Back-arc spreading, tied to the evolution of progressive subduction, records the spatial expression of three-dimensional subduction dynamics. Mantle flow exerts a dominant control on its development; however, the underlying mechanisms driving spatial variability in back-arc spreading extent remain unclear. The opening of the Japan Sea provides an example of evolved back-arc spreading that is driven by mantle flow. We present geochemical data of back-arc basin basalts (BABBs) from Ocean Drilling Program Sites 794 and 797 in the Japan Sea. These sites are located on two profiles characterized by contrasting back-arc spreading extents. Variations in Nb/Zr and 143Nd/144Nd in samples from the two sites reveal contrasts in spreading extent modulated by mantle flow. In addition, Mg isotopic variations reveal the lateral heterogeneity in volatile cycling, and are observed concurrently with along-strike variations in back-arc extension. BABBs from the northern site in limited-spreading profile exhibit extremely high delta 26Mg values (-0.30%o to +0.34%o), suggesting contributions of water-dominated fluids derived from serpentinite. In contrast, samples from the central site in greater-spreading profile show relatively low delta 26Mg values (-0.57%o to +0.10%o), primarily reflecting the involvement of deep subducted carbonates. The marked differences in how water and carbon modulate mantle viscosity as well as weaken the overlying mantle wedge, thereby regulating mantle rheology, are inferred to have the potential to contribute to the striking contrast in the extent of back-arc spreading. Accordingly, distinct volatile cycling processes, yet often overlooked, need to be considered in interpreting at least some of the curved back-arc extension observed in evolved subduction zones.
Arc volcanic rocks preserve critical records of subduction zone mass transfer processes, with m & eacute;lange melting and mantle metasomatism being two potential mechanisms, yet distinguishing them remains challenging. Here, we integrated elemental and Mg-Sr-Nd isotopic data for Mariana arc and back-arc basin basalts to constrain slab material transport mechanisms. Using high-MgO primitive basalts to circumvent the effects of crystal fractionation, we show that both initial arc rifting and arc volcanoes exhibit systematically higher delta 26Mg values than mid-ocean-ridge basalt (MORB), consistent with serpentinite-bearing m & eacute;lange melting. Back-arc basin basalts show MORB-like delta 26Mg values, reflecting slab-derived hydrous melt metasomatism of the mantle wedge. These results suggest an evolution in subducted slab component transfer mechanisms from m & eacute;lange diapirism beneath the arc/initial rift to mantle metasomatism beneath the back arc. This dual-mode transfer framework provides new constraints on the dynamic interplay between slab dehydration and mantle hybridization, offering a refined perspective on material recycling at convergent margins.
Reconstructing post-depositional thermal histories in carbonate basins is challenging because conventional thermochronometers are scarce and lack the required temperature sensitivity. We established karst-hosted bauxites as an alternative archive, using coexisting hematite (Hm) and goethite (Gt) formed during tropical weathering. We report Hm (U-Th)/ He (HHe) and Gt (U-Th)/He (GHe) dates to and thermal inversions of karst bauxites from Provence (France). These deposits formed during mid-Cretaceous uplift (ca. 120-105 Ma) and were subsequently buried beneath Upper Cretaceous-Cenozoic sediments. Analyses of 133 Hm-Gt grains yielded reproducible ages from 85 +/- 6 Ma to 12 +/- 1 Ma, younger than the overlying sediments, indicating post-depositional resetting. HHe systematically yields older ages (85-22 Ma) than coexisting GHe (32-12 Ma), reflecting higher He retentivity. Hm (U-Th)/He thermal modeling, consistent with regional geology, reveals two thermal events: (1) heating to 90-120 degrees C until 60-45 Ma during Pyrenean compression, followed by cooling; and (2) reheating to 50-60 degrees C between 35 Ma and 15 Ma during the onset of the West European Rift, which partially reset HHe. This second event matches the GHe reset and points to a temperature sensitivity of <60 degrees C for the GHe system. Our results establish karst bauxites as a robust archive for investigating shallow thermal histories.
The modern oceanic crust, dominated by basaltic lavas and gabbroic sills, is home to a diverse ecosystem of prokaryotes and eukaryotes that are fueled by chemical reactions during seawater alteration of mafic igneous rocks. Despite the abundance and antiquity of submarine basalts, the fossil record of this deep igneous habitat is sparse and controversial. Here we report the presence of filamentous microstructures in drill-core samples (drill depth 610 m) of vesicular submarine basalts intercalated with the 2.46 Ga Kuruman Iron Formation, in Griqualand West, South Africa. A network of sinuous and uniformly thick filaments, which are lined by fine-grained Fe-rich phyllosilicates, is preserved in calcite- and stilpnomelanecemented vesicles near the top of the flow. Based on their uniform size, regular shape, and similarity to putative microfossils in younger vesicular basalts, the filaments are interpreted to be relicts of cavity-dwelling (cryptoendolithic) microbes that were mineralized by Fe-silicate clays and encased in calcite cement shortly after basalt eruption. Our results suggest that ancient basaltic lavas were rapidly colonized by microbes in a thermally and geochemically active environment that today is occupied by chemoautotrophs and heterotrophs. The findings confirm submarine lavas as a habitat for early life, and extend the antiquity of basalt-hosted cryptoendoliths, hinting at a lengthy fossil record for the deep igneous biosphere.
The identification of microfossils in stromatolitic chert from the 1.88 Ga Gunflint iron formation (North America), including forms likened to Cyanobacteria, prompted early models linking the deposition of iron formations to biologically mediated iron oxidation. However, the exact relationship between microbial processes, iron deposition, and stromatolite growth remains murky. Here we show that stromatolites in 1.88 Ga iron formations (Sokoman Formation) from the Labrador Trough, Canada, comprise laminae of dusty chert interbedded with dusty chert clasts and sand-sized Fe-silicate grains. High-resolution transmission electron microscopy and analysis show that the dust particles comprise nanometer-sized crystals of greenalite and apatite. The presence of clasts of dusty chert that closely resemble surrounding laminated chert suggests that they are rip-up clasts, indicating penecontemporaneous silica cementation of greenalite-rich muds. Our observations suggest that the stromatolites grew during tranquil conditions by microbial trapping and binding of suspended nanoparticles accompanied by in situ silica precipitation. Our results also provide potential insights into the origin of granular iron formations, suggesting a link between the deposition of greenalite nanoparticles and the production of Fe-silicate grains by accretion on muddy substrates or erosion of silica-cemented greenalite muds. We speculate that greenalite nanoparticles were not only a major primary phase in banded iron formations (e.g., Hamersley Group, Western Australia) but also an important building block of granular iron formations, representing a key ingredient in the formation of sand-sized grains during sedimentary reworking in highenergy, shallow-water environments.
