华北蓟县中元古界剖面中燧石条带的形成机制——硼硅同位素证据

Formation mechanism of chert bands in the Mesoproterozoic Jixian Section:evidence from boron isotopes and silicon isotopes

天津蓟县中新元古界海相沉积碳酸盐岩建造中分布大量燧石条带,特别是雾迷山组燧石条带与白云岩互层密集产出,沉积韵律非常明显,记录了其形成时海洋的化学和生物等信息,但关于其成因和形成机制还存在不同的认识。我们对蓟县中元古界剖面中碳酸盐岩的硼同位素和燧石条带的硅氧同位素进行了系统研究。燧石条带的δ 30 Si NBS 28 值为0. 6‰~3. 3‰,平均2. 2‰,较热水化学沉积硅质岩的δ 30 Si NBS 28 值显著偏高,与浅海生物沉积硅质岩的硅同位素组成相近。燧石条带的δ 18 O V SMOW 值为21. 7‰~27. 8‰,平均25. 5‰,较热液成因硅质岩的值明显偏高,而与常温海相硅质岩的δ 18 O值相似。蓟县剖面白云岩和灰岩等碳酸盐岩的δ 11 B SRM 951 值为0‰~11. 0‰之间,平均4. 8‰,较现代海相碳酸盐的值明显偏低。高于庄组至雾迷山组燧石条带白云岩的δ 11 B值普遍高于白云岩和灰岩的值,在3. 3‰~12. 9‰之间,平均8. 4‰。在酸性条件下富集重硼同位素的B(OH) 3掺入碳酸盐的比例增加,可导致其δ 11 B值升高。这表明燧石条带白云岩可能形成于局部酸性水环境。结合碳酸盐和SiO 2溶解/沉淀与pH之间相互关系,提出蓟县剖面中的燧石条带是一种具有时代特征的同沉积的生物化学沉积硅质岩。中元古代海水中SiO 2浓度高,基本呈饱和状态,Mg/Ca比值高,生物活动已成为影响海洋环境的重要因素。在生物活动繁盛期大量有机质沉积于海底,导致海水-沉积物界面附近pH值大幅下降(pH<7. 8),白云石等碳酸盐溶解度升高,难以沉淀;与此相反,SiO 2溶解度降低,达到过饱和,大量沉淀形成燧石条带/透镜体。生物活动羸弱期,海底pH值升高恢复到正常水平,SiO 2溶解度升高,碳酸盐溶解度降低,形成白云岩等碳酸盐沉淀。生物活动的周期性变化,则形成白云岩与燧石互层的条带状韵律层。燧石条带白云岩的硼同位素组成反映的是局部流体的δ 11 B和pH值,不适合用来反演海水的硼同位素组成。

Marine carbonate dominated strata from the Jixian section, Tianjin, spanning from the Mesoproterozoic to early Neoproterozoic, are intercalated with a large number of chert bands. Especially, the Wumishan Formation is composed of thickly bedded dolomite intercalated with chert bands and is also characterized by well developed sedimentary rhythm and biological activity. However, the mechanism of formation of the chert bands is debated. We systematically studied the boron isotopic composition of carbonate rocks and the silicon and oxygen isotopes composition of chert bands in the Mesoproterozoic Jixian section. The δ 30 Si NBS 28 value of chert bands ranges from 0. 6‰~3. 3‰(an average of 2. 2‰), which is significantly higher than that of the chemically precipitated chert from hot water environment, but close to the silicon isotope composition of the marine bio sedimentary siliceous rocks in shallow water environment. The δ 18 O V- SMOW value of the chert bands is from 21. 7 ‰~27. 8 ‰(an average value of 25. 5 ‰), obviously higher than that of hydrothermal siliceous rocks, but similar to that of normal marine siliceous rocks. The boron isotopic composition of pristine carbonatic rocks (dolomite and dolomite limestone etc.) associated component ranges from 0. 0‰~11. 0‰(an average of 4. 8‰), which is obviously lower than that of modern marine carbonate. The δ 11 B values of siliceous banded dolomite samples from Gaoyuzhuang Formation to Wumishan Formation are generally higher ranging from 3. 3‰ to 12. 9‰ with an average of 8. 4‰. Under the acid condition, decrease in local pH would result in more B(OH) 3 with higher δ 11 B incorporated into the lattice of carbonate minerals and an elevated δ 11 B C in carbonate phase. This indicates that siliceous banded dolomite may have formed in a local acidic water environment. Combined with the correlation between carbonate, SiO 2 dissolution/precipitation and pH, this study suggests that siliceous banded dolomite was a synsedimentary bio chemical sedimentary origin of chert bands in Jixian section. Mesproterozoic seawater had a high concentration of SiO 2 and was basically in a saturated state, with a high Mg/Ca ratio. Biological activities have become an important factor affecting the marine environment. During the flourishing period of biological activities, a large amount of organic matter was deposited at the bottom of the sea, resulting in rapid decreasing of pH (e. g., pH<7. 8) at the seawater sediment interface and increasing of the solubility of dolomite and other carbonate;on the contrary, decreased solubility of SiO 2 led to saturation and this gave a rise to the precipitation of silica and form chert bands/lenses. In the weak period of biological activity, the pH value increases to the normal level, with the solubility of SiO 2 increasing and the carbonate solubility decreasing, resulting in precipitation of carbonate such as dolomite and others. The periodic change of biological activity resulted in the formation of striped rhythmic layer of dolomite and chert. The boron isotopic composition of siliceous banded dolomite reflecting δ 11 B and pH of local fluids does not apply to deducing the δ 11 B composition of sea water.