Fluid induced melting in mantle xenoliths and some implications for the continental lithospheric mantle from the Minusinsk Region (Khakasia, southern Siberia)

Eleven representative xenoliths from the Minnsinsk Region, southern Russia were studied in order to highlight the characteristic features of the subcontinental lithospheric mantle beneath the region. Type-I xenoliths show that the lithosphere underwent various degree of depletion overprinted by enrichment processes leading to LREE-enriched pyroxenes. Estimated equilibrium temperature for the xenoliths is in the range of 960-1050°C. Type-II xenoliths are the result of crystallization from a possibly basaltic melt close to the crust-mantle boundary. Three xenoliths in the Type-I series show evidence of incipient melting such as spongy rims of pyroxenes and interstitial glass. The spongy rim of clinopyroxene consist of clinopyroxene and glass with modal proportion of approximately 82 and 18%, respectively. Orthopyroxene rim contains olivine (65%) and glass (35%) with subordinate amounts of clinopyroxene (<5%). Glass within the spongy rim exhibits a clear geochemical affinity to interstitial glass as both have similarly high Al2O3, SiO2 and alkali contents. The interstitial glass and the spongy rims (minerals + glass) display light rare earth (LRE) element and large ion lithophile (LIL) element enriched character. This indicates that incipient melting of pyroxenes occurred in an open system and was likely triggered by the influx of a Na alkali silicate melt/fluid. The interstitial glass represents the residual melt after interaction with the pyroxenes. The formation of this Na-rich silicate melt may represent an earlier stage of the mantle magmatic event that produced the host basalt

Microstructural evolution during thermal annealing of ice-I_h

We studied the evolution of the microstructure of ice-Ih during static recrystallization by stepwise annealing experiments. We alternated thermal annealing and electron backscatter diffraction (EBSD) analyses on polycrystalline columnar ice pre-deformed in uniaxial compression at temperature of −7 °C to macroscopic strains of 3.0–5.2. Annealing experiments were carried out at −5 °C and −2 °C up to a maximum of 3.25 days, typically in 5–6 steps. EBSD crystal orientation maps obtained after each annealing step permit the description of microstructural changes. Decrease in average intragranular misorientation at the sample scale and modification of the misorientation across subgrain boundaries provide evidence for recovery from the earliest stages of annealing. This initial evolution is similar for all studied samples irrespective of their initial strain or annealing temperature. After an incubation period ≥1.5 h, recovery is accompanied by recrystallization (nucleation and grain boundary migration). Grain growth proceeds at the expense of domains with high intragranular misorientations, consuming first the most misorientated parts of primary grains. Grain growth kinetics fits the parabolic growth law with grain growth exponents in the range of 2.4–4.0. Deformation-induced tilt boundaries and kink bands may slow down grain boundary migration. They are stable features during early stages of static recrystallization, only erased by normal growth, which starts after >24 h of annealing

Genesis of ultra-high pressure garnet pyroxenites in orogenic peridotites and its bearing on the compositional heterogeneity of the Earth's mantle

