High Degrees Of Partial Melting Research Articles

Platinum-group elements, S, Se and Cu in highly depleted abyssal peridotites from the Mid-Atlantic Ocean Ridge (ODP Hole 1274A): Influence of hydrothermal and magmatic processes

Highly depleted harzburgites and dunites were recovered from ODP Hole 1274A, near the intersection between the Mid-Atlantic Ocean Ridge and the 15°20′N Fracture Zone. In addition to high degrees of partial melting, these peridotites underwent multiple episodes of melt–rock reaction and intense serpentinization and seawater alteration close to the seafloor. Low concentrations of Se, Cu and platinum-group elements (PGE) in harzburgites drilled at around 35–85 m below seafloor are consistent with the consumption of mantle sulfides after high degrees (>15–20 %) of partial melting and redistribution of chalcophile and siderophile elements into PGE-rich residual microphases. Higher concentrations of Cu, Se, Ru, Rh and Pd in harzburgites from the uppermost and lowest cores testify to late reaction with a sulfide melt. Dunites were formed by percolation of silica- and sulfur-undersaturated melts into low-Se harzburgites. Platinum-group and chalcophile elements were not mobilized during dunite formation and mostly preserve the signature of precursor harzburgites, except for higher Ru and lower Pt contents caused by precipitation and removal of platinum-group minerals. During serpentinization at low temperature (<250 °C) and reducing conditions, mantle sulfides experienced desulfurization to S-poor sulfides (mainly heazlewoodite) and awaruite. Contrary to Se and Cu, sulfur does not record the magmatic evolution of peridotites but was mostly added in hydrothermal sulfides and sulfate from seawater. Platinum-group elements were unaffected by post-magmatic low-temperature processes, except Pt and Pd that may have been slightly remobilized during oxidative seawater alteration.

Arc-related harzburgite–dunite–chromitite complexes in the mantle section of the Sabzevar ophiolite, Iran: A model for formation of podiform chromitites

Podiform chromitites are common within the mantle section of the Late Cretaceous Sabzevar ophiolite in NE Iran. We studied chromitite pods and related ultramafic rocks from three Sabzevar massifs: Baghjar-Kuh Siah, Gaft Chromitite Mine and Forumad peridotite–chromitite. These represent an upper mantle sequence just below the Sabzevar Moho. The Baghjar-Kuh Siah mantle sequence contains plagioclase lherzolites, enriched in bulk REEs, with low Cr# spinels and MORB-like clinopyroxenes. These lherzolites formed due to the impregnation of MORB-like melts. The Gaft and Forumad harzburgites are depleted in trace and rare earth elements and thus are residues after high degree of partial melting (more than exhaustion of Cpx). The Gaft Chromitite Mine includes two types of podiform chromitites, high Cr# and low Cr#. The melt precipitating high Cr# spinel was boninitic whereas the melt forming the low Cr# chromitites was tholeiitic. Most Forumad massif chromitites have high Cr# spinels, although those rich in silicate inclusions are aluminous. Trace and REE element patterns of Forumad harzburgite clinopyroxene are similar to those in supra-subduction zone (SSZ) peridotites while those of Baghjar-Kuh Siah lherzolites are similar to MOR peridotite clinopyroxenes. These mineral data are also consistent with bulk rock trace and rare earth elements composition of their host peridotites. Field observations indicate that early tholeiitic magmas were followed by late boninites, as revealed in chromitite compositions as well as mantle rocks and dikes. We suggest a time-integrated model for the evolution of the Sabzevar mantle sequence during an early stage of subduction initiation associated with formation of an incipient arc. In this scenario, MORB-like melts (forearc basalts) formed first, causing low Cr# chromitites and plagioclase–clinopyroxene impregnations. Subsequent arc-like or boninitic melts with increasing contribution of slab-derived fluids were responsible for the formation of replacive dunites and high Cr# chromitites.

Geochemistry of two stratigraphically-related ultramafic (komatiite) layers from the Neoarchean Sigegudda greenstone terrane, Western Dharwar Craton, India: Evidence for compositional diversity in Archean mantle plumes

