Corundum from basaltic terrains: a mineral inclusion approach to the enigma

Petrogenesis of alkaline basalt-hosted sapphire megacrysts. Petrological and geochemical investigations of in situ sapphire occurrences from the Siebengebirge Volcanic Field, Germany

“…Of special interest are secondary corundum deposits that are associated with rift-related alkaline basaltic volcanism as these are among the world's largest gem corundum suppliers (e.g., Guo et al 1996;Limtrakun et al 2001;Shore and Weldon 2009) and are hence economically of great importance. Such basaltic secondary deposits are located all over the world, most notably in Asia (e.g., Guo et al 1996, and in eastern Australia, at the western Pacific margin (e.g., Coenraads 1990;Graham et al 2008;Sutherland 1996;Sutherland et al 1998aSutherland et al , 2002Sutherland et al , 2009bSutherland et al , 2015Sutherland et al , 2017Saminpanya 2001;McGee 2005;Abduriyim et al 2012), but also in Europe (e.g., Malikova 1999;Giuliani et al 2009;Uher et al 2012), Africa (Kiefert and Schmetzer 1987;Krzemnicki et al 1996;Schwarz et al 2000;Pardieux et al 2014), in South America , and in North America (e.g., Garland 2002;Berg 2007Berg , 2014Palke et al 2017).…”

“…Hence, unless these sapphires precipitated via some yet unknown process out of the basaltic melts, they must instead have a xenogenetic relation with their host rocks. Indeed, anhedral crystal shapes, rounded edges, and macroscopically visible spinel coronas at the sapphire-basalt interface (e.g., Stephenson 1976;Guo et al 1996;Sutherland et al 2002;McGee 2005) indicate disequilibrium between these sapphires and their host rocks, supporting a xenocrystic nature. There seems to be a consensus (see references in this work) that the sapphires originate from a reservoir in the mantle or the crust, and were later transported to the surface by ascending basaltic melts.…”

“…18 O values (e.g., Giuliani et al 2005Giuliani et al , 2007Yui et al 2006;Sutherland et al 2009a, b), the mineral inclusion suite (e.g., Guo et al 1996), their trace element geochemistry (Sutherland et al 2009b(Sutherland et al , 2017, and most notably on the Ga/Mg ratio (Peucat et al 2007). …”

“…(3) Growth as restitic component during partial melting of Al-rich crustal protoliths such as anorthosites (Palke et al 2016(Palke et al , 2017. (4) Hybrid formation where sapphire is stabilized by reaction of a fractionated silicate with a carbonatitic melt at mid-crustal levels (Guo et al 1996). This hybrid model is based on the occurrence of a very complex mineral inclusion suite in sapphire that must have precipitated from melts of two different compositions, most probably a carbonatitic and a highly evolved silicate melt.…”

“…This hybrid model is based on the occurrence of a very complex mineral inclusion suite in sapphire that must have precipitated from melts of two different compositions, most probably a carbonatitic and a highly evolved silicate melt. Guo et al (1996) proposed that evolved granite/syenitic pegmatite silicate melts become Al 2 O 3 oversaturated in response to mixing with carbonatitic melts, resulting in local precipitation of corundum in the hybridization zone. (5) Crystallization at shallow crustal levels from an iron-rich syenitic melt with the participation of a carbonate-H 2 O-CO 2 fluid phase (Izokh et al 2010).…”