A new method to quantify the real supply of mafic components to a hybrid andesite
A new method to quantify the real supply of mafic components to a hybrid andesite
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DOI：
10.1007/s00410-012-0805-x
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发表时间：
2013-01-01
影响因子：
3.5
通讯作者：
Christopher, T.
中科院分区：
文献类型：
--
作者：
Humphreys, M. C. S.;Edmonds, M.;Christopher, T.
蒙特塞拉特苏弗里斯雷山火山自1995年以来一直在喷发。火山喷发出富含晶体的角闪石-斜长石安山岩，其中含有无处不在的镁铁质包裹体，表明与镁铁质岩浆混合。这种镁铁质岩浆被认为是喷发的驱动力，为岩浆房中的安山岩提供热量和挥发分。岩浆混合过程除了产生宏观镁铁质包裹体外，还包括安山岩斑晶进入镁铁质岩浆。这些继承斑晶显示出明显的不平衡结构（如角闪石上的筛分斜长石边缘和热分解边缘）。安山岩中大约25%的斑晶显示出这些结构，表明两种岩浆类型之间存在非常广泛的质量转移。在安山岩中发现了亚毫米级的镁铁质包裹体碎片，以及镁铁质晶体簇，它们通常附着在具有不平衡特征的斑晶边缘。矿物化学还指出，最初形成于镁铁质包裹体中的微晶或微斑晶转移到安山岩中。这种结合的证据表明，一些镁铁质夹杂物在混合和/或上升过程中解体，可能是由于剪切，并提出了一个问题：安山岩的“基块”的比例实际上起源于镁铁质夹杂物，因此，什么是真正的镁铁质岩浆在岩浆系统？我们提出了一种新的方法来量化的相对比例的基质斜长石来自镁铁质和安山质岩浆，基于背散射电子图像分析的基质。初步结果表明，在所有基质斜长石中，约有16%属于镁铁质包体.连同晶体簇，不平衡斑晶结构和毫米级的包裹体，有一个“神秘的”镁铁质成分中的安山岩约6%的体积。这与宏观镁铁质夹杂物的比例（通常类似于1- 5%）相比是显著的。新方法有可能允许跟踪的镁铁质部分通过时间，从而产生进一步的见解岩浆杂交过程。
The eruption of SoufriSre Hills Volcano, Montserrat, has been ongoing since 1995. The volcano is erupting a crystal-rich hornblende-plagioclase andesite with ubiquitous mafic inclusions, indicating mixing with mafic magma. This mafic magma is thought to be the driving force of the eruption, supplying heat and volatiles to the andesite resident in the magma chamber. As well as producing macroscopic mafic inclusions, the magma mixing process involves incorporation of phenocrysts from the andesite into the mafic magma. These inherited phenocrysts show clear disequilibrium textures (e.g. sieved plagioclase rims and thermal breakdown rims on hornblende). Approximately 25 % of all phenocrysts in the andesite show these textures, indicating very extensive mass transfer between the two magma types. Fragments of mafic inclusions down to sub-mm scale are found in the andesite, together with mafic crystal clusters, which are commonly found adhered to the rims of phenocrysts with disequilibrium features. Mineral chemistry also points to the transfer of microlites or microphenocrysts, initially formed in the mafic inclusions, into the andesite. This combined evidence suggests that some of the mafic inclusions disaggregate during mingling and/or ascent, possibly due to shearing, and raises the question: What proportion of the andesite 'groundmass' actually originated in the mafic inclusions, and thus, what is the true amount of mafic magma in the magmatic system? We present a new method for quantifying the relative proportions of groundmass plagioclase derived from mafic and andesitic magma, based on analysis of back-scattered electron images of the groundmass. Preliminary results indicate that approximately 16 % of all groundmass plagioclase belongs genetically to the mafic inclusions. Together with the crystal clusters, disequilibrium phenocryst textures and mm-scale inclusions, there is a 'cryptic' mafic component in the andesite of approximately 6 % by volume. This is significant compared with the proportion of macroscopic mafic inclusions (typically similar to 1-5 %). The new method has the potential to allow tracking of the mafic fraction through time and thus to yield further insights into magma hybridisation processes.