Olivine Fractionation Research Articles

Zircon U–Pb age, geochemical, and Sr–Nd–Hf isotopic constraints on the origin of mafic dykes in the Shaanxi Province, North China Craton, China

Mafic dolerite dykes form a series of swarms that are widespread across the North China Craton (NCC). We present U–Pb zircon ages, geochemical data, and Sr–Nd–Hf isotopic data for representative samples of the Shaanxi dolerite dykes in the southern NCC. Laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) U–Pb zircon analyses for three samples yield ages ranging from 448.1±1.2 to 489.6±0.9Ma (i.e., Cambro-Ordovician). The dolerites are characterised by a wide range of rock compositions. They display enrichments in light rare earth element (LREEs) and large ion lithophile element (LILE) (i.e., Ba, U, K and Pb), as well as depletion in high field strength (HFSE) (Nb, Ta, Hf and Ti). The mafic dykes have relatively uniform (87Sr/86Sr)i ranging from 0.7049 to 0.7076, (176Hf/177Hf)i from 0.282187 to 0.282236, low εNd (t) values, from −4.1 to −4.9, εHf (t) values of between −7.9 and −9.8, for zircon, and high hafnium model ages (TDM1=1366–1535Ma, TDM2=1924–2081Ma). These results suggest that the dykes are derived by partial melting of an enriched, lithospheric mantle source. The magmas underwent fractionation of olivine, pyroxene, plagioclase, and Ti-bearing phases (rutile, ilmenite, titanite), along with crustal contamination. The formation of the Shaanxi Province, NCC mafic dykes can be attributed to the collision between the NCC and south Qinling Block. Specifically, these magmas formed as a result of crustal thinning in response to extension that followed this collision.

Formation of thick stratiform Fe‐Ti oxide layers in layered intrusion and frequent replenishment of fractionated mafic magma: Evidence from the Panzhihua intrusion, SW China

AbstractThe Panzhihua intrusion is one of the largest layered intrusions that hosts huge stratiform Fe‐Ti oxide layers in the central part of the Emeishan large igneous province, SW China. Up to 60 m thick stratiform massive Fe‐Ti oxide layers containing ~85 modal% of magnetite and ilmenite and overlying magnetite gabbro compose cyclic units of the Lower Zone of the intrusion. The cyclic units of the Middle Zone consist of magnetite gabbro and overlying gabbro. In these cyclic units, contents of Fe2O3(t), TiO2 and Cr and Fe3+/Ti4+ ratio of the rocks decrease upward, Cr content of magnetite and forsterite percentage of olivine decrease as well. The Upper Zone consists of apatite gabbro characterized by enrichment of incompatible elements (e.g., 12–18 ppm La, 20–28 ppm Y) and increasing of Fe3+/Ti4+ ratio (from 1.3 to 2.3) upward. These features indicate that the Panzhihua intrusion was repeatedly recharged by more primitive magma and evolved magmas had been extracted. Calculations using MELTS indicate that extensive fractionation of olivine and clinopyroxene in deep level resulted in increasing Fe and Ti contents in the magma. When these Fe‐Ti‐enriched magmas were emplaced along the base of the Panzhihua intrusion, Fe‐Ti oxides became an early crystallization phase, leading to a residual magma of lower density. We propose that the unusually thick stratiform Fe‐Ti oxide layers resulted from coupling of gravity settling and sorting of the crystallized Fe‐Ti oxides from Fe‐Ti‐enriched magmas and frequent magma replenishment along the floor of the magma chamber.

Mantle source, magma differentiation and sulfide saturation of the ∼637 Ma Zhouan mafic–ultramafic intrusion in the northern margin of the Yangtze Block, Central China

