Phlogopite Peridotite Research Articles

Transition from orogenic-like to anorogenic magmatism in the Southern Alps during the Early Mesozoic: Evidence from elemental and Nd-Sr-Hf-Pb isotope geochemistry of alkali-rich dykes from the Finero Phlogopite Peridotite, Ivrea–Verbano Zone

The Ivrea-Verbano Zone (IVZ) in the westernmost sector of the Southern Alps is an iconic upper mantle to lower continental crust sequence of the Adriatic Plate and provides a geological window into the tectono-magmatic events that occurred at the Gondwana–Laurussia boundary from Late Paleozoic to Early Mesozoic. In this work, we document new geochemical and Nd-Sr-Hf-Pb isotopic data for Early Mesozoic alkali-rich dyke swarms which intruded the Finero Phlogopite Peridotite (northern IVZ) to provide geological constraints on the nature, origin and evolution of Early Mesozoic magmatism in the Southern Alps. The studied dykes are amphibole-phlogopite-bearing and show geochemical features varying between two end-member groups. A dyke group is characterized by HFSE-poor, Al-rich amphibole (Al2O3 up to 16 wt%) with high LILE and LREE contents, high radiogenic 87Sr/86Sr(i) (0.704732 to 0.704934) and low radiogenic Nd isotopes (εNd(i) from –0.1 to –0.7), which support the occurrence of significant amounts of recycled continental crust components in the parental mantle melts and impart an overall “orogenic-like” affinity. This dyke group was largely derived from metasomatized lithospheric mantle sources. The second group is HFSE-rich with Al-poorer amphibole enriched in LILE and LREE, low radiogenic 87Sr/86Sr(i) (0.703761–0.704103) and higher radiogenic Nd isotopes (εNd(i) from +3.4 to +5.4) pointing to an “anorogenic” alkaline affinity and asthenospheric to deep lithospheric mantle sources. Some dykes show both orogenic and anorogenic affinities, providing evidence that the orogenic-like magmatism in the IVZ predates the alkaline anorogenic magmatism. The Finero dyke swarms therefore record a geochemical change of the Early Mesozoic magmatism of the Southern Alps from orogenic-like magmatism, typical of post-collisional settings, to anorogenic alkaline magmatism, common in intraplate to extensional settings, and places a temporal correlation of Early Mesozoic magmatism in the IVZ to those in the eastern and central sectors of the Southern Alps.

Two metasomatic events recorded by noble gas characteristics in the Finero mantle wedge: Extreme fractionation (He, Ar) and seawater penetration into the mantle deformation zone

Here, we report that the Finero phlogopite peridotite massif, southern Italy, a sliver of the former subcontinental lithospheric mantle (SCLM), experienced two stages of fluid/melt percolation, which had different origins and scales in the mantle wedge based on noble gas analysis and the deformation structure. In the field investigation, we found a mylonite zone several tens of meters wide in the massif, inferred to have been caused by deformation in the mantle. Systematic differences in noble gas compositions between samples from inside and outside the mylonite zone reflect two stages of fluid/melt infiltration. In the first event, the fluid/melt with a noble gas mixture of seawater, radiogenic, and mantle components penetrated the Finero SCLM. It likely occurred simultaneously with the famous metasomatism that crystallized phlogopite and amphibole throughout the massif. This first event produced a 4He/40Ar ratio much higher than those of the convecting mantle and crustal components, probably during metasomatism by hydrous melt derived from the slab in the mantle wedge. The second event was probably synchronous with the development of mylonitization and locally supplied seawater-rich components. The 40Ar/36Ar ratios tend to be atmospheric as mylonitization progresses. Our results provide two insights: (i) the first demonstration of the formation of extremely high He/Ar ratios during mantle metasomatism of natural samples and (ii) differences in the deformation structure affecting the noble gas composition and distribution in the SCLM. Regarding (i), the elemental fractionation can be attributed to two factors: one is the difference in the solubility between noble gas elements in particular minerals in the slab, and the other is the difference in the diffusion coefficients of He and Ar. Our results imply that the recycling of volatiles within subduction zones is significantly influenced by the thermal structures of slabs. This is because these thermal structures control the stability of hydrous minerals on the slabs, temperature conditions, and potentially the efficiency of diffusion in mantle wedges.

Archean Phlogopite Peridotite from Gneissic Enderbites of the Bug Granulite–Gneiss Terrane (Ukrainian Shield): Compositional Features and Possible Mechanisms of its Formationn

The paper reports mineral chemistry, whole-rock major and trace element composition, structure, and geological setting of the phlogopite peridotite lens within the Bug granulite–gneiss terrane of the Dniester–Bug Province, Ukrainian Shield. Geochemical features of the studied peridotites and minerals indicate a complex evolution of the lens. The early stage is marked by the crystallization of olivine + spinel (Al-chromite) cumulus from melt of supposably picritic composition, and further crystallization of clinopyroxene. The melt shows signs of hybridization by host gneissic enderbite. Orthopyroxene crystallized later and frequently replaces clinopyroxene. The phlogopite cystallization is likely related to the fluid activity and the growth of potassium potential in the fluid. Granulite-facies metamorphism accompanied by ductile deformations affected the mineral asseblage and chemical compositions of peridotites and orthopyroxenites. At the final stage, the rocks and minerals experienced retrograde metamorphism and local foliation of the rocks.

