Geochemistry Geophysics Geosystems最新文献

The degree of organic sulfurization is broadly relevant yet underreported. We present a statistically significant correlation between whole rock S/Fe and the measured degree of organic sulfurization in the thermally immature Cenomanian–Turonian Eagle Ford Group. This relationship shows a sink switch for sulfur from pyrite to organic matter. Excess iron and excess sulfur relative to pyrite, which are mathematically related to S/Fe, provide better insights into organic sulfurization than previous approaches that calculate excess iron relative to detrital iron based on aluminum concentrations. Organic sulfurization and S/Fe are tightly coupled in the Eagle Ford partially due to limited sulfur- and iron-bearing components. Similar relationships could exist in other thermally immature, organic-rich, anoxia-prone, calcareous mudstones. The degree of organic sulfurization was estimated from S/Fe, which was used to map stratigraphic and regional variations of organic sulfurization across the Eagle Ford and to investigate how organic sulfurization relates to organic enrichment, organic preservation, and depositional redox chemistry. The extent of organic sulfurization is more tightly linked to organic preservation than enrichment. Together, organic sulfurization and Mo provide concordant evidence for depositional euxinia. The relationship between Mo and degree of organic sulfurization could indicate that sulfurized organic matter provides a pathway for Mo enrichment, but future work needs to disentangle direct mechanisms from indirect covariations between Mo, organic content, and degree of organic sulfurization. Whole rock elemental chemistry and programmed pyrolysis provide insights into organic sulfurization variations that can be upscaled and can guide subsequent detailed organic sulfur analyses.

Cosmogenic nuclide dating is an essential component of studying Earth surface processes, but it requires knowledge of how nuclide production rates vary in time and space. Typically, production rates are calibrated at sites with independently well-constrained exposure histories and then scaled to other sites of interest using scaling frameworks that account for spatial and temporal variations in the secondary cosmic-ray flux at Earth's surface. To date, scaling schemes for terrestrial cosmogenic nuclide production rates have been developed for the Quaternary, yet cosmogenic nuclide applications that extend beyond the Quaternary are becoming more prevalent. For these deeper time applications, production rate calculations using scaling models optimized for the latest Quaternary neglect longer term spatiotemporal variations in geomagnetic field intensity, paleogeography, and paleoatmospheric depth. We present a production rate scaling scheme for the past 70 million years, SPRITE (Scaling Production Rates In deep TimE). This framework extends existing scaling schemes into deeper time by (a) accounting for site-specific changes in paleolatitude, (b) integrating a geomagnetic field intensity model rooted in data from a global paleomagnetic database, and (c) incorporating climate-driven, time-varying atmospheric depths. We evaluate the efficacy of our model by applying it to existing data sets from paleoexposure sites, and from sites with apparent continuous million-year exposure histories. This scaling model can be applied with measurements of stable cosmogenic nuclides to research questions such as constraining hiatus durations between ancient lava flows and calculating the formation timescales of stable landforms in arid environments over millions of years.

The relationships between the serpentinized continental mantle in orogens, its geophysical signature at depth and hydrogen seepages are poorly understood. A petro-physical modeling approach accounting for serpentinization shows that a large domain of serpentinized mantle (1,800 km²) is present in the northern Pyrenees. The serpentinization reached a maximum of 40% during the mid-Cretaceous rifting, according to the predicted temperature and pressure. Although high-temperature serpentinization could have generated large quantify of hydrogen during the Mesozoic, the shallow and inactive faulting in Northern Pyrenees make this process unlikely to explain the entire serpentinization inferred by petro-physical modeling. A combination of low-temperature alteration of mafic and ultramafic rocks in the North Pyrenean Zone, active normal faulting in the North Pyrenean Fault, accumulation in local traps and transport of H₂-rich fluids along inactive but permeable fault may explain the hydrogen seepages observed today.

In volcanic arcs, magma evolves from basaltic to intermediate and felsic composition, resulting in arc crust maturation. It remains unclear whether processes involving mush during magmatic flare-ups would enhance this evolution. This study revealed a temporal-compositional evolution of plutonic rocks from mafic (∼94 Ma) to intermediate (∼92–88 Ma) to felsic (∼88 Ma) during a magmatic flare-up event in the Gangdese arc, Tibet, with increasing radiogenic Sr–Nd isotope enrichment. Apatites in mafic and felsic rocks have ε_Nd(t) values similar to their hosts, while intermediate rocks show higher values. The elemental composition of apatites in mafic and intermediate rocks is similar but differs from those in felsic rocks. Textural and compositional features indicate varying degrees of influence of mafic rock compositions by accumulation. Triangular and linear covariation relationships between apatite-compatible (e.g., La) and -incompatible (e.g., Rb) elements with SiO₂, respectively, for all plutonic rocks as a whole, confirm the incorporation of apatite-rich mushes into the mixing process. These findings suggest that mafic magma crystallized into apatite-rich mush, which was later remobilized and mixed with felsic magma to form intermediate magma. Felsic rocks represent end-member magmas resulting from crustal anatexis and/or mafic magma differentiation. Thus, the Gangdese arc's maturation during the magmatic flare-up progressed sequentially through mafic magma crystallization and mush formation, mush remobilization and mixing with felsic magma, and the eventual accumulation and segregation of felsic magma. This sequence of events during flare-ups illustrates a common crustal maturation process in volcanic arcs, as also seen in the Andean Cordillera.