The sill-like Zhouan mafic–ultramafic intrusion in the northern margin of the Yangtze Block formed at 637±4Ma and consists of a lherzolite unit and an olivine gabbronorite unit, with a marginal zone between the lower margin of the olivine gabbronorite unit and the underlying country rocks composed of marble and schist of the Meso- to Neoproterozoic Zhujiashan Formation.Rocks from the lherzolite unit have 41.4–44.0wt.% SiO2 and 29.0–38.4wt.% MgO and contain olivine with Fo values ranging from 80 to 85mole%, indicating crystallization from a moderately evolved magma. They commonly have <0.1wt.% S and low, variable PGE (2–40ppb) and display flat primitive mantle-normalized chalcophile element patterns with relatively low Cu/Pd ratios (3900–427,415), indicating derivation possibly from S-undersaturated magmas. Rocks from the olivine gabbronorite unit contain 44.0–47.1wt.% SiO2 and 22.5–28.9wt.% MgO and contain olivine with relatively low Fo values (67–68mole%), indicating crystallization from a more evolved magma. They contain 0.03–0.9wt.% S and <15ppb PGE and have Cu/Pd ratios (462,500–10,915,500) much higher than those for the rocks from the lherzolite unit. Rocks from the olivine gabbronorite unit show relatively steep, trough-like patterns depleted in PGE relative to Ni and Cu, indicating that they formed from sulfide-saturated magmas. Rocks from the marginal zone have relatively high SiO2 (45.1–53.1wt.%) and low MgO (7.4–19.6wt.%) and have chemical composition varying between the olivine gabbronorite and the country rocks. They contain 0.57wt.% S and <10ppb PGE, with high Cu/Pd ratios (404,300) and chalcophile element patterns similar to the olivine gabbronorite unit. Rocks from the lherzolite unit have (Th/Yb)PM ratios (3.7–9.1) much lower than the upper crust (28.2) and the country rocks (41.3), indicating small degrees of crustal contamination. Their ɛNd(t) (−4.4 to +1.1) and γOs(t) (8–36) values indicate an EMI mantle source. Primary magma was PGE-poor because sulfide retained in the mantle source due to small degrees of partial melting of the enriched mantle source.We propose that moderately evolved magmas may have reached sulfide saturation due to fractionation of olivine in a deep-seated staging magma chamber, forming an olivine-laden crystal mush with minor sulfide droplets in the lower part and more evolved, PGE-depleted magmas in the upper part of the chamber. The evolved, PGE-depleted magmas were forced out of the staging chamber and entered the shallow magma chamber to form the olivine gabbronorite unit. The lherzolite unit formed from the emplacement of the olivine-laden crystal mush into the same chamber. Because of paucity of extensive crustal contamination, only minor amounts of sulfide segregation occurred in the olivine gabbronorite unit and the marginal zone. The Zhouan intrusion thus may not be a good target for economic Ni–Cu sulfide deposit.

Geochronology and geochemistry of the Early Cretaceous Jigongshan and Qijianfeng batholiths in the Tongbai orogen, central China: implications for lower crustal delamination

The Jigongshan and Qijianfeng batholiths in the Tongbai orogen consist mainly of porphyritic hornblende-biotite monzogranite, biotite monzogranite, and biotite syenogranite, which are variably intruded by lamprophyre, diorite, and syenogranite dykes. Mafic microgranular enclaves commonly occur in the hornblende-biotite monzogranite, whereas surmicaceous enclaves are found in the biotite monzogranite. Both batholiths have zircon U–Pb ages ranging from ca. 139 to 120 Ma, indicating their emplacement in the Early Cretaceous. The hornblende-biotite monzogranite has an adakitic affinity marked by relatively high Sr/Y and (La/Yb)N ratios, lack of Eu anomalies, low MgO and Ni contents, and Na2O > K2O. Its chemical compositions, combined with enriched Sr–Nd isotopic signatures, suggest formation by dehydration melting of mafic rocks in a thickened lower crust. This thickened crust resulted from the Permo-Triassic subduction-collision between the North China and South China blocks and persisted until the Early Cretaceous. The biotite monzogranite and biotite syenogranite have low Al2O3, CaO, and Sr contents, low Rb/Sr, FeOt/MgO, and (Na2O + K2O)/CaO ratios, and flat HREE patterns with moderate to weak Eu anomalies. They were produced by partial melting of crustal materials under relatively low pressure. Partial melting at different crustal levels could have significantly contributed to mechanical weakening of the crust. The diorite and lamprophyre dykes show linear trends between SiO2 and major or trace elements on Harker diagrams, with two lamprophyre samples containing normative nepheline and olivine. These rocks have high La/Yb and Dy/Yb ratios, both displaying co-variation with contents of Yb. They were originated from relatively deep lithospheric mantle followed by fractionation of olivine + clinopyroxene + apatite + Fe–Ti oxides. Extensive partial melting in the lithospheric mantle indicates relatively high temperatures at this level. We suggest that the presence of adakitic magmas, thickened but weakened crust and high temperatures in the lithosphere mantle point to lower crustal delamination in the Early Cretaceous in the Tongbai orogen.

Variações composicionais de olivinas do complexo alcalino-carbonatítico de Salitre, MG

The Salitre alkaline-carbonatite-phoscorite complex belongs to the Alto Paranaíba Igneous Province and consists of three separate bodies. Salitre I is a central, kidney-shaped body composed dominantly of bebedourites, with swarms of ring dykes of carbonatites and phoscorites; Salitre II and III are bebedourite-dominated smaller intrusions, respectively to the north and to the south of Salitre I. Olivine occurs in bebedourites, phoscorites and carbonatites, with forsterite content ranging from 83 to 98 mol. %. The least evolved rocks (bebedourites) show lower forsterite content than the more evolved members (phoscorites and carbonatittes), and olivine from carbonatites have the highest forsterite content. The compositional range of forsterite content observed in bebedourites is related to crystal fractionation. In phoscorites, forsterite content increases with decreasing whole-rock MgO, and in carbonatites it decreases, with the decreasing whole-rock MgO. Variations in CaO and MnO may occur in Salitre olivines. The controls of their distribution are unclear and may suggest local variations in the concentration of these elements in the magma, rather than a crystallographic control of olivine. NiO contents show differences between olivines from bebedourites, and olivines from phoscorites and carbonatites. This discrepancy may be related to the removal of Ni from the system by early fractionation of Ni-rich olivine from a silicate parental magma or, alternatively, by Ni removal in an immiscible sulfide liquid.