International audienceWe present an integrated geochemical study of ultra-high pressure (UHP) garnet pyroxenites from the Ronda and Beni Bousera peridotite massifs (Betic-Rif Belt, westernmost Mediterranean). Based on their Sr-Nd-Pb-Hf isotopic systematics, we classify UHP garnet pyroxenites into three groups: Group A pyroxenites (Al2O3: 15–17.5 wt.%) have low initial 87Sr/86Sr, relatively high εNd, εHf and 206Pb/204Pb ratios, and variable 207Pb/204Pb and 208Pb/204Pb. Group B pyroxenites (Al2O3 < 14 wt.%) are characterized by high initial 87Sr/86Sr and relatively low εNd, εHf and 206Pb/204Pb ratios. Group C pyroxenites (Al2O3 ∼ 15 wt.%) have depleted radiogenic signatures with relatively low initial 87Sr/86Sr and 206Pb/204Pb, high εNd and εHf, and their 207Pb/204Pb and 208Pb/204Pb ratios are similar to those of Group B pyroxenites.The major and trace element and isotopic compositions of UHP garnet pyroxenites support their derivation from ancient (1.5–3.5 Ga) oceanic crust recycled into the mantle and intimately stirred with peridotites by convection. However, the genesis of these pyroxenites requires also the involvement of recycled continental lower crust with an isotopic composition akin to the lower crustal section of the lithosphere where these UHP garnet pyroxenites now reside in. These oceanic and continental crustal components were stirred in different proportions in the convective mantle, originating pyroxenites with a more marked geochemical imprint of either oceanic (Group A) or continental lower crust (Group B), or hybrid compositions (Group C). The pyroxenite protoliths likely underwent several melting events, one of them related to the formation of the subcontinental lithospheric mantle and continental crust, generating restitic UHP garnet pyroxenites now preserved in the Ronda and Beni Bousera orogenic peridotites. The extent of melting was mostly controlled by the bulk Mg-number (Mg#) of the pyroxenite protoliths, where protoliths with low Mg# experienced higher degrees of partial melting than sources with higher Mg#. Positive Eu and Sr anomalies in bulk rocks, indicative of their origin from cumulitic crustal gabbros, are preserved mostly in high Mg# pyroxenites due to their higher melting temperatures and consequent lower partial melting degrees.The results of this study show that the genesis of UHP garnet pyroxenites in orogenic peridotites requires a new recipe for the marble cake mantle hypothesis, combining significant recycling and stirring of both oceanic and continental lower crust in the Earth’s mantle. Furthermore, this study establishes a firm connection between the isotopic signatures of UHP pyroxenite heterogeneities in the mantle and the continental lower crust

Geodynamic implications of flattened tabular equigranular textured peridotites from the Bakony-Balaton Highland Volcanic Field (Western Hungary)

Peridotite xenoliths showing unusual tabular equigranular textures (addressed as flattened tabular equigranular) were found in Neogene alkali basalts from the Bakony-Balaton Highland Volcanic Field (Western Hungary), Carpathian-Pannonian Region. The olivines have a characteristic crystallographic preferred orientation (CPO) with [0 1 0]-axes perpendicular to the foliation and the [1 0 0]- and [0 0 1]-axes forming a continuous girdle in the foliation plane. Contrarily, the CPO pattern of orthopyroxene is much more scattered, although a single maximum can be observed in [0 0 1] axes subparallel to the plane of foliation. In case of olivine, the activation of (0 1 0)[1 0 0] and also probably (0 1 0)[0 0 1] is suggested. The deformation micro-mechanisms of orthopyroxenes are suggested to be a combination of intracrystalline glide on the (1 0 0)[0 0 1] system and some kind of other mechanism resulting in quite scattered patterns. We suggest that the unusual orientation patterns of olivines and orthopyroxenes are the result of the complex tectonic evolution of the region. The flattened tabular equigranular xenoliths represent a structural domain within the subcontinental lithospheric mantle beneath the volcanic field with particular seismic characteristics. The occurrence of flattened domains in the upper mantle may considerably influence the percolation and residence time of the mantle melts and fluids, which could promote or prevent melt/wall-rock interaction

Mantle refertilization by melts of crustal-derived garnet pyroxenite: Evidence from the Ronda peridotite massif, southern Spain

