In this study, we determine magma generation and evolution of magma storage conditions beneath Paka volcano located in northern segment of the Kenya Rift. The surface geology of the area is characterised by five main sequences including pre-Paka formations and volcanic sequence 1–4, all built up by basalt, intermediate and trachyte rocks spanning 582–8 ka. Numerical trace element modelling suggests that the primary magma in Paka resulted principally from 5 to 10% garnet–peridotite partial melting and possibly 0.1–1.5% partial melting of spinel-lherzolite. The parental magma was probably held at the Moho (~30 km) and evolved by ~76% gabbro fractional crystallization to generate a liquid of similar chemistry as that of erupted Paka basalts. We propose that under moderately oxidized conditions (∆FMQ +1) the primary magma underwent a further ~70% fractional crystallization and ~ 20% assimilation of crustal syenitic material to generate Paka trachyte lavas. Basaltic and intermediate magmas were held at polybaric storage pressure conditions ranging between 1.4 and 7 kbar (5–25 km), at temperatures between 1086 and 1204 °C with relatively low water content of <1.8 wt%. The trachyte magma resided at low pressures, i.e., 0.3–2.5 kbar (1–9 km) at 901–980 °C with water contents of 3.5–5.1 wt%. Based on our data eruption source depths remained largely the same for a period spanning from 582 to 11 ka. Geophysical data show correlation with magma depths beneath Paka volcano.

中文翻译：

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肯尼亚帕卡火山群岩浆储层条件的部分熔融生成和演化：来自地球化学、岩石学和地球物理学的约束

在这项研究中，我们确定了位于肯尼亚裂谷北段的帕卡火山下方的岩浆生成和岩浆储存条件的演变。该地区的地表地质以五个主要层序为特征，包括前 Paka 地层和火山层序 1-4，全部由跨度 582-8ka 的玄武岩、中间岩和粗面岩组成。数值微量元素模型表明，Paka 的原生岩浆主要由 5% 至 10% 的石榴石-橄榄岩部分熔融以及可能 0.1%-1.5% 的尖晶石-二辉橄榄岩部分熔融产生。母岩浆可能保存在莫霍面（约 30 公里），并通过约 76% 的辉长岩分级结晶演化，产生与喷发的帕卡玄武岩化学成分相似的液体。我们认为，在中等氧化条件下（ΔFMQ +1），原生岩浆进一步经历约 70% 的分步结晶和约 20% 的地壳正长岩物质同化，从而生成 Paka 粗面岩熔岩。玄武岩和中间岩浆保持在 1.4 至 7 kbar（5-25 km）之间的多压储存压力条件下，温度在 1086 至 1204 °C 之间，含水量相对较低（<1.8 wt%）。粗面岩浆处于低压，即0.3-2.5 kbar (1-9 km)，温度901-980 °C，含水量为3.5-5.1 wt%。根据我们的数据，喷发源深度在 582 至 11 ka 的时期内基本保持不变。地球物理数据显示与帕卡火山下方岩浆深度的相关性。