Preservation of ancient and fertile lithospheric mantle beneath the southwestern United States.
TL;DR: It is suggested that depleted mantle is intrinsically less dense than fertile mantle (due to iron having been lost when melt was extracted from the rock), which allows the depleted mantle to form a thicker thermal boundary layer between the deep convecting mantle and the crust, thus reducing tectonic activity at the surface.
Abstract: Stable continental regions, free from tectonic activity, are generally found only within ancient cratons—the centres of continents which formed in the Archaean era, 4.0–2.5 Gyr ago. But in the Cordilleran mountain belt of western North America some younger (middle Proterozoic) regions have remained stable, whereas some older (late Archaean) regions have been tectonically disturbed, suggesting that age alone does not determine lithospheric strength and crustal stability. Here we report rhenium–osmium isotope and mineral compositions of peridotite xenoliths from two regions of the Cordilleran mountain belt. We found that the younger, undeformed Colorado plateau is underlain by lithospheric mantle that is 'depleted' (deficient in minerals extracted by partial melting of the rock), whereas the older (Archaean), yet deformed, southern Basin and Range province is underlain by 'fertile' lithospheric mantle (not depleted by melt extraction). We suggest that the apparent relationship between composition and lithospheric strength, inferred from different degrees of crustal deformation, occurs because depleted mantle is intrinsically less dense than fertile mantle (due to iron having been lost when melt was extracted from the rock). This allows the depleted mantle to form a thicker thermal boundary layer between the deep convecting mantle and the crust, thus reducing tectonic activity at the surface. The inference that not all Archaean crust developed a strong and thick thermal boundary layer leads to the possibility that such ancient crust may have been overlooked because of its intensive reworking or lost from the geological record owing to preferential recycling.
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TL;DR: In this paper, the age and trace element and rare earth element (REE) compositions of monazite grains from a peraluminous Cretaceous granite using laser ablation-split stream analysis were mapped.
Abstract: Mapping the age and trace element and Sm-Nd isotope compositions of monazite grains from a peraluminous Cretaceous granite using laser ablation–split stream analysis reveals a wide range in Nd isotope and rare earth element (REE) compositions within and between single grains. These data corroborate isotopic variability indicated by Hf isotope analysis of zircon in the same granite sample. The REE variations indicate that monazite grew during fractional crystallization. Hf and Nd isotopes indicate that the granitic magma was generated from at least two distinct Proterozoic sources of approximately the same age: one component that had highly radiogenic initial 176Hf/177Hf and 143Nd/144Nd and a second component that was notably less radiogenic. This study highlights the utility of in situ REE and Sm-Nd isotope data in monazite in magmatic systems. Further, it refines the zircon-based constraints on magmatic processes because of sensitivity of light REEs to fractional crystallization, lower probability of complications owing to inheritance, and smaller analytical volumes required.
40 citations
TL;DR: In this paper, seismic data recorded by the temporary SPE and LA RISTRA arrays, revealed here through seismic data obtained from the southeastern portion of the Colorado Plateau, is revealed.
39 citations
TL;DR: A 3D shear velocity model of the crust and upper mantle under the Colorado Plateau and surrounding regions of the southwestern United States was made with finite frequency Rayleigh wave tomography using EarthScope/USArray data.
Abstract: A new 3-D shear velocity model of the crust and upper mantle beneath the Colorado Plateau and surrounding regions of the southwestern United States was made with finite frequency Rayleigh wave tomography using EarthScope/USArray data. The goal of our study is to examine the Colorado Plateau lithospheric modification that has resulted from Cenozoic tectonism and magmatism. We have inverted for the isotropic Vs model from a grid of Rayleigh wave dispersion curves obtained by a modified two-plane wave method for periods from 20 to 167 s. We map the lithosphere-asthenosphere boundary under the Colorado Plateau by identifying the middle of the shallowest upper mantle negative Vs gradient. The depths of the lithosphere-asthenosphere boundary inferred here agree well with receiver function estimates made independently. The strong lateral heterogeneity of shear velocity can be mainly attributed to 200–400 K variations in temperature together with ∼1% partial melt fraction in the shallow upper mantle. The resulting Vs structures clearly image the upper mantle low-velocity zones under the Colorado Plateau margins that are associated with magmatic encroachment. These upper mantle low-velocity zones resulted from the convective removal of the Colorado Plateau lithosphere that had been rehydrated by subduction-released water, refertilizing and destabilizing it. This convective erosion by the asthenosphere at the low-viscosity part of the lithosphere is driven by the large step in lithospheric thickness and the thermal gradient across the boundary between the plateau and the extended Basin and Range since the Mid-Cenozoic at a rate similar to that of magmatic migration into the plateau from the southeast, south, and northwest. Moreover, the Rayleigh wave tomography model images parts of a high-velocity drip in the western Colorado Plateau and thus provides additional seismic evidence for ongoing convective downwelling of the lithosphere that was initially suggested by receiver functions and body wave tomography. The widespread edge convective erosion, which the regional delamination-style downwelling processes are a 3-D manifestation of, could provide additional buoyancy sources to support the excess uplift at the margins of the plateau.
37 citations
TL;DR: The major and trace element chemistry of over 500 garnet xenocrysts from 12 kimberlite and alkaline rock bodies are used to examine the mantle lithosphere below the Archean Superior Province.
37 citations
TL;DR: In this article, the authors demonstrate various modes of behaviour of the tectonically active continental mantle lithosphere and show that the removal of accreted crust by erosion can modify how even the deepest portions of the mantle nuclei evolve during contraction.
