American Geophysical Union (AGU)

In this study we discuss the occurrence of mantle‐derived heat and volatiles (i.e., helium and CO₂) feeding hydrothermal systems in a seismically active margin between two convergent plates (African and European) without any signals of volcanism. The helium (He) isotopes clearly indicate a mantle‐derived component in the outgassing volatiles. The estimated mantle‐derived He fluxes are up to two to three orders of magnitude greater than those in a stable continental area. Such high He fluxes cannot be provided by a long‐lasting diffusion, thereby implying a more efficient transport (i.e., advective transport through faults). He data coupled to heat‐He relationship suggest the occurrence of active degassing of magmatic intrusions in this area of continental collisional. Geophysical data indicate the presence of a hot mantle wedge below the outgassing of mantle volatiles and a system of faults cutting the continental crust down to the hot mantle wedge. Here we discuss the hot mantle wedge and possible associated magmatic intrusions as the source of the mantle‐derived volatiles outgassing in the region. We also assessed the output of mantle‐derived CO₂ from the investigated hydrothermal basins. The possible occurrence of magma at depth as well as the geometry of the thick‐skinned deformed wedge unambiguously indicates delamination processes that are related to continental subduction. Hence, we show that delamination processes can really produce magma at depth without evidences of volcanism at the surface. Finally, we have also provided the fault systems that work as a network of pathways and actively sustain the advective transfer of the mantle fluids toward the surface.

We report on the results of an extensive geochemical survey of fluids released in the Vardar zone (central‐western Serbia), a mega‐suture zone at the boundary between Eurasia and Africa plates. Thirty‐one bubbling gas samples are investigated for their chemical and isotopic compositions (He, C, Ar) and cluster into three distinct groups (CO₂‐dominated, N₂‐dominated, and CH₄‐dominated) based on the dominant gas species. The measured He isotope ratios range from 0.08 to 1.19 Ra (where Ra is the atmospheric ratio), and reveal for the first time the presence of a minor (<20%) but detectable regional mantle‐derived component in Serbia. δ¹³C values range from −20.2‰ to −0.1‰ (versus PDB), with the more negative compositions observed in N₂‐dominated samples. The carbon‐helium relationship indicates that these negative δ¹³C compositions could be due to isotopic fractionation processes during CO₂ dissolution into groundwater. In contrast, CO₂‐rich samples reflect mixing between crustal and mantle‐derived CO₂. Our estimated mantle‐derived He flux (9.0 × 10⁹ atoms m⁻² s⁻¹) is up to 2 orders of magnitude higher than the typical fluxes in stable continental areas, suggesting a structural/tectonic setting favoring the migration of deep‐mantle fluids through the crust.

A thermo‐chemical mantle convection model with both primordial compositional layering and recycling of mid‐ocean ridge basalt (MORB) coupled to a parameterized core heat balance model is used to investigate how the thermo‐chemical evolution of the mantle affects the thermal history of the core including primordial material proposed by early Earth hypotheses. The viscosity formulation has been improved from our previous works. The amount of MORB that accumulates above the CMB is strongly dependent on effective Rayleigh number, such that more accumulates at higher Ra (lower viscosity), but a continuous layer of MORB is not obtained here. With initial primordial layering, large‐scale thermo‐chemical anomalies are found in the deep mantle, which are generated mainly by the primordial material with small amount of segregated basaltic material on top of it, localized in the hot upwelling region. A successful core evolution can only be obtained when initial primordial layering is present. In conclusion, primordial material above the CMB originated from early mantle differentiation might be needed to construct a realistic model of a coupled mantle and core evolution. However, in the current study, the convective vigor is lower than realistic and we only consider the case that primordial material is denser than MORB.

We present a 3‐D shear wave velocity model for the crust and upper mantle of the western Mediterranean from Rayleigh wave tomography. We analyzed the fundamental mode in the 20–167 s period band (6.0–50.0 mHz) from earthquakes recorded by a number of temporary and permanent seismograph arrays. Using the two‐plane wave method, we obtained phase velocity dispersion curves that were inverted for an isotropic Vs model that extends from the southern Iberian Massif, across the Gibraltar Arc and the Atlas mountains to the Saharan Craton. The area of the western Mediterranean that we have studied has been the site of complex subduction, slab rollback, and simultaneous compression and extension during African‐European convergence since the Oligocene. The shear velocity model shows high velocities beneath the Rif from 65 km depth and beneath the Granada Basin from ∼70 km depth that extend beneath the Alboran Domain to more than 250 km depth, which we interpret as a near‐vertical slab dangling from beneath the western Alboran Sea. The slab appears to be attached to the crust beneath the Rif and possibly beneath the Granada Basin and Sierra Nevada where low shear velocities (3.8 km/s) are mapped to >55 km depth. The attached slab is pulling down the Gibraltar Arc crust, thickening it, and removing the continental margin lithospheric mantle beneath both Iberia and Morocco as it descends into the deeper mantle. Thin lithosphere is indicated by very low upper mantle velocities beneath the Alboran Sea, above and east of the dangling slab and beneath the Cenozoic volcanics.

