La-Ce同位素分析技术及其在岩浆侵位和沉积过程氧化环境研究中的应用
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摘要
La-Ce和Sm-Nd同位素体系均属长寿命轻稀土元素衰变体系,其中La-Ce同位素体系中衰变子体元素Ce在氧化条件下易由Ce3+转化为Ce4+,产生不同于其它+3价REE的地球化学行为,继而改变地质样品138La/142Ce比值和138Ce增长速率,导致La-Ce与Sm-Nd同位素体系脱偶。因此,εCe-εNd参数对可对存在明显氧化还原条件变化的地质过程进行地球化学示踪研究。La-Ce同位素体系在示踪研究中具有的独特优势使其在地球科学领域具有广阔的应用前景,但因La-Ce同位素体系较长的半衰期(-100 Ga)和极低的138La丰度(0.090%)，多数地质样品的εCe变化范围(约-3-+5)低于ENd近一个数量级,使得La-Ce同位素体系的地质应用仍处于不断探索阶段。鉴于上述原因,高精度的La-Ce同位素分析技术显得尤为重要。
目前国内外大多数实验室均沿用a-HIBA法进行La-Ce分离,尽管这种实验方法大大降低了Ba的干扰,但耗时长,且全流程La、Ce本底相对偏高。本文报道作者建立的La-Ce同位素分离的无机酸淋洗法,避免了难纯化有机酸的使用,进而降低了样品分离的流程本底；使用高精度热电离质谱(Triton Ti)对样品的同位素比值进行了测量,并采用自编程序对分析数据进行离线处理。应用所建立的方法对黄陵侵入杂岩体和三峡地区沉积岩具不同程度Ce异常的样品进行了Ce同位素比值分析,结合样品的微量元素和Sr-Nd同位素特征,探讨了区域新元古代岩浆岩的构造背景和地层沉积环境。具体内容包括：
(1)在“地质过程和矿产资源”重点实验室同位素室建立了无机酸淋洗法La-Ce同位素分离方法和相应的质谱测定流程。样品的化学制备过程包括REEs分离和La、Ce分离两个阶段,在REEs分离阶段采用AG 50W-X12树脂和1.5N HNO3将地质样品中的Ba等主量元素完全洗脱后,以6.0N HCl收集REEs;在La、Ce分离阶段,采用LN特效树脂和0.18N HCl淋洗液,洗脱收集样品中的Ce,并对所收集的Ce洗脱液作二次强化分离,目的是进一步提高Ce与La、Nd的分离效率。整个化学制备过程Ce的回收率大于80%,Ce空白值低于2.8ng。
Ce同位素比值分析包括质谱测定和离线数据处理两部分。质谱分析选择氧化物测量方式,具体操作：在CeO+信号出现之前低电流下(1200mA-1400mA)停留较长时间,目的是烧掉残留的BaO+以排除其干扰；数据测量在蒸发带电流1500mA-1800mA范围内完成,测量过程以160(144NdO)/158(142CeO)<0.002054为判断依据监测NdO+的干扰。对所测得的各组氧化物比值依次进行干扰和脱氧校正,计算过程采用的18O/16O比值(R18)根据样品的160/158测量值计算得出,而不采用常数值,这种方法避免了样品间不同同位素分馏程度引起的误差,本研究获得的R18平均值为0.001986，同位素比值完成干扰校正和脱氧校正后,以136Ce/142Ce=0.01688进行归一化计算。
对标准溶液JMC304的长期测量获得的138Ce/142Ce比值平均值为0.0225787±0.0000010,与文献平均值0.0225780±0.0000012在误差范围内一致。以国家岩石标样EQB为本次实验的全流程标样,138Ce/142Ce比值测定结果为0.0225743±0.0000010，与推荐值0.0225755±0.0000003在误差范围内一致。以上结果表明,本研究建立的La-Ce同位素分离实验方法和TIMS质谱分析流程合理,Ce同位素比值测定结果准确可信。此外,本文首次对国际岩石标样BCR-2、BHVO-2与AGV-2进行了分析,其138Ce/142Ce比值分别为0.0225804±0.0000015、0.0225842±0.0000024和0.0225781±0.0000011,εCe(0)值为3.6±0.7、5.3±1.1和2.6±0.5。结合已发表的Nd同位素分析结果,显示三件标样与BCR-1类似,具有中国东部新生代碱性玄武岩Ce、Nd同位素特征。
(2)对新元古代黄陵中酸性侵入体(包括黄陵庙、三斗坪和大老岭)和基性侵入体中具不同程度Ce异常的样品进行了Ce同位素分析。在TAS岩性判别图上,基性侵入岩体具有辉长岩-闪长岩的岩性组合特征,中酸性侵入岩体具有辉长闪长岩-闪长岩-花岗闪长岩的岩性组合。在SiO2-K2O关系图上,基性岩体显示低钾拉斑玄武质特征,中酸性侵入岩体显示中钾钙碱性岩石的特征。基性岩中除1件样品Ce异常不明显外,其余的Ce/Ce*为0.43-0.87,中酸性侵入岩的Ce/Ce*范围为0.79-1.08。在REE球粒陨石标准化图中,多数基性岩样品表现为La略有富集的近水平分布特征,LaN/YbN为1.40-9.80,中酸性岩表现出右倾型REE分布模式,LaN/YbN为6.45-43.1。以t=820Ma计算,基性岩Sr同位素初始比值(87Sr/86Sr)t为0.702663-0.704831、ENd(t)值为+1.95-+7.76,相应的模式年龄(T2DM)为0.83-1.24Ga,指示了相对年轻的亏损地幔源区性质。中酸性侵入岩相对高的Sr同位素初始比值(0.704381-0.708734)和负εNd(t)值(-3.86--19.67),相应较老的模式年龄(1.64-2.72Ga),反映出重熔地壳物质的源区特征。
