Geochemistry Geophysics Geosystems最新文献

We present a new clumped isotope calibration relating temperature to the Δ₄₇ composition of the bioapatite scales of gar fish (Lepisosteidae family). Modern gars live at <∼3 m depth in rivers and lakes, are non-migratory, and their scales grow continuously over their lifespan (∼8–14 years). As ectotherms, their body temperature is equivalent to ambient water temperature. These features indicate that the Δ₄₇ composition of gar scales may be useful for measuring the surface temperature in present and past terrestrial settings. Fossil gar scales are widely distributed in time (Cretaceous to modern) and location (North and South America, Europe, India, and Africa), and are highly mineralized and resistant to diagenesis. Our calibration is based on modern gars collected from eight locations in North America. We use climate data to convert the variable temperature in the gar's habitat into an effective temperature, T_e, to account for variable growth. For our samples, T_e ranges from 13.8 to 27.1°C. We report a lab protocol for measuring the Δ₄₇ composition of gar scale bioapatite with a reproducibility of 0.019‰ (1 SD). Our calibration is based on 19 samples, with ∼3 replicate measurements per sample. The result is Δ₄₇ = (0.1206 ± 0.0171) × 10⁶/T_e² – (0.7429 ± 0.0587) (1 SE), with R² = 0.75 (Δ₄₇ in ‰ and T_e in K). The slope of this calibration is steeper than that for the lab-controlled precipitation of inorganic carbonate. We argue that this difference is caused by kinetic isotope effects associated with hydroxyapatite biosynthesis.

Coseismic fracturing in the strong, dry, and metastable plagioclase-rich lower-crust is an effective mechanism for creating pathways for fluids to infiltrate the host rock, kick-start metamorphism, and potentially lead to rheological weakening. In this study, we have characterized the damage zone flanking a lower-crustal pseudotachylyte (solidified frictional melt produced during seismic slip) within an anorthosite to determine the mechanisms of incipient aqueous fluid infiltration and redistribution in a lower-crustal seismogenic fault. Pulverization-style fracturing of the host anorthosite resulted in the comminution of the host plagioclase (plagioclase₁) grains and the growth of very fine (<20 μm) grained secondary plagioclase neoblasts (plagioclase₂) filling the fractures. Fluid-assisted grain growth accompanied surface- and strain-energy minimization grain growth in the healing and sealing of the fractures. This process was not associated with the densification nor the creation of new reaction-induced porosity. Fourier transform infrared maps transecting the damage zones show the presence of H₂O species along the plagioclase₁ and plagioclase₂ grain boundary regions, as well as incorporated into plagioclase₂ grain interiors. Grain-size sensitive creep of fine-grained plagioclase localized along the pseudotachylyte margin where fracturing was most pervasive. In the absence of reaction-induced porosity, strain localization is determined by repeated occurrences of extreme grain-size reduction in addition to the mobilization of aqueous fluid to the grain boundary regions, to the extent in which these fine-grained wet plagioclase₂ layers are volumetrically dominant over dry, coarse plagioclase₁ fragments. This forms a layer capable of deforming by grain-size sensitive creep and sustaining the mobility of fluids.

Pyroclasts typically exhibit coalesced vesicle textures, which are the evidence of bubble coalescence and the incomplete bubble wall retraction in magma during volcanic eruptions. The sizes of bubble throats or inter-bubble apertures in permeable networks control the extent of magma outgassing, and therefore, quantifying the growth rates of the bubble throats is important but has remained poorly constrained. Using dynamically similar experiments with spontaneous bursting of a single bubble in rheologically well-characterized particulate suspensions, we investigate the growth rate of bubble throats for a range of particle volume fractions. For suspensions with $��\lesssim �$0.50 particle volume fraction, a circular hole (bubble throat) forms following bubble bursting, which after an initial fast growth starts plateauing at a throat-bubble size ratio of $��\gtrsim �$0.20. The throat growth time scale overall increases with increasing particle volume fraction due to the increase in suspension viscosity. On the other hand, bubbles in suspensions with particle volume fraction near the maximum packing fraction ($��\sim �$0.64) exhibit a fracture-like opening. Thus, our experimental results suggest that the plateauing of the bubble throat growth in crystal-poor to crystal-rich magma likely contributes to the wide occurrence of the incompletely retracted vesicle walls in pyroclasts. The implications of the flow- to fracture-like growth of bubble throats on the development of dynamic permeability in magma are discussed.

