Trace-element compositions of sapphire and ruby from the eastern Australian gemstone belt

Abstract

Significant uncertainty surrounds the processes involved in the formation of basalt-hosted corundum, particularly the role that the mantle plays in corundum generation. Some previous studies have suggested that trace-element ratios (namely, Cr/Ga and Ga/Mg) are useful for distinguishing two types of corundum: ‘magmatic’ and ‘metamorphic’, designations that include mantle and crustal processes. However, recent studies, including this one, have discovered transitional groups between these end-members that are difficult to classify.We used laser ablation inductively coupled plasma mass spectrometry (LA–ICP–MS) to measure trace-element concentrations in sapphire and ruby crystals from eight alluvial deposits that span a significant length of the eastern Australian gemstone belt. Additionally, we collected LA–ICP–MS U–Pb and traceelement data from zircon megacrysts atWeldborough, Tasmania, which is also within the gemstone belt. Our sapphire and ruby results reveal a continuum in trace-element compositions, an observation that raises questions regarding previous classifications that ascribe corundum from basalt-hosted gemfields to either ‘magmatic’ or ‘metamorphic’ sources. The spatial association of basalt-related gemfields in eastern Australia with a long-lived convergent margin suggests a link between corundum formation and Al-enrichment of the mantle wedge during periods of subduction.

Origin of Blue Sapphire in Newly Discovered Spinel–Chlorite–Muscovite Rocks within Meta-Ultramafites of Ilmen Mountains, South Urals of Russia: Evidence from Mineralogy, Geochemistry, Rb-Sr and Sm-Nd Isotopic Data

“…Twenty-seven spots were measured on both blue and colorless zones (three spots on blue-colored zones of each four sapphire samples, one sapphire sample is completely colorless). Analysis took place in GEOKHI RAS using a New Wave Research UP-213 Nd:YAG laser (New Wave Research, Inc., Fremont, CA, USA) combined with Element-XR (Thermo Finnigan, Waltham, MA, USA) ICP-MS. Trace-element compositions were determined by monitoring 6 Li, 9 Be, 24 Mg, 27 Al, 44 Ca, 47 Ti, 51 V, 53 Cr, 55 Mn, 57 Fe, 59 Co, 60 Ni, 66 Zn, 71 Ga, 88 Sr, 89 Y, 91 Zr, 93 Nb, 181 Ta, 208 Pb, and ablating a material with a spot size of 55 μm at a repetition rate of 4 Hz and an energy density of about 21.17 J/cm 2 . Warm up/background time was 90 s, dwell time was 100 s, and wash out time was 120 s. NIST SRM 610 and NIST SRM 612 glasses (National Institute of Standards and Technology; U.S. Department of Commerce: Gaithersburg, MD) were used as the reference materials and ATHO as an unknown for quality control (QCM).…”

“…In a Fe versus Ga/Mg diagram [14], all measured corundum samples were plotted to "metamorphic" and "plumasitic" population fields ( Figure 6), clearly away from the "magmatic" blue sapphires that are found within syenite pegmatites of Ilmen Mountains (see again [6,17]). However, these trace-element diagrams should be used with the caution and possibly revised, since it was found that some of the sapphires from magmatic deposits could be plotted in the "metamorphic field" [46,47]. It is probably important to refine these plots using solely data of gem-quality samples and to group them by color (e.g., plot sapphire and rubies separately), taking into account that most of trace elements used for these are corundum chromophores (e.g., Fe, Ti, Mg, Cr, V).…”

Naryn-Gol Creek Sapphire Placer Deposit, Buryatia, Russia

“…% oxides). Adapted with permission from [6]; Primorye, Cenozoic alkali basalts [6]; Primorye, placer [6]; Australia, placer [27][28][29][30]; Madagascar [31][32][33]; Vietnam [34]; Cambodia [27,35,36]; and Rwanda [37]. Naryn-Gol-this study.…”