Picrites Research Articles

Sources of Cu-Rich Sulfide Mineralization and high-Ni Olivine of the Rudnaya Dyke (Imangda Junction, Norilsk Region): Based on Compositional, Isotope and Model Data

The Rudnaya dyke of the Imangda ore junction is composed of the weakly differentiated olivine-bearing to olivine gabbrodolerites with sulfide globules and disseminated sulfides of (pentlandite-pyrrhotite)-chalcopyrite-cubanite composition. Along with cogenetic sulfide mineralization, dyke’s gabbrodolerites contain xenoliths of hornfelsed basalts, abundant amygdales and rare grains of zoned Ol-1 Fo90-47 with 0.5–0.06 wt % NiO that coexist with subhedral olivine Fo74-36 of the second generation. Modeling in the COMAGMAT and alphaMELTS programs showed that high-Mg olivine 1 with Cr-spinel inclusions could not be crystallized from a Fe-enriched tholeiitic magma that is parental for the dyke with 4.8–7.3 wt % MgO and 11.6–16.7 wt % total Fe2O3. The trend of variations and high Ni up to 0.5 wt. % in the cores of xenocrystic olivine Fo90-76 in contrast to maximum Fo83 and 0.4 wt. % NiO in olivine from the ore-bearing intrusions and picritic basalts of the Norilsk region point toward the presence of picritic cumulates, which magma had not exchanged with sulfide liquid. Platinum group element (PGE) abundances increase (up to 2.2 ppm) with Cu/Ni in the whole rocks as well as with proportions of pentlandite in a sulfide association. A specific chalcophile metal distribution, which is characterized by Ni, Os and Ir minima, elevated Cu/Ni (5–15) and Cu/Pd (3200–10 900) as well as lower both PGE tenor of sulfides (2–65 ppm) and Pd content in pentlandite (175 ppm) compared to typical of ore-bearing intrusions, suggests that Cu-rich sulfide mineralization was not mechanically captured from highly fractionated sulfide fractions of ore-bearing magmas but is cogenetic with a magma of the dyke. Sulfide saturation, near-simultaneous with fluid saturation and degassing, was achieved due to assimilation of sedimentary sulfur and volatiles from Devonian evaporites in the dyke conduit that is supported by the heavy S isotope composition of dyke’s sulfides with the average δ34S = 14.7 ±1.1‰ (n = 31), close to the values in sulfides from the endocontact zones of the ore-bearing Imangda intrusions hosted by Devonian strata. The initial isotopic characteristics of dyke’s rocks (Sri 0.70517–0.70532, ɛNd from –0.4 to 0.8) imply its comagmatic origin with the Norilsk-type intrusions whereas the overall data do not exclude even its spatial connection with an upper crustal conduit system of the ore-bearing magmas.

Crustal structure of the Western Azuero Peninsula, Panama: Insights into the structure of accretionary complexes and forearc ophiolites

ABSTRACT Detailed geologic mapping (639 field stations in ~700 km2) and a ~50 km-long gravity survey (142 stations) in the western Azuero Peninsula revealed two faulted and folded slivers of oceanic crust attached to the trailing edge of the Caribbean Large Igneous Plateau (CLIP). Our new data, along with published geochronology, allowed us to reconstruct the Cretaceous forearc configuration of the trailing edge of the CLIP prior to seamount collision, ophiolite accretion, and whole-margin deformation. The ophiolite in western Azuero is composed of two tectonic slivers arranged in south-verging, imbricated thrust faults that stack a ~73 Ma pillow, flow, and picritic basalt and black chert, together with a ~ 89–93 Ma and older basalt flows and capping red chert sequences. Accretion of these slivers to form a supra-subduction zone ophiolite resulted from the middle Eocene collision and accretion of Galapagos seamounts against the trailing edge of the CLIP. Accreted seamounts are arranged in a north-verging antiformal stack duplex, and below the thrust sheets. Change in kinematics after fission of the Cocos-Nazca Plate during early Miocene times prompted the propagation of the Azuero-Sona fault zone flower structure, favouring the preservation of these slivers of oceanic crust.

Petrochemistry, Petrogenesis and Geodynamic Implications of Mantle Plume Generated Dhanjori Volcanics, Singhbhum Craton (Eastern India)

Abstract New geological and geochemical data have been presented in this paper to improve the understanding of the petrogenesis and regional geodynamic evolution of the 2.8-2.5 Ga Dhanjori volcanics. These volcanics exhibit porphyritic texture and classified as picritic basalt. The Dhanjori volcanics are MgO-rich (9.97 - 15.81 wt%) with moderate SiO2 (51.62 - 53.82 wt%), FeOT (9.97- 12.57 wt%) and TiO2 (0.73 -1.21 wt%) but relatively higher Ni (123.56 - 545.35 ppm) and Cr (241.87- 1158.41). The rare earth element (REE) patterns and their ratios indicate their derivation from the garnet to spinel stability field due to decompressive melting of a mantle plume head near the crust-mantle boundary, which is supported by the presence of high positive gravity anomaly over the Dhanjori lavas. The thermo-barometric studies indicate that the primary magma had an initial mantle potential temperature between 1350 and 1500oC in a pressure range of 1 to 1.8 GPa, thus conforming with the mantle origin, which is further endorsed by supplementary geophysical studies.