Lower crustal metasedimentary xenoliths (garnet-sillimanite granulites) from the Bournac breccia pipe in the Massif Central, France, provide a robust example of sediments transported to depth and incorporated into stable lower continental crust during a collisional orogeny. Dates for detrital igneous zircon range from the Archean (up to 3,300 Ma) to the Devonian and record sedimentation prior to the onset of the collisional phase of the Variscan orogeny. Metamorphic zircon and monazite document the presence of the metasediments in the lower crust by ca. 330 Ma during the later phase of Variscan collision. Zircon and monazite crystallization continued within the lower crust until ca. 265 Ma, corresponding to a period of slow cooling following an episode of ultra-high temperature (UHT) metamorphism that peaked at 313 Ma. Zr-in-rutile thermometry and GASP barometry applied to these samples record conditions of 0.63–0.77 GPa and 830–910°C, which correspond to Ti-in-zircon temperatures from the latter part of the Variscan orogeny and geotherms in excess of typical continent-continent collisions. Rutile in these samples remained open to Pb loss until their eruption at ca. 11.6 Ma, providing an indirect date of the Bournac eruption. These rocks record the incorporation of felsic sedimentary material into the stable deep continental crust during a collisional orogen and their residence there for over 300 Ma. More broadly, the addition of sediments to stable lower crust contributes to changes in crustal composition and has significant implications for the heterogeneity of the deep continental crust, as well as overall crustal heat production and mantle heat flow.

High-temperature Raman spectroscopy offers a cost-effective alternative to extensive infrastructure and sensitive instrumentation for investigating nanolite crystallization in undercooled volcanic melts, a key area of interest in volcanology. This study examined nanolite formation in anhydrous andesite melts in situ at high temperatures, identifying distinct Raman peaks at 310 and 670 cm⁻¹ appearing above the glass transition temperature. The initial amorphous glass remained stable up to 655°C, beyond which Fe-Ti-oxide nanolites progressively formed at higher temperatures, as also confirmed by complementary XRD analysis. The evolution of the 310 cm⁻¹ peak depends only on the magnitude of nanolite crystallization, while the intensity of the 670 cm⁻¹ peak is temperature-dependent and challenging to observe above 500°C. Complementary low-temperature rock-magnetic analyses confirmed Fe-Ti-oxide nanocrystallization with nanolites around 20 nm in diameter. The study tested lasers of different wavelengths (from 355 to 514 nm) and found the green laser to be the most effective for collecting spectra at both room and high temperature. However, above 720°C, black body radiation significantly hinders Raman observation with the green laser when using a non-confocal setup and analyzing poorly transparent samples. If higher temperature measurements are desired, switching to a confocal setup and using lower wavelength lasers should be considered. This research offers a protocol for studying nanolite formation and melt dynamics at high temperatures, providing a foundation for future studies of volcanic processes.

Particle-fluid separation by settling is a ubiquitous process in Earth and Planetary Sciences. The settling velocity of particles is controlled by a balance between buoyancy and drag forces. Since the seminal work of George Gabriel Stokes, parameterizations for the reduction of particle velocities caused by viscous dissipation due to mutual interactions have been described by a non-linear mapping between particle volume fraction and separation velocity. We argue that these parameterizations neglect important physical behavior at high particle volume fractions (>80% of the maximum packing) and are only appropriate when considering suspensions where the particle volume fraction does not evolve in space or time. We introduce a more general model that accounts for the energy dissipation caused by changes in local particle volume fraction, which introduces a new term similar to the compaction term at higher particle volume fraction. This term depends on a consolidation/compaction viscosity that measures the resistance to changes in solid volume fraction. We derive closure equations for this compaction viscosity under dilute and concentrated particle volume fraction limits. Numerical simulations show that the extended hindered settling model predicts two significant differences compared to traditional hindered settling. First, while the steepening of particle volume fraction fronts remains, a dynamic instability is also generated at the front. Second, the rate of growth and structure of a cumulate layer growing above a no-flux boundary is affected by the compaction-like term and predicts the trapping of a higher volume fraction of interstitial melt in a correspondingly thicker cumulate layer.

