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Komatiites reveal a hydrous Archaean deep-mantle reservoir

机译：科马提人揭示了含水的太古宙深幔库

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Archaean komatiites (ultramafic lavas) result from melting under extreme conditions of the Earth's mantle. Their chemical compositions evoke very high eruption temperatures, up to 1,600 degrees Celsius, which suggests even higher temperatures in their mantle source(1,2). This message is clouded, however, by uncertainty about the water content in komatiite magmas. One school of thought holds that komatiites were essentially dry and originated in mantle plumes(3-6) while another argues that these magmas contained several per cent water, which drastically reduced their eruption temperature and links them to subduction processes(7-9). Here we report measurements of the content of water and other volatile components, and of major and trace elements in melt inclusions in exceptionally magnesian olivine (up to 94.5 mole per cent forsterite). This information provides direct estimates of the composition and crystallization temperature of the parental melts of Archaean komatiites. We show that the parental melt for 2.7-billion-year-old komatiites from the Abitibi greenstone belt in Canada contained 30 per cent magnesium oxide and 0.6 per cent water by weight, and was depleted in highly incompatible elements. This melt began to crystallize at around 1,530 degrees Celsius at shallow depth and under reducing conditions, and it evolved via fractional crystallization of olivine, accompanied by minor crustal assimilation. As its major-and trace-element composition and low oxygen fugacities are inconsistent with a subduction setting, we propose that its high H2O/Ce ratio (over 6,000) resulted from entrainment into the komatiite source of hydrous material from the mantle transition zone(10). These results confirm a plume origin for komatiites and high Archaean mantle temperatures, and evoke a hydrous reservoir in the deep mantle early in Earth's history.
机译：在地球地幔的极端条件下融化产生了古生的科马提岩（超熔岩）。它们的化学成分引起了非常高的喷发温度，高达1600摄氏度，这表明其地幔源的温度甚至更高（1,2）。然而，由于对高锰铁矿岩浆中水含量的不确定性，这一信息变得模糊不清。一种观点认为，科玛替岩基本上是干燥的，起源于地幔柱（3-6），而另一种观点则认为，这些岩浆含有百分之几的水，从而大大降低了它们的喷发温度并将它们与俯冲过程联系起来（7-9）。在这里，我们报告了对水和其他挥发性成分的含量以及异常镁质橄榄石中熔融夹杂物中主要和微量元素的测量（高达94.5摩尔％的镁橄榄石）。该信息可直接估算古生铁矾石母体熔体的组成和结晶温度。我们显示，来自加拿大Abitibi绿岩带的27亿年前科马蒂岩的母体熔体按重量计含有30％的氧化镁和0.6％的水，并且贫化了高度不相容的元素。该熔体在浅层深度和还原条件下于约1,530摄氏度开始结晶，并通过橄榄石的分步结晶以及少量的地壳同化作用而演化出来。由于其主要元素和微量元素组成以及低氧逸度与俯冲环境不一致，我们认为其高H2O / Ce比（超过6,000）是由于从地幔过渡带夹带进入含水材料的科马铁矿源中（10） ）。这些结果证实了科马提岩和古生地幔温度高的羽状起源，并在地球历史的早期唤起了深地幔中的含水储层。

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  • 来源
    《Nature》 |2016年第7596期|628-632|共5页
  • 作者

    Sobolev Alexander V.; Asafov Evgeny V.; Gurenko Andrey A.; Arndt Nicholas T.; Batanova Valentina G.; Portnyagin Maxim V.; Garbe-Schoenberg Dieter; Krasheninnikov Stepan P.;

  • 作者单位

    Univ Grenoble Alpes, Inst Sci Terre ISTerre, CNRS, F-38041 Grenoble, France|Russian Acad Sci, Vernadsky Inst Geochem & Analyt Chem, 19 Ul Kosygina, Moscow 119991, Russia;

    Russian Acad Sci, Vernadsky Inst Geochem & Analyt Chem, 19 Ul Kosygina, Moscow 119991, Russia;

    Univ Lorraine, CRPG, UMR 7358, F-54501 Vandoeuvre Les Nancy, France;

    Univ Grenoble Alpes, Inst Sci Terre ISTerre, CNRS, F-38041 Grenoble, France;

    Univ Grenoble Alpes, Inst Sci Terre ISTerre, CNRS, F-38041 Grenoble, France|Russian Acad Sci, Vernadsky Inst Geochem & Analyt Chem, 19 Ul Kosygina, Moscow 119991, Russia;

    Russian Acad Sci, Vernadsky Inst Geochem & Analyt Chem, 19 Ul Kosygina, Moscow 119991, Russia|GEOMAR Helmholtz Ctr Ocean Res Kiel, Wischhofstr 1-3, D-24148 Kiel, Germany;

    Univ Kiel, Inst Geosci, Ludewig Meyn Str 10, D-24118 Kiel, Germany;

    Russian Acad Sci, Vernadsky Inst Geochem & Analyt Chem, 19 Ul Kosygina, Moscow 119991, Russia;

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