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?

像形成太平洋火环的火山弧一样，也是构造板碰撞和俯冲的标记。这些火山弧通常以水丰富的岩浆为特征，与无水岩浆产生的火山相比，它们具有独特的矿物学和化学性。这种水丰富的特征是导致弧火山爆炸性的原因，可以说使弧火山成为地球上最危险的。它还负责其矿石形成的潜力，最著名的是在铜和金矿的形成下。弧环境中的研究者已经识别出喷发的岩石中的常见化学特征，并建议从原始的岩浆中矿物质的闪石结晶，并留在下层壳中，如残余晶体糊。先前的地球化学研究表明，它对结晶和不断发展的岩浆施加了强大的化学控制。然而，尽管闪石在这些岩浆中提出了广泛的作用，但它在地表爆发的火山岩中并不特别常见。如果它很容易从岩浆中产生糊状，为什么我们在岩浆的晶体种群中没有看到它，一旦它脱离了糊状？候选“父母”岩浆的实验性工作表明它不是两栖动物，而是另一种矿物质，Clinopyroxene，可以预期会在早期的结晶中占主导地位，从而成为水晶糊的矿物质。在火山弧岩中通常观察到斜氧烯。观察之间存在不匹配：化学表明闪石是重要的矿物质，但是地面岩石的矿物质学表明斜杆氧烯的作用更大。如果闪石不是通过熔体直接结晶而形成的闪石？如果它是由熔体和已经形成的临床二烯之间的反应形成的，该怎么办？在这种情况下，闪石在融化的媒介界面形成，通过改变斜氧烯。由于熔体定期从这个熔融糊状的“反应区”释放，因此留下了糊状（现在是斜盘和闪石的混合物）。熔体移至较浅的地壳水平，并且在闪石的压力稳定性范围内。因此，熔体通过反应形成了闪石，将其留在糊状中，并升至闪石无法形成的水平。闪石在到达表面的熔体的晶体含量中不丰富。反应过程解释了以下观察结果：闪石在火山弧岩中并非普遍存在，但它可以解释闪比尔是化学的主要驱动力的观察吗？表面岩石含有保留的晶体糊材料的结节（通常留在地壳中）。 SAVO的Clinopyroxene和闪石的详细化学分析将确定反应对不断发展的熔体的化学作用。将测试两个假设：1）斜焦结晶的化学特征可以通过斜氧烯糊状熔融反应来开发，因此将化学物质与在岩石中观察到的矿物质进行了调和。 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), Savo熔体的高水和钠含量有助于稳定闪石，因此它通过直接结晶而形成。这种直接的结晶性闪石将被用作参考框架，以评估两个假设 - 这两个过程的化学效应和产生的两极分学的化学作用是相同的，因此允许反应过程调和ARC岩石中的相互矛盾的观察结果？

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.