Mantle Root Research Articles

Upper mantle roots of Siberian craton basement structures along the Rift DSS profile

We investigate the upper mantle velocity structure through processing first arrival data from peaceful nuclear explosions. The reported 2D model has been obtained by ray tracing for a spherical Earth, unlike the classical plane-approximation approach with subsequent spherical symmetry corrections, which is not always applicable to a laterally heterogeneous subsurface. The upper mantle velocity highs and lows imaged to 200–220 km depths show obvious correlation with major structures of the craton basement. Namely, low-velocity zones are observed beneath basins, the largest (to 8.0–8.1 km/s) under the Vendian–Early Cambrian Sayan–Yenisei syneclise. A discontinuous high-velocity layer (8.6–8.7 km/s) at depths between 150 and 240 km is underlain by a zone of lower velocity (8.50–8.55 km/s) down to the 410 km discontinuity, where the velocity at the top of the transition zone is 9.4–9.5 km/s.

Formation of the North Atlantic Craton: Timing and mechanisms constrained from Re–Os isotope and PGE data of peridotite xenoliths from S.W. Greenland

In this contribution we present platinum-group element (PGE) abundance and Os isotope data of North Atlantic Craton peridotite xenoliths ( n = 62) sampled from four major clusters of kimberlite/ultramafic lamprophyre volcanism in W. Greenland. In addition, we analysed olivine separates from selected peridotites. We find distinct differences in peridotites sampled from the northern (including the reworked Archean Nagssuqtoqidian mobile belt) and southern portions of the W. Greenland fragment of the NAC (WG-NAC). The heavily serpentinized harzburgitic to dunitic peridotites from the northern WG-NAC are typically marked by Os and Ir abundances that are somewhat lower than those of primitive mantle. Pt and Pd abundances are significantly depleted resulting in chondrite-normalized PGE patterns consistent with extensive melt extraction. As a consequence of the melting process, removal of sulphide and possibly a portion of residual PGE-rich alloys is likely. Typically, Os isotopes of these northern samples are unradiogenic (γOs = − 14.2 to − 6.6, average ca.−9.2), with separated olivines being similar in composition. The unradiogenic Os isotope compositions of the northern peridotites result in T RD erupt model ages clustering between 2.7 and 3.2 and also at ca. 2.0 Gyr. The grouping of the T RD erupt model ages correspond well with the U–Pb zircon ages derived from the trondjemite–tonalite–granodiorite continental crust (∼ 2.8 Gyr) and the tectonic activity associated with the amalgamation and rifting of the WG-NAC and adjacent crust to the WG-NAC (Kangâmiut dykes ∼ 2.0 Gyr; Nagssuqtoqidian ∼ 1.8 Gyr). In detail however, a lack of Eoarchean Re depletion ages has to be recognized in this mantle root although the WG-NAC continental crust clearly preserves magmatic activity from the first 500 Ma of Earth's evolution. This lack of Eoarchean ages in the deep lithosphere may be due to unfortunate bias in the kimberlite eruption sites. Subordinate post-Archean melt extraction and metasomatic enrichment at ca. 2.0 Gyr is evident from the PGE and Os isotope systematics of some of these peridotites and is consistent with prolonged and episodic lithosphere evolution. The southern WG-NAC peridotites appear to lack the extreme Pd depletion that is endemic in their northern counterparts. Generally the Os isotopes in these samples are more radiogenic (γOs = − 11.5 to + 1.1, average ca. − 7.2) and show a very strong mode of 2.0 Gyr T RD erupt model ages. Only a small number of Archean T RD erupt model ages can be recognized in this sub-suite of the WG-NAC lithosphere. Despite their relatively depleted major element characteristics, the PGE systematics of the southern peridotites do not preserve evidence of mantle melting and Pd enrichment is evident in a number of samples. Olivine separates from selected southern peridotites typically yield older, Archean, T RD erupt model ages relative to the corresponding whole-rock peridotites, although Pd enrichment is evident in the olivine PGE patterns. The majority of WG-NAC peridotite xenoliths with 2.7 to 3.1 Gyr and 2.0 Gyr T RD erupt model depletion ages have normal FeO abundances that are consistent with shallow mantle melting. A significant subset of peridotite xenoliths are enriched in FeO and have 2.3–2.6 Gyr T RD erupt model ages that do not align with specific, well-characterized magmatic events, although granulite-facies metamorphism has been proposed during this period. Pronounced PGE over-printing via metasomatism appears to have affected this set of peridotites. Overall it is apparent that the WG-NAC mantle xenoliths record lithosphere stabilization at the Meso-Neoarchean boundary which was followed by selective modification that has altered and overprinted Os isotope, PGE systematics and FeO abundances in some parts of the lithosphere

Lithoprobe’s impact on the Canadian diamond-exploration industryThis article is one of a series of papers published in this Special Issue on the theme Lithoprobe — parameters, processes, and the evolution of a continent.Earth Sciences Sector Contribution 20070261; Lithoprobe Contribution 1479.

