Garnet Peridotite Research Articles

Linking Chemical Heterogeneity to Lithological Heterogeneity of the Samoan Mantle Plume With Fe‐Sr‐Nd‐Pb Isotopes

AbstractThe Samoan mantle plume is thought to host three isotopically (radiogenic) distinct low‐3He/4He components including EM2 (enriched mantle 2), dilute HIMU (high μ = 238U/204Pb), and a depleted mantle (DM) component, which were sampled by shield‐stage lavas from the Malu, Vai, and Upo trend volcanoes, respectively. However, it is unclear whether the isotopically distinct components are present as different lithologies. Using new Fe–Sr–Nd–Pb isotope data for Tutuila basalts (Samoa), combined with literature data for other Samoan basalts, we attempt to infer the lithological structure of the Samoan plume. The results show that “Malu trend” basalts have heavier Fe isotopic compositions (δ57Fe = 0.15–0.24‰) than “Vai trend” and “Upo trend” basalts. The latter two groups have average δ57Fe of 0.14 ± 0.07‰ (2SD) and 0.11 ± 0.03‰ (2SD), respectively, similar to normal midocean ridge basalts (N‐MORBs, δ57Fe = 0.15 ± 0.05‰, 2SD). The fractional‐crystallization‐corrected δ57Fe values of all shield lavas are positively correlated with (Gd/Yb)N, Pb/Nd and 87Sr/86Sr ratios whereas negatively correlated with Nb/Th and εNd ratios, which cannot be explained by partial melting of a single garnet peridotite but point to heterogeneous source lithologies. The EM2 lavas are characterized with high δ57Fe and (Gd/Yb)N, low Nb/Th, and enriched Sr–Nd isotopic ratios, requiring a pyroxenitic source component with imprints of both recycled terrigenous sediments and oceanic crust. The Vai‐ and Upo‐trend lavas with MORB‐like δ57Fe can be explained by partial melting of peridotitic sources, although different extents of refertilization by recycled crust are essential for generating their distinct radiogenic isotope signatures. These observations highlight the lithological heterogeneity of the Samoan plume and relates the EM2 component with a pyroxenitic lithology.

Dynamic Metasomatism Experiments Investigating the Interaction between Migrating Potassic Melt and Garnet Peridotite

Dynamic metasomatism experiments were performed by reacting a lamproite melt with garnet peridotite by drawing melt through the peridotite into a vitreous carbon melt trap, ensuring the flow of melt through the peridotite and facilitating analysis of the melt. Pressure (2–3 GPa) and temperature (1050–1125 °C) conditions were chosen where the lamproite was molten but the peridotite was not. Phlogopite was formed and garnet and orthopyroxene reacted out, resulting in phlogopite wehrlite (2 GPa) and phlogopite harzburgite (3 GPa). Phlogopites in the peridotite have higher Mg/(Mg + Fe) and Cr2O3 and lower TiO2 than in the lamproite due to buffering by peridotite minerals, with Cr2O3 from the elimination of garnet. Compositional trends in phlogopites in the peridotite are similar to those in natural garnet peridotite xenoliths in kimberlites. Changes in melt composition resulting from the reaction show decreased TiO2 and increased Cr2O3 and Mg/(Mg + Fe). The loss of phlogopite components during migration through the peridotite results in low K2O/Na2O and K/Al in melts, indicating that chemical characteristics of lamproites are lost through reaction with peridotite so that emerging melts would be less extreme in composition. This indicates that lamproites are unlikely to be derived from a source rich in peridotite, and more likely originate in a source dominated by phlogopite-rich hydrous pyroxenites. Phlogopites from an experiment in which lamproite and peridotite were intimately mixed before the experiment did not produce the same phlogopite compositions, showing that care must be taken in the design of reaction experiments.

Partial melt generation and evolution of magma reservoir conditions at the Paka volcanic complex in Kenya: Constraints from geochemistry, petrology and geophysics

In this study, we determine magma generation and evolution of magma storage conditions beneath Paka volcano located in northern segment of the Kenya Rift. The surface geology of the area is characterised by five main sequences including pre-Paka formations and volcanic sequence 1–4, all built up by basalt, intermediate and trachyte rocks spanning 582–8 ka. Numerical trace element modelling suggests that the primary magma in Paka resulted principally from 5 to 10% garnet–peridotite partial melting and possibly 0.1–1.5% partial melting of spinel-lherzolite. The parental magma was probably held at the Moho (~30 km) and evolved by ~76% gabbro fractional crystallization to generate a liquid of similar chemistry as that of erupted Paka basalts. We propose that under moderately oxidized conditions (∆FMQ +1) the primary magma underwent a further ~70% fractional crystallization and ~ 20% assimilation of crustal syenitic material to generate Paka trachyte lavas. Basaltic and intermediate magmas were held at polybaric storage pressure conditions ranging between 1.4 and 7 kbar (5–25 km), at temperatures between 1086 and 1204 °C with relatively low water content of <1.8 wt%. The trachyte magma resided at low pressures, i.e., 0.3–2.5 kbar (1–9 km) at 901–980 °C with water contents of 3.5–5.1 wt%. Based on our data eruption source depths remained largely the same for a period spanning from 582 to 11 ka. Geophysical data show correlation with magma depths beneath Paka volcano.

