Sector-zoned augite megacrysts in Aleutian high alumina basalts: implications for the conditions of basalt crystallization and the generation of calc-alkaline series magmas
Sector-zoned augite megacrysts in Aleutian high alumina basalts: implications for the conditions of basalt crystallization and the generation of calc-alkaline series magmas
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DOI：
10.1007/s004100050512
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发表时间：
1999-05
影响因子：
3.5
通讯作者：
J. Brophy;Carla S. Whittington;Young-rok Park
中科院分区：
文献类型：
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作者：
J. Brophy;Carla S. Whittington;Young-rok Park
来自冷湾和卡纳加岛阿留申火山中心的几块高铝玄武岩含有大块（直径达1.5厘米）的扇形长辉岩巨晶。巨晶均为自面体，具有发育良好的{001}、{010}和{111}形态。所有的晶体都显示出富含矿物和玻璃包裹体的同心带。部门分区通常发生在以不同速度增长的发达（2010）、（100）、（111）和（110）部门。同步生长带宽度的比较表明，相对生长速率为（111）比（100）比（110）比（010）。组成上，sio2和MgO丰度降低，TiO2、Al2O3、FeO和Na2O丰度增加的顺序依次为（111）、（100）~（110）、（010）。这个顺序与相对增长率的顺序相同，这意味着增长率显然在部门分区的发展中起着作用。计算出的玄武岩喷发前的水含量在1 - 3 wt%之间，而喷发后的实际（测量）水含量在0.01 - 0.3 wt%之间。氘的同位素值在−110 ~−141‰之间严重枯竭。这些都表明在喷发前有大量水蒸气（H2O）析出。原始玻璃包裹体中最大H2O丰度被认为是熔体包裹时宿主液体储层的最具代表性，从一个奥辉岩巨晶的核心到边缘有系统地降低。认为扇形奥辉岩的存在是由于岩浆减压和挥发性（H2O）析出引起的奥辉岩过饱和和快速结晶所致。喷发前计算的H2O含量为1-3 wt%，限制了蒸汽析出和玄武岩结晶的深度小于3 km，更有可能为1.5 km。在非常浅的深度非常迅速的结晶，使得从最初结晶到最终喷发之间的时间尺度不太可能足以允许相当数量的部分结晶。考虑到高铝玄武岩分馏作用是产生更多演化的安山岩、英安岩和流纹岩岩浆的主要过程，母体高铝玄武岩不能在低压环境中产生此类衍生岩浆，因此分馏作用可能发生在高压环境中，即地壳底部或附近。
Several high alumina basalts from the Aleutian volcanic centers of Cold Bay and Kanaga Island contain large (up to 1.5 cm diameter) megacrysts of sector-zoned augite. The megacrysts are invariably euhedral with well developed {001}, {010} and {111} forms. All crystals display concentric bands that are rich in mineral and glass inclusions. The sector zonation typically occurs as well developed (010), (100), (111) and (110) sectors which grew at different rates. A comparison of the width of synchronous growth bands indicates that following relative growth rates: (111) ≫ (100) ∼ (110) > (010). Compositionally, SiO2and MgO abundances decrease, and TiO2, Al2O3, FeO and Na2O abundances increase in the different sectors in the order (111), (100) ∼ (110), (010). This order is identical to that deduced for the relative growth rates, implying that growth rate clearly had a role in the development of the sector zonation. Calculated pre-eruption H2O contents of the basalts range from 1 to 3 wt% but actual (measured) post-eruption H2O contents range from 0.01 to 0.3 wt%. Deteurium isotopic values are heavily depleted and range from −110 to −141‰ . Together these indicate significant vapor (H2O) exsolution prior to eruption. Maximum H2O abundances in primitive glass inclusions, thought to be most representative of the host liquid reservoir at the time of melt entrapment, systematically decrease from the core to the rim of one augite megacryst studied in detail. We conclude that the presence of sector-zoned augite is due to augite supersaturation and rapid crystallization brought about by magma decompression and volatile (H2O) exsolution. The calculated pre-eruption H2O contents of 1–3 wt% limit vapor exsolution and basalt crystallization to depths of less than 3 and more likely 1.5 km. Very rapid crystallization at very shallow depths makes it unlikely that the time scales between initial crystallization and final eruption are sufficient to permit appreciable amounts of fractional crystallization. Given that high alumina basalt fractionation is the dominant process for generating more evolved andesite, dacite and rhyolite magmas of the calc-alkaline suite, the inability of parental high alumina basalt to yield such derivative magmas in the low pressure environment places the likely site of fractionation in the high pressure environment, at or near the base of the crust.