Plastic Deformation of Plagioclase in a Gabbro Pluton at a Slow-Spreading Ridge (IODP Hole U1473A, Atlantis Bank, Southwest Indian ridge)

2021 ◽  
Author(s):  
Maël Allard ◽  
Benoît Ildefonse ◽  
Émilien Oliot
Keyword(s):  
Plastic Deformation ◽  
Grain Boundary ◽  
Shear Zones ◽  
Southwest Indian Ridge ◽  
Temperature Deformation ◽  
Spreading Ridge ◽  
Slip Systems ◽  
Thin Sections ◽  
Atlantis Bank ◽  
Indian Ridge

<p>The crustal architecture of slow-spread ocean crust results from complex interactions between magmatism, hydrothermalism, and tectonics. IODP Hole U1473A (809 m depth) was drilled during IODP Expeditions 360 and 362T at the summit of the Atlantis Bank, a gabbroic massif exhumed at the Southwest Indian Ridge (SWIR). In this study, we identify and quantify plastic deformation processes in oceanic gabbros and active slip-systems in plagioclase from 112 thin sections sampled throughout Hole U1473A.</p><p>We describe deformed zones using petrographic observations and modern Electron Backscattered Diffraction (EBSD) analyses made all along the core. Ductile deformation is widespread and is sometimes strongly localized. It initiated during accretion under magmatic conditions and continued until late brittle conditions. Porphyroclastic microstructures testify to post-magmatic, solid-state, high-temperature (HT) deformation. Plagioclase represents ~60% of rock’s volume and is the dominant phase accommodating deformation in the gabbro. It shows strong dynamic recrystallization accommodated by dislocation creep, forming a fine-grained matrix. Strain localizes in mylonitic and ultramylonitic zones, and these shear zones are often overprinted by lower temperature deformation.</p><p>EBSD analyses reveal weak to moderate crystallographic preferred orientations (CPO) of plagioclase first developed during early magmatic flow, that has produced a primary fabric with a (010) foliation plane and a [100] lineation axis. This CPO is persistent during subsequent plastic deformation and strain localization and is observed in almost all samples. However, a detailed investigation of internal misorientations measured at subgrains reveals the activity of at least 4 to 5 slip systems in plagioclase grains: <img src="https://contentmanager.copernicus.org/fileStorageProxy.php?f=gnp.ccc6371e5dff55120440161/sdaolpUECMynit/12UGE&app=m&a=0&c=6a9c8089187375ae30c9f8697f57bca5&ct=x&pn=gnp.elif&d=1" alt="">, and maybe <img src="https://contentmanager.copernicus.org/fileStorageProxy.php?f=gnp.effe7c6d5dff54400440161/sdaolpUECMynit/12UGE&app=m&a=0&c=33f8e61b26ba87bbb8acbd5b9e5556ab&ct=x&pn=gnp.elif&d=1" alt="">. The strength of CPO is first increasing from slightly foliated gabbros to mylonites before decreasing significantly in ultramylonites, which could be explained by orientation scattering after subgrain rotation recrystallization and grain boundary processes (e.g., nucleation, grain boundary sliding).</p>

Evidence for Multi-stage Melt Transport in the Lower Ocean Crust: the Atlantis Bank Gabbroic Massif (IODP Hole U1473A, SW Indian Ridge)

2020 ◽  
Author(s):  
Wei-Qi Zhang ◽  
Chuan-Zhou Liu ◽  
Henry J B Dick
Keyword(s):  
Shear Zones ◽  
Geochemical Data ◽  
Spreading Ridge ◽  
Ocean Crust ◽  
Magma Supply ◽  
Spreading Ridges ◽  
Melt Transport ◽  
Atlantis Bank ◽  
Indian Ridge ◽  
Compositional Zonation

Abstract The architecture of lower oceanic crust at slow- and ultraslow-spreading ridge is diverse, yet the mechanisms that produce this diversity are not well understood. Particularly, the 660-km2 gabbroic massif at Atlantis Bank (Southwest Indian Ridge) exhibits significant compositional zonation, representing a high magma supply end member for accretion of the lower ocean crust at slow and ultraslow-spreading ridges. We present the petrographic and geochemical data of olivine gabbros from the 809-metre IODP Hole U1473A at Atlantis Bank gabbroic massif. Structurally, the upper portion of U1473A consists of a ∼600-metre shear zone; below this, the hole is relatively undeformed, with several minor shear zones. Olivine gabbros away from the shear zones have mineral trace element compositions indicative of high-temperature reaction with an oxide-undersaturated melt. By contrast, olivine gabbros within shear zones display petrographic and chemical features indicative of reaction with a relatively low-temperature, oxide-saturated melt. These features indicate an early stage of primitive to moderately evolved melt migration, followed by deformation-driven transport of highly evolved Fe–Ti-rich melts to high levels in this gabbroic massif. The close relationship between shear zones and the reaction with oxide-saturated melts suggests that syn-magmatic shear zones provide a conduit for late-stage, Fe–Ti-rich melt transport through Atlantis Bank lower crust. This process is critical to generate the compositional zonation observed. Thus, the degree of syn-magmatic deformation, which is fundamentally related to magma supply, plays a dominant role in developing the diversity of lower ocean crust observed at slow- and ultraslow-spreading ridges.

scholarly journals Magmatism and “Crust-mantle Boundary” on the Ultra-slow Spreading Ridge as Observed in Atlantis Bank, Southwest Indian Ridge

2003 ◽  
Vol 112 (5) ◽  
pp. 705-719 ◽  
Author(s):  
Takeshi MATSUMOTO ◽  
Sumio MIYASHITA ◽  
Shoji ARAI ◽  
Tomoaki MORISHITA ◽  
Jin-ichiro MAEDA ◽  
...  
Keyword(s):  
Southwest Indian Ridge ◽  
Spreading Ridge ◽  
Mantle Boundary ◽  
Atlantis Bank ◽  
Slow Spreading ◽  
Indian Ridge

scholarly journals The Atlantis Bank Gabbro Massif, Southwest Indian Ridge

2019 ◽  
Vol 6 (1) ◽  
Author(s):  
Henry J. B. Dick ◽  
Astri J. S. Kvassnes ◽  
Paul T. Robinson ◽  
Christopher J. MacLeod ◽  
Hajimu Kinoshita
Keyword(s):  
Plastic Deformation ◽  
Lower Crust ◽  
Large Scale ◽  
Southwest Indian Ridge ◽  
Crystal Mush ◽  
Ocean Crust ◽  
Magmatic Complex ◽  
Upward Migration ◽  
Atlantis Bank ◽  
Indian Ridge

AbstractThis 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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