THE ORIGIN OF LARGE IRREGULAR GEM-QUALITY TYPE II DIAMONDS AND THE RARITY OF BLUE TYPE IIB VARIETIES

Abstract

Type II (N-poor) diamonds do not constitute a single genetic population with a common paragenesis. Rather, several distinct populations (P-, E-, W- and sub-lithospheric) can be recognized, which formed in contrasting geological environments, with very different factors controlling the N-deficient character of these stones. Mantle-derived silicate and oxide inclusions are absent or extremely rare in large, Irregular Type II stones, which are often of exceptional gem quality. Such characteristics set these diamonds apart from N-poor (Type II) stones with “superdeep” inclusions, as well as the peridotitic and eclogitic suites, both of which include stones with non-detectable N. This in turn points to crystallization of the Irregular Type II stones in an environment markedly different from those in which the other three associations formed. Carbon isotopic signatures and syngenetic inclusions link both framesite and Irregular Type II diamonds to the websterite paragenesis. The latter in turn shows strong chemical affinities to the megacryst suite. From a purely petrographic perspective, the large Irregular Type II stones would be classified as megacrysts on the basis of their size (often >10 mm). The megacryst suite formed as pegmatitic veins from small volumes of kimberlitic liquids, injected into fractures within the thermal aureole surrounding the pooled kimberlite magma in the mantle prior to eruption. Crystallization of Irregular Type II diamonds was initiated when small volumes of evolved, residual megacryst magmas became buffered by highly reduced mantle wall rocks. Visually identified graphite inclusions, often rounded and thus inferred to be protogenetic, are relatively common in these diamonds. This observation is consistent with the inferred P-T conditions of crystallization of megacrysts across the graphite-diamond inversion curve, and thus within the lithosphere. Rapid crystallization of framesites from reduced megacryst magmas was probably triggered by stresses in the mantle immediately prior to kimberlite eruption. The websterite diamond suite is thus directly linked to megacryst crystallization, broadly coeval with the kimberlite magmatic event in the mantle. Experimental studies on the crystallization of granite pegmatites, complemented by processes specific to formation of the megacryst suite, provide a framework to account for the absence or extreme rarity of inclusions of other megacryst phases in the Irregular Type II stones. The elevated boron contents of blue Type IIb diamonds are consistent with the model for crystallization from highly fractionated pegmatitic megacryst magmas. Reduction of boron to the metallic state is ascribed to buffering of the megacryst magma by extremely refractory and highly reduced wallrock harzburgites, such as those found at Premier, which is one of the world’s major kimberlite sources of blue diamonds. Formation of fibrous cubic diamonds and the Premier eclogitic diamonds, which are both broadly coeval with eruption of the host kimberlite, can also be linked to the kimberlite magmatic event in the mantle.