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首页> 外文期刊>Contributions to Mineralogy and Petrology >Peralkaline magma evolution and the tephra record in the Ethiopian Rift
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Peralkaline magma evolution and the tephra record in the Ethiopian Rift

机译：Peralkaline岩浆演化和埃塞俄比亚裂谷中的特非拉记录

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The 3.119 ± 0.010 Ma Chefe Donsa phreatomagmatic deposits on the shoulder of the Ethiopian Rift mark the northern termination of the Silti-Debre Zeyit Fault Zone, a linear zone of focused extension within the modern Ethiopian Rift. These peralkaline pumice fragments and glass shards span a wide range of glass compositions but have a restricted phenocryst assemblage dominated by unzoned sanidine. Glass shards found within the ash occupy a far more limited compositional range (75–76 wt% SiO2) in comparison with the pumice (64–75 wt% SiO2), which is rarely mingled. Thermodynamic modeling shows that liquids broadly similar to the least evolved glass composition can be achieved with 50–60 % fractionation of moderately crustally contaminated basalt. Inconsistencies between modeled solutions and the observed values of CaO and P2O5 highlight the important role of fluorine in stabilizing fluor-apatite and the limitations of current thermodynamic models largely resulting from the scarce experimental data available for the role of fluorine in igneous phase stability. On the basis of limited feldspar heterogeneity and crystal content of pumice at Chefe Donsa, and the difficulties of extracting small volumes of Si-rich melt in classical fractional crystallization models, we suggest a two-step polybaric process: (1) basaltic magma ponds at mid-upper-crustal depths and fractionates to form a crystal/magma mush. Once this mush has reached 50–60 % crystallinity, the interstitial liquid may be extracted from the rigid crystal framework. The trachytic magma extracted at this step is equivalent to the most primitive pumice analyzed at Chefe Donsa. (2) The extracted trachytic liquid will rise and continue to crystallize, generating a second mush zone from which rhyolite liquids may be extracted. Some of the compositional range observed in the Chefe Donsa deposits may result from the fresh intrusion of trachyte magma, which may also provide an eruption trigger. This model may have wider application in understanding the origin of the Daly Gap in Ethiopian magmas—intermediate liquids may not be extracted from crystal-liquid mushes due to insufficient crystallization to yield a rigid framework. The wide range of glass compositions characteristic of the proximal Chefe Donsa deposits is not recorded in temporally equivalent tephra deposits located in regional depocenters. Our results show that glass shards, which represent the material most likely transported to distal depocenters, occupy a limited compositional range at high SiO2 values and overlap some distal tephra deposits. These results suggest that distal tephra deposits may not faithfully record the potentially wide range in magma compositions present in a magmatic system just prior to eruption and that robust distal–proximal tephra correlations must include a careful analysis of the full range of materials in the proximal deposit.
机译：埃塞俄比亚裂谷肩部3.119±0.010 Ma的Chef Dons吞吐岩浆沉积标志着Silti-Debre Zeyit断裂带的北端，这是现代埃塞俄比亚裂谷集中的线性延伸区域。这些碱性碱性浮石碎片和玻璃碎片覆盖了各种各样的玻璃成分，但具有受限制的非结晶的苯甲酸酯类组合。与浮石（64–75 wt％SiO 2 ）相比，在灰烬中发现的玻璃碎片的成分范围（75–76 wt％SiO 2 ）受限制得多。 ，很少混用。热力学模型表明，使用50-60％的受中度硬壳污染的玄武岩分级可实现与演化最少的玻璃成分大致相似的液体。建模溶液与CaO和P 2 O 5 的观测值之间的不一致突出了氟在稳定氟磷灰石中的重要作用以及当前热力学模型的局限性缺乏关于氟在火成相稳定性中作用的稀有实验数据。基于CheféDonsa有限的长石非均质性和晶体含量，以及在经典分步结晶模型中提取少量富Si熔体的困难，我们提出了两步多压过程：（1）玄武岩浆池中上地壳深度和分馏形成晶体/岩浆糊状物。一旦糊状物达到50-60％的结晶度，就可以从刚性晶体框架中提取间隙液。在此步骤中提取的走行岩浆相当于Chefe Donsa分析的最原始的浮石。 （2）提取的疏松液体将上升并继续结晶，生成第二个糊状区，可从中提取流纹岩液体。在CheféDonsa矿床中观察到的某些成分范围可能是由于曲奇岩浆的新鲜侵入所致，也可能提供了喷发触发。该模型在理解埃塞俄比亚岩浆中Daly Gap的起源方面可能具有更广泛的应用-由于结晶不足以产生刚性骨架，因此可能无法从晶液mushes中提取中间液体。近距离的Chefe Donsa沉积物具有广泛的玻璃成分特征，但在区域沉积中心的时间上等效的提夫拉沉积物中没有记录。我们的研究结果表明，玻璃碎片代表最有可能转移到远端沉积中心的材料，在高SiO 2 值下仅占有限的组成范围，并且与一些远端的特弗拉沉积物重叠。这些结果表明，在火山喷发之前，远端的前生沉积可能无法如实记录岩浆系统中岩浆成分的潜在范围，并且可靠的远端-近端的前生相关性必须包括对近端沉积物中全部物质的仔细分析。 。

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