Geochemistry of the Yumen picrites-basalts from the Emeishan large igneous province: Implications for their mantle source, PGE behaviors, and petrogenesis

Olivine compositions are often used to infer the source lithology of basaltic rocks, but the effect of source melting conditions on olivine chemistry remains poorly studied. Here we present mineral chemistry, whole-rock major and trace elements, SrNd isotopes, and platinum-group element (PGE) concentrations for high-Ti picritic-basaltic rocks from Yumen area. These data provide new constraints on the source lithology and petrogenesis of these high-Ti rocks in the Emeishan large igneous province (ELIP). The picrites have primitive compositional features (e.g., high Mg# values, depleted SrNd isotopic compositions, no PGE depletions and/or fractionations). Some basaltic rocks have negative NbTa anomalies, enriched SrNd isotopic compositions, and depleted and/or fractionated PGE contents, indicative of fractional crystallisation, variable degrees of crustal contamination, and magmatic sulfide segregation. Combined with published data for the ELIP, the compositions of the primitive magmas and 10000Zn/Fe and FC3MS (FeO/CaO − 3 × MgO/SiO2, all in wt%) values of the ELIP high-Ti picrites are similar to those of garnet peridotite-derived experimental melts. Based on the primitive magma compositions, the ELIP high-Ti melts could have been produced by mantle melting at 4.0–5.4 GPa and 1620–1660 °C, and started to crystallise olivine at 1410–1550 °C, based on the Al-in-olivine thermometer, which are higher temperatures than for mid-ocean ridge basalts and ELIP low-Ti magmas. The high-Ni and low-Mn olivine phenocrysts in the ELIP high-Ti samples crystallised from peridotite-derived, MgO-rich melts at high temperatures. Variations in the physicochemical source melting conditions was enough to explain the differences in olivine Ni and Mn contents in the ELIP high- and low-Ti picrites-basalts. Additionally, the almost constant Pd, Pt, and Rh contents with decreasing Mg# values indicate intermediate partition behaviors in the ELIP magmas. No correlations exist between whole-rock 10,000 × Pd/Yb and Mg#, (Th/Nb)N, and εNd(t) values, implying that selective assimilation of crustal sulfur resulted in magmatic sulfide saturation. We propose that the ELIP high-Ti magmas were derived from a deep-sourced garnet peridotite, and experienced various igneous processes during ascent through the continental lithosphere to form different types of ELIP high-Ti basaltic rocks.

Confocal Raman spectroscopic study of melt inclusions in olivine of mantle xenoliths from the Bultfontein kimberlite pipe (Kimberley cluster, South Africa): Evidence for alkali‐rich carbonate melt in the mantle beneath Kaapvaal Craton

AbstractIdentifying the composition of primary/primitive mantle melts is crucial for understanding the mantle's evolution and mantle‐derived magmatism. Melt inclusions in mantle xenoliths provide key information about such melts. This study used confocal Raman spectroscopy to characterize mineral assemblage of unexposed crystallized secondary melt inclusions in the olivine from the xenoliths of the sheared garnet peridotites from the Bultfontein kimberlite pipe (Kimberley cluster, Kaapvaal Craton, South Africa). The studied peridotites originated from 112‐ to 146‐km mantle depths. In total, 16 minerals were identified among daughter crysatls, which include carbonates (calcite, magnesite, dolomite), alkali carbonates (eitelite, nyerereite, gregoryite/natrite, K–Na–Ca–carbonate (K,Na)2Ca(CO3)2 (K analogue of nyerereite), alkali carbonates with additional anions (northupite, bradleyite, burkeite), alkali sulfates (glauberite, thenardite, aphthitalite), apatite, tetraferriphlogopite, and magnetite. Several more daughter minerals gave distinct Raman spectra, but they were not determined due to the lack of similar spectra in the databases. Carbonates are predominant among the daughter minerals in the melt inclusions. Many daughter minerals are rich in alkalis. These facts indicate that melt(s), parental for the inclusions, is alkali‐rich carbonate in composition. Two possible models were suggested for the origin of these melt inclusions: (1) in situ fracturing of olivine and the mantle melt infiltration shortly before the sheared peridotites were entrained by the ascending kimberlitic magma; (2) infiltration of the transporting kimberlite melt into xenoliths during ascent. Both models imply that the alkali‐rich carbonate melt(s) that interacted with peridotites originated at a greater depth than the entrapment level of studied xenoliths (&gt;150 km), that is, at the base of the lithosphere or in the asthenosphere. This melt is genetically related to the kimberlite magmatism that formed the Bultfontein pipe and points at the alkali‐rich carbonate composition of primary kimberlite melt.

