摘要
为约束华北克拉通岩石圈的流变机制,使用傅立叶变换显微红外光谱仪（FTIR）测量了阳原玄武岩中尖晶石橄榄岩捕虏体的矿物含水量。FTIR光谱结果表明阳原橄榄岩捕虏体的橄榄石、斜方辉石和单斜辉石都有特征的OH吸收峰。橄榄石含有微量的水（4×10^-6-9×10^-6H2O）,斜方辉石含水量为（105-201）×10^-6,单斜辉石含水量为（260-440）×10^-6,计算得出的全岩含水量为（49-75）×10^-6。值得注意的是,富流体交代以及后期蚀变作用使得含角闪石的方辉橄榄岩样品中单斜辉石的含水量显著增加。阳原橄榄岩中单斜辉石与斜方辉石的含水量存在明显的正相关关系,水在单斜辉石和斜方辉石中的分配系数D^cpx/opxOH=2.4±0.9,与全球橄榄岩样品的平均值基本一致（D^cpx/opxOH=2.2±0.1）。与汉诺坝的橄榄岩捕虏体相OHOH比,阳原捕虏体中橄榄石和辉石的结构水含量都明显较高,这可能与阳原橄榄岩经历的上地幔交代作用有关。统计表明全球克拉通玄武岩携带的尖晶石橄榄岩捕虏体中橄榄石的含水量普遍较低（0-10×10^-6）,而斜方辉石和单斜辉石的含水量则存在明显的不均一性。金伯利岩携带的石榴石橄榄岩中橄榄石、斜方辉石和单斜辉石的含水量都明显高于玄武岩携带的尖晶石橄榄岩中相应矿物的含水量,这可能与金伯利岩来源深、富流体、上升快的性质相关。
Water in the upper mantle plays a critical role in the strength and partial melting of lithosphere. Investigation on water contents of peridotite xenoliths from the North China Craton （NCC） gives important constraints on mechanism for its lithospherie thinning since the Mesozoic. Spinel peridotite xenoliths were collected from Cenozoic basalt in Yangyuan, Hebei province, to study the water contents of the lithospheric mantle in the northern Trans-North China Orogen of NCC. Using microscopic Fourier transform infrared spectrometry （FTIR）, we measured the water contents of major minerals in five samples. All olivine, orthopyroxene and clinopyroxene grains exhibit characteristic hydroxyl absorption peaks in FTIR spectra. Olivine has relatively low amount of water （4 × 10^-6-9×10^-6）, while orthopyroxene and Glinopyroxene have （105-201）×10^-6 and （260-440）×10^-6, respectively. Based on the modal composition, the water contents of bulk samples are estimated to be （49-75） ×10^-6. It is worthy to note that metasomatism and late alteration significantly increase the water contents of elinopyroxene in our amphibole-bearing harzburgite sample. Water contents in clinopyroxene and orthopyroxene from Yangyuan peridotite xenoliths display positive correlation with the water partition coefficient D^cpx/opxOH=2.4±0.9 which is consistent with the average value （D^cpx/opxOH=2.2±0.1） for the global peridotite xenoliths. Olivine, orthopyroxene and clinopyroxene in Yangyuan peridotite xenoliths contain much more hydroxyl than those from Hannuoba, which may be caused by fluid-rich metasomatism in/he upper mantle beneath Yangyuan. Statistical analysis of available water measurements on natural peridotites shows that olivine from basalt-borne spinel peridotites is relatively dry with water contents of 0-10 × 10^-6. In contrast, variable water contents in orthopyroxene and clinopyroxene from basalt-borne spinel peridotites reflect heterogeneous water distribution in the upper mantle. Olivine, orthopyroxene and clinopyroxene from kimbcrlite-borne garnet peridotites contain much more water than those from basalt-borne spinel peridotites. This is probably because kimberlite is characterized by greater depth of origin, more fluids and faster ascending rate than basalt.
出处
《高校地质学报》
CAS
CSCD
北大核心
2009年第2期263-272,共10页
Geological Journal of China Universities
基金
国家自然科学基金委重点项目(40634021)
面上项目(40872144)共同资助
