Magma–carbonate interaction: An experimental study on ultrapotassic rocks from Alban Hills (Central Italy)
Magma–carbonate interaction: An experimental study on ultrapotassic rocks from Alban Hills (Central Italy)
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DOI：
10.1016/j.lithos.2007.08.008
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发表时间：
2008-03
期刊：
影响因子：
3.5
通讯作者：
C. Freda;M. Gaeta;V. Misiti;S. Mollo;D. Dolfi;P. Scarlato
中科院分区：
文献类型：
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作者：
C. Freda;M. Gaeta;V. Misiti;S. Mollo;D. Dolfi;P. Scarlato
阿尔班山超钾质火山区是意大利第四纪沿第四纪侵位的主要地区之一。阿尔班山熔岩流和火山岩碎屑主要由单斜辉石和白榴石组成，它们的化学成分主要是钾质。分异产品(MgO<；3wt.%)具有二氧化硅浓度低(<；50wt.%)的特点，而地球化学特征表明，这种独特的分异趋势是由晶体分馏和碳酸盐结壳相互作用驱动的。值得注意的是，阿尔班山火山区被安置在厚厚的石灰岩单元中。为了抑制岩浆分异作用，我们对阿尔班山的母体成分(不含斜长石的声软玉)进行了无水、有水和有水碳酸盐化条件下的实验。实验在1℃大气压、0.5℃和1℃Gpa，温度从1050℃到1300℃，原料中的H2O和CaCO3分别高达2wt%和7wt%。在0.5G GPA下进行的实验是阿尔班山管道系统中最具代表性的。单斜辉石和白榴石是在所有研究条件下出现的主要物相和液相线相。然而，我们的实验结果表明，起始材料中CaCO3的存在强烈地影响着相关系。在含水条件下进行的实验在相对较高的温度下结晶磁铁矿和金云母。这种早期的晶化作用促使玻璃成分朝着二氧化硅的富集化方向发展，导致了从声辉橄榄岩(Alban Hills的母体成分)向声辉橄榄岩成分的分化趋势。这与表明天然产物中磁铁矿和金云母结晶较晚的微观结构证据以及幼年产物的成分形成了鲜明对比。相反，在含CaCO3实验中(即模拟岩浆-碳酸盐相互作用)，磁铁矿和金云母的稳定场显著降低。因此，熔体的分异主要受单斜辉石和白榴石的共晶结晶控制，导致分异趋势向钾橄榄岩成分移动。这些实验结果与阿尔班山天然产物的显微结构特征和化学成分相吻合，也与根据地球化学数据推断的岩浆分异模式相一致。岩浆-碳酸盐相互作用并不罕见，在不同的管道系统中已经证明了这种作用的存在。然而，Alban Hills液体下降线的独特性表明，碳酸盐污染过程的有效性受不同因素控制，管道系统的动态是最重要的因素之一。
The Alban Hills ultrapotassic volcanic district is one of the main districts emplaced during Quaternary time along the Tyrrhenian margin of Italy. Alban Hills lava flows and scoria clasts are made up essentially of clinopyroxenes and leucites and their chemical composition is mostly K-foiditic. Differentiated products (MgO<3 wt.%) are characterised by low SiO2concentration (<50 wt.%) and geochemical features indicate that this unique differentiation trend is driven by crystal fractionation plus carbonate crust interaction. Notably, the Alban Hills Volcanic District was emplaced into thick limestone units. With the aim of constraining the magmatic differentiation, we performed experiments on the Alban Hills parental composition (plagioclase-free phono-tephrite) under anhydrous, hydrous, and hydrous-carbonated conditions. Experiments were carried out at 1 atm, 0.5 GPa and 1 GPa, temperatures ranging from 1050 to 1300 °C, and H2O and CaCO3in the starting material up to 2 and 7 wt.%, respectively. The experiments performed at 0.5 GPa are the most representative of the Alban Hills plumbing system. Clinopyroxene and leucite are the main phases occurring under all the investigated conditions and the liquidus phases. Nevertheless, our experimental results demonstrate that the occurrence of CaCO3in the starting material strongly affects phase relations. Experiments performed under hydrous conditions crystallize magnetite and phlogopite at relatively high temperature. This early crystallization drives the glass composition towards a silica enrichment, resulting in a differentiation trend moving from phono-tephritic (Alban Hills parental composition) to phonolitic compositions. This is in contrast with micro-textural evidence showing late crystallization of magnetite and phlogopite in the natural products and with the composition of the juvenile products. On the contrary, in the CaCO3-bearing experiments (i.e., simulating magma–carbonate interaction) the magnetite and phlogopite stability fields are strongly reduced. As a consequence, the melt differentiation is mainly controlled by the cotectic crystallization of clinopyroxene and leucite, resulting in a differentiation trend moving towards K-foiditic compositions. These experimental results are in agreement with micro-textural features and chemical compositions of Alban Hills natural products and with the magmatic differentiation model inferred by geochemical data. Magma–carbonate interaction is not a rare process and its occurrence has been demonstrated for different plumbing systems. However, the uniqueness of the Alban Hills liquid line of descent suggests that the efficacy of the carbonate contamination process is controlled by different factors, the dynamics of the plumbing system being one of the most important.