An official website of the United States government
Abstract Determining the fate of subducted oceanic crust is critical for understanding material cycling through Earth’s deep interior and sources of mantle heterogeneity. A key control on the distribution of subducted slabs over long timescales is the bridgmanite to post-perovskite phase transition in the lowermost mantle, thought to cause rheological weakening. Using high-resolution computational models, we show that the ubiquitous presence of weak post-perovskite at the core-mantle boundary can facilitate or prevent the accumulation of basaltic oceanic crust, depending on the amount of weakening and the crustal thickness. Moderately weak post-perovskite ( ~ 10–100× weaker) facilitates segregation of crust from subducted slabs, increasing basalt accumulation in dense piles. Conversely, very weak post-perovskite (more than 100× weaker) promotes vigorous plumes that entrain more crustal material, decreasing basalt accumulation. Our results reconcile the contradicting conclusions of previous studies and provide insights into the accumulation of subducted crust in the lowermost mantle throughout Earth’s history.
more »
« less
A Role for Subducted Oceanic Crust in Generating the Depleted Mid‐Ocean Ridge Basalt Mantle
Abstract The composition of Earth's mantle, continental crust, and oceanic crust continuously evolve in response to the dynamic forces of plate tectonics and mantle convection. The classical view of terrestrial geochemistry, where mid‐ocean ridges sample mantle previously depleted by continental crust extraction, broadly explains the composition of the oceanic and continental crust but is potentially inconsistent with observed slab subduction to the lower mantle and oceanic crust accumulation in the deep mantle. We develop a box model to explore the key processes controlling crust‐mantle chemical evolution. The model mimics behaviors observed in thermochemical convection simulations including subducted oceanic crust separating and accumulating in the deep mantle. We demonstrate that oceanic crust accumulation strongly depletes the mantle independently of continental crust extraction. Slab stalling depths and continental crust recycling rates also affect the extent and location of mantle depletion. We constrain model regimes that reproduce oceanic and continental crust compositions using Markov chain Monte Carlo sampling. Some regimes deplete the lower mantle more than the upper mantle, contradicting the assumption of a more enriched lower mantle. All regimes require oceanic crust accumulation in the mantle. Though a small fraction of the mantle mass, accumulated oceanic crust can sequester trace element budgets exceeding the continental crust, depleting the mantle more than continental crust extraction alone. Oceanic crust accumulation may therefore be as important as continental crust extraction in depleting the mantle, contradicting the paradigmatic complementarity of depleted mantle and continental crust. Instead, depleted mantle is complementary to continental crust plus sequestered oceanic crust.
more »
« less
- Award ID(s):
- 1664642
- PAR ID:
- 10453944
- Publisher / Repository:
- DOI PREFIX: 10.1029
- Date Published:
- Journal Name:
- Geochemistry, Geophysics, Geosystems
- Volume:
- 21
- Issue:
- 8
- ISSN:
- 1525-2027
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
-
-
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.more » « less
-
Abstract The evolution of mantle composition can be viewed as a process of destruction whereby the initial chemical state is overprinted and reworked with time. Analyses of ocean island basalts reveals that some portion of the mantle has survived this process, retaining a chemically “primitive” signature. A question that remains is how this primitive signature has survived four and a half billion years of vigorous convection. We hypothesize that some of Earth's primitive mantle is buried within a slab graveyard at the core‐mantle boundary. We explore this possibility using high‐resolution finite element models of mantle convection, in which oceanic lithosphere is produced at zones of plate spreading and subducted at zones of plate convergence. Upon subduction, dense oceanic crust sinks to the base of the mantle and gradually accumulates to form broad, robust thermochemical piles. Sinking oceanic crust entrains the surrounding mantle whose composition is predominantly primitive early in the model's evolution. As a result, thermochemical piles are initially supplied with relatively high concentrations of primitive material—summing up to ∼30% their total mass. The dense oceanic crust dominating the piles resists efficient mixing and preserves the primitive material that it is intermingled with. The significance of this process is shown to be proportional the rate of mantle processing through time and the excess density of oceanic crust at mantle pressures and temperatures. Unlike other theories for the survival of Earth's primitive mantle, this one does not require the early Earth to have large‐scale domains of anomalously high density and/or viscosity.more » « less
-
