Magnesium Isotope Composition of Different Geological Reservoirs and Controlling Factors of Magnesium Isotope Fractionation in the Formation of Carbonate Minerals-A Summary of Previous Results
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摘要: 镁同位素在低温地球化学过程中显著的分馏效应，是其示踪地球表生环境演化及物质循环的基础。本文在前人研究的基础上，对地球上不同地质储库中的镁同位素组成及碳酸盐矿物形成过程中的镁同位素分馏控制因素进行了总结：火成岩的镁同位素组成较均一；风化产物总体富集重的镁同位素，且变化较大；碳酸盐岩中灰岩相对白云岩富集轻的镁同位素，但总体上富集轻的镁同位素；岩石类型、风化强度以及植被等因素对河流地表水的镁同位素组成影响较大，导致地表水的镁同位素组成总体变化较大；海水的镁同位素组成均一，平均值约为-0.83‰；低温条件下，控制碳酸盐矿物无机成因过程中镁同位素分馏的因素有矿物相、沉淀速率和温度，其中矿物相是主要控制因素；生物成因碳酸盐矿物镁同位素组成与生物体对含镁碳酸盐矿物的利用形式有关，除了需考虑与无机碳酸盐沉淀类似的控制因素外，还需考虑不同物种对轻、重镁同位素的选择性吸收能力；因生物成因海相碳酸盐矿物几乎都是由最初的无定形相碳酸盐转变而来，故生物成因海相碳酸盐矿物的镁同位素特征不能代表生成无定形相碳酸盐的流体的镁同位素特征。镁同位素在低温条件下具有良好的分馏效应，随着分析测试技术的发展及不同地质储库中镁同位素组成数据的积累和完善，有关表生环境中镁同位素分馏机制的许多问题将逐步得到解决，镁同位素在揭示地球表生环境演化及物质循环方面将发挥更大的作用。要点
(1) 镁同位素在低温地球化学过程中具有显著的分馏作用。
(2) 碳酸盐岩总体上富集轻的镁同位素，灰岩的镁同位素组成比白云岩稍轻。
(3) 生物成因海相碳酸盐矿物几乎都是由最初的无定形相碳酸盐转变而来。
HIGHLIGHTS(1) Magnesium isotopes showed significant fractionation during low-temperature geochemical processes.
(2) Carbonate rocks showed enrichment in light isotopes of magnesium in general; magnesium isotopes of limestone were lighter than those of dolomite.
(3) Almost all biogenic marine carbonate minerals were transformed from the original amorphous phase carbonate precursor.
Abstract:BACKGROUNDMagnesium isotope fractionation effect during low-temperature geochemical processes is the foundation of tracing supergene evolution and material cycle of the earth.OBJECTIVESTo summarize the magnesium isotope composition of different geological reservoirs and investigate the controlling factors of the magnesium isotope fractionation during the formation of carbonate minerals.METHODSSystematic collection and summary of previous research results.RESULTSMagnesium isotope compositions of igneous rocks were relatively homogeneous. The weathering products were relatively enriched in heavy isotopes of magnesium with significant variation. Carbonate rocks showed enrichment in light isotopes of magnesium in general. The large variation of magnesium isotope composition of river water was affected by lithology, weathering degree and vegetation. Magnesium isotope composition of seawater was homogeneous with an average of -0.83‰. At low temperature, the factors controlling the fractionation of magnesium isotopes in the inorganic process of carbonate minerals were mineral phase, precipitation rate and temperature, of which mineral phase was the main controlling factor. The factors influencing the magnesium isotope composition of biogenic carbonate minerals were forms of utilization of magnesium carbonate minerals by organisms. In addition to considering the mechanisms that were similar to inorganic carbonate precipitation, the selective absorption of light and heavy magnesium isotopes by different species should be considered. Almost all biogenic marine carbonate minerals were transformed from the original amorphous phase carbonate precursor, and their original magnesium isotope composition was masked by the later magnesium isotope composition during transformation. Therefore, the magnesium isotope composition of biogenic marine carbonate minerals cannot represent the fluid isotope composition from which the original amorphous carbonate precursor formed.CONCLUSIONSMagnesium isotopes have a good fractionation effect at low temperature. With the development of analytical technology and the accumulation and improvement of magnesium isotope composition data in different geological reservoirs, many problems related to the mechanism of magnesium isotope fractionation in the supergene environment will be solved gradually. Magnesium isotopes will play a greater role in revealing the evolution of the supergene environment and the material cycle of the earth.
