Abstract

The 2·1 ka Moinui lava flow field, erupted from the southwest rift zone of Mauna Loa, Hawai`i, exhibits striking textural and geochemical variations, that can be used to interpret magma processes pre-, syn- and post-eruption. From this lava flow, the duration of magma storage and storage conditions, the timescales over which magma is transported to the surface, and flow emplacement mechanisms at Mauna Loa are determined. Electron microprobe analysis (EMPA) and diffusion chronometry of olivine crystals identify two distinct crystal populations: a primitive, polyhedral olivine population with core compositions of Fo_90–88 and a more evolved, platy olivine population with core compositions of Fo_83–82. Fe–Mg diffusion modelling of these olivine populations gives distinct timescales for each population; platy olivines yield timescales of days up to a few weeks, while polyhedral olivines yield timescales of months to years. Despite the nature of a well-insulated pāhoehoe flow, meaning that post-emplacement diffusion continues for some time, a wealth of time information can be retrieved concerning pre-eruptive magmatic processes as well as the processes associated with the lava extrusion. The short timescales obtained from the platy olivine crystals and the observed equilibrium between its cores and ambient melt suggest late-stage nucleation and crystal growth in the shallow conduit and during lava emplacement. Conversely, the longer timescales and olivine-melt disequilibrium of the polyhedral olivine crystals suggests accumulation from a deeper source and subsequent transportation to shallow magma storage beneath the summit of Mauna Loa months, or even years before eruption. The chemical and textural details of the Moinui lava reflect the mode of flow emplacement and may have implications for the interpretation of the distribution of spinifex and cumulate olivine within komatiites; high-temperature, low-viscosity lavas, common in the Archean.

中文翻译：

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从双峰橄榄石种群推断，莫纳伊火山熔岩喷发在夏威夷莫纳洛亚的岩浆过程扩散时标

摘要

从夏威夷莫纳洛亚西南裂谷喷发的2·1 ka Moinui熔岩流场具有明显的构造和地球化学变化，可用于解释喷发前，同发后和后喷浆的岩浆过程。从这种熔岩流，岩浆储存的持续时间和储存条件，岩浆被输送到地表的时间尺度以及莫纳罗亚火山的流转机制来确定。橄榄石晶体的电子微探针分析（EMPA）和扩散年表确定了两个不同的晶体种群：原始的多面体橄榄石种群，其核心成分为Fo _90–88；以及一个更演化的板状橄榄石种群，其核心成分为Fo _83-82。这些橄榄石种群的Fe–Mg扩散模型为每个种群提供了不同的时间尺度。板状橄榄石的时间尺度长达数周，而多面体橄榄石的时间尺度长达数月至数年。尽管有充分绝缘的phoehoe流动的性质，这意味着安装后扩散会持续一段时间，但仍可以检索到大量有关喷发前岩浆过程以及与熔岩挤压有关的过程的信息。从板状橄榄石晶体获得的短时间尺度以及观察到的其核与周围熔体之间的平衡表明，在较浅的导管中和熔岩沉积期间，后期成核和晶体生长。反过来，多面体橄榄石晶体的时间尺度较长且橄榄石融化不平衡，表明从较深的源头开始积累，随后运往莫纳罗阿火山顶峰下方甚至是喷发前数年的浅层岩浆储存。Moinui熔岩的化学和质地细节反映了流动进位的方式，可能对解释高锰铁矿中的尖晶石和累积橄榄石的分布有影响；太古代中常见的高温低粘度熔岩。Moinui熔岩的化学和质地细节反映了流动进位的方式，可能对解释高锰铁矿中的尖晶石和累积橄榄石的分布有影响；太古代中常见的高温低粘度熔岩。Moinui熔岩的化学和质地细节反映了流动进位的方式，可能对解释高锰铁矿中的尖晶石和累积橄榄石的分布有影响；太古代中常见的高温低粘度熔岩。