中酸性岩样品εCe(t)值的变化范围为-1.18-+3.22,而基性岩样品的εCe(t)值具有-6.89-+7.58的较大变化范围。在εCe-εNd关系图上,黄陵庙单元样品沿“地壳分布”线附近分布,指示了偶合的Ce-Nd同位素组成,反之,大老岭、三斗坪单元样品和基性岩样品则明显偏离“地壳分布”线,表现为在相近的ENd(t)值条件下,εCe(t)值呈较大范围变化,显示出不同程度的Nd-Ce同位素脱偶现象。在εCe(t)-Ce/Ce*图上,基性岩体样品表现为正相关性,指示了该岩体的负Ce异常形成于岩浆结晶以后的后期蚀变或变质；相反,中酸性侵入岩ECe(t)与Ce/Ce*之间不存在明显的相关性,表明其负Ce异常由岩浆作用所致,即岩浆上侵过程中伴随了明显的向强氧化环境的演化。
微量元素和矿物学特征也表明基性侵入体样品经历了后期流体交代蚀变。样品的蚀变年龄TCe的计算结果显示,蚀变事件的平均年龄≥350Ma。前人的研究表明,黄陵侵入杂岩的中酸性侵入岩体主要由太古宙-古元古代高级变质基底岩系部分熔融形成,但这些基底岩系缺乏明显的负Ce异常(Ce/Ce*=0.92-1.12),说明中酸性岩体的负Ce异常并非继承了源岩的特征,而中下地壳还原条件下的部分熔融也不能导致负Ce异常,因此,中酸性侵入体的负Ce异常指示了岩浆侵位过程中的氧逸度增高,即岩浆的结晶分异发生于地壳浅部氧化环境,该现象可能与岩浆作用过程中区域地壳的快速抬升有关。
(3)对三峡地区南华纪莲沱组-南沱组、震旦纪陡山沱组-灯影组和寒武纪水井沱组泥质粉砂岩、黑色页岩、泥质岩和碳酸岩进行了元素和Sr-Nd-Ce同位素研究。所有样品均具有LREE富集特征(LaN/YbN为6.44-18.1),泥质岩的REE含量低于PAAS。绝大多数样品表现出不同程度的负Eu异常(Eu/Eu*=0.49-0.92)和负Ce异常(Ce/Ce*=0.42-0.91)。
微量元素和同位素组成的判别结果表明,泥质岩样品的负Ce异常负指示了其沉积岩形成时的氧化还原环境,而非后期改造的结果。沿地层剖面,南沱组、陡山沱组顶部和水井沱组底部样品出现Ce/Ce*低值。这些样品所代表的层位附近同时出现明显的Co、Zr、Th、Sc、Cr、Ni等微量元素含量变化,并与前人发现的δ13C负异常现象相对应,指示了发生于南沱冰期、陡山沱期(-550Ma)和寒武纪初期(-542Ma)的缺氧事件。
南华-震旦系和震旦-寒武系边界附近出现的εCe(t)值突然降低现象,与大洋缺氧事件相伴随的大陆岩石的风化剥蚀强度减弱密切相关。大陆岩石风化速率的降低导致大洋盆地沉积物中来自大陆地壳Ce的贡献降低,而幔源物质Ce的贡献相对增加。此外,陡山沱期约580Ma的短期缺氧事件附近εCe(t)值显示出相对于εNd(t)值更明显的波动现象,表现为较低的εCe(t)值,这种现象表明Ce同位素在示踪短期沉积环境变化和沉积物源组成方面更具优势。
南华纪至寒武纪εCe(t)值整体降低趋势虽然与同期海相碳酸岩Sr同位素记录相符,指示了地幔源区贡献不断增加的特征,但Ce同位素组成在地层剖面上出现的复杂的多周期旋迴过程,进一步说明Ce同位素不仅可识别出具全球背景的地质事件,还可对区域性地质事件提供更详细的信息。
Both La-Ce and Sm-Nd isotopic systems are long-lived decay system of light rare earth element (LREE). In contrast to Nd, Ce is a variant-valence element, converting from Ce3+ to Ce4+ in natural oxidizing condition. This feature makes Ce in high valence state separated from other REEs in +3 valence, which results in decoupled Sm-Nd and La-Ce systems in geological units due to elevated 138La/142Ce ratio and high radiogenic Ce isotope. TheεCe-εNd correlation thus has been used to trace paleo-redox conditions in geological history. Although the La-Ce system has shown a wide range of potential applications in the Earth sciences, documented studies remain limited owing to a long half-life (～100Ga) and extremely low abundance (0.08%) of 138La, resulting in a restricted range inεCe (ca.-3 to +5, about one order of magnitude lower than that of sNd) for most geological samples. Thus, high-precise La-Ce isotope analysis is extraordinarily essential.