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滇黔地区的含锰层下均发育了一套以硅质岩、硅质灰岩为主的硅质岩建造，习称“白泥塘层”[1-3]，其中硅质成分占20%～30%，碳酸盐岩占70%～80%[2]。该套硅质灰岩作为遵义锰矿的底板，与锰矿的形成具有密切的联系[3-8]，为锰矿的形成提供了物质来源[3-5]。研究硅质灰岩的成因对认识遵义锰矿的成矿作用可以提供新的信息，但就目前来看，前人对“白泥塘层”硅质灰岩成因的研究程度还较为薄弱。刘志臣等[7]对遵义锰矿区“白泥塘层”硅质灰岩的地球化学特征研究认为，“白泥塘层”硅质灰岩的成因可能属于热水沉积成因。但从其研究的对象来看，刘志臣等关注的是硅质灰岩全岩的地球化学特征，并不是硅质成分本身。硅质灰岩中含有一定成分碳酸盐岩，硅质岩全岩能否真实地反映“白泥塘层”中硅质的来源，这一问题有待研究。皮道会等[9]研究发现黑色岩系中有机质的稀土元素特征与全岩的稀土元素特征有很大的不同。裴浩翔等[10]对道坨矿区锰矿石全岩及其中的菱锰矿进行了分离提取实验，发现锰矿石全岩的稀土元素特征与菱锰矿的稀土元素特征亦存在较大区别，而菱锰矿的地球化学特征反映其可能是后期所形成。由此可见，全岩与有机质和菱锰矿的地球化学特征有较大差别。
为了真实地反映硅质灰岩中硅质成分来源问题，本文以遵义南茶锰矿“白泥塘层”硅质灰岩为研究对象，利用盐酸浸泡硅质灰岩样品，得到成分较为单一的硅质成分，并应用电感耦合等离子体质谱(ICP-MS)和电感耦合等离子体发射光谱法(ICP-OES)测定全岩与去除碳酸盐的硅质组分中的微量元素，通过对比去除碳酸盐处理后的样品与全岩的微量元素特征，探讨硅质灰岩中硅质成分的来源问题。
1. 研究区地质背景
南茶锰矿床是近年来在遵义铜锣井地区发现的又一中型锰矿床，该矿床位于贵州遵义县城南约12 km，属于铜锣井锰矿床黄土坎矿段的一部分。其大地构造位于扬子准地台西部，黔中台沟的北东端。区域构造上则处于铜锣井背斜南东倾末端的南延部分。区内出露的地层有寒武系、奥陶系、二叠系和三叠系。二叠系茅口组为锰矿的主要赋存层位，根据岩性组合、结构、构造和岩相特征，该层可分为3个岩性段：1段为灰、浅灰色厚层至块状生物灰岩，并夹泥质条带灰岩，偶夹白云质灰岩及燧石条带、团块；2段为灰、深灰、灰黑色薄至中厚层状含炭硅质灰岩(为本次研究的对象，即“白泥塘层”)；3段为生物屑灰岩、黏土岩、薄层条带状菱锰矿、含黄铁矿质菱锰矿。
2. 实验部分
本文所研究的硅质灰岩取自遵义南茶锰矿zk1103钻孔，该岩性段内共取样5件，样品间距为50～70 mm，选取的样品均为新鲜且未经风化的岩石。将其破碎5～10目后放入玛瑙研磨器中进一步研磨到200目左右，分别采用ICP-MS和ICP-OES进行硅质灰岩全岩及去除碳酸盐后硅质组分的微量元素测定，分析测试均在国家地质实验测试中心完成。
2.1 仪器及主要试剂
X-Series Ⅱ型电感耦合等离子体质谱仪(美国Thermo公司) ，Optima 8300型电感耦合等离子体发射光谱仪(美国PerkinElmer公司)。
烘箱，50 mL平底聚丙烯离心管，25 mL聚四氟乙烯坩埚，封闭溶样罐。
硝酸、氢氟酸、盐酸均为优级纯。内标元素为10 μg/L的Rh、Re溶液。
2.2 全岩样品微量元素分析
准确称取0.10000 g(误差小于 0.00020 g)样品于封闭溶样器的聚四氟乙烯内罐中，加入1 mL硝酸和1.5 mL氢氟酸后，将其装入封闭溶样罐，于190℃烘箱中保温24 h。冷却后取出聚四氟乙烯内罐，置于160℃的电热板上敞口蒸干，以除去其中的氢氟酸。待溶液蒸干后，在罐中加入4 mL 50%的盐酸(保证酸度在10%)，再次装入封闭溶样罐，于150℃下封闭溶样5 h。冷却至室温后定容至20 mL，用ICP-OES测量Mn、Fe含量，然后从定容的25 mL溶液中取出2 mL稀释5倍后用ICP-MS测量微量元素含量。
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图 1 南茶锰矿区地质略图1—寒武系；2—奥陶系；3—二叠系上统梁山组；4—二叠系中统栖霞组；5—二叠系中统茅口组；6—二叠系下统龙潭组；7—三叠系；8—锰矿；9—断层；10—背斜。Figure 1. Geological map of Nancha manganese ore2.3 去除碳酸盐的硅质组分微量元素分析