Archean Phlogopite Peridotite from Gneissic Enderbites of the Bug Granulite–Gneiss Terrane (Ukrainian Shield): Compositional Features and Possible Mechanisms of its Formation

Archean Phlogopite Peridotite from Gneissic Enderbites of the Bug Granulite–Gneiss Terrane (Ukrainian Shield): Compositional Features and Possible Mechanisms of its Formation

The metasomatism of sub-cratonic peridotites by a slab-derived fluid – Implications for cratons’ evolution

Metasomatized peridotites entrained by kimberlites are of great importance in understanding the geochemical changes occurring in the sub-continental lithospheric mantle (SCLM) during percolation and interaction of various fluids with the harzburgitic mantle. To simulate a scenario where slab-derived silicic fluid infiltrates the SCLM, high-pressure, high-temperature experiments were performed on a harzburgite + silicic fluid system at an 80:20 rock:fluid ratio (bulk ∼4.5 wt% H2O and ∼3 wt% CO2). Experiments were performed at 3–6 GPa and 900–1200 °C on a rocking multi-anvil apparatus, conditions corresponding to the deep SCLM. The compositions of the mineral assemblages were analyzed in order to evaluate the formation of the metasomatic rocks at these conditions.At 3–6 GPa between 900–1000 °C, phlogopite-K-richterite- peridotites (PKP) were formed, at 3–5 GPa and 1100 °C, phlogopite-peridotites (PP), and at 6 GPa between 1100–1200 °C, garnet-phlogopite peridotites (GPP) were formed. Similar mineral assemblages and mineral chemistry are found in metasomatic xenoliths from Kimberly, South Africa, and surrounding localities, registering similar P-T conditions. The results presented in this study, together with previous studies on various peridotite + H2O ± CO2 systems, clarify the relationship between bulk composition characteristics (e.g., K2O/H2O, peralkalinity) and the stability of different aluminum-bearing hydrous phases (e.g., phlogopite and amphibole). Fluxing harzburgite with silicic high-density fluids (HDFs) resulted in a peralkaline system, stabilizing both phlogopite and K-richterite. The stabilization of these Al-bearing hydrous phases led to the formation of a Cr-rich spinel, shifting the spinel to garnet transition to higher pressures. In addition, fluxing the harzburgite with silicic HDF led to the oxidation of the SCLM by one log unit, similar to the fO2 registered by natural metasomatic xenoliths.

Metasomatism in the Finero Phlogopite Peridotite: New insights from C and N concentrations and δ13C - δ11B signatures

The interconnection between element recycling and mantle metasomatism may provide powerful insights to unravel the mechanisms governing the transfer, partitioning and residence times of volatiles among the different Earth's reservoirs. In this study, we provide new constraints on the incorporation of C and N in the rock-forming minerals of the Finero Phlogopite Peridotite (FPP) from the Ivrea-Verbano Zone (Southern Alps, Italy), representing a natural laboratory to shed lights on the amounts of volatiles that may be fixed in a deeply metasomatized sub-continental lithospheric mantle (SCLM). Constraints on the relationships between metasomatic events and volatile element enrichment were given by in-situ LA-ICP-MS and micro-Raman investigations together with C (δ13C) and B (δ11B) isotopes.All rock-forming minerals (olivine, clinopyroxene, amphibole and phlogopite) show enrichments in C (270–690 ppm) and N (7–20 ppm) compared to depleted mantle values. The high volatile contents in clinopyroxene and in amphibole are associated with enrichment in incompatible elements (e.g., Li, B, Rb, Sr, Pb, Th, U) and high LaN/LuN ratios (up to 42.0) thus confirming that the metasomatic agent affecting the FPP derive form a hydrous melt with crustal affinity enriched in C and N. Micro-Raman investigations document the occurrence of graphite, carbonate and N2 inclusions, specifically in olivine and clinopyroxene, that are likely responsible for the high CN concentrations reported here. The δ13C signatures measured in mineral separates range from −12.6 to −22.7 ‰, suggesting an overprinting of the C mantle signature by a 12C-rich crustal metasomatic agent. High temperature decarbonation processes, simulated by heating the samples up to 900 °C, indicate that up to10 ppm of C can be residually stored in the SCLM, whereas N is completely released even at lower temperature. The LA-MC-ICP-MS in-situ δ11B signature of amphibole is −5.4 ± 4.1 ‰ (2σ, n = 18), whereas bulk δ11B of olivine and clinopyroxene is −5.51 ± 0.01 ‰ (2σ) and − 1.82 ± 0.06 ‰ (2σ), respectively. Modelling shows that residual slab melts plus variable amounts of 11B-rich serpentinite-derived fluids may account for these B isotope imprints. A subduction-related metasomatizing event during the FPP's evolution is thus proposed to explain the geochemical features documented in our study, in agreement with previous works. At global scale, our results suggest that the metasomatized SCLM might significantly contribute to the so-called hidden C reservoir invoked to match the global C mass balance calculations.

