太古宙科马提岩并非来自^(18)O亏损的地幔源区

No^(18)O-depleted mantle source for Archean komatiite

氧是地幔中丰度最高的元素,探究地球早期原始地幔的氧同位素组成对了解硅酸盐地球的起始特征以及后期分异过程极为重要.太古宙早期的科马提岩多源自地球深部很少受到后期对流扰动的地幔源区,因此是揭示地球早期地幔属性的绝佳对象.前人基于激光氟化法对科马提岩中橄榄石的氧同位素分析提出地球早期深部地幔存在一个异常亏损^(18)O的源区.本文通过详细的显微结构和微区氧同位素分析揭示所谓异常亏损^(18)O的特征更可能是激光氟化分析过程中混入次生矿物蛇纹石和磁铁矿的结果.岩浆成因橄榄石具有与现今地幔橄榄石一致的氧同位素组成.这表明原始地幔并不存在初始的氧同位素不均一性且该特征自冥古宙以来未发生明显变化.因此,地幔中出现富集或亏损^(18)O的组分可归因于表壳物质再循环,这对于识别板块构造启动时间以及幔源岩浆成因具有重要意义.

Oxygen is the most abundant element in the mantle.Characterizing the oxygen isotopic compositions of the early Earth’s mantle is of importance as it is the starting point of the differentiation of the bulk silicate Earth[1-3].However,the original oxygen isotopic signature of the early Earth’s mantle is poorly constrained,because it is rarely preserved and sampled at the Earth’s surface[4].Olivine from Archean komatiite can provide insight into the early mantle with three key advantages:Archean komatiite is the highest-MgO volcanic rock-type on Earth and was derived from a deep mantle source that experienced minimal disturbance by subsequent convection of the upper mantle[5,6];olivine is the earliest crystallized phase in ultramafic magmas so that the effect from assimilation of supracrustal materials during magmatic evolution is potentially avoided[7,8];and isotopic fractionation between mantle and komatiite olivine is minimal and thereforeδ^(18)O values obtained from such olivine would closely reflect the mantle source[1,3].