晶体吸收是蘑菇成熟的驱动力:一项实验研究

IF 3.5 2区 地球科学 Q1 GEOCHEMISTRY & GEOPHYSICS
Martin F Mangler, Madeleine C S Humphreys, Alexander A Iveson, Kari M Cooper, Michael A Clynne, Amanda Lindoo, Richard A Brooker, Fabian B Wadsworth
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引用次数: 0

摘要

岩浆储层的热状态控制着它的物理和流变特性:在接近液相的储存温度下,岩浆以熔体为主,因此具有流动性;而在较低温度下,岩浆以流变锁定的晶体网络与间隙熔体(晶体泥浆)的形式储存。在岩浆系统的整个生命周期中,温度波动会促使岩浆主导和熔体主导两种状态之间发生转换。例如,岩浆下沉或补给到晶体泥浆中提供热量,导致泥浆解离,并通过晶体再吸收增加熔融分量,然后通过晶体堆积和再结晶冷却恢复晶体泥浆。在此,我们研究了这种温度驱动的蘑菇云再加工循环对晶体货物的纹理影响。我们进行了高P-T重吸收实验,在流纹岩熔体中对斜长石晶体进行了成核、生长、重吸收和再结晶,施加了中间弧火山(20-40°C)管道系统的典型温度波动。实验再现了常见的重吸收纹理,并表明斜长石的溶解不可逆转地降低了三维晶体的长宽比,从而形成了更均衡的形状。将我们的实验结果与来自圣海伦火山(美国)的再吸附和未再吸附斜长石晶体的形态进行比较,发现天然岩石中存在一种一致的趋势:未再吸附的斜长石晶体(在圣海伦火山的英安岩、玄武岩中发现)与再吸附的斜长石晶体的形态一致。圣海伦火山闪长岩、玄武岩和淬火岩浆包裹体中发现的)具有片状形状,而具有一个或多个再吸附层的斜长石晶体(圣海伦火山闪长岩、淬火岩浆包裹体和蘑菇状包裹体中发现的)则显示出更多的等轴形状。显示出普遍重吸收现象的斜长石晶体(在斜长石和蘑菇状包裹体中发现)的长宽比更低。因此,我们认为,晶体蘑菇的成熟会导致晶体形状逐渐趋于等长:岩浆储层中的斜长石晶体每被重新移动和吸收一次,其形状就会变小。这对岩浆流变学以及最终的喷发性都有影响,因为晶体形状控制着晶体团的最大堆积分数和渗透性。我们假设,与由未再吸附的片状晶体组成的未成熟岩浆相比,经过多次再吸附-再结晶过程而形成的具有更多等位晶体的成熟岩浆更容易再吸附。这意味着,在地壳岩浆系统的热成熟过程中,火山行为和爆发前岩浆时间尺度可能会发生系统性变化,在热成熟系统中,由于岩浆储层的快速整体再移动而导致大规模喷发的可能性更大。
本文章由计算机程序翻译,如有差异,请以英文原文为准。
Crystal resorption as a driver for mush maturation: an experimental investigation
The thermal state of a magma reservoir controls its physical and rheological properties: at storage temperatures close to the liquidus, magmas are dominated by melt and therefore mobile, while at lower temperatures, magmas are stored as a rheologically locked crystal network with interstitial melt (crystal mush). Throughout the lifetime of a magmatic system, temperature fluctuations drive transitions between mush-dominated and melt-dominated conditions. For example, magma underplating or recharge into a crystal mush supplies heat, leading to mush disaggregation and an increase in melt fraction via crystal resorption, before subsequent cooling reinstates a crystal mush via crystal accumulation and recrystallisation. Here, we examine the textural effects of such temperature-driven mush reprocessing cycles on the crystal cargo. We conducted high-P-T resorption experiments during which we nucleated, grew, resorbed, and recrystallised plagioclase crystals in a rhyolitic melt, imposing temperature fluctuations typical for plumbing systems in intermediate arc volcanoes (20-40°C). The experiments reproduce common resorption textures and show that plagioclase dissolution irreversibly reduces 3D crystal aspect ratios, leading to more equant shapes. Comparison of our experimental results with morphologies of resorbed and unresorbed plagioclase crystals from Mount St. Helens (USA) reveals a consistent trend in natural rocks: unresorbed plagioclase crystals (found in Mount St. Helens dacite, basalt and quenched magmatic inclusions) have tabular shapes, while plagioclase crystals with one or more resorption horizons (found in Mount St. Helens dacite, quenched magmatic inclusions, and mush inclusions) show more equant shapes. Plagioclase crystals showing pervasive resorption (found in the dacite and mush inclusions) have even lower aspect ratios. We therefore suggest that crystal mush maturation results in progressively more equant crystal shapes: the shapes of plagioclase crystals in a magma reservoir will become less tabular every time they are remobilised and resorbed. This has implications for magma rheology and, ultimately, eruptibility, as crystal shape controls the maximum packing fraction and permeability of a crystal mush. We hypothesise that a mature mush with more equant crystals due to multiple resorption-recrystallisation events will be more readily remobilised than an immature mush comprising unresorbed, tabular crystals. This implies that volcanic behaviour and pre-eruptive magmatic timescales may vary systematically during thermal maturation of a crustal magmatic system, with large eruptions due to rapid wholesale remobilisation of mushy reservoirs being more likely in thermally mature systems.
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来源期刊
Journal of Petrology
Journal of Petrology 地学-地球化学与地球物理
CiteScore
6.90
自引率
12.80%
发文量
117
审稿时长
12 months
期刊介绍: The Journal of Petrology provides an international forum for the publication of high quality research in the broad field of igneous and metamorphic petrology and petrogenesis. Papers published cover a vast range of topics in areas such as major element, trace element and isotope geochemistry and geochronology applied to petrogenesis; experimental petrology; processes of magma generation, differentiation and emplacement; quantitative studies of rock-forming minerals and their paragenesis; regional studies of igneous and meta morphic rocks which contribute to the solution of fundamental petrological problems; theoretical modelling of petrogenetic processes.
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