Pyroxenite Layers Research Articles

Trace element geochemistry of mantle xenoliths from Zarnitsa kimberlite pipe, Daldyn field, Yakutia: Complex history of melts interactions with lithospheric mantle

Mantle xenoliths from Zarnitsa pipe studied in gray eruptive breccias (early) brown autholitic breccia (BAB) and black macrocrystic kimberlites (last dike; BMK), include garnet and spinel dunites-harzburgites, pyroxenites, eclogites, glimmerites and megacrysts. PT reconstructions using xenoliths reveal sharply layered structure (8 levels), estimated with the single grain mineral estimates mark hot (Cpx, Ilm) and cold (OPx, Gar) inflected geotherm. The interaction with plume melts is found at the lithosphere asthenosphere boundary (LAB), pyroxenite layer (3-4 GPa), Gar-Sp transition and Moho. Eclogites reveal Fe# growth from LAB to middle pyroxenites layer. Clinopyroxenes and ilmenite estimates marks melt refertilisations in interlayers between coupled subduction slabs. Source of capture from first to third stage deepening to LAB and Cr- rich garnets (to 19.5 Cr2O3) are below the LAB.The grey erupted breccia (GEB) includes mainly depleted and deformed peridotites. The later BAB includes pyroxenites, eclogites, and refertilised, deformed, and veined peridotites in BMK. Geochemistry of minerals changes from primary mid ocean ridge basalts (MORB) and back arc peridotites with low REE and large ion lithophile elements (LILE) and deeps in high field strength elements (HFSE) to metasomatized by alkaline (high Na and LILE) and adakitic melts (high Al, Na, Sr, and elevated HFSE) varieties and refertilised lherzolites due to plume at last stage. Mantle column metasomatized with scattered phlogopites in early grey eruptive breccia to amphibole-phlogopite ilmenite veins at last stages. Amphiboles trace mantle from lithosphere – asthenosphere boundary to Moho.Growth of the diamond grade from early eruptive breccia to later kimberlite phases refer to decreasing of the crust material and deepening of the xenoliths capture level. Metasomatism dissolve the diamonds but growth of megacrystic diamond crystals increase diamond grade.

Structural context of the Flatreef in the Northern Limb of the Bushveld Complex

AbstractThe Flatreef occurs at a depth of 700 m under the farm Turfspruit 241 KR in the Northern Limb of the Bushveld Complex. The Flatreef forms part of the Platreef of the Northern Limb, which contains magmatic rocks of the Rustenburg Layered Suite of the Bushveld Complex. The structure of the Flatreef is a flat-lying to gently westerly dipping monoclinal to open fold, 1 km wide and 6 km long. Distinctive features within the Flatreef include the development of cyclical magmatic layering with locally thickened pyroxenitic layers, and associated economically significant poly-metallic mineralisation. Geophysical evidence, exploration drill core, and recent underground exposure show that deformation had a major influence on the Flatreef mineralization. Block faulting and first generation folding affected the orientation and shape of the sedimentary host-rock sequence prior to intrusion of the Rustenburg Layered Suite. These structural and host-rock elements controlled the intrusion of the Lower Zone, and to a lesser degree, the Critical Zone correlatives of the Bushveld Complex in the Northern Limb. During intrusion reverse faults and shear zones and a second generation of folds were active, as well as local extension along layering. Syn-magmatic deformation on these structures led to laterally extensive stratal thickening across them, including the Merensky-Reef correlative that forms part of the Flatreef. This deformation was likely to have been driven by subsidence of the Bushveld complex. Many of these structures were intruded by granitic magmas during the late stages of intrusion, and they were reactivated during extension after intrusion. Thus, structures were active before, during and after the intrusion of Northern Limb, and the structural evolution determined the current geometry and mineral endowment of the Flatreef.

Reconstructions of mantle structure beneath the Anabar Shield kimberlites – Similarities and differences

