The Hf isotope composition of cratonic mantle: LAM-MC-ICPMS analysis of zircon megacrysts in kimberlites

The thermal thickness of Precambrian lithosphere is modeled and compared with estimates from seismic tomography and xenolith data. We use the steady state thermal conductivity equation with the same geothermal constraints for all of the Precambrian cratons (except Antarctica) to calculate the temperature distribution in the stable continental lithosphere. The modeling is based on the global compilation of heat flow data by Pollack et al. [1993] and more recent data. The depth distribution of heat-producing elements is estimated using regional models for ∼300 blocks with sizes varying from 1°×1° to about 5°×5° in latitude and longitude and is constrained by laboratory, seismic and petrologic data and, where applicable, empirical heat flow/heat production relationships. Maps of the lateral temperature distribution at depths 50, 100, and 150 km are presented for all continents except Antarctica. The thermal thickness of the lithosphere is calculated assuming a conductive layer overlying the mantle with an adiabat of 1300°C. The Archean and early Proterozoic lithosphere is found to have two typical thicknesses, 200–220 km and 300–350 km. In general, thin (∼220 km) roots are found for Archean and early Proterozoic cratons in the Southern Hemisphere (South Africa, Western Australia, South America, and India) and thicker (>300 km) roots are found in the Northern Hemisphere (Baltic Shield, Siberian Platform, West Africa, and possibly the Canadian Shield). We find that the thickness of continental lithosphere generally decreases with age from >200 km beneath Archean cratons to intermediate values of 200±50 km in early Proterozoic lithosphere, to about 140±50 km in middle and late Proterozoic cratons. Using known crustal thickness, our calculated geotherms, and assuming that isostatic balance is achieved at the base of the lithosphere, we find that Archean and early Proterozoic mantle lithosphere is 1.5% less dense (chemically depleted) than the underlying asthenosphere, while middle and late Proterozoic subcrustal lithosphere should be depleted by ∼0.6–0.7%. Our results suggest three contrasting stages of lithosphere formation at the following ages: >2.5 Ga, 2.5–1.8 Ga, and <1.8 Ga. Ages of komatiites, greenstone belts, and giant dike swarms broadly define similar stages and apparently reflect secular changes in mantle temperature and, possibly, convection patterns.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2000JB900439

Thermal thickness and evolution of Precambrian lithosphere: A global study

A catalytic delamination-driven model for coupled genesis of Archaean crust and sub-continental lithospheric mantle

The late Archean record: a puzzle in ca. 35 pieces

The composition of the subcontinental lithospheric mantle (SCLM) is broadly related to the tectonothermal age of the overlying crust, suggesting a secular change in SCLM-forming processes. Most estimated compositions of Archean SCLM, based on well-studied suites of xenoliths and xenocrysts, are depleted garnet lherzolites with high orthopyroxene/olivine. However, these compositions make it difficult to account for the high shear-wave velocities measured in the cores of large cratons, and predict deeper geoid anomalies and higher elevations than are observed in most cratons. Global and regional seismic tomography indicates that most cratonic xenolith suites represent material from the lower-velocity margins of lithospheric blocks. This implies that previous compositional estimates are strongly biased toward metasomatized material. We suggest that most Archean SCLM originally consisted of highly depleted dunites/ harzburgites, similar to the Archean orogenic massifs of western Norway. Incorporation of such rocks in the cold upper parts of the cratonic SCLM satisfies the seismic and gravity data, suggesting that large volumes of these rocks are preserved in the cores of cratons, but are poorly sampled by volcanic rocks. The roots of most Proterozoic shields probably consist of refertilized Archean SCLM; the juvenile SCLM beneath Proterozoic and Phanerozoic mobile belts reflects only moderate depletion of Primitive Mantle compositions. Rather than a gradual evolution in SCLM-forming processes, we suggest a sharp dichotomy between Archean and younger tectonic regimes.The differences in buoyancy and viscosity between these two types of SCLM have played a major role in the construction, preservation and recycling of continental crust. If originally Archean SCLM is more widespread than currently recognized, models of crustal growth rates and recycling may need to be revised.

