Carbonatite-alkaline silicate rock associations in the Gregory Rift, East Africa

本项目研究了东非裂谷格里高利分支碳酸盐岩-云母岩-白云岩组合的岩浆演化，碳酸盐岩显示出碱和卤素的高度变化富集。该项目将描述Kerimasi、Mosonik和Shombole火山的碳酸盐-硅酸盐岩石组合，并将它们与Oldoinyo Lengai （OL）的过碱性钠云母岩、钠云母岩和钠云母岩的极端例子以及进一步的区域钠云母火山（Sadiman、Burko、Essimongor）进行比较。由于OL在过去几十年已经被广泛研究，我们不会再收集新的数据，而是使用文献数据进行比较。这种综合方法详细地比较了来自若干空间相关火山的碳酸盐岩和伴生硅酸盐岩石，这在以前是没有做过的，尽管它可能解释了在碳酸盐岩中观察到的碱和卤素富集变化的原因，以及伴生硅酸盐岩石在区域尺度上的作用。我们特别指出，我们的项目将把独特的OL火山置于更广泛的区域背景下。我们的工作假设是：•来自三个火山的钠辉石和碳酸盐来自一个共同的地幔源，并通过分离结晶和液液不混溶从一个共同的母岩浆演化而来。•所研究火山的钠云母-phonolite组合反映了在分馏结晶（AFC）过程中不同数量的同化作用。•碳酸盐岩的Na/ ca比值变化是由原生含co2熔体演化过程中碳酸盐岩分离的不同“时间”引起的。对它们的测试涉及系统的结构和岩石学方法，使用同位素年龄测定、全岩数据、矿物化学（SEM、EPMA）和相平衡计算（QUILF、PerpleX等）。通过这些方法确定的选定关键样品将通过卤素全岩和矿物数据（CIC），空间分辨放射性成因同位素数据（LA ICP-MS），特别是熔体包裹体研究进行进一步研究，熔体包裹体研究将由熔体包裹体专家Victor Sharygin（新西伯利亚）共同监督。一个主要的焦点将是将岩石的分带斑晶、反晶和异晶的成分与同位素分带和熔体包裹体数据联系起来。这样就可以区分出岩浆混合/补充、流体析出/脱气、液-液不混溶和同化等生长过程中可能发生的各种过程。将三个新研究的火山的资料与Oldoinyo Lengai的文献资料和进一步的例子（见上文）结合起来，将能够得出结论，说明碳酸盐中碱和卤素的富集程度与辉石岩、空纹岩和碳酸盐的熔融成分的岩石学和地球化学演化之间的潜在依赖性。

This project concerns the magmatic evolution of carbonatite-nephelinite-phonolite-associations in the Gregory branch of the East African Rift, where carbonatites show highly variable enrichment of alkalis and halogens. The project will characterize the carbonatite-silicate rock associations of Kerimasi, Mosonik, and Shombole volcanoes and compare them to each other, to the extreme example of Oldoinyo Lengai (OL) with its peralkaline nephelinites, phonolites and natrocarbonatites and to further regional nephelinitic volcanoes (Sadiman, Burko, Essimongor). As OL has been extensively studied in the last decades, we will not gather new data on it, but will use literature data for comparison. Such an integrated approach that compares carbonatites and associated silicate rocks from several spatially associated volcanoes in detail has not been done before - although it may explain the reasons for the observed variations of alkali- and halogen-enrichment in carbonatites and the role of the associated silicate rocks on a regional scale. We specifically note our project will put the unique volcano OL into a much broader, regional context. Our working hypotheses are: • Nephelinites and carbonatites from the three volcanoes derive from a common mantle source and evolved from a common parental magma via fractional crystallization followed by liquid-liquid immiscibility. • Nephelinite-phonolite associations of the investigated volcanoes reflect variable amounts of assimilation during fractional crystallization (AFC processes).• The variable Na/Ca-ratios of the carbonatites are caused by different "timing" of carbonatite unmixing during the evolution of the parental CO2-bearing melts. Testing them involves a systematic textural and petrological approach using isotopic age-dating, whole-rock data, mineral chemistry (SEM, EPMA) and phase equilibrium calculations (QUILF, PerpleX and others). Selected key samples identified with these methods will be further investigated by means of halogen whole-rock and mineral data (CIC), spatially resolved radiogenic isotope data (LA ICP-MS) and especially melt inclusion studies which will be co-supervised by melt inclusion expert Victor Sharygin (Novosibirsk). A major focus will be to correlate compositional with isotopic zonations and melt inclusion data of zoned phenocrysts, antecrysts and xenocrysts of the rocks. In doing so, various processes that may occur during their growth (magma mixing/replenishment, fluid exsolution/degassing, liquid-liquid immiscibility and assimilation) can be distinguished. Combining the information from the three newly studied volcanoes with literature data from Oldoinyo Lengai and further examples (see above) will allow to draw conclusions on the potential dependence between the level of enrichment of alkalis and halogens in carbonatites and the petrological and geochemical evolution of melt compositions parental to nephelinites, phonolites and carbonatites.