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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Highly heterogeneous depleted mantle recorded in the lower oceanic crust
The Earth’s mantle is heterogeneous as a result of early planetary differentiation and subsequent crustal recycling during plate tectonics. Radiogenic isotope signatures of mid-ocean ridge basalts have been used for decades to map mantle composition, defining the depleted mantle endmember. These lavas, however, homogenize via magma mixing and may not capture the full chemical variability of their mantle source. Here, we show that the depleted mantle is significantly more heterogeneous than previously inferred from the compositions of lavas at the surface, extending to highly enriched compositions. We perform high-spatial-resolution isotopic analyses on clinopyroxene and plagioclase from lower crustal gabbros drilled on a depleted ridge segment of the northern Mid-Atlantic Ridge. These primitive cumulate minerals record nearly the full heterogeneity observed along the northern Mid-Atlantic Ridge, including hotspots. Our results demonstrate that substantial mantle heterogeneity is concealed in the lower oceanic crust and that melts derived from distinct mantle components can be delivered to the lower crust on a centimetre scale. These findings provide a starting point for re-evaluation of models of plate recycling, mantle convection and melt transport in the mantle and the crust.
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- Award ID(s):
- 1834367
- NSF-PAR ID:
- 10094759
- Date Published:
- Journal Name:
- Nature Geoscience
- ISSN:
- 1752-0894
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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N/A (Ed.)Long-lived radiogenic isotopes of abyssal peridotites, residues of MORB extraction, show that the asthenosphere is intrinsically heterogeneous, which is inherited from ancient melting events and crustal recycling during Earth's history. Yet, Mid Ocean Ridge Basalts (MORB) have a rather uniform average composition, suggesting that the variability of their mantle source is concealed during their ascent. Here we document that mantle heterogeneity is exceptionally well preserved in high permeability mantle conduits from the Lanzo South mantle massif, Western Italian Alps. Nd-Hf-Os isotopes of decametre-scale replacive bodies provide evidence for the existence of two generations of mantle channels. The first generation consists of dunites concordant to the main foliation of host peridotites. The replacive dunites include clinopyroxene with MORB-like incompatible element signature and initial (160 Ma) ƐNd and ƐHf ranging from +4 to +7 and from +10 to +15, respectively. The second generation, made up of pyroxene-poor harzburgites discordant to the main foliation, is geochemically depleted in incompatible elements and its clinopyroxene displays highly radiogenic Hf isotopes (initial ƐHf up to +202). The mantle channel heterogeneity is confirmed by Resingle bondOs isotopes and platinum-groups elements. The MORB-type dunites have high Pt, Pd and, locally, Re, and have 187Os/188Os ratios similar to the host peridotite (0.122–0.128). On the other hand, the depleted bodies have lower Pt, Pd and Re, and 187Os/188Os ratios ranging from those of host peridotites (0.124) to highly unradiogenic values (0.118) in the most refractory sample. The preserved heterogeneity in trace elements, PGE, and Nd-Hf-Os isotopes highlights infiltration of melts from a highly heterogeneous mantle, still partially preserved within these mantle bodies. If applied to present-day Mid Ocean Ridges, our model indicates that the isotopic variability of melts migrating through replacive mantle conduits is by far larger than magmas erupted on the seafloor, which implies that diverse mantle components are mainly delivered and homogenised above the crust-mantle boundary.more » « less
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1038/s41561-019-0368-9