准确称取0.5 g样品置于离心管中，加入10%的盐酸，浸泡至见冒小气泡为止(说明硅质灰岩中的碳酸盐部分已被溶解完全)。然后加水洗涤并在离心机上以4000 r/min速率离心10 min，该过程重复2次。离心完毕后，将处理后的样品转移至滤纸中，放于烘箱内烘干0.5 h，得到去除碳酸盐后的样品。对其微量元素的测定步骤与2.2节全岩测定步骤相同。分析结果的单位为 μg/g，精密度和准确度分别为5%和小于5%，分析质量监控结果表明样品分析质量满足研究要求。
3. 结果与讨论
硅质岩的形成条件较为苛刻，不仅需要有丰富的硅质来源，同时需要特殊的沉积环境。姚旭等[11]指出扬子地区二叠系硅质岩形成于缺氧的闭塞沉积环境，因此在讨论“白泥塘层”硅质组分来源时，确定其沉积环境对于硅质岩的成因的研究具有重要意义。此外，“白泥塘层”作为遵义锰矿的底板[2]，确定其沉积环境对于探讨遵义锰矿成因具有一定指示意义。本文对硅质灰岩全岩及去除碳酸盐后硅质组分的元素测试结果列于表 1，以下对微量元素和稀土元素的特征作一分析。
3.1 微量元素特征及其对沉积环境和硅质组分来源指示意义
3.1.1 硅质灰岩与全岩中微量元素特征
南茶锰矿赋矿层下伏硅质灰岩经过去除碳酸盐处理后，测试结果显示：去除碳酸盐后的样品V、Mo、U值较高，分别为169.98～249.40 μg/g、2.97～5.47 μg/g、1.53～8.08 μg/g；Ni/Co=11.60～13.76；Th/U=0.02～0.05；V/Cr=2.91～3.33；V/(V+Ni)=0.90～0.93；Sr/Ba=0.46～0.72。与全岩的数据对比，Ni/Co、V/Cr、V/(V+Ni)值相当，但在Sr、Th、U值上出现了较大的差异，具体表现为：全岩的Sr值为880.20～1472.0 μg/g，Th值为0.40～1.95 μg/g；而去除碳酸盐的硅质组分其Sr值为3.45～4.56 μg/g，Th值为0.02～0.08 μg/g。由微量元素蛛网图(图 2)也能看出这一差异，在Sr值处，去除碳酸盐硅质组分高于全岩，Th值的下降幅度高于全岩，而U值的下降幅度不大。
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图 2 (A)去除碳酸盐硅质组分与全岩微量元素蛛网图；(B)去除碳酸盐硅质灰岩与全岩稀土元素配分图Figure 2. (A)The race element spider diagrams of the whole rock and siliceous composite of removing carbonate minerals; (B)The NASC normalized REEs patterns of the whole rock and siliceous composite of removing carbonate minerals3.1.2 硅质灰岩与全岩中微量元素的指示意义
通常情况下，Al、Ti、Th、Zr等元素常被用于指示陆源碎屑物质[12]。研究区内去除碳酸盐后的硅质组分Ti、Th、Zr的值较低，明显低于其对应的澳大利亚后太古平均页岩(PAAS，Ti值为600 μg/g，Th值为14.6 μg/g，Zr值为210 μg/g)，表明该区“白泥塘层”硅质组分沉积时陆源碎屑物质的加入量较低。Sr/Ba值在全岩及去除碳酸盐硅质组分中的巨大差异主要表现在Sr值的差异上(表 1)。这可能是因为在富Ca矿物中(碳酸钙)常容纳Sr，而硅质岩中Sr的含量低所造成的。
Ni/Co值被认为是表征海水化学特征的一项指数[13-14]，该值越大，代表沉积物形成于越缺氧的环境。在沉积过程中形成的黄铁矿能发现Co、Ni的存在，且Ni/Co>1。因此，当Ni/Co>1时代表了缺氧环境，Th/U<1则代表了相对氧化的环境[15]。南茶地区硅质灰岩全岩与去除碳酸盐后的硅质组分整体上Ni/Co值分别介于11.60～13.76及10.02～11.83之间，明显高于阈值，表明当时的沉积环境为一缺氧环境。
