Hydrothermal modification of the Sikhote-Alin iron meteorite under low pH geothermal environments. A plausibly prebiotic route to activated phosphorus on the early Earth
Hydrothermal modification of the Sikhote-Alin iron meteorite under low pH geothermal environments. A plausibly prebiotic route to activated phosphorus on the early Earth
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DOI：
10.1016/j.gca.2012.12.043
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发表时间：
2013-05-15
影响因子：
5
通讯作者：
Kee, Terence P.
中科院分区：
文献类型：
--
作者：
Bryant, David E.;Greenfield, David;Kee, Terence P.
Sikhote-Alin(SA)陨石是IIAB型八面体陨石铁陨石的一个例子，其磷(P)含量约为0.5wt%，主要以嗜铁矿物施雷伯利石(Fe，Ni)(3)的形式存在。陨石坠落到地球早期将大大增加这种亲铁磷的储量。随后在适当电解液存在下的厌氧腐蚀将产生不同于内源地球化学中通常发现的形式的磷，然后可以释放到环境中。一种特别令人感兴趣的环境包括在烟雾石或火山加热的地热水中发现的低pH条件，在这些条件下，铁金属的阳极氧化为亚铁(Fe2+)和铁(Fe3+)将与合适的电子受体的阴极还原相结合。在没有有氧氧气的情况下(E-o=+1.229 V)，质子将提供有效的最终电子受体，被转化为二氢气体(E-o=0 V)。在这里，我们探索了锡霍特-阿林陨石切片样品的水热变质作用，其中暴露了亲铁的P相。我们报告了这两种情况，(I)使用低pH蒸馏水模拟火山条件和(Ii)冰岛Vatna jokull冰川北部Kverkfjoll火山区地热加热的冰下流体。X射线光电子能谱(XPS)和扫描开尔文探针(SKP)电化学测量相结合的结果表明，硅铝榴石夹杂物的阳极氧化敏感性明显低于其周围的Fe-Ni基质，比基质材料高约550 mV。通过无限聚焦显微镜的拓扑图和拉曼光谱的化学图证实，这导致了基体-夹杂物边界处的优先腐蚀。从化学的角度来看，这些观察的意义在于，电化学性质的贵重包裹体，如施里伯利石，可能已经通过对周围基质的破坏性腐蚀而释放到地质环境中，从而提供了以H-亚磷酸盐形式存在的可溶于水的、化学反应的有效磷的局部来源[由P-31核磁共振波谱确定的H-H_2O_3-，PI(III)]。这种化合物已被证明具有相当大的生前化学潜力，可以作为缩合对氧酸的来源。在这里，我们证明了在温和的(85℃)条件下，由亚冰期地热流体对Sikhote-Alin进行水热改性而产生的PI(III)可以很容易地脱水成缩合的焦亚磷酸对氧酸[H_2P_2O_52-，PPI(III)]。通过与焦磷酸盐[H_2P_2O_72-，PPI(V)]的化学性质的比较，讨论了后者的缩合P-化合物对益生化学的潜在意义。(C)2013爱思唯尔有限公司。保留所有权利。
The Sikhote-Alin (SA) meteorite is an example of a type IIAB octahedrite iron meteorite with ca. 0.5 wt% phosphorus (P) content principally in the form of the siderophilic mineral schreibersite (Fe,Ni)(3)P. Meteoritic in-fall to the early Earth would have added significantly to the inventory of such siderophilic P. Subsequent anaerobic corrosion in the presence of a suitable electrolyte would produce P in a form different to that normally found within endogenous geochemistry which could then be released into the environment. One environment of specific interest includes the low pH conditions found in fumaroles or volcanically heated geothermal waters in which anodic oxidation of Fe metal to ferrous (Fe2+) and ferric (Fe3+) would be coupled with cathodic reduction of a suitable electron acceptor. In the absence of aerobic dioxygen (E-o = +1.229 V), the proton would provide an effective final electron acceptor, being converted to dihydrogen gas (E-o = 0 V). Here we explore the hydrothermal modification of sectioned samples of the Sikhote-Alin meteorite in which siderophilic P-phases are exposed. We report on both, (i) simulated volcanic conditions using low pH distilled water and (ii) geothermally heated sub-glacial fluids from the northern Kverkfjoll volcanic region of the Icelandic Vatnajokull glacier. A combination of X-ray photoelectron spectroscopy (XPS) and electrochemical measurements using the scanning Kelvin probe (SKP) method reveals that schreibersite inclusions are significantly less susceptible to anodic oxidation than their surrounding Fe-Ni matrix, being some 550 mV nobler than matrix material. This results in preferential corrosion of the matrix at the matrix-inclusion boundary as confirmed using topological mapping via infinite focus microscopy and chemical mapping through Raman spectroscopy. The significance of these observations from a chemical perspective is that electrochemically noble inclusions such as schreibersite are likely to have been released into the geological environment through an undermining corrosion of the surrounding matrix, thus affording localised sources of available water-soluble, chemically reactive P in the form of H-phosphite [H2PO3-, Pi(III) as determined by P-31 NMR spectroscopy]. This compound has been shown to have considerable prebiotic chemical potential as a source of condensed P-oxyacids. Here we demonstrate that Pi(III) resulting from the hydrothermal modification of Sikhote- Alin by sub-glacial geothermal fluids can be readily dehydrated into the condensed P-oxyacid pyrophosphite [H2P2O52-, PPi(III)] by dry-heating under mild (85 degrees C) conditions. The potential significance of this latter condensed P-compound for prebiotic chemistry is discussed in the light of its modified chemical properties compared to pyrophosphate [H2P2O72-, PPi(V)]. (C) 2013 Elsevier Ltd. All rights reserved.