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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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
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).
more » « less- NSF-PAR ID:
- 10456930
- Publisher / Repository:
- DOI PREFIX: 10.1029
- Date Published:
- Journal Name:
- Geochemistry, Geophysics, Geosystems
- Volume:
- 20
- Issue:
- 1
- ISSN:
- 1525-2027
- Page Range / eLocation ID:
- p. 46-66
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Abstract 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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Abstract The first known occurrence of rhyolite along the submarine segments of the mid-ocean ridge (MOR) system was discovered on Alarcon Rise, the northernmost segment of the East Pacific Rise (EPR), by the Monterey Bay Aquarium Research Institute in 2012. Zircon trace element and Hf and O isotope patterns indicate that the rhyolite formed by extreme crystal fractionation of primary mid-ocean ridge basalt (MORB) sourced from normal to enriched MOR mantle with little to no addition of continental lithosphere or hydrated oceanic crust. A large range in zircon ɛHf spanning 11 ɛ units is comparable to the range of whole rock ɛHf from the entire EPR. This variability is comparable to continental granitoids that develop over long periods of time from multiple sources. Zircon geochronology from Alarcon Rise suggests that at least 20 kyr was needed for rhyolite petrogenesis. Grain-scale textural discontinuities and trace element trends from zircon cores and rims are consistent with crystal fractionation from a MORB magma with possible perturbations associated with mixing or replenishment events. Comparison of whole rock and zircon oxygen isotopes with modeled fractionation and zircon-melt patterns suggests that, after they formed, rhyolite magmas entrained hydrated mafic crust from conduit walls during ascent and/or were hydrated by seawater in the vent during eruption. These data do not support a model where rhyolites formed directly from partial melts of hydrated oceanic crust or do they require assimilation of such crust during fractional crystallization, both models being commonly invoked for the formation of oceanic plagiogranites and dacites. A spatial association of highly evolved lavas (rhyolites) with an increased number of fault scarps on the northern Alarcon Rise might suggest that low magma flux for ~20 kyr facilitated extended magma residence necessary to generate rhyolite from MORB.more » « less
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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-meter IODP Hole U1473A at Atlantis Bank gabbroic massif. Structurally, the upper portion of U1473A consists of a ∼600-meter 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.more » « less
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