安徽女山新生代玄武岩中橄榄岩包体矿物的含水性研究

Water in minerals of peridotite xenoliths from Cenozoic basalts in Nushan volcano,SE China.

本文对15个来自安徽女山新生代玄武岩的橄榄岩包体矿物(橄榄石、单斜辉石和斜方辉石)进行了详细的微区傅立叶变换红外光谱(Micro-FTIR)分析。结果显示,所有被测的橄榄石颗粒都没有明显的OH吸收峰,这表明橄榄石要么不含结构水,要么结构水含量＜2ppm(H_2O wt．下同);所有的单斜辉石和斜方辉石颗粒都含有以OH形式存在的结构水。辉石矿物颗粒内部的结构水含量要么是均一的,要么表现出中心高边缘低的不均一分布。这种不均一分布的特征应该来自于包体上升过程中由于压力降低而引起的H扩散。如果用每个样品多个测定颗粒的中心部位的平均值来代表该样品的话,15个样品的单斜辉石水含量为6～356ppm,斜方辉石水含量6～139ppm。单斜辉石与斜方辉石之间水含量的比值为-2．5,和文献中报道的分配系数吻合,表明女山单斜辉石和斜方辉石之间达到了H平衡。女山的橄榄岩分成低温(＜950℃)和高温(＞1050℃)两组,所有的含角闪石的样品都在低温组。高温组的斜方辉石和单斜辉石表现出H_2O含量和Al含量的正相关、和Mg含量的负相关,表明Al^(3+)+H^+(?)Si^(4+)和Al^(3+)+H^+(?)2Mg^(2+)是辉石中H的主要结合机制,同时也表明这些样品有效地保存了其在源区的原始结构水含量。而低温组的辉石偏离了相关趋势,结合该组部分样品含有角闪石的特征,我们提出一个流体(本文中采用广义的概念,即包括熔体和流体)上升交代的模式来解释女山橄榄岩的特征:流体从下向上运移,与高温组(下部)橄榄岩先发生反应,由于水含量较低,既没有影响橄榄岩矿物的原始水含量,也没有能形成角闪石,在上升的过程中由于发生名义上无水矿物(橄榄石,辉石等巨晶)的结晶分离,流体中的水含量不断增加,积累到一定程度,即与低温组(上部)橄榄岩反应时,不仅影响了橄榄岩矿物的原始水含量。也在部分样品中形成了角闪石。

Nominally anhydrous minerals （olivine, clinopyroxene and orthopyroxene ） from 15 peridotite xenoliths hosted by Cenozoic basalts in Nushan volcano, SE China have been investigated by micro-Fourier transform infrared spectrometry （FTIR）. No distinct OH absorption bands can be found in olivines, whereas all the analyzed elinopyroxene （epx） and orthopyroxene （opx） grains contain structural water in the form .of OH. The structural water contents of pyroxenes are homogeneous within individual grains or directly zoned （higher in the core-lower in the rim）.Low water contents at grain rims possibly result from hydrogen diffusion after pressure decrease upon rock ascending. The range of the average core water content （H2Owt.） of multi grains （〉10） from the same sample for the 15 Nushan peridotites is 6-356ppm and 6-139ppm for cpx and opx respectively. The ratio of H2O between cpx and opx is -2.5, which is close to their H partition coefficient, demonstrating H equilibrium. The Nushan peridotite xenoliths can be divided into two groups： low temperature （ 〈950℃, LT） group and high temperature （ 〉 1050℃, HT） group; all samples containing metasomatized amphiboles belong to LT group. In the HT group, both cpx and opx display positive correlation between H2O and Al, and negative correlation between H2O and Mg. Such correlations indicate that Al^3＋＋H^＋←→Si^4＋ and Al^3＋ ＋ H^＋←→2Mg^2＋ are probably the main mechanisms of H incorporation in pyroxenes, and these samples should preserve the initial water content. The LT group pyroxenes derivate from such trends and are possibly due to fluid metasomatism in the mantle. We propose a ＂fluid ascending ＋ metasomatism＂ model to explain the petrological and geochemical characteristics of the Nushan peridotites. Reaction between the ascending fluids （early stage） with the HT peridotites neither affected the initial water content of the minerals nor formed amphibole due to low water content. During ascending nominally anhydrous minerals （ olivine, C pyroxene etc. ） crystallized from the fluids on the wall between fluids and peridotites and resulted in increasing water content in the fluids. Therefore, when the fluids （late stage with much more water） reacted with the LT peridotites the initial water content of the minerals were changed, and amphiboles were formed.