The Pleistocene (∼0.6 Ma) Cupaello monogenetic volcano, cropping out in central Italy, belongs to the Intra Apennine Province (IAP). It is represented by a single lava flow with kamafugitic composition, associated with a volumetrically limited pyroclastic deposit and a phosphate-rich layer close to the small vent. This kamafugite is characterized by ultrapotassic (K₂O = 5.2–7.6 mass %; K₂O/Na₂O = 18.0–33.9), ultracalcic (CaO/Al₂O₃ = 2.2–2.4), ultrabasic (SiO₂ = 42.6–44.1 mass %), and SiO₂-undersaturated (∼29% CIPW normative leucite) composition, with a peculiar paragenesis (kalsilite, melilite, phlogopite, clinopyroxene, calcite, olivine and glass, plus accessory phases). Trace elements (e.g., high LILE, high LILE/HFSE, negative anomalies for Nb, Ta, Ti and Eu, and Pb peaks in primitive mantle-normalized diagrams) and isotopic constraints (e.g., strongly radiogenic ⁸⁷Sr/⁸⁶Sr, unradiogenic ¹⁴³Nd/¹⁴⁴Nd, high Δ7/4) point to subduction-modified mantle sources. Cupaello products fall within the field of the IAP volcanic products, which define, as a whole, clear negative correlations of compatible and incompatible trace elements, as well as of all the other major oxides, with CaO. These trends are here interpreted as the effect of assimilation of crustal carbonates rather than the presence of a Ca-carbonatitic component in the source, as instead commonly reported in literature. Despite this general trend, we emphasize that Cupaello rocks derive from partial melting of a subduction-modified carbonated phlogopite-peridotite source, followed by a prolonged fractional crystallization of olivine- and melilite-rich kalsilitolite assemblages. A carbonatitic component is required to explain the absence of feldspars in the ultrabasic, ultracalcic and strongly SiO₂-undersaturated magma, whose composition was possibly modified in minimal amounts by carbonate assimilation at shallow depths.

Oxic pelagic clays are an important component of seafloor sediment that may hold valuable information about past ocean chemistry due to their affinity for and accumulation of biogeochemically important metals. We present a new approach to calculating site-specific sedimentation rates (SRs) by comparing authigenic sediment thorium isotope compositions (²³⁰Th/²³²Th) to seawater dissolved ²³⁰Th/²³²Th in a suite of deep (>3,000 m) pelagic core sites. We extracted the authigenic sediment fraction using an HHAc leach protocol, which major element chemistry (Al, Mn, Fe, Ti) suggested was less affected by lithogenic contamination than the HCl leach. Four different methods were tested for extracting the appropriate initial ²³⁰Th/²³²Th from seawater: using either the nearest water column station (methods 1 and 2) or a regionally averaged profile (methods 3 and 4) and using either the bottommost profile measurement (methods 1 and 3) or linear regression of the profile and extrapolation to the seafloor (methods 2 and 4). Method 3 outperformed the other methods in reconstructing previously published SRs from pelagic clays in the North Pacific. The new thorium-based SRs were then combined with estimates from the total sediment thickness on ocean crust and non-lithogenic cobalt accumulation to determine the best estimates for SRs of oxic pelagic clays. The Pacific has the lowest SR (median 0.28 cm/kyr), while the Atlantic is higher (median 0.46 cm/kyr) and the Indian Ocean is highest (median 0.75 cm/kyr). These new estimates are consistent with the expected spatial patterns of sedimentation, but they revise the absolute SR values downward from available gridded SR maps.

Using bathymetry and ROV dives, we investigate two successive flip-flop detachment faults (D1 active, D2 older) in the near-amagmatic 64°35′E region of the SWIR. Kilometer-sized benches on the upper slopes of D1 footwall form the D1 degraded breakaway. Scarps at the top expose the D2 fault zone with deformed serpentinized peridotite, sigmoidal phacoids, planar fractures, and serpentinite microbreccia/gouge horizons. Two ROV sections of the D1 footwall show contrasting deformation styles, corresponding to distinct morphological domains, which relate to contrasting fault and footwall strength. One section documents corrugations, outcrops dominated by sigmoidal phacoids, and planar fractures with thin, discontinuous serpentinite microbreccia/gouge horizons. ROV dives in this corrugated domain show that NNE-trending km-spaced ridges and WNW-trending narrow benches in the shipboard bathymetry correspond, respectively, to broad undulations (mega-corrugations) of the D1 fault and to several antithetic minor normal faults (cumulated horizontal offset of ∼285 m). The other section, lacking corrugations, broad ridges, and antithetic fault, has thicker and more continuous serpentinite microbreccia/gouge horizons, indicating a weaker fault. The abundance of such weak gouges probably reflects hydrous fluid availability during deformation. We link mega-corrugations in the western domain and km-scale lobes of D1 emergence to a broad detachment damage zone with up to ∼600 m-thick mega-phacoids of less deformed serpentinized peridotite. Small antithetic normal faults in the corrugated domain are interpreted as due to bending forces in the D1 footwall. Our findings highlight the three-dimensional, non-planar structural and morphological variability of the exhumed D1 detachment fault zone along the ridge-axis.