Most laboratories traditionally useα-hydroxy-isobutyric acid (a-HIBA) eluent to separate Ce from matrix elements, through which interfering element Ba is effectively removed. However, this technique shows disadvantages of time-consuming operation and relatively high procedure blanks in La and Ce. In this thesis, a new chemical protocol is reported for La-Ce sample separation using hydrochloric acid (HC1), replacedα-HIBA, as eluent. Prepared samples by this protocol were measured by an up-to-date TIMS (Triton Ti), and the collected data were treated using off-line software designed by the present authors. Samples with variable Ce anomalies collected from the Neoproterozoic Huangling intrusive complex and the Neoproterozoic to Cambrian sedimentary strata from the eastern Three Gorges were analyzed using the analytical method mentioned above. Along with their trace element and Sr-Nd isotope features, regional tectonic regime of the Neoproterozoic magmatism and the evolution trends of depositional environment from the Neoproterozoic to Cambrian were discussed. The major experimental works and conclusions of the application research are listed below:
(1) A new chemical protocol of Ce separation and purification using HCl elution for geological samples, along with corresponding TIMS analytical procedure, were established in the State Key Laboratory of Geological Process and Mineral Resources. The chemical protocol comprises steps of REEs separation from matric elements, and La and Ce purifications. During the first step, cation-exchange resin (AG 50W-X12) and 1.5N HNO3 are used to wash off Ba and other matrix elements before REEs collection by 6.ON HCl eluent. The La and Ce collection and purification are realized using the Ln-spec resin and 0.18N HCl eluent. For a higher degree of purification, the second step is repeated for the collected Ce fraction. The whole procedure shows a recovery>80% and a Ce blank<2.8ng.
The Ce isotopic data are obtained by TIMS measurement followed by an off-line data processing. Ce isotope ratios were measured under an ion status of oxide species (CeO+). During the measurement, electric current of the evaporation filament was set at a range of 1200-1400mA for a time until the isobaric interference from 138Ba is eliminated by burning off. The isotopic data are then collected when the evaporation current is increased to 1500-1800mA and the monitored 160/158 (144NdO/142CeO) ratio is less than 0.002054, indicating no NdO+ interfering. The final Ce isotopic composition is calculated by deoxy and interference corrections to the raw data. During the deoxy correction, the real-time 18O/16O (i.e. R18) ratios were estimated by the measured 160/158 ratios, instead of using a constant value measured at a room temperature, which is usually different from those within the TIMS'ion source at high-temperature varying from sample to sample. A weighted average Rig value of 0.001986 was obtained during the analyses. For a mass discrimination correction, the 138Ce/142Ce and 140Ce/142Ce ratios were normalized by 136Ce/142Ce=0.01688.
Long-term analysis of the Reference JMC304 (Ce isotope standard liquid) gave a weighted average 138Ce/142Ce ratio of 0.0225780±0.0000012 (2σm, the same below), which agrees well with the documented values in literature within analysis error. A Chinese Reference basalt rock (EQB) for La-Ce isotopic chronology was also analyzed and gave a mean 138Ce/142Ce ratio of 0.0225743±0.0000010, which remains consistent with its referring value (0.0225755±0.0000003) within analysis error. These results suggest that the established chemical protocol, TIMS analysis procedure and correction programming are reliable and suitable for high-precise La-Ce isotopic analysis in geology. Besides, Ce isotope analysis of the international References BCR-2, BHVO-2 and AGV-2 were also carried out by this study, which firstly reports average 138Ce/142Ce ratios of 0.0225804±0.0000015,0.0225842±0.0000024 and 0.0225781±0.0000011 (correspondingεCe(0) values are 3.6±0.7,5.3±1.1 and 2.6±0.5), respectively. It thus reveals that these standars as well as BCR-1 have Ce-Nd isotope signatures basically resambling those of the Cenozoic alkali basalts in eastern China.
