUSUAL DIAMONDS AND UNIQUE INCLUSIONS FR OM NEW-SOUTH-WALES, AUSTRALIA

Although diamonds widely distributed in eastern Australia, no primary source is known. Search for a conventional source such as kimberlite o r laproite may be unrealistic as many of the diamonds have unusual sur face features, unique isotopic signatures and distinctive inclusion ch emistry (including the dominance of coesite, which tended to cause loc al fractures while plates from this collection were being polished). N ew data obtained here confirm previous observations involving isotopes and inclusion chemistry, but also report the discovery of one augite inclusion typical of eclogitic inclusion suites, the first such inclus ion to be discovered in the Copeton/Bingara area. Alternative theories concerning the provenance of these diamonds are outlined and discusse d. Infrared (IR), cathodoluminescence (CL) and morphological (SEM) dat a have been obtained for 65 specimens from the Cluff collection of dia monds from the Copeton/Inverell district, and have confirmed the exist ence of two distinct categories - rounded yellow crystals having high, uniform nitrogen content, featureless blue CL and often unusual micro disc etch features visible in the SEM - and white, composite crystals showing complicated CL zoning and nitrogen content which is usually lo w and variable, but for a few fragments from individual stones is as h igh as in the yellow stones. The high nitrogen content means that much of the IR data is off-scale, and empirical procedures based an intern al consistence have been devised in order to extract information from this data. The results suggest that such procedures can be useful when considering population charecteristics for a single source. Rather un expectedly, the IR studies suggest similar time-temperature constraint s for both Copeton types, with the possibility that the white specimen s are slightly older or have been slightly hotter, but the differences are marginal. Temperature bounds, determined by the requirement that mantle residence times probably lie within the range 0.3 to 3.1 Ga, ar e found to be 1180 to 1240 degrees C. Short mantle residence times are compatible both with young diamond ages and with a large range of per missible emplacement ages. The absence of microdiamonds and the scarci ty of broken stones in the collection would suggest either transport o ver long distances or an unusual local source. Uneven representation o f specimens from separate sites within the area in the 65 stones consi dered here prevent conclusions being drawn concerning relationships to paleodrainage channels, but further specimens have been selected in o rder to pursue this matter and augment the IR data.