The Chilean margin is one of the Earth's tectonically most active plate boundaries, and yet, some of its segments are still underexplored. Here, we present amphibious data from the Copiapó region at ∼27°S located within the mature Atacama seismic gap. Combined 2D seismic refraction, multibeam bathymetry, and local seismicity data show a typical oceanic crust thickness of 6–7 km and seismic P-wave velocities between 3.0 and 7.3 km/s with slightly lower velocities and increased thicknesses underneath the Copiapó Ridge seamounts. The latter is most likely due to predominantly extrusive formation. Elevated velocities underneath one of the seamounts indicate a local region of magmatic underplating, while bending-related faults visible in the bathymetry and reduced mantle velocities near the trench suggest mantle hydration. The subduction angle of the down-going Nazca plate smoothly increases from 12° below the marine forearc to 22° at greater depths (40–60 km) with no abrupt change in the dip angle as observed at ∼22°S. The local seismicity off- and onshore Copiapó shows three separated bands of earthquakes sub-parallel to the down-going plate, and are most likely related to the plate interface, the oceanic Moho and the Double Benioff Zone. The largest event (M_W 5.9) during our observation period (December 2022–June 2023) and its aftershocks occurred in the deepest band ∼20 km below the subduction interface. Along the interface, seismicity is most pronounced in areas of high locking offshore, whereas areas of low locking are characterized by previously observed slow slip events and sparse seismicity.

Our understanding of the geological evolution of mid-ocean ridges in response to tectonic reconfigurations and associated mantle processes is hampered by a lack of exploration in off-axis areas. A notable exception is the Reykjanes Ridge, where multibeam bathymetry, magnetics, and gravity surveys have been conducted up to ∼150 km from the ridge axis. Previous work shows that the ridge has undergone a major reorganization following changes in spreading direction, resulting in the progressive formation and then elimination of transform faults from north to south under the influence of the regional mantle melting anomaly. Notably, this process is incomplete near the southern termination of the ridge, providing a window into the processes of crustal accretion and segmentation prior to and immediately following this reorganization. Here, we employ remote-predictive geological and structural mapping methods linked to chrono-magnetic data to elucidate changes in segment morphology, magma supply, and structural fabrics along the southern ∼200 km of the ridge over the past ∼11 M.y. We identify two new fracture zones and three new non-transform discontinuities, with elimination of transform motion occurring between ∼9.7 and 4.2 Ma, which is later than previously thought. Transform elimination coincides with rift propagation and the emergence of a new magmatically robust segment at ∼58°N at ∼9.7–8.2 Ma. This transition is also associated with a reorientation of seafloor fabric from dominantly N-trending to NE-trending, associated with the dissection of axial volcanic ridges by the oblique (NE-trending) plate boundary, resulting in more crustal accretion to the North American plate overall.

Gravity data provide constraints on lateral subsurface density variations and thus provide crucial insights into the geological evolution of the region. Previously, gravity data from the Norwegian Arctic archipelago of Svalbard comprised an onshore regional gravity database with coarse station spacing of 2–20 km, offshore gravity profiles acquired in some fjords, airborne gravity, and satellite altimetry. The sparse regional point-based onshore coverage hampered the direct integration of gravity data with seismic profiles acquired onshore Svalbard in the late 1980s and early 1990s. In April 2022, we acquired gravity data at 260 new stations along seven profiles from western to eastern Spitsbergen, with a cumulative length of 329 km. The profiles were acquired directly along selected seismic profiles and provide much closer station spacing (0.5–2 km) compared to the regional inland grid (2–20 km) acquired in the late 1980s (total number of onshore stations: 1,037). Having processed the data, we compared the first-order density trends of our new data with the legacy regional grid. The new gravity data are consistent with the regional data, imaging a gravity low in the western part of the area underlying a foreland basin and a gravity high in the northwestern part of the area likely associated with a basement high or denser basement. We compare the new and vintage gravity using maps and profiles, linked to the known major tectonic features such as major basinal axes and fault zones, as well as other geophysical data sets including seismics and magnetics.

Oceanic island basalts are the products of mantle plume melting and their chemistry provides insights into the Earth's deep interior. We report a statistical and machine learning study of 8 radiogenic isotopes and 19 incompatible trace element ratios in basalts from 27 oceanic island volcanic chains associated with mantle plumes compiled from the GEOROC and EARTHCHEM-PetDB databases. Machine-learning hierarchical analysis and agglomerative clustering results based on t-distributed stochastic neighbor embedding (t-SNE) reveal distinct clusters of isotopic compositions corresponding to canonical ones of HIMU, EM I, EM II, PREMA. and DM as well as a LOND cluster, which however do not reflect the existence of discrete components. The HIMU clan is restricted to only a couple of plumes and is characterized by low K/U, Pb/Ce, Ba/Nb and strong REE fractionation. EM I has higher K/U, Ce/Rb, Ba/Nb and lower U/Pb, Rb/Ba, Rb/Sr than EM II reflecting a difference in prevalent recycled components. Stepwise multiple regression reveals that fractionations of incompatible element ratios can be explained by variations in partial melting controlled by lithospheric thickness and plume buoyancy flux; the latter indicates that buoyancy flux primarily reflects plume temperature. ³He/⁴He also correlates with plume buoyancy flux, suggesting that the hottest plumes carry the least radiogenic He. The hottest plumes may be those rising from the core-mantle boundary. This, and the absence of evidence of a primordial mantle reservoir suggest that unradiogenic He may be derived from the core.