MICROBIAL ABUNDANCE AND DIVERSITY IN SUBSURFACE LOWER OCEANIC CRUST AT ATLANTIS BANK, SOUTHWEST INDIAN RIDGE

International Ocean Discovery Program Expedition 360 drilled Hole U1473A at Atlantis Bank, an oceanic core complex on the Southwest Indian Ridge, with the aim of recovering representative samples of the lower oceanic crust. Recovered cores were primarily gabbro and olivine gabbro. These mineralogies may host serpentinization reactions that have the potential to support microbial life within the recovered rocks or at greater depths beneath Atlantis Bank. We quantified prokaryotic cells and analyzed microbial community composition for rock samples obtained from Hole U1473A, and conducted nutrient addition experiments to assess if nutrient supply influences the composition of microbial communities. Microbial abundance was low (≤10 4 cells cm −3 ) but positively correlated to the presence of veins in rocks within some depth ranges. Due to the heterogeneous nature of the rocks downhole (alternating stretches of relatively unaltered gabbros and more significantly altered and fractured rocks), the strength of the positive correlations between rock characteristics and microbial abundances was weaker when all depths were considered. Microbial community diversity varied at each depth analyzed. Surprisingly, addition of simple organic acids, ammonium, phosphate, or ammonium plus phosphate in nutrient addition experiments did not affect microbial diversity or methane production in nutrient addition incubation cultures over 60 weeks. The work presented here from Site U1473A, which is representative of basement rock samples at ultraslow spreading ridges and the usually inaccessible lower oceanic crust, increases our understanding of microbial life present in this rarely studied environment and provides an analog for basement below ocean world systems such as Enceladus. IMPORTANCE The lower oceanic crust below the seafloor is one of the most poorly-explored habitats on Earth. The rocks from the Southwest Indian Ridge (SWIR) are similar to rock environments on other ocean-bearing planets and moons. Studying this environment helps us increase our understanding of life in other subsurface rocky environments in our solar system that we do not yet have the capability to access. During an expedition to the SWIR, we drilled 780 meters into lower oceanic crust and collected over 50 rock samples to count the number of resident microbes and determine who they are. We also selected some of these rocks for an experiment where we provided them with different nutrients to explore energy and carbon sources preferred for growth. We found that the number of resident microbes and community structure varied with depth. Additionally, added nutrients did not shape the microbial diversity in a predictable manner.

Fractional crystallization causes the iron isotope contrast between mid-ocean ridge basalts and abyssal peridotites

The iron isotope contrast between mid-ocean ridge basalts and abyssal peridotites is far greater than can be explained by mantle melting alone. Here we investigate a suite of mid-ocean ridge magma chamber rocks sampled by the Ocean Drilling Project Hole 735B in the Atlantis Bank of the Indian Ocean. We report major and trace element geochemistry from these rocks and measure their iron isotope compositions to investigate the potential role of fractional crystallization during melt evolution. We observe a large range of δ56Fe that defines a significant inverse curvilinear correlation with bulk rock MgO/FeOT. These data confirm that δ56Fe in the melt increases as fractional crystallization proceeds but, contrary to expectation, δ56Fe continues to increase even when oxides begin to crystallize. We conclude that iron isotope fractionation through fractional crystallization during the evolution of mid-ocean ridge basalts from abyssal peridotites reconciles the disparity in isotopic compositions between these two lithologies.

IODP Hole 1473A (Atlantis Bank, SWIR) - Formation of felsic veins in gabbros: reactions at the contact between felsic melt and host rock