Two compositionally different ultramafic units are present in the Neoarchean Sigegudda greenstone terrane, Western Dharwar Craton, India. These ultramafic units occur in the same volcano-sedimentary sequence and are separated by a fault-bounded volcanic arc association. Because of deformation and amphibolite facies metamorphism, the primary igneous textures have been extensively modified in both ultramafic units. Given their spatial and temporal association with sedimentary and volcanic rocks and high MgO contents (12–29wt.%), these ultramafic rocks are interpreted as metamorphosed komatiites and komatiitic basalts. Field relationships and rock types indicate that the lower section of the Sigegudda volcano-sedimentary sequence was deposited in a peri-cratonic, continental rift setting. The lower ultramafic unit is characterized by 12.4–29.6wt.% MgO, Mg#=76–91, and 526–1150ppm Ni. The upper ultramafic unit is overall more magnesian and compositionally restricted: MgO=21.6–25.9wt.%, Mg#=86–89, and Ni=610–1000ppm. The lower unit features a combination of relatively elevated TiO2 (0.40–0.90wt.%), Al2O3/TiO2≤the chondritic ratio of 21, and (Gd/Yb)N ratios >1; these are Ti-enriched komatiites reported for the first time from greenstone belts in the Dharwar Craton. The upper unit is compositionally comparable to Al-undepleted komatiites and characterized by Al2O3/TiO2≥21, LREE-depletion coupled with near-flat chondrite-normalized HREE patterns. Geochemical data indicate that the Sigegudda komatiites and komatiitic basalts were variably contaminated by either continental crust or sub-continental lithospheric mantle, or both. The geochemical differences between the two units are explained by variable depths and degrees of partial melting. The Ti-enriched lower unit appears to have been formed through lower degrees of partial melting at depths ≥90km, whereas the Al-undepleted upper unit was generated by higher degrees of partial melting at depths ≤90km. Field relationships and geochemical characteristics of the Sigegudda komatiites can be explained by a geodynamic model in which the stratigraphically lower Ti-enriched komatiites were erupted from a mantle plume onto a rifting continental margin, whereas the stratigraphically upper Al-undepleted komatiites originated from a younger mantle plume and erupted onto a volcanic arc sequence that accreted to the rifted continental margin.

Multiphase Alkaline Basalts of Central Al-Haruj Al-Abyad of Libya: Petrological and Geochemical Aspects

Al-Haruj basalts that represent the largest volcanic province in Libya consist of four lava flow phases of varying thicknesses, extensions, and dating. Their eruption is generally controlled by the larger Afro-Arabian rift system. The flow phases range from olivine rich and/or olivine dolerites to olivine and/or normal basalts that consist mainly of variable olivine, clinopyroxene, plagioclase, and glass. Olivine, plagioclase, and clinopyroxene form abundant porphyritic crystals. In olivine-rich basalt and olivine basalt, these minerals occur as glomerophyric or seriate clusters of an individual mineral or group of minerals. Groundmass textures are variably intergranular, intersertal, vitrophyric, and flow. The pyroclastic, clastogenic flows and/or ejecta of the volcanic cones show porphyritic, vitrophric, pilotaxitic, and vesicular textures. They are classified into tholeiite, alkaline, and olivine basalts. Three main groups are recorded. Basalts of phase 1 are generated from tholeiitic to alkaline magma, while those of phases 3 and 4 are derived from alkaline magma. It is proposed that the tholeiitic basalts represent prerift stage magma generated by higher degree of partial melting (2.0–3.5%) of garnet-peridotite asthenospheric mantle source, at shallow depth, whereas the dominant alkaline basalts may represent the rift stage magma formed by low degree of partial melting (0.7–1.5%) and high fractionation of the same source, at greater depth in an intra-continental plate with OIB affinity. The melt generation could be also attributed to lithosphere extension associated with passive rise of variable enriched mantle.

Late Cenozoic magmatic transitions in the central Great Xing'an Range, Northeast China: Geochemical and isotopic constraints on petrogenesis

The Wuchagou and Halaha volcanic fields are located in the central Great Xing'an Range, Northeast China, cover an area of ~1400km2 in China, and continue westward into Mongolia. These regions provide an important opportunity to study temporal changes in magma source regions accompanying the evolution of geodynamic-processes. Late Miocene (10.5–8.0Ma) volcanic rocks from the Wuchagou volcanic field are composed of basaltic andesite and andesite. The characteristics of these rocks include a high concentration of Si (SiO2=55.7–59.2wt.%), a pronounced negative Nb―Ta anomaly, and a slightly LREE enriched pattern with (La/Yb)N ratios varying from 2.1 to 6.9. They have Sr and Nd isotopic compositions indicative of long-term incompatible element enrichment (87Sr/86Sr=0.7053–0.7067, 143Nd/144Nd=0.5118 to 0.5122), with a trend toward the EM1 component. Their isotopic compositions can also be described as Dupal-like, but with less-radiogenic Pb isotopic ratios (206Pb/204Pb=16.99–17.83, 207Pb/204Pb=15.46–15.52, 208Pb/204Pb=37.42–37.91). Quaternary (2.0Ma to Holocene) lavas from the Halaha volcanic field consist of alkali olivine basalt and olivine tholeiite, which show alkalic affinities and are characterised by ocean island basalt-like REE and trace element patterns, typical of an intraplate sodic basalt composition. The low 87Sr/86Sr (0.7034–0.7041), high 143Nd/144Nd (0.5128 to 0.5130), and moderately radiogenic 206Pb/204Pb (18.39–18.55), 207Pb/204Pb (15.54–5.55), and 208Pb/204Pb (38.32–38.47) ratios are similar to those of most late Cenozoic basalts of South China and the Abaga region, Inner Mongolia. The different melt compositions between the volcanic rocks of the two volcanic fields are most likely due to changes in geodynamic processes and magma sources. A possible scenario for the genesis of these volcanic rocks is that Wuchagou andesitic magma formed by high degrees of partial melting of the subcontinental lithospheric mantle with some lower-crust contamination as a result of the thermal reactivation of the continental lithosphere by shallow-level rifting following the cessation of the Japan Sea opening. During the Quaternary, extensive volcanism resumed, with eruptions along deep rifting systems in the Halaha volcanic field, which might have been related to the geodynamic forcing of the continuous piling up and upward thickening of the stagnant subducted Pacific slab beneath Northeast China. Basaltic magmas of this stage were primarily produced by melting of convecting asthenosphere. Using the batch melting of a hypothetical LREE-enriched mantle source in the garnet stability field proposed by Xu et al. (2005), we estimated that the Halaha basalts could be generated from 9 to 14% of partial melting of garnet peridotites.

