Origin and evolution of primitive melts from the Debunscha Maar, Cameroon: Consequences for mantle source heterogeneity within the Cameroon Volcanic Line
Origin and evolution of primitive melts from the Debunscha Maar, Cameroon: Consequences for mantle source heterogeneity within the Cameroon Volcanic Line
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DOI：
10.1016/j.lithos.2017.06.028
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发表时间：
2017-09
期刊：
影响因子：
3.5
通讯作者：
C. Ngwa;T. Hansteen;C. Devey;F. Zwan;C. E. Suh
中科院分区：
文献类型：
--
作者：
C. Ngwa;T. Hansteen;C. Devey;F. Zwan;C. E. Suh
Debunscha Maar是一座单成因火山，是Mt.喀麦隆火山区，位于喀麦隆火山线（CVL）内。部分玻璃菜花弹原始碧玄岩picrobaltite组成，并含有丰富的正常和反环带橄榄石（Fo 77-87）和单斜辉石斑晶。最原始的橄榄石斑晶中的自然淬冷熔体包裹体显示出的成分，当对包裹后的改性进行校正时，覆盖了从碧玄岩到碱性玄武岩（MgO 6.9-11.7重量%）的广泛范围，并且通常比基质玻璃（MgO 5.0-5.5重量%）更原始，并且仅部分地落在与大块岩石样品和基质玻璃的共同液体下降线上。熔体包裹体微量元素组成取决于两个不同的地球化学趋势：一个（走向高Ba/Nb）被认为是代表的无水二辉橄榄岩和角闪石橄榄岩的源中的各种比例的效果，而其他（例如，高La/Y）反映了可变程度的部分熔融。相对较低的分馏校正的CaO在熔融包裹体与最高的La/Y表明轻微参与的辉石岩源组件，仅在低程度的熔融可见。大多数样品显示Gd/Yb升高，表明源中石榴石高达8%。熔融包裹体所代表的主要元素和微量元素的范围涵盖了喀麦隆火山线不同火山的玄武岩所给出的完整地球化学范围，这表明以前被认为是火山特有的地球化学特征实际上可能存在于所有火山之下。单斜辉石熔体压力测定强烈表明，在喷发前，上地幔内830 ± 170 MPa的成分多样的熔体反复混合。地幔位温估计的原始熔融包裹体表明，地幔柱的热影响是不需要解释岩浆岩石成因。
Debunscha Maar is a monogenetic volcano forming part of the Mt. Cameroon volcanic field, located within the Cameroon Volcanic Line (CVL). Partly glassy cauliflower bombs have primitive basanite-picrobasalt compositions and contain abundant normally and reversely zoned olivine (Fo 77–87) and clinopyroxene phenocrysts. Naturally quenched melt inclusions in the most primitive olivine phenocrysts show compositions which, when corrected for post-entrapment modification, cover a wide range from basanite to alkali basalt (MgO 6.9–11.7 wt%), and are generally more primitive than the matrix glasses (MgO 5.0–5.5 wt%) and only partly fall on a common liquid line of descent with the bulk rock samples and matrix glasses. Melt inclusion trace element compositions lie on two distinct geochemical trends: one (towards high Ba/Nb) is thought to represent the effect of various proportions of anhydrous lherzolite and amphibole-bearing peridotite in the source, while the other (for example, high La/Y) reflects variable degrees of partial melting. Comparatively low fractionation-corrected CaO in the melt inclusions with the highest La/Y suggests minor involvement of a pyroxenite source component that is only visible at low degrees of melting. Most of the samples show elevated Gd/Yb, indicating up to 8% garnet in the source. The range of major and trace elements represented by the melt inclusions covers the complete geochemical range given by basalts from different volcanoes of the Cameroon volcanic line, indicating that geochemical signatures that were previously thought to be volcano-specific in fact are probably present under all volcanoes. Clinopyroxene-melt barometry strongly indicates repeated mixing of compositionally diverse melts within the upper mantle at 830 ± 170 MPa prior to eruption. Mantle potential temperatures estimated for the primitive melt inclusions suggest that the thermal influence of a mantle plume is not required to explain the magma petrogenesis.