Petrogenesis of Archean PGM-bearing chromitites and associated ultramafic–mafic–anorthositic rocks from the Guelb el Azib layered complex (West African craton, Mauritania)

The Archean Guelb el Azib layered complex (GAC) in the West African craton of Mauritania is composed of an association of serpentinites, chromitites, amphibolites and anorthosites with few fine-grained amphibolite dykes. The complex forms tectonic slices in 2.9–3.5Ga TTG gneiss terrains in close association with supracrustal rocks (BIFs, impure marbles, amphibolites). It was affected by a main granulite-facies grade metamorphism (up to 900°C at 5–6kbar) with subsequent retrogression in amphibolite and greenschist facies conditions.The preserved igneous macrostructures, the mineral compositions and the nature of relic magmatic assemblages have been used to constrain the composition of the parental melts and the conditions of crystallization. According to petrological observations and to comparison with experimental data, the formation of the complex can be explained by fractionation of a slightly hydrous high-alumina basaltic melt at low pressure. The early fractionation of olivine and the absence of massive clinopyroxene fractionation before plagioclase saturation led to crystallization of highly calcic plagioclase with Fe-, Al-rich but Cr-poor chromite from a hydrous tholeiitic parental magma, similar to worldwide Archean tholeiites.The complex shares many similarities with Archean anorthosite layered complexes, possibly formed in a supra-subduction zone environment according to results obtained on similar 2.9–3.0Ga complexes from Greenland and India (namely Fiskenaesset and Sittampundi). Three phases of PGE mineralization affected the GAC chromitites: (i) igneous crystallization of laurite; (ii) formation of late magmatic IPGE sulpho-arsenides (irarsite–hollingworthite) and (iii) hydrothermal Pt–Pd mineralization represented by sperrylite and rustenburgite.

A New View on the Petrogenesis of the Oman Ophiolite Chromitites from Microanalyses of Chromite-hosted Inclusions

To decipher the petrogenesis of chromitites from the Moho Transition Zone of the Cretaceous Oman ophiolite, we carried out detailed scanning electron microscope and electron microprobe investigations of ∼500 silicate and chromite inclusions and their chromite hosts, and oxygen isotope measurements of seven chromite and olivine fractions from nodular, disseminated, and stratiform ore bodies and associated host dunites of the Maqsad area, Southern Oman. The results, coupled with laboratory homogenization experiments, allow several multiphase and microcrystal types of the chromite-hosted inclusions to be distinguished. The multiphase inclusions are composed of micron-size (1–50 μm) silicates (with rare sulphides) entrapped in high cr-number [100Cr/(Cr + Al) up to 80] chromite. The high cr-number chromite coronas and inclusions are reduced (oxygen fugacity, fO2, of ∼3 log units below the quartz–fayalite–magnetite buffer, QFM). The reduced chromites, which crystallized between 600 and 950°C at subsolidus conditions, were overgrown by more oxidized host chromite (fO2 ≈ QFM) in association with microcrystal inclusions of silicates (plagioclase An86, clinopyroxene, and pargasite) that were formed between 950 and 1050°C at 200 MPa from a hydrous hybrid mid-ocean ridge basalt (MORB) melt. Chromium concentration profiles through the chromite coronas, inclusions, and host chromites indicate non-equilibrium fractional crystallization of the chromitite system at fast cooling rates (up to ∼0·1°C a−1). Oxygen isotope compositions of the chromite grains imply involvement of a mantle protolith (e.g. serpentinite and serpentinized peridotite) altered by seawater-derived hydrothermal fluids in an oceanic setting. Our findings are consistent with a three-stage model of chromite formation involving (1) mantle protolith alteration by seawater-derived hydrothermal fluids yielding serpentinites and serpentinized harzburgites, which were probably the initial source of chromium, (2) subsolidus crystallization owing to prograde metamorphism, followed by (3) assimilation and fractional crystallization of chromite from water-saturated MORB. This study suggests that the metamorphic protolith assimilation occurring at the Moho level may dramatically affect MORB magma chemistry and lead to the formation of economic chromium deposits.

Low oxygen fugacity dependency for the deformation of partially molten lherzolite

We present the results of an experimental investigation of the influence of oxygen fugacity on the deformation of a partially molten spinel lherzolite using a 0.1MPa gas-media creep rig under temperatures of 1160–1190°C, stresses of 4–74MPa and well-controlled oxygen fugacities of 10−7 to 10−11MPa. The partially molten spinel lherzolite was deformed by a dislocation-mediated creep process such as dislocation creep or dislocation-accommodated grain boundary sliding (DGBS) (n=3.5±0.3) as well as in the diffusion creep regime (n=1.2±0.2). The average oxygen fugacity exponent of lherzolite is 0.04±0.02, which is significantly smaller than the values measured for olivine single crystals (0.10–0.36) and dunite rocks (0.20) deformed by dislocation creep. We attribute the low oxygen fugacity exponent of partially molten lherzolite samples either to the operation of grain boundary sliding, if DGBS dominates the deformation, or to the presence of pyroxenes whose deformation has a weak or no dependency on oxygen fugacity, if dislocation creep dominates the deformation. In the latter case, the oxygen fugacity exponent decreases rapidly with decreasing volume fraction of olivine. The low oxygen fugacity exponent for our partially molten lherzolite samples implies that dislocation creep or DGBS of mantle peridotite will most likely be insensitive to variations of oxygen fugacity in the upper mantle.

