Magmatic processes revealed by textural and compositional trends in Merapi dome lavas
Magmatic processes revealed by textural and compositional trends in Merapi dome lavas
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DOI：
10.1016/s0377-0273(00)00136-0
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发表时间：
2000-07-01
影响因子：
2.9
通讯作者：
Voight, B
中科院分区：
文献类型：
--
作者：
Hammer, JE;Cashman, KV;Voight, B
同喷发脱气的火山可能会导致同喷发结晶的基质阶段，我们已经调查了这个过程中使用的结构和成分分析的圆顶材料默拉皮火山，中爪哇，印度尼西亚。样品包括1986-88年、1992-93年、1994年和1995年喷发期的穹丘熔岩，以及1994年11月穹丘坍塌沉积的火山碎屑物质。总堆积通常超过70%（斑晶+微晶），默拉皮安山岩中的液体在喷发时高度演化（流纹岩）。穹隆岩石中的长石微晶由斜长石核（Ab（63）An（29）Or（8））和碱性长石边（Ab（53）An（5）Or（42））组成。不平衡的成分对。脱气引起的三元长石系统的相关系的变化最好地解释了所观察到的长石组合物的转变。1994年坍塌的一小部分高度多孔的火山灰颗粒的玻璃成分比典型的圆顶物质少，并且含有无边缘的斜长石微晶，这表明1994年的坍塌事件除了完全固化的圆顶岩石外，还包含了脱气较少的部分液态岩浆。脱气速率和由此产生的微晶结晶可由岩浆上升速率控制。1995年圆顶样品中微晶结晶度几乎相同，表明在整个挤出期内实现了类似的微晶生长条件（PH 2 O和温度），然而，微晶数密度在这些样品中变化超过四倍，并且通常随着与排气口的距离而增加。低排气口向微晶数密度和更大的微晶浓度下降流可能反映了逐步降低的过冷度在晶体成核的时间在挤出的1995年圆顶。圆顶挤压事件之间的比较表明熔岩渗出率和微晶数密度之间的相关性，表明1995年期间挤压减缓，1992-93年和1994年圆顶的晶体纹理和成分与1995年圆顶所显示的范围相同，表明结晶条件的转变（即，由渗出率确定的过冷率）是循环的。(C)2000 Elsevier Science B. V.保留所有权利。
Syn-eruptive degassing of volcanoes may lead to syn-eruptive crystallization of groundmass phases, We have investigated this process using textural and compositional analysis of dome material from Merapi volcano, Central Java, Indonesia. Samples included dome lavas from the 1986-88, 1992-93, 1994 and 1995 effusive periods as well as pyroclastic material deposited by the November 1994 dome collapse. With total crystallinities commonly in excess of 70% (phenocrysts + microlites), the liquids present in Merapi andesites are highly evolved (rhyolitic) at the time of eruption. Feldspar microlites in dome rocks consist of plagioclase cores (Ab(63)An(29)Or(8)) surrounded by alkali feldspar rims (Ab(53)An(5)Or(42)). compositional pairs which are not in equilibrium. A change in the phase relations of the ternary feldspar system caused by degassing best explains the observed transition in feldspar composition. A small proportion of highly vesicular airfall tephra grains from the 1994 collapse have less evolved glass compositions than typical dome material and contain rimless plagioclase microlites, suggesting that the 1994 collapse event incorporated less-degassed, partially liquid magma in addition to fully solidified dome rock.As decompression drives volatile exsolution, rates of degassing and resultant microlite crystallization may be governed by magma ascent rate. Microlite crystallinity is nearly identical among the 1995 dome samples, an indication that similar microlite growth conditions (PH2O and temperature) were achieved throughout this extrusive period, However, microlite number density varied by more than a factor of four in these samples, and generally increased with distance from the vent. Low vent-ward microlite number densities and greater microlite concentrations down-flow probably reflect progressively decreasing rates of undercooling at the time of crystal nucleation during extrusion of the 1995 dome. Comparison between dome extrusion episodes indicates a correlation between lava effusion rate and microlite number density, suggesting that extrusion slowed during 1995, Crystal textures and compositions in the 1992-93 and 1994 domes share the range exhibited by the 1995 dome, suggesting that transitions in crystallization conditions (i.e., rates of undercooling determined by effusion rate) are cyclic. (C) 2000 Elsevier Science B.V. All rights reserved.