Sulfide enrichment along igneous layer boundaries in the lower oceanic crust: IODP Hole U1473A, 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.

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Whole-rock geochemistry of gabbros from the Southwest Indian Ridge: constraints on geochemical fractionations between the upper and lower oceanic crust and magma chamber processes at (very) slow-spreading ridges

Chemical Geology ◽

10.1016/s0009-2541(00)00424-1 ◽

2001 ◽

Vol 178 (1-4) ◽

pp. 1-22 ◽

Cited By ~ 55

Author(s):

L.A Coogan ◽

C.J MacLeod ◽

H.J.B Dick ◽

S.J Edwards ◽

A Kvassnes ◽

...

Keyword(s):

Oceanic Crust ◽

Magma Chamber ◽

Southwest Indian Ridge ◽

Magma Chamber Processes ◽

Spreading Ridges ◽

Slow Spreading ◽

Indian Ridge ◽

Lower Oceanic Crust

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Petro-geochemical characteristics and origin of the quartz in the lower oceanic crust, example from IODP-Hole U1473A

Journal of Mining and Earth Sciences ◽

10.46326/jmes.2020.61(4).07 ◽

2020 ◽

Vol 61 (4) ◽

pp. 67-74

Author(s):

Du Khac Nguyen ◽

Tomoaki Morishita ◽

Keyword(s):

Oceanic Crust ◽

Crystallization Mechanism ◽

Southwest Indian Ridge ◽

Precipitation Process ◽

Olivine Gabbro ◽

Magmatic Origin ◽

Fracture Systems ◽

Atlantis Bank ◽

Lower Oceanic Crust ◽

Felsic Rocks

IODP-Hole U1473A was drilled on the summit of Atlantis bank, Southwest Indian Ridge recovered large amounts of gabbroic rocks including mainly olivine gabbro. Felsic rocks are minor, approximately 1,5% of the total volume, which are comprising significant amount of quartz in some samples. The Ti concentrations and the estimated temperatures of the quartz in veins are relatively high, ranging from 30÷130 ppm and 540÷7000C, coupled with the myrmekitic textures in some veins are unambigeous evidence for the late magmatic origin. In addition to the crystallization mechanism in free spaces, such as crack/ fracture systems during the penetration of SiO2 - saturated magmas; the quartz is also formed by re-precipitation process at the same location leaving behind after the previous olivine in the host gabro has been dissolved.

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Microstructures of olivine gabbros from the Atlantis Bank OCC (57°E Southwest Indian Ridge) reveal compaction-driven melts extraction and accumulation in ultraslow-spread oceanic crust.

10.7185/gold2021.5218 ◽

2021 ◽

Author(s):

Marine Boulanger ◽

Carlotta Ferrando ◽

Valentin Basch ◽

Benoit Ildefonse ◽

Jeremy Deans ◽

...

Keyword(s):

Oceanic Crust ◽

Southwest Indian Ridge ◽

Atlantis Bank ◽

Indian Ridge

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Formation and evolution of a magma reservoir at a slow-spreading center (Atlantis Bank, Southwest Indian Ridge)

10.5194/egusphere-egu21-880 ◽

2021 ◽

Author(s):

Marine Boulanger ◽

Lydéric France ◽

Jeremy Deans ◽

Carlotta Ferrando ◽

Johan Lissenberg ◽

...

Keyword(s):

Oceanic Crust ◽

Lower Crust ◽

Magma Reservoir ◽

Southwest Indian Ridge ◽

Ridge Axis ◽

Porous Flow ◽

Atlantis Bank ◽

Slow Spreading ◽

Indian Ridge

The heterogeneous presence of ephemeral magmatic systems below the ridge axis and their complexity mostly account for the heterogeneous character of the oceanic crust accreted at (ultra) slow-spreading ridges. In order to better understand the magmatic processes involved in slow-spreading lower oceanic crust formation, we studied a drilled section of an oceanic core complex (OCC) interpreted as an exhumed portion of lower crust close to the ridge axis. We focused on ODP Hole 735B which presents the most primitive lithologies sampled at Atlantis Bank OCC (Southwest Indian Ridge) in a ~250 m thick section previously interpreted as a single crustal intrusion.We combined detailed structural and petrographic data with whole-rock and in situ mineral analyses to determine the processes of emplacement and differentiation of melts within this section. The lower half of the unit is comprised of alternating troctolites and olivine gabbros showing intrusive contacts, and both magmatic and crystal-plastic fabrics. Such features are lacking in the upper half, rather uniform, gabbroic sequence. Whole-rock compositions highlight the cumulative character of both lower and upper units, and a great compositional variability in the lower sequence, whereas the upper sequence is rather homogeneous and differentiates up-section. In situ analyses of mineral phases document magma emplacement processes and provide evidence for ubiquitous reactive porous flow during differentiation. Comparison between both units' geochemistry also led us to strongly favor a model of formation of the reservoir that genetically links melts from the lower and the upper unit.We show that the whole section, and related geochemical units, likely constitutes a single magmatic reservoir, in which the lower unit formed by emplacement of primitive sills related to the continuous recharge of primitive melts. Recharge led to partial assimilation of the crystallizing primitive mush, and related hybridization with interstitial melts. Hybrid melts were progressively collected in the overlying mushy part of the reservoir (upper unit), whereas the sills' residual melt differentiated by reactive porous flow processes under a predominantly crystallization regime. Similarly, hybrid melts&#8217; evolution in the upper unit was governed by upward reactive porous flow and progressive differentiation and accumulation of evolved melts at the top of the reservoir. Our results provide the first integrated model for magma reservoir formation in the lower slow-spreading oceanic crust, and have potential implications regarding the lower crust structure and the composition of MORBs.

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