The uniquely high-temperature character of Cullinan diamonds: A signature of the Bushveld mantle plume?

Evaluating the Formation Pressure of Diamond‐Hosted Majoritic Garnets: A Machine Learning Majorite Barometer

“…All 221 available inclusion compositions were divided into regional datasets, consisting of diamonds sourced from Southern Africa, South America, Western Africa, China, North America, and Russia. Southern African diamonds originate from major South African mines including Jagersfontein, Monastery and Cullinan and also incorporate inclusion compositions from Botswana, Tanzania, and Zimbabwe (Kaminsky et al, 1997;Korolev et al, 2018;Moore et al, 1991;Moore & Gurney, 1985, 1989Motsamai et al, 2018;Pokhilenko et al, 2004Pokhilenko et al, , 2001Shatskii et al, 2010;Smith et al, 2009;Tappert et al, 2005;Tsai et al, 1979). Those from South America are predominantly from the kimberlite and alluvial sources in the Juina region (Bulanova et al, 2010;Burnham et al, 2016Burnham et al, , 2015Harte & Cayzer, 2007;Kaminsky et al, 2001;Meyer & Svisero, 1975;Smith et al, 2016;Thomson et al, 2014;Wilding, 1990;Zedgenizov et al, 2014).…”

“…Neither of these requirements can be guaranteed when studying natural diamond‐hosted garnet inclusions because the co‐equilibrating minerals for any isolated garnet inclusion are unknown. Additionally, it is well known that majoritic garnet inclusions are commonly associated with inclusions believed to be former calcium perovskite (e.g., Bulanova et al., 2010; Burnham et al., 2015, 2016; Korolev et al., 2018; Stachel et al., 2000; Thomson, EMIF, et al., 2014; Thomson, Kohn, et al., 2016; Thomson, Walter, et al., 2016; Walter et al., 2008; Zedgenizov et al., 2014). Therefore, using the published barometers to evaluate the formation pressure of natural inclusions is likely to be stretching them beyond their calibrated range and could potentially produce inaccurate results.…”

Diamond's depth distribution systematics

“…Temperature estimates for inclusions in Cullinan diamonds are, on average, anomalously high (Nimis, 2002;Viljoen et al, 2014;Korolev et al, 2018). Sm-Nd dating of lherzolitic and eclogitic inclusions yielded ages of ⁓1930 Ma and ⁓1150 Ma, respectively, the latter being within uncertainty of kimberlite emplacement (Richardson, 1986;Richardson et al, 1993).…”

“…The ratio of lherzolitic to harzburgitic diamonds is also significantly higher at Cullinan than other southern African localities (Viljoen et al, 2014). Overall, eclogitic diamonds are the most abundant (69%), followed by peridotitic (21%) and sublithospheric mafic types (9%) (Korolev et al, 2018).…”

“…In this work, we compare diamond depth distributions for the three localities that have been the most prolific for diamond geobarometry (Cullinan, Kimberley and Voorspoed, South Africa), using published and unpublished chemical data. Based on inclusion compositions, the diamonds considered in this work are representative of different paragenetic types: the diamonds from Cullinan and Kimberley are peridotitic and belong either to the lherzolitic suite (most Cullinan diamonds; Nimis, 2002;Viljoen et al, 2014;Korolev et al, 2018) or to the harzburgitic suite (most Kimberley diamonds; Phillips et al, 2004); most of the diamonds from Voorspoed belong to an unusual class, which is characterized by the presence of exsolved pyroxene inclusions and is believed to be transitional between the lherzolitic and websteritic types (Viljoen et al, 2018). For the purpose of this paper, previous thermobarometric estimates will be revised.…”