Contribution of AMS measurements in understanding the migmatitic terrains of Pointe Géologie, Terre Adélie (East-Antarctica)

A detailed magnetic mapping using Anisotropy of Magnetic Susceptibility (AMS) technique was carried out in Pointe Géologie archipelago (Terre Adélie, East Antarctica) that represents a hot crust having experienced a long-lived anatectic event during Paleoproterozoic times, 1.69 Ga ago. AMS measurements allowed to better analyse the tectonic structure of the crystalline basement that is built up by rocks affected by various degrees of partial melting and then, devoid of clear strain markers. AMS sampling was performed from main rocks types of Pointe Géologie: migmatites including leucosomes and melanosomes, coarse-grained pink granites, anatexites and mylonitic gneisses. For melanosomes, the magnetic foliation is dominantly in agreement with the observed field foliation, i.e. dominantly N–S sub-vertical in shear zones and gently inclined in dome structures. AMS technique reveals a sub-horizontal magnetic lineation in migmatites from shear zones and a gently plunging one in dome structures. Magnetic properties of leucosomes and of coarse-pink granitic dykes contrast with melanosomes. The bulk susceptibility and anisotropy degree are significantly lower in granitic magmas that in melanosomes. In addition, in well-defined leucosomes, granitic dykes and anatexites, the magnetic ellipsoid is characterized by a higher plunge of the magnetic lineation, which tends to be vertical. This is associated to a rheological contrast between the solid-state deformation suffered by oxide grains in the melanosomes and their reorientation in a viscous flow during the transfer of felsic melt to the granitic dykes. Magnetic structure of leucosomes, granitic dykes and anatexites highlights the role of the gravity-induced upwelling of a crust undergoing high degree of partial melting in a transpressional regime. • Pointe Géologie experienced intensive anatectic event during Paleoproterozoic times. • Melanosomes, felsic leucosomes and anatexites develop contrasted AMS signatures. • Results highlight the role of the gravity-induced magmatic upwelling.

Petrology and Geochemistry of the Crust–Mantle Boundary in a Nascent Arc, Massif du Sud Ophiolite, New Caledonia, SW Pacific

The Massif du Sud ophiolite, New Caledonia, SW Pacific, is one of the largest exposed ultramafic bodies on Earth. The ophiolite consists of a mantle section of ultra-depleted tectonite harzburgite, overlain by a large dunite zone, which is separated by a transition zone from the gabbros at the top of the massif. Profiles through the stratigraphy of the mantle section show complex geochemical and petrological variations. The harzburgites are characterized by a high degree of partial melting and document a complex evolution from mantle exhumation towards a supra-subduction zone environment. Olivine and spinel compositions suggest that the harzburgites are residual after boninite-like melt extraction at 5–10 kbar. The lower dunite zone is analogous to the replacive dunite channels in the harzburgite, in that it is formed by pyroxene consumption. In contrast, the upper dunite zone has chemical and textural characteristics of cumulus olivine crystallized from primitive mafic melts. The upper dunite zone grades into a transition zone with pyroxenite cumulates that are intruded by gabbro sills. The crystallization sequence and mineral compositions indicate that pyroxenites and gabbros formed from hydrous, oxidized primitive basaltic magmas at ∼1250°C and 2–4 kbar. Their geochemistry indicates that these parental melts are transitional between boninites and primitive arc magmas and carry a fore-arc basalt signature. The Massif du Sud therefore represents a crust–mantle section in a nascent arc. The cryptic transition between residual mantle rocks and crustal cumulates highlights the difficulty of estimating the thickness and average composition of an arc crust by seismic methods.