表 1 去除碳酸盐后硅质组分与全岩的微量元素(包括稀土元素)数据Table 1. Trace elements data (including rare earth elements) in siliceous components removing carbonate minerals and total rock微量元素 去除碳酸盐样品(×10-6) 未去除碳酸盐样品(×10-6) Jhl-28 Jhl-29 Jhl-30-2 Jhl-31 Jhl-32 Jhl-28 Jhl-29 Jhl-30 Jhl-31 Jhl-32 Ti 152.54 152.80 127.70 99.30 153.84 330.80 196.60 226.00 146.30 328.50 V 223.20 185.46 201.80 169.98 249.40 510.90 356.60 387.90 381.30 573.40 Cr 70.42 63.66 60.64 51.68 73.18 155.40 101.90 120.80 83.48 153.80 Co 1.97 1.87 1.35 0.95 2.28 5.76 4.10 3.85 2.73 6.58 Ni 23.74 21.72 17.14 13.02 26.98 61.26 41.09 45.56 29.14 69.60 Cu 5.70 4.38 3.60 2.03 6.14 11.06 4.80 5.79 1.13 12.88 Zn 46.98 116.74 50.88 65.42 59.56 109.70 163.90 91.01 110.90 128.30 Ga 0.81 0.82 0.64 0.58 0.85 2.62 1.89 1.52 1.26 3.22 Sr 3.94 4.37 4.56 3.45 4.48 914.90 1472.00 1274.00 1459.00 880.20 Zr 25.64 22.64 12.89 29.04 17.59 67.58 35.26 21.72 33.29 40.50 Mo 4.48 5.24 4.08 2.97 5.47 20.31 13.77 15.41 9.79 24.98 Ba 8.52 8.44 6.31 5.84 8.90 41.47 27.72 25.25 18.15 43.51 Th 0.06 0.08 0.06 0.02 0.07 1.81 1.20 1.17 0.40 1.95 U 1.53 1.73 1.89 5.56 8.08 8.92 5.99 6.79 5.26 9.88 Sr/Ba 0.46 0.52 0.72 0.59 0.50 22.06 53.10 50.46 80.39 20.23 Ni/Co 12.08 11.60 12.71 13.76 11.84 10.64 10.02 11.83 10.66 10.58 Th/U 0.04 0.05 0.03 0.02 0.04 0.20 0.20 0.17 0.08 0.20 V/Cr 3.17 2.91 3.33 3.29 3.41 3.29 3.50 3.21 4.57 3.73 V/(V+Ni) 0.90 0.90 0.92 0.93 0.90 0.89 0.90 0.89 0.93 0.89 La 1.03 0.94 0.59 0.24 1.49 25.97 22.75 17.57 15.34 27.45 Ce 0.72 0.77 0.43 0.24 0.94 20.92 15.91 12.65 9.958 21.72 Pr 0.09 0.13 0.06 0.03 0.12 4.797 4.103 3.066 2.529 4.969 Nd 0.28 0.46 0.17 0.11 0.34 16.98 13.53 10.22 8.466 17.08 Sm 0.04 0.07 0.02 0.01 0.05 3.56 2.855 2.136 1.662 3.718 Eu 0.0106 0.0128 0.0088 0.0056 0.0106 0.758 0.572 0.438 0.353 0.791 Gd 0.05 0.07 0.04 0.02 0.06 4.11 3.172 2.547 1.89 4.428 Tb 0.0072 