Lithoprobe aided and will continue to guide diamond exploration in Canada via a number of diverse ways and at scales from cratons to individual ore bodies. Specific transects outlined the domainal nature, geometry, and age relations of crust, subjacent mantle, and diamond-bearing roots in unusually rich detail for cratonic settings such as those of the Superior and Slave cratons, as well as pericratonic (shield) settings such as those of the Sask craton and Buffalo Hills terrane. The Superior and Slave examples of craton construction by multiple underthrust layers, as inferred from structural geometries observed in Lithoprobe seismic and magnetotelluric surveys, remain the clearest resolved to date. The first definition of the Sask craton occurred 10–15 years ago and helped fuel the diamond industry’s ongoing revision of Clifford’s Rule for exploration so as to include targets in Proterozoic shield settings. Main Lithoprobe transect results and supporting geoscience studies have helped to demonstrate the very different settings for paragenesis of both peridotitic and eclogitic types of diamonds found across Canada. Seismic methods have delineated many of the the oldest, typically depleted cratonic mantle blocks, as well as Archean and Proterozoic sutures, where larger percentages of eclogite might occur. Development of geophysical methods related to Lithoprobe data acquisition has led to metre-scale, high-resolution mapping of diamondiferous kimberlite pipes and shallow-dipping dykes, thus allowing more accurate estimates of their volume and more efficient mining of these diamond deposits.

A major element, PGE and Re–Os isotope study of Middle Atlas (Morocco) peridotite xenoliths: Evidence for coupled introduction of metasomatic sulphides and clinopyroxene

We present major element and PGE (platinum-group-element) abundances in addition to Re–Os isotope data for 11 spinel-facies whole rock peridotites from a single maar from the Middle Atlas Mountains, Morocco. Major element systematics of these xenoliths are generally correlated with indices of depletion. FeO–MgO systematics appear to suggest spinel-facies melting in the range of 5 to 25%. However, Al 2O 3 abundances in these xenoliths appear elevated relative to primitive mantle (P rima). The Al 2O 3 abundances in conjunction with other major elements require distinct re-enrichment of the Middle Atlas continental mantle root due to melt/rock reaction and precipitation of amphibole and/or clinopyroxene from passing silicate melts akin to MORB or OIB that evolved in reverse direction along the melting curves in e.g. FeO–MgO space. Sc and V confirm the range of apparent depletion and also indicate that the currently preserved fO 2 in these peridotites is distinctly different from fO 2 conditions observed in subduction zones. The majority of these xenoliths have low Os and Ir (I-PGEs) concentrations relative to P rima and modelled sulphide- and clinopyroxene-depleted residues of mantle melting under low fO 2, mid-ocean ridge-like conditions. Moreover, Pt and Pd (P-PGE) abundances are elevated when compared to their expected abundances after substantial melt extraction. Importantly, the systematically low Ir abundances in the majority of samples show well-correlated trends with Al 2O 3, MgO and Cu that are inconsistent with established melting trends. Os isotopes in the Middle Atlas xenoliths range from 187Os/ 188Os = 0.11604 to 0.12664 although most samples are close to chondritic. The Os isotope ratios are decoupled from 187Re/ 188Os but, together with Re abundances, also exhibit a good correlation with Al 2O 3, MgO and Cu. The major element, I-PGE and Os isotope correlations suggest that the initial melt depletion led to the exhaustion of sulphide and clinopyroxene (20 to 30%) without significant stabilization of I-PGE-rich alloys. During later modal metasomatism of the refractory Middle Atlas continental mantle root with silicate melts akin to MORB or OIB the introduction of clinopyroxene/amphibole reduced the volume of the melt inducing sulphur saturation in these melts causing precipitation of secondary sulphides. This coupled crystallization of pyroxenes and sulphides (chalcopyrite) resulted in the two-component mixing systematics exhibited by I-PGEs, Os isotopes with major elements and Cu preserved in the Middle Atlas continental mantle root.

Tracing the metasomatic and magmatic evolution of continental mantle roots with Sr, Nd, Hf and and Pb isotopes: A case study of Middle Atlas (Morocco) peridotite xenoliths

