Peridotite Massif Research Articles

Origin of Chromitites in the Songshugou Peridotite Massif, Qinling Orogen (Central China): Mineralogical and Geochemical Evidence

The Songshugou peridotite massif is located in the north of Shangdan suture zone, North Qinling orogenic belt of Central China. The massif is mainly composed of fine-grained mylonitic dunites, coarse-grained dunites, fine- and coarse-grained harzburgites, and minor clinopyroxenites. The coarsegrained dunites as well as parts of the harzburgites host small-scale chromitites. Chromite grains from various textural types of chromitites and dunites pervasively contain primary and secondary silicate inclusions. Primary inclusions are dominated by monophase olivine, with minor clinopyroxene and a few multiphase mineral assemblages consisting of olivine and clinopyroxene. Secondary inclusions, mainly Cr-chlorite and tremolite, show irregular crystal shapes. Besides, Cr2O3 contents (0.08 wt.%–0.71 wt.%) of primary olivine inclusions are remarkably higher than those of interstitial olivine (<0.1 wt.%). Chromites in the Songshugou peridotite massif are high-Cr type, with Cr# and Mg# values ranging of 67.5–87.6, and 23.4–41.2, respectively. The Cr-chlorite, formed by reactions between olivine and chromite in the presence of fluid under middle temperature, indicates the Songshugou peridotite massif has undergone alteration/metamorphism process during emplacement. Chromite grains are modified by these processes, resulting in the various degrees of enrichment of Fe2O3, Cr2O3, Zn, Co and Mn, depletion of MgO, Al2O3, Ga, Ti and Ni. Due to low silicate/chromite ratios in the massive ores, chromites from them are slightly influenced by alteration/metamorphism and thus preserve the pristine magmatic compositions. The parental magma calculated based on them has 11.17 wt.%–13.57 wt.% A12O3 and 0.15 wt.%–0.27 wt.% TiO2, which is similar to the parental melts of high-Cr chromitites from elsewhere and comparable with those of boninites. Combined with informations from previous studies, major and trace elements geochemistry of chromite, as well as the nature of the parental magma, it can be revealed that the Songshugou chromitities formed in a supra-subduction zone environment.

Multi‐stage Melting Processes of Ophiolitic Chromitites from Purang Peridotite, the Western Yarlung‐Zangbo Suture Zone, Tibet

AbstractThe Purang ophiolite, which crops out over an area of about 600 km2 in the western Yarlung‐Zangbo suture zone, consists chiefly of mantle peridotite, pyroxenite and gabbro. The mantle peridotites are mostly harzburgite and minor lherzolite that locally host small pods of dunite. Some pyroxenite and gabbro veins of variable size occur in the peridotites, and most of them strike NW. On the basis of their mineral chemistry podiform chromitites are divided into high‐alumina (Cr# = 20‐60) (Cr# = 100*Cr/(Cr+Al)) and high‐chromium (Cr# = 60‐80) varieties (Thayer, 1970). Typically, only one type occurs in a given peridotite massif, although some ophiolites contain several massifs which can have different chromitite compositions. However, the Purang massif contains both high chrome and high alumina chromitites within a single mafic‐ultramafic body. Seven small, lenticular bodies of chromitite ore have been found in the harzburgite, with ore textures ranging from massive to disseminated to sparsely disseminated; no nodular ore has been observed. Individual ore bodies are 2‐6 m long, 0.5‐2 m wide and strike NW, parallel to the main structure of the ophiolite. Ore bodies 1 and 6 consist of Al‐rich chromitite (Cr# = 52‐55), whereas orebodies 2, 3, 4 and 5 are Cr‐rich varieties (Cr # = 63 to 89). In addition to magnesiochromite, all of the orebodies contain minor olivine, amphibole and serpentine. Mineral structures show that the peridotites experienced plastic deformation and partial melting. On the basis of magnesiochromite and olivine/clinopyroxene compositions two stages of partial melting are identified in the Purang peridotites, an early low‐partial melting event (about 8%), and a later high‐partial melting event (about 40%). We interpret the Al‐rich chromitites as the products of early MORB magmas, whereas the Cr‐rich varieties are thought to have been generated by the later SSZ melts..