Abstract: Geodynamic modeling demonstrates various modes of behaviour of the tectonically active continental mantle lithosphere. At continental collision, mantle lithosphere below thickening crust can be accommodated by mixed subduction-like consumption and viscous drip-like instability, depending on the material rheology, temperature, and convergence velocity. Late-stage slab steepening, dual-sided and ablative consumption, and breakoff can occur as the buoyant crust resists subduction. Removal of accreted crust by erosion can modify how even the deepest portions of the mantle lithosphere evolves during contraction. When gravitational forcing rather than plate shortening dominates, mantle lithosphere may be removed through viscous dripping-like instability or delamination. The removal induces crustal heating, modified topography, and deformation, but distinctive styles of these develop depending on whether mantle lithosphere delaminates or drips. With a modified density stratification postulated for the Archean, r...
36 citations
Cites background from "Preservation of ancient and fertile..."
...There is a substantial body of geophysical, geochemical, and geological evidence that suggests cratonic mantle lithosphere (as ‘‘roots’’ or ‘‘keels’’) has been largely stable over long time scales (e.g., Jordan 1978; Rudnick 1998; Lenardic and Moresi 1999; Shapiro et al. 1999; Lee et al. 2001; Shirey et al. 2002; Kelly et al. 2003)....
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References
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TL;DR: In this paper, the authors compared the relative abundances of the refractory elements in carbonaceous, ordinary, and enstatite chondritic meteorites and found that the most consistent composition of the Earth's core is derived from the seismic profile and its interpretation, compared with primitive meteorites, and chemical and petrological models of peridotite-basalt melting relationships.
10,830 citations
"Preservation of ancient and fertile..." refers background in this paper
...For internal consistency, the density for convecting upper mantl...
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2,058 citations
TL;DR: In this article, the Wilson cycle is used to balance the tectosphere by depleting the continental upper mantle in a basalt-like component, which stabilizes the old continental nuclei against convective disruption.
Abstract: Beneath the old continental nuclei are thick root zones which translate coherently during plate motions. These zones are apparently stabilised against convective disruption by the depletion of the continental upper mantle in a basalt-like component. Construction of this delicately balanced tectosphere is accomplished by the dynamic and magmatic processes of the Wilson cycle.
770 citations
"Preservation of ancient and fertile..." refers background in this paper
...This allows the depleted mantle to form a thicker thermal boundary laye...
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TL;DR: Isotope analyses of Os, Sr, Nd, and Pb elements were caried out on twelve peridotite xenoliths from the Jagersfontein, Letseng-la-terae, Thaba Patsoa, Mothae, and Premier kimberlites of southern Africa, to investigate the timing and the nature of melt extraction from the continental lithosphere and its relation to the continent formation and stabilization.
688 citations
"Preservation of ancient and fertile..." refers background in this paper
...Assuming that partial melting leads to stabilization of the lithospheric mantle, the Re–Os isotope systematics of peridotite xenoliths (samples of the lithospheric mantle) can be used to date this time of stabilization; this is because partial melting fractionates Re/Os (Re is moderately depleted and Os is sequestered in the residu...
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TL;DR: In this article, three age provinces have been delineated, each generally northeast-southwest trending, having decreasing crystallization ages and increasing initial e nd values with increasing distance southeastward from the Archean craton.
Abstract: Initial Nd isotopic ratios of crystalline rocks from an area of ∼ 1.5 × 10 6 km 2 of the western United States have been determined in order to map Precambrian age province boundaries and thus document the growth and modification of the North American continent in the Proterozoic. The use of three representative rock suites of different ages— Mesozoic and Tertiary peraluminous granitic rocks, middle Proterozoic (ca. 1.4 Ga) “an-orogenic” granitic rocks, and lower Proterozoic (ca. 1.7 Ga) igneous and metamorphic rocks—allows the ages of the provinces to be distinguished on the basis of different Nd isotopic evolution paths rather than solely on the basis of model ages. Three age provinces have been delineated, each generally northeast-southwest trending, having decreasing crystallization ages and increasing initial e Nd values with increasing distance southeastward from the Archean craton. Province 1 is composed of crustal rocks of central Utah and northeastern Nevada, which are characterized by average values of e Nd (1.7 Ga) ≈ 0 and T DM ≈ 2.0–2.3 Ga. Province 2 covers Colorado, southern Utah, and northwestern Arizona and has e Nd (1.7 Ga) ≈ +3 and T DM ≈ 1.8–2.0 Ga. Province 3, which comprises the basement rocks of New Mexico and southern Arizona, has e Nd (1.7 Ga) ≈ +5 and T DM ≈ 1.7–1.8 Ga. An additional region of province 1-type isotopic characteristics, herein named “Mojavia,” is found in eastern California and western Nevada. Crust formation in each province involved a large component of mantle-derived material plus a moderate amount (∼20%) of pre-existing crust. As the new crust was built outward from the Archean nucleus, however, contributions of Archean material to the newly forming crust were more effectively screened, so that the most distal province (3) is derived almost entirely from Proterozoic mantle. The province boundaries are subparallel to the crystallization age trends determined by other workers. An exception to this is the Mojavia region of province 1, which crosscuts and truncates the other provinces in the region of the lower Colorado River. This region appears to be displaced relative to other areas of the North American basement that have similar isotopic characteristics. This suggests the presence of a previously unrecognized large-scale, left-lateral, north-south–trending basement offset of Proterozoic age in the vicinity of the California-Arizona border.
518 citations
"Preservation of ancient and fertile..." refers background in this paper
...But in the Cordilleran mountain belt of western North America some younger (middle Proterozoic) regions have remained stabl...
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