Solubility controlled noble gas fractionation during magmatic degassing: Implications for noble gas compositions of primary melts of OIB and MORB

Noble gas abundances in basaltic glasses from ocean islands (OIBs) are generally lower than those of mid-oceanic ridge basalts (MORBs), contrary to most geodynamic models which usually require that the source of OIBs is less degassed (resulting in higher primordial noble gas abundances) and more trace element enriched (resulting in higher radiogenic noble gas abundances) than the MORB source. Therefore, noble gas abundances in OIBs are often thought to have been reduced by extensive gas loss from the magma before eruption.The extent of magmatic degassing can be tested as it will cause characteristic changes in the composition of the volatiles; notably the ⁴He/⁴⁰Ar* ratio (where ⁴⁰Ar* is ⁴⁰Ar corrected for atmospheric contamination) will increase in residual volatiles due to the higher solubility of He relative to Ar. The degree of He-Ar fractionation for a given fraction of gas loss depends on the ratio of the solubilities, S_He/S_Ar, which is sensitive to (among other things) the CO₂ and H₂O content of the basalt at the time of degassing.From a global database of OIB and MORB glasses, we show that ⁴He/⁴⁰Ar* ratios of MORB glasses are broadly consistent with degassing of a magma with an initial ⁴⁰Ar of ≈1.5 × 10⁻⁵ ccSTP/g, i.e., similar to that of the “popping rock.” However, OIB glasses generally have lower ⁴⁰Ar* concentration for a given ⁴He/⁴⁰Ar*. While this would appear to require lower ⁴⁰Ar* abundances in the undegassed OIB magmas, the higher volatile contents of OIBs will reduce S_He/S_Ar (relative to MORBs) during degassing. By modeling S_He/S_Ar in OIBs, it is possible to show that extensive degassing of OIBs can occur without dramatically increasing the ⁴He/⁴⁰Ar* ratio. We show that undegassed ⁴⁰Ar concentrations of OIB magmas were probably similar to those of MORBs.     

Geochemistry and origin of Archean volcanic rocks from the Upper Keewatin assemblage (ca 2.7 Ga), Lake of the Woods Greenstone Belt, Western Wabigoon Subprovince, Superior Province, Canada

Abstract   Volcanic rocks from the Upper Keewatin assemblage ( ca 2720 Ma) were geochemically classified into five groups; komatiites, tholeiitic rocks having near-flat primitive mantle-normalized abundance patterns, Nb-enriched basalts and andesites (NEBA) plus normal calc-alkaline (NCA) rocks, adakites and shoshonites. The adakites having [La/Yb]_N >30 and <30 were probably derived from felsic magmas formed by partial melting of a subducted slab at relatively greater and smaller depths, respectively. Ascending adakite magmas, by interaction with the overlying mantle wedge, decreased in Al₂O₃ / Y ratio and selectively lost high-field strength elements, thereby forming mantle sources for both NEBA + NCA and shoshonite magmas. Under the influence of a mantle plume, the source of komatiites, the NEBA + NCA magmas were generated from that part of the mantle wedge metasomatized by adakite magmas having [La / Yb]_N <30, and tholeiitic magmas from unmetasomatized part of the same mantle wedge. Magmas of both adakites having [La / Yb]_N >30 and shoshonites were generated in a normal Archean Arc system setting.     

汾渭平原典型城乡PM2.5中多环芳烃特征与健康风险

为查明汾渭平原典型城乡地区重度污染天气PM_2.5中多环芳烃（PAHs）污染特征及其人群健康效应,本文于2018—2019年冬季分别选取西安和陇县作为城乡对比参照点,采集了重度污染天气PM_2.5颗粒态气溶胶样品。利用气相色谱—质谱联用仪（GC-MS）检测样品中具有“三致效应”的15种PAHs含量及组分特征,使用特征比值法及主成分法进行PAHs源解析,并分析了气象因素对PAHs质量浓度的可能影响,通过对苯并芘（BaP）等效毒性浓度和终生超额致癌风险度（ILCR）的计算,对人群健康风险进行评估。结果表明：西安与陇县在重度污染天气条件下PM_2.5中15种PAHs总平均质量浓度分别为243.78 ng/m³、609.39 ng/m³,其中4~6环PAHs占比最高;且PAHs浓度与气温、气压及风速呈显著负相关,与相对湿度则无明显相关性。西安PAHs污染主要来自燃烧源与交通排放源,而煤炭及生物质燃烧是造成陇县PAHs质量浓度偏高的主要原因。健康风险评估结果显示,重污染天气下陇县人群通过呼吸引发的致癌风险要高于西安,女性致癌风险高于男性,成人致癌风险高于儿童,且两地区成人ILCR值均超过风险阈值,存在潜在致癌风险,儿童则无明显致癌风险。     