Experimental assessment of the relationships between electrical resistivity, crustal melting and strain localization beneath the Himalayan–Tibetan Belt

Seismic and magnetotelluric field campaigns carried out across the Himalaya and the Tibetan Plateau show mid-crustal low resistivity and low velocity zones. Interpretation of these anomalous observations, either saline fluids or partial melts, is still largely debated partly because experimental data simulating crustal melting under relevant pressure, temperature and water content conditions have not been provided. We report laboratory measurements constraining the resistivity as a function of temperature and the viscosity at 800°C of natural metapelite during partial melting. Dehydration-melting of muscovite is simulated using a Paterson press at 300MPa in the temperature range 600–850°C. Electrical resistivity has been measured from the solid state (<650°C) to 25vol% of partial melt (>800°C) and viscosity has been determined at 800°C. Our results together with recent experimental constraints on seismic properties of partially molten rocks strongly suggest that the electrical and seismic anomalies measured underneath the Himalayan–Tibetan collisional orogen are best explained by partially molten rocks or local accumulation of pure melt bodies. This is also remarkably corroborated by the temperature–depth conditions of crustal partial melting and melt ponding expected from petrological surveys in the Himalayan range. However, our data suggest much higher melt fraction than previously thought and this implies regions in the middle crust having viscosities several orders of magnitude lower than previously assumed. High degree partial melting in the middle crust of the Himalayan–Tibetan orogenic system suggests revision of conceptual models on the development of mountain belts or that geophysical models addressing electrical resistivity at depth must be re-evaluated.

The distribution and origin of smooth plains on Mercury

Orbital images from the MESSENGER spacecraft show that ~27% of Mercury's surface is covered by smooth plains, the majority (&gt;65%) of which are interpreted to be volcanic in origin. Most smooth plains share the spectral characteristics of Mercury's northern smooth plains, suggesting they also share their magnesian alkali‐basalt‐like composition. A smaller fraction of smooth plains interpreted to be volcanic in nature have a lower reflectance and shallower spectral slope, suggesting more ultramafic compositions, an inference that implies high temperatures and high degrees of partial melting in magma source regions persisted through most of the duration of smooth plains formation. The knobby and hummocky plains surrounding the Caloris basin, known as Odin‐type plains, occupy an additional 2% of Mercury's surface. The morphology of these plains and their color and stratigraphic relationships suggest that they formed as Caloris ejecta, although such an origin is in conflict with a straightforward interpretation of crater size–frequency distributions. If some fraction is volcanic, this added area would substantially increase the abundance of relatively young effusive deposits inferred to have more mafic compositions. Smooth plains are widespread on Mercury, but they are more heavily concentrated in the north and in the hemisphere surrounding Caloris. No simple relationship between plains distribution and crustal thickness or radioactive element distribution is observed. A likely volcanic origin for some older terrain on Mercury suggests that the uneven distribution of smooth plains may indicate differences in the emplacement age of large‐scale volcanic deposits rather than differences in crustal formational process.

Rapid forearc spreading between 130 and 120 Ma: Evidence from geochronology and geochemistry of the Xigaze ophiolite, southern Tibet

The Cretaceous Xigaze ophiolite is best exposed at the central Yarlung Zangbo Suture Zone (YZSZ, Tibet) which also includes the Gangdese arc and the Xigaze forearc basin. This study reports new geochronological and geochemical data for this ophiolite to revisit its geodynamic and petrogenetic evolution. The Xigaze peridotites have low CaO and Al2O3 contents and U-shaped Rare Earth Element (REE) patterns, suggesting that they are residues after moderate to high degrees of partial melting and were modified by infiltration of Light Rare Earth Element (LREE)-enriched boninitic melts. The Xigaze crustal rocks belong to two groups: Mid-Ocean Ridge Basalt (MORB)-like rocks and boninitic rocks showing a uniform LREE depletion and flat to LREE enrichment on chondrite-normalized patterns, respectively. Geochemically, both groups show the influence of subducting oceanic slab-derived fluids. LA-ICPMS zircon U–Pb and Lu–Hf analyses from dolerite and quartz diorite dikes, which intruded into the mantle peridotite, and dolerite sheeted sills show that they were generated between 127 and 124Ma. The zircons possess positive εHf(t) values ranging from +7.5 to +17.3. Taking into account the geological and geochronological characteristics of the central-western YZSZ, we propose that ophiolites in this region formed in a forearc spreading setting through rapid slab rollback during subduction initiation between 130 and 120Ma. Following this stage of spreading, the forearc was stabilized and the zone of melting migrated beneath the Gangdese arc producing the voluminous Late Cretaceous granitoids displaying depleted mantle-type Hf isotopic compositions. Our model provides a new explanation for the generation and evolution of forearc-type ophiolites.

Hydration and dehydration in the lower margin of a cold mantle wedge: implications for crust–mantle interactions and petrogeneses of arc magmas

