CAJ | 학술논문

2.33～2.06Ga期间发生了全球性大气圈充氧作用及其相关的占δ~(13)C_(carb)正异常,被称为Lomagundi事件.2.2～2.174Ga的辽河群大石桥菱镁矿及其围岩显示了δ~(13)C_(carb)正异常,是运用REY(REE+Y)指纹技术研究Lomagundi事件的良好对象.本文研究表明,6件白云岩围岩样品∑REE为0.988×10-6～2.744×10-6;Y/Ho比值为37.9～49.4(平均42.5±4.7);(La/La~*)_(SN)为1.075±0.317,(Gd/Gd~*)_(SN)为1.390±0.166,均为正异常;HREE富集(Nd_(SN)/Yb(_SN)=0.38～0.78).所有上述特征与现代海水REY配分模式一致,表明这些样品记录了Lomagundi时期海水的REY特征.6件菱镁矿样品∑REE为4.549±2.239,高于围岩白云岩;HREE弱亏损(Nd_(SN)/Yb_(SN)=1.141±0.265),Y/Ho平均值为40.2±3.2;(La/La~*)_(SN)为0.657～1.149(平均0.919±0.203),(Gd/Gd~*)_(SN)=1.036±0.081,正异常程度弱于白云岩,但仍显示以海水来源为主.矿体顶板滑石白云岩∑REE含量最高(10.758);页岩标准化稀土配分模式为平坦型,(Eu/Eu~*)_(SN)正异常高达1.97,Gd和Y正异常最小,Y/Ho比值最低(31.3),(Nd/Yb)_(SN)为0.89,显示受后期热流体交代影响.菱镁矿(Sm/Yb)_(CN)值(2.61±0.45)高于白云岩(1.19±0.27),指示海水由深变浅,大石桥菱镁矿及其围岩白云岩REY主要来自陆源溶解物,洋底热液贡献微弱,制约REY特征的主导因素为大气圈-水圈的性质,如fO_2,pCO-2等.白云岩和菱镁矿(Ce/Ce~*)_(SN)值平均分别为1.11±0.13和1.04±0.08,表明2.2～2.174Ga期间大陆风化作用加剧,海水呈碱性,pH值＞8.2.2.33Ga前的化学沉积物(Eu/Eu~*)_(SN)＞1.53,2.06Ga后的化学沉积物(Eu/Eu~*)_(SN)＜1.53,大石桥组白云岩和菱镁矿(Eu/Eu~*)_(SN)均值分别为1.44±0.11和1.58±0.20.表明2.2～2.174Ga时海相沉积物(Eu/Eu~*)_(SN)≈1.53,2.33～2.06Ga是地球水圈-气圈系统由还原向氧化转变的关键时期.
2.33～2.06Ga기간발생료전구성대기권충양작용급기상관적점δ~(13)C_(carb)정이상,피칭위Lomagundi사건.2.2～2.174Ga적료하군대석교릉미광급기위암현시료δ~(13)C_(carb)정이상,시운용REY(REE+Y)지문기술연구Lomagundi사건적량호대상.본문연구표명,6건백운암위암양품∑REE위0.988×10-6～2.744×10-6;Y/Ho비치위37.9～49.4(평균42.5±4.7);(La/La~*)_(SN)위1.075±0.317,(Gd/Gd~*)_(SN)위1.390±0.166,균위정이상;HREE부집(Nd_(SN)/Yb(_SN)=0.38～0.78).소유상술특정여현대해수REY배분모식일치,표명저사양품기록료Lomagundi시기해수적REY특정.6건릉미광양품∑REE위4.549±2.239,고우위암백운암;HREE약우손(Nd_(SN)/Yb_(SN)=1.141±0.265),Y/Ho평균치위40.2±3.2;(La/La~*)_(SN)위0.657～1.149(평균0.919±0.203),(Gd/Gd~*)_(SN)=1.036±0.081,정이상정도약우백운암,단잉현시이해수래원위주.광체정판활석백운암∑REE함량최고(10.758);혈암표준화희토배분모식위평탄형,(Eu/Eu~*)_(SN)정이상고체1.97,Gd화Y정이상최소,Y/Ho비치최저(31.3),(Nd/Yb)_(SN)위0.89,현시수후기열류체교대영향.릉미광(Sm/Yb)_(CN)치(2.61±0.45)고우백운암(1.19±0.27),지시해수유심변천,대석교릉미광급기위암백운암REY주요래자륙원용해물,양저열액공헌미약,제약REY특정적주도인소위대기권-수권적성질,여fO_2,pCO-2등.백운암화릉미광(Ce/Ce~*)_(SN)치평균분별위1.11±0.13화1.04±0.08,표명2.2～2.174Ga기간대륙풍화작용가극,해수정감성,pH치＞8.2.2.33Ga전적화학침적물(Eu/Eu~*)_(SN)＞1.53,2.06Ga후적화학침적물(Eu/Eu~*)_(SN)＜1.53,대석교조백운암화릉미광(Eu/Eu~*)_(SN)균치분별위1.44±0.11화1.58±0.20.표명2.2～2.174Ga시해상침적물(Eu/Eu~*)_(SN)≈1.53,2.33～2.06Ga시지구수권-기권계통유환원향양화전변적관건시기.