(2) Ce isotope analysis of the samples with variable extents of negative Ce anomaly from the Neoproterozoic Huangling felsic (comprising the Huanglingmiao, Sandouping and Dalaoling suites) and mafic intrusives were carried out. The TAS classification shows that the mafic rocks consist of gabbros with minor diorite, whereas the felsic suites are dominated by gabbrodiorite, diorite and granodiorite. On the SiO2-K2O plot, the mafic suite displays a low-K tholeiitic trend, whereas the felsic rocks show a mid-K calc-alkaline feature. All of the mafic rocks but one exhibit variable negative Ce anomalies with Ce/Ce* values ranging from 0.43 to 0.87, whereas the felsic samples show a range of 0.79-1.08. On the chondrite-normalized REE diagram, most mafic rocks show nearly flat to right-inclined patterns with (La/Yb) N ratios of 1.40 to 9.80, whereas the felsic rocks are featured by LREE enriched patterns with a (La/Yb) N range of 6.45 to 43.1. The mafic samples have initial (820 Ma) Sr isotope ratios of 0.702663-0.704831,εNd(t) values of+1.95 to +7.76 and Nd model ages (T2DM) of 0.83-1.24Ga, suggesting a derivation of young depleted mantle source. In contrast, the felsic suites display evidently higher initial Sr isotope ratios of 0.704381-0.708734, negativeεNd(t) values of -19.7 to -3.86 and older Nd model ages of 1.64-2.72Ga, indicative of source rocks dominated by old crustal basement.
The felsic and the mafic suites possesse (t) values of -1.18 to +3.32 and -6.89 to +7.58, respectively. On theεCe-εNd plot, the Huanglingmiao samples scatter near the "crustal array", exhibiting a coupled Ce-Nd isotope correlation. By the contrary, samples from the Dalaoling, Sandouping and the mafic suites display locations obviously apart from the "crustal array", which is featured by an evidently largerεCe(t) variation for a givenεNd(t) value and thus reveals a decoupled Ce-Nd isotope correlation. The mafic rocks show a positively coherentεCe (t)-Ce/Ce* variation, indicating that their negative Ce anomalies stemmed from post-magmatic alteration or metamorphism. In contrast, a coherentεCe(t)-Ce/Ce* relation among the felsic samples is absent, it thus suggests that their negative Ce anomalies were cast during the Neoproterozoic magmatism, indicative of a highly oxidizing trend in environmental evolution during the melt ascent.
This alteration- or metamorphism-related explanation for the Ce anomalies of the mafic rocks is further supported by their trace element and mineralogical signatures. An alteration age (TCe) calculation reveals that the alteration event occurred at>350Ma). It has been suggested that the Huangling felsic intrusive complex was derived from the Archean-Paleoproterozoic basement by anatexis. Given that no negative Ce anomalies are recognized in the basement rocks (0.92-1.12 Ce/Ce*), it is unlikely that the negative Ce anomalies in felsic samples was inherited from their source rocks. Besides, it is well known that partial melting of lower crust occurring under reducing conditions may not lead to a negative Ce anomaly. Thus, it is suggested that the negative Ce anomalies in the felsic rocks indicate an increase of oxygen fugacity during magmatism when the melts ascending towards shallow depth. This explanation implies that the Neoproterozoic magmatism occurred in a tectonic setting of rapid regional crust uplifting.
(3) A comprehensive trace element and Sr-Nd-Ce isotope study was carried out on the fine-grain sedimentary and carbonate rocks from the strata, from the bottom upward, of the Liantuo and Nantuo Formations (the Neoproterozoic Nanhua), the Doushantuo and Dengying Formations (the Neoproterozoic Sinian) and the Shuijingtuo Formation (the latest Cambrian). Both the carbonate and pelite samples are LREE-enriched. Of them, the pelites have LaN/YbN ratios of 6.44-18.1 and lower REE content relative to that of the PAAS. Most pelites display variable negative Eu (0.49-0.92) and Ce anomalies (0.42-0.91).
Trace element and Nd-Sr-Ce isotope features of the pelite rocks suggest that the negative Ce anomalies were developed during their depositional and diagenetic processess, rather than the results from later alteration. Extremely low Ce/Ce* values are recorded in the black shales from the Nantuo Formation, the top Doushantuo Formation and the bottom Shuijingtuo Formation. These samples also show distinct variations in Co, Zr, Th, Sc, Cr and Ni contents relative to the samples from their neighboring strata. It is noted that this evidently elemental change coincides with theδ13C shifts along the strata documented by previous works. These geochemical anomalies are interpreted as indicative of anoxic events occurred at the Nantuo glacial eppoch, the Doushantuo (-550Ma) and the early Cambrian (-542 Ma), respectively.
ObviousεCe(t) decrease near the Nanhua-Sinian and Sinian-Cambrian boundaries is inferred to correlate with reductions in weathering intensity of continental rocks during the anoxic events, which resulted a decrease in Ce flux from continents into oceans and thus a relative increase of Ce flux there derived from mantle-related rocks. It is worth to note that a wider range ofεCe (t) variation relative to that of theεNd(t) is recognized from samples collected across the geochemical anomaly boundaries. This suggests that the La-Ce system is more sensitive relative to the Sm-Nd system in tracing variations in depositional environment and sediment provenance.
