中太平洋海山群玄武岩磷酸盐化特征及其对全岩地球化学的影响

Characteristics of phosphatization and its effects on the geochemical compositions of basalts from the Mid-Pacific Mountains

大洋玄武岩是研究地幔不均一性、岩浆起源与演化的重要对象。然而,由于其长期与周围的海水相互作用,极易发生蚀变和次生变化。磷酸盐化是大洋玄武岩最常见的次生变化之一,会影响到其全岩地球化学成分,且目前仍没有去除磷酸盐化的有效方法。因此,研究磷酸盐化特征以及评估其对玄武岩全岩地球化学成分的影响至关重要。本文以中太平洋海山群(九皋和紫檀海山)玄武岩为研究对象,通过能谱面扫描元素分布图、主量元素以及微量元素揭示玄武岩的磷酸盐化特征,评估磷酸盐化对其全岩主微量元素的影响。面扫描元素分布图显示,中太平洋海山群玄武岩的磷酸盐化作用主要发生在玄武岩气孔和裂隙周围,以交代早期的碳酸盐化基质,形成细小的磷酸盐矿物,呈浸染状分布在玄武岩基质中为特点,并且磷酸盐化会不同程度地改变玄武岩的主量元素和微量元素成分:比如磷酸盐化会使玄武岩的MgO、CaO、Na_(2)O、MnO含量降低,K_(2)O和Fe_(2)O_(3)T含量升高,同时也会对相容元素(如Cr、Co、Ni等)、大离子亲石元素(Rb、Ba、Cs等)和稀土元素造成不同程度的影响。值得注意的是,在磷酸盐化过程中,玄武岩的Al_(2)O_(3)、SiO_(2)和高场强元素(Nb、Ta、Zr、Hf和Ti)几乎不受影响。

Oceanic basalts are ideal samples in deciphering geodynamics,mantle heterogeneity,and magma origin and evolution.However,due to its long-term interaction with the surrounding seawater,it is easy to undergo alteration and secondary alteration.Phosphatization is one of the most common secondary alteration in oceanic basalts,which can affect the geochemical compositions of basalt and there is no effective method to eliminate it yet.Therefore,it is important to evaluate the effect of phosphatization on the geochemical compositions of basalt.The mapping of element distribution by energy spectrum surface scanning with energy dispersive spectrometer,and analyses of major and trace elements of the phosphatized basalts from the Mid-Pacific Mountains were conducted.The elemental mapping shows that the phosphatization occurred mainly around the vesicles and fissures of basalts.It metasomatized the early-formed carbonated matrix by which fine phosphate minerals were formed.Phosphatization would change the major elements and trace elements of basalt.For example,phosphatization could decrease the contents of MgO,CaO,Na_(2)O and MnO,and increase the contents of K_(2)O and Fe_(2)O_(3)T in the basalt.Meanwhile,it also affected the compatible elements(such as Cr,Co,Ni,etc.),large ion lithophile elements(Rb,Ba,Cs,etc.),and rare earth elements.It is noted that Al_(2)O_(3),SiO_(2),and high field strength elements(Nb,Ta,Zr,Hf and Ti)of the basalts are nearly unaffected the the phosphatization.