Garnet orthopyroxenites from Maowu (Dabieshan orogen, eastern China) were formed from a refractory harzburgite/dunite protolith. They preserve mineralogical and geochemical evidence of hydration/metasomatism and dehydration at the lower edge of a cold mantle wedge. Abundant polyphase inclusions in the cores of garnet porphyroblasts record the earliest metamorphism and metasomatism in garnet orthopyroxenites. They are mainly composed of pargasitic amphibole, gedrite, chlorite, talc, phlogopite, and Cl-apatite, with minor anhydrous minerals such as orthopyroxene, sapphirine, spinel, and rutile. Most of these phases have high XMg, NiO, and Ni/Mg values, implying that they probably inherited the chemistry of pre-existing olivine. Trace element analyses indicate that polyphase inclusions are enriched in large ion lithophile elements (LILE), light rare earth elements (LREE), and high field strength elements (HFSE), with spikes of Ba, Pb, U, and high U/Th. Based on the P–T conditions of formation for the polyphase inclusions (˜1.4 GPa, 720–850°C), we suggest that the protolith likely underwent significant hydration/metasomatism by slab-derived fluid under shallow–wet–cold mantle wedge corner conditions beneath the forearc. When the hydrated rocks were subducted into a deep–cold mantle wedge zone and underwent high-pressure–ultrahigh-pressure (HP–UHP) metamorphism, amphibole, talc, and chlorite dehydrated and garnet, orthopyroxene, Ti-chondrodite, and Ti-clinohumite formed during prograde metamorphism. The majority of LILE (e.g. Ba, U, Pb, Sr, and Th) and LREE were released into the fluid formed by dehydration reactions, whereas HFSE (e.g. Ti, Nb, and Ta) remained in the cold mantle wedge lower margin. Such fluid resembling the trace element characteristics of arc magmas evidently migrates into the overlying, internal, hotter part of the mantle wedge, thus resulting in a high degree of partial melting and the formation of arc magmas.

Platinum group elements geochemistry of ultramafic and associated rocks from Pindar in Madawara Igneous Complex, Bundelkhand massif, central India

Ultramafic rocks comprising dunite, harzburgite, lherzolite, olivine webserite and websterite occur as intrusives in the form of small hillocks at Pindar into the granite–gneisses of Bundelkhand Gneissic Complex (BnGC). The peridotites are dominated by olivine cumulates where chromite and precious metal-bearing sulphides crystallized along with pyroxenes, subsequent to crystallization of olivine into the interstitial spaces of cumulates during cooling. Ultramafic rocks of Pindar are characterized by high MgO (up to 46.0 wt%) and FeO (up to 5.8 wt%); low SiO2 (40.8 to 48.0 wt%), TiO2 (0.2 to 0.5 wt%), Al2O3 (~3.2 wt% av.), CaO (~2.7 wt% av.) and Cu (11 to 73 μg/g). Cr and Ni values range from 2297 to 3150 μg/g and 2434 to 2767 μg/g, respectively. Distribution of Ir (up to 20 ng/g), Ru (27 to 90 ng/g), Rh (3 to 14 ng/g), Pt (18 to 72 ng/g), Pd (10 to 27 ng/g) and Au (22 to 57 ng/g) indicate platinum group element (PGE) and associated gold mineralization in these ultramafic rocks. A mineral phase representing sperrylite (PtAs2) was also identified within the sulphides in scanning electron microscopy with energy dispersive spectrometer (SEM–EDS) studies. The primitive mantle-normalized siderophile elements pattern shows platinum group element PGE (PPGE) enrichment (Rh, Pt, Pd). Discrimination diagrams of Pd/Ir vs. Ni/Cu, Pd/Pt vs. Ni/Cu, Cu/Pd vs. Pd, and Cu vs. Pd for the peridotites of Pindar attribute to affinity towards komatiite magma, derived from high degree of partial melting of prolonged depleted mantle, and the sulphur saturation condition incurred during the crystallization of chromite which was favourable for PGE mineralization.

Platinum‐Group Element Geochemical Characteristics of the Picrites and High‐Ti Basalts in the Binchuan Area, Yunnan Province

AbstractThe Binchuan area of Yunnan is located in the western part of the Emeishan large igneous province in the western margin of the Yangtze Block. In the present study, the Wuguiqing profile in thickness of about 1440 m is mainly composed of high‐Ti basalts, with minor picrites in the lower part and andesites, trachytes, and rhyolites in the upper part The picrites have relatively higher platinum‐group element (PGE) contents (ΣPGE=16.3–28.2 ppb), with high Cu/Zr and Pd/Zr ratios, and low S contents (5.03–16.9 ppm), indicating the parental magma is S‐unsaturated and generated by high degree of partial melting of the Emeishan large igneous province (ELIP) mantle source. The slightly high Cu/Pd ratios (11 000–24 000) relative to that of the primitive mantle suggest that 0.007% sulfides have been retained in the mantle source. The PGE contents of the high‐Ti basalts exhibit a wider range (ΣPGE=0.517–30.8 ppb). The samples in the middle and upper parts are depleted in PGE and have εNd (260 Ma) ratios ranging from −2.8 to −2.2, suggesting that crustal contamination of the parental magma during ascent triggered sulfur saturation and segregation of about 0.446%–0.554% sulfides, and the sulfide segregation process may also provide the ore‐forming material for the magmatic Cu‐Ni‐PGE sulfide deposits close to the studied basalts. The samples in this area show Pt‐Pd type primitive mantle‐normalized PGE patterns, and the Pd/Ir ratios are higher than that of the primitive mantle (Pd/Ir=1), indicating that the obvious differentiation between Ir‐group platinum‐group elements (IPGE) and Pd‐group platinum‐group elements (PPGE) are mainly controlled by olivine or chromites fractionation during magma evolution. The Pd/Pt ratios of most samples are higher than the average ratio of mantle (Pd/Pt=0.55), showing that the differentiation happened between Pt and Pd. The differentiation in picrites may be relevant to Pt hosted in discrete refractory Pt‐alloy phase in the mantle; whereas the differentiation in the high‐Ti basalts is probably associated with the fractionation of Fe‐Pt alloys, coprecipitating with Ir‐Ru‐Os alloys. Some high‐Ti basalt samples exhibit negative Ru anomalies, possibly due to removal of laurite collected by the early crystallized chromites.