Sheared Peridotites from Kimberley (Kaapvaal Craton, RSA): Record of Multiple Metasomatic Events Accompanied with Deformation

Abstract Sheared peridotites from the Kaapvaal craton may be broadly divided into two types: (1) high T and refertilized and (2) low T and highly depleted, which equilibrated at conditions lying either above or along the Kaapvaal craton conductive geotherm, respectively. Here, we have studied 14 low-T sheared peridotites from Kimberley entrained by several Late Cretaceous (90 Ma) kimberlites in order to constrain the nature and timing of the deformation. The sample suite comprises nine garnet peridotites (GPs) with various amounts of clinopyroxene ± isolated spinel, three garnet-free phlogopite peridotites (PPs) with minor amounts of spinel, one garnet–spinel peridotite (GSP) and one dunite. The peridotites have intense deformation textures, ranging from porphyroclastic to fluidal mosaic. Olivine and orthopyroxene compositions (Mg# = 91–94) indicate varying degrees of depletion, similar to coarse-grained peridotites from the same localities. Pre-deformation conditions of the GPs are preserved in the cores of large (>100 μm–mm diameter) porphyroclasts and give a range in temperature of 930–1000°C at pressures of 4.0 ± 0.4 GPa. The GSP was equilibrated at 840°C and 3.1 GPa. Projected onto a 40-mW/m2 geothermal gradient, the PP samples yield temperatures of 850–870°C at 3.3–3.4 GPa. Trace element measurements by laser ablation inductively coupled plasma mass spectrometry and electron microprobe indicate that the ‘cold’ sheared peridotites were influenced by several metasomatic events, ranging from ‘old’ pre-deformation metasomatism to interactions shortly before or during deformation. The old pre-deformation metasomatism is recorded in garnet, clinopyroxene and orthopyroxene porphyroclasts and implies interactions with phlogopite–ilmenite–clinopyroxene- or muscovite–amphibole–rutile–ilmenite–diopside-related metasomatic agents, which also led to crystallization of phlogopite in the garnet-free peridotites. A ‘young’ metasomatic event caused an enrichment in Fe, Ti, Ca and Y (+heavy rare earth elements) and is evident in zoned orthopyroxene and clinopyroxene and phlogopite, the crystallization of new clinopyroxene porphyroclasts and compositional heterogeneities in garnet. This young event marks the beginning of extensive kimberlite-related metasomatism in the late Cretaceous beneath Kimberley. The metasomatism caused the deformation (triggered by a kimberlite pulse?), resulting in the recrystallization of fine-grained, mainly olivine, neoblasts (down to <10 μm). These record the metasomatic conditions at the time of deformation, revealing an increase in temperature up to 1200°C accompanied by an increase in Ti content up to 300 μg/g. Crystal preferred orientations of olivine neoblasts suggest the presence of elevated concentrations of water (B, C, E type) or the presence of a melt during the deformation (AG type). We suggest that these high water contents led to hydrolytic weakening of the cratonic lithosphere and prepared the pathways for subsequent kimberlite magmas to reach the surface. We propose that the deformation is a byproduct of extensive metasomatism, resulting in a metasomatism–deformation cycle. In times of extensive magmatism and metasomatism, fluids and melts flow along the pathways established by previous metasomatic agents, leading to further hydrolytic weakening of these mantle segments. Later, deformation was initiated by a new pulse of melt/fluid, with one of the later pulses eventually reaching the surface and transporting fragments of sheared and undeformed peridotites with it. The remaining peridotite anneals after the period of extensive metasomatism and recrystallizes to become coarse-grained peridotite again.

Solidus and melting of carbonated phlogopite peridotite at 3–6.5 GPa: Implications for mantle metasomatism

It is well known that water significantly lowers mantle solidi. But it turns out this paradigm is not always true. Here, we studied the interaction of K-rich carbonate melts with the natural garnet lherzolite from the Udachnaya kimberlite (Russia) in the presence of water at 3.0–6.5 GPa, corresponding to depths of 100–200 km. We found that at ≤ 1100 °C, the metasomatic interaction consumes garnet, orthopyroxene, and melt to produce phlogopite ± K-richterite + magnesite ± dolomite. Besides, primary clinopyroxene is replaced by one with a lower amount of jadeite component. Thus, the addition of water to the K-rich carbonate melt, infiltrating the subcontinental lithospheric mantle, should yield its partial or complete disappearance accompanied by phlogopitization and carbonation.The studied systems have H2O/K2O = 2, like that in phlogopite, and therefore correspond to carbonated phlogopite peridotite under fluid-absent conditions. At 4.0–6.5 GPa, the solidus of carbonated phlogopite peridotite is controlled by the following reaction: phlogopite + clinopyroxene + magnesite = garnet + orthopyroxene + olivine + hydrous K-carbonatite melt, which is bracketed between 1100 and 1200 °C. At 3 GPa, the solidus temperature decreases to about 1050 °C presumably owing to the Ca-Mg exchange reaction, clinopyroxene + magnesite = orthopyroxene + dolomite, which stabilizes dolomite reacting with phlogopite at a lower temperature than magnesite.Our results suggest that the phlogopite- and carbonate-rich metasomatic vein networks, weakening rigid lithosphere and promoting continental rifting, could be formed as a result of infiltration of hydrous K-carbonatite melt at the base of subcontinental lithospheric mantle. Stretching and thinning of the cratonic lithosphere make geotherms warmer and shifts their intersections with the solidus of carbonated phlogopite peridotite to shallower depths. Consequently, the successive erosion of the continental lithosphere and ascent of the lithosphere-asthenosphere boundary during continental rifting should be accompanied by remelting of phlogopite-carbonate metasomes, upward percolation of K-rich melt, and its solidification at the front of the magmatic-metasomatic mantle system.