Deep subduction, melting, and fast cooling of metapelites from the Cima Lunga Unit, Central Alps

AbstractThe Cima Lunga unit in the Central Alps is dominated by quartzofeldspathic gneisses with subordinate mafic, ultramafic, and metacarbonate rocks. Only mafic and ultramafic lithologies were thought to preserve clear evidence of Alpine high‐P metamorphism. This led to the questions of whether the different rock types were subducted and exhumed as a coherent unit or underwent different pressure–temperature (P–T) histories. New petrological and geochemical data from a metapelite associated with garnet peridotite from Cima di Gagnone (Cima Lunga unit, Switzerland) were obtained using major and trace element mapping. Complex zoning patterns in garnet and white mica are observed. In particular, high Ti content in phengite and increasing P, Zr, and HREE contents in pyrope‐rich garnet indicate that this metapelite underwent high‐P and high‐T (HP–HT) metamorphism involving fluid‐fluxed partial melting. A P–T path is reconstructed by combining textural analysis with petrological–geochemical data and thermodynamic simulations. We show that the mineral record preserves an evolution from prograde to HP–HT peak conditions (2.7 ± 0.1 GPa and 800℃) followed by near‐isobaric cooling (~2.5 GPa and 700–750℃) prior to decompression (1.0 GPa and ~620℃). The reconstructed P–T path suggests that the studied metapelites were subducted to depths where the slab gets heated by proximity to asthenospheric mantle related to slab break‐off. This heating resulted in the dehydration of chlorite‐ to garnet peridotite and the liberated fluids triggered partial melting in the associated metapelites, which might have favoured the fast exhumation of the entire Cima Lunga unit. Metapelites and garnet peridotite from Cima di Gagnone underwent a common prograde to peak and retrograde P–T path without significant tectonic pressure difference between the different lithologies, and deviation from lithostatic pressure is excluded. Lastly, the peak metamorphic conditions of metapelite from Cima di Gagnone are comparable with P–T estimates of ultramafic lithologies from the southern Adula nappe and the Dascio Bellinzona zone, thus opening new scenarios for the geodynamic interpretation of the Central Alps.

Potassium distribution and isotope composition in the lithospheric mantle in relation to global Earth’s reservoirs

Recent analytical advances have provided means to measure potassium (K) isotopes in various terrestrial materials, but little is known about K distribution and stable isotope composition in the lithospheric mantle because of: (a) common low K abundances, (b) potential contamination and alteration, (c) diversity of mantle rocks and minerals hosting K in different tectonic settings. We report K abundances and δ41K values for well-studied whole-rock (WR) mantle xenoliths (spinel and garnet peridotites and pyroxenites) from mobile belts, a craton, a subduction zone, as well as for K-rich phases (mica, amphibole, silicate glass) and xenolith-bearing volcanic materials (67 samples). The xenolith materials show extremely broad ranges of K content (7 μg/g to 6.6 wt.%) and δ41K (−2.77‰ to 0.62‰). They contrast with the narrow δ41K range for host volcanic materials (−0.53‰ to −0.27‰) and literature data on oceanic basalts (melting products of upwelling asthenosphere: −0.43 ± 0.17‰, 2sd). Amphibole-bearing subduction zone peridotites show the highest WR δ41K values (0.40–0.62‰) likely inherited from fluids released into the mantle wedge from subducted oceanic crust. All other WR samples yield negative δ41K: −0.06‰ to −2.77‰ for peridotites and −0.17‰ to −0.52‰ for pyroxenites. The δ41K in K-rich mantle phases range from positive values (0.16–0.57‰) for phlogopite in strongly metasomatized peridotites to negative values (−0.27 to −0.94‰) for phlogopite, amphibole and glass pockets from other samples, which cannot be explained by equilibrium inter-mineral fractionation inferred from ab initio calculations. We attribute the broad δ41K variations to (a) isotope fractionation during fluid-rock interaction in the mantle, and (b) distinct sources of K-bearing fluids. Kinetic isotope fractionation during fluid percolation and diffusion is inferred for composite xenoliths (phlogopite and pyroxenite veins in peridotites) that have lower δ41K in the hosts than in the veins due to slower migration of 41K than 39K from the veins (former fluid channels) to host mantle. Overall, the K isotope fractionation in the lithospheric mantle appears to be greater than for magmatic fractionation in the crust. The average δ41K of normal off-craton continental lithospheric mantle calculated from the least altered fertile and lightly metasomatized lherzolites is −0.57 ± 0.28‰ (2sd). This value is within error (though a little lower) of estimates for both continental crust and MORB and OIB mantle sources indicating that these major silicate Earth reservoirs have similar bulk δ41K values, apparently due to low or negligible K isotopic fractionation during their formation by magmatic differentiation and melting. By contrast, K isotopes in modern and fossil subduction zones are fractionated via fluid-related equilibrium and kinetic processes.

Subduction initiation-induced rapid emplacement of garnet-bearing peridotites at a nascent forearc: Petrological and Os-Li isotopic evidence from the Purang ophiolite, Tibet

Abstract Garnet-bearing peridotites commonly occur in the deeper parts of mature or thickened oceanic lithosphere, and are rarely exhumed and emplaced onto the seafloor. The Purang ophiolitic peridotites in south Tibet contain rare symplectite pseudomorphs after garnet, offering a unique window into the still poorly understood evolution of the deep oceanic lithosphere. Here, integrated petrologic and Os-Li isotopic data are used to constrain the evolution and dynamics of emplacement for these garnet peridotite protoliths. The Purang peridotites show wide variations of chemical compositions (spinel Cr#: 0.2–0.8) and Os model ages (up to 2.0 Ga), thus representing a piece of heterogeneous oceanic mantle lithosphere. Dunite channels show two distinctive groups of Cr# of spinels and Os-isotope compositions, with the low- to medium-Cr# (0.2–0.6) and high-Cr# (0.7–0.8) dunites reflecting the reaction of host lherzolites/harzburgites with percolating mid-ocean ridge basalt–like and boninitic melts, respectively. This confirms recent subduction initiation-related melt percolation in the Purang peridotites. Coexisting olivines and pyroxenes in the peridotites show systematic Li elemental and isotopic disequilibrium, suggesting fast cooling of the peridotites to Li closure temperature shortly after the melt percolations, likely during exhumation of the peridotites onto the seafloor. This supports a close link between subduction initiation and tectonic emplacement of the Purang peridotites. Combined with other geological evidence, we suggest the Purang peridotites may originate from the deep part of old, thick oceanic lithosphere of the Neo-Tethys. This thick oceanic lithosphere was progressively weakened and thinned likely during widespread plume-lithosphere interaction, triggering the transformation of garnet peridotite protoliths to spinel peridotites. Subsequently, initiation of a new subduction zone along the lithospheric weakness caused rapid ascent and emplacement of the Purang peridotites at a nascent forearc.