Mineralogy and distribution of Platinum-Group Minerals (PGM) and other solid inclusions in the Faryab ophiolitic chromitites, Southern Iran

High-Cr podiform chromitites hosted by upper mantle depleted harzburgite were investigated for PGM and other solid inclusions from Faryab ophiolitic complex, southern Iran. Chemical composition of the chromian spinels, Cr#[100*Cr/(Cr+Al) = 77–85], Mg# [100*Mg/(Mg+Fe2+) = 56–73], TiO2≤0.25wt%, and the presence of abundant primary hydrosilicates included in the chromian spinels indicate that the deposits were formed from aqueous melt generated by high degree of partial melting in a suprasubduction zone setting. Solid phases hosted by chromian spinel grains from the Faryab ophiolitic chromitites can be divided into three categories: PGM, base-metal minerals and silicates. Most of the studied PGM occurred as very small (generally less than 20 μm in size) primary single or composite inclusions of IPGE-bearing phases with or without silicates and base metal minerals. The PGM were divided into the three subgroups: sulfides, alloys and sulfarsenides. Spinel-olivine geothermometry gives the temperatures 1,131–1,177 °C for the formation of the studied chromitites. At those temperatures, fS2 values ranged from 10−3 to 10−1 and provided a suitable condition for Ru-rich laurite formation in equilibrium with Os-Ir alloys. Progressive crystallization of chromian spinel was accompanied by increase of fS2 in the melt. The formation of Os-rich laurite, erlichmanite and then sulfarsenides occurred by increase of fS2 and slight decrease in temperature of the milieu. The compositional and mineralogical determinations of PGM inclusions respect to their spatial distribution in chromian spinels show that the minerals regularly distributed within the chromitites, reflecting cryptic variation consistent with magmatic evolution during host chromian spinel crystallization.

Melt inclusions in olivine and plagioclase phenocrysts from Antarctic–Phoenix Ridge basalts: Implications for origins of N- and E-type MORB parent magmas

The Antarctic–Phoenix Ridge (APR) is a fossil spreading center in the Drake Passage, Antarctic Ocean. Spreading ceased in chron C2A (ca. 3.3Ma). Although the APR is a normal ridge that is not influenced by a hotspot, enriched (E-type) mid-ocean ridge basalt (MORB) coexists with normal (N-type) MORB in the ridge's axial region. The E-type APR basalt is relatively young (<3.1Ma) compared to the N-type basalt (>3.5Ma). The E-type basalt is characterized by elevated K2O/TiO2 (=0.4–0.8) and (La/Sm)N (=2.2–3.4) ratios, relative to the N-type basalt (K2O/TiO2=0.1–0.3; (La/Sm)N=0.7–0.8). To better understand the compositional variation in the APR basalts and their mantle source regions through time, silicate melt inclusions in primitive olivine (Fo87–89) and plagioclase (An85–89) phenocrysts from the N-type APR basalt were studied. Rehomogenized melt inclusions were analyzed by electron microprobe and LA-ICPMS for major and trace elements. The melt inclusions are more primitive than the host basalt, with Mg#s from 67.5 to 74.1. All inclusions exhibit patterns that are depleted in the light rare earth elements. The inclusions have K2O/TiO2 from 0.1 to 0.3 and (La/Sm)N ratios from 0.4 to 0.9; these values overlap with those of the N-type APR basalt. Furthermore, the melt inclusions have elevated (Lu/Hf)N and (Sm/Nd)N ratios compared to the E-type basalts. The N-type APR basalts do not contain any melt inclusions that are enriched in incompatible elements. The E-type basalt was generated by a low degree of partial melting of a relatively incompatible-element-enriched mantle source. In contrast, chemistries of melt inclusions and N-type basalts are compatible with high degrees of partial melting of an increasingly depleted mantle source. Assuming a veined or otherwise heterogeneous mantle, the absence of E-type inclusions from the N-type host has implications for cyclic magmatic activity beneath the APR. Multi-stage mantle melting and melt extraction from a composite source with sequential extraction of melt fluids might give rise to the primary melt diversity documented in the APR axis. The mantle source of the N-type melts may have been the residue from an earlier phase of melting that removed the easily melted, enriched components. The N-type APR basalt studied represents melt at the end of single cycle, whereas the E-type basalt may represent the early stage of a new pulse that was dominated by highly enriched components.