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.

Siderophile and chalcophile elements in spinels, sulphides and native Ni in strongly metasomatised xenoliths from the Bultfontein kimberlite (South Africa)

The metasomatised continental mantle may play a key role in the generation of some ore deposits, in particular mineral systems enriched in platinum-group elements (PGE) and Au. The cratonic lithosphere is the longest-lived potential source for these elements, but the processes that facilitate their pre-concentration in the mantle and their later remobilisation to the crust are not yet well-established. Here, we report new results on the petrography, major-element, and siderophile- and chalcophile-element composition of native Ni, base metal sulphides (BMS), and spinels in a suite of well-characterised, highly metasomatised and weakly serpentinised peridotite xenoliths from the Bultfontein kimberlite in the Kaapvaal Craton, and integrate these data with published analyses. Pentlandite in polymict breccias (failed kimberlite intrusions at mantle depth) has lower trace-element contents (e.g., median total PGE 0.72 ppm) than pentlandite in phlogopite peridotites and Mica-Amphibole-Rutile-Ilmenite-Diopside (MARID) rocks (median 1.6 ppm). Spinel is an insignificant host for all elements except Zn, and BMS and native Ni account for typically <25% of the bulk-rock PGE and Au. High bulk-rock Te/S suggest a role for PGE-bearing tellurides, which, along with other compounds of metasomatic origin, may host the missing As, Ag, Cd, Sb, Te and, in part, Bi that are unaccounted for by the main assemblage.The close spatial relationship between BMS and metasomatic minerals (e.g., phlogopite, ilmenite) indicates that the lithospheric mantle beneath Bultfontein was resulphidised by metasomatism after initial melt depletion during stabilisation of the cratonic lithosphere. Newly-formed BMS are markedly PGE-poor, as total PGE contents are <4.2 ppm in pentlandite from seven samples, compared to >26 ppm in BMS in other peridotite xenoliths from the Kaapvaal craton. This represents a strong dilution of the original PGE abundances at the mineral scale, perhaps starting from precursor PGE alloy and small volumes of residual BMS. The latter may have been the precursor to native Ni, which occurs in an unusual Ni-enriched zone in a harzburgite and displays strongly variable, but overall high PGE abundances (up to 81 ppm). In strongly metasomatised peridotites, Au is enriched relative to Pd, and was probably added along with S. A combination of net introduction of S, Au +/− PGE from the asthenosphere and intra-lithospheric redistribution, in part sourced from subducted materials, during metasomatic events may have led to sulphide precipitation at ~80–120 km beneath Bultfontein. This process locally enhanced the metallogenic fertility of this lithospheric reservoir. Further mobilisation of the metal budget stored in these S-rich domains and upwards transport into the crust may require interaction with sulphide-undersaturated melts that can dissolve sulphides along with the metals they store.

The Variscan subduction inheritance in the Southern Alps Sub-Continental Lithospheric Mantle: Clues from the Middle Triassic shoshonitic magmatism of the Dolomites (NE Italy)

Although often speculated, the link between the Middle Triassic shoshonitic magmatism at the NE margin of the Adria plate and the subduction-related metasomatism of the Southern Alps Sub-Continental Lithospheric Mantle (SCLM) has never been constrained. In this paper, a detailed geochemical and petrological characterization of the lavas, dykes and ultramafic cumulates belonging to the shoshonitic magmatic event that shaped the Dolomites (Southern Alps) was used to model the composition and evolution of the underlying SCLM in the time comprised between the Variscan subduction and the opening of the Alpine Tethys. Geochemical models and numerical simulations enabled us to define that 5–7% partial melting of an amphibole + phlogopite-bearing spinel lherzolite, similar to the Finero phlogopite peridotite, can account for the composition of the primitive Mid-Triassic SiO2-saturated to -undersaturated melts with shoshonitic affinity (87Sr/86Sri = 0.7032–0.7058; 143Nd/144Ndi = 0.51219–0.51235; Mg # ~ 70; ~1.1 wt% H2O). By taking into account the H2O content documented in mineral phases from the Finero phlogopite peridotite, it is suggested that the Mid-Triassic SCLM source was able to preserve a significant enrichment and volatile content (600–800 ppm H2O) for more than 50 Ma, i.e. since the slab-related metasomatism connected to the Variscan subduction. The partial melting of a Finero-like SCLM represents the exhaustion of the subduction-related signature in the Southern Alps lithosphere that predated the Late Triassic-Early Jurassic asthenospheric upwelling related to the opening of the Alpine Tethys.

