Early-Stage Melt-Rock Reaction in a Cooling Crystal Mush Beneath a Slow-Spreading Mid-Ocean Ridge (IODP Hole U1473A, Atlantis Bank, Southwest Indian Ridge)

Abstract Mantle source heterogeneity and magmatic processes have been widely studied beneath most parts of the Southwest Indian Ridge (SWIR). But less is known from the newly recovered mid-ocean ridge basalts from the Dragon Bone Amagmatic Segment (53°E, SWIR) and the adjacent magmatically robust Dragon Flag Segment. Fresh basalt glasses from the Dragon Bone Segment are clearly more enriched in isotopic composition than the adjacent Dragon Flag basalts, but the trace element ratios of the Dragon Flag basalts are more extreme compared with average mid-ocean ridge basalts (MORB) than the Dragon Bone basalts. Their geochemical differences can be explained only by source differences rather than by variations in degree of melting of a roughly similar source. The Dragon Flag basalts are influenced by an arc-like mantle component as evidenced by enrichment in fluid-mobile over fluid-immobile elements. However, the sub-ridge mantle at the Dragon Flag Segment is depleted in melt component compared with a normal MORB source owing to previous melting in the subarc. This fluid-metasomatized, subarc depleted mantle is entrained beneath the Dragon Flag Segment. In comparison, for the Dragon Bone axial basalts, their Pb isotopic compositions and their slight enrichment in Ba, Nb, Ta, K, La, Sr and Zr and depletion in Pb and Ti concentrations show resemblance to the Ejeda–Bekily dikes of Madagascar. Also, Dragon Bone Sr and Nd isotopic compositions together with the Ce/Pb, La/Nb and La/Th ratios can be modeled by mixing the most isotopically depleted Dragon Flag basalts with a composition within the range of the Ejeda–Bekily dikes. It is therefore proposed that the Dragon Bone axial basalts, similar to the Ejeda–Bekily dikes, are sourced from subcontinental lithospheric Archean mantle beneath Gondwana, pulled from beneath the Madagascar Plateau. The recycling of the residual subarc mantle and the subcontinental lithospheric mantle could be related to either the breakup of Gondwana or the formation and accretion of Neoproterozoic island arc terranes during the collapse of the Mozambique Ocean, and is now present beneath the ridge.

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Strain localization in the root of detachment faults at a melt‐starved mid‐ocean ridge: a microstructural study of abyssal peridotites from the Southwest Indian Ridge

Geochemistry Geophysics Geosystems ◽

10.1029/2020gc009434 ◽

2021 ◽

Author(s):

M. Bickert ◽

M. Cannat ◽

A. Tommasi ◽

S. Jammes ◽

L. Lavier

Keyword(s):

Strain Localization ◽

Southwest Indian Ridge ◽

Microstructural Study ◽

Detachment Faults ◽

Mid Ocean Ridge ◽

Indian Ridge ◽

Ocean Ridge

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Mid-ocean ridge research in China: Discovery of the first active hydrothermal vent field at the ultraslow spreading Southwest Indian Ridge

10.1190/sbgf2007-205 ◽

2007 ◽

Cited By ~ 1

Author(s):

John Chen* ◽

Ning Zhou ◽

Chunhui Tao ◽

Jian Lin

Keyword(s):

Hydrothermal Vent ◽

Southwest Indian Ridge ◽

Mid Ocean Ridge ◽

Indian Ridge ◽

Ocean Ridge ◽

Hydrothermal Vent Field

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Magma Reservoir Formation and Evolution at a Slow-Spreading Center (Atlantis Bank, Southwest Indian Ridge)

Frontiers in Earth Science ◽

10.3389/feart.2020.554598 ◽

2020 ◽

Vol 8 ◽

Author(s):

Marine Boulanger ◽

Lydéric France ◽

Jeremy R.L. Deans ◽

Carlotta Ferrando ◽

C. Johan Lissenberg ◽

...

Keyword(s):

Magma Reservoir ◽

Southwest Indian Ridge ◽

Spreading Center ◽

Atlantis Bank ◽

Slow Spreading ◽

Formation And Evolution ◽

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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