0.0092 0.0064 0.003 0.008 0.438 0.328 0.252 0.192 0.449 Dy 0.06 0.07 0.06 0.02 0.07 3.126 2.403 1.834 1.421 3.335 Ho 0.0134 0.0166 0.0136 0.0048 0.0158 0.539 0.402 0.314 0.241 0.582 Er 0.05 0.05 0.04 0.01 0.06 1.419 1.09 0.844 0.624 1.56 Tm 0.0088 0.01 0.0096 0.0028 0.0114 0.211 0.156 0.124 0.094 0.232 Yb 0.06 0.07 0.06 0.02 0.08 1.259 0.958 0.721 0.56 1.441 Lu 0.0096 0.0126 0.0126 0.0038 0.0138 0.244 0.187 0.142 0.099 0.267 Y 0.97 1.01 0.70 0.52 0.94 29.44 23.29 16.46 14.18 32.42 ∑LREEs 2.18 2.38 1.28 0.64 2.95 72.99 59.72 46.08 38.31 75.73 ∑HREEs 0.25 0.31 0.24 0.09 0.31 11.35 8.70 6.78 5.12 12.29 ∑LREEs/∑HREEs 8.64 7.64 5.24 7.37 9.52 6.43 6.87 6.80 7.48 6.16 REEs+Y 17.01 18.48 11.16 6.23 20.99 113.77 91.71 69.32 57.61 120.44 Y/Ho 72.70 60.80 51.69 107.75 59.52 54.62 57.94 52.42 58.84 55.70 Pr/Pr* 1.24 1.31 1.32 1.05 1.25 1.42 1.55 1.51 1.53 1.45 Ce/Ce* 0.48 0.49 0.49 0.64 0.45 0.43 0.38 0.39 0.36 0.43 Eu/Eu* 1.05 0.86 1.35 1.54 0.92 0.92 0.89 0.87 0.93 0.90 V/Cr值是环境变化的重要化学指标之一[15]。当V/Cr＜2时指示一个氧化环境，V/Cr＞2则指示缺氧的环境，代表沉积物表面存在含H2S的水柱[15]。研究区内硅质灰岩及硅质组分的V/Cr值均超过了2，也指示了其沉积时的环境为缺氧环境。
V倾向于富集于Fe、Mn还原带之下、次氧或缺氧环境的沉积物中[16]，通常采用V/(V+Ni)值来指示水体的氧化还原环境。当V/(V+Ni)=0.83～1时为硫化环境；V/(V+Ni)=0.57～0.83时为缺氧环境；V/(V+Ni)=0.46～0.57时为弱氧化环境；V/(V+Ni)<0.46时为氧化环境[15]。南茶地区硅质灰岩全岩、去除碳酸盐后的硅质组分的V/(V+Ni)值分别介于0.89～0.93及0.90～0.93之间，同样说明了其沉积时所处的环境为缺氧环境。
沉积物中的Th/U值可以作为判断氧化-还原状态的指标[17]。在正常的氧化条件下，U4+易氧化成为U6+而迁移出沉积物，海洋页岩应具有平均页岩的Th/U值(3.8，综合了上地壳成分)或者更高值(强氧化环境Th/U值为8)，而在典型缺氧环境地层水体中，U易被还原而赋存下来，造成Th/U值下降，此时Th/U值常介于0～2之间[18]。研究区内硅质组分的Th/U值介于0.02～0.2之间，表明其沉积时的环境为缺氧环境。此外，在正常的深海沉积物中，由于沉积速率缓慢使得其能从海水中汲取大量的Th，沉积岩中的Th含量增高，最终导致Th含量高于U；而在热水沉积物中，因沉积堆积过快，Th不能被沉积物充分吸收而造成沉积体系富U贫Th，因此热水沉积的Th/U＜1，而非热水沉积岩的Th/U＞1[19]。在刘志臣等[7]所测硅质岩全岩中，Th/U=0.21～2.31，变化幅度较大，均值为1.013，与本文实测值存在差异。虽然研究区内全岩及去除碳酸盐硅质组分的Th/U值均小于1，但硅质组分的Th及U值相比于全岩表现出来的特征(Th值下降幅度大，U值下降较小)更能充分反映硅质组分的来源可能来自于热水。
