Cratonic Mantle Research Articles

Isopycnicity of cratonic mantle restricted to kimberlite provinces

The isopycnicity hypothesis states that the lithospheric mantle of ancient platforms has a unique composition such that high density due to low lithosphere temperature is nearly compensated by low-density composition of old cratonic mantle. This hypothesis is supported by petrological studies of mantle xenoliths hosted in kimberlite magmas. However, the representativeness of the kimberlite sampling may be questioned, given that any type of magmatism is atypical for stable regions. We use EGM2008 gravity data to examine the density structure of the Siberian lithospheric mantle, which we compare with independent constraints based on free-board analysis. We find that in the Siberian craton, geochemically studied kimberlite-hosted xenoliths sample exclusively those parts of the mantle where the isopycnic condition is satisfied, while the pristine lithospheric mantle, which has not been affected by magmatism, has a significantly lower density than required by isopycnicity. This discovery allows us to conclude that our knowledge on the composition of cratonic mantle is incomplete and that it is biased by kimberlite sampling which provides a deceptive basis for the isopycnicity hypothesis.

Making Cratonic Lithospheric Mantle

AbstractThe origin of cratonic lithospheric mantle has been attributed to either high‐pressure (5–7 GPa) melting in hot mantle plumes or low‐pressure (<5 GPa) melting in mid‐ocean ridges or suprasubduction zones. To resolve this long‐standing debate, it is necessary to confirm under what depths the incipient cratonic mantle melted. Compared with most cratonic mantle xenoliths and xenocrysts, which commonly experienced metasomatic modification after melt extraction, diamond inclusions with predominantly harzburgitic compositions can better track the compositional signature of pristine cratonic mantle. This paper presents thermodynamic calculations performed with THERMOCALC software to establish a quantitative relationship between garnet and cratonic peridotite compositions. Along the normal cratonic geotherm (40 mW/m2), the XCa [atomic Ca/(Ca + Mg + Fe2+)] and Cr# [atomic Cr/(Cr + Al) × 100] values in garnet depend mainly on the bulk CaO/Al2O3 and Cr# values, respectively. Furthermore, mantle melting modeling shows that the Cr# of the residue displays a negative correlation with melting pressure at melt fractions of greater than ~15%. Therefore, the high Cr# values (mostly 12–36) of global garnet inclusions in diamond support a low‐pressure (<4 GPa) origin for cratonic lithospheric mantle. More importantly, the incipient cratonic mantle calculated from the garnet inclusions exhibits similar bulk CaO/Al2O3 and Cr# compositions to oceanic lithosphere but different values from arc lithosphere with higher CaO/Al2O3 and Cr# values. These results imply that cratonic mantle was initially formed by the extensive melting of hot ambient mantle in the Archean ocean ridge environments. The shallow oceanic lithosphere was subsequently stacked to generate the thick and stable cratonic lithospheric mantle.

Spatiotemporal Variation of Mantle Viscosity and the Presence of Cratonic Mantle Inferred From 8 Years of Postseismic Deformation Following the 2010 Maule, Chile, Earthquake

AbstractPostseismic displacements following great subduction earthquakes show significant long‐wavelength and time‐dependent patterns caused primarily by transient viscoelastic relaxation processes occurring broadly at depth. However, the Earth's viscosity structure and time‐dependent variations are still poorly understood, especially in the years immediately following a great earthquake. Here we investigate the spatiotemporal variation of mantle viscosity proximal and distal to the southern Andes using 8 years of continuous high‐resolution GPS observations following the 2010 Mw 8.8 Maule earthquake in south Central Chile. We remove the potential influences of relocking and afterslip on far‐field GPS displacements and estimate viscosities that can explain the 3‐D displacements. The optimal viscosity structure exhibits a low‐viscosity (~1018Pa s) mantle beneath the Andean volcanic arc and high‐viscosity (>1022Pa s) cratonic mantle, indicating a dependence of transient viscosity on temperature. Comparisons of the viscosity distributions at different times show that mantle viscosities increase with time throughout the study region. Viscosity increase is generally fastest in the mantle wedge beneath the Andes and slows down with increasing distance from the source region of the Maule earthquake. Such temporal viscosity evolution may indicate a stress dependence of the viscosity proximal to the rupture zone, while regions east of the Andes act as a relatively rigid body (i.e., cratonic mantle) with much higher viscosity. Our results thus suggest that both temperature structure and stress state contribute to spatiotemporal variations of the mantle viscosity. Heterogeneous spatiotemporal variations of viscosity seem to control the expansion and duration of the postseismic deformation and therefore the postseismic stress evolution.

Diamondiferous Paleoproterozoic mantle roots beneath Arctic Canada: A study of mantle xenoliths from Parry Peninsula and Central Victoria Island