Abstract We studied clinopyroxenes from spinel-facies peridotite xenoliths sampled by the Quaternary intra-plate volcanism of the Middle Atlas (Morocco) and present new trace element and Sr–Nd–Hf isotope data. However, we focus in particular on Pb isotope data and 238 U/ 204 Pb and 232 Th/ 204 Pb ratios of these clinopyroxenes. This data allows us to investigate: (a) the timing of metasomatic events, (b) the prevalence and persistence of elevated 238 U/ 204 Pb, 232 Th/ 238 U and 232 Th/ 204 Pb in continental mantle roots and (c) the 238 U/ 204 Pb and 232 Th/ 204 Pb composition of putative basaltic melts generated from such metasomatised sub-continental lithospheric mantle (SCLM). Incompatible trace element concentrations in these clinopyroxenes are elevated, marked by high-field strength element depletion and fractionated elemental ratios (e.g., U/Nb, Zr/Hf) most consistent with enrichment due to carbonatitic liquids. Sr, Nd and Hf isotopes have an affinity to HIMU. U, Th and Pb abundances in the clinopyroxenes generally exceed estimates of primitive mantle clinopyroxene. Pb isotope compositions of these clinopyroxenes are radiogenic and vary between 206 Pb/ 204 Pb = 19.93–20.25, 207 Pb/ 204 Pb = 15.63–15.66 and 208 Pb/ 204 Pb = 39.72–40.23. These Pb isotope systematics result in generally negative Δ7/4 but positive Δ8/4; setting these samples distinctly apart from typical HIMU. These Pb isotope compositions are also distinct from the associated host volcanic rocks. 238 U/ 204 Pb and 232 Th/ 204 Pb of these clinopyroxenes, which range from 26 to 81 and 136 to 399, respectively, are elevated and more extreme than estimates of MORB- and HIMU-source mantle. The Pb isotope evolution of the clinopyroxenes suggests that the metasomatic enrichment is younger than 200 Ma, which discounts the volcanic activity due to the opening of the Atlantic and the onset of the collision of the African and Eurasian plates as processes generating the lithophile element and isotope composition of this continental mantle root. Instead, the enrichment is thought to be associated with the Quaternary intra-plate volcanism in the Middle Atlas. However, the erupted mafic melts have unradiogenic Pb isotopes and lower 238 U/ 204 Pb, 232 Th/ 204 Pb and 232 Th/ 238 U relative to the clinopyroxene and do not seem to have equilibrated with the clinopyroxenes. The high Th abundances and the high 232 Th/ 238 U also suggest that the metasomatism was due to carbonatitic liquids. When literature data for Pb isotopes in mantle minerals are considered, the Pb isotope range of Archean, Proterozoic and Phanerozoic continental mantle roots is remarkable in that they are similar to the convecting mantle. This observation does not support the existence of sub-continental lithospheric mantle with high 238 U/ 204 Pb and 232 Th/ 204 Pb for long periods of time. Consequently, the narrow range of Pb isotopes in SCLM worldwide suggests that only the youngest metasomatic events are recorded by incompatible elements such as U, Th and Pb. Numerical modelling of putative magmas generated from Middle Atlas SCLM by fractional, non-modal melting calculations yield extremely high 238 U/ 204 Pb and 232 Th/ 204 Pb ratios. For example, pure SCLM magmas generated from 0.5% to 10% melting are anticipated to have 232 Th/ 204 Pb ratios exceeding those known from terrestrial basalts.

Were deep cratonic mantle roots hydrated in Archean oceans?

Using a compilation of mantle peridotites (n = 2153), we statistically evaluate the frequency of Si enrichment in cratonic and other types of mantle lithosphere. We defi ne an empirical parameter (ΔMg/Si) describing the degree of Si enrichment or depletion in mantle lithosphere relative to residue trends defi ned by high-pressure, high-temperature melting experiments. Silica enrichment (a strong skew to negative ΔMg/Si) is absent in the majority of cratonic xenoliths, and mostly occurs only in those from southern Africa, and in abyssal peridotites. The Si-rich composition of cratonic mantle, where it occurs, can be linked to the hydration of its protoliths on the Archean ocean fl oor before being subducted or imbricated to form a craton root. Oxygen isotopic shifts that correlate with bulk Mg/Si in mid-Atlantic ridge seafl oor rocks parallel those seen in the few such data for cratonic peridotite xenoliths, in support of our hypothesis. Chemical variability in the mantle is canonically viewed to have originated from the bottom up by percolating melts. We turn this idea on its head, and explain how Si enrichment in the cratonic lithosphere could have originated by a top-down chemical exchange during weathering or hydrothermal activity when such peridotites resided on an Archean ocean fl oor.

An integrated petrological, geochemical and Re–Os isotope study of peridotite xenoliths from the Argyle lamproite, Western Australia and implications for cratonic diamond occurrences

An integrated study of the petrology and Re–Os geochemistry of a suite of peridotite xenoliths, some carrying abundant diamonds, from the richly diamondiferous Argyle AK1 lamproite pipe provides definitive evidence for a depleted lithospheric root of Neoarchean age (T RD eruption ∼ 2.2–3.1 Ga) beneath the Proterozoic Halls Creek Orogen at the margin of the Kimberley Craton, Western Australia. The microdiamonds from the peridotitic xenoliths are similar in their properties to the minor population of small, commercial sized, peridotitic diamonds from Argyle, both formed in the Archean from isotopically mantle-like carbon. The major element bulk chemistry and mineral chemistry of the Argyle peridotites are slightly less depleted than Archean cratonic peridotites as a whole but similar to those reported from Neoarchean–Paleoproterozoic cratonic provinces. The Argyle peridotite xenoliths were derived from within the diamond stability field (1050–1300 °C and 4.9–5.9 GPa) near the base of the lithosphere (typically 160–200 km depth) with a geothermal gradient of 41.5 mW/m 2. This thick diamondiferous lithosphere, estimated at up to 225 km thick from present day seismic S-wave tomography, appears to have persisted since the time of eruption of the Argyle lamproite (∼ 1180 Ma). The existence of late Archean age lithosphere beneath the Argyle diamond pipe, in a region where no crustal rocks of Archean age are known, suggests a decoupling of the crust and mantle in the region of the Halls Creek Orogen, perhaps as a consequence of Paleoproterozoic (∼ 1.85 Ga) reworking and/or subduction at the margin of the Kimberley Craton. The confirmation of an Archean lithospheric root beneath the Argyle pipe at the margin of the Kimberley Craton seemingly conforms with “Clifford's Rule”, regarding the restriction of economic diamond deposits to those underlain by Archean cratons. However, Argyle owes its rich diamond grades not to its Neoarchean mantle roots but to the presence of richly diamondiferous eclogitic material accreted to the craton root during the Proterozoic.

