Intrinsic solidification behaviour of basaltic to rhyolitic melts: A cooling rate experimental study

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• 作者：Francesco Vetere ; Gianluca Iezzi ; Harald Behrens ; Andrea Cavallo ; Valeria Misiti ; Marcel Dietrich ; Jaayke Knipping ; Guido Ventura ; Silvio Mollo
• 关键词：Cooling kinetics ; Nucleation ; Solidification ; Superheating ; Reproducibility ; Silicate melt
• 刊名：Chemical Geology
• 出版年：2013
• 出版时间：16 September, 2013
• 年：2013
• 卷：354
• 期：Complete
• 页码：233-242
• 全文大小：1425 K

文摘

Dynamic cooling-induced solidification experiments were run using six silicate glasses along the basalt-rhyolite join (B₁₀₀ = 100 wt. % of basalt, R₁₀₀ = 100 wt. % of rhyolite), i.e. B₁₀₀, B₈₀R₂₀, B₆₀R₄₀, B₄₀R₆₀, B₂₀R₈₀ and R₁₀₀; the glasses directly quenched from 1300 ¡ãC after a dwell of 120 min (experiment E0) contain 50-400 ppm H₂O, ? 1 area % ¦Ìm-sized bubble, and Fe^2 +/Fe_tot between 0.34 and 0.46. Experiments were performed in Pt capsules at room pressure and fO₂ of air, between 1300 and 800 ¡ãC using three different cooling rates of 0.0167, 3 and 30 ¡ãC/min; these cooling rates were run two times: E1-E2 experiments at 0.0167 ¡ãC/min, S1-E3 at 3 ¡ãC/min, and E4-E5 at 30 ¡ãC/min. In experiments E1 to E5, samples were annealed for 120 min at 1300 ¡ãC, whereas in the experiment S1 the samples were firstly heated for 30 min at 1400 ¡ãC followed by a dwell time of 2400 min at 1300 ¡ãC before cooling. In none of the run-products preferential crystallization at the melt/gas interface was observed. B₁₀₀, B₈₀R₂₀ and B₆₀R₄₀ run-products have a low tendency to preferentially crystallize on Pt walls, while B₄₀R₆₀, B₂₀R₈₀ and R₁₀₀ are not affected by the presence of Pt substrata. All run-products show homogeneous textures, except for B₆₀R₄₀ and B₄₀R₆₀ at 0.0167 ¡ãC/min in the E1 experiment. The duplicates of B₄₀R₆₀ and B₆₀R₄₀ at 0.0167 ¡ãC/min and B₁₀₀ at 30 ¡ãC/min show differences in crystal content (> 4 and < 14 area % ). B₄₀R₆₀ and B₆₀R₄₀ duplicated run-products that have the same amount of early-crystallized clinopyroxene and spinel, but different contents in lately-formed plagioclase. The run-products with the same starting composition from E3-S1 (3 ¡ãC/min) show a high reproducibility in terms of crystal shape, size, and amount (< 4 area % ). This demonstrates that the crystallization path is not affected by the different heat treatments above the liquidus temperature, i.e. the time scale of structural re-equilibration (relaxation) and chemical re-homogenisation are shorter than the experimental time scale. Possible chemical heterogeneities on a length scale of several micrometres for R₁₀₀ and several hundreds of micrometres for B₁₀₀ can be removed at 1300 ¡ãC within 120 min. A heat treatment at 1300 ¡ãC for 120 min significantly reduces the amount of ¦Ìm-sized bubbles, potentially responsible for the onset of nucleation. The experimental reproducibility is low when the cooling path intersects the tip of the time-temperature-transformation (TTT) curves, i.e. when the nucleation rate is near its maximum (I_max). In that case, even small thermal variations in cooling rate and local composition can have large effects on phase abundance and crystal size. Dynamic crystallization experiments can be properly interpreted and compared only if they are texturally homogeneous and the physico-chemical state of the superheated silicate liquid is known. The solidification conditions used in this study mirror those of aphyric lavas and dikes emplaced at shallower crustal levels.