Boron isotope geochemistry and U–Pb systematics of altered MORB from the Australian Antarctic Discordance (ODP Leg 187)
Boron isotope geochemistry and U–Pb systematics of altered MORB from the Australian Antarctic Discordance (ODP Leg 187)
复制标题
澳大利亚南极不一致 (ODP Leg 187) 改变的 MORB 的硼同位素地球化学和 U-Pb 系统学
DOI：
10.1016/j.chemgeo.2007.05.004
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发表时间：
2007
期刊：
影响因子：
3.9
通讯作者：
K. Hoernle
中科院分区：
文献类型：
--
作者：
Sylwia Królikowska;A. Deyhle;F. Hauff;K. Hoernle
对澳大利亚南极不一致区（AAD）附近的太平洋和印度地幔型洋中脊玄武岩（MORB）进行了硼、铅同位素组成及B-U-Th-Pb含量测定。蚀变样品中的硼含量范围为29.7 - 69.6 ppm，相对于B含量为0.4-0.6 ppm的新鲜MOR B玻璃而言，硼含量极其富集。同样，蚀变玄武岩的δ11B值在5.5‰ ~ 15.9‰之间，在次生粘土的形成过程中，需要与类似于海水的δ11B富集流体发生作用（δ 11B = 39.5‰）和/或硼同位素分馏。B浓度与H_2O_CO_2、K_2O、P_2O_5、U和~（87）Sr/~（86）Sr等蚀变化学指标呈正相关关系，表明B在玄武岩中随着蚀变程度的加深而逐渐富集。有趣的是，在蚀变程度最轻的玄武岩中，δ11B的变化最大，为+ 16‰，而在蚀变程度最重的玄武岩中，δ11B的变化持续下降，为+ 5-6‰。这些观测结果可能表明，随着海底年龄的增加，沉积物覆盖面增加，导致早期海水主导的流体向沉积物主导的流体转变。随着蚀变程度的增加，δ 11 B从重值向轻值的变化也可能反映了粘土矿物的形成增加（例如，皂石）。1160 B站新鲜玻璃质玄武岩和灰蚀变玄武岩中238 U/204 Pb和206 Pb/204 Pb的比较表明，低温蚀变过程中铀的次生富集引起了极端的变化。U-Pb同位素系统的建模证实，一些蚀变事件发生在2150万年的早期。这些岩石的历史，即使一个显着的第二次脉冲蚀变发生在地壳形成后的1.12亿年。同生玄武岩玻璃和低温蚀变玄武岩全岩的U-Pb系统学因此是一个潜在的工具，把年龄限制在洋壳的蚀变和流体流动的时间。
Boron and Pb isotopic compositions together with B–U–Th–Pb concentrations were determined for Pacific and Indian mantle-type mid-ocean ridge basalts (MORB) obtained from shallow drill holes near the Australian Antarctic Discordance (AAD). Boron contents in the altered samples range from 29.7 to 69.6 ppm and are extremely enriched relative to fresh MORB glass with 0.4–0.6 ppm B. Similarly the δ11B values range from 5.5‰ to 15.9‰ in the altered basalts and require interaction with a δ11B enriched fluid similar to seawater ∼ 39.5‰ and/or boron isotope fractionation during the formation of secondary clays. Positive correlations between B concentrations and other chemical indices of alteration such as H2O CO2, K2O, P2O5, U and87Sr/86Sr indicate that B is progressively enriched in the basalts as they become more altered. Interestingly, δ11B shows the largest isotopic shift to + 16‰ in the least altered basalts, followed by a continual decrease to + 5–6‰ in the most altered basalts. These observations may indicate a change from an early seawater dominated fluid towards a sediment-dominated fluid as a result of an increase in sediment cover with increasing age of the seafloor. The progression from heavy δ11B towards lighter values with increasing degrees of alteration may also reflect increased formation of clay minerals (e.g., saponite). A comparison of238U/204Pb and206Pb/204Pb in fresh glass and variably altered basalt from Site 1160B shows extreme variations that are caused by secondary U enrichment during low temperature alteration. Modeling of the U–Pb isotope system confirms that some alteration events occurred early in the 21.5 m.y. history of these rocks, even though a significant second pulse of alteration happened at ∼ 12 Ma after formation of the crust. The U–Pb systematics of co-genetic basaltic glass and variably low temperature altered basaltic whole rocks are thus a potential tool to place age constraints on the timing of alteration and fluid flow in the ocean crust.