In contrast to larger river systems that drain relatively pristine basins, little is known about the sediment geochemistry of rivers impacted by intense human activities. In this paper, we present a systematic investigation of the anthropogenic overprints on element geochemistry in sediments of the human‐impacted Seine River, France. Most elements are fractionated by grain size, as shown by the comparison between suspended particulate matter (SPM) and riverbank deposits (RBD). The RBD are particularly coarse and enriched in carbonates and heavy minerals and thus in elements such as Ba, Ca, Cr, Hf, Mg, Na, REEs, Sr, Ti, Th, and Zr. Although the enrichment/depletion pattern of some elements (e.g., K, REEs, and Zr) can largely be explained by a binary mixture between two sources, other elements such as Ag, Bi, Cr, Cd, Co, Cu, Fe, Mo, Ni, Pb, Sb, Sn, W, and Zn in SPM in Paris show that a third end‐member having anthropogenic characteristics is needed to account for their enrichment at low water stage. These “anthropophile” elements, with high enrichment factors (EFs) relative to the upper continental crust (UCC), display a progressive enrichment downstream and different geochemical behaviors with respect to the hydrodynamic conditions (e.g., grain size) compared to elements having mainly a natural origin. Our findings emphasize the need for systematic studies of these anthropophile elements in other human‐impacted rivers using geochemical normalization techniques, and stress the importance of studying the chemical variability associated with hydrodynamic conditions when characterizing riverine element geochemistry and assessing their flux to the ocean.

Erebus volcano in Antarctica offers an exceptional opportunity to probe the dynamics of degassing – its behavior is characterized by an active lava lake through which sporadic Strombolian eruptions occur. Here, we develop a framework for interpreting contrasting degassing signatures measured at high temporal resolution, which integrates physical scenarios of gas/melt separation into a thermodynamic model that includes new volatile solubility data for Erebus phonolite. In this widely applicable framework, the measured gas compositions are backtracked from surface to depth according to physical templates involving various degrees of separation of gas and melt during ascent. Overall, explosive signatures can be explained by large bubbles (gas slugs) rising slowly in equilibrium from at least 20 bars but at most a few hundred bars in a magmatic column closer to the stagnant end‐member than the convecting end‐member. The span of explosive signatures can be due to various departure depths and/or slug acceleration below a few tens of bars. Results also reveal that explosive gases last equilibrated at temperatures up to 300°C colder than the lake due to rapid gas expansion just prior to bursting. This picture (individual rise of gas and melt batches from a single, potentially very shallow phonolitic source) offers an alternative to the conclusions of previous work based on a similar data set at Erebus, according to which differences between quiescent and explosive gas signatures are due to the decompression of two deep, volatile‐saturated sources that mixed to various degrees (phonolite at 1–3 kbar and basanite at 5–8 kbar).

Chúng tôi báo cáo dữ liệu đồng vị Yb với mục đích chính xác chỉnh sửa các phép đo nồng độ Lu và tỷ lệ Lu/Hf trong quá trình phân tích bằng Khối phổ Plasma Động cảm Đa kênh (MC‐ICPMS). Tỷ lệ đồng vị Yb phù hợp với các kết quả gần đây của Chu et al. [2002] và Amelin và Davis [2004], khi được chỉnh sửa cho sự phân tách khối lượng về cùng giá trị tham chiếu. Các giao thức để đo lường Lu bao gồm việc loại bỏ đồng thời sự nhiễu Yb và áp dụng một phép chỉnh sửa phân tách dựa trên Yb được mô tả. Được chỉ ra rằng nồng độ Lu và tỷ lệ Lu/Hf được xác định bởi MC‐ICPMS sử dụng phương pháp này chính xác hơn và có thể đáng tin cậy hơn so với kết quả dựa trên phân tích Lu bằng Khối phổ Ion hóa Nhiệt (TIMS). Một số phân tích đồng vị Lu‐Hf bằng MC‐ICPMS của các mẫu đá tiêu chuẩn và các mẫu địa chất khác, trước đó đã được phân tích bằng TIMS, được trình bày. Những kết quả này ghi lại sự tái sản xuất của tỷ lệ Lu/Hf trong khoảng 0.2%, trong đó hầu hết các khác biệt có thể được quy cho sự phân tách khối lượng Lu không xác định trong quá trình phân tích TIMS.

