Geochemistry Geophysics Geosystems最新文献

Natural gas fields with economic helium (>0.3 He %) require the radioactive decay of crustal uranium (U) and thorium (Th) to generate He and tectonic/structural regimes favorable to releasing and concentrating He. An unknown is determining the role of faults and structural features in focusing deep-seated He sources on shallow accumulations. We tested the correlation between high-He wells (n = 94) and structural features using a new high-resolution aeromagnetic survey in the Four Corners area, USA. A depth-to-basement map with basement lineaments/faults, an intrusion map, and a flattened basement structural high map were created using Werner deconvolution algorithms by combining magnetic, gravity, and topography data with magnetic and gravity depth profiles. We show quantitatively (via analysis of variance) that a non-random process controls the relationship between He (>0.3%) and both basement faults and intrusions: 88% of high-He wells occur <1 km of basement faults; and 85% of high-He wells occur <1 km of intrusions. As He % increases, the distance to the structural features decreases. Strong spatial/statistical correlations of He wells to both basement faults and intrusions suggest that advective transport via faults/intrusions facilitates He migration. The role of gas phase buoyancy and structural trapping is confirmed: 88% of high-He occurs within basement structural highs, and 91% of the remaining wells are <1 km from intrusions (potential structural high). We present a composite figure to illustrate how a probabilistic approach can be used as a predictive model to improve He exploration success by targeting zones of intersection of basement faults and intrusions within basement structural highs.

The recently discovered Fåvne vent field, located at 3,040 m depth on the slow-spreading Mohns mid-ocean ridge between Greenland and Norway, is a high-temperature (≥250°C) vent field that is characterized by Fe oxyhydroxide-rich and S-poor chimneys and mounds. The vent field is located on both the hanging wall and footwall of a normal fault with a ∼1.5 km throw that forms the western edge of the ∼20 km wide ridge axial valley. Data collected during exploration of the site using a remotely operated vehicle as well as mineralogical and geochemical analyses of rock samples and sediments are used to characterize the geological setting of the vent field and composition of the hydrothermal deposits. The chimney walls are highly porous and lack defined chalcopyrite lined conduits, typical of high-temperature chimneys. Overall, abundant Fe oxyhydroxide precipitation at high-temperature vents at Fåvne reflects an excess of Fe over reduced S in the fluid, leading to precipitation of Fe oxide and oxyhydroxide minerals at high to moderate temperature vents (>100°C), and as microbially mediated and abiotic precipitation of Fe oxyhydroxide minerals at low-temperature diffuse vents (<100°C). The mounds and chimneys exhibit low base metal and reduced S concentrations relative to globally averaged seafloor deposits and suggest subseafloor mixing of hydrothermal fluid with seawater, causing metal sulfide precipitation. Cobalt enrichment at Fåvne may reflect a subsurface influence of an ultramafic substrate on circulating fluids, although ultramafic rocks are absent on the seafloor and no other elements typical of ultramafic deposits are present.

Seismic observations show the Earth's inner core has significant and unexplained variation in seismic attenuation with position, depth and direction. Interpreting these observations is difficult without knowledge of the visco- or anelastic dissipation processes active in iron under inner core conditions. Here, a previously unconsidered attenuation mechanism is observed in zinc, a low pressure analog of hcp-iron, during small strain sinusoidal deformation experiments. The experiments were performed in a deformation-DIA combined with X-radiography, at seismic frequencies (∼0.003–0.1 Hz), high pressure and temperatures up to ∼80% of melting temperature. Significant dissipation (0.077 ≤ Q⁻¹(ω) ≤ 0.488) is observed along with frequency dependent softening of zinc's Young's modulus and an extremely small activation energy for creep (⩽7 kJ mol⁻¹). In addition, during sinusoidal deformation the original microstructure is replaced by one with a reduced dislocation density and small, uniform, grain size. This combination of behavior collectively reflects a mode of deformation called “internal stress superplasticity”; this deformation mechanism is unique to anisotropic materials and activated by cyclic loading generating large internal stresses. Here we observe a new form of internal stress superplasticity, which we name as “elastic strain mismatch superplasticity.” In it the large stresses are caused by the compressional anisotropy. If this mechanism is also active in hcp-iron and the Earth's inner-core it will be a contributor to inner-core observed seismic attenuation and constrain the maximum inner-core grain-size to ≲10 km.

