The Oxidation State of Hawaiian Magmas

玄武岩岩浆的氧逸度是控制熔体铁氧化还原状态（Fe2O3/FeO + Fe2O3）的基本强度变量，并且对从冷却岩浆结晶的矿物的序列和组成具有强烈的影响，因此对分馏熔体的组成具有强烈的影响。 更重要的是，玄武岩浆的氧逸度被认为反映了地幔源的氧逸度，或者至少是对源的氧逸度设定了一个上限。 夏威夷岩浆的氧逸度被广泛认为接近FMQ（铁橄榄石-磁铁矿-石英）缓冲区。 这一假设主要基于对基拉韦厄火山熔岩的分析。 很少有关于夏威夷其他火山熔岩的公开测量，那些确实存在的可能是可疑的。 熔岩很可能在喷发过程中发生了陆上或近地表氧化作用，其结果是报告的数值可能过高。 也就是说，氧逸度可以低于FMQ并且更接近MW（磁铁矿-浮氏体缓冲区）。 本研究将试图澄清这种情况。 将对仔细选择的快速淬火样品进行FeO和Fe2O3分析，以估计氧逸度。 还将对同一样品测定主要元素、微量元素和硫的丰度。 将测量硫以检验硫脱气导致岩浆还原的假设。 这些样品将包括来自莫纳罗亚火山、基拉韦厄、莫纳克亚火山和洛伊希火山的淬火熔岩、飞溅物、玻璃碎屑岩和玻璃枕状边缘。 这些数据应提供一个坚实的基础，建立夏威夷岩浆的氧逸度，并显示是否有显着差异的熔岩从不同的火山的氧逸度。 它将限制夏威夷地幔柱的氧化状态，并有助于对地幔氧化状态的激烈辩论。 在过去的25年里，人们对地幔的氧化状态进行了激烈的辩论，部分原因是它与地球大气层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。 玄武岩岩浆是地幔熔融产生的，它被认为提供了地幔源氧化状态的信息。 夏威夷火山爆发的那些被认为起源于一个热的地幔柱，可能起源于地核-地幔边界的深处。 据推测，这些地幔柱物质是原始地幔和来自沿着地球板块边缘俯冲带的再循环地壳物质的混合物。 因此，了解其氧化态具有一定的重要性。 通过精确测量玄武质熔岩中三价铁和二价铁的含量，就有可能估计岩浆的氧化状态，并由此推断地幔柱的氧化状态。 迄今为止所做的工作表明，夏威夷的岩浆相对氧化，比沿着地球扩张的洋中脊喷发的岩浆氧化得更厉害。 这些结果可能是错误的。 这是因为玄武岩浆在喷发和熔岩流中的运输过程中容易氧化。 我们的初步研究结果表明，熔岩的氧化是严重依赖于如何，何时和何地采样。 缓慢冷却的熔岩，或从喷口移动了一段距离的熔岩总是被氧化。 只有被迅速淬灭的熔岩，和/或在喷发口取样的熔岩，才能保持其原始的氧化状态，这比以前假设的要低得多。 这项研究的重点将是确定莫纳罗亚火山、基拉韦厄火山、莫纳克亚火山和洛伊希火山的快速淬火熔岩的氧化状态，以纠正这种情况。

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.