The Oxidation State of Hawaiian Magmas

玄武岩岩浆的氧逸度是控制熔体中铁的氧化还原状态(Fe_2O_3/FeO_2，Fe_2O_3)的一个基本的强烈变量，它对冷却岩浆结晶的矿物的顺序和组成有很大的影响，因此对分馏熔体的组成也有很大的影响。更重要的是，玄武岩岩浆的氧逸度被认为反映了地幔来源的氧逸度，或者至少为来源的氧逸度设定了上限。夏威夷岩浆的氧逸度被广泛认为接近铁闪石-磁铁矿-石英(FMQ)缓冲区。这一假设主要基于对基拉韦厄火山熔岩的分析。关于夏威夷其他火山熔岩的公开测量很少，那些确实存在的可能是可疑的。熔岩很可能在喷发期间经历了地下或近地表的氧化，结果是报告的值可能太高了。也就是说，氧逸度可能低于FMQ，而更接近Mw(磁铁矿-维氏体缓冲层)。本研究将试图澄清这一情况。为了估算氧逸度，将对经过精心挑选的快速淬火样品进行FeO和Fe2O的分析。常量元素、微量元素和硫丰度也将在相同的样品中进行测定。硫将被测量，以检验硫脱气导致岩浆还原的假设。样品将包括来自莫纳罗亚、基拉韦厄、莫纳克亚和洛伊希火山的淬火熔岩、飞溅、透明碎屑和玻璃枕边。这些数据应该为确定夏威夷岩浆的氧逸度提供了坚实的基础，并表明不同火山的熔岩的氧逸度是否存在显着差异。它将限制夏威夷羽流的氧化状态，并有助于关于地幔氧化状态的激烈辩论。在过去的25年里，关于地幔的氧化状态一直存在激烈的争论，部分原因是它与地球大气通过火山脱气的起源有关，因此最终与生命的发展和进化有关。玄武岩岩浆是由地幔熔融产生的，被认为提供了有关其地幔来源的氧化状态的信息。那些在夏威夷火山上喷发的火山被认为起源于一股炽热的地幔热柱，它可能起源于深至核幔边界的地方。据推测，这种羽状物质是原始地幔和来自地球板块边缘俯冲带的再循环地壳物质的混合物。因此，了解其氧化状态具有一定的重要性。通过精确地测量玄武岩熔岩中的铁和亚铁的含量，就有可能估计岩浆的氧化状态，并由此得出关于地幔热柱的氧化状态的推断。到目前为止所做的工作表明，夏威夷的岩浆相对被氧化，比沿着地球不断扩张的大洋中脊喷发的岩浆更容易氧化。这些结果可能是错误的。这是因为玄武岩岩浆在喷发和熔岩流动过程中容易被氧化。我们的初步结果表明，熔岩的氧化严重依赖于取样的方式、时间和地点。缓慢冷却的熔岩或从喷口流过一段距离的熔岩总是被氧化。只有快速冷却的熔岩和/或在喷发口取样的熔岩才能保持其原始的氧化状态，这比之前的假设要低得多。这项研究将重点确定莫纳罗亚、基拉韦厄、莫纳克亚和洛伊希火山快速熄灭的熔岩的氧化状态，以试图纠正这种情况。

The oxygen fugacity of basaltic magma is a fundamental intensive variable that controls the iron redox state (Fe2O3/FeO + Fe2O3) of the melt, and has a strong influence on the sequence and composition of minerals that crystallize from a cooling magma, and therefore on the composition of a fractionated melt. More importantly, the oxygen fugacity of basaltic magma is thought to reflect the oxygen fugacity of the mantle source or, at the very least, to place an upper limit on that of the source. The oxygen fugacity of Hawaiian magmas is widely accepted as being close to the FMQ (fayalite-magnetite-quartz) buffer. This assumption is based largely on analyses of lavas from Kilauea volcano. There are very few published measurements on lavas from other Hawaiian volcanoes, and those that do exist may be suspect. The lavas may well have undergone subaerial or near-surface oxidation during eruption, with the consequence that the reported values may be too high. That is, the oxygen fugacities may be lower than FMQ and closer to MW (magnetite-wustite buffer). This research will attempt to clarify this situation. Carefully selected, rapidly quenched samples will be analyzed for FeO and Fe2O3 in order to estimate the oxygen fugacity. Major elements, trace elements and sulfur abundances will also be determined on the same samples. Sulfur will be measured to test the hypothesis that sulfur degassing results in the reduction of the magma. The samples will include quenched lavas, spatter, hyaloclastites, and glassy pillow margins from Mauna Loa, Kilauea, Mauna Kea and Loihi volcanoes. This data should provide a firm base for establishing the oxygen fugacity of Hawaiian magmas and show whether or not there are significant differences in oxygen fugacity of lavas from different volcanoes. It will place limits on the oxidation state of the Hawaiian plume, and contribute to the vigorous debate on the oxidation state of the mantle. Over the last twenty five years there has been a vigorous debate on the oxidation state of the earth's mantle, partly because it has relevance to the origin of the earth's atmosphere through volcanic degassing, and therefore ultimately to the development and evolution of life. Basaltic magmas, which are produced by melting of the earth's mantle are thought to provide information on the oxidation state of their mantle source. Those erupted on Hawaiian volcanoes are thought to have their origins in a hot, mantle plume that may have originated as deep as the core-mantle boundary. It is speculated that this plume material is a mixture of primitive mantle and re-cycled crustal material from subduction zones along the earth's plate margins. An understanding of its oxidation state is therefore of some importance. By precisely measuring the amounts of ferric and ferrous iron in basaltic lava it is possible to estimate the oxidation state of the magma, and from this arrive at inferences concerning the oxidation state of the mantle plume. The work done to date, suggests that Hawaiian magmas are relatively oxidized, more so than magmas erupted along the Earth's spreading mid-ocean ridges. These results may be in error. This is because basaltic magmas are prone to oxidation during eruption and transportation in lava flows. Our preliminary results indicate that the oxidation of a lava is critically dependent on how, when and where it is sampled. Lavas that have cooled slowly, or have traveled some distance from the vents are invariably oxidized. Only lavas that have been quenched rapidly, and/or have been sampled at the eruptive vent, retain their original oxidation state, which is much lower than has previously been supposed. This research will focus on determining the oxidation state of rapidly quenched lavas from Mauna Loa, Kilauea, Mauna Kea and Loihi volcanoes in an attempt to rectify this situation.