Evidence of subduction-related components in sapphirine-bearing gabbroic dykes (Finero phlogopite–peridotite): Insights into the source of the Triassic–Jurassic magmatism at the Europe–Africa boundary

A gabbroic dyke swarm containing magmatic sapphirine occurs in the Finero phlogopite–peridotite (FPP), one of the major mantle massifs in the Ivrea–Verbano Zone (IVZ; western Southern Alps). Sapphirine is part of a particular mineral assemblage, including plagioclase, titanian pargasite, titanian phlogopite, and Cl-rich apatite; the latter mineral hosts calcite inclusions. The dykes cut the mantle foliation at a high angle, are bounded by orthopyroxenite layers, and show symmetric internal banding, represented by two outer hornblendite selvages and an inner leucogabbro band. The sapphirine occurs in up to 3 cm-thick irregular patches in both hornblendite salvages, along with Al-rich amphibole and green spinel.We present major and trace elements of minerals and bulk rock, as well as mineral O, Sr, and Nd isotopic compositions of dykes and the host peridotite from two different outcrops in the FPP area.Our data show that early melt migration developed through porous flow within cm-thick channels and was characterised by orthopyroxene dissolution. Following progressive percolation and reaction, the melt became silica saturated with segregation of orthopyroxenite in the centres of the channels. The banded internal structure of the dykes was caused by three different evolutionary stages, involving opening and enlargement of the conduits. The sapphirine and green spinel segregation took place at T > 1000 °C, in the presence of melt with transient composition, which interstitially migrated and reacted with the cumulus minerals to form the hornblendite layers. The mineral chemistry of the newly-formed amphiboles indicates that the sapphirine parental melt was Al-rich, depleted to strongly depleted in Hf, Zr, Nb, Ta, Ti, Sc, V, and middle and heavy rare earth elements, and characterised by a positive Eu anomaly and (Zr/Hf)N < 1. These data suggest a parental melt with a significant amount of normative plagioclase. However, the studied veins do not show evidence of plagioclase assimilation, and we argue that this process could have occurred in magmatic bodies that are not outcropping today to the surface or in the melt source.The δ18O values of vein amphiboles and plagioclases vary from 6.9 to 8.6‰ SMOW, which is well above the mantle range, even when considering fractionation upon cooling. Given that orthopyroxene from the wall has “normal” mantle δ18O values (5.8‰), reaction with the host metasomatised peridotite cannot be responsible for the heavy δ18O signature, and the latter must have been imparted by crustal components deeper in the mantle.Our petrographic and geochemical evidence demonstrates that the northern IVZ records an extremely prolonged release, from the Variscan orogenic cycle to the Mesozoic exhumation, of K-H2O-rich mantle-derived melts, mixed with subduction-related components. This finding provides valuable insights into the Triassic–Jurassic magmatism and the geodynamic environment at the Europe–Africa boundary.

Lamproitoides of the Magnitogorsk megazone (South Urals): petrologic-geochemical features and geodynamic conditions of their formation

The material composition and petrographic features of lamproitoide dikes of the Magnitogorsk Megazone of the Southern Urals are described. Their younger age (197–240 million years) in comparison with lamprophyres developed in the same megazone (304–308 million years) is established. It is shown that these formations are a single petrographic series (from alkaline-ultrabasic to orendites). The mineralogical features of rocks are characterized and the conclusion is made about their proximity to lamproites by mineralogical criteria, but there are also significant differences from the latter. The petrogeochemical features of rocks are studied. According to their chemistry, they are divided into three groups: ultrabasic (rare), basic (the absolute majority of the analyzed rocks) and medium (orendites). The average compositions of the selected groupsare calculated, the features of their normative composition are described and the geochemical characteristic is given. The conclusion about the similarity of the studied rocks with postcollisional igneous formations is made. A possible mechanism of the petrogenesis of lamproitoides is reconstructed. The most probable mantle source for the primary melt of lamproitoides is established as phlogopite peridotite, which underwent partial melting in the presence of a substantially aqueous fluid with a sharply subordinate role of carbon dioxide in its composition. The crystallization of the melt occurred according to the scheme corresponding to the experimentally established phase relationships in the presence of an aqueous fluid. The geodynamic position of dikes was clarified and the described rocks were compared with “classical” lamproites of different regions. Their similarity to the lamproites of Aldan and Spain, the formation of which is associated with ancient subduction zones, is shown. Taking into account the geological situation of lamproitoides, we propose distinguishing them into a “postcollisional” subtype. The conducted research allows us to confirm the earlier conclusion that the lamproitoides of the Magnitogorsk Megazone are not the primary sources of Ural diamonds.

Laurite and zircon from the Finero chromitites (Italy): New insights into evolution of the subcontinental mantle

