中新元古代海相碳酸盐岩与海水的硼同位素组成特征及演化：以蓟县剖面为例

The B isotope composition of seawater throughout the Earth’s history is key issue for boron isotope in geochemical research. The boron isotopic composition of marine carbonate can indicate the B isotope composition of B(OH)4-of the sea water , provides a powerful proxy for seawater pH reconstruct and further reveals the partial pressure of atmospheric carbon dioxide, which can be indirectly acquired according to the dissociation equilibrium between the seawater and CO2 .The method of δ11BCC-pHSW has been successfully applied in studying evolution history of seawater pH and the partial pressure of atmospheric carbon dioxide during different periods in the past 20 million years. From Archean to proterozoic, boron isotopic compositions of the seawater have great changes to the modern ocean, however, previous work are rarely involved and poorly understood, severely restrict the development of boron isotope geochemistry. This study we explore well preserved strata of the marine carbonate-dominated Jixian section in northern China, which are believed to span a large portion of the Mesoproterozoic to early Neoproterozoic with relatively continuous deposition, few major interruption, and well preserved sedimentary features. Based on analyses of δ18O, δ13C and Al,Fe,Mn,B element analyses to avoid the B isotope composition of the marine carbonate has potential alteration by the clay contamination or diagenesis; using a variety of methods and different models to limit the real change of the seawater pH; revealing character and evolution of the boron isotope composition of paleo-ocean water; exploring the reasons for the secular variations during 1700Ma to 800Ma; providing the basic data and building the foundation in order to establish marine boron isotope evolution curve in the future.

海洋硼同位素组成特征及演化是硼同位素地球化学研究的基础。海相碳酸盐灵敏记录了海洋硼同位素组成及海洋pH值、大气p(CO2)分压的变化。利用δ11BCC-pHSW技术对20Ma以来海水pH演化历史及大气CO2浓度的研究取得了很大进展。太古代-元古代时，海水硼同位素组成较现代海洋发生了巨大变化，然而前人工作对此鲜有涉及，知之甚少，严重制约了硼同位素地球化学的发展。本研究拟选取时间跨度大、沉积连续、环境稳定的天津蓟县中新元古代剖面中典型海相碳酸盐岩为研究对象，对剖面中不同层位的碳酸盐岩进行硼同位素分析，并结合氧、碳稳定同位素及Al、Fe、Mn、B等元素分析来判断古老海相碳酸盐岩硼同位素变化的可靠性，运用多种方法和不同模型限定该时期海水的pH值变化范围，揭示1700Ma~800Ma中新元古代海水的硼同位素组成特征、演化规律及影响因素，为建立海洋硼同位素演化曲线提供数据，奠定基础。

海洋硼同位素组成特征及演化是硼同位素地球化学研究的基础。海相碳酸盐灵敏记录了海洋硼同位素组成及海洋pH值的变化。本研究选取时间跨度大、沉积连续、环境稳定的天津蓟县中新元古代剖面中典型海相碳酸盐岩为研究对象，对剖面中不同层位的碳酸盐岩分相（分离吸附相、碳酸盐相）进行硼同位素分析，并结合碳、氧同位素及Al、Mn、B等元素分析来判断古老海相碳酸盐岩硼同位素变化的可靠性。排除Al含量 > 2%、δ18O < -10‰、Mn/Sr > 10的可疑样品后，得到中新元古代1630Ma~800Ma碳酸盐相硼同位素组成变化范围是0.0‰～12.9‰，其中高于庄组至雾迷山组9个硅质条带白云岩样品的碳酸盐相的硼同位素组成普遍偏高，变化于3.3‰～12.9‰之间，平均9.2‰，其余样品变化范围为0.0‰～7.1‰，平均3.7‰。硅质白云岩的成因推测可能是由于部分层位沉积时生物活动加强，大量菌藻类繁殖、死亡，有机质氧化分解时，放出有机酸、CO2等造成局部水环境pH降低，导致SiO2在有机质较为丰富的层位集中沉淀，而在有机质不够丰富、pH偏高的环境下SiO2难以沉淀，沉积物以碳酸盐为主，造成硅质和碳酸盐沉积交替成层形成条带。这种局部酸性水环境导致的pH突然变低可能导致富集重同位素的B(OH)3掺入比例提高，造成碳酸盐相的硼同位素组成普遍偏高。这种很可能受到后期成岩作用，反映流体的δ11B和海水pH综合影响的样品不适合用来反演海水的硼同位素组成。利用δ11BCC-pHSW技术限定该时期海水的pH值变化范围，由于该阶段发育特征性的硅质条带白云岩，SiO2在中酸性条件下沉积，因而认为海水的pH值在7.3～8.3 的范围内变化比较合理，海水的硼同位素组成(δ11BSW)为25‰~26‰。将本研究得到的数据与前人测得的不同类型的碳酸盐的δ11B值一并考虑，得到中元古代到现代海洋碳酸盐岩的硼同位素组成演化曲线，δ11B值由古至今呈稳步上升趋势。