贵州织金寒武纪磷块岩中磷灰石和白云石稀土元素的LA-ICP-MS分析:对沉积环境和成岩过程的指示意义

贵州织金富稀土磷矿是典型的早寒武世肖滩阶海相沉积磷块岩矿床,主要由碳氟磷灰石和白云石组成,其中丰富的小壳动物化石、普遍发育的生物碎屑结构和潮汐层理等特征综合反映了充氧、动荡的潮汐环境,为探讨成磷环境下磷灰石和白云石在沉积成岩过程中稀土元素的地球化学行为提供了理想的制约。通过激光剥蚀-电感耦合等离子体-质谱(LA-ICP-MS)对织金磷块岩钻孔样品中胶状磷灰石、磷酸盐化小壳动物化石和成岩期白云石的稀土及相关主微量元素进行原位分析,根据主微量元素特征分析研究对象中的污染来源,进而提出在成磷环境下使用白云石和磷酸盐矿物稀土元素反映同期海水化学特征的数据筛选指标,最后通过筛选样品的稀土元素特征值讨论当时海水化学特征。结果表明:三种研究对象均具有显著的Ce的负异常(δCe为0.29~0.40)和La正异常(δLa为1.85~2.00)及较高的Y/Ho比值(47.25~56.91),表现出与现代海洋环境相似的氧化特征。磷酸盐矿物表现为一定程度的MREE富集和HREE“右倾”模式,可能与当时浮游生物繁盛并在深部水体中的降解有关,代表了深部富磷洋流的特征;成岩期白云石稀土模式与现代浅海海水相似但HREE为“左倾”模式,代表了同期浅海海水的特征,但白云石与磷酸盐矿物的HREE耦合特征反映了胶磷矿沉积成矿过程中对浅海海水HREE的选择性富集;肖滩阶的浅海海水与深部洋流都具有现代海洋的氧化特征,可能是促使当时小壳动物繁盛的重要因素。

LA-ICP-MS REE Analyses for Phosphates and Dolomites in Cambrian Phosphorite in Zhijin,Guizhou Province:Implication for Depositional Conditions and Diagenetic Processes

Zhijin phosphorite deposit enriched in rare earth elements of Guizhou Province is a typical early Cambrian marine sedimentary phosphate deposit formed in Xiaotan Stage,which is mainly composed of francolite and dolomite.Abundant small shelly fossils,the widely developed bioclastic structures and the tidal bedding characteristics reflect the phosphogenesis occurred in the shallow water and oxygenated environment,which provides a baseline to explore the depositional and diagenetic impact on rare earth elements mobility in francolites and dolomites.In this paper,laser ablation inductively coupled plasma-mass spectrometry(LA-ICP-MS)was used to analyze concentrations of rare earth elements plus yttrium,as well as other trace elements in francolite,phosphatized small shelly fossils and dolomites from drilled cores in Zhijin,Guizhou Province.In order to obtain primary geochemical characteristic to infer the marine chemistry in which carbonate and phosphate were deposited,we propose some index of trace element contents and/or ratios in dolomites and phosphate minerals.The results showed that dolomites,francolites,and small shelly fossils share similar REE PAAS-normalized patterns characterized by positive La anomalies(δLa from 1.85 to 2.00),negative Ce anomalies(δCe from 0.29 to 0.40),and super-chondrite Y/Ho ratios(Y/Ho ratios from 47.25 to 56.91),a typical modern oxic sea water features.The small shelly fossil recorded the fluid chemistry during phosphogenic process.However,the REE patterns of francolites and small shelly fossils after normalized by PAAS showed obvious MREE enrichment and HREE fractionation(average LuN/HoN 0.41)patterns,which were ascribed to aerobic degradation of settling plankton,and indicated features of the deep oceanic currents enriched by phosphorus.The diagenetic dolostone samples have the same REE patterns with those of the modern oxic shallow seawater,which we ascribed to record the features of the shallow seawater.The HREE fractionation(average LuN/HoN 2.19)patterns for dolomite samples coupling with phosphate minerals(average LuN/HoN 0.42)indicated that HREE in shallow sea water were selectively uptake by francolite during precipitation.The similar modern oxic characteristic between the shallow water and the deep water probably fuel the small shelly animals during the Xiaotan Stage of the early Cambrian.