Origin and distribution of corundum from an intraplate alkali basaltic province in Thailand : evidence from field and inclusion studies
Creators: name: Limtrakun, P
Publisher: name: University Of Tasmania
Year: 2023

description: It has long been recognised that gem-quality corundums (sapphires and rubies) are commonly found in close proximity to intraplate alkali basalts. On this basis, a general consensus has developed that the gem-quality corundums were the products of weathering of \nearby\" alkali basalts. However despite this association gem-quality corundums are rarely found hosted within fresh basalts. Through the detail study of their mineralogy and inclusion compositions this study derives new evidence on the origin of sapphires. All sapphires examined in this thesis were recovered from alluvial placer deposits in the Denchai gem fields of Phrae Province Northern Thailand and are interpreted to have been derived from the late Cenozoic alkali Denchai basalts. The basalts have olivine + clinopyroxene + plagioclase phenocryst and microphenocryst assemblages are alkali in character and have compositions varying from basanites basalts basaltic trachyandesites to trachybasalts. They have abundant mantle-derived xenoliths mostly spinel-lherzolites and rare crustal xenoliths. The equilibration P-T estimates of spinel-lherzolite xenoliths are 1030¬¨‚àûC at 8 to 20 kbars. The basalts themselves show evidence of fractionation at mantle depths (&gt; 10 kbars) and no evidence of lowpressure crystal fractionation prior to eruption. Isotopic data suggest that the basalts have not experienced crustal contamination and that mixing of three mantle components was involved; (1) an \"A\" component (Depleted Mantle HIMU and a component lying somewhere between DM and HIMU line) (2) \"A\"-EM1\" mixing component and (3) EM2-enriched subcontinental lithospheric mantle. Most Denchai sapphires are blue in colour having very low Cr contents and high Ti/Ga ratios. Their oxygen isotope compositions (+4.7 to +8.4%0) suggest a mixed source composition (crust and mantle) for sapphire crystallisation. Four types of inclusions were identified in the sapphires; CO\\(_2\\)-rich inclusions (Type-I) polyphase (V+L+S) inclusions (Type-II) silicate-melt inclusions (Type-III) and mineral inclusions. Type-I CO\\(_2\\)-rich fluid inclusions have wide ranges of CO\\(_2\\) density up to about 0.87 g/cm\\(^3\\). Type-II inclusions contain ~60 wt% NaCl equivalent H\\(_2\\)O and anhydrite daughter crystals. Glass compositions of Type-III are silica-rich (~61 wt% SiO\\(_2\\)) 15-30 wt% Al\\(_2\\)O\\(_3\\) ~10 wt% K\\(_2\\)O + Na\\(_2\\)O and high volatiles (S Cl and H\\(_2\\)O). Critical assessment of the data suggests that the Denchai sapphires originated at pressure of in the range of 6-15 kbars at a temperature between 700 and 900¬¨‚àûC. The source composition had low Si activity high NaCl and was highly oxidised (hematite stable). New evidence from this study suggests that the Denchai sapphires crystallised from melts that formed in the continental lithosphere. The crystallisation of these sapphires requires hybrid melts dominated by a mantle ˜í¬•\\(^{18}\\)O signature with high salinity\\( ‚Äöv†¬¥\\)O\\(_2\\) CO\\(_2\\) and low in Ca Mg Fe contents. It is also concluded that the role of the associated alkali basalts is to provide (I) a heat source (2) possibly to act as a low aSiO\\(_2\\) buffer that reacts with the contaminant and (3) to provide an effective method ofrapid transport to the surface. This inclusion-based study emphasises the large variability of source compositions that can account for the genesis of corundum. Many sapphire provinces have a strong association with Nb-Ta oxides and have mantle ˜í¬•\\(^{18}\\)O. These features suggest carbonatites are a very important component. However the Thai sapphire provinces lack these feature. The Denchai sapphires were derived from highly oxidised Ca-poor parental melts which have variable ˜í¬•\\(^{18}\\)O. Partial melts of seafloor weathered MORB basalts are the most likely source for this composition."