Trace element composition of peridotites from the southern Mariana forearc:Insights into the geochemical effects of serpentinization and/or seafloor weathering

Peridotites from the southern Mariana forearc were sampled on the landward trench slope of the Izu-Bonin-Mariana (IBM) subduction zone by dredging.These mantle wedge peridotites underwent hydration by fluid derived from a dehydrated descending slab,and later interacted with seawater after emplacement at or near the seafloor.This study investigates how these two different rock-fluid interaction processes influenced trace element distribution in the southern Mariana forearc peridotites.We measured trace element concentrations of peridotites from the southern Mariana forearc.The southern Mariana forearc peridotites are characterized by a distinct seawater-like REE pattern with an obvious negative Ce anomaly,and La shows good correlation with other REEs (except Ce).In addition,there is a great enrichment of U,Pb,Sr and Li elements,which show a distinct positive anomaly relative to adjacent elements in the multi-element diagram.For the seawater-like REE pattern,we infer that REEs are mainly influenced by seawater during peridotite-seawater interactions after their emplacement at or near the seafloor,by serpentinization or by marine weathering.Furthermore,the anomalous behavior of Ce,compared with other rare earth elements in these samples,may indicate that they have undergone reactions involving Ce (IV) when the peridotites interacted with seawater.Positive U,Pb,Sr and Li anomalies are inferred to be related to seawater and/or fluids released during dehydration of the subducting slab.

Distribution of rare earth elements of granitic regolith under the influence of climate

The distribution and anomalies of rare earth elements(REEs) of granitic regolith were studied in Inner Mongolia and Hainan Island, China. One profile showed slight REE enrichment of an upper layer and no obvious light REE/heavy REE(LREE/HREE) fractionation(La_N/Yb_N of 0.9). The second profile was significantly enriched in REEs and enriched in LREEs in the upper portion(La_N/Yb_N>1.8). Eu, Ce, and Gd anomalies of the two profiles are different. Slightly negative Eu, Ce, and Gd anomalies in NMG-3-1 indicate slow dissolution of primary minerals and little secondary products; in contrast, a positive Eu anomaly in HN-2 suggests the vegetation cycle may contribute to soil. The Ce anomaly of HN-2 reflects oxidation of Ce and coprecipitation by Fe-and Mn-oxides and organic matter. Correlation between Ce and Gd anomalies in HN-2 suggests Ce and Gd are both influenced by redoxreduction.

Petrologic and REE Geochemical Characters of Burnt Rocks

The study of burnt rocks is beneficial to the discussion on the tectonic movement, paleoclimate and paleogeography that coal seams are subjected to after they were formed. In order to obtain the basic data on the features of the burnt rocks, a systematic study of petrology and REE geochemistry on burnt rocks in Shenmu, Northern Shaanxi Province has been done, using the methods of SEM, EDS, susceptibility measurements and ICP-MS. The burnt rocks are divided into two series in the section： the melted rocks and the baked rocks. SEM and EDS analyses reveal that all the minerals show burnt and melted traces, and there are no clay minerals except illite found in the burnt rocks. Susceptibility measurements reveal that the burnt rocks have abnormally high susceptibility values, whereas a geochemical analysis shows that the REE distribution pattern of burnt rocks is similar to that of sedimentary rocks （initial rocks）. In the longitudinal section, with increasing degree of burning （from baked rocks to melted rocks）, the ∑REE gradually decreases, and the total REE of melted rocks is obviously lower than that of baked rocks. Besides, the melted rocks show apparent negative Ce anomalies, while the baked rocks show no anomaly of Ce, and sometimes even show positive anomalies.

Ce Anomalies of the Yangtze Region,South China,through the Ordovician and Silurian Transition

Systematic Ce anomalies for whole-rock have been obtained from the shale-dominated, continuous, and pelagic sedimentary sequences spanning the Ordovician/Silurian （O/S） boundary at the Tieshui （铁水） of Xiushan （秀山）, Chongqing （重庆）, South China. Ce anomalies across the O/S boundary are recognized in three intervals, Wufeng （五峰）, Guanyinqiao （观音桥） and Longmaxi （龙马溪）. The calculated Ce/Ce＊ values of Wufeng Formation range from 0.84 to 0.96 （avg. 0.90）. In the Guanyinqiao Formation, the values of calculated Ce/Ce＊ range from 0.73 to 0.85 （avg. 0.79）. The Ce/Ce＊ values of uppermost Longmaxi Formation range from 0.87 to 0.96 （avg. 0.91）. All along the section, the magnitude of the Ce anomaly is always negative, but is more significant in the Guanyinqiao Formation. The relatively higher Ce/Ce＊ values in the Wufeng and Longmaxi shales are likely to be due to the sediments deposited under rather reducing conditions. The Ce anomaly apparently does play some regular roles in the anoxic events that accompany prominent mass extinctions, and this work provides new data of critical importance for constraining models on the end-Ordovician anoxic events and mass extinctions.

Decoupled Ce-Nd Isotopic Systematics of the Neoproterozoic Huangling Intrusive Complex and Its Geological Significance, Eastern Three Gorges, South China

Cerium is one of multivalent rear earth elements, which can transfer from trivalence to tretavalence at oxidizing environment. This process may cause variable degrees of fractionation of Ce from other trivalent rear earth elements, and thus may provide specific insight into the geological processes associated with marked redoxomorphism. Multiple geochemical tracing of Sr-Nd-Ce isotopes are performed on the felsic and mafic intrusives of the Neoproterozoic（~800 Ma） Huangling complex located at the eastern Three Gorges, South China. The intrusive rocks exclusively show various extents of negative Ce anomalies. On the εCe-εNd plot, most samples from the mafic intrusions scatter within the second quadrant, whereas those from the felsic intrusions within the fourth Quadrant. Both of the two groups exhibit relatively large range of ？Ce（t） variation but limited ？Nd（t） range, which cause a deviation from the ＂crustal array＂ and reveal a decoupled Nd-Ce isotope correlation. The intermediate-felsic suite have varied Ce/Ce＊ ratios but broadly proximate εCe（t） values, indicating that their negative Ce anomalies were generated during the magmatism; on the contrary, a positive correlation between εCe（t） and Ce/Ce＊ is observed for the intermediate-mafic suite, an indication of an origin of post-magmatic alteration or metamorphism for their Ce anomalies. Calculation of model age, the occurrence age of negative Ce anomalies（TCe） for the intermediate-mafic samples infers that the alteration events took place 〉350 Ma. Data showed that negative Ce anomalies of the felsic intrusions may reflect an increase of oxygen fugacity during magma ascending, rather than an inheritance from their source rocks. This explanation implies that the Neoproterozoic magmatism occurred at the continental nucleus of the Yangtze block were developing at a geodynamic context of rapidly regional uplifting.

Rare earth element(REE) geochemistry of phosphorites of the Sonrai area of Paleoproterozoic Bijawar basin,Uttar Pradesh,India

The rare earth element（REE） data from the Paleoproterozoic Bijawar basin,Sonrai phosphorites were used to interpret the depositional conditions of the phosphorites.The post archean Australian shales（PAAS） normalized REE patterns of the Sonrai phosphorites were characterized by negative Ce and positive Eu anomalies.Middle rare earth elements（MREE）-enrichment was a characteristic feature.Phosphorites showing the diagenetic effects on the REE patterns were limited.The observed Eu anomaly was indicative of an anoxic（or sulphate reducing） diagenetic environment of phosphate formation.Mixing of sea water and upwelling during the Paleoproterozoic was responsible for the recording of positive Eu and negative Ce anomalies in the Sonrai phosphorites.