Furthermore, although the deceaseεCe (t) trend along the strata from the Nanhua to Cambrian matchs the reducing trend in Sr isotopic ratio recorded by the interbedded marine carbonates, which implies an increasing mantle flux into oceans with time. However, much more complicated multi-cycle variations in Ce isotope along the strata profile suggest that the La-Ce system, due to a short oceanic residence time, may provide more detailed information about the regional geological evolution in addition to global events.
目前国内外大多数实验室均沿用a-HIBA法进行La-Ce分离,尽管这种实验方法大大降低了Ba的干扰,但耗时长,且全流程La、Ce本底相对偏高。本文报道作者建立的La-Ce同位素分离的无机酸淋洗法,避免了难纯化有机酸的使用,进而降低了样品分离的流程本底；使用高精度热电离质谱(Triton Ti)对样品的同位素比值进行了测量,并采用自编程序对分析数据进行离线处理。应用所建立的方法对黄陵侵入杂岩体和三峡地区沉积岩具不同程度Ce异常的样品进行了Ce同位素比值分析,结合样品的微量元素和Sr-Nd同位素特征,探讨了区域新元古代岩浆岩的构造背景和地层沉积环境。具体内容包括：
(1)在“地质过程和矿产资源”重点实验室同位素室建立了无机酸淋洗法La-Ce同位素分离方法和相应的质谱测定流程。样品的化学制备过程包括REEs分离和La、Ce分离两个阶段,在REEs分离阶段采用AG 50W-X12树脂和1.5N HNO3将地质样品中的Ba等主量元素完全洗脱后,以6.0N HCl收集REEs;在La、Ce分离阶段,采用LN特效树脂和0.18N HCl淋洗液,洗脱收集样品中的Ce,并对所收集的Ce洗脱液作二次强化分离,目的是进一步提高Ce与La、Nd的分离效率。整个化学制备过程Ce的回收率大于80%,Ce空白值低于2.8ng。
Ce同位素比值分析包括质谱测定和离线数据处理两部分。质谱分析选择氧化物测量方式,具体操作：在CeO+信号出现之前低电流下(1200mA-1400mA)停留较长时间,目的是烧掉残留的BaO+以排除其干扰；数据测量在蒸发带电流1500mA-1800mA范围内完成,测量过程以160(144NdO)/158(142CeO)<0.002054为判断依据监测NdO+的干扰。对所测得的各组氧化物比值依次进行干扰和脱氧校正,计算过程采用的18O/16O比值(R18)根据样品的160/158测量值计算得出,而不采用常数值,这种方法避免了样品间不同同位素分馏程度引起的误差,本研究获得的R18平均值为0.001986，同位素比值完成干扰校正和脱氧校正后,以136Ce/142Ce=0.01688进行归一化计算。
对标准溶液JMC304的长期测量获得的138Ce/142Ce比值平均值为0.0225787±0.0000010,与文献平均值0.0225780±0.0000012在误差范围内一致。以国家岩石标样EQB为本次实验的全流程标样,138Ce/142Ce比值测定结果为0.0225743±0.0000010，与推荐值0.0225755±0.0000003在误差范围内一致。以上结果表明,本研究建立的La-Ce同位素分离实验方法和TIMS质谱分析流程合理,Ce同位素比值测定结果准确可信。此外,本文首次对国际岩石标样BCR-2、BHVO-2与AGV-2进行了分析,其138Ce/142Ce比值分别为0.0225804±0.0000015、0.0225842±0.0000024和0.0225781±0.0000011,εCe(0)值为3.6±0.7、5.3±1.1和2.6±0.5。结合已发表的Nd同位素分析结果,显示三件标样与BCR-1类似,具有中国东部新生代碱性玄武岩Ce、Nd同位素特征。
(2)对新元古代黄陵中酸性侵入体(包括黄陵庙、三斗坪和大老岭)和基性侵入体中具不同程度Ce异常的样品进行了Ce同位素分析。在TAS岩性判别图上,基性侵入岩体具有辉长岩-闪长岩的岩性组合特征,中酸性侵入岩体具有辉长闪长岩-闪长岩-花岗闪长岩的岩性组合。在SiO2-K2O关系图上,基性岩体显示低钾拉斑玄武质特征,中酸性侵入岩体显示中钾钙碱性岩石的特征。基性岩中除1件样品Ce异常不明显外,其余的Ce/Ce*为0.43-0.87,中酸性侵入岩的Ce/Ce*范围为0.79-1.08。在REE球粒陨石标准化图中,多数基性岩样品表现为La略有富集的近水平分布特征,LaN/YbN为1.40-9.80,中酸性岩表现出右倾型REE分布模式,LaN/YbN为6.45-43.1。以t=820Ma计算,基性岩Sr同位素初始比值(87Sr/86Sr)t为0.702663-0.704831、ENd(t)值为+1.95-+7.76,相应的模式年龄(T2DM)为0.83-1.24Ga,指示了相对年轻的亏损地幔源区性质。中酸性侵入岩相对高的Sr同位素初始比值(0.704381-0.708734)和负εNd(t)值(-3.86--19.67),相应较老的模式年龄(1.64-2.72Ga),反映出重熔地壳物质的源区特征。