Chromitites enclosed within metasomatised Finero phlogopite peridotite (FPP) contain accessory platinum-group minerals, base metal (BM) sulfides, baddeleyite, zircon, zirconolite, uraninite and thorianite. To provide new insights into mantle-crustal interaction in the Finero lithosphere this study evaluates (1) the mineral chemistry and Os-isotope composition of laurite, (2) the crystal morphology, internal structure, in-situ U-Pb, trace-element and Hf-isotope data of zircon from two chromitite localities at Alpe Polunia and Rio Creves. The osmium isotope results reveal a resticted range of ‘unradiogenic’ 187Os/188Os values for laurite at Alpe Polunia (0.1247–0.1251, mean 0.1249±0.0001). Re-Os model ages (TRD) of laurite reflect an Early Paleozoic partial melting event (ca 450Ma or older), presumably before the Variscan orogeny. The Os isotopic composition of laurite/chromitite probably preserves their mantle signature and was not affected by later metasomatic processes. U-Pb and Hf-isotope data show that the Finero chromitites have distinct zircon populations with peculiar morphology, internal cathodoluminescence textures, trace-element composition and an overall U-Pb age span from ∼310Ma to 190Ma. Three age peaks at Rio Creves (220±4Ma, 234.2±4.5Ma and 277.5±3.2Ma) are consistent with a prolonged formation and multistage zircon growth, in contrast to the common assumption of a single metasomatic event during chromitite formation. The trace-element signatures of zircons are comparable with those of mantle-derived zircons from alkaline ultramafic rocks, supporting the carbonatitic nature of the metasomatism. Hf-isotope compositions of the Finero zircons, with εHf values ranging mainly from −3 to +1, are consistent with crustal input during metasomatism and could indicate that the parental melts/fluids were derived from a relatively old source; the minimum estimates for Hf model ages are 0.8–1.0Ga. Our findings imply that mantle rocks and metasomatic events at Finero have a far more complex geological history than is commonly assumed.

Excess water generation during reaction-inducing intrusion of granitic melts into ultramafic rocks at crustal P–T conditions in the Sør Rondane Mountains of East Antarctica

Arc magmas are one of the main sources of aqueous geofluids in the crust, and the movement of fluids above magma chambers has been geophysically imaged. Here, we constrain the water budget (i.e., supply, consumption and release of H2O) in these areas above magma chamber by examining the hydration caused by crust–melt reactions in the Sør Rondane Mountains of East Antarctica. The study area contains a phlogopite–pargasite–peridotite unit that has been intruded by numerous granitic dikes, creating hydration reaction zones at the dike–peridotite boundary. These reactions occurred at 0.5GPa and 700°C, corresponding to middle crustal conditions, and generated a series of reaction zones with distance from the granitic dikes as follows: (i) granitic dike, (ii) pargasite–actinolite zone, (iii) tremolite–phlogopite zone, (iv) anthophyllite–phlogopite zone, (v) phlogopite–olivine–orthopyroxene zone, and (vi) unaltered pargasite–phlogopite peridotite. The presence of amphiboles with a preferred orientation perpendicular to the dike margins and an absence of Cr-rich magnetite indicate that the pargasite–actinolite zone [zone (ii)] grew from the dike margins as a result of the dike reacting with the host rock, with an initial melt/rock boundary located between zones (ii) and (iii). The H2O contents of reaction zones (ii)–(v) are higher than the content in the hosting pargasite–phlogopite peridotite, suggesting that the intrusion of the dike was associated with hydration reactions. Geochemical analysis along a profile through the reaction zones indicates Mg and Fe depletion, and Si enrichment in zones (iii)–(iv), and Ca depletion and K enrichment in zones (iv)–(v) relative to the hosting pargasite–phlogopite peridotite. In contrast, zone (ii) is characterized by Ca, Fe, and Mg enrichments relative to the granitic dike. These observations suggest that the reaction zone sequence was formed by the elemental transfer between granitic dike and parasite–phlogopite peridotite: Ca, Fe, and Mg were leached from the host peridotite [zones (iii)–(v)]; K, Si, and H2O were mobilized from the granitic melt; and Ca, Mg, and Fe were deposited in the overgrowth zone [(ii)]. The width of the overgrowth zone correlates with the width of the leached zone [(iii)–(v)] but does not correlate with the width of the dike. These chemical profiles and geometry of the reaction zones suggest that hydration was not rate-limited by the supply of water from granitic melt, but instead was limited by the chemical transfer of elements such as K and Ca. It is likely that 3.7wt% of the >5.6–5.0wt% H2O content of the granitic melt was consumed by hydration reactions, with the other >1.9–1.3wt% removed from the system. These observations suggest that at least one-fourth of the water in granitic melts can be liberated as excess water during the intrusion and solidification of the granitic melts, and subsequently transported to the overriding upper crust by the upward infiltration.

Origin and age of zircon-bearing chromitite layers from the Finero phlogopite peridotite (Ivrea–Verbano Zone, Western Alps) and geodynamic consequences

An investigation has been performed on three chromitite layers segregated in dunite bodies of the Phlogopite Peridotite mantle unit in the Finero complex (FPP, Ivrea–Verbano Zone, Southern Alps) aimed at providing new constraints to their origin and evolution.Field relationships, the sub-chondritic Hf isotopic composition of the zircons (εHf(188) as low as −5.4), the heavy O isotopic composition of zircons and pyroxenes (δ18O up to 6.9‰), the strict similarity of the trace element composition between the clinopyroxenes and amphiboles from the chromitites and those from the phlogopite harzburgites and pyroxenites forming the typical FPP association, as well as the REE composition of zircons, which approaches equilibrium with the associate clinopyroxene, suggest that the studied chromitites were segregated from melts, highly contaminated from continental crust, during the pervasive cycle of metasomatism recorded by the FPP. An LA-ICP-HRMS survey of chromitite zircon grains has provided Early Jurassic U–Pb ages mostly between 199±3Ma and 178±2Ma, with a pronounced peak at 187Ma. Relevant exceptions are inherited domains of two grains giving Triassic ages of 242±7Ma and 229±7Ma, and a third homogeneous zircon giving 208±3Ma. Our geochronological data and those reported in the literature show that the FPP chromitites have zircon populations with different internal CL textures, but the same sub-chondritic Hf isotopic composition, which define an overall U–Pb age span from ~290Ma to 180. The segregation of the chromitite layers and the main pervasive metasomatism likely occurred in the Early Permian (in a post-collisional, transtensional setting) or before (possibly, in a subduction-related setting). The rejuvenation of the zircon ages was accompanied by a progressive disappearance of the internal zoning, interpreted as the result of a prolonged residence at mantle depths with progressive re-equilibration of the U–Pb system due to thermal perturbations. The age peak at ~187Ma is argued to constrain the timing of FPP exhumation at shallower, crustal levels. This process was characterised by an important reheating event, possibly due to lithospheric hyperextension. The evolution of the FPP appears completely different than that of mantle bodies of the central IVZ (i.e., the Val Sesia-type bodies), which were emplaced within the continental crust, as part of accretionary prisms, at or before the end of the Variscan orogeny.