Kalaymyo Peridotite Massif in the Indo‐Myanmar Ranges (western Myanmar): Its Mineralogy, Geochemistry and Tectonic Implications

AbstractMesozoic ophiolites crop out discontinuously in the Indo‐Myanmar Ranges in NE India and Myanmar, and represent the remnants of the Neotethyan oceanic lithosphere. These ophiolites in the Indo‐Myanmar Ranges are the southern continuation of the Neotethyan ophiolites occurring along the Yarlung Zangbo Suture Zone in southern Tibet farther northwes, as indicated by their coeval crystallization ages and geochemical compositions. The Kalaymyo ophiolite is located in the central part of the Indo‐Myanmar Ranges (Myanmar). The Kalaymyo ophiolite are composed of olivine (Fo = 89.8–90.5), orthopyroxene (En86‐91Wo1‐4Fs8‐10; Mg#=89.6–91.9), clinopyroxene (En46‐49Wo47‐50Fs3‐5; Mg# = 90.9–93.6) and spinel (Mg# = 67.1–78.9; Cr# = 13.5–31.5), and have relatively homogeneous whole‐rock compositions with Mg# of 90.1–90.8 and SiO2 (41.5–43.65 wt.%), Al2O3 (1.66–2.66 wt.%) and CaO (1.45–2.67 wt.%) contents. They display Light Rare Earth Element (LREE)‐depleted chondrite‐normalized REE patterns and show a slight enrichment from Pr to La. The Kalaymyo peridotites are characterized by Pd‐enriched chondrite‐normalized PGE patterns with superchondritic (Pd/Ir)CN ratios (1.15–2.36). Their calculated oxygen fugacities range between QFM–0.57 and QFM+0.90. These features collectively suggest that the Kalaymyo peridotites represent residual upper mantle rocks after low to moderate degrees (5–15%) of partial melting at a mid‐ocean‐ridge environment. The observed enrichment in LREE and Pd was a result of their reactions with enriched MORB‐like melts, percolating through these already depleted, residual peridotites. The Kalaymyo and other ophiolites in the Indo‐Myanmar Ranges hence represent mid‐ocean ridge–type Tethyan oceanic lithosphere derived from a downgoing plate and accreted into a westward migrating subduction–accretion system along the eastern margin of India.

First Detection of Methane within Chromitites of an Archean-Paleoproterozoic Greenstone Belt in Brazil

Abiotic methane is widely documented in seeps, springs and aquifers associated with mafic-ultramafic rocks in Phanerozoic ophiolites, peridotite massifs and intrusions worldwide. Chromitites in ophiolites, in particular, have been interpreted as the rocks potentially generating methane though CO2 hydrogenation. Here, we document, for the first time, the presence of methane within chromitites in South America. We analyzed, through milling and gas extraction, the content of gas occluded in Cedrolina chromitite samples, belonging to the Pilar de Goiás greenstone belt in Brazil. The chromitites display significant gas concentrations up to 0.31 µg CH4/grock and 2800 ppmv of hydrogen, while the host talc schist is devoid of gas. Stable C isotope composition of methane (δ13C from −30 to −39.2‰) and the absence of organic-matter rich metasediments in the region suggest an abiotic origin. Hydrogen and methane concentrations appear related to high-Cr chromite modal content and to the presence of Ni-sulfides/alloys, which are potential catalysts of CO2 hydrogenation at temperatures above 200 °C. Accessory ruthenium-bearing minerals occurring in the chromitites could also act as catalysts, even at lower temperatures. Geothermometry of chlorite found in the chromitites constrains serpentinization at ~250 °C, during lower greenschist facies retrometamorphism. Hydrogen could be autochthonous, and thus formed under similar temperature, which we hypothesize represents the upper limit for abiotic methane generation in the area (250 °C). The Cedrolina chromitites are the first example of CH4 occurrence in ultramafic rocks related to an Archean-Paleoproterozoic greenstone belt. This may imply that serpentinized Cr-rich chromitites could have been sources of methane for the early Earth’s atmosphere.