Mars Hesperian Magmatism as Revealed by Syrtis Major and the Circum-Hellas Volcanic Province

Review of morphologic, morphometric and compositional data from Mars suggests that volcanism in the early Hesperian Syrtis Major edifice was predominantly ultramafic, in contrast to the abundant basaltic volcanism of the Hesperian to Amazonian Tharsis and Elysium provinces. Comparisons of edifice characteristics between Syrtis Major and the large, circum-Hellas Noachian to Hesperian volcanoes suggest that these structures may also be formed by ultramafic volcanic activity. The data suggest that a global scale magma compositional change occurred on Mars during the late Hesperian. The occurrence of widespread ultramafic volcanism suggests that a high degree of partial melting in a relatively hot mantle characterized Mars’ early thermal history, conditions that may be analogous to those that prevailed in the Archean Earth.

Textures and geochemistry of the Saramta peridotites (Siberian craton): Melting and refertilization during early evolution of the continental lithospheric mantle

The Saramta peridotite massif is located within the Sharyzhalgai complex, SW margin of the Siberian craton. The Saramta massif was formed in the Archean and then juxtaposed with granulites of crystalline basement of the Siberian craton. The Saramta harzburgites are highly refractory in terms of lack of residual clinopyroxene, olivine Mg-number (up to 0.937), and spinel Cr-number (∼0.5), suggesting high degree of partial melting. Detailed study of their microstructures shows that they have extensively reacted with a SiO2-rich melt, leading to the crystallization of orthopyroxene, clinopyroxene, amphibole and spinel at the expense of olivine. The major element compositions of the least reacted harzburgites are similar to the residues of refractory peridotites produced by the fractional melting (initial melting pressures >3GPa and melt fractions ∼40%). Moreover, non-residual clinopyroxenes are highly depleted in Yb, Zr and Ti, but highly enriched in LREE. A two-stage history is proposed for the Saramta peridotite: (1) primitive mantle underwent depletion in the garnet stability field followed by melting in the spinel stability field; (2) refractory harzburgites underwent refertilization by SiO2-rich melt in supra-subduction zone. Rare Saramta lherzolites probably formed from more refractory harzburgites as a result of such a melt–rock reaction. The Saramta peridotites are similar to low-T coarse-grained peridotites of subcratonic mantle. Processes of their formation, as reflected by textures and composition of minerals of the Saramta peridotites, are characteristic of the early stages of subcratonic mantle formation.

Geochemistry of the Neoarchaean Volcanic Rocks of the Kilimafedha Greenstone Belt, Northeastern Tanzania

The Neoarchaean volcanic rocks of the Kilimafedha greenstone belt consist of three petrological types that are closely associated in space and time: the predominant intermediate volcanic rocks with intermediate calc-alkaline to tholeiitic affinities, the volumetrically minor tholeiitic basalts, and rhyolites. The tholeiitic basalts are characterized by slightly depleted LREE to nearly flat REE patterns with no Eu anomalies but have negative anomalies of Nb. The intermediate volcanic rocks exhibit very coherent, fractionated REE patterns, slightly negative to absent Eu anomalies, depletion in Nb, Ta, and Ti in multielement spidergrams, and enrichment of HFSE relative to MORB. Compared to the other two suites, the rhyolites are characterized by low concentrations of TiO2 and overall low abundances of total REE, as well as large negative Ti, Sr, and Eu anomalies. The three suites have a εNd (2.7 Ga) values in the range of −0.51 to +5.17. The geochemical features of the tholeiitic basalts are interpreted in terms of derivation from higher degrees of partial melting of a peridotite mantle wedge that has been variably metasomatized by aqueous fluids derived from dehydration of the subducting slab. The rocks showing intermediate affinities are interpreted to have been formed as differentiates of a primary magma formed later by lower degrees of partial melting of a garnet free mantle wedge that was strongly metasomatized by both fluid and melt derived from the subducting oceanic slab. The rhyolites are best interpreted as having been formed by shallow level fractional crystallization of the intermediate volcanic rocks involving plagioclase and Ti-rich phases like ilmenite and magnetite as well as REE-rich phases like apatite, zircon, monazite, and allanite. The close spatial association of the three petrological types in the Kilimafedha greenstone belt is interpreted as reflecting their formation in an evolving late Archaean island arc.

