Acta Geochimica (v.35, #1)

The Moon by Ross Taylor (1-13).
Recent geochemical and geophysical data from the Moon enable a revision of earlier interpretations regarding lunar origin, structure and bulk composition. Earth and Moon show many similarities among their isotopic compositions, but they have evolved in totally dissimilar ways, probably related to the deficiency of water and volatile elements in the Moon as well as the vast differences in size and internal pressure. Some global geochemical differences from the Earth such as volatile depletion based on K/U ratios have been established. However, all current lunar samples come from differentiated regions, making the establishment of a bulk composition more reliant on bulk geophysical properties or isotopic similarities; it remains unclear how the latter arose or relate to whole Moon composition. The lack of fractionation effects among the refractory and super-refractory elements indicates that the proto-lunar material seems unlikely to have been vaporized while the presence of volatile elements may place lower limits on proto-lunar temperatures. The apparent lack of geochemical evidence of an impacting body enables other possible impactors, such as comets, to be considered. Although the origin of the Moon remains currently unknown, it is generally believed that the Moon originated as the result of a giant impact on the Earth.
Keywords: Lunar origin; Grand tack model; Apollo; Volatite elements

The Taylor Moon by Wei-dong Sun (14-14).

Several important equilibrium Si isotope fractionation factors among minerals, organic molecules and the H4SiO4 solution are complemented to facilitate the explanation of the distributions of Si isotopes in Earth’s surface environments. The results reveal that, in comparison to aqueous H4SiO4, heavy Si isotopes will be significantly enriched in secondary silicate minerals. On the contrary, quadra-coordinated organosilicon complexes are enriched in light silicon isotope relative to the solution. The extent of 28Si-enrichment in hyper-coordinated organosilicon complexes was found to be the largest. In addition, the large kinetic isotope effect associated with the polymerization of monosilicic acid and dimer was calculated, and the results support the previous statement that highly 28Si-enrichment in the formation of amorphous quartz precursor contributes to the discrepancy between theoretical calculations and field observations. With the equilibrium Si isotope fractionation factors provided here, Si isotope distributions in many of Earth’s surface systems can be explained. For example, the change of bulk soil δ30Si can be predicted as a concave pattern with respect to the weathering degree, with the minimum value where allophane completely dissolves and the total amount of sesqui-oxides and poorly crystalline minerals reaches their maximum. When, under equilibrium conditions, the well-crystallized clays start to precipitate from the pore solutions, the bulk soil δ30Si will increase again and reach a constant value. Similarly, the precipitation of crystalline smectite and the dissolution of poorly crystalline kaolinite may explain the δ30Si variations in the ground water profile. The equilibrium Si isotope fractionations among the quadra-coordinated organosilicon complexes and the H4SiO4 solution may also shed light on the Si isotope distributions in the Si-accumulating plants.
Keywords: Si isotopes; Equilibrium fractionation factor; Quantum chemistry calculation; Cluster model; Kinetic isotope effect

Economic concentrations of Fe–Ti oxides occurring as massive layers in the middle and upper parts of the Hongge intrusion are different from other layered intrusions (Panzhihua and Baima) in the Emeishan large igneous province, SW China. This paper reports on the new mineral compositions of magnetite and ilmenite for selected cumulate rocks and clinopyroxene and plagioclase for basalts. We use these data to estimate the oxidation state of parental magmas and during ore formation to constrain the factors leading to the abundant accumulation of Fe–Ti oxides involved with the Hongge layered intrusion. The results show that the oxygen fugacities of parental magma are in the range of FMQ−1.56 to FMQ+0.14, and the oxygen fugacities during the ore formation of the Fe–Ti oxides located in the lower olivine clinopyroxenite zone (LOZ) and the middle clinopyroxenite zone (MCZ) of the Hongge intrusion are in the range of FMQ−1.29 to FMQ−0.2 and FMQ−0.49 to FMQ+0.82, respectively. The MELTS model demonstrates that, as the oxygen fugacity increases from the FMQ−1 to FMQ+1, the proportion of crystallization magnetite increases from 11 % to 16 % and the crystallization temperature of the Fe–Ti oxides advances from 1134 to 1164 °C. The moderate oxygen fugacities for the Hongge MCZ indicate that the oxygen fugacity was not the only factor affecting the crystallization of Fe–Ti oxides. We speculated that the initial anhydrous magma that arrived at the Hongge shallow magma chamber became hydrous by attracting the H2O of the strata. In combination with increasing oxygen fugacities from the LOZ (FMQ−1.29 to FMQ−0.2) to the MCZ (FMQ−0.49 to FMQ+0.82), these two factors probably account for the large-scale Fe–Ti oxide ore layers in the MCZ of the Hongge intrusion.
Keywords: Oxygen fugacity; Fe–Ti oxide deposit; Basalts; Emeishan large igneous province; Hongge layered intrusion

