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首页> 外文期刊>Contributions to Mineralogy and Petrology >Li, Be, B concentrations and δ7Li values in plagioclase phenocrysts of dacites from Nea Kameni (Santorini, Greece)
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Li, Be, B concentrations and δ7Li values in plagioclase phenocrysts of dacites from Nea Kameni (Santorini, Greece)

机译：来自Nea Kameni（希腊圣托里尼）的Daaf的斜长石斑晶中的Li，Be，B浓度和δ7Li值

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摘要

Li, Be, B and δ7Li 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 (An62–40); type-O has only oscillatory zoning accompanied by resorption surfaces (An58–39); type-C is complex with high-An cores (subtype C1: An64–58, subtype C2: An88–73) and normal rims (An55–42). Type-A plagioclase with high An content (An92–82) 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. δ7Li profiles in type-O2 and type-A phenocrysts manifest an unmistakable inverse relation to Li concentration, with variations of up to ~39 ‰, 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.
机译：Li，Be，B和δ7LiSIMS分析1040–1941 Niki达克特熔岩（Nea Kameni，希腊圣托里尼）的斜长石隐晶石显示出不同的过程。仅从它们的钙长石含量来看，晶体就可以分为四种主要类型：N型表明在结晶过程中An的正常下降（An62–40）； O型仅具有振荡区域，并伴有吸收表面（An58–39）； C型复杂，具有高An磁芯（子类型C1：An64-58，子类型C2：An88-73）和普通轮辋（An55-42）。在镁铁质飞地中发现了具有较高An含量（An92–82）的A型斜长石。根据其Li浓度，可以将O型晶体细分为Li浓度分布平坦的O1型和Li浓度从核到边缘降低的O2型。所有四种类型的Be和B的浓度与钙长石含量（An）呈负相关，但在不同斜长石类型之间，Li的浓度分布有所不同。 N型和O1型以及C型核在锂浓度中达到平衡。 O2和A型以及C型地幔显示出最初的Li富集，可能是由于挥发性流入熔体中。与熔融物趋于平衡的倾向一致，这些晶体显示出锂浓度的急剧向边缘的下降。除异晶A型外，所有分析的斜长石晶体在其边缘处的Li，Be和B浓度几乎相同。这些与地面物质中斜长石微晶的组成相吻合，从而提供了轻元素斜长石熔体分配的估计值：K Li = 0.19-0.28，K Be = 0.24-0.38和K B = 0.007-0.009。 O2型和A型隐晶中的δ7Li曲线与Li浓度具有明显的反比关系，变异高达〜39‰，揭示了优先的动力学扩散。这可能是由于锂从熔体中流失所致，最有可能是通过减压过程中的脱气，也许是在岩浆上升之后的喷发过程中。考虑到锂在斜长石酶中的快速扩散，原位酚晶分析可能会产生有关导致或什至引起爆发的过程的有用信息。

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