Earth and Planetary Science Letters最新文献

Highly siderophile element abundances and ¹⁸²W/¹⁸⁴W and ¹⁸⁷Os/¹⁸⁸Os were determined for a suite of Mauna Kea lavas from the Hawaiian Scientific Drilling Project phase 2 drill core. The new analyses, combined with previous measurements, compose the largest database for µ¹⁸²W (the parts-per-million deviation of ¹⁸²W/¹⁸⁴W from a terrestrial standard) for a single volcano (n = 16). Although most lavas analyzed are characterized by negative µ¹⁸²W values, lavas with values similar to the modern bulk silicate Earth are found throughout the entire stratigraphic column. This suggests that components with normal µ¹⁸²W are collocated with components that host µ¹⁸²W deficits in the plume. Negative µ¹⁸²W values are associated with elevated ³He/⁴He, as well as elevated Ti and Nb. These correlations may link µ¹⁸²W anomalies to ancient deep mantle crystal-liquid fractionation processes. Consistent with previously measured ³He/⁴He (R/R_A) in the drill core, the magnitude of negative µ¹⁸²W values was greatest when Mauna Kea was close to the plume axis then generally decreased over the ∼400 kyr captured by the stratigraphic section. The component with anomalous µ¹⁸²W was either concentrated near the plume axis, or was more effectively sampled by melting near the plume axis where the temperature excess was greatest, suggesting it was less fusible than the dominant plume components. The process leading to the generation of a mantle component with a negative µ¹⁸²W anomaly could either be related to some form of core-mantle isotopic equilibration, or early-Earth fractionation within the silicate Earth. At present each possibility remains viable.

Carbon cycle models used to calculate the marine reservoir age of the non-polar surface ocean (called Marine20) out of IntCal20, the compilation of atmospheric ${\Delta }^{14}$C, have so far neglected a key aspect of the millennial-scale variability connected with the thermal bipolar seesaw: changes in the strength of the Atlantic meridional overturning circulation (AMOC) related to Dansgaard/Oeschger and Heinrich events. Here we implement such AMOC changes in the carbon cycle box model BICYCLE-SE to investigate how model performance over the last 55 kyr is affected, in particular with respect to available ¹⁴C and CO₂ data. Constraints from deep ocean ¹⁴C data suggest that the AMOC in the model during Heinrich stadial 1 needs to be highly reduced or even completely shutdown. Ocean circulation and sea ice coverage combined are the processes that almost completely explain the simulated changes in deep ocean ¹⁴C age, and these are also responsible for a glacial drawdown of ∼60 ppm of atmospheric CO₂. We find that the implementation of abrupt reductions in AMOC during Greenland stadials in the model setup that was previously used for the calculation of Marine20 leads to differences of less than ±100 ¹⁴C yrs. The representation of AMOC changes therefore appears to be of minor importance for deriving non-polar mean ocean radiocarbon calibration products such as Marine20, where atmospheric carbon cycle variables are forced by reconstructions. However, simulated atmospheric CO₂ exhibits minima during AMOC reductions in Heinrich stadials, in disagreement with ice core data. This mismatch supports previous suggestions that millennial-scale changes in CO₂ were probably not driven directly by the AMOC, but rather by biological and physical processes in the Southern Ocean and by contributions from variable land carbon storage.

The mantle transition zone (MTZ) beneath Iberia and NW Maghreb has been precisely mapped using more than 56000 high-quality P-wave receiver functions calculated from the data collected by permanent seismic networks and multiple temporary deployments in the region. Three-dimensional depth migration using both regional and global tomographic models has allowed us to obtain robust and continuous measurements of the MTZ thickness and the depth of the 410 and 660 discontinuities. We found the MTZ thickened by as much as

A paradox exists between the great number of intermediate-depth earthquakes occurring along active subduction interfaces worldwide and the extreme scarcity of paleo-seismic events recorded in exhumed metasediments from ancient subducted slabs. Recrystallization associated with exhumation-related overprinting generally contributes to the nearly-complete erasing of markers of unstable slip events in metamorphic rocks. We herein focus on a sample from an ancient deep thrust from a Cretaceous High-Pressure paleo-accretionary complex in Chilean Patagonia. A representative, moderately foliated micaschist exhibits broken garnet crystals that host a dense network of healed micro-fractures. While garnet fragments appear thoroughly disaggregated along the main foliation, the rock matrix that completely recrystallized has lost the record of brittle deformation. We employ a 2D visco-elasto-plastic numerical modelling approach in order to investigate the mechanical conditions that enable the fracturing of isolated garnet grains in a relatively weak matrix. The rupture of these stiff grains is achieved in our models at strain rates faster than 10⁻¹⁰ /s to 10⁻¹² /s for elevated pore fluid pressures (80 to 99 % of the lithostatic value, respectively). Since high pore fluid pressures prevail in deep subduction interface settings, it is suggested that the rupture of these garnet crystals occurred through cataclastic deformation via (transient) slip rate acceleration, perhaps as a consequence of localized slip associated with slow to conventional earthquakes. Upon slip rate deceleration, viscous disaggregation of the broken garnet clasts occurred along with the erasing of the matrix cataclastic fabric.