Phanerozoic burial and unroofing history of the western Slave craton and Wopmay orogen from apatite (U–Th)/He thermochronometry

Low temperature thermochronometry of cratonic regions can illuminate relationships among burial and unroofing patterns, surface subsidence and uplift, and lithosphere–asthenosphere interactions. The Slave craton, initially stabilized by the development of a thick lithospheric mantle root in late Archean time, is an excellent location in which to examine these connections. Although the Slave craton currently lacks Phanerozoic cover, Phanerozoic sedimentary xenoliths entrained in ca. 610 to 45 Ma kimberlites indicate that the region underwent a more dynamic history of burial and unroofing than widely recognized. We report new apatite (U–Th)/He thermochronometry data along a southeast to northwest transect from the interior of the Slave craton into the adjacent Paleoproterozoic Wopmay orogen to resolve the region's depositional and denudational history. Six samples from the western Slave craton and three samples from Wopmay orogen yield mean dates from 296 ± 41 Ma to 212 ± 39 Ma. Individual apatite dates are broadly uniform over a wide span of apatite [eU], and this pattern can be used to more tightly restrict the spectrum of viable temperature–time paths that can explain the dataset. When coupled with geologic and stratigraphic information, temperature–time simulations of the thermochronometry results suggest complete He loss from the apatites at minimum peak temperatures of ~ 88 °C in Devonian–Pennsylvanian time, cooling to near-surface conditions by the Early Cretaceous, followed by reheating to ≤ 72 °C during Cretaceous–Early Tertiary time. Consideration of modern and ancient geotherm constraints implies ≥ 3.3 km of burial during the first Phanerozoic heating phase, with an ancillary phase of reburial in late Mesozoic–Cenozoic time. The uniformity of the apatite (U–Th)/He dates indicates that the rocks encompassed by our > 250 km-long sample transect experienced similar Phanerozoic thermal histories. Despite the distinctly different lithospheric architecture on either side of the Paleoproterozoic suture between the Slave craton and Wopmay orogen, the region behaved as a single, broadly coherent cratonic unit since at least ca. 250 Ma. The Phanerozoic burial and unroofing patterns across the craton may be a response to far-field convergent activity at the northern and western margins of North America and processes associated with episodic kimberlite emplacement.

Late Neoproterozoic rise and fall of the northern Arabian–Nubian shield: The role of lithospheric mantle delamination and subsequent thermal subsidence

Review of the Late Neoproterozoic history of the northern Arabian–Nubian Shield (ANS) reveals that rapid and extensive erosional denudation, widespread late-orogenic calc–alkaline and alkaline igneous activity and intramontane basin formation affected this terrane at 630–590 Ma. Core-complex extension is also recorded at this time interval. To account for this coupling it is suggested that the mantle lithosphere was removed/delaminated from below the northern ANS subsequent to significant crust–mantle thickening in the course of the Late Neoproterozoic orogeny. Because the physical properties of the delamination process are not clear, we use it as a conceptual term accounting for the replacement of mantle lithosphere by an asthenosphere. Removal and replacement of the thickened lithospheric mantle roots potentially caused uplift of the northern ANS to elevations of more than 3 km, thus triggering exceptionally-rapid erosional unroofing of a ca. 10 km thick rock carapace, and some lateral extension. Delamination and erosional decompression potentially caused partial melting of ANS residual upper mantle and lower crust to produce the widespread post-orogenic magmas. We emphasize that removal of the lithospheric mantle also played a key role in lowering ANS topography to sea level. Our data show that lowering ANS down to sea level was significantly accelerated by post-delamination cooling and thermal subsidence. Unlike erosional denudation which is followed by isostatic uplift, thermal contraction causes net surface lowering. We show that thermal subsidence plays a critical role in lowering mountain belts, particularly in the latest stages, when the topography reaches a threshold of ∼ 1.0 km and erosion rates greatly decrease. Post-delamination thermal subsidence can cause 1.3 km of net surface lowering within 100 m.y. regardless of the state of relief and elevation. Therefore, orogenic belts that experienced removal or delamination of their mantle lithosphere (hot orogens) would rapidly rise, but would relatively rapidly fall too.

