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首页> 外文期刊>Contributions to Mineralogy and Petrology >Textural characterization, major and volatile element quantification and Ar-Ar systematics of spherulites in the Rocche osse obsidian flow, Lipari, Aeolian Islands: a temperature continuum growth model
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Textural characterization, major and volatile element quantification and Ar-Ar systematics of spherulites in the Rocche osse obsidian flow, Lipari, Aeolian Islands: a temperature continuum growth model

机译：洛克希奥斯西黑曜石流中风球的组织表征，主要和挥发性元素定量以及球体的Ar-Ar系统：风连续群岛的温度连续增长模型

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摘要

Spherulitic textures in the Rocche Rosse obsidian flow (Lipari, Aeolian Islands, Italy) have been characterized through petrographic, crystal size distribution (CSD) and in situ major and volatile elemental analyses to assess the mode, temperature and timescales of spherulite formation. Bulk glass chemistry and spherulite chemistry analyzed along transects across the spherulite growth front/glass boundary reveal major-oxide and volatile (H_2O, CO_2, F, Cl and S) chemical variations and heterogeneities at a ≤5 μm scale. Numerous bulk volatile data in non-vesicular glass (spatially removed from spherulitic textures) reveal homogenous distributions of volatile con centrations: H_2O (0.089 ± 0.012 wt%), F (950 ± 40 ppm) and Cl (4,100 ± 330 ppm), with CO_2 and S consistently below detection limits suggesting either completedegassing of these volatiles or an originally volatile-poor melt. Volatile concentrations across the spherulite bound ary and within the spherulitic textures are highly variable. These observations are consistent with diffusive expulsion of volatiles into melt, leaving a volatile-poor rim advancing ahead of anhydrous crystallite growth, which is envisaged to have had a pronounced effect on spherulile crystalliza tion dynamics. Argon concentrations dissolved in the glass and spherulites differ by a factor of ～20, with Ar sequestered preferentially in the glass phase. Petrographic observation, CSD analysis, volatile and Ar data as well as diffusion modeling support continuous spherulite nucle-ation and growth starting at magmatic (emplacement) temperatures of ～ 790-825 °C and progressing through the glass transition temperature range (T_g ～ 750-620 °C), being further modified in the solid state. We propose that nucleation and growth rate are isothermally constant, but vary between differing stages of spherulite growth with continued cooling from magmatic temperatures, such that there is an evolution from a high to a low rate of crystal lization and low to high crystal nucleation. Based on the diffusion of H_2O across these temperature ranges (～ 800-300 °C), timescales of spherulite crystallization occur on a timescale of ～4 days with further modification up to ～400 years (growth is prohibitively slow <400 °C and would become diffusion reliant). Selective deformation of spherulites supports a down-temperature continuum of spherulite formation in the Rocche Rosse obsidian; indeed, petrographic evidence suggests that high-strain zones may have catalyzed progressive nucleation and growth of fur ther generations of spherulites during syn- and post-emplacement cooling.
机译：Rocche Rosse黑曜石流（意大利风神群岛利帕里）的球状纹理已经通过岩石学，晶体尺寸分布（CSD）以及原位主要和挥发性元素分析来表征，以评估球晶形成的方式，温度和时间尺度。沿横跨球晶生长前沿/玻璃边界的样线分析的大块玻璃化学和球晶化学在≤5μm范围内显示出主要的氧化物和挥发性（H_2O，CO_2，F，Cl和S）化学变化和非均质性。非水泡玻璃中的大量总体挥发性数据（已从球形纹理中去除）显示了挥发性浓度的均匀分布：H_2O（0.089±0.012 wt％），F（950±40 ppm）和Cl（4,1​​00±330 ppm），以及CO_2和S始终低于检测极限，表明这些挥发物已完全消失或最初的挥发物贫乏。整个球晶结合元和球晶结构内的挥发性浓度变化很大。这些观察结果与挥发物扩散排出熔体是一致的，在无水微晶生长之前留下了较差的挥发物边缘，这被认为对球晶的动力学具有显着影响。溶解在玻璃和球晶中的氩气浓度相差约20倍，其中Ar优先滞留在玻璃相中。岩石学观测，CSD分析，挥发物和Ar数据以及扩散模型支持球晶在790〜825°C的岩浆（定位）温度下开始并持续到玻璃化转变温度范围（T_g〜750- 620°C），然后在固态下进一步改性。我们提出成核和生长速率是等温不变的，但随着岩浆温度的不断冷却，在球晶生长的不同阶段之间会发生变化，从而导致晶体形成速率从高到低以及从低到高的成核过程。基于H_2O在这些温度范围（〜800-300°C）的扩散，球晶结晶的时标发生在〜4天的时标上，并进一步修改直到〜400年（生长缓慢<400°C且会缓慢增长）。变得依赖扩散）。在Rocche Rosse黑曜石中，球晶的选择性变形支持了球晶形成的低温连续体。确实，岩相学证据表明，高应变区可能在同位和后位冷却过程中催化了进一步生成的球晶的逐渐成核和生长。

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