The lithospheric mantle is of paramount importance in controlling the chemical composition of ocean island basalts (OIBs), influencing partial melting and magma evolution processes. To improve the understanding of these processes, the pressure–temperature conditions of mantle melting were investigated, and liquid lines of descent were modelled for OIBs on Pohnpei Island. The studied basaltic samples are alkalic, and can be classified as SiO₂-undersaturated or SiO₂-saturated series rocks, with the former having higher TiO₂ and FeO^T contents but with no distinct trace-element composition, suggesting melting of a compositionally homogenous mantle source at varying depths. Both series underwent sequential crystallization of olivine, clinopyroxene, Fe–Ti oxides, and minor plagioclase and alkali feldspar. Early magnetite crystallization resulted from initially high FeO^T contents and oxygen fugacity, and late feldspar crystallization was due to initially low Al₂O₃ contents and alkali enrichment of the evolved magma. The Pohnpei lavas formed at estimated mantle-melting temperatures of 1486–1626 °C (average 1557 ± 43 °C, 1σ), and pressures of 2.9–5.1 GPa (average 3.8 ± 0.7 GPa), with the SiO₂-undersaturated series forming at higher melting temperatures and pressures. Trace-element compositions further suggest that garnet rather than spinel was a residual phase in the mantle source during the melting process. Compared with the Hawaiian and Louisville seamount chains, Pohnpei Island underwent much lower degrees of mantle melting at greater depth, possibly due to a thicker lithosphere.

中文翻译：

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密克罗尼西亚波纳皮岛地幔融化和岩浆成因的地球化学约束

岩石圈地幔对于控制海洋玄武岩（OIBs）的化学成分，影响部分融化和岩浆演化过程至关重要。为了增进对这些过程的了解，研究了地幔融化的压力-温度条件，并对庞贝岛上的OIBs的下降液体线进行了建模。所研究的玄武岩样品是碱性的，可以分为SiO _2不饱和或SiO ₂饱和系列岩石，前者具有较高的TiO ₂和FeO ^T地幔物质含量高，但没有明显的痕量元素组成，表明在不同深度融化了成分均匀的地幔源。这两个系列都经历了橄榄石，斜向辉石，Fe-Ti氧化物以及次生斜长石和碱金属长石的顺序结晶。磁铁矿的早期结晶起因于最初高的FeO ^T含量和氧逸度，而长石的结晶则起因于最初低的Al ₂ O ₃含量和析出岩浆的碱富集。Pohnpei熔岩形成于1486–1626°C（平均1557±43°C，1σ）的地幔融化温度，压力为2.9–5.1 GPa（平均3.8±0.7 GPa），SiO ₂-在较高的熔融温度和压力下进行的不饱和串联成型。微量元素组成进一步表明，在融化过程中，石榴石而不是尖晶石是地幔源中的残留相。与夏威夷和路易斯维尔海山链相比，波纳佩岛的地幔融化程度要低得多，可能是由于岩石圈较厚。