Determination of rhenium and osmium by ICP-MS for galena and sphalerite by Yingying Liu; Liang Qi; Jianfeng Gao; Zhilong Huang (43-49).
Digestion with aqua regia in a Carius tube and separation of Re with anion exchange resin is commonly employed for Re–Os dating of molybdenite and pyrite. However, the recovery of Re is extremely low when this routine anion exchange method is applied to galena, causing difficulty in Re–Os dating of galena. In this study, we investigated the mechanism of Re loss during sample preparation and tested a revised procedure for Re–Os dating of galena and sphalerite.
Keywords: Galena; Sphalerite; Rhenium and osmium; Anion exchange resin; ICP-MS

There are abundant bitumens and oil seepages stored in vugs in a Lower-Triassic Daye formation (T1d) marlite in Ni’erguan village in the Southern Guizhou Depression. However, the source of those oil seepages has not been determined to date. Multiple suites of source rocks of different ages exist in the depression. Both the oil seepages and potential source rocks have undergone complicated secondary alterations, which have added to the difficulty of an oil-source correlation. For example, the main source rock, a Lower-Cambrian Niutitang Formation (Є1n) mudstone, is over mature, and other potential source rocks, both from the Permian and the Triassic, are still in the oil window. In addition, the T1d oil seepages underwent a large amount of biodegradation. To minimize the influence of biodegradation and thermal maturation, special methods were employed in this oil-source correlation study. These methods included catalytic hydropyrolysis, to release covalently bound biomarkers from the over mature kerogen of Є1n mudstone, sequential extraction, to obtain chloroform bitumen A and chloroform bitumen C from the T1d marlite, and anhydrous pyrolysis, to release pyrolysates from the kerogen of T1d marlite. Using the methods above, the biomarkers and n-alkanes released from the oil samples and source rocks were analysed by GC–MS and GC-C-IRMS. The oil-source correlation indicated that the T1d oil seepage primarily originated from the Є1n mudstone and was partially mixed with oil generated from the T1d marlite. Furthermore, the seepage also demonstrated that the above methods were effective for the complicated oil-source correlation in the Southern Guizhou Depression.
Keywords: Oil seepage; Biomarker; Carbon isotopic composition; Catalytic hydropyrolysis; Oil-source correlation

The influence of three mangrove species on the distribution of inorganic nitrogen and phosphorus in the Quanzhou Bay estuarine wetland soils by Guiyao Zhou; Yanyou Wu; Deke Xing; Mingming Zhang; Rui Yu; Weiyi Qiao; Qaiser Javed (64-71).
This study aims to investigate the effects of region and three regional dominated mangrove species (Avicennia marina, Aegiceras corniculatum and Kandelia candel) on the distribution of inorganic nitrogen and phosphorus. Measurement of the inorganic nitrogen and phosphorus and enzymatic activities was carried out in soils covered by three mangrove species in the Quanzhou Bay estuarine wetlands, a typical coastal wetland in China. Species with a higher biomass in upstream and midstream absorb more nitrogen from soils, and the retention of the available phosphorus in the soils of different regions causes the regional variation of phosphorus. In areas dominated by A. marina, nitrate nitrogen is lower while available phosphorus is higher. Meanwhile, nitrate nitrogen and available phosphorus are higher in the soils covered by K. candel. Moreover, all three species affect the elemental and enzymic stoichiometry. The mangrove species influences the diversity of the elemental and enzymic stoichiometric relationship through differential microenvironments, which induce the biodiversity of wetland ecosystems. Thus, this study may facilitate a better understanding of the transformation ability of mangroves to nitrogen and phosphorus and will therefore be beneficial for providing a basis for the ecological restoration of estuarine wetlands.
Keywords: Ecological restoration; Estuarine wetland; Mangrove; Nutritional elements

