Volatile Evolution of Magma Associated with the Solchiaro Eruption in the Phlegrean Volcanic District (Italy)
Volatile Evolution of Magma Associated with the Solchiaro Eruption in the Phlegrean Volcanic District (Italy)
复制标题
DOI：
10.1093/petrology/egr051
复制
发表时间：
2011-12-01
影响因子：
3.9
通讯作者：
Shimizu, N.
中科院分区：
文献类型：
--
作者：
Esposito, R.;Bodnar, R. J.;Shimizu, N.
超过 1 个中心点、500 万人居住在意大利南部的 Phlegrean 火山区 (PVD) 或其附近，该火山区是世界上研究最仔细的火山危险区之一。纵观其历史，火山活动的风格变化很大，从相对静止的熔岩流到爆炸性的岩浆喷发。本研究的目的是更详细地了解与 PVD 中索尔基亚罗喷发相关的物理和化学过程。 PVD 包括三个火山区：Campi Flegrei (CF) 火山口以及伊斯基亚和普罗奇达火山岛。普罗奇达岛的索尔基亚罗喷发是 PVD 中为数不多的原始（进化程度较低）喷发之一，可以提供有关与该火山系统相关的进化程度较高的岩浆来源的信息。决定火山喷发类型的更重要的化学参数之一是喷发前和喷发期间岩浆的挥发性预算。熔体包裹体 (MI) 为 PVD 源区喷发前熔体的挥发物含量提供了最直接的信息。通过分析索尔基亚罗火山喷发的四个样品中橄榄石中 109 MI 的主量、微量和微量元素以及挥发物含量，确定了喷发前熔体相的组成，这些样品代表了沉积物中不同的地层高度，因此也代表了不同的相对喷发时间。橄榄石成分从 Fo(82) 到 Fo(88) 不等，其中一个最大值为 Fo(90)。橄榄石中的 MI 成分针对包埋后结晶 (PEC) 和扩散造成的铁损失进行了校正。大多数（109 个中的 97 个）研究的 MI 被归类为“正常”MI，因为它们显示出与来自 PVD 的大块岩石一致的化学演化趋势。根据主量元素和微量元素组成，还识别出两种类型的异常 MI：(1) 富 Sr MI，以及 (2) 富集 MI，相对于“正常”MI，富集 MI 不同程度地富含 TiO2、K2O、P2O5、大离子亲石元素、高场强元素和稀土元素。这些MI可能起源于岩浆体糊状带的溶解-反应-混合过程。 “正常”MI 包括含气泡和无气泡（仅包含玻璃+/- 截留的铬铁矿）类型。无气泡 MI 最密切地记录了在一系列时间和空间条件下熔体喷发前的挥发物含量。观察到的 MI 的 CO2 含量与结晶指示剂（例如 Al2O3/CaO）的趋势支持这样的解释：无气泡 MI 的挥发物含量的变化反映了岩浆演化过程中熔体挥发物预算的真实变化。 MI 的 CO2 含量与每个样本的相对地层位置之间的相关性与挥发性饱和岩浆的喷发一致，该岩浆最初从至少 8 公里的原始深度穿过地壳上升。岩浆在喷发和结晶之前在 4-2 公里深的地方积聚，伴随着浅室饱和熔体的挥发性溶出，引发了索尔基亚罗喷发。随着喷发的进行，索尔基亚罗岩浆继续上升穿过地壳，最终储存深度约为1公里。
More than 1 center dot 5 million people live in or near the Phlegrean Volcanic District (PVD) in southern Italy, which represents one of the most carefully studied volcanic hazard areas in the world. Throughout its history, the style of volcanic activity has varied greatly, from relatively quiescent lava flows to explosive phreatomagmatic eruptions. The goal of this study is to develop a more detailed understanding of the physical and chemical processes associated with the Solchiaro eruption in the PVD. The PVD includes three volcanic fields: the Campi Flegrei (CF) caldera and the volcanic islands of Ischia and Procida. The Solchiaro eruption on the island of Procida is one of the few primitive (less evolved) eruptions in the PVD and can provide information on the source of the more evolved magmas associated with this volcanic system. One of the more important chemical parameters that determine the style of volcanic eruptions is the volatile budget of the magma before and during eruption. Melt inclusions (MI) provide the most direct information on the volatile contents of the pre-eruptive melt in the source region for the PVD. The composition of the melt phase before eruption was determined by analyzing the major, minor and trace element and volatile contents of 109 MI in olivine from four samples of the Solchiaro eruption, representing different stratigraphic heights in the deposits and, therefore, different relative times of eruption. Olivine compositions vary from Fo(82) to Fo(88), with one maximum value of Fo(90). The compositions of the MI in olivine were corrected for post-entrapment crystallization (PEC) and for Fe loss by diffusion. Most (97 out of 109) of the MI studied are classified as 'normal' MI because they show chemical evolution trends consistent with that of bulk-rocks from the PVD. Two types of anomalous MI were also recognized based on their major and trace element compositions: (1) Sr-rich MI, and (2) enriched MI that are variably enriched in TiO2, K2O, P2O5, large ion lithophile elements, high field strength elements and rare earth elements relative to 'normal' MI. These MI probably originated from dissolution-reaction-mixing processes in the mush zone of the magma body. 'Normal' MI include both bubble-bearing and bubble-free (containing only glass +/- trapped chromite) types. Bubble-free MI most closely record the pre-eruptive volatile content of the melt over a range of temporal and spatial conditions. The observed trends in CO2 contents of MI versus crystallization indicators (e.g. Al2O3/CaO) support the interpretation that variations in the volatile contents of bubble-free MI reflect real variations in the volatile budget of the melt during the evolution of the magma. The correlation between CO2 contents of MI and the relative stratigraphic position of each sample is consistent with eruption of a volatile-saturated magma that initially ascended through the crust from an original depth of at least 8 km. The magma ponded at 4-2 km depth prior to eruption and crystallization and the concomitant volatile exsolution from the saturated melt in the shallow chamber triggered the Solchiaro eruption. As the eruption proceeded, the Solchiaro magma continued to ascend through the crust to a final storage depth of about 1 km.