Seawater alteration effects on REE, K, Rb, Cs, Sr, U, Th, Pb and Sr–Nd–Pb isotope systematics of Mid-Ocean Ridge Basalt

抄録

A detailed isotopic and trace element acid-leaching study of “less-altered” and “more-altered” splits of a crystalline MORB (∼12 Ma) and a glassy (∼15 Ma) MORB from NE Pacific has been carried out by collecting two successive acid-leached fractions and residues left after leaching, and analyzing for Sr, Nd and Pb isotopes and related elements. During oxidation of the crystalline basalt, the inferred whole-rock isotope ratios show only a small increase for Sr, a very small insignificant decrease for Nd, but very large increase for Pb. Relative to “fresh” basaltic glass, the whole-rock isotope ratios for palagonitized glass show a very large increase for Sr, a very small decrease for Nd, and small increase for Pb. Most trace elements and their ratios in both types of samples and their splits also show very significant changes related to seawater interaction. During oxidation of the crystalline MORB, the whole-rock Rb/Sr and U/Pb increase by ∼75% and ∼500%, whereas Sm/Nd, Th/Pb and Th/U decrease by ∼4%, ∼30% and ∼90% respectively. On the other hand during palagonitization of the basaltic glass, the whole-rock Rb/Sr increases by ∼275%, whereas Th/Pb and Th/U both decrease by ∼60%. For the first acid-leached fractions, all elements including the REE show high enrichment factors; the highest values are for U and Pb in the crystalline MORB. A viable mechanism to explain these data may be an elemental and isotopic exchange (under seawater-dominated conditions; modified seawater/rock ratios ∼10–400) between basalt and seawater that had interacted earlier with overlying sediments.