Comparisons of mantle xenocrysts from Lower Triassic kimberlites in the Anabar Shield (Ary-Mastakh, Dyuken, Kuranakh and Orto-Yargyn fields) have shown essential differences from the xenocrysts in the Olenek River Basin (Chomurdakh field). Xenoliths in the Anabar Shield and its northern fields are very rare; they include mainly garnet dunites and harzburgites, and less commonly, pyroxenites and eclogites.PTXFO2 diagram reconstructions for the Boomerang pipe in the Ary-Mastakh field located in the suture zone of the Daldyn and Magan terranes have used monomineral thermobarometry to show that the Opx in rare lherzolitic pyropes formed between 6 and 7.5 GPa. Eclogites represent the mantle heated to the middle pyroxenite layer, and in these terranes the formation of Cr-less pyroxenites are linked to plume melt interactions with the eclogites. In the Dyuken, Kuranakh and Orto-Yargyn fields, the garnet advective trend starts from 7.5 GPa, while the asthenosphere – lithosphere boundary is found at 6 GPa, suggesting that the middle pyroxenite layer was heated and metasomatized. The lower and particularly mid-mantle parts of these fields also contain abundant eclogites. In the Chomur field, lherzolitic and pyroxenitic pyropes form from 7 GPa, while the captured materials mainly represent the upper mantle (4 GPa). All pipes show a similar mantle layering, consisting of seven parts and determined by the clustering of PT estimates for garnets, Cr-spinels, and pyroxenes. In the Boomerang pipe, the Cr-clinopyroxenes and pyropes show REE spectra with varying fan-shaped slopes, (La/Yb)n of 10–100 for pyropes, and HREE for garnets. Spider diagrams reveal peaks for Th, and troughs for U, Nb, Ta, and Pb. Eclogitic garnets and omphacites show minimum values of Eu and Nb, Ta, Zr, and Hf. REEs in ilmenites show a joint increase in LREE and HFSE for Mg to Fe-rich varieties with the degree of differentiation. Most depleted pyropes from Kuranakh have V-U shaped REE patterns, Ba and U peaks. The HFSE minima represents dunites from the arc and back-arc mantle, and the fertilization produces an increase in incompatible elements and sometimes large ion lithophile element (LILE) levels for lherzolitic pyropes. The pyroxenitic garnets display happed REE and the Cpx shows varying LILE and HFSE correlating with the (La/Yb)n.The Chomur pipe contains predominantly harzburgite-lherzolite garnets with minima Ba and Sr, in addition to various incompatible elements. Cpx shows similar variations with mainly depleted HFSE patterns. The marginal parts of the subcratonic lithospheric mantle (SCLM) of the Anabar Shield are extremely enriched in eclogitic deep-seated material. This is especially seen in the lower SCLM parts, demonstrating thermobarometric trends and features similar to the diamond inclusions from the Ebelyakh (Mayat) placers. The mantle column beneath several pipes (Los’, Universitetskaya, Kuranakh) contain Cr amphiboles distributed from the base of the lithosphere to the Moho. A comparison of the reconstructed SCLM beneath the Anabar and Chomurdakh fields reveal more fertile compositions of the mantle rocks and simpler structure for later field. The lower mantle lithospheric abundance and the PTX trends of the Kuranakh eclogites and the Orto-Yargyn kimberlites are similar to the eclogitic diamond inclusions from the Ebelyakh placers. This increases the possibility of finding diamonds deposits in the Anabar Shield kimberlites.

The Tennis Ball Marker in the south-eastern Bushveld Complex: comparison with the Boulder Bed of the Western Bushveld Complex and proposed genesis by disaggregation of intrusive sills in processes akin to those involved in the formation of peperites and mafic enclave swarms

Abstract The Tennis Ball Marker (TBM) is a distinctive lithology that is particularly well developed near the base of the Rustenburg Layered Suite (RLS) on the farm Middelkraal 221 JS, approximately 20 km south of the town of Roossenekal in the Eastern Limb of the Bushveld Complex. The name refers to a texture in which approximately tennis ball-sized spheroidal aggregates of feldspathic orthopyroxenite or melanorite occur within a lighter-coloured gabbronorite host rock. We have identified two well-defined layers in which pyroxenitic spheroids are densely packed, with spheroids more sparsely distributed elsewhere in the host gabbronorite. The TBM at Middelkraal has previously been described as a contact phenomenon where the Main Zone (MZ) has been contaminated by footwall lithologies that include basaltic lavas of the Dullstroom Formation. Our geochemical data, in tandem with new geological mapping, suggest that the TBM is in fact hosted by the Marginal Sill Phase (MSP), and not by the MZ. The MSP is a regional feature that separates the overlying units of the RLS from its floor rocks throughout this region. Sills that make up the MSP were injected on a regional plane of weakness in the primary stratigraphy prior to the intrusion of the remainder of the RLS. The MSP does not represent the chilled carapace of a magma chamber, as implied in previous studies, and there is no evidence of a genetic relationship with the MZ. Our geochemical data indicate, furthermore, that the gabbronorite constituting the dominant lithology of the MSP that hosts the TBM at Middelkraal formed from a sequence of several magma influxes. The TBM is ascribed to one or more subsequent intrusions of pyroxenitic magma into the still hot gabbronoritic host sequence. There are no comparable lithologies reported from other layered intrusions, and the closest analogy to the TBM is the Boulder Bed in the Upper Critical Zone (UCZ) of the Western Bushveld Complex, which has received considerably more attention in the literature than has the TBM. The Boulder Bed has been ascribed variously to in-situ agglomeration of clusters of orthopyroxene resulting from liquid immiscibility, in situ breakup of a pre-existing pyroxenite layer, possibly due to seismic events, or the disaggregation of a late-stage pyroxenitic sill intruding into the magma chamber. We subscribe to the latter mechanism for both the TBM and the Boulder Bed, drawing on similarities with mafic magmatic enclaves, where mafic sills have been shown to have disaggregated on intrusion into earlier bodies of felsic to intermediate igneous rock, or peperites, where magma intruded fluidised sediments.