Two billion years of episodic and simultaneous websteritic and eclogitic diamond formation beneath the Orapa kimberlite cluster, Botswana

The Sm–Nd isotope systematics and geochemistry of eclogitic, websteritic and peridotitic garnet and clinopyroxene inclusions together with characteristics of their corresponding diamond hosts are presented for the Letlhakane mine, Botswana. These data are supplemented with new inclusion data from the nearby (20–30 km) Orapa and Damtshaa mines to evaluate the nature and scale of diamond-forming processes beneath the NW part of the Kalahari Craton and to provide insight into the evolution of the deep carbon cycle. The Sm–Nd isotope compositions of the diamond inclusions indicate five well-defined, discrete eclogitic and websteritic diamond-forming events in the Orapa kimberlite cluster at 220 ± 80 Ma, 746 ± 100 Ma, 1110 ± 64 Ma, 1698 ± 280 Ma and 2341 ± 21 Ma. In addition, two poorly constrained events suggest ancient eclogitic (> 2700 Ma) and recent eclogitic and websteritic diamond formation (< 140 Ma). Together with sub-calcic garnets from two harzburgitic diamonds that have Archaean Nd mantle model ages (TCHUR) between 2.86 and 3.38 Ga, the diamonds studied here span almost the entire temporal evolution of the SCLM of the Kalahari Craton. The new data demonstrate, for the first time, that diamond formation occurs simultaneously and episodically in different parageneses, reflecting metasomatism of the compositionally heterogeneous SCLM beneath the area (~ 200 km2). Diamond formation can be directly related to major tectono-magmatic events that impacted the Kalahari Craton such as crustal accretion, continental breakup and large igneous provinces. Compositions of dated inclusions, in combination with marked variations in the carbon and nitrogen isotope compositions of the host diamonds, record mixing arrays between a minimum of three components (A: peridotitic mantle; B: eclogites dominated by mafic material; C: eclogites that include recycled sedimentary material). Diamond formation appears dominated by local fluid–rock interactions involving different protoliths in the SCLM. Redistribution of carbon during fluid–rock interactions generally masks any potential temporal changes of the deep carbon cycle.

Decorated dislocations in naturally deformed olivine with C-type fabric: A case study in the Lüliangshan garnet peridotite from the North Qaidam ultrahigh-pressure belt, NW China

This paper presents a detailed case study of decorated dislocations of olivine with C-type fabric in the Lüliangshan garnet peridotite from the North Qaidam ultrahigh-pressure (UHP) metamorphic belt in NW China. Electron backscatter diffraction (EBSD) measurements reveal a weak C-type fabric in olivine, i.e., the crystallographic [100] axes form a weak point maximum subnormal to the foliation while the [001] axes form a girdle in the foliation plane with a weak point maximum subparallel to the lineation. Oriented hydrous mineral of sodic amphibole and pyroxene were found in porphyroclastic garnet indicating considerable water content in the parental majoritic garnet at high pressure conditions. However, the water content of olivine and garnet is below the detection limit (less than a few ppm wt. H2O) of the instrument (a Nicolet 6700 FTIR with Continuum FTIR Microscope). Two distinctive deformation features of decorated olivine grains were recognized: (1) linear morphologies of many dislocations with long straight segments parallel to the [001] orientation, which are likely screw dislocations with b = [001]; (2) olivine grains with planar dislocation, subgrain boundaries or undulose extinction are scarce in thin sections under optical microscopy. The dislocation structures and densities in olivine are heterogeneous at multi-scales. The mean 3D dislocation density and the estimated differential stress based on the dislocation density piezometer are 0.4–0.8 × 10−12 m−2 and 65–108 MPa, respectively. Decorated dislocations and EBSD analyses suggest the most likely dominant activation slip systems with b = [001]. Considering that most C-type fabric of olivine are reported from UHP garnet peridotites, we emphasize the important effects of high pressure and relatively low temperature on the formation of the C-type fabric in olivine.