Geochemistry of gabbros and granitoids (M- and I-types) from the Nubian Shield of Egypt: Roots of Neoproterozoic intra-oceanic island arc

The Neoproterozoic intrusive rocks of the Wadi Ranga area, Nubian Shield of Egypt, comprise gabbros and granitoids emplaced during oceanic island arc and post-collision stages. The plutonic rocks of the island arc stage include hornblende gabbros (Dabbah pluton), trondhjemite (Abu Ghalaga pluton) and tonalites with subordinate quartz gabbro and quartz diorite (Reidi and Abu Ghusun plutons), whereas the post-collision intrusives include granodiorite and monzogranite (Helifi-Hamata pluton). The gabbros and granitoids of the island arc stage are largely calcic, low-K rocks which have either tholeiitic (gabbro and trondhjemite) or transitional tholeiitic to calcalkaline nature (tonalites). On the other hand, the granitoids of the post-collision stage are medium to high-K calcalkaline rocks. All the investigated granitoids are metaluminous. The spider diagrams, with enrichment in LILE and strong Nb depletion, and the almost flat to slightly LREE-depleted REE patterns of the gabbro and trondhjemite are similar to those of the Wadi Ranga low-K tholeiitic basalts and silicic volcanics, respectively, suggesting that the gabbro and trondhjemite are the plutonic equivalents of the Wadi Ranga immature island arc extrusives, and they were derived from mantle source at the early immature island arc stage. Similar to the trondhjemite, the tonalites show LILE enrichment and strong Nb depletions on the MORB-normalized spider diagrams. However, the tonalites have REE patterns which are enriched in LREE (La/Yb=1.71–5.54). The derivation of the tonalites through fractionation of the same magma produced the trondhjemite seems unlikely. Therefore, high degree partial melting of juvenile basaltic arc crust is favoured for the origin of tonalites during a late immature island arc stage. The post-collision granitoids show considerable enrichment in LILE and to a lesser extent in HFSE, slight negative Nb anomaly and strong negative P and Ti anomalies relative to N-MORB. Their REE patterns are LREE-enriched (La/Yb=5–19), with negative Eu anomaly. These characteristics are consistent with origin through lower degrees of partial melting of old basaltic arc crust and subsequent fractional crystallization. The geochemical characteristics of the trondhjemite and tonalites, and the granodiorite–monzogranite classify them as M-type and I-type granitoids, respectively. The partly tholeiitic intrusives of the Wadi Ranga area (South Eastern Desert) have lower K2O, Rb and LREE compared to the M-type calcalkaline intrusives of the North Eastern Desert, implying northwardly dipping subduction zone.The geochemical similarities between the intrusives of Neoproterozoic and Phanerozoic oceanic island arcs imply that they share similar style of subduction, which differs from that of the Archaean. The generation of high SiO2 (up to 74.5wt%), low K2O (0.56–1.78wt%) and slightly LREE-depleted trondhjemite in early immature oceanic island arc setting supports the arc origin of the primitive continental crust. Silicic magma production through partial melting of the early arc volcanic rocks during the evolution of the arc and the post-collision stage, drives the middle and upper oceanic arc crust towards a composition closer to that of the continental crust. The present study indicates that the intra-oceanic island arcs continued to play a role in the generation of the continental crust after the Archaean.

Significance of chromian spinels from the mantle sequence of the Andaman Ophiolite, India: Paleogeodynamic implications

The mantle section of the Andaman Ophiolite between Rutland Island in the south and north Andaman in the north has been studied. The restitic peridotite of Rutland Island is comprised of depleted harzburgite to clinopyroxene-bearing harzburgite whereas that in middle and north Andaman is mostly less-depleted, lherzolite-dominated mantle. Chromian spinels from the mantle section of the Andaman Ophiolite belong to four major groups: Group-1 — spinels from Rutland chromitite pods, characterized by high Cr# [=Cr/(Cr+A1) atomic ratio] (~0.71); Group-2 — spinels from north Andaman chromitite pods with two subgroups, 2A — characterized by high Cr# (0.70–0.79) and 2B — characterized by medium Cr# (0.48–0.51) and high Mg# [=Mg/(Mg+Fe2+) atomic ratio] (0.74–0.77); Group-3 — residual accessory grains from Rutland peridotite, characterized by medium Cr# (0.37–0.55) and low Mg# (0.41–0.54); and Group-4 — residual accessory grains from middle and north Andaman peridotites, characterized by low Cr# (0.09–0.23) and high Mg# (0.71–0.81).Group-1 chromian spinels of Rutland Island and Group-2A of north Andaman are likely to have crystallized from boninitic melts, formed by relatively high degrees of partial melting, whereas Group-2B varieties from north Andaman are from tholeiitic melts, formed by lower degrees of melting. The chemistry of the residual accessory chromian spinels (Groups 3 and 4) suggests that the mantle peridotites of Rutland Island towards the south are similar to fore-arc peridotites of suprasubduction zone environments whereas those of north Andaman are less depleted. This spatial and/or temporal directional change in spinel compositions may reflect variations linked to the melting history where the same sliver of oceanic mantle underwent different styles of melting in different tectonic settings at different points in time. The coexistence of both chromitite types, with high- and low-Cr spinels, in the same area from north Andaman possibly reflects temporal variations of separate melt intrusions produced through specific melting stages. It is inferred that a similar geodynamic setting to the present-day also existed during the Cretaceous period with a fore-arc at Rutland Island and a back-arc towards the north at north Andaman.