华北中元古代浅海碳酸盐沉淀方式变化:海水氧化还原条件波动的响应?*

海相碳酸盐的沉淀方式被认为与水体氧化还原条件密切相关,即太古宙至古元古代缺氧的铁化海水中碳酸盐沉淀抑制剂Fe²⁺和Mn²⁺强力抑制灰泥在水柱中成核,但允许文石直接在海底生长,从而导致大量文石以海底沉淀方式产出,而新元古代适度的氧化海水则有利于灰泥以水柱沉淀方式形成。然而,碳酸盐沉淀方式的长期变化还可能受控于其他因素,其与海水氧化还原条件之间的关系还需要通过大量具体实例来验证。针对上述科学问题,笔者选择碳酸盐沉淀方式尚处于过渡时期的华北中元古界碳酸盐岩为研究对象,开展碳酸盐沉淀方式及与之对应的氧化还原条件研究。结果表明,华北高于庄组三段(约1.56 Ga)、雾迷山组四段下部(约1.48 Ga)和铁岭组二段(约1.44 Ga)发育大量灰泥水柱沉淀,其Ⅰ/(Ca+Mg)值较高(普遍大于0.5 μmol/mol)、Ce负异常(低至0.8),指示适度氧化的条件;而高于庄组四段下部(约1.55 Ga)和雾迷山组二段中部(约1.50 Ga)则发育大量纤维状文石海底沉淀,其Ⅰ/(Ca+Mg)值约为0,指示次氧化至缺氧的环境。因此,本研究首次用大量实例证实了前寒武纪海水氧化还原条件对碳酸盐沉淀方式的重要调控作用,并且后者可作为海水氧化还原条件分析的重要指标,适用于高效开展长序列、多剖面的低氧背景下前寒武纪碳酸盐岩地层的氧化还原条件分析。     

Petrological and Geochemical Constraints on the Protoliths of Serpentine‐Magnetite Ores in the Zhaoanzhuang Iron Deposit,Southern North China Craton

The uncommon Mg-rich and Ti-poor Zhaoanzhuang serpentine-magnetite ores within Taihua Group of the North China Craton(NCC) remain unclear whether the protolith was sourced from ultramafic rocks or chemical sedimentary sequences. Here we present integrated petrographic and geochemical studies to characterize the protoliths and to gain insights on the ore-forming processes. Iron ores mainly contain low-Ti magnetite(TiO_2 ~0.1 wt%) and serpentine(Mg#=92.42–96.55), as well as residual olivine(Fo=89–90), orthopyroxene(En=89–90) and hornblende. Magnetite in the iron ores shows lower Al, Sc, Ti, Cr, Zn relative to that from ultramafic Fe-Ti-V iron ores, but similar to that from metamorphic chemical sedimentary iron deposit. In addition, interstitial minerals of dolomite, calcite, apatite and anhydrite are intergrown with magnetite and serpentine, revealing they were metamorphic, but not magmatic or late hydrothermal minerals. Wall rocks principally contain magnesian silicates of olivine(Fo=83–87), orthopyroxene(En=82–86), humite(Mg#=82–84) and hornblende [XMg=0.87–0.96]. Dolomite, apatite and anhydrite together with minor magnetite, thorianite(Th-rich oxide) and monazite(LREE-rich phosphate) are often seen as relicts or inclusions within magnesian silicates in the wall rocks, revealing that they were primary or earlier metamorphic minerals than magnesian silicates. And olivine exists as subhedral interstitial texture between hornblende, which shows later formation of olivine than hornblende and does not conform with sequence of magmatic crystallization. All these mineralogical features thus bias towards their metamorphic, rather than magmatic origin. The dominant chemical components of the iron ores are SiO_2(4.77–25.23 wt%), Fe_2O_3 T(32.9–80.39 wt%) and MgO(5.72–27.17 wt%) and uniformly, those of the wall rocks are also SiO_2(16.34–48.72 wt%), Mg O(16.71–33.97 wt%) and Fe_2O_3 T(6.98–30.92 wt%). The striking high Fe-Mg-Si contents reveal that protolith of the Zhaoanzhuang iron deposit was more likely to be chemical sedimentary rocks. The distinct high-Mg feature and presence of abundant anhydrite possibly indicate it primarily precipitated in a confined seawater basin under an evaporitic environment. Besides, higher contents of Al, Ti, P, Th, U, Pb, REE relative to other Precambrian iron-rich chemical precipitates(BIF) suggest some clastic terrestrial materials were probably input. As a result, we think the Zhaoanzhuang iron deposit had experienced the initial Fe-Mg-Si marine precipitation, followed by further Mg enrichment through marine evaporated process, subsequent high-grade metamorphism and late-stage hydrothermal fluid modification.     

