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 authors presented the first geochemical data for a peridotite xenolith suite from Marie Byrd Land, including Re-Os isotope measurements for 17 samples.
60 citations
Cites background from "Preservation of ancient and fertile..."
...Over the past dozen years, an increasing number of Re–Os isotopic studies have shown that lithospheric mantle keels of the continents can remain intact for up to 2–3 Ga (e.g. Walker et al., 1989; Reisberg and Lorand, 1995; Handler et al., 1997; Peslier et al., 2000a; Lee et al., 2001), and may thus provide age information that is not available from crustal rocks exposed at the surface (e....
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TL;DR: In this paper, a method for explicit calculation of the temperatures and pressures at which the peridotite protoliths melted was presented, which relies only on the bulk FeO and MgO of residual peridotsites.
59 citations
TL;DR: In this paper, a 3D multi-observable probabilistic tomography method was applied to directly image the thermochemical structure of the Colorado Plateau and surrounding areas by jointly inverting P wave and S wave teleseismic arrival times.
Abstract: We apply a novel 3-D multiobservable probabilistic tomography method that we have recently developed and benchmarked, to directly image the thermochemical structure of the Colorado Plateau and surrounding areas by jointly inverting P wave and S wave teleseismic arrival times, Rayleigh wave dispersion data, Bouguer anomalies, satellite-derived gravity gradients, geoid height, absolute (local and dynamic) elevation, and surface heat flow data. The temperature and compositional structures recovered by our inversion reveal a high level of correlation between recent basaltic magmatism and zones of high temperature and low Mg# (i.e., refertilized mantle) in the lithosphere, consistent with independent geochemical data. However, the lithospheric mantle is overall characterized by a highly heterogeneous thermochemical structure, with only some features correlating well with either Proterozoic and/or Cenozoic crustal structures. This suggests that most of the present-day deep lithospheric architecture reflects the superposition of numerous geodynamic events of different scale and nature to those that created major crustal structures. This is consistent with the complex lithosphere-asthenosphere system that we image, which exhibits a variety of multiscale feedback mechanisms (e.g., small-scale convection, magmatic intrusion, delamination, etc.) driving surface processes. Our results also suggest that most of the present-day elevation in the Colorado Plateau and surrounding regions is the result of thermochemical buoyancy sources within the lithosphere, with dynamic effects (from sublithospheric mantle flow) contributing only locally up to ∼15–35%. ©2016. American Geophysical Union. All Rights Reserved.
58 citations
TL;DR: In this article, eclogite and pyroxenite xenoliths from ultramafic diatremes of the Navajo province on the Colorado Plateau were analyzed to investigate hydration of continental mantle and effects of low-angle subduction on the mantle wedge.
Abstract: [1] Eclogite and pyroxenite xenoliths from ultramafic diatremes of the Navajo province on the Colorado Plateau have been analyzed to investigate hydration of continental mantle and effects of low-angle subduction on the mantle wedge. Xenoliths have been characterized by petrographic and electron probe analysis and by Sm-Nd, Rb-Sr, K-Ar, and O isotopic analysis of mineral separates from one eclogite and by U-Pb isotopic analysis of zircons from three samples. K-Ar analysis of phengite establishes eruption of a Garnet Ridge, Arizona, diatreme at 30 Ma. Sm-Nd and Rb-Sr analyses of clinopyroxene and garnet from that eclogite document recrystallization shortly preceding eruption. Three zircon fractions have been analyzed from that eclogite and from two others representing the nearby Moses Rock and Mule Ear diatremes. Seven of nine small multigrain fractions scatter about a poorly fit discordia between ca. 35 Ma and 1515 Ma (fractions range from overlapping concordia at the lower intercept to a 207Pb/206Pb age of ca. 1220 Ma). The discordant fractions establish a mid-Proterozoic zircon component in each eclogite, inconsistent with an origin from basalt of the Farallon plate. The pressure recorded by one of these eclogites (3.3 GPa) exceeds that of an eclogite previously attributed to the Farallon plate. Nonetheless, each of the eclogites contains a fraction of nearly concordant zircons with ages in the range 35 to 41 Ma, and one rock also contains a fraction that is nearly concordant at 70 Ma. These concordant ages are interpreted to record episodic zircon growth during recrystallization of Proterozoic mantle. The concordant zircon ages are consistent with published data that establish recrystallization of Navajo eclogites from 81 to 33 Ma, a time interval similar to that of the Laramide orogeny. The eclogite-facies recrystallization and growth of new zircon are attributed to the catalytic effects of water introduced into the mantle from the Farallon slab. Water penetrated fracture zones extending for at least tens of kilometers into the mantle wedge above the Farallon slab during low-angle subduction. Magmatism in the San Juan volcanic field to the northeast of the diatremes may be related to similar hydration.
53 citations
Cites background from "Preservation of ancient and fertile..."
...The lithosphere extended to greater than 120 km depth as established by thermobarometry together with Os and Nd model ages of peridotite xenoliths from the Navajo field [Lee et al., 2001; Roden et al., 1990]....
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...subduction (Figure 5). The lithosphere extended to greater than 120 km depth as established by thermobarometry together with Os and Nd model ages of peridotite xenoliths from the Navajo field [ Lee et al., 2001; Roden et al., 1990]....
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TL;DR: In this paper, scaling laws and parameterizations are developed that allow the results of their numerical experiments to be extrapolated to viscosity variations as large as would be expected for thermally activated creep in planetary mantles.
Abstract: [1] The possible role of solid-state convective cooling is important in understanding planetary thermal evolution. A stable compositional stratification in planetary mantles, which can arise from partial melting due to adiabatic decompression or from early magmatic differentiation, may suppress or delay convective instability, restrict the depth scale of convective motions, and reduce the convective heat flux out of the mantle. Two-dimensional numerical experiments on convection in a stably stratified viscous fluid with a strongly temperature-dependent viscosity cooled from above are carried out to understand how convecting, well-mixed layers form and thicken in time because of cooling at the planetary surface. Simple scaling laws and parameterizations are developed that allow the results of our numerical experiments to be extrapolated to viscosity variations as large as would be expected for thermally activated creep in planetary mantles. Strongly temperature-dependent viscosity reduces the available thermal buoyancy in the cold top boundary layer and therefore increases the impact of compositional stratification. Scaling laws for the mixed layer thickening rate indicate that thickening occurs mainly by buoyant, rather than viscous, entrainment of the underlying still stratified fluid. Compositional stratification may be important in the thermal evolution of the oceanic upper mantle, formation of cratonic continental lithosphere, and in planetary evolution, more generally. In several geologically realistic settings, a restricted depth of stratification or mantle viscosities on the order of 1018 Pa s or less are required for convective instability.
53 citations
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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
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...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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