Deep segregation and crystallization of ultra-depleted melts in the sub-ridge mantle

Partial melting of mantle peridotite from which considerable amounts of melt have been extracted during prior melting episodes generates melts characterized by low incompatible element contents and very low ratios of highly to moderately incompatible elements, so-called ‘ultra-depleted’ melts. Reaction of peridotite with percolating ultra-depleted melts has been inferred from petrological-geochemical studies of abyssal peridotites and ophiolites. But so far, direct evidence for the existence of ultra-depleted melts, only comes from rare melt inclusions. Here, we show that a pyroxenite layer within abyssal peridotite from the Mid Atlantic Ridge (8°N, Doldrums Fracture Zone) formed by crystallization of a segregated melt that is highly depleted in incompatible elements, at >27 km-depth beneath the ridge axis (at T ∼ 1250 °C), and with little or no modification by interaction with the host harzburgite. During exhumation, the pyroxenite experienced decompression and partial re-equilibration under plagioclase-facies conditions (∼1060 °C and ∼ 15 km depth). The high Hf isotope ratio (εHf = 40.3) of the pyroxenite clinopyroxene is inherited from a melt sourced from an ultra-depleted peridotite that evolved with high Lu/Hf. The associated MORB-like Nd isotope ratios (εNd = 10.6), however, imply a long-term evolution of the source peridotite with composition moderately depleted in incompatible elements (rather low Sm/Nd). These compositions are different from the host harzburgite, but typical for peridotites that have melted and partially reacted with migrating melts in ancient times. Hence, the pyroxenite investigated here is a partial melt from an ultra-depleted peridotite that has become re-enriched in incompatible elements and clinopyroxene. This melt crystallized in the oceanic lithosphere, and partially re-equilibrated at low pressure during exhumation. Overall, our results show that renewed melting of ultra-depleted peridotites with a complex history of prior melting and melt-rock reaction occurs, and that such melts migrate through the sub-ridge mantle, and can thus contribute to mid ocean ridge magmatism, although to a still unknown extent.

Origin of Erzgebirge ultrahigh‐pressure garnetite: Formation from a basaltic protolith by serpentinization‐assisted metasomatism?

AbstractErzgebirge ultrahigh‐pressure (UHP) garnet peridotite includes scarce layers of garnet pyroxenite, nodules of garnetite and, very rarely, of eclogite. Peridotite‐hosted eclogite shows the same subalkali‐basaltic bulk rock composition, mineral assemblage and peak conditions as gneiss‐hosted eclogite present in the same UHP unit. Garnetite has considerably more Mg, moderately enhanced Ca and Fe and significantly lower contents of Na, Ti, P, K and Si than eclogite, whereas Al is very similar. In addition, the compatible trace elements (Ni, Co, Cr, V) are elevated and most incompatible elements (Zr, Hf, Y, Sr, Rb and rare Earth elements [REE]) are depleted in garnetite relative to eclogite. In contrast to other large ion lithophile elements (LILEs), Pb (+121%) and Ba (+83%) are strongly enriched. The REE patterns of garnetite are characterized by depletion of light and heavy REE and a medium REE hump indicative of metasomatism, features being absent in eclogite. An exceptional garnetite sample shows an REE distribution similar to that of eclogite. Garnetite is interpreted to have formed from the same, but metasomatically altered, igneous protolith as eclogite. Except for Ba and Pb, the chemical signature of garnetite is explained best by metasomatic changes of its basaltic protolith caused by serpentinization of the host peridotite. Garnetite is chemically similar to basaltic rodingite/metarodingite. Although rodingite is commonly more enriched in Ca, there are also examples with moderately enhanced Ca matching the composition of Erzgebirge garnetite. Limited Ca metasomatism is attributed to the preservation of Ca in peridotite during hydrous alteration. This can be explained by incomplete serpentinization favouring metastable survival of the original clinopyroxene. In this case, most Ca is retained in peridotite and not available for infiltration and metasomatism of the garnetite protolith. This inescapable consequence is supported by the fact that clinopyroxene is part of the garnet peridotite UHP assemblage, which would not be the case if Ca had been removed from the protolith prior to high‐pressure metamorphism. The enrichment of compatible elements in garnetite is attributed to decomposition of peridotitic olivine (Ni, Co) and spinel (Cr, V) during serpentinization. Enrichment of Ba and Pb contrasts the behaviour of other LILEs and is ascribed to dehydration of the serpentinized peridotite (deserpentinization). This requires two separate stages of metasomatism: (1) intense chemical alteration of the basaltic garnetite precursor, together with serpentinization of peridotite at the ocean floor or during incipient subduction; and (2) prograde metamorphism and dehydration of serpentinite during continued subduction, thereby releasing Pb–Ba‐rich fluids that reacted with associated metabasalt. Finally, subduction to >100 km and UHP metamorphism of all lithologies led to formation of garnetite, eclogite and garnet pyroxenite hosted by co‐facial garnet peridotite as observed in the Erzgebirge.