安山岩风化过程中元素行为——以豫西熊耳山地区为例

在风化过程中主量元素的风化行为可用风化指数来表征,但对在地球化学中更重要的微量元素的风化行为还缺少研究。本文研究了豫西熊耳山地区—安山岩风化剖面,发现花岗岩风化指数(WIG)也适用于安山岩风化过程的定量表征。基于上陆壳作标准绘制风化剖面样品稀土元素配分曲线时,发现风化过程稀土元素出现分馏现象,表现为富集中稀土;但基于球粒陨石作标准时,风化壳样品均具有相似的稀土配分曲线形态,即上陆壳是一种比球粒陨石更为敏感的稀土元素标准化标尺。牛头沟金矿区安山岩风化过程中部分微量表现出随风化程度增强而显著富集的特征,Au、Ag含量变化可达两个数量级以上,Pb、As、Hg可达一个数量级以上。     

Linkage between Seasonal Insolation Gradient in the Tropical Northern Hemisphere and the Sea Surface Salinity of the Equatorial Indian Ocean during the Last Glacial Period

Paired stable oxygen isotope and Mg/Ca analyses in calcite tests of the mixed-layer-dwelling planktic foraminifer Globigerinoides ruber has been used to reconstruct equatorial Indian Ocean δ18O of seawater (δ18Osw) over the last ~137 thousand years. On the basis of ice-volume-corrected δ18Osw (δ18Osw–ivc), relative changes in sea surface salinity (SSS) have been estimated. The SSS estimates suggest three episodes of higher SSS (131–113 thousand years before present (kyr BP), 62–58 kyr BP, and 30–24 kyr BP) within the last glacial period as compared with the present. SSS comparison between interglacial episodes reveals that the surface seawater over the core site was significantly saltier during the penultimate interglacial than the Holocene. We suggest that the evolution of a seasonal insolation gradient between the Indian monsoon areas and the equator over the investigated time interval was instrumental in shaping the strength of the Indian winter and summer monsoons that left their imprints on the equatorial Indian Ocean SSS via freshwater input and wind-induced mixing. The study shows that the insolation difference between northern latitudes and the equator during winter affects monsoon strength in the Indian region, especially during cold intervals.     

A 1,100-year palaeoenvironmental record inferred from stable isotope and trace element compositions of ostracode and plant caryopses in sediments of Cattle Pond,Dongdao Island,South China Sea

Abundant ostracode valves (Cyprinotus cingalensis) and caryopses of Urochloa paspaloides, a terrestrial grass, were well preserved in the lacustrine sediments of the Cattle Pond on Dongdao Island, South China Sea. Oxygen and carbon isotopes, and elements (Ca, Mg) were analyzed on ostracode valves and plant caryopses in this study. The δ¹⁸O_ostracode and δ¹³C_ostracode exhibited a positive and statistically significant correlation, and showed a decreasing trend toward the top of sediment sequence with some fluctuations, indicating a gradual increase in effective moisture. The Mg/Ca ratios in the ostracode shells, generally used as a proxy for salinity changes in lake water, showed a trend toward lower values in the upper samples, reflecting a gradual decrease of salinity in the lake. The δ¹³C_org values in the plant caryopses of the upper 14 cm of sediment have lower values than those in the bottom sediments, suggesting less water and salinity stress. These results indicate increasing effective moisture and rainfall intensity over the past ∼1,100 years on Dongdao Island.     

艾比湖二千余年来环境演变的地球化学记录

通过对新疆艾比湖Ash孔二千余年以来沉积的总有机碳含量及其碳同位素、碳酸盐含量及Mg／Ca比值等环境指标综合分析,数据显示：约在公元前500-400年,艾比湖水体面积较小,对应的气候偏干。约公元前400～公元1400年,水体面积较大,处于高水位时期,气候较为湿润,但此时期内的气候也有小幅的冷暖和干湿变化。其中在公元800-1300年期间艾比湖沉积记录显示水生生物对沉积有机质的贡献达到最大,而陆源有机质输入不明显,表明此时湖水水位可能达到二千余年来的最高,这一时段基本对应于中世纪暖期。从碳酸盐含量及Mg/Ca比值指标来看,该地区中世纪暖期可能有一个干湿的波动。公元1400年以来,艾比湖水位下降,其中在15世纪中和19世纪初是两次较为干燥的时期,与其他地区的小冰期气候相对应。     

西天山玉希莫勒盖达坂玄武安山岩—高钾玄武安山岩—粗安岩组合的发现及其地质意义

在阿吾拉勒东段玉希莫勒盖达坂地区大哈拉军山组主要是一套以安山质（粗安质）-英安质（粗面质-粗面英安质）火山熔岩和火山碎屑岩为主的火山岩建造。根据岩石地球化学特征,本文在大哈拉军山组火山岩的上部厘定出了钙碱性玄武安山岩—高钾钙碱性玄武安山岩—粗安岩（橄榄安粗岩系）组合。微量元素地球化学特征显示,三类岩石形成于岛弧环境,钙碱性玄武安山岩的形成与板片俯冲作用有关,而高钾钙碱性玄武安山岩和粗安岩的形成则与俯冲板片断裂诱发的软流圈上涌作用有关。钙碱性玄武安山岩—高钾钙碱性玄武安山岩—粗安岩组合的存在说明阿吾拉勒东段在晚石炭世由于俯冲板片的断裂,构造体制由挤压转变为伸展。玉希莫勒盖达坂大哈拉军山组上部的火山岩组合与世界许多著名铜-金矿集区（如Papua New Guinea）的火山岩组合相似,显示该地区晚石炭世具有良好的铜、金成矿条件。     