The influence of magmatic differentiation on the oxidation state of Fe in a basaltic arc magma

Subduction zone basalts are more oxidized than basalts from other tectonic settings (e.g., higher Fe3+/∑Fe), and this contrast may play a central role in the unique geochemical processes that generate arc and continental crust. The processes generating oxidized arc magmas, however, are poorly constrained, although they appear inherently linked to subduction. Near-surface differentiation processes unique to arc settings might drive oxidation of magmas that originate in equilibrium with a relatively reduced mantle source. Alternatively, arc magmas could record the oxidation conditions of a relatively oxidized mantle source. Here, we present new measurements of olivine-hosted melt inclusions from a single eruption of Agrigan volcano, Marianas, in order to test the influence of differentiation processes vs. source conditions on the Fe3+/∑Fe ratio, a proxy for system oxygen fugacity (fO2). We determined Fe3+/∑Fe ratios in glass inclusions using μ-XANES and couple these data with major elements, dissolved volatiles, and trace elements. After correcting for post-entrapment crystallization, Fe3+/∑Fe ratios in the Agrigan melt inclusions (0.219 to 0.282), and their modeled fO2s (ΔQFM +1.0 to +1.8), are uniformly more oxidized than MORB, and preserve a portion of the evolution of this magma from 5.7 to 3.2wt.% MgO. Fractionation of olivine±clinopyroxene±plagioclase should increase Fe3+/∑Fe as MgO decreases in the melt, but the data show Fe3+/∑Fe ratios decreasing as MgO decreases below 5wt.% MgO. The major element trajectories, taken in combination with this strong reduction trend, are inconsistent with crystallization of common ferromagnesian phases found in the bulk Agrigan sample, including magnetite. Rather, decreasing Fe3+/∑Fe ratios correlate with decreasing S concentrations, suggesting that electronic exchanges associated with SO2 degassing may dominate Fe3+/∑Fe variations in the melt during differentiation. In the case of this magma, the dominant effect of differentiation on magmatic fO2 is reduction rather than oxidation. Tracing back Agrigan melts with MgO>5wt.% (i.e., minimally degassed for S) along a modeled olivine fractionation trend to primary melts in equilibrium with Fo90 olivine reveals melts in equilibrium with the mantle beneath Agrigan at fO2s of ΔQFM +1 to +1.6, significantly more oxidized than current constraints for the mantle beneath mid-ocean ridges.

Age, geochemical characteristics and petrogenesis of Late Cenozoic intraplate alkali basalts in the Lut–Sistan region, eastern Iran

Miocene to Quaternary alkali basalts in the Lut–Sistan region, eastern Iran are spatially associated with two active, N–S-trending dextral strike–slip fault systems in the region, i.e., the Neh faults in the Sistan suture zone and the Nayband fault ~200km further to the west in the Lut block. Here, we present new 40Ar/39Ar ages, geochemical and Sr–Nd isotopic data for these rocks to decipher the petrogenetic processes responsible for their formation and regional tectonic implications. Our new ages indicate that the volcanism commenced at ~14Ma in the western Lut region and ~11Ma in the northern Sistan suture zone. The rocks are composed dominantly of hawaiites and mugearites with minor basanites and basaltic andesites. Petrographic observations and major and trace elemental variations suggest that the alkali basalts underwent variable fractionation of olivine, clinopyroxene and Fe–Ti oxides. Chondrite-normalized rare earth element and mantle-normalized trace element patterns of these rocks largely resemble those of ocean island basalts. High εNd(t) (+1.4 to +3.6), low to moderate initial Sr isotopic ratios (0.7047–0.7073), and trace element ratios indicate that crustal contamination was insignificant in the petrogenesis. The rocks have neither geochemical features pointing to residual hornblende or phlogopite, nor arc-related signatures characteristic of the Iranian sub-continental lithospheric mantle. Thus, the alkali basalts most likely have asthenospheric origin. Modeling of REE suggests that they could have formed by low degrees of partial melting (~3–10%) of an enriched mantle source at garnet-stable depths. We propose that the east Iranian alkali basaltic volcanism was triggered by asthenospheric upwelling in an extensional setting, presumably caused by delamination of thickened lithospheric root following the Late Cretaceous collision between the Lut and Afghan continental blocks. Our results imply that two contrasting tectonic regimes coexist in Iran since the Middle Miocene, i.e., extensional in eastern and compressional in southwestern Iran.

