Li, Be, B concentrations and δ7Li values in plagioclase phenocrysts of dacites from Nea Kameni (Santorini, Greece)
Li, Be, B concentrations and δ7Li values in plagioclase phenocrysts of dacites from Nea Kameni (Santorini, Greece)
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DOI：
10.1007/s00410-013-0851-z
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发表时间：
2013-06-01
影响因子：
3.5
通讯作者：
Meyer, Hans-Peter
中科院分区：
文献类型：
--
作者：
Cabato, Joan;Altherr, Rainer;Meyer, Hans-Peter
1040-1941年希腊圣托里尼Nea Kameni尼基英安岩熔岩中斜长石斑晶的Li、Be、B和Delta Li-7 SIMS分析显示出不同的过程。仅从斜长岩的含量来看，晶体可分为四种主要类型：类型-N显示结晶过程中AN的正常下降(AN(62-40))；类型-O仅具有伴随着再吸收表面的振荡环带(AN(58-39))；类型-C具有高AN核(亚型C1：AN(-58)、亚型C2：AN(88-73))和正常边缘(AN(55-42))。A型斜长石，AN含量高(AN(92-82))，产于镁铁质包体中。根据Li浓度的不同，O型晶体可分为Li浓度分布平坦的O_1亚型和Li浓度从核到缘逐渐降低的O_2亚型。4种斜长石中Be、B的含量均与钙长石含量(An)呈负相关，但不同类型斜长石的Li含量分布不同。类型N和类型O1以及类型-C的核心在Li浓度上是平衡的。O2型和A型以及C型地幔最初表现出Li的富集性，可能是挥发性物质流入熔体的结果。与熔体趋于平衡的倾向一致，这些晶体的锂浓度呈现出明显的边缘下降。所有被分析的斜长石晶体，除了捕捉型A型外，其边缘的Li、Be和B的浓度几乎相同。这与基质中斜长石微石的组成相吻合，从而估算出轻元素的斜长熔融分配：K(Li)=0.19-0.28，K(Be)=0.24-0.38，K(B)=0.007-0.009。O2型和A型斑晶中的Li-7曲线与Li浓度呈明显的反相关关系，变化幅度可达39度，显示出优先的动力学扩散。这可能是由于熔体中锂的损失，最有可能是通过减压过程中的脱气，可能是在岩浆上升到随后喷发的过程中。考虑到锂在斜长石中的快速扩散，原位斑晶分析可能会提供导致甚至导致喷发的过程的有用信息。
Li, Be, B and delta Li-7 SIMS analyses of plagioclase phenocrysts from the 1040-1941 Niki dacite lava (Nea Kameni, Santorini, Greece) exhibit varied processes. From their anorthite contents alone, the crystals may be segregated into four main types: type-N shows the normal decline in An during crystallisation (An(62-40)); type-O has only oscillatory zoning accompanied by resorption surfaces (An(58-39)); type-C is complex with high-An cores (subtype C1: An(64-58), subtype C2: An(88-73)) and normal rims (An(55-42)). Type-A plagioclase with high An content (An(92-82)) is found within mafic enclaves. On the basis of their Li concentrations, type-O crystals may be subdivided into subtype O1 with flat Li concentration profiles and subtype O2 with decreasing Li concentration from core to rim. The concentrations of Be and B of all four types show a negative correlation with anorthite content (An), but Li concentration profiles differ amongst the different plagioclase types. Types N and O1, and the cores of type-C, are equilibrated in Li concentration. Types O2 and A, and the mantles of type-C display an initial enrichment in Li, probably from volatile influx into the melt. Consistent with the propensity towards equilibrium with the melt, these crystals display dramatic rim-ward declines in Li concentration. All analysed plagioclase crystals, except for the xenocrystic type-A, have nearly the same Li, Be and B concentrations at their rims. These coincide with the composition of plagioclase microlites in the groundmass, thereby affording estimates of plagioclase-melt partitioning for the light elements: K (Li) = 0.19-0.28, K (Be) = 0.24-0.38 and K (B) = 0.007-0.009. delta Li-7 profiles in type-O2 and type-A phenocrysts manifest an unmistakable inverse relation to Li concentration, with variations of up to similar to 39 aEuro degrees, revealing preferential kinetic diffusion. This may have been driven by Li loss from the melt, most likely through degassing during decompression, perhaps in the course of magma ascent to subsequent eruption. Considering the rapid diffusion of Li in plagioclase, in situ phenocryst analyses may yield useful information about processes leading up to, or even causing, eruptions.