Measuring and modeling black carbon (BC) contamination in the SE Tibetan Plateau

Black carbon (BC) concentrations were measured in the southeast (SE) Tibetan Plateau along the valley of the Yarlung Tsangpo River during winter (between November, 2008 and January, 2009). The measured mean concentration (0.75 μg m⁻³) is significantly higher than the concentrations (0.004–0.34 μg m⁻³) measured in background and remote regions of the globe, indicating that Tibetan glaciers are contaminated by BC particles in the Plateau. Because BC particles play important roles for the climate in the Tibetan Plateau, the sources and causes of the BC contamination need to be understood and investigated. In this study, a mesocale dynamical model (WRF) with BC particle modules is applied for analyzing the measurement. The analysis suggests that the major sources for the contamination in the SE Plateau were mainly from the BC emissions in eastern Indian and Bangladesh. Because of the west prevailing winds, the heavy emissions in China had no significant effects on the SE Plateau in winter. Usually, the high altitude of the Himalayas acts a physical wall, inhibiting the transport of BC particles across the mountains to the plateau. This study, however, finds that the Yarlung Tsangpo River valley causes a 'leaking wall', whereby under certain meteorological conditions, BC particles are being transported up onto the glacier. This too causes variability of BC concentrations (ranging from 0.3 to 1.5 μg m⁻³) in a time scale of a few days. The analysis of the variability suggests that the “leaking wall” effect cannot occur when the prevailing winds were northwest winds, during which the BC transport along the valley of the Yarlung Tsangpo River was obstructed. As a result, large variability of BC concentration was observed due to the change of prevailing wind directions.     

Experimental Study Assessing the Role of Sedimentary Organic Materials to Control the Redox State of Ground Materials to Control the Redox State of Groundwater: Consumption of Dissolved Oxygen by Humic Acid

Abstract: A total of 189 runs were completed to determine the rate of dissolved oxygen consumption in water by the reaction humic acid at 25°, 45°, and 65°C. Glass vial bottles were used as reaction vessels, and humic aicd and pure water of different intitial O₂ concentration was loaded with humic acid/water weight of 0.008 and 0.009. Vials were opened periodically to measure O₂ concentrations. The pH of the solution fluctuated in the early stages of the experiments and gradually decreased to a constant value. Dissolved oxygen concentration decreased, for example, from initial value of 12 mg/l to 9 mg/l at 25°C, to 5 mg/l at 45°C, and to 2 mg/l at 65°C after the reaction of 500 hours. Analyses of rates suggest that the reaction is first order, and rate constants are 8.42 × 10^-9 at 25°C, 2.22 × 10^-8 at 45°C, and 1.28 × 10^-7 moles m^-2 s^-1 at 65°C.     

地壳表层砷的循环与污染地下水模式

为了更好地监测和预测地下水砷污染,揭示由自然原因引起的地下水砷污染规律和机制,通过多年来对内蒙古河套平原地下水砷污染的调查研究,在充分了解和掌握砷的化学与地球化学性质以及世界各国地下水砷污染状况与分布特征的基础上,总结了砷在自然环境中的分布和污染地下水的机理。文章阐述了4种地下水砷污染类型的形成机理,并提出了地壳表层砷的循环与污染地下水模式,通过该模式在基本掌握某一地区地质环境背景的情况下,可以初步预测其可能发生地下水砷污染的地段。地下水砷污染是砷在地壳表层循环过程中发生的一种环境水文地球化学现象,污染区的分布与特定的地质、地理背景和环境条件有关。在还原解吸污染类型中,封闭的环境、硫元素和有机碳的存在是地层中砷溶出的主要控制因素。今后应加强对封闭的冲积或湖积平原、河流入湖或入海三角洲等地区地下水砷污染的调查与监测。