While the mantle roots directly beneath Archean cratons have been relatively well studied because of their economic importance, much less is known about the genesis, age, composition and thickness of the mantle lithosphere beneath the regions that surround the cratons. Despite this knowledge gap, it is fundamentally important to establish the nature of relationships between this circum-cratonic mantle and that beneath the cratons, including the diamond potential of circum-cratonic regions. Here we present mineral and bulk elemental and isotopic compositions for kimberlite-borne mantle xenoliths from the Parry Peninsula and Central Victoria Island, Arctic Canada. These xenoliths provide key windows into the lithospheric mantle underpinning regions to the North and Northwest of the Archean Slave craton, where the presence of cratonic material has been proposed. The mantle xenolith data are supplemented by mineral concentrate data obtained during diamond exploration. The mineral and whole rock chemistry of peridotites from both localities is indistinguishable from that of typical cratonic mantle lithosphere. The cool mantle paleogeotherms defined by mineral thermobarometry reveal that the lithospheric mantle beneath the Parry Peninsula and Central Victoria Island terranes extended well into the diamond stability field at the time of kimberlite eruption, and this is consistent with the recovery of diamonds from both kimberlite fields. Bulk xenolith Se and Te contents, and highly siderophile element (including Os, Ir, Pt, Pd and Re) abundance systematics, plus corresponding depletion ages derived from Re-Os isotope data suggest that the mantle beneath these parts of Arctic Canada formed in the Paleoproterozoic Era, at ∼2 Ga, rather than in the Archean. The presence of a diamondiferous Paleoproterozoic mantle root is part of the growing body of global evidence for diamond generation in mantle roots that stabilized well after the Archean. In the context of regional tectonics, we interpret the highly depleted mantle compositions beneath both studied regions as formed by mantle melting associated with hydrous metasomatism in the major Paleoproterozoic Wopmay-Great Bear-Hottah arc systems. These ∼2 Ga arc systems were subsequently accreted along the margin of the Slave craton to form a craton-like thick lithosphere with diamond potential thereby demonstrating the importance of subduction accretion in building up Earth’s long-lived continental terranes.

Trace metal and isotopic depth profiles through the Abitibi cratonic mantle

Geophysical imaging of trans-lithospheric structures provide a spatial link between ore deposits in the crust and the underlying cratonic mantle. However, the deep lithosphere's role in ore deposit genesis remains poorly understood because remotely acquired datasets do not provide any direct constraints on the behaviour of ore elements within these mantle-roots. The abundance and behaviour of ore elements governs the metallic endowment of the cratonic mantle and the economic potential of mantle-derived magmas. Herein we present in situ electron probe microanalysis (EPMA) and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) geochemical datasets for clinopyroxene and olivine mantle xenocrysts from the Jurassic Kirkland Lake kimberlite field, Abitibi greenstone belt, Canada. We specifically focus on unconventional trace elements, including ore elements with chalcophile and/or siderophile affinities (Ag-As-Au-Bi-Cu-Mo-Pb-Pt-Pd-Sb-Se-Sn-Te-W-Zn). Robust principal component analysis suggests that low-T, large-ion lithophile element alteration (Ba-Sr), which likely occurred during kimberlite emplacement, represents the largest source of variance for the xenocryst dataset. PT-dependent element partitioning during sub-solidus equilibration represents the second most important control on olivine and clinopyroxene chemistry. We demonstrate that least-altered, high-PT mantle silicates are, in fact, a significant mineral host for a range of ore elements (Cu-Zn ± Ag ± As ± Se ± Sn ± Mo) within equilibrated, garnet peridotite at depth (70–190 km). Statistical analysis of the raw, individual mass sweeps for each LA-ICP-MS signal suggest that the most abundant ore elements (Cu-Zn) occur predominantly as PT-dependent substitution reactions with the dominant mineral-forming elements, rather than as inclusions. A subset of high-PT olivine (160–180 km) yields Fe-Ni-S-poor and Na (Au ± Pt ± Pd)-rich compositions, which may reflect metasomatism, sulphide segregation and trapping of precious metal-bearing fluids at the base of the lithosphere. These anomalous mantle fragments possibly represent the first, direct sampling of precious metal-modified mantle peridotite beneath the Abitibi. Mid-PT olivine xenocrysts (70–120 km), which yield Mg-rich and high field-strength element-poor compositions, document a highly melt-depleted segment of mantle peridotite coincident with and below a shallow-dipping, low-seismic-velocity anomaly and conductive feature of the Kirkland Lake mid-lithosphere at 70–100 km. We speculate that the trace element signature of mid-PT xenocrysts documents the re-distribution of high-charge and incompatible elements from refractory garnet peridotite to phlogopite- and/or amphibole-bearing peridotite with conductive metasomatic up-flow zones. The rapid, sub-solidus diffusion of elements at high-T suggest that these processes likely occurred during, and/or immediately preceding, kimberlite volcanism. New in situ Pb isotope analyses of clinopyroxene xenocrysts sampled from metasomatized, low-Al garnet peridotite, however, also document ancient metasomatic events that likely pre-date Jurassic kimberlitic volcanism by at least one billion years.

Evidence of Enriched, Hadean Mantle Reservoir from 4.2-4.0 Ga zircon xenocrysts from Paleoarchean TTGs of the Singhbhum Craton, Eastern India

Sensitive High-Resolution Ion Microprobe (SHRIMP) U-Pb analyses of zircons from Paleoarchean (~3.4 Ga) tonalite-gneiss called the Older Metamorphic Tonalitic Gneiss (OMTG) from the Champua area of the Singhbhum Craton, India, reveal 4.24-4.03 Ga xenocrystic zircons, suggesting that the OMTG records the hitherto unknown oldest precursor of Hadean age reported in India. Hf isotopic analyses of the Hadean xenocrysts yield unradiogenic 176Hf/177Hfinitial compositions (0.27995 ± 0.0009 to 0.28001 ± 0.0007; ɛHf[t] = −2.5 to −5.2) indicating that an enriched reservoir existed during Hadean eon in the Singhbhum cratonic mantle. Time integrated ɛHf[t] compositional array of the Hadean xenocrysts indicates a mafic protolith with 176Lu/177Hf ratio of ∼0.019 that was reworked during ∼4.2-4.0 Ga. This also suggests that separation of such an enriched reservoir from chondritic mantle took place at 4.5 ± 0.19 Ga. However, more radiogenic yet subchondritic compositions of ∼3.67 Ga (average 176Hf/177Hfinitial 0.28024 ± 0.00007) and ~3.4 Ga zircons (average 176Hf/177Hfinitial = 0.28053 ± 0.00003) from the same OMTG samples and two other Paleoarchean TTGs dated at ~3.4 Ga and ~3.3 Ga (average 176Hf/177Hfinitial is 0.28057 ± 0.00008 and 0.28060 ± 0.00003), respectively, corroborate that the enriched Hadean reservoir subsequently underwent mixing with mantle-derived juvenile magma during the Eo-Paleoarchean.