The worldwide 2.33～2.06Ga positive δ~(13) Ccarb excursion is related to the Great Oxidation Event(GOE)of the atmosphere and called as Lomagundi Event.The Dashiqiao magnesite ore-bed and its dolomite hostrocks of the Dashiqiao Fm.,Liaohe Group,are characterized by positive δ~(13) C_(carb) excursion and suitable for the study of the Lomagundi Event using REY(rare earth element and yttrium,REE+Y)chemical fingerprints.Our results show that the PAAS(Post-Archean Average Shale)normalized REY patterns of six dolomite samples,whose ∑ REE range from 0.988×10~(-6)～2.744×10~(-6) and Y/Ho ratios range from 37.9～49.4 with average of 42.5±4.7,show uniform positive(La/La~*)_(SN)(1.075±0.317)and(Gd/Gd~*)_(SN)(1.390±0.166)anomalies and notable HREE enrichment indicated by(Nd/Yb)_(SN) = 0.38～0.78(average 0.49±0.15).These features are consistent with the geochemistry of well-oxygenated,shallow ambient seawater,and suggest that these samples substantially record the primary REY signatures of seawater during the Lomagundi Event era.The ∑ REE of six magnesite samples,in average of 4.549×10-6±2.239×10~(-6),are higher than those of dolomite hostrocks;and the Y/Ho ratios,averaging 40.2±3.2,are slightly lower than the dolomites.The magnesites show slight depletion in HREE(Nd~(SN)/Yb~(SN) = 1.141±0.265),and positive(La/La~*)_(SN)(average 0.919±0.203)and(Gd/Gd~*)_(SN)(average 1.036±0.081)anomalies which are weaker than those of dolomites and still originated from seawater.The mega-cylindrical talc-bearing dolomite in the hanging-wall shows highest ∑ REE(10.758×10~(-6))content and a flat PAASnomalised REY pattern,with positive(Eu/Eu~*)SN anomaly up to 1.97,the lowest positive(Gd/Gd~*)SNand Y anomalies,and the lowest Y/Ho(31.3)and(Nd/Yb)_(SN)(0.89).This unique talc-bearing dolomite was reworked by hydrothermal.The(Sm/Yb)CNvalues of magnesites average 2.61±0.45,higher than those of dolomites(1.19±0.27 in average),which indicates that the seawater depth became shallower during sedimentation of the Dashiqiao Formation,and that the dominant factor controlling the REY geochemical magnesite and dolomite were mainly sourced from fluxes of solutes from terrestrial weathering,whereas the contribution from marine hydrothermal processes was negligible.The(Ce/Ce~*)~(SN) of dolomites and magnesits range 0.95-1.25 and 0.94～1.16,with averages of 1.11±0.13 and 1.04±0.08,respectively.This suggests that weathering of continental crust was strong during 2.2～2.174Ga,and seawater might be alkaline with pH value above 8.2.Considering that the pre-2.3Ga and post-2.06Ga chemical sediments have(Eu/Eu~*)-(SN) values higher and lower than 1.53,respectively,the(Eu/Eu~*)_(SN) values of the Dashiqiao dolomites (1.44±0.11)and magnestes(1.58±0.20)indicate that the(Eu/Eu~*)_(SN)≈1.53 can be used as one of criteria for 2.2～2.174Ga marine chemical sediments;and that the 2.33～2.06Ga period was a critical turnpoint that the Earth's atmosphere-hydrosphere system sharply changed from reduction to oxygenation.