COMPOSITIONAL LAYERING AND SYN-ERUPTIVE MIXING OF A PERIODICALLY REFILLED SHALLOW MAGMA CHAMBER - THE AD-79 PLINIAN ERUPTION OF VESUVIUS
COMPOSITIONAL LAYERING AND SYN-ERUPTIVE MIXING OF A PERIODICALLY REFILLED SHALLOW MAGMA CHAMBER - THE AD-79 PLINIAN ERUPTION OF VESUVIUS
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DOI：
10.1093/petrology/36.3.739
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发表时间：
1995-06-01
影响因子：
3.9
通讯作者：
SBRANA, A
中科院分区：
文献类型：
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作者：
CIONI, R;CIVETTA, L;SBRANA, A
对公元79年维苏威火山“庞贝”普林尼式喷发的火山碎屑沉积物进行了详细的研究，（2）浅部岩浆房中不同岩浆的同喷发混合作用的定量定义及其与喷发动力学的关系;（3）对供给岩浆房的基性岩浆批次变化的认识。在喷发的不同阶段，大约25-30%的岩浆以白色K型响岩浮石喷出，70-75%的岩浆以灰色K型火山岩响岩浮石喷出。白色浮岩是从一个由两个不同的层组成的主体（T = 850-900摄氏度）中逐渐更深的岩浆流出的结果，这两个不同的层主要由晶体分离形成。灰色浮岩的结果同喷发混合涉及三个主要的端员：响岩“白色”岩浆（硅铝酸盐端员，SEM），镁铁质堆（堆端员，GEM）和结晶贫乏的“灰色”的phono-tephritic岩浆（镁铁质端员，MEM），从来没有爆发，而不是第一次与“白色”岩浆混合。提供的证据表明，混合发生在室内，其特征在于与时间的过渡，从物理混合在微观尺度上的化学杂交的MEM岩浆有一个均匀的组成和恒定的Sr-87/Sr-86同位素比值，可能是由于持续对流。没有发现明确的液相线相，表明MEM岩浆过热（T = 1000-1100摄氏度）;其非常低的粘度是建立分离白色和灰色岩浆的物理不连续性的主要原因。白灰色的边界层可能包括一个多扩散界面，周期性地打破和重建，供应phonolitic机构通过混合适量的分馏灰色熔体与上覆的白色岩浆。一个大的过热的质量的存在表明，年轻的，成长阶段的AD 79室，其主要引擎是定期到来的热镁铁质岩浆批次。其特征是钾质火山岩到钾质玄武质火山岩组成、高温（>1150 ℃）、高挥发物含量（2.0- 2.5%H2O + Cl + F + S）、低粘度[（1- 2x 10（2）泊）]和相对低的密度（2500-2600 kg/m3）。庞培室的诞生是在这些批次（平均特征为Sr-87/Sr-86约为0.70729）反复到达一个含有火山岩-响岩质、响岩质、富含晶体的岩浆的水库之后，这是前一次阿韦利诺普林尼火山喷发（3400 BP）的残留物。事实上，在Ao 79喷发期间喷出的岩浆中，大约有一半可能是从前Avellino时代继承下来的。
A detailed study of the pyroclastic deposits of the AD 79 'Pompei' Plinian eruption of Vesuvius has allowed: (1) reconstruction of the thermal, compositional and isotopic (Sr-87/Sr-86) pre-eruptive layering of the shallow magma chamber; (2) quantitative definition of the syn-eruptive mixing between the different magmas occupying the chamber, and its relationships with eruption dynamics; (3) recognition of the variability of mafic magma batches supplying the chamber During the different Phases of the eruption similar to 25-30% of the magma was ejected as white K-phonolitic pumice, and 70-75% as grey K-tephri-phonolitic pumice. The white pumice results from the tapping of progressively deeper magma from a body (T = 850-900 degrees C) consisting of two distinct layers mainly formed by crystal fractionation. The grey pumice results from syn-eruptive mixing involving three main end-members: the phonolitic 'white' magmas (salic end-member, SEM), mafic cumulates (cumulate end-member, GEM) and a crystal-poor 'grey' phono-tephritic magma (mafic end-member, MEM) which was never erupted without first being mixed with 'white' magma. Evidence is provided that mixing occurred within the chamber and was characterized by a transition with time from physical mixing at a microscopic scale to chemical hybridization The MEM magma had a homogeneous composition and constant Sr-87/Sr-86 isotopic ratio, possibly as a result of sustained convection. No unambiguous liquidus phases were found suggesting that the MEM magma was superheated (T = 1000-1100 degrees C); its very low viscosity was a main cause in the establishment of a physical discontinuity separating the white and the grey magmas. The white-grey boundary layer possibly consisted of a multiply diffusive interface, periodically broken and recreated, supplying the phonolitic body through mixing of moderate amounts of fractionated grey melts with the overlying white magma. The presence of a large overheated mass indicates the young, growing stage of the AD 79 chamber whose main engine was the periodic arrival of hot mafic magma batches. These were characterized by K-tephritic to K-basanitic compositions, high temperatures (>1150 degrees C), high volatile contents (2.0-2.5% H2O + Cl + F + S), low viscosities [(1-2x10(2) poises)] and relatively low densities (2500-2600 kg/m(3)). The birth of the Pompei chamber followed the repeated arrival of these batches (on average characterized by Sr-87/Sr-86 approximate to 0.70729) into a reservoir containing a tephritic-phonolitic, phonolitic, crystal-enriched magma, a residue from the preceding 'Avellino' Plinian eruption (3400 BP). In fact, about half of magma ejected during the Ao 79 eruption could have been inherited from pre-Avellino times.