Deep komatiite signature in cratonic mantle pyroxenite

AbstractWe present new and compiled whole‐rock modal mineral, major and trace element data from extremely melt depleted but pyroxenite and garnet(‐ite)‐bearing Palaeoarchean East Greenland cratonic mantle, exposed as three isolated, tectonically strained orogenic peridotite bodies (Ugelvik, Raudhaugene and Midsundvatnet) in western Norway. The studied lithologies comprise besides spinel‐ and/or garnet‐bearing peridotite (dunite, harzburgite, lherzolite) garnet‐clinopyroxenite and partially olivine‐bearing garnet‐orthopyroxenite and ‐websterite. Chemical and modal data and spatial relationships between different rock types suggest deformation to have triggered mechanical mixing of garnet‐free dunite with garnet‐bearing enclosures that formed garnet‐peridotite. Inclusions of olivine in porphyroclastic minerals of pyroxenite show a primary origin of olivine in olivine‐bearing variants. Major element oxide abundances and ratios of websterite differ to those in rocks expected to form by reaction of peridotite with basaltic melts or silica‐rich fluids, but resemble those of Archean Al‐enriched komatiite (AEK) flows from Barberton and Commondale greenstone belts, South Africa. Websterite GdN/YbN, 0.49–0.65 (olivine‐free) and 0.73–0.85 (olivine‐bearing), overlaps that of two subgroups of AEK, GdN/YbN 0.25–0.55 and 0.77–0.90, with each of them being nearly indistinguishable from one another in not only rare earth element fractionation but also concentration. Websterite MgO content is high, 22.7–29.0 wt%, and Zr/Y is very low, 0.1–1.0. The other, non‐websteritic pyroxenites overlap—when mechanically mixed together with garnetite—in chemistry with that of AEK. It follows an origin of websterite and likely all pyroxenite that involves melting of a garnet‐bearing depleted mantle source. Pyroxene exsolution lamellae in the inferred solidus garnet in all lithological varieties require the pyroxenites to have crystallized in the majorite garnet stability field, at 3–4 GPa (90–120 km depth) at minimum 1,600°C. Consequently, we interpret the websterites to represent the first recognized deep plutonic crystallization products that formed from komatiite melts. The other pyroxenitic rocks are likely fragments of such crystallization products. An implication is that a mantle plume environment contributed to the formation of (one of) the worldwide oldest lithospheric mantle underneath the eastern Rae craton.

Inclusions in diamonds constrain thermo-chemical conditions during Mesozoic metasomatism of the Kaapvaal cratonic mantle

Fluid/melt inclusions in diamonds, which were encapsulated during a metasomatic event and over a short period of time, are isolated from their surrounding mantle, offering the opportunity to constrain changes in the sub-continental lithospheric mantle (SCLM) that occurred during individual thermo-chemical events, as well as the composition of the fluids involved and their sources. We have analyzed a suite of 8 microinclusion-bearing diamonds from the Group I De Beers Pool kimberlites, South Africa, using FTIR, EPMA and LA-ICP-MS. Seven of the diamonds trapped incompatible-element-enriched saline high density fluids (HDFs), carry peridotitic mineral microinclusions, and substitutional nitrogen almost exclusively in A-centers. This low-aggregation state of nitrogen indicates a short mantle residence times and/or low mantle ambient temperature for these diamonds. A short residence time is favored because, elevated thermal conditions prevailed in the South African lithosphere during and following the Karoo flood basalt volcanism at ∼180 Ma, thus the saline metasomatism must have occurred close to the time of kimberlite eruptions at ∼85 Ma. Another diamond encapsulated incompatible-element-enriched silicic HDFs and has 25% of its nitrogen content residing in B-centers, implying formation during an earlier and different metasomatic event that likely relates to the Karoo magmatism at ca. 180 Ma.Thermometry of mineral microinclusions in the diamonds carrying saline HDFs, based on Mg–Fe exchange between garnet–orthopyroxene (Opx)/clinopyroxene (Cpx)/olivine and the Opx–Cpx thermometer, yield temperatures between 875–1080 °C at 5 GPa. These temperatures overlap with conditions recorded by touching inclusion pairs in diamonds from the De Beers Pool kimberlites, which represent the mantle ambient conditions just before eruption, and are altogether lower by 150–250 °C compared to P–T gradients recorded by peridotite xenoliths from the same locality. Oxygen fugacity (fO2) differs as well. The fO2 calculated for the saline HDF compositions (Δlog⁡fO2(FMQ)=−2.47 to −1.34) are higher by about a log unit compared with that recorded by xenoliths at 4–7 GPa.We conclude that enriched saline HDFs mediated the metasomatism that preceded Group I kimberlite eruptions in the southwestern Kaapvaal craton, and that their ‘cold and oxidized’ nature reflects their derivation from a deep subducting slab. This event had little impact on the temperature and redox state of the Kaapvaal lithosphere as a reservoir, however, it likely affected its properties along limited metasomatized veins and their wall rock. To reconcile the temperature and oxygen fugacity discrepancy between inclusions in diamonds and xenoliths, we argue that xenoliths did not equilibrate during the last saline metasomatic event or kimberlite eruption. Thus the P–T–fO2 gradients they record express pre-existing lithospheric conditions that were likely established during the last major thermal event in the Kaapvaal craton (i.e. the Karoo magmatism at ca. 180 Ma).

