Hf-Nd isotope decoupling in bulk abyssal peridotites due to serpentinization
Hf-Nd isotope decoupling in bulk abyssal peridotites due to serpentinization
复制标题
DOI：
10.1016/j.chemgeo.2016.07.006
复制
发表时间：
2016-11-15
期刊：
影响因子：
3.9
通讯作者：
Mallick, Soumen
中科院分区：
文献类型：
--
作者：
Frisby, Carl;Bizimis, Michael;Mallick, Soumen
蛇纹岩化的深海橄榄岩是地球海洋与橄榄岩上地幔化学作用的产物。这种反应主要是由水和流体活动元素添加到橄榄岩中，这意味着俯冲带中水和流体活动元素的循环。稀土元素(REE)在蛇纹石化过程中通常被认为是不活动的。然而，新鲜的单斜辉石和它们的寄主块状蛇纹石化橄榄岩之间的Nd(和Sr)同位素不平衡的鉴定表明，在蛇纹石化过程中，向橄榄岩中添加了Nd和其他轻稀土(Frisby等人)。(2016))。与稀土元素一样，高场强元素(HFSE)在蛇纹石化过程中也被认为是静止和非反应的。然而，这一点尚未得到明确的检验。本文提出了一种改进的溶解和提取方法，通过在消化干燥过程中使用B(OH)(3)，用Fe沉淀REE-HFSe隔离，以及液-液交换除铁，提高了高度亏损的超镁铁岩(橄榄岩)中REE和HFSe的产率。利用这种方法，我们报道了西南印度洋海脊(SWIR)、大西洋中脊(MIR)和开门中隆起(MCR)的块状岩石蛇纹化深海橄榄岩的NdHf同位素组成，以及以前报道过其单斜辉石Hf、Nd同位素的样品。块状岩石的Hf同位素与单斜辉石相当，小于1埃(Hf)，而块状橄榄岩中的Nd同位素比其单斜辉石的放射性低8 epsilon(ND)单位。贫化的橄榄岩端元和局部富集型熔岩之间的熔岩反应不能产生观察到的Hf和ND同位素之间的解偶。相反，我们的数据与海水-橄榄岩的相互作用是一致的，因为这导致了ND同位素的显著移动，而Hf同位素几乎没有变化，直到水/岩石比达到>；100,000。我们的数据表明，Hf同位素不受蛇纹岩作用的影响，块状橄榄岩中的Hf同位素比Nd或Sr更好地记录了橄榄岩的岩浆成分。在大多数LREE亏损的样品中，单斜辉石和块状橄榄岩的Nd值相差最大，而参考样品变化不大。根据块状岩石(蛇纹岩)和单斜辉石(岩浆值)的Nd同位素组成，我们估计块状岩石的1-34%来自海水，但只有1-6%的Sm和Sm来自海水。
Serpentinized abyssal peridotites are the product of the chemical interaction between the Earth's oceans and the peridotitic upper mantle. This reaction is dominated by the addition of water and fluid mobile elements to the peridotite, with implications on the cycling of water and fluid mobile elements in subduction zones. Rare earth elements (REE) were generally thought as immobile during serpentinization. However the identification of Nd (and Sr) isotopic disequilibrium between fresh clinopyroxene and their host bulk serpentinized peridotite suggest addition of Nd and other light REE to the peridotite during serpentinization (Frisby et al. (2016)). As with the REE, the high-field strength elements (HFSE) have also been considered immobile and non-reactive during serpentinization. However, this has yet to be explicitly tested. Here we present a modified dissolution and extraction method that improves on the yields of REE and HFSE from highly depleted ultramafic rocks (peridotites) through the use of B(OH)(3) during digestion dry-down steps, Fe-precipitation for REE-HFSE sequestration, and liquid-liquid exchange for Fe removal. Using this method we report on the Nd and Hf isotope compositions of bulk rock serpentinized abyssal peridotites from the South-West Indian Ridge (SWIR), Mid-Atlantic Ridge (MIR) and Mid-Caymen Rise (MCR) and on samples where their clinopyroxene Hf, Nd isotopes were previously reported. The bulk rock Hf isotopes are equal to clinopyroxene to within 1 epsilon(Hf), while Nd isotopes in the bulk peridotite are up to 8 epsilon(Nd) units less radiogenic than their clinopyroxene. Melt-rock reaction between the depleted peridotite endmember and local enriched lavas cannot generate the observed decoupling between Hf and Nd isotopes. Our data is instead consistent with seawater - peridotite interaction as this results in significant shift in Nd isotopes with little to no change in Hf isotopes until extremely high water/rock mass ratios of >100,000. Our data shows that Hf isotopes are not affected by serpentinization and that Hf isotopes in bulk peridotites better record the magmatic composition of peridotites than Nd or Sr. The largest Nd isotopic difference between clinopyroxene and bulk peridotite is observed in the most LREE-depleted samples, while refertilized samples show little change. Based on the bulk rock (serpentinized) and clinopyroxene (magmatic value) Nd isotope compositions we estimate that 1-34% of the bulk rock Nd budget is seawater derived, but only 1-6% of Sm and