How Small-volume Basaltic Magmatic Systems Develop: a Case Study from the Jeju Island Volcanic Field, Korea

Jeju is a volcanic field that has erupted from around 1·8 Myr to c. 1 kyr ago. Activity began with dispersed, basaltic, monogenetic, phreatomagmatic eruptions. Continuing monogenetic volcanism was later joined by more voluminous lava effusion events building a central composite shield. Samples from older (40·7 Ma) and younger (50·2 Ma) monogenetic centres were analysed for their whole-rock major element, trace element and Sr^Nd^Pb isotopic compositions. Pyroclastic products from the monogenetic centres are dominantly alkali basalt to trachybasalt, whereas the more voluminous lava flows and domes of the central edifice consist of subalkali basalt and alkali basalt to trachyte. Lavas from the Early Pleistocene monogenetic centres are depleted in MgO, Cr and Ni, reflecting considerable olivine fractionation. By contrast, Late Pleistocene^Holocene monogenetic centre magmas fractionated clinopyroxeneþ olivine at deeper levels. Isotopic compositions show little variation across the suite; however, the Late Pleistocene^Holocene monogenetic centres have generally lower Sr/Sr and Pb/Pb and higher Nd/Nd than the older centres and subalkali lavas. Major and trace element and isotope data suggest a common, shallower source for the high-Al alkali and subalkali lavas, in contrast to a deeper source for the low-Al alkali magmas. We propose that mantle melting was initiated under partially hydrous conditions at a pressure of near 2·5 GPa, followed by drier conditions and extension of the melting zone to 3^3·5 GPa, with a concomitant increase in the volume of melt derived from the shallower part of the system to produce subalkaline magmas. Increasing melt production at shallow depths may be related to accelerated heat transfer resulting from deepening of the melting zone, or increased mantle upwelling. Mantle lenses were uplifted, probably lubricated by shear zones created during the opening of the Sea of Japan c. 15 Myr ago, and reactivated during rotation of the Philippine Sea plate direction of subduction at around 2 Ma. This is the first hypothesized link between subduction processes and intraplate volcanism atJeju.

Distribution and characteristics of Adirondack-class basalt as observed by Mini-TES in Gusev crater, Mars and its possible volcanic source

We identified 104 unique rock targets belonging to the olivine-rich Adirondack class using Mini-TES data. Rare rocks on the West Spur of the Columbia Hills and on the plains east of the Hills also belong to this class. We present evidence that Adirondack-class basaltic lavas may have had their origin at Apollinaris Tholus. Linear modeling of Adirondack-class rock spectra shows only minor variations in mineralogy and the primary phases identified are consistent with olivine basalt having an average olivine composition of ∼Fo45. We used factor analysis and target transformation to identify variability within a single class of rocks for the first time, and we find that olivine abundance varies independently of a basaltic matrix. A spectral component previously attributed to downwelling radiance is proposed here to be attributable to optically thin dust on rocks, which has a greater effect on Mini-TES spectra than previously recognized, but can explain the sloping continuum observed in Adirondack-class rock spectra as well as the apparent distortion of an olivine absorption. Spectral mixing is not completely linear, leading to the overestimation of sulfate and olivine fractions and the underestimation of plagioclase feldspar, although linear mixing appears to successfully replicate the majority of the observed signal. Published TES spectra of low albedo, low dust cover, olivine-bearing materials do not exhibit the spectral telltales of non-linear behavior, probably because orbital spectra represent mostly mobile regolith that has not accumulated a sufficient dust cover over ∼3×6km areas.

Petrology and geochemistry of Marion and Prince Edward Islands, Southern Ocean: Magma chamber processes and source region characteristics

New bulk rock geochemical data on an extensive suite of samples from the Marion and Prince Edward Islands, located in the Southern Ocean at ~37°45′E, 46°55′S, show highly coherent major and trace element variations. Rock types are dominated by alkali basalt, with lesser hawaiite and minor mugearite and benmoreite. Prince Edward Island has in addition rare trachytes exposed on the plateau, and xenoliths of comenditic rhyolite pumice were found in a basalt flow of the 1980 eruption on Marion Island. Lavas from Prince Edward Island differ from those erupted on Marion Island by having subtly distinct incompatible trace element ratios (e.g. Zr/Nb=9.1 vs 6.7; Ba/Nb=7.8 vs 6.7; Ce/Pb=31 vs 36), and lower 87Sr/86Sr (0.70302) and higher 143Nd/144Nd (0.51300) isotope ratios than found on Marion Island (0.70336–0.70349; 0.51292–0.51295). The two islands are indistinguishable in terms of Pb isotope ratios (206Pb/204Pb=18.61–18.75, 207Pb/204Pb=15.54–15.56).Quantitative modelling suggests that the range in rock types found on each island can be ascribed to extensive low pressure crystal fractionation of phenocryst phases. For example, hawaiite can be related to the more primitive alkali basalts by some 24% fractional crystallisation of olivine, clinopyroxene, plagioclase and minor FeTi-oxides, whereas the more evolved mugearites and benmoreites require, respectively a further 36% and 24% fractional crystallisation to account for their felsic compositions. The trachytes of Prince Edward Island appear to represent the residual 28% magma following extensive crustal fractionation of dominantly clinopyroxene, plagioclase and FeTi-oxide with minor apatite. None of the sampled lavas have compositions characteristic of true primary magmas, but the least evolved, with Mg# ~63, appear to have experienced as much as 18% olivine fractionation from a primary magma with ~15wt.% MgO. The rhyolite pumice is distinct in composition from Bouvet rhyolite or South Sandwich Island pumice and may belong to an earlier, submarine eruption at or near Marion Island.The enriched incompatible element ratios (Zr/Nb=6–9; La/Ybn=10.6; Y/Nb=0.73) and radiogenic 87Sr/86Sr and unradiogenic 143Nd/144Nd isotope ratios show that the lavas erupted on Marion and Prince Edward Islands resulted from melting an enriched mantle source, consistent with derivation from an upwelling deep mantle plume. Pb isotope ratios (∆8/4Pb~33) confirm the absence of any DUPAL signature in the source region of these magmas.