The uniquely high-temperature character of Cullinan diamonds: A signature of the Bushveld mantle plume?

The mantle beneath the Cullinan kimberlite (formerly known as “Premier”) is a unique occurrence of diamondiferous cratonic mantle where diamonds were generated contemporaneously and shortly following a mantle upwelling that led to the formation of a Large Igneous Province that produced the world's largest igneous intrusion – the 2056 Ma Bushveld Igneous Complex (BIC). We studied 332 diamond inclusions from 202 Cullinan diamonds to investigate mantle thermal effects imposed by the formation of the BIC. The overwhelming majority of diamonds come from three parageneses: (1) lithospheric eclogitic (69%), (2) lithospheric peridotitic (21%), and (3) sublithospheric mafic (9%). The lithospheric eclogitic paragenesis is represented by clinopyroxene, garnet, coesite and kyanite. Main minerals of the lithospheric peridotitic paragenesis are forsterite, enstatite, Cr-pyrope, Cr-augite and spinel; the sublithospheric mafic association includes majorite, CaSiO3 phases and omphacite. Diamond formation conditions were calculated using an Al-in-olivine thermometer, a garnet-clinopyroxene thermometer, as well as majorite and Raman barometers. The Cullinan diamonds may be unique on the global stage in recording a cold geotherm of 40 mW/m2 in cratonic lithosphere that was in contact with underlying convecting mantle at temperatures of 1450–1550 °C. The studied Cullinan diamonds contain a high proportion of inclusions equilibrated at temperatures exceeding the ambient 1327 °C adiabat, i.e. 54% of eclogitic diamonds and 41% of peridotitic diamonds. By contrast, ≤1% of peridotitic diamond inclusions globally yield equally high temperatures. We propose that the Cullinan diamond inclusions recorded transient, slow-dissipating thermal perturbations associated with the plume-related formation of the ~2 Ga Bushveld igneous province. The presence of inclusions in diamond from the mantle transition zone at 300–650 km supports this view. Cullinan xenoliths indicative of the thermal state of the cratonic lithosphere at ~1.2 Ga are equilibrated at the relatively low temperatures, not exceeding adiabatic. The ability of diamonds to record super-adiabatic temperatures may relate to their entrainment from the deeper, hotter parts of the upper mantle un-sampled by the kimberlite in the form of xenoliths or their equilibration in a younger lithosphere after a decay of the thermal disturbance.

Mesozoic mafic magmatism in North China: Implications for thinning and destruction of cratonic lithosphere

The North China Craton (NCC) has been thinned from >200 km to in its eastern part. The ancient subcontinental lithospheric mantle (SCLM) has been replaced by the juvenile SCLM in the Meoszoic. During this period, the NCC was destructed as indicated by extensive magmatism in the Early Cretaceous. While there is a consensus on the thinning and destruction of cratonic lithosphere in North China, it has been hotly debated about the mechanism of cartonic destruction. This study attempts to provide a resolution to current debates in the view of Mesozoic mafic magmatism in North China. We made a compilation of geochemical data available for Mesozoic mafic igneous rocks in the NCC. The results indicate that these mafic igneous rocks can be categorized into two series, manifesting a dramatic change in the nature of mantle sources at ~121 Ma. Mafic igneous rocks emplaced at this age start to show both oceanic island basalts (OIB)-like trace element distribution patterns and depleted to weakly enriched Sr-Nd isotope compositions. In contrast, mafic igneous rocks emplaced before and after this age exhibit both island arc basalts (IAB)-like trace element distribution patterns and enriched Sr-Nd isotope compositions. This difference indicates a geochemical mutation in the SCLM of North China at ~121 Ma. Although mafic magmatism also took place in the Late Triassic, it was related to exhumation of the deeply subducted South China continental crust because the subduction of Paleo-Pacific slab was not operated at that time. Paleo-Pacific slab started to subduct beneath the eastern margin of Eruasian continent since the Jurrasic. The subducting slab and its overlying SCLM wedge were coupled in the Jurassic, and slab dehydration resulted in hydration and weakening of the cratonic mantle. The mantle sources of ancient IAB-like mafic igneous rocks are a kind of ultramafic metasomatites that were generated by reaction of the cratonic mantle wedge peridotite not only with aqueous solutions derived from dehydration of the subducting Paleo-Pacific oceanic crust in the Jurassic but also with hydrous melts derived from partial melting of the subducting South China continental crust in the Triassic. On the other hand, the mantle sources of juvenile OIB-like mafic igneous rocks are also a kind of ultramafic metasomatites that were generated by reaction of the asthenospheric mantle underneath the North China lithosphere with hydrous felsic melts derived from partial melting of the subducting Paleo-Pacific oceanic crust. The subducting Paleo-Pacific slab became rollback at ~144 Ma. Afterwards the SCLM base was heated by laterally filled asthenospheric mantle, leading to thinning of the hydrated and weakened cratonic mantle. There was extensive bimodal magmatism at 130 to 120 Ma, marking intensive destruction of the cratonic lithosphere. Not only the ultramafic metasomatites in the lower part of the cratonic mantle wedge underwent partial melting to produce mafic igneous rocks showing negative e Nd( t ) values, depletion in Nb and Ta but enrichment in Pb, but also the lower continent crust overlying the cratonic mantle wedge was heated for extensive felsic magmatism. At the same time, the rollback slab surface was heated by the laterally filled asthenospheric mantle, resulting in partial melting of the previously dehydrated rocks beyond rutile stability on the slab surface. This produce still hydrous felsic melts, which metasomatized the overlying asthenospheric mantle peridotite to generate the ultramafic metasomatites that show positive e Nd( t ) values, no depletion or even enrichment in Nb and Ta but depletion in Pb. Partial melting of such metasomatites started at ~121 Ma, giving rise to the mafic igneous rocks with juvenile OIB-like geochemical signatures. In this context, the age of ~121 Ma may terminate replacement of the ancient SCLM by the juvenile SCLM in North China. Paleo-Pacific slab was not subducted to the mantle transition zone in the Mesozoic as revealed by modern seismic tomography, and it was subducted at a low angle since the Jurassic, like the subduction of Nazca Plate beneath American continent. This flat subduction would not only chemically metasomatize the cratonic mantle but also physically erode the cratonic mantle. Therefore, the interaction between Paleo-Pacific slab and the cratonic mantle is the first-order geodynamic mechanism for the thinning and destruction of cratonic lithosphere in North China.

