Liquid line of descent of a basanitic liquid at 1.5 Gpa: constraints on the formation of metasomatic veins
Liquid line of descent of a basanitic liquid at 1.5 Gpa: constraints on the formation of metasomatic veins
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DOI：
10.1007/s00410-009-0445-y
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发表时间：
2010-05
影响因子：
3.5
通讯作者：
S. Pilet;P. Ulmer;Samuel Villiger
中科院分区：
文献类型：
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作者：
S. Pilet;P. Ulmer;Samuel Villiger
在大洋和大陆岩石圈中观察到的交代作用通常被解释为代表了一个连续的分异过程，形成无水和含水脉，加上周围橄榄岩中的隐藏富集。为了限制脉形成的机制，并可能澄清的性质和起源的初始交代剂，我们进行了一系列的高压实验模拟的玄武岩浆下降的液体线在大陆或成熟的大洋岩石圈内的分化。这一系列的实验是在端部加载的活塞缸装置中进行的，该装置从初始的水辉石标准碧玄岩开始，压力为1.5 GPa，温度在1，250和980 ℃之间变化。在30°C的温度步骤中以逐步方式实现近纯的分步结晶过程，并且起始组合物对应于先前的较高温度玻璃组合物的液体组合物。液体逐渐从碧玄岩演变为过碱性，富铝的组合物没有显着的SiO2变化。在高温（1，250 - 1，160 °C）下，堆晶岩的特征是无水单斜辉石+橄榄石组合，而在较低温度（1，130 -980°C）下，则形成主要由角闪石+少量单斜辉石、尖晶石、钛铁矿、钛磁铁矿和磷灰石组成的含水堆晶岩（1，130 -980°C）。这一新的数据集支持的解释，无水和含水交代脉可能产生的原生，含水碱性岩浆在高压下的连续分异过程。然而，从一个initialne规范的液体或从hy-normal的初始组合物（夏威夷岩或picrobaltite）的分离结晶所产生的累积之间的比较表明，对于所有的含水液体，不同的阶段形成后分化大多是相似的，即使含水与无水矿物的比例可能会有很大的变化。这表明，在岩石圈地幔中观察到的角闪石交代脉的形成可能与初始液体的分异有关，其成分范围从ne-normal到hy-normal。目前的研究没有解决岩石圈地幔交代作用是碱性岩浆作用的前兆还是后果的问题;但是，在此情况下，它证实，类似于或略高于亏损MORB地幔的来源的低程度部分熔融产生的液体的渗滤和分异可以产生含水交代脉，这些脉被解释为各种碱性岩浆作用的潜在来源。自行
The metasomatism observed in the oceanic and continental lithosphere is generally interpreted to represent a continuous differentiation process forming anhydrous and hydrous veins plus a cryptic enrichment in the surrounding peridotite. In order to constrain the mechanisms of vein formation and potentially clarify the nature and origin of the initial metasomatic agent, we performed a series of high-pressure experiments simulating the liquid line of descent of a basanitic magma differentiating within continental or mature oceanic lithosphere. This series of experiments has been conducted in an end-loaded piston cylinder apparatus starting from an initial hydrousne-normative basanite at 1.5 GPa and temperature varying between 1,250 and 980°C. Near-pure fractional crystallization process was achieved in a stepwise manner in 30°C temperature steps and starting compositions corresponding to the liquid composition of the previous, higher-temperature glass composition. Liquids evolve progressively from basanite to peralkaline, aluminum-rich compositions without significant SiO2variation. The resulting cumulates are characterized by an anhydrous clinopyroxene + olivine assemblage at high temperature (1,250–1,160°C), while at lower temperature (1,130–980°C), hydrous cumulates with dominantly amphibole + minor clinopyroxene, spinel, ilmenite, titanomagnetite and apatite (1,130–980°C) are formed. This new data set supports the interpretation that anhydrous and hydrous metasomatic veins could be produced during continuous differentiation processes of primary, hydrous alkaline magmas at high pressure. However, the comparison between the cumulates generated by the fractional crystallization from an initialne-normative liquid or fromhy-normative initial compositions (hawaiite or picrobasalt) indicates that for all hydrous liquids, the different phases formed upon differentiation are mostly similar even though the proportions of hydrous versus anhydrous minerals could vary significantly. This suggests that the formation of amphibole-bearing metasomatic veins observed in the lithospheric mantle could be linked to the differentiation of initial liquids ranging fromne-normative tohy-normative in composition. The present study does not resolve the question whether the metasomatism observed in lithospheric mantle is a precursor or a consequence of alkaline magmatism; however, it confirms that the percolation and differentiation of a liquid produced by a low degree of partial melting of a source similar or slightly more enriched than depleted MORB mantle could generate hydrous metasomatic veins interpreted as a potential source for alkaline magmatism by various authors.