Subduction-zone peridotites and their records of crust-mantle interaction

Subduction is the core process of plate tectonics. The mantle wedge in subduction-zone systems represents a key tectonic unit, playing a significant role in material cycling and energy exchange between Earth’s layers. This study summarizes research progresses in terms of subduction-related peridotite massifs, including supra-subduction zone (SSZ) ophiolites and mantle-wedge-type (MWT) orogenic peridotites. We also provide the relevant key scientific questions that need be solved in the future. The mantle sections of SSZ ophiolites and MWT orogenic peridotites represent the mantle fragments from oceanic and continental lithosphere in subduction zones, respectively. They are essential targets to study the crust-mantle interaction in subduction zones. The nature of this interaction is the complex chemical exchanges between the subducting slab and the mantle wedge under the major control of physical processes. The SSZ ophiolites can record melt/fluid-rock interaction, metamorphism, deformation, concentration of metallogenic elements and material exchange between crust and mantle, during the stages from the generation of oceanic lithosphere at spreading centers to the initiation, development, maturation and ending of oceanic subduction at continental margins. The MWT orogenic peridotites reveal the history of strong metamorphism and deformation during subduction, the multiple melt/fluid metasomatism (including silicatic melts, carbonatitic melts and silicate-bearing C-H-O fluids/supercritical fluids), and the complex cycling of crust-mantle materials, during the subduction/collision and exhumation of continental plates. In order to further reveal the crust-mantle interaction using subduction-zone peridotites, it is necessary to utilize high-spatial-resolution and high-precision techniques to constrain the complex chemical metasomatism, metamorphism, deformation at micro scales, and to reveal their connections with spatial-temporal evolution in macro-scale tectonics.

Calcium isotope fractionation during magmatic processes in the upper mantle

Abstract The Earth’s mantle has a complex history of partial melting and melt-peridotite reaction that have redistributed Ca and other elements between residues and melting products. Given the considerable Ca isotopic variation reported in mantle rocks, evaluation of the fractionation of stable Ca isotopes in magmatic processes in the mantle is critical to decode mantle evolution and the effect of recycled materials. We have performed precise and accurate Ca isotopic analyses on a series of well-characterized spinel-facies mantle peridotites (lherzolite, harzburgite and dunite, n = 29), pyroxenites (websterite, clinopyroxenite and orthopyroxenite, n = 15) and their mineral constituents (n = 8) from the Balmuccia and Baldissero peridotite massifs of the Ivrea Zone in the Italian Alps. These peridotites underwent variable degrees of melting and melt-peridotite reaction, whereas the pyroxenites are mainly the products of melt-peridotite reaction and crystallization of migrating basic melts from the asthenosphere. The lherzolites from Balmuccia and Baldissero show δ44/40Ca values of 0.94 ± 0.11‰ (2sd, n = 22), which are uniform within long-term external reproducibility (±0.14‰, 2sd). The δ44/40Ca values of the harzburgites (0.83‰ to 0.92‰) do not differ from those of lherzolites, including those with a history of intensive melt-peridotite reaction to form replacive dunites. The websterites and spinel clinopyroxenites display δ44/40Ca of 0.86 ± 0.10‰ (n = 14), within the range of the lherzolites and harzburgites. The indistinguishable δ44/40Ca among these very diverse mantle rocks is the consequence of the overwhelming control of stable Ca isotopes by clinopyroxene in the magmatic processes involved, because clinopyroxene dominates the budget of Ca (>90% for harzburgites; 93% to 99% for lherzolites, websterites and clinopyroxenites). Only the clinopyroxene-poor ( 25% melting and replacive dunites tend to display slightly heavier Ca isotopic compositions. Consequently, irrespective of their magmatic history, most fertile mantle rocks from different geological settings display a homogenous Ca isotope composition, summarized as, δ44/40Ca of 0.94 ± 0.10‰ (2sd, n = 47) for the Earth’s mantle. The deviations in Ca isotopic variations observed in other mantle rocks may be attributed to kinetic isotope fractionation and metasomatism by melts with isotopic compositions influenced by recycled crustal materials.

Coexistence of MORB- and OIB-like dolerite intrusions in the Purang ultramafic massif, SW Tibet: A paradigm of plume-influenced MOR-type magmatism prior to subduction initiation in the Neo-Tethyan lithospheric mantle

