Saturday, January 2, 2016

Domains for Diamond Formation

Diamond forms at high pressures and temperatures, natural diamond in the Earth can be formed at Earth’s surface resulted from meteoritic impact, in the upper mantle, and in the crust. However, diamonds occur predominantly in the Earth’s lithospheric upper mantle. They are transported into the crust either rapidly, in explosively emplaced volatile-rich kimberlite, lamproite or related magmas (Kimberlite clan Rocks/KCRs). The diamondiferous kimberlites have only been found in old continental cratons or in the mobile belts adjacent to the cratons. These diamonds have been recognized as cratonic diamonds (Haggerty, 1999) or lithospheric diamonds (Gurney et al., 2010).

Pressure and Temperature Diagram for Upper Mantle
Pressure and Temperature Diagram for Upper Mantle.

To examine how diamond formation has changed with time, we distinguish three distinct domains of crystallization. In order of relative importance these are as follows:

Lithospheric Diamonds

Diamonds that form in SCLM are associated with mantle peridotites, websterites, and eclogites. They are the source of almost all macrodiamonds and therefore the most significant contributors to diamond deposit viability. Spanning ages from the Archean to just prior to pipe emplacement, with the majority having a long mantle residence history, they are the primary focus of interest with respect to diamond mineralization. They provide ~99 percent of all macrodiamonds worldwide. Subsequent discussion will be focused on this diamond domain.

Sublithospheric Diamonds

Sublithospheric diamonds are typically labeled as “deep diamonds,” which are identified by occasional mineral inclusions such as majorite, ferropericlase, magnesiowustite, native iron, and moissanite. They are estimated to contribute not more than 1 percent of the overall worldwide production (Stachel and Harris, 2008). Their distribution is erratic, none being reported from some localities, and being relatively common at others, including localities at craton margins, such as Jagersfontein, South Africa. The diamonds typically have low N contents and high N aggregation (e.g., Tappert et al., 2009), but are inferior quality crystals in general. This is due to poor crystal shape, high degrees of residual stress, extensive fracturing, and a large proportion of brown stones, the latter a product of plastic deformation (Robinson, 1979). Foundered ancient crustal megaliths (e.g., Ringwood, 1991), as deep as the 650 km discontinuity, have been postulated as the source of some sublithospheric diamonds, whereas the lower mantle has been identified as a source for the ferropericlase paragenesis (Stachel et al., 2005). As might be expected for rare diamonds from such extreme depths, information relevant to their crystallization history, mantle storage, and transport to the crust has proved elusive.

Ultrahigh-Pressure Diamonds

Diamonds also occur in crustal rocks, subducted along craton margins to depths corresponding to pressures of the diamond stability field and subsequently exhumed by tectonic forces. Such diamonds are known as ultrahigh-pressure metamorphic diamonds, and they are typically only found at the Earth’s surface if they have. been preserved as inclusions in other robust minerals such as zircon and garnet. They are typically small, many being microdiamonds (<1 mm), and where they may have been larger, but unarmored (e.g., Beni Bousera, Ronda), they failed to survive their exhumation and reverted to graphite. Consequently, ultrahigh-pressure diamonds do not materially contribute to any exploitable diamond deposits. Such diamonds formed only during the period when the rock in which they are hosted was transiently within the diamond stability field. The reported occurrences are important in demonstrating that subduction is a process that can initiate diamond formation, giving further credibility to the evidence that lithospheric diamonds are closely linked to subduction events and often involve recycled carbon.

Editor by : Flyshgeost
Sources : Society of Economic Geologists, Inc. Economic Geology, v. 105, pp. 689–712

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