藍寶石多產於火成岩與變質岩中，其中以與張裂環境鹼性玄武岩相關的沖積型礦床最具商業開採價值。由於風化、侵蝕作用影響使研究藍寶石之成因更加困難。實驗岩石學結果顯示，一般玄武岩質岩漿無法結晶出剛玉。因此藍寶石應為鹼性玄武岩之捕獲晶，但其成因仍有許多爭論。由藍寶石之微量元素、同位素、礦物包裹體及年代學研究結果，鹼性玄武岩藍寶石成因有三種主要模式：（一）由富鋁沉積岩經變質作用而產生，（二）晶出於地函小比例熔融產生之鹼性岩漿演化晚期，（三）地函碳酸岩漿與地殼矽酸岩漿混合後造成鋁過飽和而結晶。本研究利用X光能量分散式光譜儀（EDS）分析全球藍寶石主要產地泰國Chanthaburi-Trat與Kanchanaburi及澳洲New South Wales三地區之藍寶石原生包裹體礦物相組成，探討產自大陸鹼性玄武岩的藍寶石之來源成因。藍寶石包裹體顆粒微小（≦5 μm），大致有兩種產狀；一為存在於一群同方向延伸排列之高氯細長脈狀物中，其為次生包裹體，形成於藍寶石結晶後，礦物相有方解石、石英、白雲石和氧化鐵等皆存在於泰國Kanchanaburi地區與澳洲New South Wales藍寶石內；另有鈉長石與磷灰石出現在泰國Kanchanaburi地區中。而泰國Chanthaburi-Trat地區之樣品則無次生包裹體發現。另一類為獨立散佈於藍寶石內，無特定形狀或排列規則的原生包裹體，可反映藍寶石形成之環境條件。泰國Chanthaburi-Trat地區樣品中有斜長石、鹼性長石、石英、方解石、鈮-鈦鐵礦與高鈉輝石（霓石）；泰國Kanchanaburi地區樣品中有褐簾石?與方解石；澳洲New South Wales地區樣品中則有鹼性長石與鈦氧化物（金紅石?）。這些原生包裹體礦物中的方解石、鈮-鈦鐵礦及高鈉輝石（霓石）與碳酸岩質岩漿相關聯，而斜長石，石英，鹼性長石，鈦氧化物（金紅石?）和褐簾石?則為矽酸質岩漿（花崗岩）之產物，兩者無法源自同一母岩漿，推測此藍寶石是由矽酸質岩漿（花崗岩）與碳酸岩漿混合後造成鋁過飽和而結晶形成。但Yui et al.（2006）認為泰國Chanthaburi-Trat藍寶石氧同位素比值變化有限（5.1-6.2 ‰），不可能為會造成氧同位素多變之混合岩漿作用所形成。依此進一步推測碳酸質岩漿於混合岩漿中所佔比例極小，故無法造成氧同位素比值明顯變化。

Sapphire commonly occurs in igneous and metamorphic rocks. Commercial sapphire is mainly mined from alluvial deposits associated with alkali basalts. Tracing the origin of alluvial sapphire is difficult for the complexity induced during weathering, erosion, and transportation. Isothermal experiments show that corundum cannot crystallize from basaltic magmas. Therefore, sapphire is considered to be xenocryst in basalts, but its genesis is still debated. Based on trace element geo-chemistry, mineral inclusions, isotopes, and geochronology, three ge-netic processes are proposed for sapphire from alkali basalts. They are: (1) metamorphic recrystallization of aluminous rocks, (2) crystallization from evolved alkali magmas produced by low degrees of partial melting from mantle, and (3) crystallization from over-saturation caused by mixing carbonatitic and silicic magmas. In this study we used energy dispersive spectrometry (EDS) to identify mineral inclusions within sapphires from Thailand (Chanthaburi-Trat and Kanchanaburi) and Australia (New South Wales). Most inclusions are less than 5 μm, and two types of inclusions can be distinguished. The first type occurs in slender veins, which are cracks formed after the crystallization of host sapphire. They are referred to as the “secondary” inclusions. The secondary inclusions include calcite, quartz, dolomite, Fe-oxide (Kanchanaburi and New South Wales) as well as albite and apatite (Kanchanaburi). Secondary inclusions are not observed in Chanthaburi-Trat sapphires. Other inclusions distribute randomly in sapphire. They are referred to as “primary” inclusions and can provide constraints on the genesis of host sapphire. The observed primary in-clusions include plagioclase, alkali feldspar, quartz, calcite, Nb-ilmenite and aegirine in Chanthaburi-Trat sapphires, calcite and allanite? in Kanchanaburi sapphires, and alkali feldspar and Ti-oxide (rutile?) in New South Wales sapphires. Calcite, Nb-ilmenite and aegirine usually occur within carbonatitic magmas, whereas plagioclase, alkali feldspar, quartz and Ti-oxide (rutile?) are constituent minerals in silicic magmas (granite). Carbonatitic and silicic magmas are unlikely to originate from the same parent magma: therefore, it is inferred that hybrid process resulted in Al-oversaturation followed by sapphire crystallization. Hybrid process might lead to variable O-isotope composition, inconsistent with the limited variation shown by the sapphire from Chanthaburi-Trat (5.1-6.2 ‰). We propose that the proportion of carbonatitic melts in hybrid magmas was minute; thus, the variation in O-isotope composi-tion was not significant as expected.

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