The widespread High Arctic Large Igneous Province (HALIP) exhibits prolonged melting over more than 50 Myr, an observation that is difficult to reconcile with the classic view that large igneous provinces (LIPs) originate from melting in plume heads. Hence, the suggested plume-related origin and classification of HALIP as a LIP have been questioned. Here, we use numerical models that include melting and melt migration to investigate a rising plume interacting with lithosphere of variable thickness, that is, a basin-to-craton setting applicable to the Arctic. Models reveal that melt migration introduces significant spatial and temporal variations in melt volumes and pulses of melt production, including protracted melting for at least about 30–40 Myr, because of the dynamic feedback between migrating melt and local lithosphere thinning. For HALIP, plume material deflected from underneath the Greenland craton can re-activate melting zones below the previously plume-influenced Sverdrup Basin after a melt-free period of about 10–15 Myr, even though the plume is already ∼500 km away. Hence, actively melting zones do not necessarily represent the location of the deeper plume stem at a given time, especially for secondary pulses. Additional processes such as (minor) plume flux variations or local lithospheric extension may alter the timing and volume of HALIP pulses, but are to first order not required to reproduce the long-lived and multi-pulse magmatism of HALIP. Since melting zones are always plume-fed, we would expect HALIP magmatism to exhibit plume-related trace element signatures throughout time, potentially shifting from mostly tholeiitic toward more alkalic compositions.

Copper and iron isotopic signatures in sulfide and silicate minerals are important genetic indicators in magmatic sulfide deposits. Kalatongke is a large-scale magmatic Cu-Ni sulfide deposit in the Central Asian Orogenic Belt, and one that experienced multiple stages of magmatism and contamination. It is an ideal deposit in which to study Cu-Fe isotopic fractionation during multiple stages of magmatism and sulfide mineralization processes. The Kalatongke sulfide orebodies are hosted by three small mafic intrusions in which pyroxene and sulfides (pyrrhotite, pentlandite, and chalcopyrite) are the most common Fe-rich minerals, and chalcopyrite is the dominant Cu-rich mineral. Sulfide liquid and silicate melt ▵⁵⁶Fe_Sul-Sil (0.03–0.19‰) and ▵⁶⁵Cu_Ccp-Sil (−0.78–0.74‰) values are indicative of non-equilibrium fractionation. Most of the Cu isotope compositions in the sulfide ores at Kalatongke can be modeled as subduction- metasomatized, oxidized mantle source-derived silicate melt (initial δ⁵⁷Fe = 0.15‰, δ⁶⁵Cu = −0.07‰) that underwent lower crustal contamination, and then reacted with silicate melt, having an R factor of 100–1,000. Rapid silicate melt and sulfide liquid Fe isotope exchange and re-equilibration between chalcopyrite and pyrrhotite in the massive ores is reflected in the similarity of their δ⁵⁶Fe values. Sulfide in disseminated ores shows a range of Fe isotope ratios, influenced by the proportions of monosulfide solid solution (MSS) and intermediate solid solution (ISS) formed. Copper isotopes can be utilized to characterize crustal contamination and silicate melt-sulfide liquid interaction, while the Fe isotope ratios of sulfide minerals record sulfide liquid segregation and evolution in magmatic sulfide deposits.

The thermal properties of major minerals play a key role in understanding the internal dynamic mechanism and thermal evolution of the Earth and rocky planets. In this study, we first investigated the effect of Fe on the thermal conductivity (κ) and the thermal diffusivity (D) of orthopyroxene at 1–3 GPa and 293–873 K by the transient plane source method. The κ and D both decrease with increasing temperature and decreasing pressure. With increasing Fe content, the two parameters both quickly decrease from the beginning and then slack off. We further modeled the thermal evolution of S-type asteroids, which strongly depends on the composition model and the dimension of the planet. Combining the present data with surface heat flow and heat production, the lunar's geotherm until 1,400 km is constructed. The core-mantle boundary temperature of the Moon is refined from 1,883 to 1,754 K.