Trung tâm Nghiên cứu Đồng vị và Địa hóa Thái Bình Dương (PCIGR) tại Đại học British Columbia đã tiến hành phân tích có hệ thống các thành phần và nồng độ đồng vị (Sr, Nd, và Pb) của một loạt vật liệu tham khảo của Cục Khảo sát Địa chất Hoa Kỳ (USGS), bao gồm basalt (BCR‐1, 2; BHVO‐1, 2), andesite (AGV‐1, 2), rhyolite (RGM‐1, 2), syenite (STM‐1, 2), granodiorite (GSP‐2), và granite (G‐2, 3). Các vật liệu tham khảo đá của USGS đã được đặc trưng hóa địa hóa tốt, nhưng không có phương pháp hệ thống hay cơ sở dữ liệu cho các thành phần đồng vị phóng xạ, ngay cả đối với BCR‐1 thường được sử dụng. Cuộc điều tra này đại diện cho phân tích có hệ thống đầu tiên về thành phần và nồng độ đồng vị của các vật liệu tham khảo USGS và cung cấp một cơ sở dữ liệu quan trọng cho cộng đồng đồng vị. Thêm vào đó, dải thiết bị tại PCIGR, bao gồm máy Plasma MC‐ICP‐MS của Nu Instruments, Triton TIMS của Thermo Finnigan, và Element2 HR‐ICP‐MS của Thermo Finnigan, cho phép đánh giá và so sánh độ chính xác và độ chính xác của các phân tích đồng vị được xác định bởi cả phương pháp TIMS và MC‐ICP‐MS (ví dụ, các thành phần đồng vị Nd). Đối với mỗi vật liệu tham khảo, 5 đến 10 phân tích hoàn toàn lặp lại cung cấp các kết quả đồng vị nhất quán, tất cả với độ chính xác bên ngoài dưới 30 ppm (2 SD) cho các thành phần đồng vị Sr và Nd (27 và 24 ppm đối với TIMS và MC‐ICP‐MS, tương ứng). Kết quả của chúng tôi cũng cho thấy rằng các vật liệu tham khảo thế hệ đầu tiên và thứ hai của USGS có các thành phần đồng vị Sr và Nd đồng nhất. Các thành phần đồng vị Nd bằng MC‐ICP‐MS và TIMS đồng ý trong phạm vi 15 ppm cho tất cả các vật liệu tham khảo. Các so sánh giữa các phòng thí nghiệm MC‐ICP‐MS cho thấy sự đồng nhất tuyệt vời cho các thành phần đồng vị Pb; tuy nhiên, độ tái tạo không tốt bằng cho Sr và Nd. Một thí nghiệm rửa tỉ mỉ, tuần tự ba vật liệu tham khảo thế hệ đầu tiên và thứ hai (BCR, BHVO, AGV) cho thấy tính không đồng nhất trong các thành phần đồng vị Pb và nồng độ có thể liên quan trực tiếp đến ô nhiễm bởi thép (cối/ chày) sử dụng để xử lý các vật liệu. Ô nhiễm cũng là nguyên nhân cho nồng độ cao của một số nguyên tố vi lượng khác (ví dụ, Li, Mo, Cd, Sn, Sb, W) trong các vật liệu tham khảo USGS khác nhau.

One of the most persistent questions regarding the phase equilibria of mid‐ocean ridge basalts (MORB) pertains to the petrogenesis of the anorthitic plagioclase phenocrysts (>An₉₀) that are characteristic of the more primitive members of such suites. Anorthitic phenocrysts are present in many if not most MORB suites in spite of the fact that no naturally occurring MORB glasses have ever been discovered to be in equilibrium with plagioclase more calcic than An₈₅. We have addressed this paradox by attempting to saturate natural basalts with anorthite in a series of 1 atm experiments using three different natural basaltic starting compositions: an N‐MORB, an E‐MORB, and a continental high‐alumina basalt. To ensure duplication of the olivine and anorthite saturation observed in natural anorthite‐bearing basalt, the experiments were run in An_93‐6 capsules with Fo₉₂ olivine added to the starting glass. The compositions of experimental liquids are generally colinear with the trends observed in the lava suites used as the source material for the starting glasses. Significantly, aluminous spinel (Al₂O₃ contents of 61–68 wt%) was produced at 1290°C in all compositions and chromites (Al₂O₃ contents of 33–42 wt%) at lower temperatures in N‐MORB‐derived liquids despite no spinel having been added to the starting mixture. In addition, the experiments produced basaltic liquid in equilibrium with both >Fo₈₉ olivine and >An₈₅ feldspar at temperatures of 1230° and 1210°. These liquids have compositions with Mg# (at% Mg/Mg + Fe^T*100) that range from 63 to >85. The TiO₂‐MgO correlation indicates large (∼16–23%) amounts of crystallization for each percent decrease in MgO. These results suggest the possibility that dry, anorthite‐bearing basaltic magmas are the product of the interaction between primary melt and Al‐spinel‐bearing upper mantle. In addition, the results indicate that MORB magmas can undergo a large amount (>50%) of crystallization prior to reaching 8% MgO. Further, although anorthite‐bearing magmas have characteristics consistent with their being a significant volumetric component of MORB “parent” magmas, the reaction mechanism suggested for their petrogenesis indicates that they are not necessarily primary magmas.