The Yarlung Zangbo Suture Zone (YZSZ) of South Tibet is divided by the ZhongbaZhada terrane into two subparallel ophiolitic belts in its western end. The peridotite massifs of the southern belt tectonically overlie the Tethyan Himalaya sequence. The Purang peridotite body in this belt is intruded by two groups of dolerite dikes, providing significant compositional, geochronological, and isotopic information about the melting history of the Neo-Tethyan mantle. U-Pb ages of zircons separated from dolerites show that peridotites of West Purang were intruded by an early generation of dikes at 138.5 +/- 2.0 Ma (Valanginian). These dolerites show ocean island basalt (OIB)-type normalized multi-elemental profiles and Sr-Nd isotopic signatures [(La/Yb)(N )= 13-16], high initial Sr-87/Sr-86 ratios (0.70598-0.70765), and low epsilon(Nd)(t) values (-2.6 to -2.3). Zircons separated from this group of dolerites have slightly radiogenic epsilon(Hf)(t) values (+2.6 to +4.6). The next generation of dolerite dikes intruded the East Purang peridotites between 124.5 +/- 2.5 Ma and 124.4 +/- 3.2 Ma (Aptian). These East Purang dolerites show normal mid-ocean ridge basalt (N-MORB)-type normalized multi-element patterns [(La/Yb)(N) = 0.6-0.9] with noticeable negative Nb and Th (+/- Ti) anomalies, and have high Sr-87/Sr-86((i)) (0.70295-0.70618) and high epsilon(Nd)(t) values (+7.7 to +9.2). Zircons separated from the East Purang dolerites show strongly radiogenic epsilon(Hf)(t) values (+3.5 to +17.0).Semiquantitative geochemical modeling demonstrates that the parental magmas of West Purang dolerites were generated from 5 %-10 % polybaric partial melting of a deep-seated juvenile asthenospheric source enriched by plume-type components. In contrast, the parental melts of East Purang dolerites were derived from more than 20% melting of a juvenile spinel-bearing MORB-type mantle source that was modified by subduction-related melts/fluids to a minor extent. A possible tectono-magmatic model for the petrogenesis of the Purang ophiolitic massif could be linked to incipient continental rifting and subsequent oceanic seafloor spreading associated with decompression upwelling of an asthenospheric source contaminated by plume-type components. This plume-proximal seafloor spreading-system was succeeded by the initiation of Neo-Tethyan intra-oceanic subduction close to the active continental margin of Eurasia during the Early Cretaceous.

Shallow submarine seep of abiotic methane from serpentinized peridotite off the Island of Elba, Italy

Abiotic methane (CH4) is today widely reported in gas seeps and hyperalkaline springs in ophiolites and peridotite massifs characterized by low temperature continental serpentinization. Origin and distribution of this gas have far reaching implications in microbiology, astrobiology and carbon cycle. We report an in-depth study of a recently described abiotic CH4 seep occurring in shallow seafloor along the western coast of Elba Island, Tyrrhenian Sea (Italy). The gas is characterized by stable C and H isotopic compositions of CH4 (δ13C ∼ −18‰; δ2H ∼ −141‰) and a very low CO2 content that are typical of abiotic gas in continental ultramafic rock systems. Based on local geothermal gradients, the temperature of methane production is estimated to be below 100 °C. The isotope signature of methane is similar to that occurring in the Liguria region, about 200 km north of Elba Island, where the same ophiolite unit exposed. A mantle CO2 component, suggested by relatively high 3He/4He ratios, has likely acted as CH4 precursor. The reconstruction of the geological-structural setting of Elba ophiolite sequence highlighted that the seep occurs in correspondence with a faulted reverse limb of the antiform of the ophiolite unit. The gas bearing fault forms a contact between mafic and ultramafic serpentinized rocks, as typically observed in other continental seeps and springs related to ophiolites. Magmatic intrusions in the island may have contributed to the C feedstock of methane.

Geochemistry and geochronology of dolerite dykes from the Daba and Dongbo peridotite massifs, SW Tibet: Insights into the style of mantle melting at the onset of Neo-Tethyan subduction

