A Metamodel for Crustal Magmatism: Phase Equilibria of Giant Ignimbrites
A Metamodel for Crustal Magmatism: Phase Equilibria of Giant Ignimbrites
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DOI：
10.1093/petrology/egq039
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发表时间：
2010-09-01
影响因子：
3.9
通讯作者：
Spera, Frank J.
中科院分区：
文献类型：
--
作者：
Fowler, Sarah J.;Spera, Frank J.
对于形成加州东部长谷主教凝灰岩、新墨西哥州班德利尔凝灰岩和美国蒙大拿州黄石凝灰岩的大规模灾难性喷发，存在着各种各样的解释。这些喷发是过去200万年来地球上最大的喷发之一。最近提出的每个系统的成岩作用的一个共同因素是多阶段处理，其中晶体糊形式的晶体分馏，然后再活化释放高硅液体。岩浆在下地壳中演化的早期阶段。我们已经测试了这些情况下进行定量相平衡计算（MELTS），并与观察到的液体（玻璃）和斑晶组合物的结果进行比较。虽然从每个熔结凝灰岩的凝灰岩样品的比较揭示了不同的斑晶成分和比例，计算结果表现出显着程度的一致性之间的系统，指向一些基本的统一行为相关的大体积的熔结凝灰岩。计算的液体组合物来自超过类似于25%的分离结晶的母体熔体在地壳深部的标志是SiO2浓度几个重量百分比太低，与观察到的组合物相比，这表明在地壳深部的结晶分馏岩浆演化的程度上的限制。在所有情况下，相平衡的结果和相关的考虑点的演变占主导地位的水饱和的镁铁质母熔体在浅深度（类似5公里）的晶体分馏。母岩熔体成分与各地区各系统熔结凝灰岩喷发前所观测到的区域原始玄武岩相一致。富水镁铁质熔体在浅层次的分馏结晶本质上导致不稳定的热力学pseudoinvariant点附近，在约800摄氏度内的熔化区间接近，但以上，固相线。对于每一个系统，岩浆演变的状态，高exsolved H2O体积分数，即使在5公里的深度，最终超过了标准的岩浆破碎类似的60体积%附近的pseudoinvariant点温度。大量的出溶和可能的驱逐流体发生在这个温度下，在岩浆中的固体部分几乎不连续地变化（等温）显着更高的值。这种不稳定机制通过在薄（几公里）的地壳盖下产生重力不稳定的低密度、水饱和的岩浆排列而成为喷发的触发因素。当结晶固体和/或出溶流体从残余熔体中等压（恒压）或等容（恒体积）分馏时，触发机制对于基于各种条件的分馏结晶方案是常见的。在一个单一的系统中，液体组成的差异，导致恒体积与恒压结晶和驱逐与保留exsolved H2O是小的相比，所产生的变化，在初始水浓度，岩石静压力，和氧逸度。正是这些后者的数量，在于在大容量的熔结凝灰岩形成喷发的共性的关键，与合理的元模型参数范围。尺度分析提供了分步结晶的热时间尺度，包括离散晶体群的年龄范围。对于Bishop凝灰岩，Bishop岩浆体的总时间尺度大于100万年。对于黄石凝灰岩，计算的热时间尺度与连续破火山口崩塌之间的类似600 kyr的发生间隔一致。虽然人们认识到，除了完全分离结晶作用以外，其他成岩过程在熔结凝灰岩成岩中也起作用，但通过强调共同特征，可以通过合理的定量分析更好地关注每个系统的独特性。
Diverse explanations exist for the large-volume catastrophic eruptions that formed the Bishop Tuff of Long Valley in eastern California, the Bandelier Tuff in New Mexico, and the tuffs of Yellowstone, Montana, USA. These eruptions are among the largest on Earth within the last 2 Myr. A common factor in recently proposed petrogenetic scenarios for each system is multistage processing, in which a crystal mush forms by crystal fractionation and is then remobilized to liberate high-silica liquids. Magma evolves in the lower crust in earlier phases. We have tested these scenarios quantitatively by performing phase equilibria calculations (MELTS) and comparing the results with observed liquid (glass) and phenocryst compositions. Although comparison of tuff samples from each ignimbrite reveals distinct phenocryst compositions and proportions, the computed results exhibit a remarkable degree of congruity among the systems, pointing to some underlying uniform behavior relevant to large-volume silicic ignimbrites. Computed liquid compositions derived from more than similar to 25% fractional crystallization of the parental melt in the deep crust are marked by SiO2 concentrations several weight per cent too low compared with the observed compositions, suggesting a limit on the extent of magma evolution by crystal fractionation in the deep crust. In all cases, the phase equilibria results and related considerations point to evolution dominated by crystal fractionation of a water-saturated mafic parental melt at shallow depths (similar to 5 km). Parental melt compositions are consistent with those of observed regional primitive basalts erupted prior to ignimbrite eruption for each system in each region. Fractional crystallization of water-rich mafic melt at shallow levels leads inherently to destabilization near thermodynamic pseudoinvariant points at around 800 degrees C within the melting interval close to, but above, the solidus. For each system, the magmas evolve to states of high exsolved H2O volume fraction even at 5 km depth, eventually exceeding the criterion for magma fragmentation of similar to 60 vol. % near the pseudoinvariant point temperature. Copious exsolution and possible expulsion of fluid occurs at this temperature, where the solid fraction in the magma changes almost discontinuously (isothermally) to significantly higher values. This instability mechanism acts as an eruption trigger by generating a gravitationally unstable arrangement of low-density, water-saturated magma beneath a thin (several kilometres) crustal lid. The trigger mechanism is common to fractional crystallization scenarios based on a variety of conditions, when crystallized solids and/or exsolved fluids are fractionated from residual melt isobarically (constant pressure) or isochorically (constant volume). In a single system, differences in liquid compositions resulting from constant volume versus constant pressure crystallization and expulsion versus retention of exsolved H2O are small compared with those arising from variations in initial water concentration, lithostatic pressure, and oxygen fugacity. It is these latter quantities that lie at the crux of the commonality in large-volume ignimbrite-forming eruptions, with a reasonable range of metamodel parameters. Scale analysis provides thermal timescales for fractional crystallization, including age ranges for discrete crystal populations. For the Bishop Tuff, the overall timescale for the Bishop magma body is > 1 Myr.For the Yellowstone Tuffs, calculated thermal timescales are consistent with ecurrence intervals of similar to 600 kyr between successive caldera collapses. Although it is recognized that petrogenetic processes other than perfect fractional crystallization play a role in ignimbrite petrogenesis, by emphasizing common features the uniqueness of each system can be brought into better focus by sound and quantitative analysis.