3.2 稀土元素特征及其对沉积环境和硅质组分来源指示意义
3.2.1 硅质灰岩与全岩中稀土元素特征
由表 1及图 3可知，硅质组分的∑LREEs=0.64～2.95 μg/g，∑HREEs=0.09～0.31 μg/g，Y/Ho=51.69～107.75，Pr/Pr*=1.05～1.32，Ce/Ce*=0.45～0.64，Eu/Eu=0.86～1.54，显示为正Eu异常。全岩的∑LREEs=38.31～75.73 μg/g，∑HREEs=5.12～12.29 μg/g，Y/Ho=52.42～58.84，Pr/Pr*=1.42～1.55，Ce/Ce*=0.36～0.43，Eu/Eu*=0.87～0.93，显示为弱的负Eu异常。整体上来看，当硅质灰岩经去除碳酸盐处理后，其稀土元素相比于全岩也发生了明显的降低，但去除碳酸盐后的硅质组分中的轻稀土含量比重增大。
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图 3 (a)硅质灰岩与全岩的Ba-Eu/Eu*相关性；(b)去除碳酸盐硅质组分的Ba-Eu/Eu*相关性；(c)去除碳酸盐硅质组分各数据指标变化图Figure 3. (a)The correlation between Ba and Eu/Eu* of carbonate and the whole rock；(b)The correlation between Ba and Eu/Eu* of siliceous composite of removing carbonate minerals；(c)Illustration of changes of data of siliceous composite of removing carbonate minerals3.2.2 硅质灰岩与全岩中稀土元素的指示意义
稀土元素在成岩的过程中具有较好的稳定性，因此稀土配分模式、Y/Ho值、Ce异常及Eu异常等可用于解译古环境信息[20-22]。海水的Y/Ho值(约40～80)随着海水深度的增加而减小，但显著不同于球粒陨石和平均页岩的Y/Ho值(约27)[23]。通常情况下，如果沉积物的Y/Ho值接近PAAS值，则认为其受到了陆源碎屑物质的较大影响；而当Y/Ho值显著偏离PAAS值，则认为其主要源于海水特征[24-25]。南茶地区硅质灰岩全岩Y/Ho均值为55.90；去除碳酸盐后的硅质组分Y/Ho均值为70.79，均介于40～80之间，表明了其沉积时受到陆源碎屑物质的影响较小，而主要是继承了海水特征。
由于在氧化条件下，Ce4+在沉积物中能够保持稳定，但当环境由氧化变为还原时，Ce4+则更倾向于迁出而造成沉积物中Ce的亏损[26]。因此，Ce异常通常作为海洋氧化还原环境的指标[27]，被广泛用于地质历史时期各种化学沉积岩的研究中[28-30]。需要指出的是，由于海水中常具过量的La，因此采用传统的Ce/Ce*值计算方法[Ce/Ce*=2Cen/(Lan+Prn)]可能会导致Ce假象异常[24]。理论上，Nd和Pr十分稳定，并无异常现象，因此可通过Pr/Pr*来判别Ce是否存在异常现象：真实Ce异常应也导致Pr的异常(Pr/Pr*＜0.95或Pr/Pr*＞1.05)，当0.95＜Pr/Pr*＜1.05时，则Pr不具异常，也表明了Ce的异常可能不真实。研究区内硅质灰岩全岩及去除碳酸盐硅质组分的测试结果显示Pr/Pr*值均＞1.05，表现为Pr异常，说明了Ce异常是真实存在的。当Ce显示为负异常时，指示沉积环境属还原环境；当Ce显示为正异常时，指示沉积环境属氧化环境。南茶锰矿区硅质灰岩全岩与去除碳酸盐硅质组分整体上呈现为中等Ce负异常，反映其当时的沉积环境可能为相对还原的状态，这与前述微量元素比值所揭示的现象是相一致的。Murray等[31]研究表明现代大洋硅质岩和造山带古海洋硅质岩的Ce/Ce*值从大洋中脊(0.29)到大洋盆地(0.55)，再到大陆边缘沉积环境(0.9～1.30)呈递增规律。研究区内去碳酸盐硅质组分的Ce/Ce*值介于0.45～0.64之间，均值为0.51，与大洋盆地的Ce/Ce*值相近，表明硅质沉积时处于靠近洋盆的深海海域。
