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1. The Atlantis Bank Gabbro Massif, Southwest Indian Ridge

   https://doi.org/org/10.1186/s40645-019-0307-9  

   Dick, Henry J. ; Robinson, Paul T. ; MacLeod, C.J. MacLeod ; Kinoshita, Hajimu ( November 2019 , Progress in earth and planetary science)

   This paper presents the first detailed geologic map of in situ lower ocean crust; the product of six surveys of Atlantis Bank on the SW Indian Ridge. This combined with major and trace element compositions of primary magmatic phases in 99 seafloor gabbros shows there are both significant vertical and ridge-parallel variations in crustal composition and thickness, but a continuity of the basic stratigraphy parallel to spreading. This stratigraphy is not that of magmatic sedimentation in a large crustal magma chamber. Instead, it is the product of dynamic accretion where the lower crust formed by episodic intrusion, large-scale upward migration of interstitial melt due to crystal mush compaction, and continuous tectonic extension accompanied by hyper- and sub-solidus, crystal-plastic deformation. Five crossings of the gabbro-peridotite contact along the transform wall show that massive mantle peridotite is intruded by cumulate residues of moderately to highly evolved magmas, few of which could be even close to equilibrium with a primary mantle magma. This contact then does not represent the crust-mantle boundary as envisaged in the ophiolite analog for ocean crust. The residues of the magmas parental to the shallow crust must also lie beneath the center of the complex. This, and the nearly complete absence of dunites in peridotites from the transform wall, shows that melt transport through the shallow lithosphere was largely restricted to the central region of the paleo-ridge segment. There is almost no evidence for a melt lens or high-level storage of primitive melt in the upper 1500 m of Atlantis Bank. Thus, the composition of associated mid-ocean ridge basalt appears largely controlled by fractional crystallization of primitive cumulates at depth, near or at the base of the crust, modified somewhat by melt-rock reaction during transport through the overlying cumulate pile to the seafloor. Inliers of the dike-gabbro transition show that the uppermost gabbros crystallized at depth and were then emplaced upward, as they cooled, into the zone of diking. ODP and IODP drilling along the center of the gabbro massif also found few primitive gabbros that could have been in equilibrium with the original overlying lavas. Evidence of large-scale upward, permeable transport of interstitial melt through the gabbros is ubiquitous. Thus, post-cumulus processes, including extensive reaction, dissolution, and re-precipitation within the cumulate pile have obscured nearly all evidence of earlier primitive origins. We suggest that many of the gabbros may have started as primitive cumulates but were hybridized and transformed by later, migrating melts to evolved compositions, even as they ascended to higher levels, while new primitive cumulates were emplaced near the base of the crust. Mass balance for a likely parental melt intruded from the mantle to form the crust, however, requires that such primitive cumulates must exist at depth beneath Atlantis Bank at the center of the magmatic complex. The Atlantis Bank Gabbro Massif accreted by direct magma intrusion into the lower crust, followed by upward diapiric flow, first as a crystal mush, then by solid-state, crystal-plastic deformation, and finally by detachment faulting to the sea floor. The strongly asymmetric spreading to the south, parallel to the transform, was due to fault capture, with the bounding faults on the northern rift valley wall cut off by the detachment fault, which extended across the zone of intrusion causing rapid migration of the plate boundary to the north. 

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2. Formation of Amphibole‐Bearing Peridotite and Amphibole‐Bearing Pyroxenite Through Hydrous Melt‐Peridotite Reaction and In Situ Crystallization: An Experimental Study

   https://doi.org/10.1029/2020JB019382  

   Wang, Chunguang ; Liang, Yan ; Xu, Wenliang ( March 2021 , Journal of Geophysical Research: Solid Earth)

   Amphibole is a common hydrous mineral in mantle rocks. To better understand processes leading to the formation of amphibole‐bearing peridotites and pyroxenites in the lithospheric mantle, we conducted experiments by juxtaposing a lherzolite against hydrous basaltic melts in Au‐Pd capsules. Two melts were examined, a basaltic andesite and a basalt, each containing 4 wt% of water. The experiments were run at 1200°C and 1 GPa for 3 or 12 h, and then cooled to 880°C and 0.8 GPa over 49 h. The reaction at 1200°C produced a melt‐bearing orthopyroxenite‐dunite sequence. Crystallization of the partially reacted melts during cooling lead to the formation of an amphibole‐bearing gabbronorite‐orthopyroxenite‐peridotite sequence. Orthopyroxene in the peridotite and orthopyroxenite has a poikilitic texture enclosing olivines and spinels. Amphibole in the peridotite occurs interstitial to olivine, orthopyroxene, clinopyroxene, and spinel. Comparisons of texture and mineral compositions in the experimental products with those from field observations allow a better understanding of hydrous melt‐rock reaction in the lithospheric mantle. Amphibole‐bearing pyroxenite veins (or dikes) can be formed in the lithospheric mantle or at the crust‐mantle boundary by interaction between hydrous melt and peridotite and subsequent crystallization. Hornblendite or amphibole gabbronorite can be formed in the veins when the flux of hydrous melt is high. Differences in reacting melt and peridotite compositions are responsible for the variation in amphibole composition in mantle xenoliths from different tectonic settings. The extent of melt‐rock reaction is a factor that control amphibole composition across the amphibole‐bearing vein and the host peridotite.