Multi-stage sulfur and carbon mobility in fossil continental subduction zones: New insights from carbonate-bearing orogenic peridotites

The volatile transfer in subduction zones and the role of sulfate as a vector for the mobilization of oxidized components from down-going slabs remain hotly debated issues. Orogenic spinel and garnet peridotite lenses from the Ulten Zone (Eastern Alps, Italy), exhumed as part of felsic metamorphic terranes in continental collision zones, bear witness to mass transfer processes in these pivotal environments.In this study, we carried out a multi-method investigation of mantle sulfides coexisting with four generations of carbonates, indicating coupled sulfur and carbon mobility throughout the peridotites’ metamorphic evolution as part of the Variscan subduction architecture. Detailed petrography, bulk rock measurements, in situ chemical and geochemical analyses of sulfides as well as Sr isotope analyses of associated clinopyroxene and amphibole are combined with the aim to constrain the origin, nature and effect of multiple C-O-H-S-bearing fluids and melts the peridotites interacted with. The first, pre-peak, metasomatic pulse (Stage 1) is represented by an H2S-CO2-bearing melt from the subduction-modified hot mantle wedge, which formed a pyroxenite layer hosting matrix pentlandite with δ34S of +2.77‰. Matrix carbonates occasionally occur in the coarse-grained peridotite under eclogite-facies conditions (Stage 2), with heavier δ34S (up to +3.43‰), radiogenic Sr (87Sr/86Srclinopyroxene > 0.7052) and elevated Pb abundances. These are ascribed to interaction with isotopically heavy melts carrying recycled crustal component, permissive of, but not requiring, involvement of oxidized S species. Conversely, isotopically lighter matrix pentlandite (δ34S = −1.62 to +0.67‰), and radiogenic Sr in amphibole (87Sr/86Sr = 0.7056) and associated dolomite (published data) from fine-grained garnet-amphibole peridotites may point to involvement of H2S-CO2-bearing crustal fluids, which variably equilibrated with the mantle before interacting with the peridotites. The post-peak Stage 3 marks the entrapment of peridotites into a tectonic mélange. Here, kelyphitization of garnet is catalyzed by further ingress of a S-bearing fluid (δ34S = −0.38‰), while carbonate veining with occasional sulfides bear witness to channelized fluid flow. Sulfide and amphibole grains in retrogressed spinel peridotites reveal the highest contents of fluid-mobile elements (As, Sb) and 87Sr/86Sramphibole up to 0.7074, suggesting late interactions with isotopically heavy crustal fluids at high fluid-rock ratios. Textural observations indicate that, during Stage 4, serpentinization of peridotites at low ƒS2 played an active role not only in CO2 release by conversion of dolomite to calcite + brucite intergrowths, but also in local removal of 32S during the final exhumation stage. Late channelized sulfur remobilization is evidenced by the serpentine + magnetite (±millerite ± calcite) vein carrying > 300 ppm S.Overall, the relatively narrow range of sulfur isotope composition (δ34S = −1.62 to +3.76‰) is indicative of limited interaction with isotopically heavy crustal liquids, and points to a subordinate role of subduction-derived sulfate throughout the extended fluid(melt)/rock evolution of the Ulten Zone peridotites, first in the mantle wedge and then as part of a tectonic mélange.

Zircon U–Pb geochronology, Hf isotope composition, and petrochemical characteristics of Paleocene granitoids in the western Gangdese Belt, Tibet

Abstract —The research team studied the petrology, whole-rock geochemistry, zircon U–Pb age, and stable isotopic characteristics of the Rongguo Longba and Garongcuo granites of the Nuocang area to understand better the impact of Neo-Tethys ocean subduction and In-dia–Eurasia continental collision on Paleocene tectonomagmatic processes along the southern margin of the Gangdese Belt. The Rongguo Longba granite and Garongcuo granite porphyry formed at 61.86 and 62.17 Ma, respectively. The Nuocang granitoids are characterized by (1) high SiO2, NaO2, and Al2O3 contents and low FeOtot, MgO, and TiO2 contents; (2) LREE and LILE enrichment and HREE and HFSE (Nb, P, and Ti) depletion; and (3) obvious negative Eu anomalies. These features indicate that the Nuocang granites are of the high-K calc-alkaline and peraluminous granite types. Furthermore, their zircon Hf isotope characteristics suggest that the magma source region has an ancient crystalline basement. The basaltic andesitic crystal tuff is the product of garnet–peridotite partial melting and crust contamination from rising magma emplacement.

Ni-in-garnet geothermometry in mantle rocks: a high pressure experimental recalibration between 1100 and 1325\xa0\xb0C