Major environmental changes during the Great Oxidation Episode are recorded in 2.5–2.0 Ga sedimentary successions across the world. In North America, the Huronian Supergroup is the most complete succession, preserving three glacial intervals with possible equivalents in the Lake Superior region. The third and youngest glacial unit (Gowganda Formation, Cobalt Group) of the Huronian Supergroup has been correlated with glacial deposits of the Chocolay Group in the Lake Superior region. Here we present the results of in situ SHRIMP U-Pb geochronology of zircon in thin tuff layers from the basal Wewe Slate, Chocolay Group, from drill-core in the Marquette Range area, upper Michigan. Zircon U-Pb data from two intervals ∼16 m apart yielded weighted mean ²⁰⁷Pb/²⁰⁶Pb ages of 2174 ± 9 Ma and 2172 ± 6 Ma, which define the depositional ages of the Wewe Slate and the conformably underlying Kona Dolomite. In the Huronian Supergroup, SHRIMP U-Pb dating of zircon in a tuff bed from the Gordon Lake Formation, Cobalt Group, yielded a weighted mean ²⁰⁷Pb/²⁰⁶Pb age of 2318 ± 8 Ma, interpreted to be the age of deposition. Re-examination of previously published U-Pb zircon data from the Gordon Lake Formation (Rasmussen et al., 2013) yields a slightly older weighted mean ²⁰⁷Pb/²⁰⁶Pb age of 2310 ± 5 Ma. Our results show that the Wewe Slate, and probably the Kona Dolomite, are ∼150 million years younger than the ∼2.32 Ga Gordon Lake Formation, with which they were previously correlated. Our results suggest that the glacial deposits in the Lake Superior region represent a fourth Paleoproterozoic glaciation in North America, whose age range corresponds to that of the fourth and final early Paleoproterozoic glaciation in southern Africa. We propose that the Chocolay Group was deposited after the Huronian Supergroup and is broadly coeval with the rift-passive margin succession (Cycle 1 of the Kaniapiskau Supergroup) in the New Québec Orogen, Canada, whose deposition has been linked to rifting and breakup along the eastern margin of the Superior craton at ∼2.22 Ga.

Geochemically distinguishing the products of melt depletion from refertilization and constraining the timing of such mantle-melt interactions in the subcontinental lithosphere (SCLM) remain outstanding research issues. Here we utilize alphaMELTS thermodynamic modeling of both partial melting and refertilization to revisit the origins of the Lherz mantle rocks from the Pyrenean orogenic mantle massifs. Thermodynamic modeling reveals subtle but critical differences between refertilization (i.e., elevated TiO₂/Al₂O₃ and higher HREE in both whole rocks and clinopyroxenes) and partial melting processes. Harzburgites from the Lherz massif comprise predominantly pristine residues of partial melting and subordinately refertilized products with low melt/rock ratios. The Lherz lherzolites that show close spatial associations with olivine websterite layers represent secondary rocks, derived from refertilization involving the pristine, refractory harzburgites and upwelling N-MORB-like melts. Lherzolites with no intimate spatial association to olivine websterites are open to an additional origin, i.e., via stationary cooling at the base of lithosphere after moderate adiabatic upwelling of asthenospheric mantle. The Re-depletion model ages (T_RD) of refractory harzburgites yield a systematic peak melting age of 2.0 Ga. We have developed a novel solution for constraining the relatively ancient age of refertilization (∼ 1.5 – 2.0 Ga) through the approach of an adapted percolation model to assess the behavior of Re-Os and Lu-Hf isotopic systems. Together with a comprehensive dataset of global on- and off-cratonic SCLM, this study has successfully distinguished silicate-melt induced refertilization from partial melting on elemental level, demonstrated the contribution of different melting mechanisms to the distinctive SCLM compositional evolution in the history, and highlighted how profound a role that refertilization has played on the variations of geochemical buoyancy and mechanical robustness and eventually on the stability and longevity of the ancient SCLM.