Clinopyroxene-corundum assemblages from alkali basalt and alluvium, eastern Thailand: constraints on the origin of Thai rubies

Abstract

An inclusion of corundum (ruby) was found in a clinopyroxene xenocryst in alkali basalt from the lateCenozoic Chanthaburi-Trat volcanics of eastern Thailand. The clinopyroxene is fairly sodic, highly aluminous and magnesian (0.12 0.14 Na, 0.31 0.33 Al IV and 0.36 0.40 Al VI per 6(O), and Mg/ (Mg+Fe 2+ ) > 0.9)) and is chemically similar to clinopyroxene inclusions in rubies from nearby alluvial gem deposits, suggesting a common origin for the two types of occurrence. Sapphirine (Mg/(Mg+Fe 2+ ) = 0.91 0.94) and garnet (py 56 67 alm 11 18 grs 18 23 ) also occur as inclusions in alluvial rubies. Thermodynamic calculation of the equilibrium 2 di + 2 crn = 2 cats + en constrains the temperatures of clinopyroxene + corundum crystallization to between 800 and 1150 +1008C. Use of other equilibria as stability limits places the pressures of crystallization between 10 and 25 kbar, implying depths of between 35 and 88 km. The most Fe-rich clinopyroxene crystallized at a pressure in the lower part of the range. The pyropic garnet inclusions in corundum crystallized at pressures of >18 kbar (i.e. at depths >~63 km).The xenocrystic clinopyroxene could have coexisted in equilibrium with garnet of similar composition to the observed inclusions at the deduced temperatures of crystallization. The rubies probably crystallized in rocks of mafic composition, i.e. garnet-clinopyroxenites or garnet-pyriclasites, within the upper mantle.

Geochemical and oxygen isotope signatures of mantle corundum megacrysts from the Mbuji-Mayi kimberlite, Democratic Republic of Congo, and the Changle alkali basalt, China

“…5) defined for ruby xenocrysts or megacrysts carried up to the surface by alkali basalts . They are also similar to those found for ruby in eclogites derived from oceanic crust (Shirey et al, 2013) and hosted by clinopyroxene xenocrysts from Bo Rai (Thailand) alkali basalt alluvials (Giuliani et al, 2005;Sutthirat et al, 2001; in subducted oceanic crust at relatively high-temperature, i.e. 300-400 8C similar to high-temperature seawater-altered oceanic basalts, the oceanic protolith being subducted to the pressure and temperature regime of the mantle while preserving its lower d 18 O signature (Neal et al, 1990); in mafic protoliths affected by metasomatic percolation of hydrothermal fluids; by metasomatic reactions between pyroxenite or wehrlite intrusions in peridotites such as described for the Beni Bousera plagioclase-rich gabbros (Kornprobst et al, 1990) which contain ruby-bearing garnet pyroxenites (with d 18 O value of ruby = 4.4 %; Table 2).…”

“…Nevertheless, the associations of corundum together with clinopyroxene and garnet (Sobolev et al, 1998) or Alrich clinopyroxene have been identified in diamond (Hutchinson et al, 2001(Hutchinson et al, , 2004 as well as in corundum (Saminpanya and Sutherland, 2011;Sutherland et al, 2003;Sutthirat et al, 2001). At the Juina kimberlite deposit, high Cr 2 O 3 ruby inclusions in diamond are associated with Al-rich pyroxene (up to 10 wt% Al 2 O 3 ), ferropericlase and Mg-perovskite (Hutchinson et al, 2001).…”

Contrasts in gem corundum characteristics, eastern Australian basaltic fields: trace elements, fluid/melt inclusions and oxygen isotopes

“…This spinel-sapphirine-ruby facies contrasts with a higher-T, Al-rich diopside-garnet-ruby facies in the Cudgegong-Macquarie River area and a magmatic-related ruby suite in the New England area. It closely resembles a spinel-sapphirine-ruby facies suite from western Palin, Cambodia, even in its oxygen isotope values , but clearly differs from garnet-sapphirineruby suites from a proposed garnet granulite facies in Thailand (Sutthirat et al, 2001).…”

Metamorphic evolution of blueschists of the Alt?nekin Complex, Konya area, south central Turkey

“…Because many of the mineral assemblages have high variance, it is not generally possible to constrain P-T conditions using the intersection of isopleths of equilibria involving end-member components that are all present in the assemblages concerned; consequently, we have had to rely heavily on the use of equilibria as stability limits, with the activities of absent components set at unity or at partition-corrected values (Sutthirat et al 2001). The P-T coordinates of equilibria were calculated using version 3.1 (2001) of the program THERMOCALC (Powell and Holland 1988;.…”