International audienceGeochemical studies of primitive basalts have documented the presence of crustal-derived garnet pyroxenite in their mantle sources. The processes whereby melts with the signature of garnet pyroxenite are produced in the mantle are, however, poorly understood and somewhat controversial. Here we investigate a natural example of the interaction between melts of garnet pyroxenite derived from recycled plagioclase-rich crust and surrounding mantle in the Ronda peridotite massif. Melting of garnet pyroxenite at ∼1.5 GPa generated spinel websterite residues with MREE/HREE fractionation and preserved the positive Eu anomaly of their garnet pyroxenite precursor in whole-rock and clinopyroxene. Reaction of melts from garnet pyroxenite with depleted surrounding peridotite generated secondary fertile spinel lherzolite. These secondary lherzolites differ from common spinel lherzolite from Ronda and elsewhere by their lower-Mg# in clinopyroxene, orthopyroxene and olivine, lower-Cr# in spinel and higher whole-rock Al2O3, CaO, Sm/Yb and FeO⁎ at a given SiO2. Remarkably, secondary spinel lherzolite shows the geochemical signature of ghost plagioclase in the form of positive Eu and Sr anomalies in whole-rock and clinopyroxene, reflecting the transfer of a low-pressure crustal imprint from recycled pyroxenite to hybridized peridotite. Garnet pyroxenite melting and melt-peridotite interaction, as shown in the Ronda massif, may explain how the signature of subducted or delaminated crust is transferred to the mantle and how a garnet pyroxenite component is introduced into the source region of basalts. The efficiency of these processes in conveying the geochemical imprint of crustal-derived garnet pyroxenite to extruded lavas depends on the reactivity of pyroxenite melt with peridotite and the mantle permeability, which may be controlled by prior refertilization reactions similar to those documented in the Ronda massif. Highly fertile heterogeneities produced by pyroxenite-peridotite interaction, such as secondary spinel lherzolite in Ronda, may nucleate magmatic channels that remain chemically isolated from the ambient mantle and act as preferential pathways for melts with the signature of recycled crust

Stress tensors and stress orientation obtained from fault population analysis of the Llevant ranges in Mallorca (Spain)

ProjectPID2019- 808 107138RB-I00/SRA (State Research Agency /10.13039/501100011033) “Junta de 809 Andalucía” Project P18-RT-36332Stress inversion analysis was carried out to obtain the paleo-stress states in 33 stations, from fault orientation and kinematics field measurements, using a Search Grid method (Galindo-Zaldívar & González-Lodeiro, 1988). These data are used to characterize the stress evolution of Llevant Ranges (Mallorca, Spain)

Metallogenic fingerprint of a fertile mantle source underlying an ore-productive volcanic province

Peridotite xenoliths hosted in alkali basalts from Tallante (SE Spain) provide a unique insight into the geochemical evolution of a transitional lithospheric domain between the paleo-southern Iberian margin and the westward migrating Alborán micro-continent in the westernmost Mediterranean. Fertile spinel lherzolites xenoliths sample a subcontinental lithospheric mantle that underwent pervasive crystallization of metasomatic sulfide-bearing pyroxenes. Mantle refertilization occurred in response to the percolation of subalkaline silicate melts released upon asthenosphere upwelling and slab tearing of the Iberian continental lithosphere during the Miocene. In the Pliocene, the influx of heat/volatiles from host-alkali magmas triggered small-scale partial melting of metasomatic sulfide-bearing assemblages, producing melt now quenched to silicate glass and spongy coronae around clinopyroxene and spinel.&#13; Refertilization of Tallante peridotites caused the precipitation of pyroxenes-hosted base-metal sulfides (BMS) with anomalously high Au concentrations. These sulfides are everywhere pentlandite ± chalcopyrite ± bornite aggregates with homogeneous compositions in terms of major elements (Ni, Fe, Cu) and semi-metals (Se, As, Te, Sb, Bi), consistent with precipitation from a Ni-Cu rich sulfide melt produced by incongruent melting of monosulfide solid solution. However, BMS show strongly heterogeneous PGE systematics characterized by a variety of PGE-chondritic normalized patterns (i.e., positive, flat and negative slope), which cannot be explained by conventional partitioning of PGE in sulfide systems. Moreover, the presence of euhedral Pt-(Pd)-Sn rich platinum-group minerals (PGM) and Au particles points out that the distribution of noble metals in Tallante BMS was controlled by the incorporation of distinct populations of nano- to micron-sized PGM and/or metal particles during mantle melting and/or melt percolation. We conclude that metasomatic precipitation of sulfide-hosting pyroxenes provided an efficient mechanism for storing metals, especially gold, in the SCLM underlying the ore-productive volcanic province of southeast Spain

A Late Oligocene Suprasubduction Setting in the Westernmost Mediterranean Revealed by Intrusive Pyroxenite Dikes in the Ronda Peridotite (Southern Spain)

