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

该项目涉及东非裂谷格雷戈里分支中碳酸岩-霞石-响岩岩浆岩的演化，其中碳酸岩显示出碱金属和卤素的高度可变富集。该项目将描述 Kerimasi、Mosonik 和 Shombole 火山的碳酸盐-硅酸盐岩组合的特征，并将它们相互比较，以 Oldoinyo Lengai (OL) 的极端例子及其全碱性霞岩、声云岩和硝化碳酸盐岩，以及进一步的区域霞岩火山（Sadiman、Burko、Essimongor）。由于OL在过去几十年里得到了广泛的研究，我们不会收集新的数据，而是使用文献数据进行比较。这种详细比较来自几个空间相关火山的碳酸岩和伴生硅酸盐岩石的综合方法以前还没有做过——尽管它可以解释观察到的碳酸岩中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火山 OL 置于更广泛的区域背景中。我们的工作假设是： • 三座火山的霞岩和碳酸岩源自共同的地幔来源，并由共同的母岩浆通过分步结晶和随后的液-液不混溶演化而来。 • 所研究的火山的霞石-响沸石组合反映了分步结晶（AFC 过程）过程中不同数量的同化作用。 • 碳酸岩的 Na/Ca 比率的变化是由母体含 CO2 熔体演化过程中碳酸岩分解的不同“时间”造成的。测试它们涉及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）的系统结构和岩石学方法。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)、空间分辨放射性同位素数据 (LA ICP-MS) 以及特别是由熔体包裹体专家 Victor Sharygin（新西伯利亚）共同监督的熔体包裹体研究进行进一步研究。主要重点是将岩石的斑晶、前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。这样做，可以区分它们生长过程中可能发生的各种过程（岩浆混合/补充、流体析出/脱气、液-液不混溶和同化）。将三座新研究的火山的信息与 Oldoinyo Lengai 的文献数据和进一步的例子（见上文）相结合，将得出关于碳酸岩中碱金属和卤素的富集水平与霞石岩、响岩和碳酸岩母体熔体成分的岩石学和地球化学演化之间的潜在依赖性的结论。

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.