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

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, Na₂O and MnO, and increase the contents of K₂O and Fe₂O₃^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 Al₂O₃, SiO₂, and high field strength elements (Nb, Ta, Zr, Hf and Ti) of the basalts are nearly unaffected the the phosphatization.