Contrasting tectonic reconstructions of the westernmost Mediterranean have been proposed to explain the origin of the Alboran marine basin contemporaneously with Cenozoic convergence between the African and European plates. Cr-rich pyroxenites in the Ronda massif record the geochemical processes occurring in the subcontinental mantle of the Alboran domain in the Late Oligocene, thus constraining the geodynamic scenario of Cenozoic extension in the western Mediterranean lithosphere. Clinopyroxene in intrusive Cr-rich websterite dikes crosscutting the Ronda peridotite is strongly depleted in Nb-Ta and enriched in light rare earth elements, as typically observed in arc magmas, and is in trace element equilibrium with Neogene subduction-related lavas from the western and central Mediterranean. Sr-Nd-Pb radiogenic isotopes indicate that the mantle source of the Ronda pyroxenite dikes was contaminated by a subduction component released by detrital sediments likely deposited in passive continental margins. Rather than convective removal or delamination of the lithospheric root, our data strongly support Alboran geodynamic models that envisage slab rollback as the tectonic mechanism responsible for the Miocene lithospheric thinning. The Ronda Cr-rich pyroxenite dikes represent the earliest unambiguous manifestation of subduction-related magmatism in the western Mediterranean and testify to the involvement of terrigenous sediments in the primitive stages of subduction

Multi-stage evolution of the lithospheric mantle beneath the westernmost Mediterranean: Geochemical constraints from peridotite xenoliths in the eastern Betic Cordillera (SE Spain)

International audienceSpinel (± plagioclase) peridotite xenoliths from the Tallante and Los Perez volcanic centres in the eastern Betics (SE Spain) range from depleted (clinopyroxene-poor) harzburgites to fertile (clinopyroxene-rich) lherzolites and orthopyroxene-free wehrlites. Significantly, only one harzburgite, which is depleted in heavy rare earth elements (HREE), retains the imprint of ca. 20% ancient melting of an original garnet lherzolite source. In contrast, REE abundances of other harzburgites and lherzolites from the eastern Betics have been increased by melt-rock reaction. The whole-rock and mineral compositions of these mantle rocks are largely controlled by three types of modal metasomatism: 1) common clinopyroxene-orthopyroxene addition and olivine consumption which increased FeOt, SiO2 and Al2O3, and decreased MgO compared to the refractory melting products; 2) subordinate orthopyroxene dissolution and precipitation of clinopyroxene and olivine, which led to higher FeOt and MgO and lower SiO2 than in common (orthopyroxene-rich) lherzolites; and 3) rare orthopyroxene consumption and olivine addition that caused higher FeOt and lower SiO2 compared to the original melting residues. These mineral modal and major element variations have been produced mostly by interactions with relatively FeOt-rich/SiO2-poor melts, likely derived from a peridotite-pyroxenite lithospheric mantle with a highly heterogeneous isotopic composition. Melting of the lithospheric mantle in the western Mediterranean was triggered by upwelling of the asthenosphere induced by back-arc extension in the Late Oligocene-Early Miocene. Trapping of small fractions of exotic melts in whole-rocks — likely the parental magmas of Miocene back-arc dykes that intruded the Betic crust — caused local disequilibrium between the trace element signatures and Pb isotopic compositions of clinopyroxene and whole-rock. Subsequent interaction with SiO2-undersaturated magmas, similar to the parental melts of the Pliocene alkali basalts that host the xenoliths, promoted orthopyroxene consumption and clinopyroxene-olivine enrichment at locations close to magma conduits, and finally generated orthopyroxene-free wehrlites. This event constitutes the last episode of the Cenozoic magmatic evolution of the westernmost Mediterranean which is recorded in the mantle xenoliths from the eastern Betics

Strain localization in pyroxenite by reaction-enhanced softening in the shallow subcontinental lithospheric mantle

We report structural evidence of ductile strain localization in mantle pyroxenite from the spinel to plagioclase websterite transition in the Ronda Peridotite (southern Spain). Mapping shows that, in this domain, small-scale shear zones occurring at the