On the Potential for Cumulate Mantle Overturn in Mercury

AbstractMercury has a compositionally diverse surface exhibiting geochemical terranes that represent different periods of igneous activity, suggesting diverse mantle source compositions. Mercury's juvenile mantle likely formed after fractional solidification of a magma ocean, which produced distinct mineralogical horizons with depth. To produce the diversity of observed volcanic terranes, dynamic mixing of materials from distinct mantle horizons is required. One process that could dynamically mix the juvenile cumulate pile is cumulate mantle overturn, where dense layers in shallow planetary mantles sink into deeper, less dense layers as Rayleigh‐Taylor instabilities. Gravitationally unstable density stratification is a requisite starting condition for overturn; solidification of the Mercurian magma ocean is likely to have produced such a density inversion, with a relatively dense clinopyroxene‐bearing pyroxenite layer atop lower density dunite and harzburgite layers. Sulfides are present in abundance on Mercury's surface and would be additional mantle phase(s) if they are indigenous to the planet's interior. Sulfides have variable densities; they could potentially enhance the formation of gravitational instabilities or prevent them from developing. Exploring physically reasonable mantle density and viscosity structures, we evaluate the potential for cumulate mantle overturn in Mercury and predict the possible timing, scale, and rate of overturn for plausible physical parameter combinations. Our analysis suggests that overturn is possible in Mercury's mantle within 100 Myr of magma ocean solidification, providing a mechanism for producing the mantle sources that would melt to form surface compositions on Mercury, and overturn may control the spatial scale of volcanic provinces observed on the surface today.

Reworked Neoproterozoic sub-arc lithosphere beneath the Eastern Desert (Egypt): Evidence from geochemistry and geochronology of metabasites and granitoid gneisses, Gabal Um Gunud area

ABSTRACT High-grade granitoid gneisses (740–710 Ma) and elongate bodies of amphibolite and hornblende schist with cm-scale layers of garnet pyroxenite form an overturned fold in the Gabal Um Gunud area, South Eastern Desert of Egypt. Whole-rock geochemical data combined with zircon U-Pb-Hf isotope data suggest that the metabasites represent relics of oceanic crust with an N-MORB affinity, which has been derived from partial melting (2.0–1.4 GPa; ~65- ~ 45 km depth) of a depleted mantle source beneath an intra-oceanic, spreading forearc basin. Such increment of melting degree resulted in small basaltic melt batches with transitional tholeiitic to boninitic affinities. Progressive slab subduction and partial melting of the tholeiitic amphibolite produced high-Al and low HREE melts for trondhjemite at av. T = 854°C and low-Al and high HREE melts for tonalite at higher temperatures (av. 949°C). The low Mg# of the trondhjemite-tonalite rocks may be attributed to limited contamination of subduction components, as evidenced by the weak lanthanide tetrad effects (TE1,3~1) and near-chondritic Zr/Hf, Nb/Ta, and Y/Ho ratios, while the garnet pyroxenite was the residuum produced in equilibrium with the trondhjemite-tonalite melts.

A Rosetta stone linking melt trajectories in the mantle to the stress field and lithological heterogeneities (Trinity ophiolite, California)

Abstract Infiltration triggered by selective dissolution of pyroxenes is a major mode of melt migration in the mantle. A common view, supported by experiments and numerical models, is that the geometry of the melt plumbing system is governed by the stress field induced by solid-state flow of the host peridotite. Yet, salient melt migration structures frozen at an early stage of development in the mantle section of the Trinity ophiolite reveal that lithological heterogeneities drastically impact melt trajectories. Where melts reach a pyroxenite layer, dissolution-induced permeability abruptly increases, initiating a feedback loop confining melt migration to that layer regardless of its orientation relative to the stress field. This process results in the development of a network of interweaved dunitic channels evolving to thick tabular dunites where the melt reacts with closely spaced pyroxenite layers. This reacting melt was rich in alkali elements and water, as evidenced by the minerals (mostly amphibole and micas) encapsulated in the Cr-spinel grains that crystallized during the reaction. This “pioneer melt” differs from the volumetrically dominant depleted andesite that fed the crustal section. In fact, the migration of andesite benefited from the enhanced permeability provided by the dunites formed by the pioneer melt. As a result, dunites are palimpsests, the compositions of which record successive percolation events. The geometry of the melt pathways is extremely challenging to model because the abundance, spacing, and orientation of lithological heterogeneities cannot be predicted, being inherited from a long geological history.

Lateral uniformity of the Pyroxenite Marker Transition in the western Bushveld Complex, South Africa

Abstract Mineralogically distinctive layers in the Bushveld Complex, South Africa, can usually be traced for hundreds of km in both the eastern and western limbs. They are remarkably uniform laterally in mineral chemical composition. There is one notable exception, namely the Pyroxenite Marker in the middle of the Main Zone in the eastern limb. It defines the boundary between the Lower Main Zone and Upper Main Zone. Toward the south in the eastern limb mineral compositions become more evolved, and ultimately the Pyroxenite Marker layer itself disappears and is replaced by magnetite-bearing rocks. In all previously published profiles through the eastern Bushveld, through a 400 m interval with the Pyroxenite Marker in the middle, there is a regular prolonged reversal in the anorthite content of plagioclase and mg# of pyroxene of about ten units, attributable to magma addition. In contrast, in the western limb there is no actual outcrop of this layer, but it has been located in the BK borehole from the centre of the limb. A similar magnitude of reversals in mineral compositions as in the east was reported in a previous study. In this study, a second profile was taken close to the eastern limit of the western limb to test if there was lateral variation comparable to that observed in the eastern limb. More mineralogical data are also reported from the borehole intersection. These two sections from the western limb show extremely similar changes in mineral compositions. In the eastern limb the location of the Pyroxenite Marker also shows an upward, abrupt change from primary pigeonite (below) to primary orthopyroxene (above). The same change occurs in the west, and at the same mineral compositions as in the east, and so this boundary can be defined by the pigeonite to orthopyroxene transition even in the absence of an actual pyroxenite layer. Hence, the term Pyroxenite Marker Transition is more applicable. Both in the east and west more primitive plagioclase compositions occur well above this boundary, and so magma addition and/or mixing continued well into the Upper Main Zone.