The temperature-dependent exchange of Ni and Mg between garnet and olivine in mantle peridotite is an important geothermometer for determining temperature variations in the upper mantle and the diamond potential of kimberlites. Existing calibrations of the Ni-in-garnet geothermometer show considerable differences in estimated temperature above and below 1100 °C hindering its confident application. In this study, we present the results from new synthesis experiments conducted on a piston cylinder apparatus at 2.25–4.5 GPa and 1100–1325 °C. Our experimental approach was to equilibrate a Ni-free Cr-pyrope-rich garnet starting mixture made from sintered oxides with natural olivine capsules (Niolv ≅ 3000 ppm) to produce an experimental charge comprised entirely of peridotitic pyrope garnet with trace abundances of Ni (10–100 s of ppm). Experimental runs products were analysed by wave-length dispersive electron probe microanalysis (EPMA) and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). We use the partition coefficient for the distribution of Ni between our garnet experimental charge and the olivine capsule left( {{text{lnD}}_{{{text{grt}}/{text{olv}}}}^{{{text{Ni}}}} ; frac{{{text{Ni}}_{{{text{grt}}}} }}{{{text{Ni}}_{{{text{olv}}}} }}} right), the Ca mole fraction in garnet ({mathrm{X}}_{mathrm{grt}}^{mathrm{Ca}}; Ca/(Ca + Fe + Mg)), and the Cr mole fraction in garnet ({mathrm{X}}_{mathrm{grt}}^{mathrm{Cr}}; Cr/(Cr + Al)) to develop a new formulation of the Ni-in-garnet geothermometer that performs more reliably on experimental and natural datasets than existing calibrations. Our updated Ni-in-garnet geothermometer is defined here as:T left(^circ{rm C} right)=frac{-8254.568}{left(left( {mathrm{X}}_{mathrm{grt}}^{mathrm{Ca}} times 3.023 right)+left({mathrm{X}}_{mathrm{grt}}^{mathrm{Cr}} times 2.307 right)+left({mathrm{lnD}}_{frac{mathrm{grt}}{mathrm{olv}}}^{mathrm{Ni}} - 2.639 right)right)}-273pm 55where {mathrm{D}}_{mathrm{grt}/mathrm{olv}}^{mathrm{Ni}}= frac{{mathrm{Ni}}_{mathrm{grt}}}{{mathrm{Ni}}_{mathrm{olv}}}, Ni is in ppm, {mathrm{X}}_{mathrm{grt}}^{mathrm{Ca}} = Ca/(Ca + Fe + Mg) in garnet, and {mathrm{X}}_{mathrm{grt}}^{mathrm{Cr}}= Cr/(Cr + Al) in garnet. Our updated Ni-in-garnet geothermometer can be applied to garnet peridotite xenoliths or monomineralic garnet xenocrysts derived from disaggregation of a peridotite source. Our calibration can be used as a single grain geothermometer by assuming an average mantle olivine Ni concentration of 3000 ppm. To maximise the reliability of temperature estimates made from our Ni-in-garnet geothermometer, we provide users with a data quality protocol method which can be applied to all garnet EPMA and LA-ICP-MS analyses prior to Ni-in-garnet geothermometry. The temperature uncertainty of our updated calibration has been rigorously propagated by incorporating all analytical and experimental uncertainties. We have found that our Ni-in-garnet temperature estimates have a maximum associated uncertainty of ± 55 °C. The improved performance of our updated calibration is demonstrated through its application to previously published experimental datasets and on natural, well-characterised garnet peridotite xenoliths from a variety of published datasets, including the diamondiferous Diavik and Ekati kimberlite pipes from the Lac de Gras kimberlite field, Canada. Our new calibration better aligns temperature estimates using the Ni-in-garnet geothermometer with those estimated by the widely used (Nimis and Taylor, Contrib Mineral Petrol 139:541–554, 2000) enstatite-in-clinopyroxene geothermometer, and confirms an improvement in performance of the new calibration relative to existing versions of the Ni-in-garnet geothermometer.

Phlogopite-rich and phlogopite-poor kimberlite intrusions within the Du Toitspan kimberlite pipe, South Africa: Petrogenetic relationships and localised source heterogeneity

Samples from three petrographically distinct, intrusive kimberlite bodies and associated kimberlite dykes from the eastern lobe of the Du Toitspan kimberlite pipe, Kimberley, South Africa, have been analysed for their bulk rock major and trace element compositions and their olivine and phlogopite compositions. The two dominant intrusive bodies (D13, D14) are distinguished by the one (D13) being phlogopite-rich and best classified as a macrocrystic hypabyssal phlogopite kimberlite, and the other (D14) being phlogopite-poor and best classified as a macrocrystic hypabyssal monticellite kimberlite. The minor D17 intrusive body is classified as a macrocrystic transitional hypabyssal serpentinized phlogopite kimberlite. The associated kimberlite dykes range texturally from aphanitic to macrocrystic and are classified as calcite kimberlites.The major kimberlite intrusions and their associated dykes show no evidence of crustal contamination and are characterised by broadly overlapping geochemistry except for distinctly higher K2O (> 2 wt%) and Al2O3 (>3 wt%) and flattening HREE patterns (Gd/YbN = 6.5–7.0) in the D13 – phlogopite kimberlite compared to the D14 – monticellite kimberlite and the calcite kimberlite dykes (Gd/YbN = 9.6–12.1). These distinguishing geochemical features of the D13 – phlogopite kimberlite are comparable to typical Group II kimberlites in southern Africa. However, their diagnostic incompatible trace element ratios (for example, Th/Nb, La/Nb, Ce/Pb, and Ba/Nb) are instead comparable to other kimberlite intrusions analysed in this study and to southern African Group I kimberlites in general.Semi-quantitative modelling suggests that these kimberlite intrusions could have derived by low (<1%) degrees of partial melting of a source region that is enriched in LREE (Lan = ~6.1; Ybn ~ 1.47) comparable to metasomatised peridotites from the underlying lithospheric mantle. The composition of the D13 phlogopite kimberlite is consistent with a partial melt of a modally metasomatised source containing a higher proportion of residual clinopyroxene relative to garnet (compared to that giving rise to the D14 monticellite kimberlite and calcite kimberlite dykes), as well as accessory amounts of phlogopite, i.e. a garnet phlogopite peridotite (GPP). The absence of K-anomalies on primitive mantle normalized diagrams for the D13 phlogopite kimberlite requires that phlogopite was not a residual phase during partial melting and was exhausted shortly before or at the moment of melt segregation. The higher Gd/Yb ratios and lower K2O in the D14 monticellite kimberlite and calcite kimberlite dykes can be explained by partial melting of a cryptically metasomatized, phlogopite – free, garnet peridotite (GP) source, containing a higher proportion of garnet relative to clinopyroxene. The low absolute K and strong negative K-anomaly on primitive mantle normalized diagrams for the D14 monticellite kimberlite were inherited from a source region that previously experienced cryptic metasomatism by a differentiated fluid already carrying a negative K-anomaly.

