Experimental and Thermodynamic Constraints on the Sulphur Yield of Peralkaline and Metaluminous Silicic Flood Eruptions
Experimental and Thermodynamic Constraints on the Sulphur Yield of Peralkaline and Metaluminous Silicic Flood Eruptions
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DOI：
10.1093/petrology/egl016
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发表时间：
2006-07
影响因子：
3.9
通讯作者：
B. Scaillet;R. Macdonald
中科院分区：
文献类型：
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作者：
B. Scaillet;R. Macdonald
许多玄武岩泛滥省份的特点是存在大量的硅质岩浆，但硅质成分在与圈闭活动有关的硫排放中的作用仍然鲜为人知。我们已经进行了实验和理论计算来解决这个问题。实验测定了三个过碱性流纹岩中的熔体硫含量和在饱和状态下硫化物或硫酸盐或两者兼有时的流体/熔体分配，这些流纹岩是一些洪泛省的主要成分。实验在150 Mpa，800-900�C，fO2，NNO-2到NNO-3范围内，在富水条件下进行。除fO2外，硫含量还强烈依赖于熔体的过碱度，在800�℃下，在最强的过碱性成分中，硫含量达到1000ppm。在fO2的所有值下，过碱性熔体携带的硫都比它们的金属铝熔体多5-20倍。温和的过碱性成分表明，流体/熔体硫的分配几乎不会随着fO2的变化而变化(DS�270)。在最碱性的熔体中，DS在fO2>；nit1处急剧上升至>；500。在1~6wt%之间，SBulk的分配系数稳定增加，而在0�5~1wt%之间，SBulk的分配系数保持不变。在体积硫含量低于4wt%时，温度从800wt%升高到900wt C时，DS降低了�10%。这些结果，以及(1)玄武岩结晶和地壳部分熔融过程中硫行为的热力学计算，以及(2)最近对硫在铝质流纹岩中溶解度的实验限制，表明玄武岩分馏可以产生比地壳深熔形成的流纹岩具有更多硫的流纹岩岩浆。特别是，由脱水下地壳熔融产生的热的和干的金属铝硅质岩浆几乎不含硫。相反，碱性玄武岩结晶分馏形成的过碱性流纹岩可以浓缩母岩浆原始硫含量的90%，特别是当玄武岩富CO2时。在此基础上，我们估计了洪水喷发序列的硅质组分可能释放到大气中的硫磺的量。埃塞俄比亚过碱性流纹岩和德干流纹岩可能分别产生�10 17和�10 18 g S，与已发表的各省玄武岩活动估计值相当。相比之下，尽管喷发量相似，但巴拉那-埃滕代卡准铝质硅质喷发只向大气中注入了4�6·10 15g S。因此，过碱性洪水序列可能具有比金属铝亲和力更大的环境影响，这与从大规模灭绝和海洋缺氧事件中获得的证据一致。
Many basaltic flood provinces are characterized by the existence of voluminous amounts of silicic magmas, yet the role of the silicic component in sulphur emissions associated with trap activity remains poorly known. We have performed experiments and theoretical calculations to address this issue. The melt sulphur content and fluid/melt partitioning at saturation with either sulphide or sulphate or both have been experimentally determined in three peralkaline rhyolites, which are a major component of some flood provinces. Experiments were performed at 150MPa, 800–900 � C, fO2 in the range NNO – 2 to NNO þ 3 and under water-rich conditions. The sulphur content is strongly dependent on the peralkalinity of the melt, in addition to fO2, and reaches 1000ppm at NNO þ 1 in the most strongly peralkaline composition at 800 � C. At all values of fO2, peralkaline melts can carry 5–20 times more sulphur than their metaluminous equivalents. Mildly peralkaline compositions show little variation in fluid/melt sulphur partitioning with changing fO2 (DS � 270). In the most peralkaline melt, DS rises sharply at fO2 > NNO þ 1 to values of >500. The partition coefficient increases steadily for Sbulk between 1 and 6wt % but remains about constant for Sbulk between 0� 5 and 1wt %. At bulk sulphur contents lower than 4wt %, a temperature increase from 800 to 900 � C decreases DS by � 10%. These results, along with (1) thermodynamic calculations on the behaviour of sulphur during the crystallization of basalt and partial melting of the crust and (2) recent experimental constraints on sulphur solubility in metaluminous rhyolites, show that basalt fractionation can produce rhyolitic magmas having much more sulphur than rhyolites derived from crustal anatexis. In particular, hot and dry metaluminous silicic magmas produced by melting of dehydrated lower crust are virtually devoid of sulphur. In contrast, peralkaline rhyolites formed by crystal fractionation of alkali basalt can concentrate up to 90% of the original sulphur content of the parental magmas, especially when the basalt is CO2-rich. On this basis, we estimate the amounts of sulphur potentially released to the atmosphere by the silicic component of flood eruptive sequences. The peralkaline Ethiopian and Deccan rhyolites could have produced � 10 17 and � 10 18 g of S, respectively, which are comparable amounts to published estimates for the basaltic activity of each province. In contrast, despite similar erupted volumes, the metaluminous Parana´–Etendeka silicic eruptives could have injected only 4� 6 · 10 15 g of S in the atmosphere. Peralkaline flood sequences may thus have greater environmental effects than those of metaluminous affinity, in agreement with evidence available from mass extinctions and oceanic anoxic events.