This study reports on compositional, whole-rock SrNd isotopic, and zircon UPb geochronological data for dolerite dykes in the Daba and Dongbo ultramafic massifs of the southwest Yarlung–Zangbo Suture Zone (YZSZ), SW Tibet. The 121 ± 2 Ma dolerite dykes from the Daba peridotite exhibit primitive mantle (PM)-normalised multi-element patterns with (La/Yb)N = 0.43–0.72 and negative Nb anomalies. They have high initial 87Sr/86Sr ratios (87Sr/86Sr(i) = 0.707197 to 0.707879) and high εNd(t) values (+7.4 to +7.9). The 125 ± 2 Ma dolerite intrusions within the Dongbo peridotite show PM-normalised trace element profiles [(La/Yb)N = 0.65–0.84] that are characterised by apparent negative Nb anomalies and moderate negative Ti (±Y) anomalies. They also have high 87Sr/86Sr(i) ratios (0.706108–0.706793) and high εNd(t) values (+7.8 to +8.2). Semi-quantitative Sm/Yb vs. La/Sm modelling demonstrates that the parental magmas of the investigated dykes were derived from 10%–20% (cumulative) melting of a (broad) mantle source region that had a spinel-bearing normal mid-ocean ridge basalt (N-MORB)-like lherzolitic composition. The geochemical and isotopic data indicate that the composition of the inferred mantle source was influenced by minor inputs of subducted crustal material. The petrogenesis of the Daba and Dongbo massifs can be linked to the upwelling of an asthenospheric source that caused continental rifting and subsequent seafloor spreading that was followed by the subduction initiation adjacent to a passive margin during the Early Cretaceous (~130–120 Ma). The results provide a more detailed and perhaps more elegant hypothesis for the tectonomagmatic evolution of the southwestern YZSZ “ophiolitic” peridotites after their accretion beneath a Neo-Tethyan marginal basin.

Eclogites in peridotite massifs in the Western Gneiss Region, Scandinavian Caledonides: Petrogenesis and comparison with those in the Variscan Moldanubian Zone

Eclogite lenses and boudins are volumetrically minor, but petrologically important, features of peridotite massifs worldwide. In the Western Gneiss Region of the Scandinavian Caledonides, eclogites in the Almklovdalen and Raubergvik peridotites originated as basaltic to picrobasaltic dikes, comprising both olivine–normative and nepheline–normative types, with a wide variation in Mg–number from 34 to 65. Positive anomalies for Pb and Sr and negative anomalies for Zr and Hf reflect a subduction signature in the basic melts, and rare–earth element modelling requires 20% to 70% fractional crystallization, combined with 20% to 70% assimilation of peridotite. Clinopyroxenes in eclogites have a wide variation in εNd(0) from +68 to −26, which is comparable to that for associated garnet peridotites and pyroxenites, +55 to −38, and a range in 87Sr/86Sr from 0.7021 to 0.7099, which is much larger than that in peridotites and pyroxenites, 0.7014 to 0.7033.Plagioclase and amphibole inclusions in eclogite garnet provide evidence for prograde metamorphism, which attained a maximum temperature of ~775 °C and pressure of ~25 kb. Such conditions are allofacial with those of associated garnet peridotites and pyroxenites, which equilibrated at ~825 °C and ~37 kb. Eclogites yield mixed Sm-Nd isochron ages, as do the peridotites and pyroxenites, but ages in eclogites are <1000 Ma, and those in peridotites and pyroxenites are >1000 Ma. Three eclogites yield Ordovician U-Pb ages for rutile at 440 ± 12, 445 ± 51, and 480 ± 29 Ma, which are coeval with the Taconic Orogeny and are consistent with a Laurentian provenance for the host peridotites.Eclogites in both Norwegian and Czech peridotites originated from melts passing through a mantle wedge above a subduction zone, and both suites exhibit subduction geochemical signatures, although they differ dramatically in petrogenesis. Eclogites in Norwegian peridotites initially crystallized as relatively low–pressure, plagioclase–bearing basaltic or gabbroic dikes and subsequently recrystallized to high–pressure eclogite, whereas most eclogites in Variscan Moldanubian peridotites crystallized directly from magmas at high pressure to produce eclogite facies assemblages.

Oceanic accretionary belt in the West Qinling Orogen: Links between the Qinling and Qilian orogens, China

We present an integrated study of ophiolite complexes and island arc rocks from the Tianshui-Wushan Accretionary Belt, West Qinling Orogen. The West Qinling Orogen is important because it links the Qinling orogen to the east and the Qilian and Kunlun orogens to the west. The link between these orogens is commonly assumed, but has little study in detail. Zircon U-Pb analyses from ophiolitic rocks indicate the oceanic lithosphere formed in the Cambrian (530–500 Ma). Pillow lavas in the ophiolite complex show geochemical signatures of enriched MORB, suggesting they represent remnants of an oceanic plateau or seamounts. The island arc rocks include a volcanic complex with basalt-andesite and boninite of Late Ordovician age (460–440 Ma), and a serpentinized peridotite massif. The serpentinized peridotite most likely represent a highly refractory mantle residue with subsequent melt-rock interaction at ~450 Ma, suggesting that it formed in a forearc setting. The co-occurrence of ophiolite complexes and island arc rocks indicates that Tianshui-Wushan Accretionary Belt is an oceanic suture zone caused by oceanic subduction in the Early Paleozoic. The island arc rocks most likely represent the early product of an Izu-Bonin-Mariana (IBM)-type intra-oceanic arc, developed in response to a collision between an oceanic plateau and a continental margin. Our study permits a tectonic correlation between the Tianshui-Wushan Accretionary Belt in the West Qinling Orogen and the South Qilian Accretionary Belt in the Qilian Orogen, thereby establishing the continuity between the Early Paleozoic orogenic belts along the southern margin of the North China Craton.

