Abstract
Ruby (red corundum) is one of the most prominent colored gemstones in the world. The highest-quality ruby (“pigeon blood” ruby) comes from marbles of the Mogok Stone Tract in central Myanmar. Although Mogok ruby has been exploited since the 6th century AD, the formation time of this gemstone is ambiguous and controversial. In this paper, we describe a mineralogical, geochemical, and geochronological study of ruby and titanite in ruby-bearing marbles obtained from an outcrop in the Mogok Stone Tract, central Myanmar. Petrographic observations have shown that titanite generally occurs in the marble matrix or occurs as inclusions in ruby. These two types of titanite exhibit identical chemical compositions. In situ secondary ion mass spectrometer (SIMS) U–Pb dating of the separated titanite from two representative samples of ruby-bearing marbles yielded lower intercept ages of 25.15 ± 0.24 Ma (MSWD = 0.26) and 25.06 ± 0.22 Ma (MSWD = 0.15), respectively. Because the closure temperature of the U–Pb system in titanite is close to the temperature of ruby growth, the obtained U–Pb ages (~25 Ma) are suggested to represent the timing of the studied ruby formation in Mogok. The acquired ages are in agreement with the timing of post-collisional extension in the Himalaya related to the migration of the eastern Himalayan syntaxis. Combining our dating results with previous geochronological data from the Mogok Stone Tract, we suggest that the formation of the studied ruby is most likely related to the high-temperature metamorphic event in the marbles during the India–Asia collision. Our study not only confirms that texturally constrained titanite could be a precise geochronometer to date the mineralization of different types of ruby, but also provides important geochronological information linking gemstone formation to the India–Asia collision.
Highlights
Ruby, a red variety of Cr-bearing corundum (Al2O3), is the most prominent and valuable colored gemstone because of its extraordinary appearance and rare occurrence [1,2]
Several factors make the dating of mineralization in the Mogok Stone Tract highly challenging: (1) foreigners were strictly forbidden from visiting the Mogok mines until 2012 [8], so most reported samples have been obtained from gem markets or museums; direct information on the field relationships and petrographic textures of ruby-bearing rocks is usually not available; (2) the host metacarbonate rocks of ruby are notoriously difficult to date due to a lack of suitable chronometers [9]; (3) “hard” ruby typically occurs as isolated, rigid crystals in a “soft” carbonate matrix, which makes it difficult to maintain original ruby crystals during the process of making thin sections; the coexisting relationship between ruby and other silicate/oxide minerals is difficult to determine [4,7]
The 40Ar–39Ar dating result may represent a younger age than the ruby formation time due to the lower closure temperature of phlogopite [6,13], and the zircon inclusions may have the potential to be detrital grains inherited from the protoliths of the marbles
Summary
A red variety of Cr-bearing corundum (Al2O3), is the most prominent and valuable colored gemstone because of its extraordinary appearance and rare occurrence [1,2]. Due to its unique “pigeon blood” color, ruby from the Mogok Stone Tract (central Myanmar) represents the highest grade of corundum gemstone worldwide [2]. The formation ages of ruby are speculated via 40Ar–39Ar dating of phlogopite in ruby-bearing marbles (18.7 ± 0.2 Ma) [6] and U–Pb dating of inclusions in ruby crystals (31–32 Ma and 22 Ma for zircon; 32.4 Ma and 21 Ma for titanite) [7,8,10,11,12]. The 40Ar–39Ar dating result may represent a younger age than the ruby formation time due to the lower closure temperature of phlogopite [6,13], and the zircon inclusions may have the potential to be detrital grains inherited from the protoliths of the marbles. More geochronological data from the Mogok belt are strongly needed to better understand ruby genesis
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