Microstructural Shift due to Post‐Deformation Annealing in the Upper Mantle

AbstractSyntectonic microstructural evolution is a well‐known phenomenon in the mantle and lower crust associated with two main processes: grain size reduction through dynamic recrystallization and development of crystallographic preferred orientation (CPO). However, the effects of annealing via static recrystallization on grain size and CPO have been largely overlooked. We investigated mantle annealing by analyzing a suite of kimberlite‐hosted garnet peridotite xenoliths from the Wyoming Craton. We focus on five xenoliths that show microstructures reflecting different degrees of recrystallization, with annealed grains characterized by distinctive faceted boundaries crosscutting surrounding, nonfaceted matrix grains. These textures are indicative of discontinuous static recrystallization (DiSRX). Electron backscatter diffraction analysis further demonstrates a ∼10°–20° misorientation between DiSRXed grains and the matrix grains, resulting in an overall weaker CPO. These characteristics are remarkably similar to microstructures observed in samples that were annealed after deformation in the laboratory. Measurements of the thermal conditions and water contents associated with the last equilibration of the xenoliths suggests that high homologous temperatures (T/Tm &gt; 0.9) are necessary to induce DiSRX. We postulate that annealing through DiSRX occurs under high temperatures after a short episode of intense deformation (years to hundreds of years) with timescales for annealing estimated as weeks to years, significantly slower than the timescale of hours expected for a kimberlitic magma ascent. We conclude that microstructural transformation due to DiSRX will occur during transient heating events associated with mantle upwelling, plumes, and lithospheric thinning.

Oxidation state and metasomatism of the lithospheric mantle beneath the Rae Craton, Canada: strong gradients reflect craton formation and evolution

We present the first oxidation state measurements for the subcontinental lithospheric mantle (SCLM) beneath the Rae craton, northern Canada, one of the largest components of the Canadian shield. In combination with major and trace element compositions for garnet and clinopyroxene, we assess the relationship between oxidation state and metasomatic overprinting. The sample suite comprises peridotite xenoliths from the central part (Pelly Bay) and the craton margin (Somerset Island) providing insights into lateral and vertical variations in lithospheric character. Our suite contains spinel, garnet-spinel and garnet peridotites, with most samples originating from 100 to 140 km depth. Within this narrow depth range we observe strong chemical gradients, including variations in oxygen fugacity (ƒO2) of over 4 log units. Both Pelly Bay and Somerset Island peridotites reveal a change in metasomatic type with depth. Observed geochemical systematics and textural evidence support the notion that Rae SCLM developed through amalgamation of different local domains, establishing chemical gradients from the start. These gradients were subsequently modified by migrating melts that drove further development of different types of metasomatic overprinting and variable oxidation at a range of length scales. This oxidation already apparent at ~ 100 km depth could have locally destabilised any pre-existing diamond or graphite.

A Reappraisal of Peridotite Solidus Phase Equilibria from 6 to 14 GPa in the System CaO–MgO–Al2O3–SiO2

AbstractExperimentally determined isobaric invariant melting phase relations from 6 to 14 GPa in the system CaO–MgO–Al2O3–SiO2 (CMAS), involving the crystalline phases forsterite + orthopyroxene + clinopyroxene + garnet and liquid, are reported. Experiments were conducted using a multianvil device with stepped lanthanum chromite heaters in the pressure cells. At a fixed pressure, the five-phase assemblage identified above can exist only at a single temperature. As such, these isobaric invariant points correspond to the solidus of model garnet peridotite in this part of the composition space in the studied system, as is the case at lower pressures in some previous studies. The solidus of model peridotite is univariant in pressure–temperature space, has a positive Clapeyron slope, and the isobaric invariant solidus temperatures, at 6, 8, 10, 12, and 14 GPa, are 1965 °C, 2090 °C, 2200 °C, 2280 °C, and 2320 °C, respectively. Over the investigated pressure range, orthopyroxene is in reaction relation with the liquid, with the fusion reaction taking the form forsterite + clinopyroxene + garnet = orthopyroxene + liquid. The compositions of liquids reported here do not seem to depend on orthopyroxene being present in the experiments. Compositionally, liquids here are fairly magnesian and siliceous, and have lower alumina and lime concentrations than at lower pressures with the identical crystalline phase assemblage in the system CMAS. In contrast to some previous studies, in this study, there is no evidence of maximum and minimum normative forsterite concentration of the isobaric invariant liquid at around 8 and 12 GPa, respectively, nor of a substantial curvature in the track of liquid compositions, when such liquids coexist with the above-mentioned four-phase crystalline phase assemblage. Instead, here, with increasing pressure from 6 to 14 GPa, liquids at the isobaric invariant points (defining the univariant solidus) become progressively (quasi-linearly) enstatite-normative. This experimental observation on liquid compositions from the present study might be important for future work aimed at investigating the chemistry of liquids derived from partial fusion of anhydrous peridotite at pressures, and corresponding depths in Earth, greater than investigated here.