This study investigates the potential of laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) U-Pb dating for carbonate nodules in the Late Triassic Ischigualasto Formation of northwestern Argentina. We establish a fully characterized paragenetic sequence to guide the analysis of three pedogenic carbonates and compare the U-Pb ages with published geochronology from volcanic ashes within the sedimentary succession. Our findings demonstrate the importance of interpreting U-Pb data within a well-defined paragenetic framework for accurate age interpretation of pedogenic carbonates. We observe variations in U-Pb isotopic signatures across different generations of carbonate precipitates and identify syn-pedogenic and early burial calcite cements as most suitable for precise dating. Respectively, these two calcite cements are interpreted as microcodium and crack-lining calcite cements formed early in the paragenetic sequence during pedogenesis to early burial of the paleosols as they transitioned from the unsaturated vadose to saturated phreatic zone below the water table. The U-Pb ages obtained from the carbonate nodules agree with the radioisotopic ages of volcanic ashes, supporting the validity of our dating strategy. These results contribute to advancing U-Pb carbonate geochronology and highlight its increased potential for dating sedimentary records in the terrestrial realm. Future research should focus on replicating similar work on different carbonate nodules within the Ischigualasto Fm and expanding the application of LA-ICP-MS U-Pb dating to other carbonate-bearing formations, especially in successions with limited absolute ages or where volcanic ashes are sparse or absent.

Generic Mapping Tools (GMT) is a well-known set of software originally developed for geosciences, allowing scientists in climate and solid earth disciplines to routinely create publish-ready maps and graphics. However, GMT users rarely make animations despite their undeniable benefit for understanding and teaching dynamic processes. As reading habits shift from print to digital, capitalizing on animations for illustrating scientific concepts is more accessible than ever. In the latest GMT version (6.5), we have added and refined the movie-making modules, alleviating the time-consuming steps that would hinder GMT users from making such animations. In this paper, we will explain how GMT's “movie” module works and provide six representative examples, from basic to more advanced, to show some of its key features. We hope our presentation will encourage the masses to routinely create animations for their publications.

Iron stable isotopes (δ⁵⁶Fe) are a useful tool for studying Earth processes, many of which involve redox transformations between Fe(III) and Fe(II). Here, we present two related experimental efforts, a study of the kinetic isotope effects (KIEs) associated with the reduction of Fe(III)-ethylenediaminetetraacetic acid (EDTA) to Fe(II), and measurements of the biological fractionation of Fe isotopes by phytoplankton in culture. Reductants tested were ascorbate, hydroxylamine, Mn(II), dithionite, and photoreduction at pH between 5 and 9 and temperatures from 0 to 100°C. Isotope fractionations were very large, and included both normal and inverse KIEs, ranging from −4‰ to +5‰. Experiments were reproducible, yielding similar results for triplicate experiments run concurrently and for experiments run weeks apart. However, fractionations were not predictable, without a clear relationship to reaction rate, temperature, pH, or the reductant used. Acquisition of Fe by eukaryotic phytoplankton also often involves the reduction of Fe(III) to Fe(II). Several species of diatoms and a coccolithophore were tested for Fe isotope fractionation in culture using EDTA, NTA, and DFB as Fe(III) chelating ligands, yielding fractionations from −1.3‰ to +0.6‰. Biological isotope effects were also unpredictable, showing no clear relationship to species, growth rate, or Fe concentration. Variability in Fe isotope fractionation observed in culture may be explained in part by the sensitivity of KIEs. This work has implications for the industrial purification of isotopes, interpretation of natural δ⁵⁶Fe, and the use of Fe isotopes as a tracer Fe source and biological processes in the ocean and other natural systems.

Paleosecular variation analysis is a primary tool for characterizing ancient geomagnetic behavior and its evolution through time. This study presents a new high-quality directional data set, paleosecular variation of the Paleogene (PSVP), with and without correction for serial correlation, compiled from 1,667 sites from 45 different localities from the Paleogene and late Cretaceous (84–23 Ma). The data set is used to study the variability, structure, and latitude dependence of the geomagnetic field during that period by varying selection criteria and PSV models. Modeled values for the equatorial virtual geomagnetic pole (VGP) dispersion have over-lapping uncertainty intervals within their uncertainty bounds between 8.3° and 18.6° for the past 250 Ma. We investigate the suitability of two descriptive models of PSV, Model G-style quadratic fits and covariant Giant Gaussian Process models, and find that both styles of model fail to satisfactorily reproduce the latitude dependent morphology of PSV, but suggest that estimates of the equatorial VGP dispersion may still robustly characterize aspects of Earth's long-term field morphology. During this time where the PSV behavior has not changed substantially, the reversal frequency has varied widely. The lack of a clear relationship between PSV behavior and reversal frequency is not trivially explained in the context of published findings regarding numerical geodynamo simulations.