Active channel width as a proxy of sediment supply from mining sites in New Caledonia

AbstractAlthough the channel morphology of upland fluvial systems is known to be strongly controlled by sediment supply from hillslopes, it is still difficult to isolate this effect from the other controlling factors of channel forms, such as the sediment transport capacity (depending notably on the size of the catchment) and local conditions (e.g. confinement, riparian vegetation, valley‐floor slope). The rivers in New Caledonia offer an interesting field laboratory to isolate the morphological effect of contrasted sediment supply conditions. Some of these rivers are known to be highly impacted by the coarse sediment waves induced by the mining of nickel deposits that started in the early 1870s, which was particularly intensive between the 1940s and 1970s. The propagation of the sediment pulses from the mining sites can be traced by the presence of wide and aggraded active channels along the stream network of nickel‐rich peridotite massifs. A first set of 63 undisturbed catchments in peridotite massifs distributed across the Grande Terre was used to fit a classic scaling law between active channel width and drainage area. A second set of 86 impacted sites, where the presence of sediment waves was clearly attested by recent aerial imagery, showed systematically wider active channels, with a width ratio around 5 (established from the intercept ratio of width–area power laws). More importantly, this second set of disturbed sites confirmed that the residual of active channel widths, computed from the scaling law of undisturbed sites, is statistically positively related to the catchment‐scale relative area of major mining sediment sources. It is therefore confirmed that the characterization of sediment supply conditions is crucial for the understanding of spatial patterns of active channel width, and this should be more thoroughly considered in morphological studies of rivers draining environments with contrasted geomorphic activities on hillslopes. © 2018 John Wiley &amp; Sons, Ltd.

Peridotite-derived detrital pyropes versus high-pressure felsic granulite-derived pyrope-almandine garnets from the Lower Triassic deposits of the NE foreland of the Bohemian Massif (S Poland, Central Europe)

Numerous pyrope grains (group I; Prp62–72Alm14–24Grs(<1)4–14Uv(1)2–14Sps<1–1Adr<1–2) have been found in the heavy mineral assemblages from the Older Buntsandstein arenites (Lower Triassic) cropping out in S Poland (NE foreland of the Bohemian Massif). This is the first find of the abundant detrital pyropes outside, but near to, the Bohemian Massif, and the first from the Lower Triassic deposits. In the heavy mineral assemblages from the younger wackes and marly limestones (Older Buntsandstein and Röt deposits), from the same area, Ca-low pyrope-almandines of subgroups IVA (Prp28–52Alm44–68Grs1–4Uv<1Sps1–2Adr<1) and IVB (Prp27–50Alm41–65Grs>5–15Uv<1Sps<1–2Adr<1–1) are the most numerous. The main aims of this study were to determine the primary and secondary source rocks for the garnets, and to identify a cause for the change of the garnet types in the older and younger deposits. Pyropes contained in the Bohemian Massif peridotites, presently exposed in the Moldanubian Zone, and the Kutná Hora Unit, show most comparable chemical compositions to group I pyropes. Some group I pyropes have no chemical counterparts within the comparative materials, which suggests that some other pyrope-bearing peridotites may have been exposed within the Bohemian Massif, but destroyed in the past. Garnets contained in the high-pressure felsic granulites known from several subunits of the Moldanubian Zone show similar chemical compositions to the garnets from subgroup IVA. The group I grains reveal features pointing to (possibly multiple) reworking and likely longer transport distance, while the group IV grains reveal signs of not advanced abrasion and possible short transport distance. However, no older or coeval sedimentary rocks with the similar heavy mineral assemblages, nor the garnet groups, are recognized within the Bohemian Massif or the adjacent areas, which suggests that the garnets may have been recycled from deposits largely destroyed in the past. The deposition of group IV garnets was a normal occurrence for a long time span. In contrast, the deposition of the detrital pyropes was an unusual event. These pyropes must have been redeposited from pyrope-bearing ‘horizons’ present in Carboniferous-Lower Triassic sedimentary rocks. Their availability was limited to the time span necessary for erosion of these horizons.