Refertilized mantle keel below the Southern Alps domain (North-East Italy): Evidence from Marosticano refractory mantle peridotites

The Veneto Volcanic Province (VVP), a Cenozoic magmatic province in northeastern Italy, is one of the widest volcanic areas of the Adria plate. It consists of five main magmatic districts, and its most primitive products commonly host mantle xenoliths. In this study, we present a newly discovered xenolith suite from the Marosticano district that contains peridotites with compositional characteristics of mineral assemblages that provide insight into an unexpected nature of the sub-continental lithospheric mantle (SCLM) of the Adria plate. In contrast to xenoliths from other VVP sites previously studied (i.e., Val d'Adige and Lessini Mts.), Marosticano xenoliths exhibit highly refractory compositions typical of on-craton peridotites. High olivine forsteritic contents (Fo: 91–93) indicate high degrees of partial melting (>25%) that should have been associated with the complete consumption of clinopyroxene. Major and trace element compositions further link these peridotite fragments to early Proterozoic cratonic mantle. The occurrence of clinopyroxene within such rocks suggests Marosticano clinopyroxene testify to a metasomatic legacy. The i) LREE-enrichments of Marosticano clinopyroxene and ii) the dissolved CO2 mole fractions (up to 1.0) for the inferred clinopyroxene-forming melt are consistent with carbonatite/CO2-rich silicatic melts as metasomatic agents. The latter could be responsible for the equilibrium temperatures (1033–1117°C) and oxidizing conditions [ΔlogfO2 (FMQ)=−0.6−+1.1], anomalously high for a cratonic environment but similar to the off-craton VVP xenoliths. The cratonic signature and carbonatite/CO2-rich silicate metasomatism found together in the Marosticano mantle xenoliths reveal that ancient features can be preserved in SCLM in a young, active geodynamic setting such as the Adria plate boundary. In this framework Lessini Mts. and Val d'Adige xenoliths could be interpreted as circumcratonic reminiscent domains affected by refertilization due to infiltration of asthenosphere-derived melts, rather than newly accreted “off-craton” SCLM. These new interpretations could be useful for completing the reconstruction of the Africa/Eurasia interplay during the Alpine collision.

Volatile-rich Metasomatism in the Cratonic Mantle beneath SW Greenland: Link to Kimberlites and Mid-lithospheric Discontinuities

The cratonic part of Greenland has been a hotspot of scientific investigation since the discovery of some of the oldest crust on Earth and of significant diamond potential in the underlying lithospheric mantle, the characterization of which remains, however, incomplete. We applied a detailed petrographic and in situ analytical approach to a new suite of fresh kimberlite-borne peridotite xenoliths, recovered from the North Atlantic craton in SW Greenland, to unravel the timing and nature of mantle metasomatism, and its link to the formation of low-volume melts (e.g. kimberlites) and to geophysically detectible discontinuities. Two types of mineralogies and metasomatic styles, occurring at two depth intervals, are recognized. The first type comprises lherzolites, harzburgites and dunites, some phlogopite-bearing, which occur from ∼100–170 km depth. They form continuous trends towards lower mineral Mg# at increasing TiO2, MnO and Na2O and decreasing NiO contents. These systematics are ascribed to metasomatism by a hydrous silicate melt precursor to c. 150 Ma kimberlites, in the course of rifting, decompression and lithosphere thinning. This metasomatism was accompanied by progressive garnet breakdown, texturally evident by pyroxene–spinel assemblages occupying former coarse grains and compositionally evident by increasing concentrations of elements that are compatible in garnet (Y, Sc, In, heavy rare earth elements) in newly formed clinopyroxene. Concomitant sulphide saturation is indicated by depletion in Cu, Ni and Co. The residual, more silica-undersaturated and potentially more oxidizing melts percolated upwards and metasomatized the shallower lithospheric mantle, which is composed of phlogopite-bearing, texturally equilibrated peridotites, including wehrlites, showing evidence for recent pyroxene-breakdown. This is the second type of lithology, which occurs at ∼90–110 km depth and is inferred to have highly depleted protoliths. This type is compositionally distinct from lherzolites, with olivine having higher Ca/Al, but lower Al and V contents. Whereas low Al may in part reflect lower equilibration temperatures, low V is ascribed to a combination of intrinsically more oxidizing mantle at lower pressure and oxidative metasomatism. The intense metasomatism in the shallow cratonic mantle lithosphere contrasts with the strong depletion recorded in the northwestern part of the craton, which at 590–550 Ma extended to >210 km depth, and suggests loss of ∼40 km of lithospheric mantle, also recorded in the progressive shallowing of magma sources during the breakup of the North Atlantic craton. The concentration of phlogopite-rich lithologies in a narrow depth interval (∼90–110 km) overlaps with a negative seismic velocity gradient that is interpreted as a mid-lithospheric discontinuity beneath western Greenland. This is suggested to be a manifestation of small-volume volatile-rich magmatism, which paved the way for Mesozoic kimberlite, ultramafic lamprophyre, and carbonatite emplacement across the North Atlantic craton.