中酸性岩样品εCe(t)值的变化范围为-1.18-+3.22,而基性岩样品的εCe(t)值具有-6.89-+7.58的较大变化范围。在εCe-εNd关系图上,黄陵庙单元样品沿“地壳分布”线附近分布,指示了偶合的Ce-Nd同位素组成,反之,大老岭、三斗坪单元样品和基性岩样品则明显偏离“地壳分布”线,表现为在相近的ENd(t)值条件下,εCe(t)值呈较大范围变化,显示出不同程度的Nd-Ce同位素脱偶现象。在εCe(t)-Ce/Ce*图上,基性岩体样品表现为正相关性,指示了该岩体的负Ce异常形成于岩浆结晶以后的后期蚀变或变质；相反,中酸性侵入岩ECe(t)与Ce/Ce*之间不存在明显的相关性,表明其负Ce异常由岩浆作用所致,即岩浆上侵过程中伴随了明显的向强氧化环境的演化。
微量元素和矿物学特征也表明基性侵入体样品经历了后期流体交代蚀变。样品的蚀变年龄TCe的计算结果显示,蚀变事件的平均年龄≥350Ma。前人的研究表明,黄陵侵入杂岩的中酸性侵入岩体主要由太古宙-古元古代高级变质基底岩系部分熔融形成,但这些基底岩系缺乏明显的负Ce异常(Ce/Ce*=0.92-1.12),说明中酸性岩体的负Ce异常并非继承了源岩的特征,而中下地壳还原条件下的部分熔融也不能导致负Ce异常,因此,中酸性侵入体的负Ce异常指示了岩浆侵位过程中的氧逸度增高,即岩浆的结晶分异发生于地壳浅部氧化环境,该现象可能与岩浆作用过程中区域地壳的快速抬升有关。
(3)对三峡地区南华纪莲沱组-南沱组、震旦纪陡山沱组-灯影组和寒武纪水井沱组泥质粉砂岩、黑色页岩、泥质岩和碳酸岩进行了元素和Sr-Nd-Ce同位素研究。所有样品均具有LREE富集特征(LaN/YbN为6.44-18.1),泥质岩的REE含量低于PAAS。绝大多数样品表现出不同程度的负Eu异常(Eu/Eu*=0.49-0.92)和负Ce异常(Ce/Ce*=0.42-0.91)。
微量元素和同位素组成的判别结果表明,泥质岩样品的负Ce异常负指示了其沉积岩形成时的氧化还原环境,而非后期改造的结果。沿地层剖面,南沱组、陡山沱组顶部和水井沱组底部样品出现Ce/Ce*低值。这些样品所代表的层位附近同时出现明显的Co、Zr、Th、Sc、Cr、Ni等微量元素含量变化,并与前人发现的δ13C负异常现象相对应,指示了发生于南沱冰期、陡山沱期(-550Ma)和寒武纪初期(-542Ma)的缺氧事件。
南华-震旦系和震旦-寒武系边界附近出现的εCe(t)值突然降低现象,与大洋缺氧事件相伴随的大陆岩石的风化剥蚀强度减弱密切相关。大陆岩石风化速率的降低导致大洋盆地沉积物中来自大陆地壳Ce的贡献降低,而幔源物质Ce的贡献相对增加。此外,陡山沱期约580Ma的短期缺氧事件附近εCe(t)值显示出相对于εNd(t)值更明显的波动现象,表现为较低的εCe(t)值,这种现象表明Ce同位素在示踪短期沉积环境变化和沉积物源组成方面更具优势。
南华纪至寒武纪εCe(t)值整体降低趋势虽然与同期海相碳酸岩Sr同位素记录相符,指示了地幔源区贡献不断增加的特征,但Ce同位素组成在地层剖面上出现的复杂的多周期旋迴过程,进一步说明Ce同位素不仅可识别出具全球背景的地质事件,还可对区域性地质事件提供更详细的信息。
Both La-Ce and Sm-Nd isotopic systems are long-lived decay system of light rare earth element (LREE). In contrast to Nd, Ce is a variant-valence element, converting from Ce3+ to Ce4+ in natural oxidizing condition. This feature makes Ce in high valence state separated from other REEs in +3 valence, which results in decoupled Sm-Nd and La-Ce systems in geological units due to elevated 138La/142Ce ratio and high radiogenic Ce isotope. TheεCe-εNd correlation thus has been used to trace paleo-redox conditions in geological history. Although the La-Ce system has shown a wide range of potential applications in the Earth sciences, documented studies remain limited owing to a long half-life (～100Ga) and extremely low abundance (0.08%) of 138La, resulting in a restricted range inεCe (ca.-3 to +5, about one order of magnitude lower than that of sNd) for most geological samples. Thus, high-precise La-Ce isotope analysis is extraordinarily essential.