No chemical change during high-T dehydration and re-hydration reactions: Constraints from Erzgebirge HP and UHP eclogite

Meta-basaltic eclogite occurs in the Erzgebirge in three high-pressure (HP) units (Unit 1, 2, and 3) as numerous lenses within high-grade felsic rocks. Units 2 and 3 are common HP units with quartz eclogite, and Unit 1 is an ultra-high pressure (UHP) unit with coesite eclogite, garnetite (metarodingite) and garnet peridotite. The conditions of peak metamorphism increase from Unit 3 (600–650 °C, 20–22 kbar), to Unit 2 (670–730 °C, 24–26 kbar) and Unit 1 (840–920 °C, ≥30 kbar), correlating with a decreasing abundance of hydrous minerals (from >10 vol% in Unit 3 to 0 vol% in Unit 1). In all units, the dominating eclogite type is dark-colored with a composition typical of MORB. A subordinate light type is chemically more variable and has higher contents of Mg, Al, Ca, Cr, Ni, and large ion lithophile elements as well as lower Fe, Zr, Y, and rare earth element concentrations than dark eclogite. Light eclogite formation is interpreted by plagioclase accumulation in a MOR magma chamber.No compositional difference is visible between well-preserved eclogite and samples with variable degrees of post-eclogitic overprint. In addition, dark eclogite from all units is compositionally indistinguishable, implying that prograde dehydration reactions did not modify major and trace element concentrations. This conclusion applies to all reactions in the temperature interval between c. 600 °C (peak T in Unit 3) and c. 900 °C (peak T in Unit 1), including prograde breakdown of calcic amphibole, zoisite, paragonite, and phengite. Together with previous studies on dehydration reactions in blueschist and eclogite at <600 °C (Spandler et al., 2003; Spandler et al., 2004), the present results imply that de-volatilization of the basaltic portion of subducting slabs plays only a minor, if any, role for the enrichment of fluid-mobile elements in the mantle wedge. We infer that most, if not all, of the H2O produced by dehydration reactions between 600 and 900 °C is preserved in eclogite due to the pressure-enhanced capability of garnet and omphacite to incorporate structural water. Incorporation of H2O, produced during the final dehydration stage (c. 800 ± 50 °C, 25–30 kbar), in nominally anhydrous minerals may explain why partial melting obviously did not occur in eclogite.

Identification of potential targets for kimberlite exploration using satellite imagery and map combination approach in the Lesotho Kimberlite Province

This study presents the use of satellite data and map combination approach for kimberlite exploration to identify follow-up exploration targets. The study area falls in the Lesotho Kimberlite Province, which hosts diamondiferous kimberlite pipes such as Letseng and Kao in Lesotho, and Monastery and Jagersfontein in South Africa. This study focusses on processing and integration of evidential map layers (geomorphic features, linear structures and kimberlite indicator minerals) that have known correlation with kimberlite occurrence. The evidential map layers were acquired from satellite platforms such as ASTER and SRTM DEM. These layers were integrated to identify favourable sites for detailed follow-up exploration by subdividing the area into high, moderate and low prospective regions based on the number of controlling evidences operating in each sub-region. A follow-up sampling and ground magnetic survey were carried out at one of the high prospective zones to validate the results of the present approach. Kimberlite indicator minerals such as garnet, diopside, ilmenite and chromite were recovered from the collected sample. Several garnets show adhering materials known as kelyphite rims, which are typical surface features of peridotitic garnets having close relationship with diamondiferous kimberlite. Keating correlation coefficient, which uses a simple pattern recognition to locate magnetic anomalies that resemble the response of modelled kimberlite pipes, was applied to ground magnetic data. We used a series of reference kimberlite models composed of homogeneous vertical cylinders of various radius ranging from 50 to 200 m and depth varying from 50 to 500 m. The correlation coefficients between analytical signal grid of modelled kimberlite pipe and observed magnetic data were computed. Following this approach, kimberlite model with correlation coefficients exceeding 80% was selected. By matching the response of modelled kimberlite pipe with the magnetic anomalies, four pipe-like targets were located. The target(s) may be a potential source of the recovered kimberlite indicator minerals since the morphology of these minerals suggest a proximal kimberlitic source.

Petrogenesis and dynamic implications of the Cenozoic alkali basalts from the Jingpohu Volcanic Field, NE China

Abstract The Cenozoic alkali basalts are widely exposed in the Jingpohu Volcanic Field, NE China. Previous volcanology and geochronology research has revealed that they were formed in three periods in the Miocene ( c. 29.23–13.59 Ma), Pleistocene ( c. 83.7 ka) and Holocene ( c. 5.5–5.2 ka BP). The Miocene and Pleistocene basalts consist of alkali olivine basalts, while the Holocene basalts are composed of alkali olivine basalts and leucite tephrites. Petrogenetic studies reveal that the primary magmas of the Miocene and Pleistocene alkali olivine basalts originated from partial melting of EM2-like garnet peridotites, and those of the Holocene alkali olivine basalts were derived from melting of EM1- and EM2-like garnet peridotites with higher garnet proportions. In contrast, the primary magmas of the Holocene leucite tephrites were derived from the melting of eclogites and peridotites. Combined with previous research, we suggest that melting of the mantle source region to generate Jingpohu alkali basalts was triggered by decarbonization and dehydration of the slabs stagnated in the mantle transition zone.