Clay mineral evolution and formation of intermediate phases during pedogenesis on picrite basalt bedrock under temperate conditions (Yunnan, southwestern China)

In order to better understanding clay mineral evolution coupled with geochemical changes during weathering of igneous rocks under temperate conditions, the picrite basalt-derived soil in Dali (South China) was investigated using X-ray diffraction (XRD), X-ray fluorescence (XRF), scanning electron microscope (SEM), and high-resolution transmission electron microscopy (HRTEM) methods. Our results show that smectite and mixed-layer clays occur throughout the weathered soil profile, while discrete kaolinite and illite phases are absent in the soil. From saprolite to topsoil, smectite decreases sharply, while mixed-layer kaolinite/smectite (K/S) and illite/smectite (I/S) clays increase markedly. Clay minerals in the saprolite consist of both dioctahedral and trioctahedral species, while those of the middle to upper profile display a uniform dioctahedral structure. I/S phases are characterized by interstratification of 12-Å smectite and 10-Å illite layers, and K/S phases by interstratification of 12-Å smectite and 7-Å kaolinite layers, with I/S/K clays containing all three layer types. Kaolinization of smectite occurs at the initial stage of weathering, earlier than that of smectite illitization. Desilication and K-fixation of smectite take place simultaneously during advanced weathering, resulting in formation of illitic and kaolinitic phases continuously throughout the developmental history of the soil. Notably high K concentrations in the weathering profile may be related to K-fixing in the interlayer of illite due to smectite illitization in response to more advanced weathering and pedogenic processes, while the increasing K content in the topsoil may be ascribed to fertilizer in land use and uptake of K by plants from deeper soil horizons. Fe2O3 and TiO2 accumulation in the topsoil is probably mediated by microorganisms.

The Permian-Triassic Riftogen Rocks in the Norilsk Area (NW Siberian Province): Geochemistry and Their Possible Link with PGE-Cu-Ni Mineralization

The volcanic rocks in the Vologochan syncline, the Khikey River valley, and Mount Sunduk, within the Norilsk area in the NW Siberian large igneous province, have been studied. They belong to the Ivakinsky, Syverminsky, Gudchikhinsky, Khakanchansky, Nadezhdinsky, Tuklonsky, and Morongovsky Formations. These Formations consist of trachybasalts, picritic basalts and tholeiitic basalts with aphyric and porphyritic textures, and intersertal and poikiloofitic structures. For the first time, we demonstrate the variations in the structure and composition of these Formations along the strike, based on 151 analyses of the major and trace elements in the rocks. The thickness of all the Formations, excepting the Morongovsky, reduce dramatically from the Yenisey–Khatanga trough to the Tunguska syneclise, and they pinch out in the east of the Norilsk area and are attributed to riftogen (rift) basalts. The rock compositions also change in this direction, especially in the Gudchikhinsky and Nadezhdinsky Formations. The two subformations of the Gudchikhinsky formation, the lower and upper, disappear in the east, so the Gudchikhinsky consists only of high-Mg rocks, picritic basalts, and picrites. The composition of the Nadezhdinsky formation varies intensely in its (Gd/Yb)n and (Th/Nb) ratios from the Vologochan syncline to the Khikey River valley. These structural and compositional variabilities differ between the rift formations and the platform ones. Two gabbro–dolerite sills from these areas that are close to the Norilsk and Ergalakh intrusive complexes have been studied. The metal contents in volcanic and intrusive rocks are similar and do not differ from the barren rocks of the South Pyasinsky massif comprising the PGE-Cu-Ni deposits. Only the Gudchikhinsky Formation contains elevated Cu and Ni concentrations. These features and the coinciding spatial distribution of the ore-bearing intrusions and picrites of the Gudchikhinsky rocks in the Norilsk–Igarka paleorift suggest their genetic link. It is proposed that the initial sulfides could have been formed in the mantle, as the Gudchikhinsky picrites, transported to the lower crust, and then involved by the trap magmas in the origin of the ore-bodies in the Norilsk deposits.