Most laboratories traditionally useα-hydroxy-isobutyric acid (a-HIBA) eluent to separate Ce from matrix elements, through which interfering element Ba is effectively removed. However, this technique shows disadvantages of time-consuming operation and relatively high procedure blanks in La and Ce. In this thesis, a new chemical protocol is reported for La-Ce sample separation using hydrochloric acid (HC1), replacedα-HIBA, as eluent. Prepared samples by this protocol were measured by an up-to-date TIMS (Triton Ti), and the collected data were treated using off-line software designed by the present authors. Samples with variable Ce anomalies collected from the Neoproterozoic Huangling intrusive complex and the Neoproterozoic to Cambrian sedimentary strata from the eastern Three Gorges were analyzed using the analytical method mentioned above. Along with their trace element and Sr-Nd isotope features, regional tectonic regime of the Neoproterozoic magmatism and the evolution trends of depositional environment from the Neoproterozoic to Cambrian were discussed. The major experimental works and conclusions of the application research are listed below:
(1) A new chemical protocol of Ce separation and purification using HCl elution for geological samples, along with corresponding TIMS analytical procedure, were established in the State Key Laboratory of Geological Process and Mineral Resources. The chemical protocol comprises steps of REEs separation from matric elements, and La and Ce purifications. During the first step, cation-exchange resin (AG 50W-X12) and 1.5N HNO3 are used to wash off Ba and other matrix elements before REEs collection by 6.ON HCl eluent. The La and Ce collection and purification are realized using the Ln-spec resin and 0.18N HCl eluent. For a higher degree of purification, the second step is repeated for the collected Ce fraction. The whole procedure shows a recovery>80% and a Ce blank<2.8ng.
The Ce isotopic data are obtained by TIMS measurement followed by an off-line data processing. Ce isotope ratios were measured under an ion status of oxide species (CeO+). During the measurement, electric current of the evaporation filament was set at a range of 1200-1400mA for a time until the isobaric interference from 138Ba is eliminated by burning off. The isotopic data are then collected when the evaporation current is increased to 1500-1800mA and the monitored 160/158 (144NdO/142CeO) ratio is less than 0.002054, indicating no NdO+ interfering. The final Ce isotopic composition is calculated by deoxy and interference corrections to the raw data. During the deoxy correction, the real-time 18O/16O (i.e. R18) ratios were estimated by the measured 160/158 ratios, instead of using a constant value measured at a room temperature, which is usually different from those within the TIMS'ion source at high-temperature varying from sample to sample. A weighted average Rig value of 0.001986 was obtained during the analyses. For a mass discrimination correction, the 138Ce/142Ce and 140Ce/142Ce ratios were normalized by 136Ce/142Ce=0.01688.
Long-term analysis of the Reference JMC304 (Ce isotope standard liquid) gave a weighted average 138Ce/142Ce ratio of 0.0225780±0.0000012 (2σm, the same below), which agrees well with the documented values in literature within analysis error. A Chinese Reference basalt rock (EQB) for La-Ce isotopic chronology was also analyzed and gave a mean 138Ce/142Ce ratio of 0.0225743±0.0000010, which remains consistent with its referring value (0.0225755±0.0000003) within analysis error. These results suggest that the established chemical protocol, TIMS analysis procedure and correction programming are reliable and suitable for high-precise La-Ce isotopic analysis in geology. Besides, Ce isotope analysis of the international References BCR-2, BHVO-2 and AGV-2 were also carried out by this study, which firstly reports average 138Ce/142Ce ratios of 0.0225804±0.0000015,0.0225842±0.0000024 and 0.0225781±0.0000011 (correspondingεCe(0) values are 3.6±0.7,5.3±1.1 and 2.6±0.5), respectively. It thus reveals that these standars as well as BCR-1 have Ce-Nd isotope signatures basically resambling those of the Cenozoic alkali basalts in eastern China.
(2) Ce isotope analysis of the samples with variable extents of negative Ce anomaly from the Neoproterozoic Huangling felsic (comprising the Huanglingmiao, Sandouping and Dalaoling suites) and mafic intrusives were carried out. The TAS classification shows that the mafic rocks consist of gabbros with minor diorite, whereas the felsic suites are dominated by gabbrodiorite, diorite and granodiorite. On the SiO2-K2O plot, the mafic suite displays a low-K tholeiitic trend, whereas the felsic rocks show a mid-K calc-alkaline feature. All of the mafic rocks but one exhibit variable negative Ce anomalies with Ce/Ce* values ranging from 0.43 to 0.87, whereas the felsic samples show a range of 0.79-1.08. On the chondrite-normalized REE diagram, most mafic rocks show nearly flat to right-inclined patterns with (La/Yb) N ratios of 1.40 to 9.80, whereas the felsic rocks are featured by LREE enriched patterns with a (La/Yb) N range of 6.45 to 43.1. The mafic samples have initial (820 Ma) Sr isotope ratios of 0.702663-0.704831,εNd(t) values of+1.95 to +7.76 and Nd model ages (T2DM) of 0.83-1.24Ga, suggesting a derivation of young depleted mantle source. In contrast, the felsic suites display evidently higher initial Sr isotope ratios of 0.704381-0.708734, negativeεNd(t) values of -19.7 to -3.86 and older Nd model ages of 1.64-2.72Ga, indicative of source rocks dominated by old crustal basement.