Rifting evolution of the lithospheric subcontinental mantle: New insights from the External Ligurian ophiolites (Northern Apennine, Italy)

The present study focuses on the petrographic and petrological characteristics of mantle bodies included in Upper Cretaceous sedimentary melanges of the External Ligurian units (Northern Appennine), within the Monte Gavi and Monte Sant'Agostino areas. Two distinct pyroxenite-bearing mantle sections were recognized, mostly based on their plagioclase-facies evolution. The Monte Gavi mantle section is nearly undeformed and records a process of melt infiltration and reaction under plagioclase-facies conditions. The melt-rock interaction event involved both peridotites (mostly harzburgites) and enclosed spinel pyroxenite layers, and is estimated to have occurred at 0.7–0.8 GPa. In the Monte Gavi peridotites and pyroxenites, the spinel-facies clinopyroxene was partially replaced by plagioclase and new orthopyroxene (± secondary clinopyroxene). The reactive melt migration led to relatively high TiO2 contents in relict clinopyroxene and spinel (up to 2.3 wt% and 1.0 wt%, respectively, in the pyroxenites), with the latter also having high Cr# (up to 35 in the peridotites). The Monte Sant'Agostino mantle section displays a widespread ductile shearing and no evidence for melt-rock interaction under plagioclase-facies conditions. The main deformation phase recorded by the Monte Sant'Agostino peridotites (mostly lherzolites) is estimated to have occurred at 750–780 °C and 0.3–0.6 GPa, and gave rise to protomylonitic to ultramylonitic textures characterized by 10–50 μm neoblasts. The enclosed pyroxenite layers yielded relatively high temperature and pressure estimates (870–930 °C and 0.8–0.9 GPa). Presumably, in the Monte Sant'Agostino mantle section, plagioclase crystallization occurred earlier in the pyroxenites than in enclosing lherzolites, thereby enhancing strain localization and formation of mylonite shear zones in the entire mantle section. We propose that subcontinental mantle section from the External Ligurian units consists of three distinct mantle domains, developed in response to the rifting evolution that ultimately formed a Middle Jurassic ocean-continent transition: (1) a spinel tectonite domain that underwent no significant deformation and melt-rock reaction under plagioclase-facies conditions, characterized by static plagioclase development, (2) a plagioclase mylonite domain experiencing melt-absent deformation, and (3) a nearly undeformed domain that underwent melt infiltration and reaction under plagioclase-facies conditions. We relate mantle domains (1,2) to a rifting-driven uplift in the late Triassic accommodated by large-scale shear zones consisting of plagioclase mylonites.

Enriched Hf[sbnd]Nd isotopic signature of veined pyroxenite-infiltrated peridotite as a possible source for E-MORB

Pyroxenite-peridotite sequences from the External Liguride (EL) Jurassic ophiolites (Northern Apennines, Italy) consist of portions of fertile MORB mantle that were modified by deep melt infiltration and melt-peridotite reaction. They represent an excellent natural example of a MORB-like veined mantle including unmodified peridotite, pyroxenite layers and metasomatized peridotite. We carried out a spatially controlled Hf isotope study on these mantle sequences to investigate how the Nd and Hf isotopic systems are affected by pyroxenite emplacement and melt-peridotite interactions. Present-day LuHf isotopic compositions of these lithologies show a large range of 176Lu/177Hf and 176Hf/177Hf ratios that are correlated with their Nd isotopic compositions. Pyroxenite-free peridotites delineate a HfNd isotope array that corresponds to a Proterozoic age (> 1.5 Ga) which is likely related to the accretion to the subcontinental lithosphere of this mantle sector. Heterogeneous 176Hf/177Hf isotopic compositions in pyroxenites mostly correlate with the significant variations of 176Lu/177Hf ratios and reflect variable garnet abundance in the primary modal assemblage. Over time, the pyroxenites acquired a large range of εHf values, which encompass the global range of HfNd isotopes in ocean ridge basalts. Infiltration of pyroxenite-derived melts led the host peridotite to acquire low Lu/Hf ratios with the consequent development of 176Hf/177Hf ratios lower than in the unmodified peridotite, generating an equivalent of an enriched mantle component. This melt-peridotite interaction likely occurred during the pyroxenite emplacement 430 Ma ago, as confirmed by two LuHf local pyroxenite-peridotite isochrons. The chemical and isotopic changes produced, over time, a spread of HfNd isotopic signatures of the EL veined mantle, covering almost the entire range of published MORB compositions. Pyroxenite emplacement and local metasomatism of the host peridotites thus created HfNd enriched mantle domains, making the EL veined mantle the first reported natural example of an enriched MORB-like mantle that formed through the combined effect of deep emplacement of pyroxenite and pyroxenite-peridotite interaction. The structure and isotopic characteristics of the EL veined mantle were used to model the isotopic compositions of melts produced by decompression melting of three-component heterogeneous mantle sources, providing an additional scenario to the generation of EMORB erupted at mid-ocean ridge settings. Our results emphasize the potential role of deep pyroxenite infiltration in modifying the host peridotites by interaction with pyroxenite-derived melts and creating heterogeneous mantle domains.

