加勒比海小安德列斯岛弧Kick’emJenny海底火山岩的斜长石成分环带:示踪大洋岛弧岩浆房的演化

Plagioclase zoning in submarine volcano Kick'em Jenny,Lesser Antilles Arc: Insights into magma evolution processes in oceanic arc magma chamber

我们对采自于加勒比海地区小安德列斯岛弧(Lesser Antilles Arc)Kick’em Jenny(KEJ)海底火山玄武岩中的斜长石斑晶进行了矿物形态和成分分析。利用电子探针(EMPA)和LA-ICP-MS测定了具有环带结构的斜长石斑晶中主量元素的空间分布,同时也利用LA-ICP-MS分析了斜长石中Sr的分布。结果表明,在不同的矿物斑晶中,元素含量均表现出和环带结构相联系的空间分布变化。斜长石斑晶中最主要的结构为韵律环带以及熔蚀结构,所测定的矿物边缘都存在An值从由内向外迅速降低的致密韵律环带,可能反映了快速结晶时的不平衡;而晶体内部的稀疏韵律环带结构是由岩浆填充或对流活动导致的。部分斜长石的熔蚀层An值由内向外升高,反映了高Ca岩浆填充的过程。这说明斜长石斑晶的矿物形态和元素环带可以用来制约俯冲带海底火山岩浆从源区上升到岩浆房再到喷发的复杂过程,包括岩浆演化、熔体多次填充、熔体与结晶矿物之间的反应、以及矿物再熔融等。这对于理解海底火山的喷发以及岛弧岩浆岩的演化有重要意义。

In this study,we analyzed textural and compositional zoning of plagioclase phenocrysts in basalt and basaltic andesites from Kick＇em Jenny（ KEJ） submarine volcano,Lesser Antilles Arc. Major element compositions of the plagioclase phenocrysts were measured by both electron microprobe（ EMPA） and laser ablation inductively coupled plasma mass spectrometry（ LA-ICP-MS）. Sr contents were also analyzed by LA-ICP-MS. The results exhibit that the variations of both major elements and Sr contents are related with the textures of plagioclase phenocrysts. The most typical textures of KEJ plagioclase phenocrysts are oscillatory zoning and resorption. Particularly,all plagioclase phenocrysts in this study have intensive oscillatory zoning with rapid decrease in An（ anorthite）content from the rim to the core. This texture may reflect the disequilibrium between plagioclase and melt during a rapid crystallization process. The width of oscillatory zoning in the core of plagioclase phenocrysts is larger than that in the rim,reflecting magma recharge and convection. Some phenocrysts have resorption texture with increasing An content from core toward rim,which may reflect recharge of high-Ca magma. The textural and compositional zoning of KEJ plagioclase phenocrysts reveal complex magma evolution processes of KEJ submarine volcano,including multiple magma recharges,magma convection in magma chamber,reactions between melt and crystals,and resorption of pre-existing crystals. Therefore,mineral zoning can provide insights into magma evolution and eruption processes of submarine volcanoes.