Extending the amphibole sponge: The metasomatism of clinopyroxene in arcs

角闪石海绵的延伸：单斜辉石弧形的交代作用

• 批准号: NE/J004472/1
• 负责人: Daniel Smith
• 金额: $ 5.77万
• 依托单位: University of Leicester
• 依托单位国家: 英国
• 项目类别: Research Grant
• 财政年份: 2011
• 资助国家: 英国
• 起止时间: 2011 至 无数据
• 项目状态: 已结题
• 来源: https://gtr.ukri.org/projects?ref=NE%2FJ004472%2F1
• 关键词: Extending; amphibole; sponge; metasomatism; clinopyroxene

Volcanic arcs, like those that form the Pacific Ring of Fire, are markers for the collision and subduction of tectonic plates. These volcanic arcs are typically characterised by water-rich magma, which gives them a distinctive mineralogy and chemistry when compared to volcanoes produced from water-free magma. This water-rich character is what leads to the explosive nature of arc volcanoes, and arguably makes arc volcanoes the most hazardous on Earth. It is also responsible for their ore forming potential, most notably in the formation of copper and gold deposits.Researchers in arc environments have recognised common chemical signatures in the erupted rocks, and have suggested that the mineral amphibole crystallises from the original magmas and is left in the lower crust as residual crystal mush. Previous geochemical studies suggest it exerts strong chemical controls on the crystallising and evolving magmas. However, despite a suggested widespread role for amphibole in these magmas, it is not particularly common in the volcanic rocks erupted at surface. If it forms so readily from the magmas to produce a mush, why do we not see it in the crystal population of the magma once it has moved away from the mush?Experimental work on candidate "parent" magmas suggest that it is not amphibole but another mineral, clinopyroxene, that will be expected to dominate the early crystallisation and thus be the main mineral in the crystal mush. Clinopyroxene is also commonly observed in the volcanic arc rocks. There is a mismatch between observations: chemistry suggests amphibole is the important mineral, but the mineralogy of the rocks at surface suggest a greater role for clinopyroxene.What if amphibole does not form by direct crystallisation from the melt? What if it formed by reactions between the melt and already-formed clinopyroxene? In such a scenario amphibole forms at the melt-mush interface, by altering the clinopyroxene. As melts are periodically released from this melt-mush "reaction zone", the mush (now a mixture of clinopyroxene and amphibole) is left behind. The melt moves to shallower crustal levels, and outside of the pressure stability range for amphibole. Thus, a melt has formed amphibole by reaction, left it behind in the mush, and risen to levels where more amphibole cannot form. Amphibole is not abundant in the crystal content of the melts that reach the surface. The reaction process explains the observation that amphibole is not ubiquitous in volcanic arc rocks, but can it explain the observation that amphibole is a major driver of chemistry?A suite of samples from Savo volcano, Solomon Islands arc, will allow us to test this process. The surface rocks contain nodules of preserved crystal mush material (which are usually left behind in the crust). Detailed chemical analysis of clinopyroxene and amphibole from Savo will determine the chemical effect the reaction has on the evolving melt. Two hypotheses will be tested: 1) chemical signatures of amphibole crystallisation can instead be developed by clinopyroxene mush-melt reactions, therefore reconciling the chemistry with the minerals observed in the rocks; 2) clinopyroxene mush in the crust acts as a sponge, drawing water and copper out of the evolving melts, and thus acting as a buffer or barrier for their transfer from the mantle to the upper crust and surface.Rather unusually, the mush nodules at Savo contain two different amphiboles - as well as the amphibole replacing clinopyroxene in the mush nodules (as per the scenario above), high water and sodium contents of melts at Savo helped to stabilise amphibole, and so it forms by direct crystallisation. This direct crystallisation amphibole will be used as a frame of reference to critically assess the two hypotheses - are the chemical effects of the two processes and produced amphiboles identical, therefore allowing the reaction process to reconcile the conflicting observations made in arc rocks?

火山弧，就像那些形成太平洋火环的火山弧，是构造板块碰撞和俯冲的标志。这些火山弧的典型特征是富含水的岩浆，与无水岩浆产生的火山相比，这使它们具有独特的矿物学和化学性质。这种富含水的特性导致了弧火山的爆炸性，并且可以说使弧火山成为地球上最危险的火山。在弧环境中的研究人员已经在喷发岩石中识别出了共同的化学特征，并提出矿物角闪石从原始岩浆中结晶出来，并作为残余晶体泥留在下地壳中。以往的地球化学研究表明，它对岩浆的结晶和演化有很强的化学控制作用。然而，尽管角闪石在这些岩浆中具有广泛的作用，但在地表喷发的火山岩中并不特别常见。如果它如此容易地从岩浆中形成以产生糊状物，为什么我们在岩浆离开糊状物后的晶体群中看不到它呢？对候选“母”岩浆的实验工作表明，它不是角闪石，而是另一种矿物，单斜辉石，预计将主导早期结晶，从而成为晶体糊状物中的主要矿物。单斜辉石在火山弧岩中也很常见。观察结果之间存在着不匹配：化学成分表明角闪石是重要的矿物，但表面岩石的矿物学表明单斜辉石的作用更大。如果角闪石不是通过熔体直接结晶形成的呢？如果它是由熔体和已经形成的单斜辉石之间的反应形成的呢？在这种情况下，角闪石的形式在熔体糊状界面，通过改变单斜辉石。当熔体周期性地从这个熔体-糊状物“反应区”释放出来时，糊状物（现在是单斜辉石和角闪石的混合物）就被留下了。熔体移动到较浅的地壳水平，并在角闪石的压力稳定范围之外。因此，熔体通过反应形成角闪石，将其留在糊状物中，并上升到不能形成更多角闪石的水平。在到达地表的熔体中，角闪石的晶体含量并不丰富。反应过程解释了角闪石在火山弧岩石中并不普遍存在的观察结果，但它能解释角闪石是化学的主要驱动力的观察结果吗？从萨沃火山，所罗门群岛弧，一套样品，将使我们能够测试这一过程。地表岩石中含有保存下来的水晶泥物质结核（通常留在地壳中）。对萨沃的单斜辉石和角闪石进行详细的化学分析，将确定反应对熔体演化的化学影响。将检验两个假设：（1）角闪石结晶的化学特征可由单斜辉石的糊状-熔融反应来代替，从而使化学特征与岩石中观察到的矿物相一致;（2）地壳中的单斜辉石浆体起着海绵的作用，从不断演化的熔体中吸走水和铜，从而起着从地幔向上地壳和地表转移的缓冲或屏障的作用。Savo的泥状结核含有两种不同的角闪石--以及取代泥状结核中单斜辉石的角闪石（根据上述情况），Savo熔融物的高水和钠含量有助于稳定角闪石，因此它通过直接结晶形成。这种直接结晶角闪石将被用来作为一个参考框架，以严格评估两种假设-是两个过程的化学效应和产生的角闪石相同，因此允许反应过程，以调和在弧岩石中作出的相互矛盾的意见？

项目成果

Clinopyroxene precursors to amphibole sponge in arc crust.

• DOI: 10.1038/ncomms5329
• 发表时间: 2014-07-08
• 期刊: NATURE COMMUNICATIONS
• 影响因子: 16.6
• 作者: Smith, Daniel J.