Experimental Investigation of Chemical Zoning in Olivine: Applications to Hawaiian Basalt

玄武岩喷发是地球上火山活动的主要类型。在基拉韦厄火山（美国夏威夷岛），玄武岩岩浆喷发在过去三个世纪中造成了社会危害，包括重大的人员损失和基础设施损坏。最近发现，基拉韦厄火山经过几十年的热情洋溢、以熔岩流为主的活动而获得了驯服的名声，但事实上，这座火山每次都经历了持续几个世纪的爆炸性和热情洋溢的活动周期。火山下岩浆库的状态、规模以及从地壳和地幔接受补给的频率是决定在任何时期是爆发性活动还是喷发性活动占主导地位的主要因素。在基拉韦厄和其他喷发玄武岩的火山中，橄榄石是最常见的矿物相。它作为揭示岩浆潜历史的替代物的潜力怎么强调都不过分：橄榄石是一个主要的信息胶囊，岩石学家可以通过检查来恢复火山爆发前岩浆过程发生的时间。该项目的重点是矿物橄榄石作为一种取证工具，以表征岩浆系统和岩浆补给的克里思下的火山，如K #299;lauea。实验室实验和橄榄石化学的详细研究，从选择20世纪世纪基拉韦厄喷发的产品将提供关键的制约因素，确定喷发周期的过渡。岩浆补给、混合和上升的历史以“化学分带”的形式保存在橄榄石中，形成晶体的元素在空间上的非随机变化。然而，阅读和解释橄榄石中的化学分区可能是复杂的，因为不同的过程可能会相互叠加。橄榄石中的初级化学分带可以在熔体中的初始生长期间获得，但是这种分带倾向于通过晶格内的元素扩散而随着时间的推移而变得平滑。这种扩散涂抹的程度越来越多地被用来回溯喷发发生前岩浆混合和上升的时间。这些现代岩石学的应用还有待于彻底的实验验证。因此，该项目使用一套精心计划的实验室实验，重点在于破译橄榄石在玄武岩熔体中诞生和生长期间获得整套元素化学分区的机制，以及这种分区的修改速度。玄武岩熔体将被冷却到不同的温度，并在最终条件下保持不同的持续时间，以诱导结晶并随时间跟踪扩散再平衡。为了利用和应用实验结果的自然系统，橄榄石从几个世纪爆发的产品从K #299;劳厄亚火山（夏威夷）将调查其化学分区模式。选择K #299;lauea作为橄榄石研究实验室的原因是多方面的。在上个世纪，火山爆发不少于50次（其中一次自1983年以来一直在持续），现有的工作表明，自20世纪50年代以来，火山一直在经历岩浆供应激增。研究这些喷发产物中橄榄石的分区模式，并提取有关喷发前岩浆注入和补给的关键时间尺度信息，对于建立有关未来行为的知情预测至关重要。

Basaltic eruptions are the dominant type of volcanism on Earth by volume. At Kilauea Volcano (Island of Hawai'i, USA), eruptions involving basalt magma have caused societal harm over the last three centuries, including significant human loss and damage to infrastructure. It was recently discovered that far from the tame reputation that Kilauea gained through decades of effusive, lava-flow dominated activity, the volcano actually undergoes cycles of explosive and effusive activity lasting several centuries each. The state of magma reservoirs underneath the volcano, their size, and the frequency at which they receive recharge from the crust and mantle, are main factors in determining whether explosive or effusive activity dominates in any period of time. At Kilauea and in other volcanoes that erupt basalt, olivine is the most commonly occurring mineral phase. Its potential as proxies for unraveling the subterraneous history of magmas cannot be overstated: olivine is a major information capsule that petrologists examine to recover the timing of magmatic processes occurring prior to an eruption. This project focuses on the mineral olivine as a forensics tool to characterize magmatic systems and the tempo of magma recharge under volcanoes like Kīlauea. Laboratory experiments and detailed studies of olivine chemistry within the products from select 20th century Kilauea eruptions will provide critical constraints for identifying transitions in eruptive cycles. The history of magma recharge, mixing and ascent is preserved in olivine in the form of 'chemical zoning', non-random spatial variations in the crystal-forming elements. However, reading and interpreting chemical zoning in olivine can be complicated because different processes may overprint each other. Primary chemical zoning in olivine can be acquired during initial growth in a melt, but this zoning tends to smooth out through time by element diffusion within the crystal lattice. The extent of this diffusion smearing is increasingly used to backtrack the timing of magma mixing and ascent before an eruption occurs. These modern petrological applications have yet to be subject to thorough experimental verification. Using a suite of carefully planned laboratory experiments, this project therefore focuses on deciphering the mechanisms by which olivine acquires chemical zoning for a whole suite of elements during its birth and growth in a basalt melt, and how fast this zoning is modified. A basalt melt will be cooled to different temperatures and left for various durations at the final conditions to induce crystallization and track diffusive re-equilibration through time. To exploit and apply the experimental results to natural systems, olivine from the products of several 20th century eruptions from Kīlauea Volcano (Hawaii) will be investigated for their chemical zoning patterns. The reasons for choosing Kīlauea as a laboratory for olivine studies are manifold. There have been no less than 50 eruptions in the last century (one that is ongoing since 1983), and existing work suggest that the volcano has been undergoing a surge in magma supply since the 1950s. Examining the zoning patterns of olivine in those eruption products and extracting critical timescale information pertaining to pre-eruptive magma injection and recharge will be essential to establish informed projections about future behavior.