The distribution of lead and thallium in mantle rocks: Insights from the Balmuccia peridotite massif (Italian Alps)

The distribution of lead and thallium in mantle rocks: Insights from the Balmuccia peridotite massif (Italian Alps)

Circa 1 Ga sub-seafloor hydrothermal alteration imprinted on the Horoman peridotite massif

The chemical compositions of the residues of the mantle melting that produces mid-ocean ridge basalt can be altered by fluid–rock interactions at spreading ridges and, possibly, during seawater penetration along bending-related faults in plates approaching trenches. This chemically modified rock, if subducted deeply and after long-term residence within the deep Earth, is a potential source of chemical heterogeneity in the mantle. Here, we demonstrate that peridotites from the Horoman massif preserve the chemical signatures of sub-seafloor hydrothermal (SSH) alteration at a mid-ocean ridge approximately one billion years ago. These rocks have evolved chemically subsequent to this SSH alteration; however, they retain the SSH-associated enrichments in fluid mobile elements and H2O despite their long-term residence within the mantle. Our results indicate that ancient SSH alteration resulting in the production of sulfide leads to Pb enrichment that could affect the present-day Pb isotopic evolution of the silicate earth. Evidence from the Horoman massif of the recycling of hydrous refractory domains into the mantle suggests that both the flux of H2O content into the mantle and the size of the mantle H2O reservoir are higher than have been estimated recently.

Widespread abiotic methane in chromitites

Recurring discoveries of abiotic methane in gas seeps and springs in ophiolites and peridotite massifs worldwide raised the question of where, in which rocks, methane was generated. Answers will impact the theories on life origin related to serpentinization of ultramafic rocks, and the origin of methane on rocky planets. Here we document, through molecular and isotopic analyses of gas liberated by rock crushing, that among the several mafic and ultramafic rocks composing classic ophiolites in Greece, i.e., serpentinite, peridotite, chromitite, gabbro, rodingite and basalt, only chromitites, characterized by high concentrations of chromium and ruthenium, host considerable amounts of 13C-enriched methane, hydrogen and heavier hydrocarbons with inverse isotopic trend, which is typical of abiotic gas origin. Raman analyses are consistent with methane being occluded in widespread microfractures and porous serpentine- or chlorite-filled veins. Chromium and ruthenium may be key metal catalysts for methane production via Sabatier reaction. Chromitites may represent source rocks of abiotic methane on Earth and, potentially, on Mars.

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

We 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.

Constant Cu/Ag in upper mantle and oceanic crust: Implications for the role of cumulates during the formation of continental crust

Delamination of dense, sulfide-bearing pyroxenitic cumulates was proposed to explain the low Cu content and the evolved major element composition of the continental crust, yet evidence for this hypothesis has been circumstantial. In this study, we present Cu and Ag contents of mantle pyroxenites and associated peridotites from the Balmuccia peridotite massif (Italian Alps) and mantle xenoliths from Hannuoba (North China Craton) to constrain the fractionation behavior of Cu and Ag during melt–peridotite reaction and magmatic accumulation in the mantle. Furthermore, we reexamine the behavior of these elements during arc magmatism and in other reservoirs to highlight the fractionation of Cu and Ag as an important tracer for processes that led to the formation of continental crust.Melt–peridotite reaction and magmatic accumulation led to highly variable sulfide abundances, Cu and Ag contents in the mantle pyroxenites (e.g., Balmuccia, 87–484 μg/g Cu; Hannuoba, 15–116 μg/g Cu). The mean Cu/Ag of the pyroxenite suites (Balmuccia: 3800 ± 1100; Hannuoba: 3100 ± 900, 1s) are indistinguishable from those of their host peridotites and other lherzolites (3500 ± 1200), MORBs (3600 ± 400), and Hawaiian basalts (3200 ± 100). These results reflect the limited fractionation of Cu from Ag during magmatic processes in the upper mantle and during the formation of oceanic crust, and indicate a similar mean Cu/Ag (3500 ± 1000, 1s) in these reservoirs. Experimental data indicate that similar partitioning of Cu and Ag between sulfide melt and silicate melt is responsible for the limited variation of Cu/Ag.Magmatic processes in the convecting mantle and in the oceanic crust, however, cannot explain the low Cu/Ag ratio of the continental crust (about 500). Experimental constraints on sulfide stability in oxidized mantle and data from back-arc magmatic series suggest that formation of primitive back-arc and island arc basalts inherit mantle-like Cu/Ag and thus also cannot explain the low Cu/Ag of the continental crust. In contrast, previous data on evolved back-arc magmas suggest that the low Cu/Ag coupled with a considerable depletion of Cu in the continental crust might be ascribed to the segregation of monosulfide solid solution (MSS) during fractional crystallization of evolving arc magmas. These results thus support the model that accumulation of sulfide-bearing mafic cumulates in the lower crust of magmatic arcs has been a critical process in defining some of the chemical characteristics of the continental crust.