南茶锰矿区去除碳酸盐硅质组分与硅质灰岩全岩相比，表现出不同的Eu异常特征，如图 3所示，但我们在对其评价时应当注意其异常的真实性。这是因为通过ICP-MS测试稀土元素含量时，Ba的各种复合物可能会干扰Eu的测量，进而造成Eu异常的假象[32]。对于这种假象的判定，可以通过Ba和Eu/Eu*的相关关系来加以说明：当Ba与Eu/Eu*正相关时，说明存在Ba的叠加干扰，Eu的异常不可靠；反之则相对可靠。由图 3可知，研究区内硅质灰岩全岩的Eu/Eu*与Ba相关性较差(相关系数为0.0177)，而去除碳酸盐硅质组分的Eu/Eu*与Ba表现负相关关系(相关系数为0.9183)，表明Eu的异常值相对可靠。
理论上，Eu只在高温的条件下(＞250℃)才能从+3价还原为+2价[33]，因此Eu异常通常只出现于海底热液流体中[34]，或者出现于岩浆、火山及其变质矿物中，这在海洋热液相关的沉积物中十分常见[35-36]。研究区内去除碳酸盐硅质灰岩样品的Eu/Eu*测定值(介于0.86～1.54之间，均值为1.15)与硅质灰岩全岩(δEu值介于0.87～0.93，均值为0.90)和刘平等[5]所测值(0.56)具有显著的不同，也与刘志臣等[7]所测值存在明显差别(其认为Eu的异常不显著)。造成这种差异的原因可能是后三者的研究对象均为硅质岩或硅质灰岩全岩，而本文在经过去除碳酸盐处理后，δEu呈现为正异常(δEu＞1)，表明其硅质组分有热水来源的特征。相比之下，去除碳酸盐后的硅质组分更能真实地反映出“白泥塘层”硅质灰岩硅质的来源，即应为热水来源。
与此同时，将本文实测稀土元素与前人所测的“白泥塘层”硅质岩进行对比后发现，本文所测的硅质灰岩全岩的稀土配分曲线与前人[7]所做的“白泥塘层”硅质岩大体一致，而与格学锰矿石、水城锰矿石、纳雍锰矿石、铜锣井锰矿石的稀土配分曲线也具有相似的分布趋势，表明它们可能为相似的成因，这也说明了“白泥塘层”硅质岩与黔中台沟内锰矿床的形成具有密切的成因联系。
4. 结论
去除碳酸盐处理后的硅质组分与硅质灰岩全岩研究结果显示，硅质灰岩中硅质成分可能来自热水，但在一些关键性的元素特征上，去除碳酸盐处理后的硅质组分与硅质灰岩全岩存在显著差异。如：经过去除碳酸盐处理后的样品，其硅质组分中Sr、Th显著减少，轻稀土比重增大；Th/U测定值波动范围(0.02～0.2)小于前人所测硅质岩全岩(0.21～2.31)，较低的Th/U值反映硅质组分可能来源于深部；去除碳酸盐处理后硅质灰岩的Eu/Eu*值(0.86～1.54)，表现为正Eu异常特征，而本文实测硅质灰岩全岩及前人所测硅质岩的Eu/Eu*值表现为Eu的负异常或Eu的不显著异常，由于硅质灰岩中含有碳酸盐矿物成分，因此本文通过去除碳酸盐的方法所得的“白泥塘层”硅质组分的各项指标参数可能更能真实地反映出其沉积时所具有的一些特征。
本文在前人研究的基础上，采用去除硅质灰岩中碳酸盐成分的方法，获得了成分较为单一的硅质组分，通过研究其微量元素特征的变化进一步探讨了遵义二叠系锰矿“白泥塘层”的硅质来源，研究结果表明该方法能够更加清晰地重现硅质沉积时的一些重要特征，所获得的结论对于解释该时期锰矿的形成具有重要的指示意义。
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图 1 不同地质储库中的镁同位素(δ26Mg)组成
本图修改自参考文献[6, 20]。图中各样品涉及的参考文献，a：[51-54]；b：[51-52, 55]；c：[46, 51-52, 54, 56-57]；d：[6]；f：[58]；g：[59-68]；h：[33-34, 37, 47, 69-72]；i：[37, 65, 69, 73-76]；j：[69]；k：[46-47, 77-79]；m：[17, 29, 69, 71, 75, 77, 80-83]；n：[24, 28-30, 77, 81, 84-87]；p：[30, 75]；q：[68, 88]；r：[68, 88]；s：[46, 67, 89-90]。
Figure 1. Composition of Mg isotopes (δ26Mg) in major terrestrial materials
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