Archean‐Paleoproterozoic Lithospheric Mantle at the Northern Margin of the North China Craton Represented by Tectonically Exhumed Peridotites

AbstractTectonically emplaced peridotites from North Hebei Province, North China Craton, have retained an original harzburgite mineral assemblage of olivine (54%–58%) + orthopyroxene (40%–46%) +minor clinopyroxene (<1%)+spinel. Samples with boninite‐like chemical compositions also coexist with these peridotites. The spinels within the peridotites have high‐Al end‐members with Al2O3 content of 30 wt%–50 wt%, typical of mantle spinels. When compared with experimentally determined melt extraction trajectories, the harzburgites display a high degree of melting and enrichment of SiO2, which is typical of cratonic mantle peridotites. The peridotites display variably enriched light rare earth elements (REEs), relatively depleted middle REEs and weakly fractionated heavy REEs, which suggest a melt extraction of over 25% in the spinel stability field. The occurrence of arc‐ and SSZ‐type chromian spinels in the peridotites suggests that melt extraction and metasomatism occurred mostly in a subduction‐related setting. This is also supported by the geochemical data of the coexisting boninite‐like samples. The peridotites have 187Os/188Os ratios ranging from 0.113–0.122, which is typical of cratonic lithospheric mantle. These 187Os/188Os ratios yield model melt extraction ages (TRD) ranging from 981 Ma to 2054 Ma, which may represent the minimum estimation of the melt extraction age. The Al2O3–187Os/188Os‐proxy isochron ages of 2.4 Ga‐2.7 Ga suggest a mantle melt depletion age between the Late Achaean and Early Paleoproterozoic. Both the peridotites and boninite‐like rocks are therefore interpreted as tectonically exhumed continental lithospheric mantle of the North China Craton, which has experienced mantle melt depletion and subduction‐related mantle metasomatism during the Neoarchean‐ Paleoproterozoic.

Composition of the lithospheric mantle in the northern part of Siberian craton: Constraints from peridotites in the Obnazhennaya kimberlite

The character of the lithospheric mantle of the northern Siberian craton is not well established; nearly all published data are for mantle xenoliths from a single kimberlite in the center of the craton (Udachnaya). We report major elements of the whole rock, trace elements data of clinopyroxene and Re-Os isotope and PGE concentration of mantle xenoliths from the Obnazhennaya kimberlite pipe (160Ma) in the northern part of Siberian craton. The Obnazhennaya mantle xenoliths include spinel harzburgites, spinel dunites, spinel lherzolites and spinel-garnet lherzolite. The spinel harzburgites and dunites have refractory compositions, with 0.23–1.35wt% Al2O3, 0.41–3.11wt% CaO and 0.00–0.09wt% TiO2, whereas the lherzolites (both spinel- and spinel-garnet-) have more fertile compositions, containing 2.16–6.55wt% Al2O3, 2.91–7.55wt% CaO and 0.04–0.15wt% TiO2. The trace element compositions and mineralogical textures of the Obnazhennaya xenoliths indicate the occurrence of metasomatic enrichments, including carbonatite melts, basaltic melts from Siberian Trap and kimberlitic melts. The spinel harzburgites and dunites have 187Os/188Os of 0.11227–0.11637, giving a TRD age of 1.6–2.2Ga. This suggests that old cratonic mantle still existed beneath the Obnazhennaya. In contrast, both spinel and spinel-garnet lherzolites have more radiogenic 187Os/188Os ratios (0.11931–0.17627), enriched P-PGEs. But the higher Al2O3 and Os character of these lherzolites suggest that they were not juvenile mantle but the refertilized ancient mantle. Therefore, our results suggest that the cratonic mantle beneath the northern part of Siberian craton contain both ancient and reworked lithospheric mantle, and the metasomatism may not be effective at overprinting/eroding the pre-existing lithosphere.

How Sharp Is the Cratonic Lithosphere‐Asthenosphere Transition?

AbstractEarth's cratonic mantle lithosphere is distinguished by high seismic wave velocities that extend to depths greater than 200 km, but recent studies disagree on the magnitude and depth extent of the velocity gradient at their lower boundary. Here we analyze and model the frequency dependence of Sp waves to constrain the lithosphere‐asthenosphere velocity gradient at long‐lived stations on cratons in North America, Africa, Australia, and Eurasia. Beneath 33 of 44 stations, negative velocity gradients at depths greater than 150 km are less than a 2–3% velocity drop distributed over more than 80 km. In these regions the base of the typical cratonic lithosphere is gradual enough to be explained by a thermal transition. Vertically sharper lithosphere‐asthenosphere transitions are permitted beneath 11 stations, but these zones are spatially intermittent. These results demonstrate that lithosphere‐asthenosphere viscosity contrasts and coupling fundamentally differ between cratons and younger continents.