Amoeboid olivine aggregates (AOAs) in the Efremovka, Leoville and Vigarano (CV3) chondrites: A record of condensate evolution in the solar nebula

Amoeboid olivine aggregates (AOAs) in the Efremovka, Leoville, and Vigarano CV3 chondrites were studied using petrographic and microanalytical techniques to evaluate the origins of these inclusions and their relationships to other chondrite components. Our data support the idea that the inclusions formed by dust growth in the solar nebula as condensates and that they preserve a record of mineralogical and chemical evolution. Spinel-rich nodules and ribbons in AOAs often show Group II trace element patterns and formed either by fractional condensation or by condensation following partial vaporization. The dominant olivine component in AOAs can be explained as a condensate produced in the same way, but with fractionation occurring at a lower temperature. Other spinel-rich material in AOAs appears to be a vaporization residue. Ca-rich pyroxene+anorthite patches in AOAs show chemical signatures similar to those observed for spinel- or melilite-bearing regions but with lower refractory element and higher Mg and Si contents, supporting the idea that the patches formed by reaction between CAI-like material and Mg–Si-rich gas. Different trace element patterns (Group II and non-Group II refractory) are sometimes found for Al-rich regions (Ca-pyroxene+anorthite, or melilite-bearing) in a given AOA, suggesting that AOAs agglomerated from materials that formed under different conditions and that they did not subsequently homogenize. AOAs appear to have originated in a system with near-canonical solar composition and a low pressure gas (∼10−6–10−4bar) over a range of temperatures (mainly ∼1200–1384K) under somewhat non-equilibrium conditions. Relative to predicted equilibrium condensates, most AOAs show an apparent paucity of metal, possibly reflecting differential, density-dependent sorting between olivine and metal grains during aggregate formation. Trace element data are consistent with a simple model involving melting and igneous fractionation of AOA-like olivine to produce Type I chondrule olivine, but other data suggest that open-system melting of chondrules occurred in a different gaseous environment than that which produced AOAs.

Seismic anisotropy of the uppermost mantle beneath the Rio Grande rift: Evidence from Kilbourne Hole peridotite xenoliths, New Mexico

Peridotite xenoliths from the Kilbourne Hole maar, New Mexico, consist of spinel lherzolite, harzburgite, and dunite. Because Kilbourne Hole erupted at approximately 10 ka, these xenoliths represent essentially current conditions beneath the Rio Grande rift. In this study, we present detailed petrofabric data and seismic properties obtained from peridotite xenoliths from Kilbourne Hole to illuminate the origin and significance of shear-wave splitting in the uppermost mantle beneath this active rift. Using phase relations and the temperature of equilibration, we infer that these xenoliths were derived from the uppermost mantle, from depths of 35–60 km. Their crystallographic preferred orientations indicate the preservation of olivine b-axis fiber fabrics with a strong concentration of [010] with girdles of [100] and [001]. We consider three geodynamic models for the source region of these xenoliths: horizontal extension, lateral shear, and upwelling. After calculating seismic properties using a volume fraction of olivine, orthopyroxene and clinopyroxene appropriate to each model, we conclude that these xenoliths are derived from a lateral shear zone (vertical foliation (XY plane) and horizontal lineation within the plane of the foliation (X-axis)). However, the degree of seismic anisotropy generated by peridotite xenoliths alone is limited, so that the existence of melt in thin cracks or dikes could be required to cause a significant increase; the orientation of such melt pockets parallel to the XY plane in either model would result in an increase in anisotropy. These results indicate that the shear-wave splitting observed in the Rio Grande rift is a reflection of the lithospheric fabric and the presence of melts as thin cracks or dikes.