Effects of Melt Percolation on Zn Isotope Heterogeneity in the Mantle: Constraints From Peridotite Massifs in Ivrea‐Verbano Zone, Italian Alps

AbstractWe determined Zn isotopic compositions of 21 orogenic peridotites from the Baldissero and Balmuccia peridotite massifs in Ivrea‐Verbano Zone, Italian Alps, to investigate Zn isotope behaviors during partial melting and melt percolation in the mantle. The samples include lherzolites, harzburgites, and dunites. Lherzolites are strongly depleted in light rare earth element relative to middle and heavy rare earth element with (La/Sm)PM from 0.009 to 0.265 and (La/Yb)PM from 0.003 to 0.125, which can be explained by 5–15% fractional melting of a primitive mantle source. Harzburgites and dunites with nearly identical Mg# (molar 100 * Mg/(Mg + Fe) = 90.2–91.0) have (La/Sm)PM and (La/Yb)PM higher than but Zn contents similar to or lower than those of the parental lherzolites, suggesting that they were influenced by Zn‐depleted silicate melt percolation. Lherzolites have δ66Zn from 0.13 to 0.27‰ showing no correlations with indicators of melt extraction (e.g., Al2O3, Mg#, and La/Yb) and Zn contents. Three sulfide melt‐affected lherzolites show similar δ66Zn to the other normal ones. These observations indicate that 5–15% partial melting and sulfide melt percolation cause limited Zn isotope variations in the mantle. The metasomatic harzburgites and dunites display high δ66Zn (up to 0.46‰) negatively correlated with Zn contents. Such correlations are attributed to kinetic effect during silicate melt percolation, whereby 64Zn preferentially diffuses out from mantle minerals (e.g., olivine) to the percolating silicate melts. A diffusion model suggests that the negative correlation between δ66Zn and Zn contents in dunites can be explained by an empirical βZn (i.e., βZn‐exponent in D66Zn/D64Zn = (m64Zn/m66Zn)βZn) of 0.05–0.06 in olivine.

New Caledonia Obducted Peridotite Nappe: Offshore Extent and Implications for Obduction and Postobduction Processes

AbstractOne of the largest ophiolitic peridotite masses in the world covers a quarter of the island of Grande Terre, New Caledonia. The Peridotite Nappe was obducted during the Eocene, is weakly deformed, and corresponds to the highest of a structurally simple pile of thrust nappes. We present new marine seismic data that allow us to track the offshore continuation of the Peridotite Nappe along strike for a distance of more than 500 km south of New Caledonia and to image its preobduction, synobduction, and postobduction sedimentary records. Offshore, the Peridotite Nappe underlies a ~150 km wide and 2 km deep basin. Flat‐topped horsts of peridotite are clearly bounded by major normal faults; in contrast, faults are obscure onland. To the east, the Peridotite Nappe roots along the eastern margin of the Félicité Ridge (new name), a ~300 × 25 km dome‐shaped ridge, which we interpret as being the southern extension of the high‐pressure/low‐temperature metamorphic core complex observed in New Caledonia. Two alternative tectonic models address the relative timing and relationships between Peridotite Nappe emplacement, uplift of a metamorphic core complex, and extensional tectonics. These models provide new ideas for the understanding the formation of the eastern margin of the Zealandia continent. Our results contribute to an understanding of how oceanic mantle is emplaced onto continental margins.