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首页> 外文期刊>Contributions to Mineralogy and Petrology >The role of liquid-liquid immiscibility and crystal fractionation in the genesis of carbonatite magmas: insights from Kerimasi melt inclusions
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The role of liquid-liquid immiscibility and crystal fractionation in the genesis of carbonatite magmas: insights from Kerimasi melt inclusions

机译：液-液不混溶性和晶体分级在碳酸盐岩浆形成中的作用：凯里马斯熔体包裹体的见解

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摘要

We have reconstructed the compositional evolution of the silicate and carbonate melt, and various crystalline phases in the subvolcanic reservoir of Kerimasi Volcano in the East African Rift. Trace element concentrations of silicate and carbonate melt inclusions trapped in nepheline, apatite and magnetite from plutonic afrikandite (clinopyroxene-nepheline-perovskite-magnetite-melilite rock) and calciocarbonatite (calcite-apatite-magnetite-perovskite-monticellite-phlogopite rock) show that liquid immiscibility occurred during the generation of carbonatite magmas from a CO2-rich melilite-nephelinite magma formed at relatively high temperatures (1,100 degrees C). This carbonatite magma is notably more calcic and less alkaline than that occurring at Oldoinyo Lengai. The CaO-rich (32-41 wt%) nature and alkali-"poor" (at least 7-10 wt% Na2O + K2O) nature of these high-temperature (>1,000 degrees C) carbonate melts result from strong partitioning of Ca (relative to Mg, Fe and Mn) in the immiscible carbonate and the CaO-rich nature (12-17 wt%) of its silicate parent (e.g., melilite-nephelinite). Evolution of the Kerimasi carbonate magma can result in the formation of natrocarbonatite melts with similar composition to those of Oldoinyo Lengai, but with pronounced depletion in REE and HFSE elements. We suggest that this compositional difference results from the different initial parental magmas, e.g., melilite-nephelinite at Kerimasi and a nephelinite at Oldoinyo Lengai. The difference in parental magma composition led to a significant difference in the fractionating mineral phase assemblage and the element partitioning systematics upon silicate-carbonate melt immiscibility. LA-ICP-MS analysis of coeval silicate and carbonate melt inclusions provides an opportunity to infer carbonate melt/silicate melt partition coefficients for a wide range of elements. These data show that Li, Na, Pb, Ca, Sr, Ba, B, all REE (except Sc), U, V, Nb, Ta, P, Mo, W and S are partitioned into the carbonate melt, whereas Mg, Mn, Fe, Co, Cu, Zn, Al, Sc, Ti, Hf and Zr are partitioned into the silicate melt. Potassium and Rb show no preferential partitioning. Kerimasi melt inclusions show that the immiscible calcic carbonate melt is strongly enriched in Sr, Ba, Pb, LREE, P, W, Mo and S relative to other trace elements. Comparison of our data with experimental results indicates that preferential partitioning of oxidized sulfur (as SO42-), Ca and P (as PO43-) into the carbonate melt may promote the partitioning of Nb, Ta, Pb and all REE, excluding Sc, into this phase. Therefore, it is suggested that P and S enrichment in calcic carbonate magmas promotes the genesis of REE-rich carbonatites by liquid immiscibility. Our study shows that changes in the partition coefficients of elements between minerals and the coexisting melts along the liquid line of descent are rather significant at Kerimasi. This is why, in addition to the REE, Nb, Ta and Zr are also enriched in Kerimasi calciocarbonatites. We consider significant amounts of apatite and perovskite precipitated from melilite-nephelinite-derived carbonate melt as igneous minerals can have high LREE, Nb and Zr contents relative to other carbonatite minerals.
机译：我们已经重建了东非大裂谷Kerimasi火山亚火山岩储层中硅酸盐和碳酸盐熔体的组成演化以及各种晶相。从深部闪闪长石（斜辉石-霞石-钙钛矿-磁铁矿-沸石岩）和钙碳碳酸盐（方解石-磷灰石-磁铁矿-钙钛矿-褐铁矿-金云母岩）中的霞石，磷灰石和磁铁矿中捕获的硅酸盐和碳酸盐熔体夹杂物的痕量元素浓度表明在相对较高的温度（1100摄氏度）下，由富含CO2的斑岩质-霞石质岩浆生成碳酸盐岩浆的过程中发生了不溶混。与Oldoinyo Lengai发生的岩浆相比，这种碳酸盐岩浆明显具有更多的钙质和碱性。这些高温（> 1,000摄氏度）碳酸盐熔体的富含CaO（32-41 wt％）性质和弱碱（至少7-10 wt％Na2O + K2O）性质是由于Ca的强烈分配所致（相对于Mg，Fe和Mn）不溶混的碳酸盐及其硅酸盐母体（例如，硅沸石-霞石）的富含CaO的性质（12-17 wt％）。 Kerimasi碳酸盐岩浆的演化可能导致形成与Oldoinyo Lengai相似成分的钠碳酸盐熔岩，但REE和HFSE元素明显减少。我们认为，这种成分差异是由不同的初始父母岩浆引起的，例如Kerimasi的陨石-霞石岩和Oldoinyo Lengai的霞石岩。母岩浆成分的差异导致分馏矿物相组合和硅酸盐-碳酸盐熔体不溶混体系中元素分配系统的显着差异。对同时存在的硅酸盐和碳酸盐熔体夹杂物的LA-ICP-MS分析提供了一个机会，可以推断出各种元素的碳酸盐熔体/硅酸盐熔体分配系数。这些数据表明，Li，Na，Pb，Ca，Sr，Ba，B，所有REE（Sc除外），U，V，Nb，Ta，P，Mo，W和S均分配到碳酸盐熔体中，而Mg， Mn，Fe，Co，Cu，Zn，Al，Sc，Ti，Hf和Zr分配到硅酸盐熔体中。钾和Rb没有优先分配。 Kerimasi熔体包裹体表明，与其他微量元素相比，不溶混的碳酸钙熔体富含Sr，Ba，Pb，LREE，P，W，Mo和S。我们的数据与实验结果的比较表明，氧化硫（如SO42-），钙和磷（如PO43-）优先分配到碳酸盐熔体中可能会促进Nb，Ta，Pb和所有REE（不包括Sc）分配到这个阶段。因此，建议碳酸钙岩浆中的P和S富集通过液体不溶混作用促进富含REE的碳酸盐岩的成因。我们的研究表明，在克里马西，矿物和共存的熔体之间沿着下降线的元素分配系数的变化相当显着。这就是为什么除了REE外，Keramasi钙碳酸盐岩中还富含Nb，Ta和Zr。我们认为，从火成岩-霞石矿物衍生的碳酸盐熔体中沉淀出大量的磷灰石和钙钛矿，因为火成矿物相对于其他碳酸盐矿物具有较高的LREE，Nb和Zr含量。

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