Influence of Andean uplift on climate and paleoaltimetry estimates

Recent elevation reconstructions of the Andean Plateau suggest a rapid 2.5 ± 1.0 km rise of the central Andes between ~ 10 and 6 Ma. This rapid rise has been attributed to a catastrophic removal of a dense lithospheric mantle root beneath the Andes. However, these findings are based on the assumption that climate did not change during deposition of paleoaltimetry proxies. Here we evaluate South American climate change due to Andean uplift and its influence on interpretations of plateau elevation from climate-sensitive paleoaltimetry data. A series of experiments are presented using the RegCM3 regional general circulation model (RCM) to characterize changes in Andean precipitation amount, surface temperature, and wind direction (vapor source) as a function of changing plateau elevation. Results indicate that South American and Andean climate changed significantly in response to plateau growth. More specifically, rising of the plateau results in up to a 900 mm increase in rainy season (December–January–February) precipitation over the plateau. Plateau uplift also results in a decrease in non-adiabatic surface temperature of up to 6.5 °C (in addition to adiabatic cooling directly related to elevation change through the lapse rate). Finally, the prevailing wind direction and the vapor source for precipitation switches from the South Pacific Ocean to the equatorial Atlantic as plateau elevation increases above ½–¾ of its present-day elevation. Taken together, these changes in paleoclimate would have substantially depleted the oxygen isotopic concentration of paleoprecipitation through the Cenozoic. Unless this climatic effect is taken into consideration, paleoaltimetry reconstructions based on stable isotope methods may overestimate the rapid rise of the Andes by up to several kilometers. We conclude that some or all of the apparent rapid rise of the Andean Plateau from paleoaltimetry data could be an artifact of large changes in paleoclimate.

Parental magmas of Grenville Province massif-type anorthosites, and conjectures about why massif anorthosites are restricted to the Proterozoic

ABSTRACTTrace element inversion modelling of Grenvillean anorthosite massifs and associated rocks yield NMORB-normalised trace element profiles enriched in highly incompatible elements; commonly with negative Nb and Th anomalies. Model melts can be divided into subtypes that cannot be linked through fractional crystallisation processes. Most model melts are depleted in the heavy rare-earth elements and can be explained by partial melting of arc basaltic sources (5–60 melting ) with garnet-bearing residues. Some of the model melts have flat NMORB-normalised profiles (for rare-earth elements), have high compatible element contents, and might have been derived from mantle fertilised by arc magmatism, followed by low-pressure fractional crystallisation. Intermediate Ce/Yb types may represent mixtures of these end-members, or less probably, variations in the crustal source composition and residual assemblage. The active tectonic context now favoured for the Grenville Province appears to be inconsistent with plume or thermal insulation models. The heat source for crustal and mantle melting could record either post-orogenic thermal relaxation of a tectonically-thickened arc crust, or basaltic underplating caused by delamination of a mantle root or subduction slab beneath this arc crust. In this context, pre-Proterozoic anorthosites may be lacking, because prior to ca. 2·5 Ga, the crust may have been too weak to be thickened tectonically. The absence of post-Proterozoic anorthosites may be due to the secular decrease in radiogenic heating and cooling of the mantle and crust.

Arc imbrication during thick-skinned collision within the northern Cordilleran accretionary orogen, Yukon, Canada

Abstract We present the results of geological mapping and geochronological studies of the Tally Ho shear zone (THSZ) and adjacent rocks. The shear zone crops out near the west margin of Stikinia, an oceanic arc and the largest of the accreted terranes within the Cordilleran orogen of western North America. The hanging wall of the largely flat-lying shear zone consists of coarsely crystalline leucogabbro and cumulate pyroxenite interpreted as the lower crustal and possibly lithospheric mantle roots of a magmatic arc. Rocks in the footwall consist of volcanic and volcano-sedimentary sequences of the Lewes River Arc, a Late Triassic magmatic arc characteristic of Stikinia. Because the shear zone places lower crustal plutonic rocks over a supracrustal sequence, we interpret it as a crustal-scale thrust fault. Kinematic indicators imply top-to-the-east displacement across the shear zone. The geometry of folds of the shear zone is consistent with deformation in response to displacement over ramps in deeper-seated thrust faults kinematically linked to the THSZ. Crystallization of the hanging-wall leucogabbro at 208±4.3 Ma provides a maximum age constraint for deformation, whereas a post-kinematic granitoid pluton that plugs the shear zone and that crystallized at about 173 Ma provides a lower age limit. The THSZ is, therefore, coeval with: (1) a series of latest Triassic–Early Jurassic shear and fault zones that characterize the length of the west margin of Stikinia; (2) the termination of isotopically juvenile arc magmatism of the Lewes River Arc; (3) crustal loading of Stikinia giving rise to a foreland basin that rapidly filled with westerly derived orogenic molasse that includes clasts of ultrahigh-pressure metamorphic rocks; and (4) juxtaposition of Stikinia against continental crust of the Nisling Assemblage of the Yukon–Tanana terrane to the west. These constraints are consistent with a model of deformation in response to the entry of the continental Nisling Assemblage into the trench of the west-facing Lewes River Arc, terminating subduction and imbricating the arc along a series of east-verging thrust faults, including the THSZ.