Chalcophile elemental constraints on sulfide-saturated fractionation of Cenozoic basalts and andesites in SE China

Cenozoic volcanic rocks are widespread in eastern China. These rocks have variable isotope compositions, reflecting different mantle sources under SE and NE China. In order to explore their PGE variations, chalcophile elements are determined for basalts from Leiqiong and Sanshui and andesites from Lianping in SE China. Basalts from Leiqiong have higher Ni (66–376ppm) and Cu (30–73ppm) contents than those from Lianping and Sanshui. All the samples have extremely low PGE contents, low Cu/Zr (<1) and high Cu/Pd ratios (19,000–475,000), except for one olivine-rich sample with much higher PGE concentrations. All the samples have variable Pd/Ir (6.1–185) and Pt/Rh ratios (1.07–43). They are PGE-depleted relatively to Ni and Cu and have U-shaped and MORB-like primitive mantle-normalized chalcophile element patterns. Positive correlations of Ni and Cr with MgO in basalts from Leiqiong suggest fractionation of olivine and chromite. All samples have low Cu/Zr and high Cu/Pd ratios, implying that sulfides were segregated during magmatic evolution. Quantitative modeling indicates that the amounts of sulfide removed from the samples are smaller than that from MORB. Sanshui basalts are depleted in Ru and Pt, indicating fractionation of chromite with Fe–Pt alloys and/or accumulation of olivine with Ir alloys. A few Leiqiong and Lianping andesites may also have experienced fractionation of Fe–Pt alloys based on Pt depletions relative to Rh and Pd. Alternatively, the Pt depletion could also be an inherited feature of a metasomatic mantle source. Basalts from Sanshui have the highest and andesites from Lianping have the lowest Pd/Ir ratios, also consistent with a metasomatized source for the andesites in Lianping.The volcanic rocks from SE China show no strong Ir-depletions, unlike those from NE China. The absence of strong Ir-depletions is likely due to more oxidized SCLM beneath SE.

The Song Da magmatic suite revisited: A petrologic, geochemical and Sr–Nd isotopic study on picrites, flood basalts and silicic volcanic rocks

Permian volcanic rocks, including picrites, flood basalts and silicic volcanic rocks, crop out in the Song Da district, northern Vietnam. They occur as a ∼300-km-long belt emanating from the southernmost part of the Emeishan large igneous province, SW China. The belt represents a detached part from the province and was displaced ∼600 km southeastward along the Ailao Shan-Red River shear zone during the mid-Tertiary in response to the India–Eurasia collision. We report here geochemical and Sr–Nd isotopic data on the Song Da volcanic rocks to decipher the petrogenetic processes responsible for their formation. The picrites show evidence of olivine fractionation and formed from high-Mg magmas (MgO = 20–25 wt.%) originated from depleted mantle peridotites from the Emeishan plume head. The plume head is characterized by εNd( t) value of +8. Some picrites and low-Ti basalts have low Nb/La, Nb/U and εNd( t), which are indicative of plume–lithosphere interaction in various degrees. Contamination of continental crustal rocks (<20%) is able to produce the spectra of the observed trace element patterns and isotopic ratios. The silicic volcanic rocks are metaluminous or peraluminous (aluminium saturation index = 0.92–1.24) and are sufficiently different from those present elsewhere in the Emeishan large igneous province that are dominantly peralkaline. The Song Da silicic volcanic rocks have εNd( t) values of −0.1 to +0.6 and show strong depletion in compatible trace elements (e.g. Ni = 0.8–3 ppm, V = 4–55 ppm). They most likely formed by extensive fractional crystallization from the associated high-Ti basalts, possibly with some crustal contamination.

Geochemistry and Sr–Nd–Pb isotopic composition of the Harrat Al-Madinah Volcanic Field, Saudi Arabia

Whole-rock geochemical and Sr, Nd and Pb isotope data are presented for the Harrat Al-Madinah volcanic field, in the north western part of the Arabian plate, aiming to understand their origin and the composition of their mantle source. This area is an active volcanic field characterized by the occurrence of two historic eruptions approximately in 641 and 1256A.D. Field investigation of the main volcanic landforms indicates dominantly monogenetic strombolian eruptions, in addition to local phreatomagmatic eruption style. The lavas consist mainly of alkali olivine basalt, olivine transitional basalt, and hawaiite with ocean island basalt (OIB)-like characteristics. Evolved rocks, represented by mugearites, benmoreites, and trachytes, occur mainly as domes, tuff cones and occasionally as lava flows. Chemical variations in the evolved rocks indicated their evolution by low pressure crystal fractionation of olivine, plagioclase, clinopyroxene, and Fe–Ti oxides from the relatively primitive basalts. The isotopic compositions of 143Nd/144Nd (0.512954–0.512995), 87Sr/86Sr (0.702899 to–0.702977) and Pb (206Pb/204Pb=18.5515–18.7446, 207Pb/204Pb=15.5120–15.5222, 208Pb/204Pb=38.1347–38.4468), show restricted variations suggesting only minor crustal contamination. They defined an array consistent with mixing of two geochemically distinct components of depleted MORB-mantle (DMM) and high 238U/204Pb ratio (HIMU). The variations in Tb/Yb, La/Yb and Sm/Yb ratios in the relatively primitive basalts (MgO>6wt.%) indicated garnet peridotite source. However, the positive Nb, Sr, Ba and Ti anomalies in the primitive mantle-normalized incompatible element patterns and the significant variation between Zr/Nb vs. Ce/Y and La/Yb vs. Yb suggest contribution of an amphibole-bearing spinel lherzolite source. Moreover, the negative correlations between SiO2 vs. 87Sr/86Sr and Th vs. 143Nd/144Nd are interpreted as an indication of mixing melts derived from two end-members; one is garnet bearing asthenospheric source with OIB characteristic and the other is amphibole-bearing spinel lherzolite. The Harrat Al-Madinah volcanic field occurs near the Red Sea Rift System and its origin reflects a strong lithospheric control on the loci of partial melting. The dominantly NNW alignment patterns of the volcanoes, which is similar to the regional Red Sea trend, may suggest that the magmas were produced by decompression partial melting triggered by lithospheric extension related to the Red Rift.