Abstract Oceanic hotspots with extreme enriched mantle radiogenic isotopic signatures—including low143Nd/144Nd indicative of subducted continental crust—are linked to plume conduits sampling the southern hemispheric mantle. However, the mechanisms responsible for concentrating subducted continental crust in the austral mantle are unknown. We show that subduction of sediments and subduction eroded material, and lower continental crust delamination, cannot generate this spatially coherent austral geochemical domain. However, continental collisions—associated with the assembly of Gondwana‐Pangea—were positioned predominantly in the southern hemisphere during the late Neoproterozoic appearance of widespread continental ultra‐high‐pressure metamorphic terranes, which marked the onset of deep subduction of upper continental crust. We propose that deep subduction of upper continental crust at ancient rifted‐passive margins during ca. 650‐300 Ma austral supercontinent assembly resulted in enhanced upper continental crust delivery into the southern hemisphere mantle. Similarly enriched mantle domains are absent in the boreal mantle plume source, for two reasons. First, continental crust subducted after 300 Ma—when the continents drifted into the northern hemisphere—has had insufficient time to return to the surface in plumes sampling the northern hemisphere mantle. Second, before the first known appearance of continental ultra‐high‐pressure rocks at 650 Ma, deep subduction of upper continental crust was uncommon, limiting its subduction into the northern (and southern) hemisphere mantle earlier in Earth history. Our model implies a recent formation of the austral enriched mantle domain, explains the geochemical dichotomy between austral and boreal plume sources, and may explain why there are twice as many austral hotspots as boreal hotspots.more » « less
-
null (Ed.)Continental arcs in Cordilleran orogenic systems display episodic changes in magma production rate, alternating between flare ups (70–90 km3 km 1 Myr 1) and lulls (< 20 km3 km 1 Myr 1) on timescales of tens of millions of years. Arc segments or individual magmatic suites may have even higher rates, up several 100 s of km3 km 1 Myr 1, during flare ups. These rates are largely determined by estimating volumes of arc crust, but do not reflect melt production from the mantle. The bulk of mantle-derived magmas are recycled back into the mantle by delamination of arc roots after differentiation in the deep crust. Mantle-derived melt production rates for continental arcs are estimated to be 140–215 km3 km 1 Myr 1 during flare ups and ≤ 15 km3 km 1 Myr 1 during lulls. Melt production rates averaged over multiple magmatic cycles are consistent with independent estimates for partial melting of the mantle wedge in subduction zones, however, the rates during flare ups and lulls are both anomalously high and anomalously low, respectively. The difference in mantle-derived melt production between flare ups and lulls is larger than predicted by petrologic and numerical models that explore the range of globally observed subduction parameters (e.g., convergence rate, height of the mantle wedge). This suggests that other processes are required to increase magmatism during flare ups and suppress magmatism during lulls. There are many viable explanations, but one possibility is that crystallized melts from the asthenospheric mantle wedge are temporarily stored in the deep lithosphere during lulls and then remobilized during flare ups. Basaltic melts may stall in the mantle lithosphere in inactive parts of the arc system, like the back-arc, refertilizing the mantle lithosphere and suppressing melt delivery to the lower crust. Subsequent landward arc migration (i.e., toward the interior of the continent) may encounter such refertilized mantle lithosphere magma source regions, contributing to magmatic activity during a flare up. A review of continental arcs globally suggests that flare ups commonly coincide with landward arc migration and that this migration may start tens of millions of years before the flare up occurs. The region of magmatic activity, or arc width, can also expand significantly during a flare up. Arc migration or expansion into different mantle source regions and across lithospheric and crustal boundaries can cause temporal shifts in the radiogenic isotopic composition of magmatism. In the absence of arc migration, temporal shifts are more muted. Isotopic studies of mantle xenoliths and exposures of deep arc crust suggest that that primary, mantle-derived magmas generated during flare ups reflect substantial contributions from the subcontinental mantle lithosphere. Arc migration may be caused by a variety of mechanisms, including slab anchoring or slab folding in the mantle transition zone that could generate changes in slab dip. Episodic slab shallowing is associated with many tectonic processes in Cordilleran orogenic systems, like alternations between shortening and extension in the upper plate. Studies of arc migration may help to link irregular magmatic production in continental arcs with geodynamic models for orogenic cyclicity.more » « less