Fractional crystallization of a basal lunar magma ocean: A dense melt-bearing garnetite layer above the core?

In the wake of its accretion, the Moon was likely partially or fully molten, forming a lunar magma ocean (LMO). A fully molten Moon may crystallize neutrally buoyant olivine, whose accumulation may form a barrier leading to a separation into two independently evolving melt reservoirs. This study investigates the crystallization of such a putative basal lunar magma ocean, ranging from the core mantle boundary (with radius r = 380 km) to the level of neutral olivine buoyancy (r = 600 km). Consecutive crystallization experiments determine liquidus temperatures and crystallize 10–30 wt% minerals, before conceptually segregating these according to their buoyancy, and then stepping to a new bulk composition that corresponds to the residual melt after removal of the cumulate minerals.The first olivine to crystallize from a Taylor Whole Moon composition (twm, XMg = Mg/(Mg + Fe2+) = 0.83) has XMg = 0.94 and is neutrally buoyant at 3.8 GPa according to the melt density model of Lange and Carmichael (1990). Crystallization begins at the core mantle boundary, but early olivine floats and re-dissolves until the magma ocean cools to the liquidus temperature at the depth of neutral buoyancy (1850 °C). At this point a > 500 km thick olivine-only layer could form, mainly fed from the upper magma shell. The crystallization sequence in the basal magma ocean is olivine-only at 1850–1675 °C, 0–26 pcs (wt% percent solidified) → olivine + opx (to 1600 °C, 42 pcs) → opx + cpx + garnet (to 1580 °C, 60 pcs) → cpx + garnet (to 1520 °C, 73 pcs) → cpx + garnet + olivine leaving 11 wt% residual melt at 1450 °C.Cooling this last melt to 1300 °C leads to 80% garnet + cpx + olivine + Ti-spinel, the residual liquid corresponding to 2.2% of the basal magma ocean. Olivine, opx and cpx remain buoyant over the entire crystallization interval and various pyroxenite layers are added to the olivine layer. Garnet and the final cotectic cpx + garnet + olivine + FeTi-oxide assemblage instead form a 70 km thick basal layer on the core-mantle boundary. Such a layer provides a gravitationally stable high-density lowermost lunar mantle, which would concur with a recent re-analysis of the lunar seismic data. The lowest temperature experiment at 1300 °C, not much above the 1250 °C proposed for the core-mantle boundary from inversion of geophysical data, had about 20% of a highly evolved, dense Fe-rich melt. It is feasible that this melt has remained in its liquid state to present day, providing an explanation for the proposed low vp and vs layer and high dissipation just above the core-mantle boundary.

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.

The microstructure of layered ultramafic cumulates: Case study of the Bear Creek intrusion, Trinity ophiolite, California, USA

In the Trinity ophiolite, California, USA, several mafic-ultramafic plutons intruded a peridotitic host 435 to 405 m.y. ago in a tectonic setting interpreted as an arc-related spreading centre. One of these intrusions, in the Bear Creek area, exposes basal ultramafic cumulates with igneous layering comprising an alternation of uncommonly thin (down to a few mm) layers of dunite, peridotite and pyroxenite that might be specific to this tectonic setting. These layers offer an excellent opportunity to characterise the microstructure of uncommon cumulates from the lower crust using EBSD (Electron Backscatter Diffraction). This “high-frequency” layering rests on underlying lherzolites and grades upward to more massive pyroxenites (i.e. clinopyroxenites with minor olivine-rich layers). Our field observations and data from the Bear Creek cumulates together with the preservation of magmatic features suggest the environment was tectonically stable after the emplacement of the cumulates. A detailed microstructural investigation of all minerals from the Bear Creek cumulates allows us to decipher their magmatic and plastic deformation history. In a structural reference frame defined by the compositional layering and the elongation direction of the surrounding host peridotites, olivine in the cumulates presents a [010]-fibre fabric and rarely a [001](010) fabric. Clinopyroxene shows a concentration of [010] axes normal to the layering plane with [100] and [001] defining girdles. Orthopyroxene mostly has a fabric with [100] and/or [010] subnormal to the layering plane and [001] scattered along a girdle in the plane of layering. All minerals show a strong fabric. We interpret the formation of the developed planar microstructures as a result of magmatic processes, with high contribution of crystal settling. To a lesser extent, compaction could have been operating and may be linked to the rare evidence of plastic deformation. Clusters of axes within the girdles of olivine and pyroxene CPOs preferentially appear close to the direction of elongation of the surrounding peridotites (i.e. N115°). EBSD analysis of the shape-preferred orientation of Bear Creek's minerals revealed a preferential alignment of the olivine and cpx long axis with the N°115 direction. This magmatic lineation and the preferred direction in the CPOs girdles are both consistent with the stretching lineation acquired during solid-state deformation by the mantle peridotite of the Trinity ophiolite. We suggest that a weak magma flux early on and/or an ongoing but limited regional stress could be responsible for these clusters. Although a direct coupling between asthenospheric flow and magmatic flow cannot be invoked in this context of melt intrusion in the lithosphere, this result highlights that the stress field applied on the mantle could have been still active and similar during the formation of Bear Creek intrusion.Our new field and microstructural data, together with previously presented petrological data, fit a scenario for the evolution of the Trinity ophiolite in which a mantle segment was intruded by a single large batch of primitive boninitic-andesitic melt. Our results emphasise the importance of considering the initial magmatic microstructures and the original shape anisotropy when investigating later deformation in ultramafic rocks.

