High-pressure phase equilibria of a high-magnesia basalt and the genesis of primary oceanic basalts
High-pressure phase equilibria of a high-magnesia basalt and the genesis of primary oceanic basalts
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高镁玄武岩的高压相平衡与原生大洋玄武岩的成因
DOI：
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发表时间：
1984
期刊：
影响因子：
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通讯作者：
C. Scarfe
中科院分区：
文献类型：
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作者：
D. Elthon;C. Scarfe
高压相平衡研究的高镁玄武岩从托尔图加蛇绿岩复杂表明，橄榄石+斜方辉石 * 单斜辉石+石榴石是其液相线在25千巴。在10至20千巴，橄榄石是液相线相，然后是尖晶石，单斜辉石，斜方辉石随着温度降低。在9.5Vo下，MgO来源于15至25 kbar的原始大洋玄武岩。大洋玄武岩在低压（=10千巴）下的起源模型被认为不能产生最“原始”的大洋玄武岩，其MeO> 9.57。大多数MgO>9.5Vo的大洋玄武岩很可能最终来自于MgO> 14 Vo的原生高MgO玄武岩，这些玄武岩是在15 - 25 kbar下熔融产生的。这种类型的高MgO玄武岩存在于许多海洋或裂谷环境中（例如，巴芬湾，戈尔戈纳岛，托尔图加蛇绿岩，贝茨湾蛇绿岩，刘易斯山蛇绿岩，昂加瓦半岛），这表明这些高MgO玄武岩是在整个地质时代在世界范围内产生的。然而，化学分异过程通常将这些高MgO玄武岩改性为MgO含量为7 - 1D %的更常见的玄武岩。由于大多数MgO <9.5%的大洋玄武岩已经平衡到低压同构造，因此它们所源自的原生岩浆的起源条件的确定并没有受到很好的限制。由于这个原因，在压力=10 kbar时地幔的熔融可以产生能够分化形成大洋玄武岩的原生岩浆，这些大洋玄武岩与许多MgO <9.5%的演化大洋玄武岩相似。在结晶和混合过程得到更充分的理解之前，不能排除在<10千巴的熔融是某些原始大洋玄武岩起源的可能性。
High-pressure phase equilibria studies on a high-MgO basalt from the Tortuga ophiolite complex indicate that olivine + orthopyroxene * clinopyroxene + garnet are its liquidus phases at 25 kbars. At l0 to 20 kbars, olivine is the liquidus phase and is followed by spinel, clinopyroxene, and orthopyroxene as temperature decreases. At 9.5Vo MgO are derived from primary oceanic basalts generated at 15 to 25 kbars. Models for the origin of oceanic basalts at low pressures (=10 kbars) are considered incapable of generating the most "primitive" oceanic basalts that have >9.57o MeO. It is likely that most oceanic basalts with>9.5Vo MgO are ultimately derived from primary highMgO basalts with>l4Vo MgO that have been produced by melting at 15 to 25 kbars. HighMgO basalts of this type are found in numerous oceanic or rifting environments (e.g., Baffin Bay, Gorgona Island, Tortuga ophiolite, Betts Cove ophiolite, Lewis Hills ophiolite, Ungava Peninsula), suggesting that these high-MgO basalts are produced on a world-wide scale throughout geologic time. Chemical differentiation processes, however, normally modify these high-MgO basalts into the more common basalts with 7 to lD%MgO. Because most oceanic basalts with <9.5% MgO have equilibrated to low-pressure cotectics, determination of the conditions of origin for the primary magmas from which they are derived is not well constrained. For this reason, melting of the mantle at pressures of =10 kbars could produce primary magmas capable of differentiating to form oceanic basalts that are similar to many of the evolved oceanic basalts that have <9.5% MgO. Until crystallization and mixing processes are more fully understood, melting at <10 kbars cannot be eliminated as a possibility for the origin of some primary oceanic basalts.