Regolith mass balance inferred from combined mineralogical, geochemical and geophysical studies: Mule Hole gneissic watershed, South India

The aim of this study is to propose a method to assess the long-term chemical weathering mass balance for a regolith developed on a heterogeneous silicate substratum at the small experimental watershed scale by adopting a combined approach of geophysics, geochemistry and mineralogy. We initiated in 2003 a study of the steep climatic gradient and associated geomorphologic features of the edge of the rifted continental passive margin of the Karnataka Plateau, Peninsular India. In the transition sub-humid zone of this climatic gradient we have studied the pristine forested small watershed of Mule Hole (4.3 km 2) mainly developed on gneissic substratum. Mineralogical, geochemical and geophysical investigations were carried out (i) in characteristic red soil profiles and (ii) in boreholes up to 60 m deep in order to take into account the effect of the weathering mantle roots. In addition, 12 Electrical Resistivity Tomography profiles (ERT), with an investigation depth of 30 m, were generated at the watershed scale to spatially characterize the information gathered in boreholes and soil profiles. The location of the ERT profiles is based on a previous electromagnetic survey, with an investigation depth of about 6 m. The soil cover thickness was inferred from the electromagnetic survey combined with a geological/pedological survey. Taking into account the parent rock heterogeneity, the degree of weathering of each of the regolith samples has been defined using both the mineralogical composition and the geochemical indices (Loss on Ignition, Weathering Index of Parker, Chemical Index of Alteration). Comparing these indices with electrical resistivity logs, it has been found that a value of 400 Ohm m delineates clearly the parent rocks and the weathered materials. Then the 12 inverted ERT profiles were constrained with this value after verifying the uncertainty due to the inversion procedure. Synthetic models based on the field data were used for this purpose. The estimated average regolith thickness at the watershed scale is 17.2 m, including 15.2 m of saprolite and 2 m of soil cover. Finally, using these estimations of the thicknesses, the long-term mass balance is calculated for the average gneiss-derived saprolite and red soil. In the saprolite, the open-system mass-transport function τ indicates that all the major elements except Ca are depleted. The chlorite and biotite crystals, the chief sources for Mg (95%), Fe (84%), Mn (86%) and K (57%, biotite only), are the first to undergo weathering and the oligoclase crystals are relatively intact within the saprolite with a loss of only 18%. The Ca accumulation can be attributed to the precipitation of CaCO 3 from the percolating solution due to the current and/or the paleoclimatic conditions. Overall, the most important losses occur for Si, Mg and Na with −286 × 10 6 mol/ha (62% of the total mass loss), −67 × 10 6 mol/ha (15% of the total mass loss) and −39 × 10 6 mol/ha (9% of the total mass loss), respectively. Al, Fe and K account for 7%, 4% and 3% of the total mass loss, respectively. In the red soil profiles, the open-system mass-transport functions point out that all major elements except Mn are depleted. Most of the oligoclase crystals have broken down with a loss of 90%. The most important losses occur for Si, Na and Mg with −55 × 10 6 mol/ha (47% of the total mass loss), −22 × 10 6 mol/ha (19% of the total mass loss) and −16 × 10 6 mol/ha (14% of the total mass loss), respectively. Ca, Al, K and Fe account for 8%, 6%, 4% and 2% of the total mass loss, respectively. Overall these findings confirm the immaturity of the saprolite at the watershed scale. The soil profiles are more evolved than saprolite but still contain primary minerals that can further undergo weathering and hence consume atmospheric CO 2.

Primary melting sequence of a deep (> 250 km) lithospheric mantle as recorded in the geochemistry of kimberlite–carbonatite assemblages, Snap Lake dyke system, Canada

Geochemistry of kimberlites and associated carbonatites from the Snap Lake dyke system, Slave craton, Canada, shows that kimberlites are similar, but not identical to Group I kimberlites from South Africa and Siberia. Snap Lake kimberlites are enriched in most incompatible elements such as U, Th, Nb and LREE (up to 1400 times chondritic abundances), and are weakly differentiated relative to primitive mantle in their Sr and Nd isotope compositions. The Snap Lake carbonatites and kimberlites are enriched or depleted in certain trace elements relative to each other. Carbonatites are depleted in Cs, Rb, K, Ta and Ti but enriched in U, Sr, P, Zr, Hf, middle and heavy REEs relative to kimberlites. The observed element distributions are not consistent with experimental data on distribution of trace elements between immiscible carbonate and silicate liquids. However, the chemistry of these rocks shows a continuous systematic change from carbonatites to kimberlites, suggesting their genetic relationship. The range of major element concentrations from low SiO 2 (3 wt.%) magmas to typical kimberlites are in excellent agreement with experimental data for melting of carbonated lherzolite at high pressure. Carbonatites have significantly superchondritic Nb/Ta (22–81) and Zr/Hf (57–128) ratios, which systematically decrease with increase of SiO 2. Modeling of partial melting requires that the source for Snap Lake dyke rocks was enriched in incompatible elements but depleted in HREE relative to the primitive mantle. The most appropriate source rocks are metasomatically enriched peridotites of the deep lithospheric mantle roots. In the melting sequence of this source, the incipient magma was carbonatitic in composition but subsequently became kimberlitic as the degree of partial melting increased to 1%. Rocks of the Snap Lake dyke represent a natural example of primary melting process within a deep (> 250 km) lithospheric mantle.