The felsic and the mafic suites possesse (t) values of -1.18 to +3.32 and -6.89 to +7.58, respectively. On theεCe-εNd plot, the Huanglingmiao samples scatter near the "crustal array", exhibiting a coupled Ce-Nd isotope correlation. By the contrary, samples from the Dalaoling, Sandouping and the mafic suites display locations obviously apart from the "crustal array", which is featured by an evidently largerεCe(t) variation for a givenεNd(t) value and thus reveals a decoupled Ce-Nd isotope correlation. The mafic rocks show a positively coherentεCe (t)-Ce/Ce* variation, indicating that their negative Ce anomalies stemmed from post-magmatic alteration or metamorphism. In contrast, a coherentεCe(t)-Ce/Ce* relation among the felsic samples is absent, it thus suggests that their negative Ce anomalies were cast during the Neoproterozoic magmatism, indicative of a highly oxidizing trend in environmental evolution during the melt ascent.
This alteration- or metamorphism-related explanation for the Ce anomalies of the mafic rocks is further supported by their trace element and mineralogical signatures. An alteration age (TCe) calculation reveals that the alteration event occurred at>350Ma). It has been suggested that the Huangling felsic intrusive complex was derived from the Archean-Paleoproterozoic basement by anatexis. Given that no negative Ce anomalies are recognized in the basement rocks (0.92-1.12 Ce/Ce*), it is unlikely that the negative Ce anomalies in felsic samples was inherited from their source rocks. Besides, it is well known that partial melting of lower crust occurring under reducing conditions may not lead to a negative Ce anomaly. Thus, it is suggested that the negative Ce anomalies in the felsic rocks indicate an increase of oxygen fugacity during magmatism when the melts ascending towards shallow depth. This explanation implies that the Neoproterozoic magmatism occurred in a tectonic setting of rapid regional crust uplifting.
(3) A comprehensive trace element and Sr-Nd-Ce isotope study was carried out on the fine-grain sedimentary and carbonate rocks from the strata, from the bottom upward, of the Liantuo and Nantuo Formations (the Neoproterozoic Nanhua), the Doushantuo and Dengying Formations (the Neoproterozoic Sinian) and the Shuijingtuo Formation (the latest Cambrian). Both the carbonate and pelite samples are LREE-enriched. Of them, the pelites have LaN/YbN ratios of 6.44-18.1 and lower REE content relative to that of the PAAS. Most pelites display variable negative Eu (0.49-0.92) and Ce anomalies (0.42-0.91).
Trace element and Nd-Sr-Ce isotope features of the pelite rocks suggest that the negative Ce anomalies were developed during their depositional and diagenetic processess, rather than the results from later alteration. Extremely low Ce/Ce* values are recorded in the black shales from the Nantuo Formation, the top Doushantuo Formation and the bottom Shuijingtuo Formation. These samples also show distinct variations in Co, Zr, Th, Sc, Cr and Ni contents relative to the samples from their neighboring strata. It is noted that this evidently elemental change coincides with theδ13C shifts along the strata documented by previous works. These geochemical anomalies are interpreted as indicative of anoxic events occurred at the Nantuo glacial eppoch, the Doushantuo (-550Ma) and the early Cambrian (-542 Ma), respectively.
ObviousεCe(t) decrease near the Nanhua-Sinian and Sinian-Cambrian boundaries is inferred to correlate with reductions in weathering intensity of continental rocks during the anoxic events, which resulted a decrease in Ce flux from continents into oceans and thus a relative increase of Ce flux there derived from mantle-related rocks. It is worth to note that a wider range ofεCe (t) variation relative to that of theεNd(t) is recognized from samples collected across the geochemical anomaly boundaries. This suggests that the La-Ce system is more sensitive relative to the Sm-Nd system in tracing variations in depositional environment and sediment provenance.
Furthermore, although the deceaseεCe (t) trend along the strata from the Nanhua to Cambrian matchs the reducing trend in Sr isotopic ratio recorded by the interbedded marine carbonates, which implies an increasing mantle flux into oceans with time. However, much more complicated multi-cycle variations in Ce isotope along the strata profile suggest that the La-Ce system, due to a short oceanic residence time, may provide more detailed information about the regional geological evolution in addition to global events.
引文
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