Using ICP-MS–LA analyses, we demonstrate that the use of the Ga/Mg ratio, in conjunction with the Fe concentration, is an efficient
tool in discriminating between “metamorphic” and “magmatic” blue sapphires. Magmatic blue sapphires found in alkali basalts (e.g.
southeastern Asia, China, Africa) are commonly medium-rich to rich in Fe (with average contents between 2000 and 11000 ppm), high in
Ga (N140 ppm), and low in Mg (generally b20 ppm) with high Ga/Mg ratios (N10). Conversely, metamorphic blue sapphires found in
basalts (e.g. Pailin pastel) and in metamorphic terrains (e.g. Mogok, Sri Lanka, Ilakaka) are characterized by low average iron contents
(b3000 ppm), low Ga contents (b75 ppm), and high Mg values (N60 ppm) with low average Ga/Mg ratios (b10). Basaltic magmatic
sapphires have Fe, Ga and Mg contents similar to those obtained for primary magmatic sapphires found in the Garba Tula syenite. This
suggests that these both sets of sapphires have a possible common “syenitic” origin, as previously proposed from other criteria. In addition,
plumasite-related sapphires and metamorphic sapphires also exhibit similar composition in trace elements. Based on results from the
present study, we suggest that fluid circulations during a metamorphic stage produced metasomatic exchanges between mafic and acidic
rocks (plumasite model), thus explaining the high Mg contents and converging Ga/Mg ratios observed in metamorphic sapphires.
© 2007 Elsevier B.V. All rights reserved.
Using ICP-MS–LA analyses, we demonstrate that the use of the Ga/Mg ratio, in conjunction with the Fe concentration, is an efficienttool in discriminating between “metamorphic” and “magmatic” blue sapphires. Magmatic blue sapphires found in alkali basalts (e.g.southeastern Asia, China, Africa) are commonly medium-rich to rich in Fe (with average contents between 2000 and 11000 ppm), high inGa (N140 ppm), and low in Mg (generally b20 ppm) with high Ga/Mg ratios (N10). Conversely, metamorphic blue sapphires found inbasalts (e.g. Pailin pastel) and in metamorphic terrains (e.g. Mogok, Sri Lanka, Ilakaka) are characterized by low average iron contents(b3000 ppm), low Ga contents (b75 ppm), and high Mg values (N60 ppm) with low average Ga/Mg ratios (b10). Basaltic magmaticsapphires have Fe, Ga and Mg contents similar to those obtained for primary magmatic sapphires found in the Garba Tula syenite. Thissuggests that these both sets of sapphires have a possible common “syenitic” origin, as previously proposed from other criteria. In addition,plumasite-related sapphires and metamorphic sapphires also exhibit similar composition in trace elements. Based on results from thepresent study, we suggest that fluid circulations during a metamorphic stage produced metasomatic exchanges between mafic and acidicrocks (plumasite model), thus explaining the high Mg contents and converging Ga/Mg ratios observed in metamorphic sapphires.© 2007 Elsevier B.V. All rights reserved.
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