Seawater alteration effects on REE, K, Rb, Cs, Sr, U, Th, Pb and Sr–Nd–Pb isotope systematics of Mid-Ocean Ridge Basalt
Seawater alteration effects on REE, K, Rb, Cs, Sr, U, Th, Pb and Sr–Nd–Pb isotope systematics of Mid-Ocean Ridge Basalt
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DOI：
10.2343/geochemj.26.159
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发表时间：
1992-06
影响因子：
0.8
通讯作者：
S. Verma
中科院分区：
文献类型：
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作者：
S. Verma
通过收集两个连续的酸浸馏分和浸出后留下的残留物，并分析 Sr、Nd 和 Pb 同位素和相关元素，对来自东北太平洋的结晶 MORB (∼12 Ma) 和玻璃状 (∼15 Ma) MORB 的“较少改变”和“较多改变”分裂进行了详细的同位素和微量元素酸浸研究。在结晶玄武岩的氧化过程中，推断的全岩同位素比显示，Sr 仅小幅增加，Nd 略有下降，但 Pb 大幅增加。相对于“新鲜”玄武岩玻璃，长方铜矿化玻璃的全岩同位素比显示，Sr 增加非常大，Nd 减少非常小，Pb 小幅增加。两种类型的样品及其分裂中的大多数微量元素及其比例也显示出与海水相互作用相关的非常显着的变化。在晶体MORB的氧化过程中，全岩Rb/Sr和U/Pb增加了~75%和~500%，而Sm/Nd、Th/Pb和Th/U分别减少了~4%、~30%和~90%。另一方面，在玄武岩玻璃的菱角矿化过程中，全岩 Rb/Sr 增加了~275%，而 Th/Pb 和 Th/U 均减少了~60%。对于第一酸浸馏分，包括稀土元素在内的所有元素均表现出高富集因子；最高值是晶体 MORB 中的 U 和 Pb。解释这些数据的一个可行机制可能是玄武岩和较早与上覆沉积物相互作用的海水之间的元素和同位素交换（在海水为主的条件下；修改后的海水/岩石比率~10-400）。
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.