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首页> 外文期刊>Journal of Volcanology and Geothermal Research >Do decades of tropical rainfall affect the chemical compositions of basaltic lava flows in Mount Cameroon?
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Do decades of tropical rainfall affect the chemical compositions of basaltic lava flows in Mount Cameroon?

机译：几十年的热带降雨会影响喀麦隆山玄武岩熔岩流的化学成分吗？

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摘要

To evaluate the effects of tropical rainfall on the compositions of basaltic lavas, we studied 20th century lava flows from Mount Cameroon in Africa. Weathering conditions are extreme because the climate is particularly warm and humid, and vegetation grows extremely quickly on the flows. The high rainfalls and dense vegetation contribute to rapid and intense degradation of the volcanic rocks and should cause significant changes in chemical composition. Such effects need to be quantified to constrain how young a lava flow must be so that its trace element and isotopic composition remains representative of the original magma. Fresh inner parts and altered flowtops of four different lava flows were sampled and analysed for major and trace elements as well as O, U, Sr, Nd and Pb isotopic compositions. Four samples of the 1999 eruption were also analysed to constrain the composition of fresh basalts. Almost all major and trace elements display similar concentrations in inner and outer parts of the same flow. This is notably the case for elements such as K, Rb and Sr, which are highly mobile during weathering. The lack of variation suggests that the overall composition of the lava flows has not been significantly affected. However, some systematic chemical changes are observed: Loss-on-ignition (LOI) and δ~(18)O increase slightly from inner parts of flows to near surface samples; Na and, to a lesser extent, U display significant losses in the outer samples. We interpret the Na loss in terms of hydration leading to exchange between Na~+ and H~+ ions. This process, associated with oxidation of Fe~(2+) to Fe(3+), accounts for the larger loss-on-ignition in the outer parts of flows. A change in U contents is only observed in the 1922 flow, which is covered by dense vegetation. This emphasizes the role that complexation by organic ligands plays in U mobility. While U is not completely immobile, all volcanic rocks are in secular equilibrium ((~(234)U/~(238)U) ≈ 1), indicating limited interaction between meteoric waters and basalts. Sr and Nd isotopic compositions remain constant and although ~(206)Pb/~(204)Pb ratios vary from inner to outer parts of the lavas, the changes are not systematic and cannot be attributed to weathering. The absence of significant chemical mobility and substantial isotopic exchange suggests very limited interaction between water and lavas at the centimeter scale, due, most probably, to their relatively young age. This work shows that over a period of about 100 years, no detectable geochemical changes are observed. These results are very promising for petrological and geochemical studies of ocean island basalts located in tropical areas since they provide direct evidence of very limited trace element mobility at the century time scale.
机译：为了评估热带降雨对玄武岩熔岩成分的影响，我们研究了来自非洲喀麦隆山的20世纪熔岩流。风化条件是极端的，因为气候特别温暖潮湿，植被在水流中生长非常快。高降雨和茂密的植被促使火山岩迅速而强烈地退化，并应引起化学成分的重大变化。需要对这些影响进行量化，以限制熔岩流必须保持多年轻，以便其痕量元素和同位素组成仍能代表原始岩浆。采样并分析了四种不同熔岩流的新鲜内部部分和变化的流顶，并分析了主要和痕量元素以及O，U，Sr，Nd和Pb同位素组成。还对1999年喷发的四个样品进行了分析，以限制新鲜玄武岩的组成。几乎所有主要元素和痕量元素在同一流动的内部和外部都显示出相似的浓度。对于诸如K，Rb和Sr之类的元素，在风化过程中具有很高的移动性，尤其是这种情况。缺乏变化表明熔岩流的整体组成没有受到明显影响。但是，观察到一些系统的化学变化：燃烧损失（LOI）和δ〜（18）O从流体内部到近地表样品略有增加； Na和（在较小程度上）U在外部样品中显示出明显的损失。我们根据水合导致Na〜+和H〜+离子交换的Na损失来解释。该过程与Fe〜（2+）氧化为Fe（3+）有关，说明了流道外部较大的点火损失。 U含量的变化仅在1922年的流中观测到，该流被茂密的植被覆盖。这强调了有机配体的络合在U迁移中的作用。尽管U并非完全不动，但所有火山岩都处于长期平衡状态（（〜（234）U /〜（238）U）≈1），表明陨石水与玄武岩之间的相互作用有限。 Sr和Nd同位素组成保持恒定，尽管〜（206）Pb /〜（204）Pb的比值在熔岩的内部到外部有所不同，但变化不是系统性的，不能归因于风化。没有明显的化学迁移率和大量的同位素交换表明，水和熔岩在厘米尺度上的相互作用非常有限，这很可能是由于它们相对年轻。这项工作表明，在大约100年的时间里，没有发现可检测到的地球化学变化。这些结果对于位于热带地区的海洋岛屿玄武岩的岩石学和地球化学研究非常有前途，因为它们直接提供了痕量元素在一个世纪时间尺度上非常有限的证据。

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