Textural characterization, major and volatile element quantification and Ar-Ar systematics of spherulites in the Rocche Rosse obsidian flow, Lipari, Aeolian Islands: a temperature continuum growth model
Textural characterization, major and volatile element quantification and Ar-Ar systematics of spherulites in the Rocche Rosse obsidian flow, Lipari, Aeolian Islands: a temperature continuum growth model
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DOI：
10.1007/s00410-012-0813-x
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发表时间：
2013-02-01
影响因子：
3.5
通讯作者：
Kelley, S. P.
中科院分区：
文献类型：
--
作者：
Clay, P. L.;O'Driscoll, B.;Kelley, S. P.
通过岩相学、晶体粒度分布(CSD)和原位主量元素和挥发分元素分析，对意大利利帕里的Rocche Rosse黑玉岩流的球晶结构进行了表征，以评价球晶形成的方式、温度和时间尺度。沿球晶生长前沿/玻璃边界断面分析的大块玻璃化学和球晶化学揭示了主要氧化物和挥发分(H2O、CO2、F、Cl和S)的化学变化和非均质性，在千分之五的货币尺度上。大量非泡孔玻璃(空间上从球晶结构中移除)中的大量挥发分数据显示，挥发分浓度均匀分布：H2O(0.089+/-A 0.012 wt%)，F(950+/-A 40ppm)和Cl(4,100+/-A330 ppm)，二氧化碳和S始终低于检测下限，表明这些挥发分要么完全脱气，要么是最初的贫挥发熔体。球晶边界和球晶结构内的挥发分浓度变化很大。这些观察结果与挥发分向熔体中的扩散排出相一致，在无水微晶生长之前留下了一个贫挥发份的边缘，这被认为对球晶结晶动力学有显著的影响。溶解在玻璃和球晶中的Ar浓度相差约20倍，Ar优先隔离在玻璃相中。岩相学观察、CSD分析、挥发分和Ar数据以及扩散模拟支持球晶从类似于790-825摄氏度的岩浆(侵位)温度开始，经过类似于750-620摄氏度的玻璃化转变温度范围(T(G))，在固态中被进一步变质。我们认为，球晶的形核和生长速率是等温恒定的，但随着岩浆温度的持续冷却，球晶生长的不同阶段会有所不同，从而存在从高到低的结晶速率和从低到高的结晶形核的演化。根据水在这些温度范围内的扩散(类似于800-300摄氏度)，球晶结晶的时间尺度类似于4天，并进一步修正至类似400年(生长非常缓慢-400摄氏度，并将变得依赖于扩散)。球晶的选择性变形支持了Rocche Rosse黑麻岩中球晶形成的低温连续体；的确，岩石学证据表明，高应变带可能在同步和侵位后冷却期间催化了下一代球晶的渐进成核和生长。
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 (H2O, CO2, F, Cl and S) chemical variations and heterogeneities at a a parts per thousand currency sign5 mu m scale. Numerous bulk volatile data in non-vesicular glass (spatially removed from spherulitic textures) reveal homogenous distributions of volatile concentrations: H2O (0.089 +/- A 0.012 wt%), F (950 +/- A 40 ppm) and Cl (4,100 +/- A 330 ppm), with CO2 and S consistently below detection limits suggesting either complete degassing of these volatiles or an originally volatile-poor melt. Volatile concentrations across the spherulite boundary 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 spherulite crystallization dynamics. Argon concentrations dissolved in the glass and spherulites differ by a factor of similar to 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 nucleation and growth starting at magmatic (emplacement) temperatures of similar to 790-825 A degrees C and progressing through the glass transition temperature range (T (g) similar to 750-620 A degrees 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 crystallization and low to high crystal nucleation. Based on the diffusion of H2O across these temperature ranges (similar to 800-300 A degrees C), timescales of spherulite crystallization occur on a timescale of similar to 4 days with further modification up to similar to 400 years (growth is prohibitively slow < 400 A degrees 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 further generations of spherulites during syn- and post-emplacement cooling.