    

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3. Silica‐Rich Vein Formation in an Evolving Stress Field, Atlantis Bank Oceanic Core Complex

   https://doi.org/10.1029/2019GC008795  

   Ma, Qiang ; Dick, Henry J. B. ; Urann, Benjamin ; Zhou, Huaiyang ( July 2020 , Geochemistry, Geophysics, Geosystems)

   Drilling 809‐m Hole U1473A in the gabbro batholith at the Atlantis Bank Oceanic Core Complex (OCC) found two felsic vein generations: late magmatic fractionates, rich in deuteric water, hosted by oxide gabbros, and anatectic veins associated with dike intrusion and introduction of seawater‐derived volatiles. Microtextures show a change from compressional to tensional stress during vein formation. Temperatures and oxidation state were obtained from amphibole‐plagioclase and oxide pairs in the adjacent gabbros. Type I veins generally have reverse shear‐sense, with restricted ΔFMQ, high Mt/Ilm ratios, and low‐amphibole Cl/F indicating deuteric fluids. They formed during percolation and fractionation of Fe‐Ti‐rich melts into the primary olivine gabbro. Type II veins are usually hosted by olivine gabbro, occur at dike contacts and the margins of normal‐sense shear zones. They are undeformed or weakly deformed, with highly variable ΔFMQ, low Mt/Ilm ratios, and high‐amphibole Cl/F, indicating seawater‐derived fluids. The detachment fault on which the gabbro massif was emplaced rooted near the base of the dike‐gabbro transition beneath the rift valley. The ingress of seawater volatiles began at >800°C and penetrated at least ~590 m into the lower crust during extensional faulting in the rift valley and adjacent rift mountains. The sequence of the felsic vein formation likely reflects asymmetric diapiric flow, with a reversal of the stress regime, and a transition from juvenile to seawater‐derived volatiles. This, in turn, is consistent with fault capture leading to the large asymmetries in spreading rates during OCC formations and heat flow beneath the rift mountains.

    

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4. Emplacement and High‐Temperature Evolution of Gabbros of the 16.5°N Oceanic Core Complexes (Mid‐Atlantic Ridge): Insights Into the Compositional Variability of the Lower Oceanic Crust

   https://doi.org/10.1029/2018GC007512  

   Sanfilippo, A. ; Dick, H. J. B. ; Marschall, H. R. ; Lissenberg, C. J. ; Urann, B. ( January 2019 , Geochemistry, Geophysics, Geosystems)

   This study reports the composition of the oceanic crust from the 16.5°N region of the Mid‐Atlantic Ridge, a spreading ridge segment characterized by a complex detachment fault system and three main oceanic core complexes (southern, central, and northern OCCs). Lithologies recovered from the core complexes include both greenschist facies and weathered pillow basalt, diabase, peridotite, and gabbro, while only weathered and fresh pillow basalt was dredged from the rift valley floor. The gabbros are compositionally bimodal, with the magmatic crust in the region formed by scattered intrusions of chemically primitive plutonic rocks (i.e., dunites and troctolites), associated with evolved oxide‐bearing gabbros. We use thermodynamic models to infer that this distribution is expected in regions where small gabbroic bodies are intruded into mantle peridotites. The occurrence of ephemeral magma chambers located in the lithospheric mantle enables large proportions of the melt to be erupted after relatively low degrees of fractionation. A large proportion of the dredged gabbros reveal evidence for deformation at high‐temperature conditions. In particular, chemical changes in response to deformation and the occurrence of very high‐temperature ultramylonites (>1000 °C) suggest that the deformation related to the oceanic detachment commenced at near‐solidus conditions. This event was likely associated with the expulsion of interstitial, evolved magmas from the crystal mush, a mechanism that enhanced the formation of disconnected oxide‐gabbro seams or layers often associated with crystal‐plastic fabrics in the host gabbros. This granulite‐grade event was soon followed by hydrothermal alteration revealed by the formation of amphibole‐rich veins at high‐temperature conditions (~900 °C).

    

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5. Microbial Abundance and Diversity in Subsurface Lower Oceanic Crust at Atlantis Bank, Southwest Indian Ridge

   https://doi.org/10.1128/AEM.01519-21  

   Wee, Shu Ying ; Edgcomb, Virginia P. ; Beaudoin, David ; Yvon-Lewis, Shari ; Sylvan, Jason B. ( October 2021 , Applied and Environmental Microbiology)

   Semrau, Jeremy D. (Ed.)

   ABSTRACT 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 with 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 m 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. 

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