Geochemical modeling and Nd–Sr data links nephelinite–phonolite successions and xenoliths of Trindade Island (South Atlantic Ocean, Brazil)

Abstract The Trindade Island corresponds to the eastern end of the submarine E–W Vitoria–Trindade Chain as part of the Trindade plume track on the South American plate. It is a suitable site for petrogenetic investigations, since a series of unusual rock compositions crops out within nephelinite–phonolite successions and scarce xenoliths. Software-based geochemical modeling and Nd–Sr analyses, coupled with field work, petrography and literature data were used to evaluate and model the petrogenetic processes that led to the formation of variable rock compositions. Results show the formation of: (1) nephelinites at 1490 °C and 3 GPa, from 0.1 to 7% of partial melting of an enriched garnet–lherzolitic source or from 1 to 5% partial melting of TiO2-rich garnet–phogopite lherzolite (up to 2.5 wt.% of CO2). Nephelinites represent low viscosity (18 Pa s), high-temperature (1170 °C) and high-density (2.79 g/cm3) lavas; (2) pyroxenite, jacupirangite and melteigite cumulates at 900 °C and 0.5 GPa, after 46%, 49% and 56% fractional crystallization of nephelinites, and leaving phonotephrites as residual liquids; (3) monchiquites, from fractional melting of enriched, CO2-bearing garnet–amphibole–phlogopite peridotites or from hydrated nephelinite magmas. They may evolved to sannaite via fractional crystallization or may generate phonolites through extraction of pyroxene and amphibole cumulates at 1080 to 970 °C and 1.2 to 0.9 GPa; (4) sannaites may represent ‘hydrous’ phonotephrites, and can evolve to phonolites and phonolitic foidites via fractional crystallization; and (5) bebedourite cumulates at 850 °C and 0.38 to 0.4 GPa, from phonotephritic or sannaitic magmas after 20% and 26% of fractional crystallization, leaving phonolites and phonolite–foidites as residual liquids. Phonolites and phonolitic foidites, the most evolved rocks of the nephelinite–phonolite association, represent higher-viscosity (1.1 × 106 Pa s), lower-temperature (950 °C), and lower-density (2.48 g/cm3) lavas. Both the geochemical modeling and the Nd–Sr isotopic data show that the studied rocks are genetically associated. They evolved via fractional crystallization after partial melting of a very homogeneous reservoir with inherited asthenospheric heterogeneities. Our work refines and validates some of the assumptions of previous contributions, and it can be used as a basis for further petrogenetic investigations on oceanic islands.

Significance of the whole rock Re–Os ages in cryptically and modally metasomatised cratonic peridotites: Constraints from HSE–Se–Te systematics

