Pyrometamorphism is the highest temperature end-member of the sanidinite facies (high-temperature, low-pressure contact metamorphism) and comprises both subsolidus and partial melting reactions which may locally lead to cryptocrystalline-glassy rocks (i.e., porcellanites and buchites). A wide range of pyrometamorphic ejecta, with different protoliths from Stromboli volcano, have been investigated over the last two decades. Among these, a heterogeneous (composite) glassy sample (B1) containing intimately mingled porcellanite and buchite lithotypes was selected to be studied through new FESEM–EDX and QEMSCAN™ mineral mapping investigations, coupled with the already available bulk rock composition data. This xenolith was chosen because of the unique and intriguing presence of abundant Cu–Fe sulphide globules within the buchite glass in contrast with the well-known general absence of sulphides in Stromboli basalts or their subvolcanic counterparts (dolerites) due to the oxygen fugacity of NNO + 0.5–NNO + 1 (or slightly lower) during magma crystallization. The investigated sample was ejected during the Stromboli paroxysm of 5 April 2003 when low porphyritic (LP) and high porphyritic (HP) basalts were erupted together. Both types of magmas are present as coatings of the porcellanite–buchite sample and were responsible for the last syn-eruptive xenoliths’s rim made of a thin crystalline-glassy selvage. The complex petrogenetic history of the B1 pyrometamorphic xenolith is tentatively explained in the framework of the shallow subvolcanic processes and vent system dynamics occurred shortly before (January–March 2003) the 5 April 2003 paroxysm. A multistep petrogenesis is proposed to account for the unique occurrence of sulphide globules in this composite pyrometamorphic xenolith. The initial stage is the pyrometamorphism of an already hydrothermally leached extrusive/subvolcanic vent system wall rock within the shallow volcano edifice. Successively, fragments of this wall rock were subject to further heating by continuous gas flux and interaction with Stromboli HP basalt at temperatures above 1000 °C to partially melt the xenolith. This is an open system process involving continuous exchange of volatile components between the gas flux and the evolving silicate melt. It is suggested that the reaction of plagioclase and ferromagnesian phenocrysts with SO₂ and HCl from the volcanic gas during diffusion into the melt led to the formation of molecular CaCl in the melt, which then was released to the general gas flux. Sulphide formation is the consequence of the reaction of HCl dissolved into the melt from the gas phase, resulting in the release of H₂ into the melt and lowering of f_O2 driving reduction of the dissolved SO₂.

中文翻译：

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来自斯特龙博利火山（意大利南部伊奥利亚群岛）的 Porcellanite-Buchite 复合捕虏体中的硫化物小球：开放系统火成岩变质作用的产物

热变质作用是花青石相（高温、低压接触变质作用）的最高温度端元，包括亚固相线和部分熔融反应，可能会在局部形成隐晶玻璃岩（即瓷质岩和紫铜质）。在过去的 20 年里，已经研究了范围广泛的具有来自斯特龙博利火山的不同原岩的热变质喷出物。其中，通过新的 FESEM-EDX 和 QEMSCAN™ 矿物测绘调查，结合已有的大块岩石成分数据，选择了包含密切混合的瓷釉和锂辉石岩型的非均质（复合）玻璃样样品 (B1) 进行研究。之所以选择这种捕虏体，是因为在 buchite 玻璃中存在丰富的 Cu-Fe 硫化物球体，这与众所周知的由于 NNO 的氧逸度而导致的 Stromboli 玄武岩或其亚火山对应物（辉绿岩）中普遍不存在硫化物形成对比+ 0.5–NNO + 1（或略低）在岩浆结晶过程中。研究的样品在 2003 年 4 月 5 日的 Stromboli 爆发期间喷出，当时低斑状 (LP) 和高斑状 (HP) 玄武岩一起喷发。两种类型的岩浆都作为瓷器-白铜矿样品的涂层存在，并且是最后一次同步喷发的捕虏体边缘的原因，该边缘由薄的结晶玻璃边缘制成。在 2003 年 4 月 5 日爆发前不久（2003 年 1 月至 3 月）发生的浅层次火山过程和喷口系统动力学的框架内，初步解释了 B1 热变质捕虏体的复杂成岩历史。提出了一个多步骤的岩石成因来解释这种复合热变质捕虏体中硫化物球的独特出现。初始阶段是浅火山建筑物内已经热液浸出的喷出/次火山喷口系统围岩的高温变质作用。随后，该围岩的碎片在 1000 °C 以上的温度下通过连续气体通量和与 Stromboli HP 玄武岩的相互作用进一步加热，从而部分熔化捕虏体。这是一个开放系统过程，涉及在气体通量和演化出的硅酸盐熔体之间连续交换挥发性成分。这表明斜长石和铁镁斑晶与 SO₂和来自火山气体的 HCl 在扩散到熔体中导致在熔体中形成分子 CaCl，然后释放到一般气体通量中。硫化物的形成是 HCl 从气相溶解到熔体中反应的结果，导致 H ₂释放到熔体中并降低f _O2驱动溶解的 SO _{2 的}还原。