La-Ce Isotope Analytical Method And Its Application To Oxidizing Environment Study During Magmatic Intrusion And Sedimentary Processes

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 possesse (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.