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, peridotite xenoliths carried in Paleozoic kimberlites and Tertiary alkali basalts confirm previous suggestions that the refractory and chemically buoyant lithospheric keel present beneath the eastern block of the North China craton is indeed Archean in age.
829 citations
Additional excerpts
...eastern Greenland (see [8] for data sources)....
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...Peridotite xenoliths from the Mojavia terrane in the southern Basin and Range are much more Ferich than typical cratonic lithosphere, but formed at the same time as the overlying crust, in the late Archean/earliest Proterozoic [8]....
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TL;DR: In this article, a thermobarometer based on magma Si and Mg contents was used to estimate the pressures and temperatures of basaltic magma generation on Earth and other terrestrial planets.
599 citations
TL;DR: A review of the geochemistry of mantle xenoliths can be found in this article, where the authors review the geochemical properties of mantle nodules and find that they are dominantly alkaline in nature.
Abstract: Fragments of the Earth’s mantle are frequently transported to the surface via volcanic rocks that are dominantly alkaline in nature. These fragments range up to sizes in excess of 1 m across. The term “mantle xenoliths” or “mantle nodules” is applied to all rock and mineral inclusions of presumed mantle derivation that are found within host rocks of volcanic origin. The purpose of this contribution is to review the geochemistry of mantle xenoliths.
536 citations
TL;DR: In this article, mineralogical and chemical compositions of spinel peridotite xenoliths from two Tertiary alkali basalt localities on the Archean North China craton (Hannuoba and Qixia) were reported.
497 citations
Cites background from "Preservation of ancient and fertile..."
...Lee et al. (2001) proposed that the thinner lithosphere beneath Mojavia failed to shield this small fragment of Archean lithosphere from tectonic reworking....
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...Mojavia is underlain by late Archean lithospheric mantle that is considerably more fertile and dense than typical cratonic mantle (Lee et al., 2001)....
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TL;DR: The sub-crustal mantle as discussed by the authors consists of a thick section of material left behind after extensive partial melt extraction, possibly from the wedge of mantle overlying a subducting oceanic plate.
Abstract: [1] Unlike in the ocean basins where the shallow mantle eventually contributes to the destruction of the overlying crust, the shallow mantle beneath continents serves as a stiff, buoyant “root” whose presence may be essential to the long-term survival of continental crust at Earth's surface. These distinct roles for subcrustal mantle come about because the subcontinental mantle consists of a thick section of material left behind after extensive partial melt extraction, possibly from the wedge of mantle overlying a subducting oceanic plate. Melt removal causes the continental mantle to be cold and strong but also buoyant compared to oceanic mantle. These characteristics allow thick sections of cold mantle to persist beneath continental crust in some cases for over 3 billion years. If the continental mantle becomes gravitationally unstable, however, its detachment from the overlying crust can cause major episodes of intracontinental deformation and volcanism.
433 citations
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56 citations
TL;DR: The geochemistry and textures of peridotite xenoliths from the Jagersfontein kimberlite pipe are reported in this article, where coarse-grained and coarse-deformed Xenoliths are subdivided into coarse and deformed groups corresponding to high and low-temperature estimates.
56 citations
TL;DR: In this article, the authors show that 30% to 40% of the mass of the cratonic metasedimentary and metaigneous rocks in the southern Death Valley region and vicinity is Archean crustal material.
Abstract: Thirty Early Proterozoic intermediate to silicic metasedimentary and metaigneous rocks in the southern Death Valley region and vicinity show ϵ Nd values of −1.6 to −6.3 at 1.7 Ga and Nd model ages of 2.1 to 2.6 Ga. These cratonic rocks thus reveal an older signature than so far reported for Nd province 1 of the western United States; as much as 30%–40% of their mass may be Archean crustal material. The Archean component was introduced in the form of sedimentary detritus that was probably subducted and mixed with juvenile material at a convergent margin. Three younger Precambrian rocks associated with the cratonic rocks also have a Nd isotopic composition of province 1 type.
45 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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TL;DR: In this paper, rare earth elements (REE) and Sr and Nd isotopic compositions are presented for spinel peridotite xenoliths from western Grand Canyon, on the margin of the Colorado Plateau (CP), and for garnet inclusions from The Thumb, central CP.
Abstract: Rare earth elements (REE) and Sr and Nd isotopic compositions are presented for spinel peridotite xenoliths from western Grand Canyon, on the margin of the Colorado Plateau (CP), and for garnet peridotite inclusions from The Thumb, central CP. The Grand Canyon spinel peridotites show original textures and geochemical characteristics which do not have counterparts in the xenoliths from the nearby Basin and Range. The restricted range of mineral compositions in these Iherzolites (Mg #Ol = 0.90 ± 0.03; Cr/(Cr+Al)Sp = 0.18 ± 0.02) suggests that they represent a limited series of residues of partial melting. Pyroxene equilibration temperatures are in the range 800–1000°C. The 143Nd/144Nd ratios in clinopyroxene (cpx) are extremely variable (0.5127–0.5202). Sample TRU 2, with eNd = +147, records an ancient depletion event around 1.2 Ga. More than half of the cpx separates show concave downward REE profiles between Gd and Yb, indicating that partial melting occurred in the garnet stability field before final recrystallization of the residues in the spinel peridotite field, probably during decompressional upwelling. The strong light REE (LREE) depletion in some of the cpx cannot be modeled by batch melting of a LREE-depleted source but requires 5–10% melting with segregation in ∼1% increments. These peridotites could be associated with the production of picritic melts either in a rift environment or beneath a mid-ocean ridge or oceanic island. A second group of xenoliths, characterized by cpx with strong LREE enrichment, includes amphibole-bearing peridotites and has a more limited range of 143Nd/144Nd ratios (0.51269–0.51285). Interactions with melts of variable composition (alkali basalt or carbonatite) are inferred on the basis of mineralogical and geochemical evidence. Contrasting with the extreme isotopic heterogeneity of Nd, a high, uniform 87Sr/86Sr ratio around 0.7045 is associated with high Sr/Nd ratios (up to 540). Aqueous fluids derived from a subducted slab could account for this preferential Sr enrichment. Six porphyroclastic garnet Iherzolites from The Thumb sample a deeper part of the subcontinental mantie (120–150 km). One xenolith has a refractory chemistry, low temperature of equilibration (T = 1220°C, P = 3.8 GPa) and Sr-Nd isotopic compositions (87Sr/86Sr = 0.7037,143Nd/144Nd = 0.51285), suggesting a lithospheric origin. The other xenoliths have a fertile chemistry, higher temperature of equilibration (T = 1260–1280°C, P = 4.5–4.7 GPa), and homogeneous isotopic compositions (87Sr/86Sr = 0.70306–0.70313,143Nd/144Nd = 0.51281–0.51284) which fall slightly to the left of the mantle array in a similar way as St. Helena oceanic basalts. The garnet shows an enrichment in Ti and Fe in the outer 300 μm. Three garnet-cpx pairs give Sm-Nd apparent ages between 60 and 100 Ma, which are older than the host minette (∼25 Ma). These observations together show that the cpx has readjusted its composition more completely than garnet during an interaction with melts having a St. Helena-type isotopic signature. As this signature has not been identified in the CP volcanic rocks, it is inferred that the high-temperature Iherzolites from The Thumb record the presence of a mantle plume at ∼25 Ma beneath the central CP.
43 citations
"Preservation of ancient and fertile..." refers background in this paper
...and consistent with evidence for old Nd in the lithospheric mantle beneath the Colorado platea...
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26 citations