Trace elements in olivine of ultramafic lamprophyres controlled by phlogopite-rich mineral assemblages in the mantle source

Carbonate-rich ultramafic lamprophyres (aillikites) and associated rocks characteristically occur during the early stages of thinning and rifting of cratonic mantle lithosphere, prior to the eruption of melilitites, nephelinites and alkali basalts. It is accepted that they require volatile-rich melting conditions, and the presence of phlogopite and carbonate in the source, but the exact source rock assemblages are debated. Melts similar to carbonate-rich ultramafic lamprophyres (aillikites) have been produced by melting of peridotites in the presence of CO2 and H2O, whereas isotopes and trace elements appear to favor distinct phlogopite-bearing rocks.Olivine macrocrysts in aillikites are usually rounded and abraded, so that it is debated whether they are phenocrysts or mantle xenocrysts. We have analyzed minor and trace element composition in olivines from the type aillikites from Aillik Bay in Labrador, Canada. We characterize five groups of olivines: [1] mantle xenocrysts, [2] the main phenocryst population, and [3] reversely zoned crystals interpreted as phenocrysts from earlier, more fractionated, magma batches, [4] rims on the phenocrysts, which delineate aillikite melt fractionation trends, and [5] rims around the reversely zoned olivines. The main phenocryst population is characterized by mantle-like Ni (averaging 3400μgg−1) and Ni/Mg at Mg# of 88–90, overlapping with phenocrysts in ocean island basalts and Mediterranean lamproites. However, they also have low 100Mn/Fe of 0.9–1.3 and no correlation between Ni and other trace elements (Sc, Co, Li) that would indicate recycled oceanic or continental crust in their sources. The low Mn/Fe without high Ni/Mg, and the high V/Sc (2–5) are inherited from phlogopite in the source that originated by solidification of lamproitic melts at the base of the cratonic lithosphere in a previous stage of igneous activity. The olivine phenocryst compositions are interpreted to result from phlogopite and not high modal pyroxene in the source. The presence of kimberlites and ultramafic lamprophyres of Mesozoic age in Greenland indicates the persistence of a steep edge to the cratonic lithosphere at a time when this had been removed from the western flank in Labrador.

Generation and Modification of the Mantle Wedge and Lithosphere beneath the West Bismarck Island Arc: Melting, Metasomatism and Thermal History of Peridotite Xenoliths from Ritter Island

Peridotite xenoliths dredged from the seafloor northwest of Ritter Island in the West Bismarck Island Arc offer a rare insight into the petrogenetic processes operating in the upper mantle wedge of an active oceanic subduction zone. Harzburgitic xenoliths and subordinate dunites and pyroxenites display significant textural and compositional variability between samples, which are interpreted as fragments of heterogeneous mantle that has experienced a complex petrogenetic history. Based on textures and in situ major and trace element analyses of olivine, orthopyroxene and clinopyroxene, five significant petrogenetic stages have been deduced. The first stage was a period of partial melting at temperatures >1100 oC in a previously active arc (likely the now extinct Vitiaz West-Melanesian arc), indicated by the high Mg# and Cr#, low Al2O3 and very low concentrations of incompatible trace elements in residual phases and the absence of residual clinopyroxene. Modelling the concentrations of Y and Yb in orthopyroxene indicates depletion by high degrees (∼30 %) of wet fractional melting of a depleted mantle source. This was followed by metasomatism also related to this previous period of subduction, resulting in refertilisation of the harzburgite residues with clinopyroxene and the formation of dunite and pyroxenite channels. Three populations of secondary clinopyroxene are identified based on their trace element compositions, with both LREE-depleted and (more unusually) sinusoidal REE patterns identified, indicating that a spectrum of fluid compositions was involved in the metasomatism. The third stage involved a period of cooling and chemical re-equilibration below the wet solidus, combined with decompression to shallow lithospheric mantle depths. Geothermometers with different closure temperatures reveal large temperature discrepancies, indicating mantle cooling was both unusually extensive (down to ∼600 oC), but also very slow (∼20 oC/My). The fourth stage marked the cessation of cooling and formation of the modern mantle wedge as part of the West Bismarck Island Arc. Silicate melts percolated through networks of veins and reacted with the residual mantle, generating a variety of new disequilibrium textures, most notably orthopyroxene-clinopyroxene-glass reaction patches. This was accompanied by increases in spinel Cr#, olivine trace element concentrations and higher mineral-mineral temperatures. These disequilibrium textures were preserved through the final stage of entrainment in the host basalt and rapid transport to the seafloor. The Ritter suite thus provides a remarkably detailed insight into the broad diversity of melt/fluid compositions and fluid-rock reaction processes in oceanic sub-arc mantle. Several of these features can be found both in other samples of sub-arc mantle and also cratonic mantle, demonstrating the ubiquity of such processes beneath modern arcs and also the potential genetic relationship between subduction zone processes and the formation of cratons. In this way, sub-arc xenoliths such as the Ritter suite, whilst presently under-sampled, can provide crucial insights for understanding the relationship between the mechanisms by which modern arc systems are generated and evolve, and the nature of the upper mantle once subduction processes have ceased.