Early Precambrian mafic dyke swarms from the Central Archaean Bastar Craton, India: geochemistry, petrogenesis and tectonic implications

AbstractThe Archaean Bastar craton is known for the presence of different generations of mafic dykes. Less studied many NW‐SE trending mafic dykes (intruded into the Archaean supracrustal rocks), encountered from the central part of the Bastar craton, are studied for their petrological and geochemical characteristics. There are many geological evidences which suggest emplacement of these mafic dykes in an intracratonic setting. Two distinct types of mafic dykes are recognized. Petrographically it is difficult to discriminate these two types. Many samples show metamorphic textures and amphibolites facies mineral assemblage but a few samples preserved original igneous texture and mineralogy. Major oxides show sub‐alkaline thoeliitic basalt/basaltic andesite nature. High‐Fe and high‐Mg contents classify them as high‐iron and high‐magnesium tholeiites. They fall in the gabbronorite field on R1‐R2 plot. Geochemical characteristics, particularly high‐field strength and rare‐earth elements, clearly distinguished them into two types: Group 1 and Group 2 mafic dykes. When compared with well‐studied mafic dyke swarms of the Southern Bastar craton, the Group 1 is recognized as Meso‐Neoarchaean sub‐alkaline mafic dykes (BD1‐CBC) and the Group 2 as Neoarchaean‐Palaeoproterozoic boninite–norite mafic dykes (BN‐CBC). The boninite–norite nature of the second group is also corroborated through immobile trace element ratios. BD1‐CBC dykes are characterized by a relatively higher concentration of HFSE and REE in comparison to BN‐CBC dykes. Some extent of crustal contamination is observed in BN‐CBC dykes but BD1‐CBC dykes do not show any indication of crustal contamination. Trace element modelling suggest that BD1‐CBC mafic dykes are derived from a melt originated through ∼20% melting of a depleted mantle source, whereas BN‐CBC mafic dykes are probably derived from a high‐Mg magma generated through ∼25% melting of a refractory mantle source. Both melts have undergone 30–40% olivine fractionation before the emplacement. Geochemistry also points out involvement of a plume in the genesis of these mafic dykes. Copyright © 2011 John Wiley &amp; Sons, Ltd.

Petrology of the mafic sill of Narshingpur-Lakhnadon section, Eastern Deccan volcanic province

Deccan volcanism with a tremendous burst of volcanic activity marks a unique episode in Indian geological history and covers nearly two third of Peninsular India. Occurrences of mafic sill in the continental basalts are rather rare throughout the flood basalt provinces and only few sporadic reports have been described from different Continental Flood Basalts of the world. In the present article, petrology of mafic sill from the Narshingpur-Lakhnadon section of Eastern Deccan province of India has been presented. The mafic sill in the field is found to occur in a relatively deep valley amidst Gondwana rocks, which occur as the basement of the extrusion. The sill is spatially associated with three initial flows viz. flow I, II and III of adjacent Narshingpur-Harrai-Amarwara section. The sill in its central part is a medium grained rock and petrographically corresponds to dolerite containing augite, plagioclase and rare olivine grains; the chilled facies of the sill is characterized by phenocrysts of olivine, plagioclase and augite that are set in groundmass consisting predominantly of plagioclase, olivine and glass. Mineral chemistry indicates that olivine phenocrystal phase is magnesian (Fo61). Plagioclase phenocrystal composition ranges from An 51 to An 71 whereas the same variation of the groundmass plagioclase composition corresponds to An 31 to An 62. The overlap in the compositions for groundmass and phenocrystal plagioclase may be explained due to fluctuating PH2O condition. The pyroxene compositions (both groundmass and phenocryst) in majority of the cases are clubbed well within the augite field, however, in a few cases, groundmass compositions are found to fall in the sub-calcic augite and pigeonite field. Some zoned pyroxene phenocrysts, characteristically display different types of zoning patterns. Opaque minerals in the mafic sill are found to be magnetite and ilmenite and this coexisting iron-oxide composition helps to constrain the prevalent fO2 condition in the parent magma. The geochemistry of the mafic sill and associated basaltic lava flows indicates close genetic link amongst them. Critical consideration of trace elements indicates a distinct enriched mantle source (EM-I/EM-II/HIMU) for the parental magma. Trace element modeling indicates that equilibrium batch-melting of plume source followed by fractionation of olivine, clinopyroxene and plagioclase and subsequent heterogeneous mixing of melt and settled crystals can very well explain the genesis of the mafic sill and the associated basaltic flows.