Origin of cratonic lithospheric mantle roots: A geochemical study of peridotites from the North Atlantic Craton, West Greenland

A critical examination of the extent to which geodynamic information on the initial mantle depletion and accretion event(s) is preserved in kimberlite-borne cratonic SCLM peridotite xenoliths is attempted by using new major and trace element data of whole-rock peridotites ( n = 55) sampled across the North Atlantic Craton (NAC; West Greenland). We also present additional whole-rock trace element data of mantle xenoliths from Somerset Island, the Slave and Kaapvaal cratons for comparison. Peridotites comprising the West Greenland SCLM are distinctly more olivine-rich and orthopyroxene-poor than most other cratonic peridotites, in particular those from the Kaapvaal craton. The West Greenland peridotites have higher Mg/Si but lower Al/Si, Al 2O 3 and CaO than cratonic mantle from the Kaapvaal Craton. We suggest that the more orthopyroxene depleted, harzburgite to dunite character of the NAC peridotites reflects more of the original melting history than peridotites from other cratons and in that sense may be more typical of cratonic lithosphere compositions prior to extensive modification. Despite this, some modal and cryptic metasomatism has clearly taken place in the West Greenland lithosphere. The insensitivity of major elements to pressure of melting at high degrees of melt extraction combined with the ease with which these elements may be changed by modal metasomatism mean that we cannot confidently constrain the depth of melting of peridotites using this approach. Mildly incompatible trace elements offer much more promise in terms of providing geodynamic information about the original Archean melting regime. The very low, systematically varying heavy REE abundances in NAC whole-rock peridotites and in peridotites from all other cratons where high-quality data are available provide ubiquitous evidence for a shallow melting regime in the absence of, or to the exhaustion of garnet. This finding explicitly excludes large extents of deep (iso- and polybaric) melting, which results in high initial garnet abundances and increasing heavy REE abundances. This evidence renders models that invoke large plume-like melting environments redundant in explaining SCLM formation and suggests broadly modern plate tectonic environments are responsible for the depletion of cratonic SCLM. A combination of the shallow melting environment and uniformly high levels of depletion indicate that melting to form the NAC lithosphere and that of other cratons probably took place in a subduction-zone environment.

Thermal evolution of cratonic roots

Cratons stabilized in the Archean because their mantle roots remained strong despite temperatures in the convecting mantle and amounts of radiogenic elements in the crust and in the lithospheric mantle that were larger than today. Thermal evolutionary models are developed using constraints from heat flow and heat production data in Archean provinces. The large time-scale of diffusive heat transport implies that the lithospheric mantle can remain thermally decoupled from the crust for as long as 1 Gyr. Heat production in the lithospheric mantle is a key variable in determining thermal conditions that permit stabilization of the crust and the preservation of a thick cratonic mantle root. Archean provinces are currently characterized by low heat flow, with an average of 41 mW m − 2 less than the global continental average (56 mW m − 2 ). The range of regionally averaged heat flow values in Archean provinces (18–54 mW m − 2 ) is narrower than in Proterozoic and Paleozoic terranes. However, at the end of the Archean, when crustal heat production was double the present value, surface heat flow would have varied over a range (≈ 45–90 mW m − 2 ) at least as wide as that presently observed in Paleozoic provinces. The high crustal heat production during the Archean is not sufficient to account for elevated lower crustal temperatures recorded in some metamorphic assemblages without some additional heat input, or without the crust being thicker, or the vertical distribution of radioelements being different from today's. Thermal models and the observed correlation between the degree of upper crustal enrichment and heat production values indicate that crustal melting and differentiation were largely driven by heating due to in-situ radiogenic heat production. Even for values of the surface heat flow higher than average in cratons, the crust can be stabilized before 2.5 Ga if the radioelements are confined to shallow crustal layers. For present surface heat flow of 40–45 mW m − 2 , calculations indicate that, prior to differentiation, the lowermost crust was near the solidus. Present heat production in the mantle root is constrained by estimates of the mantle heat flow. Further constraints can be obtained by modeling the past thermal regime of the root when heat production was higher. If heat generation is high and/or if the root is thick, the lower lithosphere must have cooled more rapidly than the convecting mantle, which implies that the temperature gradient was inverted at the base of the lithosphere and a weak mechanical layer formed in the middle of the root, precluding its survival. A low temperature gradient at the base of the root leads to the development of convective instabilities and possible removal of the lowermost part. Following its isolation from the convecting mantle and stabilization, thick continental lithosphere adjusts slowly to in-situ heat production and basal heat supply. We show that, due to in-situ heat production, temperatures in the lithospheric mantle may rise above their initial values inherited from the process of root formation due to in-situ radiogenic heat production.