Three Types of Mantle Eclogite from Two Layers of Oceanic Crust: A Key Case of Metasomatically-Aided Transformation of Low-to-High-Magnesian Eclogite

AbstractReconstructed whole-rock (RWR) and mineral major- and trace-element compositions, as well as new oxygen isotope data, for 22 mantle eclogite xenoliths from the Catoca pipe (Kasai Craton) were used to constrain their genesis and evolution. On the basis of mineralogical and major-element compositions, the Catoca eclogites can be divided into three groups: high-alumina (high-Al) (kyanite-bearing), low-magnesian (low-Mg#), and high-magnesian (high-Mg#) eclogites. The high-Al Catoca eclogites contain kyanite and corundum; high Al2O3 contents in rock-forming minerals; rare earth element (REE) patterns in garnets showing depleted LREEs, positive Eu anomalies (1.03–1.66), and near-flat HREEs; and high Sr contents in garnets and whole-rock REE compositions. All of these features point to a plagioclase-rich protolith (probably gabbro). RWR compositions (major elements, MREEs, HREEs, Li, V, Hf, Y, Zr, and Pb) and δ18O of 5.5–7.4‰ of the low-Mg# Catoca eclogites are in good agreement with the compositions of picrite basalts and average mid-ocean ridge basalt (MORB). The depleted LREEs and NMORB-normalised Nd/Yb values of 0.07–0.41 indicate that the degree of partial melting for the majority of the low-Mg# eclogites protolith was ≥30%. The narrow δ18O range of 5.5–7.4‰ near the ‘gabbro–basalt’ boundary (6‰) obtained for the high-Al and low-Mg# Catoca eclogites reflects the influence of subduction-related processes. This case shows that mantle eclogites represented by two different lithologies and originating from different protoliths—plagioclase-rich precursor, presumably gabbro (for high-Al eclogites), and basalt (low-Mg# eclogites)—can provide similar and overlapping δ18O signatures on account of the influence of subduction-related processes. Chemical compositions of the high-Mg# eclogites indicate a complicated petrogenesis, and textural signatures reveal recrystallisation. The presence of Nb-rich rutile (8–12 wt% of Nb2O5) enriched with high field strength elements (HFSE) (Zr/Hf of 72.6–75.6) and multiple trace-element signatures (including RWR, NMORB-normalised Ce/Yb of 3.9–10.6 and Sr/Y of 5.8–9.6, MgO contents of 15.7–17.9 wt%, and high Ba and Sr) provide strong evidence for deep metasomatic alteration. High Cr contents in clinopyroxene (800–3740 ppm), garnet (430–1400 ppm), and accessory rutile (700–2530 ppm), together with extremely low Li contents of 1.0–2.4 ppm in clinopyroxene, may indicate hybridisation of the eclogites with peridotite. Comparison of the chemical compositions (major and trace elements) of (1) unaltered fresh cores of coarse-grained garnets from the low-Mg# eclogites, (2) secondary garnet rims (ubiquitous in the low-Mg# eclogites), (3) proto-cores in the coarse-grained garnet (high-Mg# eclogites), and (4) homogeneous recrystallised fine-grained garnets (high-Mg# eclogites) suggests that the high-Mg# eclogites formed through recrystallisation of low-Mg# eclogite in the presence of an external fluid in the mantle. Four of the five high-Mg# samples show that mantle metasomatism inside the Kasai craton mantle beneath the Catoca pipe occurred at a depth range of 145–160 km (4.5–4.8 GPa).

Petrological features of picrobasaltic melts on Lanzarote, Canary Islands

The paper discusses issues related to the petrology of picrobasaltic melts forming lava flows on Lanzarote, Canary Islands. During ascientific and educational expedition on the aforementioned island, lava flows of picro basalt of the third phase of the 1731–1732 eruption were studied and tested. Preference was given to the well-preserved streams in the west and south of the island thatare accessible for direct study. Volcanogenic-sedimentary associations were identified in the studied outcropsand described, the structural and textural features of the flows were characterized, and samples with mantle rock xenoliths were taken for petrological reconstructions. New geochemical data onrocks and minerals are presented. It was established that the temperature of the initial melt was 1100–1180°C. Large harzburgitephenocrysts in picro basalt are xenocrysts formed as a result of melting and disintegration of mantle xenoliths.

