Subduction-related diamond deposits? Constraints, possibilities, and new data from eastern Australia

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    Abstract

    Most significant alluvial deposits of diamonds can be traced back to their primary sources in kimberlite or lamproite intrusions in cratonic areas. However, some alluvial deposits occur in noncratonic regions, do not carry the usual mantle-derived indicator mineral suite, and cannot be related to known primary sources. The occurrence of such “headless placers”in young orogenic belts suggest that the diamonds are a result of Phanerozoic subduction processes, rather than being derived from old subcontinental lithosphere, and that they might be derived from metamorphic, rather than magmatic, primary sources. Microdiamonds have been reported in crustal rocks, metamorphosed at high pressure during continent-continent collisions, from Kazakhstan, eastern China, and western Norway. However, no macrodiamonds have been found in these deposits and their tectonic setting is distinct from those where significant headless placers occur. Macrodiamonds (altered to graphite) are abundant in pyroxenite layers within peridotite massifs in Spain and Morocco, but these massifs represent high-temperature diapirs, unrelated to subduction processes. Although microdiamonds reported from Tibetan ophiolites probably represent contamination, macrodiamonds have been found in several subduction-related peridotites, thereby indicating that diamonds are produced in at least some subducted plates. The thermal evolution of a subducting slab allows formation of diamonds at relatively shallow depths, but if the diamonds are to survive graphitization, the diamond-bearing rocks must either be obducted to the surface without heating or be sampled by ascending magmas within approximately 30 Ma of the end of subduction.

    Several large headless diamond placers occur in Tertiary river deposits in the Paleozoic Tasman orogen of eastern Australia. New data on the diamonds from several fields show that two major populations are present. They consist of both peridotite and eclogite-calc-silicate parageneses and their proportions vary regionally. Surface morphology and its relation to internal structure suggest extensive resorption during transport by magmas. The diamonds may have formed during subduction of an oceanic plate in a variety of metabasaltic rocks (ranging from eclogite to metarodingite) and in the surrounding highly depleted serpentinites. The anomalously heavy carbon isotope signature (δ13C = –2 to 3‰) of one population can be explained by decarbonation-reduction reactions at 300° to 500°C within the diamond stability field. High degrees of nitrogen aggregation in the diamonds may be related to the intense deformation that is visible macroscopically and in cathodoluminescence images. If so, the N aggregation does not rule out a subduction-related origin. Unpublished 39Ar/40Ar dates (approximately 330 Ma) on pyroxene inclusions in the diamonds may suggest that the diamonds were carried to the surface by magmas related to the major Carboniferous subduction episode in the Tasman orogen. Surface abrasion and radiation damage indicate that many of the diamonds have spent considerable time in surface environments since erosion of their primary sources, and this may account for the scarcity of typical indicator minerals.
    Original languageEnglish
    Pages (from-to)291-310
    Number of pages20
    JournalReviews in Economic Geology
    Volume11
    DOIs
    Publication statusPublished - 1998

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