Interplay between melt infiltration and deformation in the deep lithospheric mantle (External Liguride ophiolite, North Italy)

Mantle peridotites from the External Liguride Jurassic ophiolites (Northern Apennines, Italy) show diffuse occurrence of pyroxenite bands, recording melt migration and crystallization at the lithosphere-asthenosphere boundary, during old, pre-Jurassic stages of their subcontinental lithospheric mantle evolution. We present coupled microstructural and geochemical study of profiles across various types of pyroxenite-peridotite layering in these ophiolites, aiming to constrain the relative timing and potential interplay between melt infiltration and mantle deformation. The mantle sequence is composed of lherzolite and harzburgite, occasionally interleaved with dunite, crosscut by centimeter- to decimeter-wide pyroxenite layers. The peridotites have a porphyroclastic texture and show a penetrative tectonic foliation subparallel to the pyroxenite layering. Peridotite-pyroxenite contacts are irregular at the grain scale. Olivine and pyroxenes in both peridotites and pyroxenites record moderate to strong crystallographic preferred orientations (CPO) with alignment of olivine [100] and pyroxenes [001] axes subparallel to the stretching lineation marked by olivine and pyroxenes elongation. This is compatible with coeval deformation of olivine and pyroxenes during a high-temperature, spinel lherzolite-facies deformation event. The major and trace element compositions of peridotites record a metasomatic imprint that decreases with distance from the pyroxenite layers, whereas the strength of the olivine CPO decreases from the country peridotites towards the pyroxenite layers. The parallelism between pyroxenite layers and the peridotite foliation, their irregular contacts, as well as the spatial correlation between CPO and geochemical changes, are consistent with syn- to late-kinematic emplacement of the pyroxenites. These observations point to a strong interplay between deformation and melt transport processes in the mantle, characterized by melt focusing in conduits parallel to the foliation and changes in the mantle deformation processes due to the presence of melts. Exhumation of this mantle section in the Jurassic resulted in partial replacement of the spinel-facies assemblage by a plagioclase-bearing assemblage. Topotaxial relationship between plagioclase and precursor spinel-facies minerals suggests that this exhumation was not associated with pervasive deformation of the peridotite, but rather accommodated by deformation localized in discrete shear zones, not sampled in the present study.

Mantle and crust interaction scenario at the crust-mantle transition zone: Depicted from inter-layered pyroxenite-granulite xenolith in Hannuoba area, North China Craton

Mantle-crust interaction occurring in various ways has been proposed for a long time, but the details of its mechanism at the crust-mantle transition zone and the lower crust are poorly known. This study reports a layered pyroxenite-granulite xenolith from the Cenozoic basalts of Hannouba in the North China Craton. Petrological and mineral chemical investigations reveal that the pyroxenite layers are cumulates from underplating basaltic magmas, while the granulite portions represent pre-existing lower crust. The magmas were crustally contaminated by interaction with the granulite. Orthopyroxene and plagioclase in the granulite involving reaction with exotic melts are transformed to K-feldspar, clinopyroxene, Fe- and Mg-oxide, melt and/or sulfide. The mineral chemical similarities and overlapping temperature estimates suggest that the pyroxenites and the granulites had been incrementally equilibrated with each other, both physically and chemically, via significant chemical and thermal exchange. The phase transformation and interaction could represent crust anatexis to generate more felsic magmatism and related ore deposits for the middle to upper crust and subsequently increase crustal maturity. This is also an efficient way of forming crust-mantle transition zone below the lower crust and crustal reworking in the Hannuoba area and may be applicable to other places worldwide.

