Oliveira Beñat, Griffin William L, Gain Sarah E M, Saunders Martin, Shaw Jeremy, Toledo Vered, Afonso Juan Carlos, O'Reilly Suzanne Y
ARC Centre of Excellence for Core to Crust Fluid Systems (CCFS) and GEMOC, Earth and Environmental Sciences, Macquarie University, Sydney, NSW, 2109, Australia.
Centre for Microscopy, Characterisation and Analysis, The University of Western Australia, Crawley, WA, 6009, Australia.
Sci Rep. 2021 Jan 28;11(1):2439. doi: 10.1038/s41598-020-79739-4.
Aggregates of corundum crystals with skeletal to hopper morphology occur in pyroclastic rocks erupted from Cretaceous basaltic volcanoes on Mt Carmel, N. Israel. The rapid growth of the crystals trapped volumes of the parental AlO-supersaturated melt; phenocrysts of tistarite (TiO) in the trapped melts indicate crystallization at oxygen fugacities 6-7 log units below the Iron-Wüstite buffer (fO = ΔIW - 6 to - 7), induced by fluxes of mantle-derived CH-H fluids. Cathodoluminescence images reveal growth zoning within the individual crystals of the aggregates, related to the substitution of Ti in the corundum structure. Ti contents are < 0.3 wt% initially, then increase first linearly, then exponentially, toward adjacent melt pockets to reach values > 2 wt%. Numerical modelling indicates that the first skeletal crystals grew in an open system, from a moving magma. The subsequent linear increase in Ti reflects growth in a partially closed system, with decreasing porosity; the exponential increase in Ti close to melt pockets reflects closed-system growth, leading to dramatic increases in incompatible-element concentrations in the residual melts. We suggest that the corundum aggregates grew in melt/fluid conduits; diffusion modelling implies timescales of days to years before crystallization was terminated by explosive eruption. These processes probably operate in explosive volcanic systems in several tectonic settings.
具有骸晶到漏斗状形态的刚玉晶体聚集体出现在以色列北部卡梅尔山白垩纪玄武质火山喷发形成的火山碎屑岩中。晶体的快速生长捕获了母源AlO过饱和熔体的体积；捕获熔体中的榍石（TiO）斑晶表明，在地幔衍生的CH - H流体通量的作用下，结晶发生在比铁 - 维氏体缓冲剂低6 - 7个对数单位的氧逸度条件下（fO = ΔIW - 6至 - 7）。阴极发光图像揭示了聚集体中单个晶体内部的生长分带，这与刚玉结构中Ti的替代有关。Ti含量最初<0.3 wt%，然后先线性增加，然后呈指数增加，朝着相邻的熔体囊袋方向，达到>2 wt%的值。数值模拟表明，最初的骸晶在一个开放系统中，由移动的岩浆生长而成。随后Ti的线性增加反映了在一个孔隙度逐渐降低的部分封闭系统中的生长；靠近熔体囊袋处Ti的指数增加反映了封闭系统中的生长，导致残余熔体中不相容元素浓度急剧增加。我们认为刚玉聚集体在熔体/流体管道中生长；扩散模拟表明，在结晶因爆发性喷发而终止之前，时间尺度为几天到几年。这些过程可能在几种构造环境中的爆发性火山系统中起作用。
