Abstract Timing, depth, and extent of high‐temperature hydrothermal alteration in the ocean crust is key to understanding how lower oceanic crust is cooled after accretion. Epidote veins were collected in spatially recurring zones of intense alteration of the lower crustal Wadi Gideah section and the root zone of sheeted dykes at Wadi Amadhi. 87 Sr/ 86 Sr ratios feature a narrow range from 0.70429 to 0.70512, while O isotope compositions vary between −0.7‰ and +4.9‰ in δ 18 O SMOW . These compositions indicate water‐rock ratios between 1 and 5 and formation temperatures in the range of 300 to 450 °C. Fluid inclusion entrapment temperatures for a subset of samples linearly increase from 338 °C to 465 °C in lowermost 3 km of crust of the Wadi Gideah section and record 340 °C for the Wadi Amadhi basal sheeted dike sample. Salinities are uniform throughout and scatter closely around seawater values (3.2 ± 0.2 wt%). A model in which cooling down to 2 km (i.e., the depth of the melt lens) is followed by slow off‐axis hydrothermal cooling of the lower crust to 1 Myr predicts a thermal gradient for the lower crust that matches this observed trend for ages between 1 and 2.5 Myr. We suggest that the epidote veins formed in off‐axial hydrothermal systems that reach the base of the crust within 50‐ to 100‐km off axis. This deep circulation provides an efficient mechanism for mining heat that escapes the crust in the young flanks of mid‐ocean ridges.
Abstract The footwalls of oceanic detachment faults commonly expose shear zone rocks that appear to have compositions intermediate between those of mantle peridotite and magmatic rocks. These compositions either reflect metasomatic mass transfers or they relate to the impregnation of lithospheric mantle with basaltic or more evolved melts. We studied chlorite‐amphibole‐rich shear zone rocks from a detachment fault zone in the 15°20′N Fracture Zone area, Mid‐Atlantic Ridge, to examine their origin and role in strain localization. Geochemical compositions of these rocks imply that they formed by mixing between peridotite and gabbro. Textural observations indicate a strong contrast between the deformation intensity of these hybrid peridotite‐gabbro rocks and the host serpentinized peridotite. Geothermometry data give formation temperatures of >500 °C for synkinematic amphibole, zircon, rutile, and titanite. Chlorite appears intergrown with these phases and likely grew at similar temperatures. These results are compliant to thermodynamic computations that predict comparable mechanically weak mineralogies when hydrating hybrid rocks at 500 to 600 °C, whereas secondary assemblages after pure peridotite or gabbro are considerably stronger. Consequently, metamorphic weakening takes place to a much greater extent in rocks with a hybrid ultramafic–mafic composition than in purely ultramafic or gabbroic lithologies. Deformation may enhance fluid flow, which will in turn increase the extent of hydration and mechanical weakening. A positive feedback loop between hydration and strain localization may hence develop and facilitate the concentration of extensional tectonics into long‐lived, high‐displacement faults. We suggest that hybrid lithologies may play a key role in detachment faulting at slow spreading ridges worldwide.
Abstract In the deep ocean symbioses between microbes and invertebrates are emerging as key drivers of ecosystem health and services. We present a large-scale analysis of microbial diversity in deep-sea sponges (Porifera) from scales of sponge individuals to ocean basins, covering 52 locations, 1077 host individuals translating into 169 sponge species (including understudied glass sponges), and 469 reference samples, collected anew during 21 ship-based expeditions. We demonstrate the impacts of the sponge microbial abundance status, geographic distance, sponge phylogeny, and the physical-biogeochemical environment as drivers of microbiome composition, in descending order of relevance. Our study further discloses that fundamental concepts of sponge microbiology apply robustly to sponges from the deep-sea across distances of >10,000 km. Deep-sea sponge microbiomes are less complex, yet more heterogeneous, than their shallow-water counterparts. Our analysis underscores the uniqueness of each deep-sea sponge ground based on which we provide critical knowledge for conservation of these vulnerable ecosystems.
This paper presents petrographic, chemical, and isotopic (Sr, S) analyses of whole rock samples from a 1.8 km section of upper ocean crust (DSDP/ODP Hole 504B). The samples were selected to cover all lithologies (pillows, flows, breccias, dikes) and alteration/mineralization styles. The chemical and petrographic data were used to calculate weighted averages for upper crustal composition, based on which seawater‐ocean crust exchange fluxes were calculated. These results confirm earlier estimates that identify the upper crust as a significant sink for K and Mg and a source of Ca and Si to the oceans. Changes in trace element geochemistry implies that the upper ocean crust in 504B is a sink for CO 2 , Rb, Cs, and U, although the flux rates are an order of magnitude smaller than suggests by previous estimates for DSDP Sites 417 and 418 in 118 Ma Atlantic crust. Fluxes of these components are similar, within a factor of four, to flux rates estimated for the Juan de Fuca Ridge flank, which may relate to similarities in the thermal and hydrogeological evolution at both sites that is controlled by rapid termination of fluid circulation and conductive reheating of the upper crust. The contrast between the fluxes of trace elements derived for those settings and the open‐ocean sites 417/418 likely reflects prolonged fluid‐rock interaction at the latter location. If the Mg uptake and Sr exchange reconstructed from 504B core is representative, ridge flank hydrothermal alteration cannot account for the imbalance in the Mg and Sr budgets of the oceans. Up to 10% of the crustal Pb resides in the mineralized parts of the transition zone between the volcanic section and the sheeted dike complex. Combined, the Pb mobilized in the deepest parts of the hydrothermal systems (probably not penetrated in 504B) and hosted in metalliferous sediments and mineralized stockwork may account for the Pb surplus of the continental crust and the evolution of Ce/Pb of the mantle. Hydrothermal alteration results in net increases of Rb/Sr and U/Pb, in particular in the uppermost 600 m of crust, but the increases are not large enough to make altered upper ocean crust a plausible precursor for the HIMU mantle component. Moreover, the fractionation between Th and Pb, if any, is insufficient to account for the development of highly radiogenic 208 Pb/ 204 Pb in a HIMU mantle source. Potential HIMU precursors can be derived from altered ocean crust after 1–2 Ga, if on the order of 80–90% Pb, 40–55% Rb, 40% Sr, and 35–40%U are removed during partial dehydration in subduction zones.