The role of serpentine in preferential craton formation in the late Archean by lithosphere underthrusting

Cratons form the cores of continents and were formed within a narrow window of time (2.5–3.2 Gy ago), the majority having remained stable ever since. Petrologic evidence suggests that the thick mantle roots underlying cratons were built by underthrusting of oceanic and arc lithosphere, but paradoxically this requires that the building blocks of cratons are weak even though cratons must have been strong subsequent to formation. Here, we propose that one form of thickening could be facilitated by thrusting of oceanic lithospheres along weak shear zones, generated in the serpentinized upper part of the oceanic lithosphere (crust + mantle) due to hydrothermal interaction with seawater. Conductive heating of the shear zones eventually causes serpentine breakdown at ~ 600 °C, shutting down the shear zone and culminating in craton formation. However, if shear zones are too thin, serpentine breakdown and healing of the shear zone occurs too soon and underthrusting does not occur. If shear zones are too thick, serpentine breakdown takes too long so healing and lithospheric thickening is not favored. Shear zone thicknesses of ~ 18 km are found to be favorable for craton formation. Because the maximal depth of seawater-induced serpentinization into the lithosphere is limited by the depth of the isotherm for serpentine breakdown, shear zone thicknesses should have increased with time as the Earth's heat flux and depth to the serpentine breakdown isotherm decreased and increased, respectively, with time. We thus suggest that the greater representation of cratons in the late Archean might not necessarily be explained by preferential recycling in the early Archean but may simply reflect preferential craton formation in the late Archean. That is, our model predicts that the early Archean was too hot, the Phanerozoic too cold, and the late Archean just right for making cratons.

Canada's craton: A bottoms-up view

The origin of mantle lithosphere underlying Archean crustal provinces is most consistent with depletion at low pressures in the spinel facies under degrees of melting higher than observed in modern ocean basins. Depleted sections of the lithosphere created in convergent margin settings were underthrust and stacked to build a thick root with time. Geochronologic and geologic evidence can be interpreted to show that the final formation and amalgamation of the bulk of the “mantle root” occurs 0.5–1 b.y. later than the age of the lithosphere from which it is comprised. “Silica enrichment” is not ubiquitous in the mantle beneath Archean crustal provinces. Where it does occur, it may be a heterogeneous feature possibly imparted by marine weathering of peridotite on the Archean ocean floor before it was stacked to form a mantle root.

Creation of a continent recorded in zircon zoning

We have discovered a robust microcrystalline record of the early genesis of North American lithosphere preserved in the U-Pb age and oxygen isotope zoning of zircons from a lower crustal paragneiss in the Neoarchean Superior province. Detrital igneous zircon cores with δ 18 O values of 5.1‰–7.1‰ record creation of primitive to increasingly evolved crust from 2.85 ± 0.02 Ga to 2.67 ± 0.02 Ga. Sharp chemical unconformity between cores and higher δ 18 O (8.4‰–10.4‰) metamorphic overgrowths as old as 2.66 ± 0.01 Ga dictates a rapid sequence of arc unroofing, burial of detrital zircons in hydrosphere-altered sediment, and transport to lower crust late in upper plate assembly. The period to 2.58 ± 0.01 Ga included ∼80 m.y. of high-temperature (∼700–650 °C), nearly continuous overgrowth events reflecting stages in maturation of the subjacent mantle root. Huronian continental rifting is recorded by the youngest zircon tip growth at 2512 ± 8 Ma (∼ 600 °C) signaling magma intraplating and the onset of rigid plate behavior. This >150 m.y. microscopic isotope record in single crystals demonstrates the sluggish volume diffusion of U, Pb, and O in zircon throughout protracted regional metamorphism, and the consequent advances now possible in reconstructing planetary dynamics with zircon zoning.

Similarities between Archean high MgO eclogites and Phanerozoic arc-eclogite cumulates and the role of arcs in Archean continent formation

Some insights into the origin of cratonic mantle can be gained from “eclogite” (loosely defined here as an assemblage containing garnet and any pyroxene) xenoliths hosted in kimberlites erupted through Archean (~ 2.5­3.5 Gy) cratons. One subset of Archean eclogite xenoliths, the low MgO Archean xenoliths, is presently believed to represent metamorphosed fragments of ancient altered oceanic crust, leading to the suggestion that Archean cratons were built, at least in part, by the accretion of oceanic lithospheric segments. However, another Archean subset, the high MgO Archean eclogite xenoliths, have major and compatible trace-element (Ni and Cr) systematics similar to high MgO arc-eclogite xenoliths originating from the lithospheric root underlying the Sierra Nevada batholith in California, an example of a Phanerozoic arc. The Sierran high MgO arc-eclogites represent cumulates from hydrous basaltic magmas beneath a thick continental arc. The compositional similarities between the Archean and Sierran high MgO eclogites suggest that not only might the Archean high MgO eclogites have a cumulate origin, as has previously been suggested, but they may be arc-related. If so, Archean high MgO eclogites provide evidence from within the mantle roots of cratons that some form of arc magmatism contributed to the formation and evolution of Archean continents.