Experimental Parametrization of Magma Mixing: Application to the AD 1530 Eruption of La Soufrière, Guadeloupe (Lesser Antilles)
Experimental Parametrization of Magma Mixing: Application to the AD 1530 Eruption of La Soufrière, Guadeloupe (Lesser Antilles)
复制标题
岩浆混合实验参数化：在公元 1530 年瓜德罗普岛（小安的列斯群岛）La Soufrière 喷发中的应用
DOI：
10.1093/petrology/egy030
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发表时间：
2018
影响因子：
3.9
通讯作者：
J. Bourdier
中科院分区：
文献类型：
--
作者：
M. Pichavant;Stéphane Poussineau;P. Lesne;Clara Solaro;J. Bourdier
整合了关于喷发产物和实验结果的新岩石学数据，并提出了公元 1530 年喷发之前 La Soufriere（瓜德罗普岛，小安的列斯群岛弧）岩浆库演化的模型。与最近安的列斯群岛的火山危机相比，公元 1530 年的火山喷发颇具特色。喷发火山碎屑序列显示全岩成分从硅质（~ 62% wt% SiO2）到镁铁质安山岩（~ 55% wt% SiO2）的连续分带。镁铁质产物估计占总喷发量的 80%。所有幼年碎屑都富含晶体（46-60 vol.% 斑晶），结晶度与块岩 SiO2 含量成反比。斑晶组合（斜长石、斜方辉石、单斜辉石、磁铁矿）在整个序列中是恒定的。遇到了复杂的分区晶体，但所有样品中都出现了 An60-65、En56-59 和 Mt66-68 成分。玻璃内含物为流纹岩，水含量高达 5-5.5%（重量）。基质玻璃具有很强的异质性，SiO2 含量从～64 wt% 到 >76wt%。储层喷发前的演化主要是玄武质岩浆到达后常驻安山岩体的重新活动。早期再活动的条件受到 L'Echelle 火山渣锥玄武岩实验的限制。到达的岩浆是一种贫晶体、中等热（975-1025℃）、潮湿（> 5wt％H2O）和氧化（NNO+1）低MgO高铝玄武岩，类似于其他安的列斯群岛火山中心的岩浆。对喷发硅质安山岩产物的地温测量和实验表明，当熔体 H2O 含量在 5 至 5.5wt% 之间时，常驻岩浆中的斑晶和间隙熔体在~875°C 和 NNO+0.8 下处于相互平衡。然而，基质玻璃和玻璃夹杂物成分表明，安山岩体局部温度低至 825°C。熔体挥发物浓度意味着岩浆库的最小深度在 5.6 至 7.1 公里之间，而角闪石斑晶的缺失表明最大深度为 8.5 公里。公元 1530 年的喷发产生了一种混合岩浆，该岩浆是由大约等比例的常驻安山岩和到达的玄武岩混合而成的。混合事件的矿物学指标包括斜长石中的富An层、斜方辉石中的富An边缘和磁铁矿中的核-边缘环带，但总体斑晶在混合岩浆的组装过程中几乎没有改变。相比之下，矩阵玻璃受到的影响更为严重。混合基本上通过将镁铁质熔体添加到安山岩体中来进行。在公元 1530 年喷发产物中观察到的连续化学分区反映了三种成分（镁铁质熔体、硅质熔体、斑晶组合）之间的混合。对不同喷发产物测量的时间尺度范围从几千年（U-Th-Ra 不平衡）、10 秒天（斜方辉石的扩散模型）到 10 秒小时（异质基质玻璃）。镁铁质补给以来的时间尺度短、缺乏广泛的斑晶转变、连续的全岩化学分带和镁铁质产物的优势，都与公元 1530 年由起源于中下安的列斯群岛弧地壳的一次重大镁铁质补给事件引发的喷发相一致。
New petrological data on eruption products and experimental results are integrated and a model for the evolution of the La Soufriere (Guadeloupe, Lesser Antilles arc) magma reservoir prior to the 1530 AD eruption is presented. In comparison with recent volcanic crises in the Antilles, the 1530 AD eruption is distinctive. The eruptive pyroclastic sequence shows a continuous zonation in whole-rock composition from silicic (∼ 62 wt% SiO2) to mafic andesite (∼ 55 wt% SiO2). Mafic products are estimated to be 80% of the total eruption volume. All juvenile clasts are crystal-rich (46-60 vol.% phenocrysts), the crystallinity being inversely correlated with the bulk-rock SiO2 content. The phenocryst assemblage (plagioclase, orthopyroxene, clinopyroxene, magnetite) is constant throughout the sequence. Complexly zoned crystals are encountered, but An60-65, En56-59 and Mt66-68 compositions occur in all samples. Glass inclusions are rhyolitic with up to 5-5.5 wt % H2O. Matrix glasses are strongly heterogeneous, from ∼64 to > 76 wt % SiO2. The pre-eruptive evolution of the reservoir is dominated by the remobilization of a resident andesitic body following the arrival of a basaltic magma batch. Conditions of early remobilization are constrained from experiments on a basalt from the L’Echelle scoria cone. The arrival magma is a crystal-poor, moderately hot (975-1025 °C), wet (> 5 wt % H2O) and oxidized (NNO+1) low-MgO high alumina basalt similar to those involved in other Antilles volcanic centers. Geothermometry and experiments on a silicic andesite product of the eruption show that, for melt H2O contents between 5 and 5.5 wt %, phenocrysts and interstitial melt in the resident magma were in mutual equilibrium at ∼875 °C and NNO+0.8. However, matrix glass and glass inclusion compositions show that, locally, the andesite body was as cold as 825 °C. Melt volatile concentrations imply a minimum depth for the magma reservoir between 5.6 and 7.1 km, and the absence of amphibole phenocrysts indicates a maximum depth at 8.5 km. The 1530 AD eruption tapped an hybrid magma assembled by mixing approximately equal proportions of resident andesite and arrival basalt. Mineralogical indicators of the mixing event include An-rich layers in plagioclase, En-rich rims on orthopyroxene and core-rim zonation in magnetite, but overall phenocrysts were little modified during assembly of the hybrid magma. In comparison, matrix glasses were more severely affected. Mixing proceeded essentially by the addition of a mafic melt to the andesite body. The continuous chemical zonation observed in 1530 AD eruption products reflects mixing between three components (mafic melt, silicic melt, phenocryst assemblage). Timescales measured on different eruptive products range from several thousand years (U-Th-Ra disequilibria), 10s days (diffusion modelling in orthopyroxenes) to 10s hours (heterogeneous matrix glasses). Short timescales since mafic recharge, lack of extensive transformations of phenocrysts, continuous whole-rock chemical zonation and predominance of mafic products are all consistent with triggering of the 1530 AD eruption by a major mafic recharge event which originated in the middle to lower Lesser Antilles arc crust.