<p>Hole U1473 (32° 42.3622’ S; 57° 16.6880’ E), located on the summit of Atlantis Bank at the ultra-slow spreading Southwest Indian Ridge was drilled to 789.7 m below seafloor (mbsf) during IODP Expedition 360. It consists of massive gabbros cut by nearly 400 felsic veins, which are evolved, SiO<sub>2</sub>- enriched lithologies comprising ~1.5 vol% of the drill core. They vary in composition from diorite to trondhjemite. For their formation, 3 endmember models are discussed: (1) fractional crystallization; (2) hydrous anatexis of mafic rocks; (3) liquid immiscibility in an evolved MORB system.</p><p>Mineral assemblages in the felsic veins include mainly plagioclase, amphibole, Fe-Ti oxides ± quartz and minor zircon, apatite, ± titanite, ± biotite, ± K-feldspar.</p><p>Vein minerals often show strong zoning, which is especially expressed in amphiboles clearly visible by their variation in color ranging from brown to green corresponding to compositions from pargasite via pargasitic amphiboles, magnesiohornblendes to tremolite/actinolite. Moreover, zoning patterns can be observed in plagioclases from the veins, in which their An contents vary from An<sub>34</sub> down to An<sub>5</sub>. This is distinctly lower than in the plagioclases of the host gabbros, which are virtually unzoned.</p><p>Clinopyroxenes at the contact between felsic vein and host gabbro show reactions either towards orthopyroxene or amphibole. TiO<sub>2</sub> in brown pargasites in the host rock at the contact is enriched (up to ~4.6 wt%), whereas counterparts of the same crystals in the felsic veins are distinctly lower in TiO<sub>2</sub> varying from ~2.5 wt% down to 0.1 wt% TiO<sub>2</sub>, associated with variation in color from brown to green. Calculated equilibrium temperatures based on Ti-content in amphibole (Ernst & Liu, 1998), consequently lead to higher formation temperatures for amphiboles in the host gabbro (up to ~1000 °C) compared to their counterparts in the veins, ranging from ~890 °C to ~500 °C.</p><p>Amphiboles contain ~0.2 wt% F and distinctively lower contents in Cl (with one exception found). Most amphiboles at the contact show a core-rim evolution trend with decreasing F and increasing Cl content, implying a magmatic formation with increasing influence of processes involving a hydrothermal fluid. Only one out of twenty-two investigated samples shows a trend vice versa.</p><p>The record of eutectic crystallization expressed by granophyric structures of quartz and plagioclase indicates that the felsic veins crystallized from a melt.</p><p><em>Ernst, W. G., & Liu, J. (1998). Experimental phase-equilibrium study of Al-and Ti-contents of calcic amphibole in MORB—A semiquantitative thermobarometer. American mineralogist, 83(9-10), 952-969.</em></p>

Microstructures reveal brittle and viscous flow during exhumation of the high-temperature lower oceanic crust from Site U1473A, Atlantis Bank, Southwest Indian Ridge (IODP Expedition 360)

<p>Detachment faulting has been hypothesized as the main process of tectonic spreading in mid-ocean ridges. The ongoing faulting leads to exhumation of oceanic core complexes (OCC) through large-scale normal faults, exposing heterogeneous sectors of the mylonitic lower crust, locally interlayered with pristine upper-mantle rocks. However, the mechanisms involved in this process – and the interplay between magmatism, deformation and fluid-rock interaction – are still debatable. To address these issues, we performed a quantitative microstructural analysis and thermodynamic modelling on mafic shear zones that occur in the lower section (≥ 600 meters below sea-floor) of Site U1473A (Atlantis Bank OCC, SW Indian Ridge), the target of IODP Expedition 360, to constrain deformation conditions and strain localization mechanisms during detachment faulting. The gabbroic shear zones consist of large (up to 5 mm in size) porphyroclasts of clinopyroxene, orthopyroxene, plagioclase and olivine embedded in a fine-grained (≤ 30 µm), polyphase matrix composed of plagioclase, clinopyroxene, orthopyroxene, amphibole, ilmenite, magnetite and olivine. Plagioclase-rich layers (~ 80 µm) are in abrupt contact with the fine-grained mixture, which define the mylonitic foliation. The porphyroclasts have undulose extinction, subgrains and are surrounded by fine-grained recrystallized grains (core-mantle structure) showing internal lattice distortion. Microfractures are common in orthopyroxene porphyroclasts. Amphibole replaces clinopyroxene and orthopyroxene porphyroclasts at their margins and fills cleavage planes. The plagioclase-rich layers show undulose extinction and subgrain boundaries in the larger grains within the layers. Mechanical twin lamellae occur in some grains regardless of grain size. Plagioclase grains show a weak shape preferred orientation with their long axes parallel to the main planar fabric of the shear zone. The grains in the polyphase matrix are mostly strain free. EBSD data in clinopyroxene clasts indicate activation of (010)[001] slip system and twinning along (001)[100]. Plagioclase-rich layers deforms by slip along the (010)[100] system. The polyphase matrix has a very weak but non-random CPO pattern. #Mg and Al content in the recrystallized clinopyroxene and orthopyroxene grains are lower compared to the porphyroclasts. Plagioclase has similar An content in both porphyroclasts and recrystallized grains. Amphibole has low concentrations of Cl and high content of F. The content of #Mg, Al and Si is similar in amphibole grains replacing pyroxene and in the polyphase matrix. Thermodynamic modelling indicates that the gabbroic shear zones formed at 820-870 °C and 2.0-2.8 kbar. Our results suggest that deformation in the porphyroclasts was accommodated by combined mechanical fragmentation and intracrystalline plasticity, which resulted in fractured grains of orthopyroxene, and clasts rimmed by recrystallized neoblasts. Plagioclase-rich layers formed mainly through dislocation creep. Phase mixing and weak CPO in the polyphase matrix point to oriented-growth during diffusion-assisted grain boundary sliding, mainly in the presence of melt, as evidenced by amphibole formed at the expense of pyroxene. Magmatic fluids are the possible source of reactant amphibole. Such mechanisms effectively resulted in strain localization in fine-grained, polyphase shear zones that contributed to the weakening of the ocean crust during detachment faulting and subsequent exhumation of the Atlantis Bank OCC.</p>