Melting, Differentiation and Degassing at the Pantelleria Volcano, Italy
Melting, Differentiation and Degassing at the Pantelleria Volcano, Italy
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DOI：
10.1093/petrology/egr074
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发表时间：
2012-03
影响因子：
3.9
通讯作者：
D. Neave;Gareth N. Fabbro;R. Herd;C. Petrone;M. Edmonds
中科院分区：
文献类型：
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作者：
D. Neave;Gareth N. Fabbro;R. Herd;C. Petrone;M. Edmonds
我们提出的第一个系统的研究结果在潘泰莱里亚的熔体成分，熔体包裹体和基质玻璃在潘泰莱里亚从10喷发在最后一个喷发周期（<45 kyr）的基础上。我们目前的主要和微量元素的组成，以及数据上的挥发分硫（S），氟（F），氯（Cl），水（H2O），二氧化碳（CO2）和锂（Li）稀土元素（REE）的组成使用程序INVMEL建立熔体分数与深度的关系在潘特勒地幔源区反演。反演结果表明，熔体是由富集轻稀土元素（LREE）的地幔源区熔融1.17%所产生的。该物质来源主要位于尖晶石-石榴子石过渡带内，根据微量元素比值，该过渡带与北非岩浆活动的物质来源有一定的相似性。主要和微量元素的数据表明，在中间组合物的熔体组合物的差距，与以前公布的全岩数据一致。这一空白排除了仅仅通过岩浆中晶体含量的变化来解释潘泰莱里亚熔岩中化学变化的可能性。主成分分析表明，镁铁质熔体成分的液相下降线受单斜辉石、斜长石、磁铁矿和橄榄石结晶的控制。碱性长石，单斜辉石，钛铁矿和橄榄石或aenigmatite结晶控制的液体线下降的岩石熔体组合物，aenigmatite广泛取代橄榄石在最进化的岩浆。微量元素建模表明，需要96%的分离结晶生成pantelleria碱性玄武岩在Pantelleria（通过粗面岩，76%的分离结晶后产生）。我们已经测量了从各种喷发熔融包裹体和基质玻璃中的碱碲矿挥发分浓度。熔体夹杂物平均含有350 ppm S、3500 ppm F和9000 ppm Cl。我们在熔融包裹体中测量到高达4.9wt%的H2O和150 ppm的CO2。熔融包裹体中Li-H2O系统学和Cl丰度与Li和Cl在低压下分配到亚临界水盐流体中是一致的。挥发物H2O和CO2用于估算熔体平衡压力，其最大值为1.5 kbar。使用已发表的长石-熔融地质温度计，计算出演化程度最高的碱碲矿的温度为800°C，演化程度最低的样品的温度高达870°C。低熔体粘度计算观察到的pantellerite组合物的范围内，并可能占快速分化晶体沉降。稳定的密度分层的岩浆房是反映在喷发后的绿色凝灰岩喷发在最后一个喷发周期的一般逐步更分馏的成分。这种趋势中的一些异常现象可以用喷发与分馏的相对速率的变化来解释。流体相向岩浆房顶部的有效迁移预计将增强并进一步稳定密度分层。硫的数据结合使用发表的实验分区数据过碱性rhythmic估计硫产量到大气中的一个大型pantelleritic喷发类似的绿色凝灰岩。预计这将显着高于类似大小的metaluminous流纹岩或英安岩喷发，主要是由于较高的散装硫含量，较低的流体-熔体分配系数，快速分化和气相分离的岩浆房。
We present the results of the first systematic study of melt compositions at Pantelleria, based on both melt inclusions and matrix glasses in pantellerites from 10 eruptions during the last eruptive cycle (<45 kyr). We present major and trace element compositions, as well as data on the volatiles sulphur (S), fluorine (F), chlorine (Cl), water (H2O), carbon dioxide (CO2) and lithium (Li) Rare earth element (REE) compositions were inverted using the program INVMEL to establish the melt fraction vs depth relationship in the Pantellerian mantle source region. Inversion indicates that melts are generated by ∼1·7% melting of a light rare earth element (LREE)-enriched mantle source. The source lies principally within the spinel–garnet transition zone, which, on the basis of trace element ratios, shows some affinity to the source of North African magmatism. Major and trace element data indicate a gap in melt compositions at intermediate compositions, consistent with previously published whole-rock data. This gap rules out the possibility of explaining chemical variability in the Pantelleria lavas merely by changes in the crystal content of the magmas. Principal component analysis of major element glass compositions shows that the liquid line of descent for mafic melt compositions is controlled by clinopyroxene, plagioclase, magnetite and olivine crystallization. Alkali feldspar, clinopyroxene, ilmenite and olivine or aenigmatite crystallization controls the liquid line of descent for the silicic melt compositions, with aenigmatite broadly replacing olivine in the most evolved magmas. Trace element modelling indicates that 96% fractional crystallization is required to generate pantellerites from alkali basalts at Pantelleria (through trachytes, generated after 76% fractional crystallization). We have measured pantellerite volatile concentrations in melt inclusions and in matrix glasses from a variety of eruptions. Melt inclusions, on average, contain 350 ppm S, 3500 ppm F and 9000 ppm Cl. We have measured up to 4·9 wt % H2O and 150 ppm CO2 in melt inclusions. Li–H2O systematics and Cl abundances in melt inclusions are consistent with partitioning of Li and Cl into a subcritical hydrosaline fluid at low pressures. The volatiles H2O and CO2 are used to estimate melt equilibration pressures, which reach a maximum of 1·5 kbar. Temperatures of 800°C are calculated for the most evolved pantellerites, using published feldspar–melt geothermometers, and up to 870°C for the least evolved samples. Low melt viscosities are calculated for the range of pantellerite compositions observed and may account for rapid differentiation by crystal settling. Stable density stratification of the magma chamber is reflected in the eruption of generally progressively more fractionated compositions after the Green Tuff eruption during the last eruptive cycle. Some anomalies in this trend may be explained by variation in the relative rates of eruption vs fractionation. The density stratification is expected to be enhanced and further stabilized by the efficient migration of a fluid phase to the roof of the magma chamber. The sulphur data are used in combination with published experimental partitioning data for peralkaline rhyolites to estimate the sulphur yield to the atmosphere for a large pantelleritic eruption similar to the Green Tuff. This is expected to be markedly higher than for a similar-sized metaluminous rhyolitic or dacitic eruption, mainly owing to the higher bulk sulphur content, lower fluid–melt partition coefficients, and rapid differentiation and vapour phase segregation in the magma chamber.