Evolution of the lithospheric mantle beneath Mt. Baekdu (Changbaishan): Constraints from geochemical and Sr–Nd–Hf isotopic studies on peridotite xenoliths in trachybasalt

Major and trace element compositions of minerals as well as Sr–Nd–Hf isotopic compositions of clinopyroxenes from spinel peridotite xenoliths entrained in Late Cenozoic trachybasalt from Mt. Baekdu (Changbaishan) were used to elucidate lithospheric mantle formation and evolution in the eastern North China Craton (NCC). The analyzed peridotites were mainly spinel lherzolites with rare harzburgites. They consisted of olivine (Fo89.3–91.0), enstatite (Wo1–2En88–90Fs8–11), diopside (Wo45–50En45–51Fs4–6), and spinel (Cr#=8.8–54.7). The peridotite residues underwent up to ~25% partial melting in fertile mid-ocean-ridge basalt (MORB) mantle. Plots of the Cr# in spinel against the Mg# in coexisting olivine or spinel suggested an affinity with abyssal peridotites. Comparisons of Cr# and TiO2 in spinel were also compatible with an abyssal peridotite-like composition; however, harzburgites were slightly enriched in TiO2 because of the reaction with MORB-like melt. Temperatures estimated using two-pyroxene thermometry ranged from 750 to 1010°C, reflecting their lithospheric mantle origin. The rare earth element (REE) patterns in clinopyroxenes of the peridotites varied from light REE (LREE) depleted to spoon shaped to LREE enriched, reflecting secondary overprinting effects of metasomatic melts or fluids on the residues from primordial melting. The calculated trace element pattern of metasomatic melt equilibrated with clinopyroxene in Mt. Baekdu peridotite showed strong enrichment in large-ion lithophile elements, Th and U together with slight fractionation in heavy REEs (HREEs) and considerable depletion in Nb and Ti. The Sr–Nd–Hf isotopic compositions of clinopyroxenes separated from the peridotites varied from more depleted than present-day MORB to bulk Earth values. However, some clinopyroxene showed a decoupling between Nd and Sr isotopes, deviating from the mantle array with a high 87Sr/86Sr ratio. This sample also showed a significant Nd–Hf isotope decoupling lying well above the mantle array. The Lu–Hf and Sm–Nd model ages of residual clinopyroxenes yielded Early Proterozoic to Phanerozoic ages. No signature of Archean cratonic mantle was present. Therefore, Mt. Baekdu peridotite is residual lithospheric mantle that has undergone variable degrees of diachronous melt extraction and infiltration metasomatism involving subduction-related, fluid-bearing silicate melts. The predominance of Phanerozoic Hf model ages indicates that the lherzolites represent lithospheric mantle fragments newly accreted underneath the eastern NCC.

Can metasomatic weakening result in the rifting of cratons?

Cratons are strong and their preservation demonstrates that they resist deformation and fragmentation. Yet several cratons are rifting now, or have rifted in the past. We suggest that cratons need to be weakened before they can rift. Specifically, metasomatism of the depleted dehydrated craton mantle lithosphere is a potential weakening mechanism. We use 2D numerical models to test the efficiency of simulated melt metasomatism and coeval rehydration to weaken craton mantle lithosphere roots. These processes effectively increase root density through a parameterized melt-peridotite reaction, and reduce root viscosity by increasing the temperature and rehydrating the cratonic mantle lithosphere. The models are designed to investigate when a craton is sufficiently weakened to undergo rifting and is no longer protected by adjacent standard Phanerozoic lithosphere. We find that cratons only become vulnerable to rifting following large-volume melt metasomatism (~30% by volume) and thinning of the gravitationally unstable cratonic lithosphere from >250km to ~100km; at which point its residual crustal strength is important. Furthermore, our results indicate that rifting of cratons depends on the timing of extension with respect to metasomatism. An important effect in the large-volume melt models is the melt-induced increase in temperature which must have time to reach peak values in the uppermost mantle lithosphere before rifting. Release of water stored in the transition zone at the base of a big mantle wedge may provide a suitable natural setting for both rehydration and refertilization of an overlying craton and is consistent with evidence from the eastern North China Craton. An additional effect is that cratons subside isostatically to balance the increasing density of craton mantle lithosphere where it is moderately metasomatized. We suggest that this forms intracratonic basins and that their subsidence and subsequent uplift, and cratonic rifting constitute evidence of progressive metasomatism of cratonic mantle lithosphere.

Diamonds from the lower mantle?

Natural diamonds, because of their great physical resiliency, can preserve information about their formation, storage, and transport conditions for billions of years. Diamond samples therefore provide a unique opportunity to directly study ancient samples of the Earth’s deep interior. To correctly interpret the information diamonds provide, it is essential to accurately constrain the depth of their origin. This depth provenance is usually identified using coexisting minerals that are trapped as inclusions within diamonds during their growth. Comparison of an inclusion’s composition and mineralogy with experimental phase equilibria allows the diamond’s growth conditions to be estimated. While the majority of diamonds likely originate from depths of 140–220 km in cratonic mantle, a small subset appears to have been exhumed from depths extending to >800 km, called “superdeep” or “ultradeep” diamonds (e.g., Walter et al. 2011; Pearson et al. 2014). Inclusions of magnesiowustite are among the most commonly described in sub-lithospheric diamonds and have often been assumed to indicate diamond provenance in the lower mantle because [Mg,Fe]O is not stable at upper mantle conditions in a subsolidus mantle compositions (Tronnes 2009). This is despite the stability field of [Mg,Fe]O extending to ambient pressure conditions and experimental evidence of magnesiowustite stability in equilibrium with diamond throughout the upper mantle (Brey et al. 2004; Thomson et al. 2016). A new study by Uenver-Thiele et al. (2017) in American Mineralogist places important new constraints on the formation and uplift history of inclusions containing magnesioferrite. Studies of magnesiowustite inclusions in diamonds from the Juina region of Brazil often report observation of nanometersized crystals of magnesioferrite ![Formula][1] , which supposedly “confirm” the lower mantle origin of these samples. The magnesioferrite precipitates can occur at the interface between the diamond and [Mg,Fe]O inclusion, or as evenly distributed dislocation “necklaces” within … [1]: /embed/mml-math-1.gif