Geochemistry of the chromitite stringer at the contact of the mafic sequence and the ultramafic sequence in the Unki Mine area, Shurugwi Subchamber of the Great Dyke, Zimbabwe

ABSTRACTSeveral models have been proposed to explain the origin of a chromitite stringer located at the contact between the Mafic and Ultramafic Sequences in the Unki Mine area of the Shurugwi Subchamber of the Great Dyke, Zimbabwe. A petrographic and geochemical study of this chromitite stringer was undertaken with the aim of constraining its origin. Forty-three chromite compositions were obtained from the studied chromitite stringer, which is characterized by a chromium number between 59.9 and 62.8 and a magnesium number which ranges from 37.8 to 46.4. The chromites at the contact zone in the Unki Mine commonly contains inclusions of sulfides, orthopyroxene, plagioclase, and/or amphiboles. The chromites likely formed early in the crystallization history of the Mafic Sequence, as they are commonly partially rimmed by sulfides and they occur as inclusions in plagioclase crystals. Unlike chromites from underlying Ultramafic Sequence chromitite layers, chromites at the contact zone contain low Cr2O3 contents which range from 39.4 to 42.6 wt.%. Furthermore, these chromites are enriched in Fe compared to most Great Dyke chromitites, which is interpreted to be a consequence of subsolidus exchange of Mg into orthopyroxene and Fe into the chromite. The absence of zoning in the chromites at this contact zone, and their low Mn, Fe contents, is consistent with attainment of equilibrium because the altered chromites often contain Cr-bearing magnetite rims. Two possible models for the formation of this chromitite stringer are mixing of relatively primitive and evolved magmas (i.e., ultramafic and anorthositic magma), possibly of different oxygen fugacities, and chemical diffusion across the contact between the Mafic and the Ultramafic sequences which resulted in melting at and below this boundary. The latter would have caused preferential loss of orthopyroxene from the underlying P1 Pyroxenite Layer, accompanied by re-precipitation of chromite at this contact.

Origin of pyroxenites in the oceanic mantle and their implications on the reactive percolation of depleted melts

Pyroxenites are diffuse in fertile mantle peridotites and considered an important component in the mantle source of oceanic basalts. They are rarely documented in abyssal and ophiolitic peridotites representing residual mantle after melt generation, and few studies defining their origin are to date available. We present a field-based microstructural and geochemical investigation of the pyroxenite layers associated with depleted peridotites from the Mt. Maggiore ophiolitic body (Corsica, France). Field and petrographic evidence indicate that pyroxenite formation preceded the melt–rock interaction history that affected this mantle sector during Jurassic exhumation, namely (1) spinel-facies reactive porous flow leading to partial dissolution of the pyroxenites, and (2) plagioclase-facies melt impregnation leading to [plagioclase + orthopyroxene] interstitial crystallization. Pyroxenes show major element compositions similar to abyssal pyroxenites from slow-spreading ridges, indicative of magmatic segregation at pressures higher than 7 kbar. Both the parental melts of pyroxenites and the melts involved in the subsequent percolation were characterized by Na2O-poor, LREE-depleted compositions, consistent with unaggregated melt increments. This implies that they represent the continuous evolution of similarly depleted melts leading to different processes (pyroxenite segregation and later melt–rock interaction) during their upward migration. To support the genetic relation and the continuity between the formation of pyroxenites and the subsequent melt–rock interaction history, we modeled all the documented processes in sequence, i.e.: (1) formation of single-melt increments after 6% mantle decompressional fractional melting; (2) high-pressure segregation of pyroxenites; (3) spinel-facies reactive porous flow, (4) plagioclase-facies melt impregnation. The early fractionation of pyroxenites leads to a decrease in pyroxene saturation that is necessary for the subsequent reactive porous flow process, without any significant change in the melt REE composition.

Copper Isotope Variations During Magmatic Migration in the Mantle: Insights From Mantle Pyroxenites in Balmuccia Peridotite Massif

AbstractThe crust shows large variations in Cu isotopic composition (δ65Cu relative to NIST 976) resulting from redox reactions and other supergene processes. The significant range of δ65Cu in mantle peridotites thus could be ascribed to recycled crustal materials and/or oxidative mantle metasomatism. However, the influence of normal magmatic fractionation processes on δ65Cu variations remains poorly understood, and it is unclear whether the magmatic processes also lead to large δ65Cu variations in mantle rocks. To address the issue, we present bulk‐rock δ65Cu of fresh mantle pyroxenites from the Balmuccia peridotite massif, Northern Italy. These pyroxenites formed by melt‐peridotite reaction and mineral accumulation from MORB‐ to OIB‐like, sulfide‐saturated basic magmas. Mass balance calculations show that sulfide phases host >98 wt % of the Cu budget of bulk rocks (87−484 μg/g). The pyroxenites show significant variations of δ65Cu from −0.66‰ to 0.66‰, which cover the range known for peridotites, including metasomatized ones. Three subsamples from the same pyroxenite layer display >0.3‰ decrease in δ65Cu with progressive differentiation. Nevertheless, δ65Cu does not display correlations with indicators of magmatic differentiation (e.g., Mg# and Cu/Pd) for all pyroxenites in this study. This might be attributed to variable extents of magmatic sulfide segregation for different samples and/or varying δ65Cu of the pyroxenite parent magmas, which were also affected by reactive migration through peridotites. The combined processes can change δ65Cu of evolving magmas and reacted peridotites and lead to Cu isotopic heterogeneity in the mantle, without involvement of oxidative metasomatism or recycled crustal materials.