/pdf/the-composition-and-evolution-of-lithospheric-mantle-a-re-3w89quzuad.pdf

The Composition and Evolution of Lithospheric Mantle: a Re-evaluation and its Tectonic Implications

https://www.geol.umd.edu/~mcdonoug/Lithium%20Group/Griffin%20et%20al.pdf

The origin and evolution of Archean lithospheric mantle

from some highly depleted ophiolites from convergent-margin settings, Heavy-mineral concentrates (garnets, chromites) and xenoliths from and may have formed in a similar situation during the accretion of 21 Cretaceous–Tertiary kimberlite intrusions have been used to map the Hackett and Contwoyto terranes (magmatic arc and accretionary the lithospheric mantle beneath the Lac de Gras area in the central prism, respectively) to the ancient continental Anton terrane at part of the Slave Province. Analyses of Nickel Temperature ( TNi) 2·6–2·7 Ga. The deeper layer is interpreted as a plume head, and Zinc Temperature ( TZn) have been used to place garnet and which rose from the lower mantle and underplated the existing chromite xenocrysts, respectively, in depth context. Paleogeotherms lithosphere at 2·6 Ga; evidence includes a high proportion of the derived from both xenoliths and concentrates lie near a 35 mW/ superdeep inclusion assemblage (ferropericlase–perovskite) in the m conductive model at T Ζ 900°C, and near a 38 mW/m diamond population. This event could have provided heat for model at higher T, implying a marked change in conductivity and/ generation of the widespread 2·6 Ga post-tectonic granites. Proteroor a thermal transient. Plots of garnet composition vs TNi also zoic subduction from east and west may have modified the cratonic show a sharp discontinuity in mantle composition at 900°C. root, mainly by introduction of eclogites near its base. Garnets from <145 km depth are ultradepleted in Y, Zr, Ti and Ga, whereas those from greater depths (to [200 km) are similar to garnets from Archean mantle world-wide. Relative abundances of garnet types indicate that the shallow layer consists of ~60% (clinopyroxene-free) harzburgite and 40% lherzolite, whereas the

Layered Mantle Lithosphere in the Lac de Gras Area, Slave Craton: Composition, Structure and Origin

Lithosphere mapping beneath the North American plate

Abstract Major- and trace-element analyses of garnets from heavy-mineral concentrates have been used to derive the compositional and thermal structure of the subcontinental lithospheric mantle (SCLM) beneath 16 areas within the core of the ancient Laurentian continent and 11 areas in the craton margin and fringing mobile belts. Results are presented as stratigraphic sections showing variations in the relative proportions of different rock types and metasomatic styles, and the mean Fo content of olivine, with depth. Detailed comparisons with data from mantle xenoliths demonstrate the reliability of the sections. In the Slave Province, the SCLM in most areas shows a two-layer structure with a boundary at 140–160 km depth. The upper layer shows pronounced lateral variations, whereas the lower layer, after accounting for different degrees of melt-related metasomatism, shows marked uniformity. The lower layer is interpreted as a subcreted plume head, added at ca. 3.2 Ga; this boundary between the layers rises to Outside the Slave Province, most North American Archon SCLM sections are less depleted than similar sections in southern Africa and Siberia; this may reflect extensive metasomatic modification. In E. Canada, the degree of modification increases toward the craton margin, and the SCLM beneath the Kapuskasing Structural Zone is typical of that beneath Proterozoic to Phanerozoic mobile belts. SCLM sections from several Proterozoic areas around the margin of the Laurentian continental core (W. Greenland, Colorado–Wyoming district, Arkansas) show discontinuities and gaps that are interpreted as the effects of lithosphere stacking during collisional orogeny. Some areas affected by Proterozoic orogenesis (Wyoming Craton, Alberta, W. Greenland) appear to retain buoyant, modified Archean SCLM. Possible juvenile Proterozoic SCLM beneath the Colorado Plateau is significantly less refractory. The SCLM beneath the Kansas kimberlite field is highly melt-metasomatised, reflecting its proximity to the Mid-Continent Rift System. A traverse across the continent shows that the upper part of the cratonic SCLM is highly magnesian; the decrease in mg# with depth is interpreted as the cumulative effect of metasomatic modification through time. The relatively small variations in seismic velocity within the continental core largely reflect the thickness of this depleted layer. The larger drop in seismic velocity in the surrounding Proton and Tecton belts reflects the closely coupled changes in SCLM composition and geotherm.

/pdf/lithospheric-mapping-beneath-the-north-american-plate-3brfvzbzdb.pdf

Lithospheric mapping beneath the North American plate

https://www.geol.umd.edu/%7Emcdonoug/Lithium%20Group/Aulbach%20et%20al%20chem%20geol.pdf

B.J. Doyle

Papers

The origin and evolution of Archean lithospheric mantle

Layered Mantle Lithosphere in the Lac de Gras Area, Slave Craton: Composition, Structure and Origin

Lithosphere mapping beneath the North American plate

Lithospheric mapping beneath the North American plate

Mantle formation and evolution, Slave Craton: constraints from HSE abundances and Re–Os isotope systematics of sulfide inclusions in mantle xenocrysts