Numerical simulation of groundwater in karst areas has long been restricted by the difficulty of generalizing the hydrogeological conditions of reservoirs and of determining the relevant parameters due to the anisotropy and discontinuity of the karst water-bearing media in these areas. In this study, we used the Guang’an Longtan Coal mine in Sichuan as an example, and generalized the complex hydrogeological conditions in the reservoir area. A finite element numerical flow model was used to simulate current and future scenarios of roadway gushing at the bottom of the coal mine at pile number 1 + 700 m. The results show that the roadway section corresponding to valleys has a gushing quantity of 4323.8–4551.25 m3/d before impoundment. Modeled water inflow after impoundment increased to 1.6 times the water inflow before impoundment, which threatens the impoundment as well as the roadway’s normal operation. Therefore, roadway processing measures are needed to guarantee the safety of the impoundment and of the mining operation.
Keywords: Karst reservoir; Roadway gushing; Numerical simulation; Hydrogeological conditions generalization

Characteristics and distributions of humic acids in two soil profiles of the southwest China Karst area by Liangang Ma; Baohua Xiao; Xinyue Di; Weilin Huang; Shijie Wang (85-94).
Characteristics and distributions of humic acid (HA) and soil organic matter (SOM) in a yellow soil profile and a limestone soil profile of the southwest China Karst area were systematically investigated to reveal their evolutions in different soils of the study area. The results showed that characteristics and distribution of SOM along the two soil profiles were notably different. Total organic carbon (TOC) contents of soil samples decreased just slightly along the limestone soil profile but sharply along the yellow soil profile. TOCs of the limestone soils were significantly higher than those of the corresponding yellow soils, and C/N ratios of SOMs showed a similar variation trend to that of TOCs, indicating that SOM can be better conserved in the limestone soil than in the yellow soil. The soil humic acids were exhaustively extracted and further fractionated according to their apparent molecular weights using ultrafiltration techniques to explore underlying conservation mechanisms. The result showed that C/N ratios of HAs from different limestone soil layers were relatively stable and that large molecular HA fractions predominated the bulk HA of the top soil, indicating that HA in the limestone profile was protected while bio and chemical degradations were retarded. Combined with organic elements contents and mineral contents of two soils, we concluded that high calcium contents in limestone soils may play a key role in SOM conservation by forming complexation compounds with HAs or/and enclosing SOMs with hypergene CaCO3 precipitation.
Keywords: Soil; Soil organic matter; Humic acid; Characteristics; Distribution; Southwest China Karst area

Organic matter enrichment in the first member of the Xiagou formation of the lower Cretaceous in the Jiuquan Basin, China by Lixin Pei; Gang Gao; Wenzhe Gang; Zhilong Huang; Guofu Ma; Zhiming Yang; Jianjun Chen (95-103).
The sources and enrichment of organic matter in a sediment core in the first member of the Xiagou Formation (K1g1) from the Chang 2-2 borehole of the Jiuquan Basin, NW China, have been examined using Rock–Eval, maceral, carbon isotopes and biomarker data. This data indicates that highly variable organic matter sources and preservation conditions in response to climate change. TOC content, HI, and δ13C value were strongly correlated with the abundance of gammacerane, woody organic matter content, steranes/hopanes ratio, and C29 sterane content. This correlation demonstrates the importance that the control of the salinity of the depositional environment and organic matter sources can have upon the enrichment, type, and carbon isotopic composition of organic matter. In the Jiuquan Basin’s relatively high temperature and arid climate, high salinity lakes with high primary productivity of algae, planktons, and bacteria, and good organic matter preservation conditions (anoxic bottom water) resulted in the enrichment of isotopically-light algae-bacterial organic matter. In the Jiuquan Basin’s regions with a relatively low temperature and wet climate, fresh lakes with low primary productivity of algae, planktons, and bacteria received significant terrigenous high plants input, resulting in the deposition of a low abundance of isotopically heavier terrestrial organic matter.
Keywords: Enrichment; Organic matter; Lacustrine; Lower Cretaceous; Jiuquan Basin