I/(Ca+Mg)作为指示碳酸盐沉积氧化还原条件的重要指标: 研究进展与问题评述*

海水氧化还原条件显著影响真核生物的起源与早期演化,但以往有关早期海水氧化还原条件研究的对象,主要依赖相对深水的细粒碎屑岩沉积(如黑色页岩),而对真核生物集中分布的浅水环境中的碳酸盐岩关注不够且手段缺乏。这显著制约了对真核生物起源与早期演化机理的认识。近年来,有学者提出碳酸盐岩的I/(Ca+Mg)值可作为反映海洋氧化还原条件的重要指标,并将其广泛应用于海相碳酸盐岩的古氧相研究中。该指标的提出主要基于对现代海洋碘组分的观测以及室内方解石合成实验结果: 观测结果表明,海水中的碘主要以氧化态(IO₃^-)和还原态(I^-)2种形式存在,随着氧含量的下降(如在氧极小带),氧化态的碘被逐步转换为还原态的碘,且海水中的IO₃^-浓度与海水氧含量大体呈正相关。实验研究证明,IO₃^-可按一定的分配系数进入到碳酸盐矿物晶格中,但I^-则不能。由于IO₃^-/I^-的还原势能与O₂/H₂O的还原势能接近,因此I/(Ca+Mg)值是最早响应海洋氧含量下降的指标之一,可用于表征深时(如前寒武纪)次氧化环境中表层海水的氧含量波动。此外,学者们也尝试建立I/(Ca+Mg)值与氧含量之间的半定量关系,如I/(Ca+Mg)值大于0和2.5μmol/mol这两个临界值所对应的海水氧含量。结合大量现代缺氧水体和氧极小带中碘组分与溶解氧浓度相关关系的研究,作者提出I/(Ca+Mg)=1.5μmol/mol为重要的临界值之一,可用于限定初级生产力在表层海水中所能产生的最大氧浓度值(~10 μM),并能进一步区分海水和大气的氧化。此外,对I/(Ca+Mg)值的应用进展及潜在问题进行评述,并对可能的发展方向进行展望。     

小兴安岭西北部与永新金矿有关岩浆岩的年代学和地球化学及成矿构造环境

永新金矿是近年发现的大型蚀变岩型脉状金矿床,闪长玢岩脉等与金矿脉相互穿切并侵入于上盘龙江组火山岩中。锆石LA ICP MS U-Pb测年显示,龙江组安山岩结晶年龄为(1137 ± 18)Ma,闪长玢岩结晶年龄为(1148 ± 19)Ma,均形成于早白垩世晚期。二者钠钾含量变化相对较大,相对富铝富铁、钙镁含量中等,从火山岩到脉岩表现为钙碱性向碱性系列过渡特征。火山岩、脉岩与矿石的稀土元素配分曲线有很好的一致性,矿石稀土含量低于岩石。岩石微量元素总体显示Zr、Hf、Nd、U等明显富集,而Nb、Ta、Th、P、Ti等明显亏损。在Sr/Y-Y和(La/Yb)N-(Yb)N图解中,所有样品均落入经典岛弧岩石范围;在Nb-Y、Ta-Yb的构造环境判别图中均处于火山弧岩浆岩区,在(La/Yb)N-δEu变异图上落在壳-幔型岩浆范围。结合矿床稳定同位素及区域金矿时空分布规律等综合特征,认为永新金矿的形成与早白垩世古太平洋板块俯冲所引发的(火山)岩浆活动有关。     

洞穴次生碳酸盐沉积的Mg/Ca与Sr/Ca比值研究进展——兼论洞穴次生沉积物Mg/Ca与Sr/Ca的影响机制

微量元素是岩溶洞穴沉积中非常重要的一类古气候环境替代指标,为近20年来国内外的一个研究热点。总结前人的研究,主要取得了以下一些重要认识:（1）洞穴上覆土壤 和围岩是洞穴次生碳酸盐沉积Mg、Sr的主要来源;（2）Mg/Ca与Sr/Ca能够指示气候环境变化,但需结合其它指标综合考虑。（3）洞穴次生碳酸盐沉积Mg/Ca与Sr/Ca受多种气候环境因素（包括土壤和围岩的组成和性质、水-岩相互作用、先期碳酸盐沉积、分配系数等）影响,其古气候环境指示意义具有多解性;（4）矿物结晶作用对Mg/Ca与Sr/Ca有一定的影响,特别是文石在向方解石转变的过程中容易丢失Mg、Sr,此外,杂质的混入也将抑制Mg、Sr进入方解石,从而引起洞穴次生碳酸盐沉积Mg/Ca与Sr/Ca比值的变化。今后应进一步加强对石笋中这些微量元素的影响机制研究,尤其是对一些影响因素与微量元素含量变化之间的定量关系进行探讨。