The Re–Os isotopic system is the geochronometer of choice to constrain the timing of lithospheric mantle root formation and reconstruct the evolution of Earth’s dynamics from the “mantle” perspective. In order to constrain the effects of metasomatic processes on the Re–Os isotopic system, eleven peridotites from the Letlhakane kimberlite pipe were investigated for whole rock major and trace elements, highly siderophile elements (HSE), Se, Te and 187Os/188Os signatures. These spinel peridotites (SP), garnet peridotites (GP), garnet-phlogopite peridotites (GPP) and phlogopite peridotites (PP) experienced cryptic metasomatism and the GP–GPP–PP additionally constitute a sequence of increasing modal metasomatism.The cryptically metasomatised SP appear devoid of base metal sulphides (BMS) and show suprachondritic Se/Te ratios (15–40) and extremely Pd- and Pt-depleted HSE patterns. These features are characteristic of high-degree partial melting residues. Their 187Os/188Os signatures are thus considered to be inherited from the partial melting event. This implies a Neoarchean (2.5–2.8Ga, TRD eruption) stabilisation of the Letlhakane mantle root and supports the Letlhakane mantle root being a westerly extension of the Zimbabwe cratonic root.The modally metasomatised peridotites contain BMS whose abundance significantly increases from the GPP to the GP and PP. The BMS-poor GPP are only slightly richer in Pt and Pd than the BMS-free SP but have similarly high Se/Te ratios. The BMS-rich GP and PP exhibit significant enrichments in Pt, Pd, Se, Te resulting in HSE–Se–Te signatures similar to that of the Primitive Upper Mantle (PUM). Addition of 0.001–0.05wt.% metasomatic BMS±PGM (platinum group minerals, i.e., Pt-tellurides) to highly refractory residues, such as the Letlhakane SP, reproduce well the HSE–Se–Te systematics observed in the BMS-poor and BMS-rich modally metasomatised peridotites. In the GPP, the negligible addition of metasomatic BMS±PGM did not disturb the 187Os/188Os signatures, indicating TRD eruption ages identical with those of the SP. By contrast, the PP and GP, which suffered significant BMS±PGM addition, have 187Os/188Os considerably shifted toward more radiogenic values. As a result, unrealistically young TRD eruption ages are obtained that cannot be used to constrain the time frame of the stabilisation of the cratonic roots and the mechanisms of craton formation.The Se–Te and incompatible HSE (i.e., Pt, Pd) are powerful geochemical tools to assess the robustness of the Re–Os isotopic system in mantle peridotites. Specifically, Se/Te and Pd/Ir ratios provide complementary insights into the complex metasomatic history of mantle peridotites. The Se/Te ratio is very sensitive to Os-free PGM metasomatism producing highly variable Se/Te >15 at low, relatively constant Pd/Ir< 0.4 but does not affect the 187Os/188Os systematics. This contrasts with BMS metasomatism, where Pd/Ir becomes more variable (>0.4), at constant but low Se/Te <10 and which is accompanied by modification of the Os isotopic composition of the host peridotite. As they are sensitive indicators of BMS±PGM metasomatism, HSE, Se and Te should be systematically considered when investigating the timing of stabilisation of lithospheric mantle and its petrogenetic history. This would allow a more robust assessment of the Re–Os ages obtained and provide firmer constraints on the evolution and formation of Archean cratons and early Earth dynamics.

Experimental melting of phlogopite-bearing mantle at 1 GPa: Implications for potassic magmatism

We have experimentally investigated the fluid-absent melting of a phlogopite peridotite at 1.0 GPa (1000–1300 °C) to understand the source of K2O- and SiO2-rich magmas that occur in continental, post-collisional and island arc settings. Using a new extraction technique specially developed for hydrous conditions combined with iterative sandwich experiments, we have determined the composition of low- to high-degree melts (Φ=1.4 to 24.2 wt.%) of metasomatized lherzolite and harzburgite sources. Due to small amounts of adsorbed water in the starting material, amphibole crystallized at the lowest investigated temperatures. Amphibole breaks down at 1050–1075 °C, while phlogopite-breakdown occurs at 1150–1200 °C. This last temperature is higher than the previously determined in a mantle assemblage, due to the presence of stabilizing F and Ti. Phlogopite–lherzolite melts incongruently according to the continuous reaction: 0.49 phlogopite + 0.56 orthopyroxene + 0.47 clinopyroxene + 0.05 spinel = 0.58 olivine + 1.00 melt. In the phlogopite–harzburgite, the reaction is: 0.70 phlogopite + 1.24 orthopyroxene + 0.05 spinel = 0.99 olivine + 1.00 melt. The K2O content of water-undersaturated melts in equilibrium with residual phlogopite is buffered, depending on the source fertility: from ∼3.9 wt.% in lherzolite to ∼6.7 wt.% in harzburgite. Primary melts are silica-saturated and evolve from trachyte to basaltic andesite (63.5–52.1 wt.% SiO2) with increasing temperature. Calculations indicate that such silica-rich melts can readily be extracted from their mantle source, due to their low viscosity. Our results confirm that potassic, silica-rich magmas described worldwide in post-collisional settings are generated by melting of a metasomatized phlogopite-bearing mantle in the spinel stability field.

Geochemistry of apatite-rich layers in the Finero phlogopite–peridotite massif (Italian Western Alps) and ion microprobe dating of apatite

Highly metasomatized apatite-rich peridotite layers (AP-layer) occur in the Finero phlogopite–peridotite massif (western Italian Alps). The AP-layer and the host peridotite are characterized by higher concentration of REE (total REE > 80ppm), especially LILE, and lower HFSE, than those of the other rocks in the Finero massif. The AP-layers and their host peridotites have bulk silicate Earth-like Sr and Nd-isotope compositions except for one sample containing rare carbonate aggregates. The carbonate aggregates occur in a late serpentine-talc veinlet and show relatively high d 13C and d 18O, high 87Sr/ 86Sr and lower ɛNd than the other samples. These results, combined with the previous data, indicate that the carbonates formed late at relatively low temperatures. The petrochemical results along with the literature suggest that the AP-layer was locally formed from a metasomatic agent geochemically distinct from that formerly producing phlogopite-bearing harzburgite widely distributed throughout the massif. We propose that the metasomatic agent for the AP-layer was an orthopyroxene-saturated, CO 2-bearing hydrous fluid/melt that evolved through the crystallization of metasomatic minerals upon reactions with peridotites. Sensitive high-resolution ion microprobe (SHRIMP) analyses of apatite grains yielded a Tera-Wasserburg concordia three-dimensional isochron age of 215 ± 35Ma in the 238U/ 206Pb– 207Pb/ 206Pb– 204Pb/ 207Pb diagram. The age is similar to intrusions in the Finero area, suggesting that the carbonate- and apatite-metasomatism was synchronous with Triassic magmatic activity.