Igneous intrusives in northern Pakistan can provide valuable insights into pre-Himalayan metaluminous to peraluminous magmatism along the northern boundary of the supercontinent Gondwana and its potential tectonic significance. This study generates new geochronologic, petrographic and geochemical data for intermediate (monzonite, syenite, and foid syenite) and felsic (granite and quartz monzonite) rocks within the NW Himalayan region of Pakistan. Both the intermediate and felsic rocks have values of A/NK > 1.1, implying a metaluminous to peraluminous composition, and are distinguished by high FeO_T/(MgO + FeO_T) (0.81–1.0), high 10,000 × Ga/Al ratio (2.1–5.1), elevated Nb + Zr + Y + Ce contents (122–1,204 ppm), and negative anomalies of P and Ti, consistent with aluminous A-type magmatic affinity. These rocks are classified as A₁-type, which is linked to anorogenic intraplate extensional setting. Both rock groups yield high calculated average whole-rock Zr saturation temperatures (i.e., T_Zr; 790–823°C), which suggests formation from high-temperature magmas. Whole-rock geochemistry, including variable (⁸⁷Sr/⁸⁶Sr)_i values (0.7034–0.7086), positive εNd(t) (+0.1 to +3.9), high Pb isotopic values (that is, (²⁰⁶Pb/²⁰⁴Pb)_i = 18.68 to 19.31, (²⁰⁷Pb/²⁰⁴Pb)_i = 15.64 to 15.74, and (²⁰⁸Pb/²⁰⁴Pb)_i = 38.93 to 39.78), and variation in zircon εHf(t) values (+0.8 to +7.0), indicates diverse magma sources for the intermediate-felsic rocks and provide evidence of partial melting of metasomatized lithospheric mantle, producing a primary magma of foid to quartz syenitic composition. Subsequently, this magma was responsible for the partial melting of the overlying juvenile crust, producing granitic, quartz monzonitic and monzonitic magmas. During the magma evolution process of these rocks, this process was primarily determined by partial melting that followed fractionation of K-feldspar, ilmenite and apatite. T_DM2 ages indicate that the parent materials of intermediate-felsic rocks were generated during the Mesoproterozoic-Neoproterozoic. LA-ICP–MS U-Pb dating of magmatic zircons documents their formation in the Late Paleozoic at ∼278-268 Ma. The intermediate-felsic rocks are correlatable with alkaline igneous rocks of the Peshawar Plain, which record the breakup of the supercontinent Gondwana and the subsequent opening of Neo-Tethys during a Late Paleozoic rifting event. The nepheline syenite records a younger episode during the Cenozoic (37 Ma), corresponding to the collision of the Indian-Eurasian tectonic plates.

The long-term state of stress in the subduction forearc depends on the balance between margin-normal compression due to the plate-coupling force and the margin-normal tension due to the gravitational force on the margin topography. In most subduction margins, the outer forearc is largely in margin-normal compression due to the dominance of the plate-coupling force. The inner forearc's state of stress varies within and among subduction zones, but what gives rise to this variation is unclear. We examine the state of stress in the forearc region of nine subduction zones by inverting focal mechanism solutions for shallow forearc crustal earthquakes for five zones and inferring the previous inversion results for the other four. The results indicate that the inner forearc stress state is characterized by margin-normal horizontal deviatoric tension in parts of Nankai, Hikurangi, and southern Mexico. The vertical and margin-normal horizontal stresses are similar in magnitudes in northern Cascadia as previously reported and are in a neutral stress state. The inner forearc stress state in the rest of the study regions is characterized by margin-normal horizontal deviatoric compression. Tension in the inner forearc tends to occur where plate coupling is shallow. A larger width of the forearc also promotes inner-forearc tension. However, regional tectonics may overshadow or accentuate the background stress state in the inner forearc, such as in Hikurangi.

Tonalite-trondhjemite-granodiorite (TTG) rocks constitute a crucial part of the Archean continental crust, yet their origins remain contentious. It is critical to decipher their source nature and hydration mechanism. This paper presents a study of whole-rock K stable isotopes in well-preserved ca. 3.51–3.42 Ga TTG rocks and associated mafic rocks from the Barberton granitoid-greenstone terrane (BGGT) in the Kaapvaal Craton, South Africa. The results show for the first time a substantial δ⁴¹K variation from −0.69 ± 0.07‰ to −0.32 ± 0.05‰ (2SD) for the Paleoarchean mafic rocks, exceeding the present mantle δ⁴¹K range from −0.6 to −0.3‰. This variation can be well explained by the seawater-hydrothermal alteration at different temperatures. Similarly, the Paleoarchean TTGs exhibit a wide δ⁴¹K range from −0.55 ± 0.04‰ to 0.07 ± 0.08‰ (2SD). In combination with available zircon δ¹⁸O values of 5.07–6.02‰, it is evident that Archean TTGs would be derived from partial melting of the seawater-hydrothermally altered oceanic crust (AOC). The distinct K-O isotope signatures demonstrate that the hydration of Archean mafic crust is caused by the hydrothermal alteration at mid-ocean ridges during seafloor spreading. The variable K-O isotope compositions in the Archean TTGs signify a series of processes that are dominated by the seawater-hydrothermally altered AOC in a Wilson cycle from divergence through convergence to rifting of Archean oceanic plates. This offers a viable mechanism for TTG petrogenesis and the growth of continental crust in the Archean.