埃达克岩的Na亏损及其对地幔Na交代的指示意义

埃达克岩是玄武质洋壳部分熔融的产物。然而,与实验室玄武岩部分熔融产生的埃达克质熔体相比,天然埃达克岩明显地高Mg、Cr和Ni,这表明埃达克岩浆在上升过程中有地幔成分的加入。本文的观察结果表明,全球新生代埃达克岩的Na2O含量低于5.8%,大约95%的新生代埃达克岩样品Na2O含量小于5.0%。然而,在埃达克岩产生的压力范围(1.5～3.0GPa),实验的玄武岩部分熔体大多数Na2O含量超过5.0%,最高达到9.0%,显示埃达克岩具有明显的Na亏损现象。我们认为这是埃达克熔体在热的地幔楔中与地幔橄榄岩反应的结果。在俯冲带,大洋板片熔融产生的熔体(埃达克熔体)上升并与地幔橄榄岩发生反应,原始的埃达克熔体获得MgO、Cr及Ni等地幔组分,但其Na2O和SiO2等通过反应进入地幔,导致地幔交代作用。根据长英质熔体与橄榄岩反应体系的相关系,我们认为,地幔单斜辉石、橄榄石、尖晶石的混染作用以及钠质角闪石和斜方辉石的分离结晶作用,是改变埃达克熔体组成并导致其Na亏损的一个重要的过程。埃达克岩的Na亏损为地幔Na交代作用和一些富Na的弧岩浆成因提供了重要证据。

NA DEPLETION IN MODERN ADAKITES AND IMPLICATION FOR MELT-MANTLE METASOMATISM

Adakite, the melt derived from basaltic oceanic crust erupted on the surface, usually exhibits anomalously high MgO, Cr, and Ni concentrations that indicate contamination of mantle components. Here we have observed that Cenozoic adakites have Na2O contents below 5.8% with about 95% samples lower than 5.0%. However, compared with experimental basalt partial melts (mostly beyond 5.0% and up to 9.0% Na2O) produced under 1.5~3.0GPa conditions that are most relevant to adakite production, adakites are depleted in Na2O contents. We ascribe this Na depletion in adakites to be a consequence of the rock/melt reaction in the hot mantle wedge. In the subduction zones, primary adakite melts during magmatic ascent and reaction with mantle peridotite will gain mantle components such as MgO, CaO, Cr and Ni but will lose Na2O, SiO2 and perhaps K2O to the mantle, leading to Na-rich mantle metasomatism. According to the phase relationship in the reaction system felsic melt+small amount of peridotite, we suggest that assimilation of the mantle clinopyroxene+olivine+spinel as well as the fractional crystallization of sodic amphibole and clinopyroxene is an important process which modifies the composition of adakites and cause the Na depletion in adakites.