Mineral Chemistry, S-Pb-O Isotopes, and S/Se Ratios of the Niubiziliang Ni-(Cu) Sulfide Deposit in North Qaidam Orogenic Belt, NW China: Constraints on the Parental Magma Composition, Evolution, and Sulfur Saturation Mechanism

The Niubiziliang Ni-(Cu) deposit is the first magmatic Ni-Cu sulfide deposit in the North Qaidam Orogenic Belt (NQOB), NW China, and plays a significant role in geological evolution, Ni-Cu mineralization, and exploration in the NQOB. Here, we report on the mineral chemistry, S-Pb-O isotopes, and S/Se ratios of the mafic-ultramafic complex, which provide insights on the parental magma, evolution, and sulfur saturation mechanism. The Niubiziliang mafic-ultramafic intrusion contains four ore blocks and about ten Ni-(Cu) ore/mineralization bodies. Olivines in Niubiziliang belong to the species of chrysolite with Fo values of 88~89, and the pyroxenes are mainly orthopyroxene (En = 79~82) and clinopyroxene (En = 44~40). The olivines and some pyroxenes likely crystallized in a magma chamber at a depth of 35.45~36.55 km at a high temperature (1289~1369 °C) and pressure (9.38~9.67 kbar), whereas the Niubiziliang complex formed at a moderate depth (8.13~8.70 km) with a temperature and pressure of 1159~1253 °C and 2.15~2.30 kbar, respectively. The parental magma was considered to be high-Mg picritic basalt with MgO and NiO contents of 14.95~16.58% and 0.053~0.068%, respectively, which indicated high-degree partial melting of the depleted mantle. The mantle-derived primary magma underwent significant fractional crystallization and crustal assimilation and contamination, which was strongly supported by S-Pb-O isotope data and S/Se ratios, resulting in sulfur saturation and sulfide immiscibility in the magma. Crustal assimilation and contamination contributed more to sulfur saturation than fractional crystallization.

ASSESSMENT OF THE MINERAL RESOURCE POTENTIAL OF HIGH-MAGNESIAN ROCKS OF THE REPUBLIC OF KARELIA

The mineral- and raw material potential of the Republic of Karelia as regards high-Mg industrial minerals and rocks has been shaped by the territory’s geological structure and the evolution of geological processes in early stages of the Earth’s crust development n the southeastern part of the Fennoscandian Shield. A majority of deposits and occurrences (olivinite, pyroxenite, serpentinite, kemistites, talc, chrysotile- and anthophylliteasbestos, soapstone, picritic basalts) are found in regions with mafic-ultramafic magmatic rocks of various facies and geodynamic settings, whose formation is associated with Archean and Paleoproterozoic endogenous events within the main structural and compositional complexes. Some deposits and occurrences (dolomites, talc, magnesian riebeckite-asbestos) are associated with dolomites of Paleoproterozoic sedimentaryvolcanogenic strata of the Karelian craton. The article discusses the main formational environments, features of the geological structure and composition of Karelian high-Mg industrial minerals and rocks deposits and occurrences. The areas of their practical appli cation have been determined on the basis of the results of mineralogical and technological studies carried out by the IG KarRC RAS.

Roberts Lake Syncline mafic lavas (NE Superior craton): A proposed extension of the Cape Smith belt

The Superior craton is bordered on its northern and eastern sides by the approximately coeval Cape Smith fold belt and the Labrador Trough, two major Paleoproterozoic volcano-sedimentary belts that record broadly similar tectonic evolution but which show some distinctive characteristics and have notably different mineral resource endowments. The Roberts Lake Syncline, situated directly between the Cape Smith and Labrador Trough, hosts two Paleoproterozoic mafic volcanic sequences and has uncertain tectonic affinities to its neighboring fold belts. It was previously linked to Labrador Trough magmatism but re-interpretation of the regional geology suggests a closer resemblance to the successions of the Cape Smith belt. We present new lithogeochemistry that shows that the Roberts Lake Syncline comprises two distinct magmatic events rather than the single event previously interpreted. The lower and more voluminous suite of the Roberts Lake Syncline lavas comprises multiply-saturated basalts characterized by normalized trace element patterns similar to E-type mid-ocean ridge basalts (E-MORB). The upper suite of the Roberts Lake Syncline stratigraphy is composed of volcanic rocks with depleted, N-MORB-like trace element signatures rocks spanning a range from komatiites and picrites with near-primary compositions to high-Mg basalts with flat normalized trace element patterns. Both suites were apparently undersaturated with sulfide liquid in their mantle source. The E-MORB magmas were affected by a sulphide-saturation event, leading to widespread and variable degrees of depletion of chalcophile elements, despite showing no significant crustal contamination. The N-MORB magmas also underwent saturation with sulfide melt, generating minor occurrences of magmatic sulfide mineralization hosted by ultramafic intrusions within the older volcanic suite and small but significant depletions in chalcophile elements in the erupted picritic basalts. The geochemistry of the lower E-MORB suite is similar to that of the 2170 Ma Biscotasing dike swarm that occurs in and near the Labrador Trough and the 1998 Ma Minto-Povungnituk large igneous province (LIP) of the Cape Smith belt, whereas the upper N-MORB-like suite in the Roberts Lake Syncline has compositions resembling those of the 1880 Ma Circum-Superior LIP. Our detailed geochemical comparison suggests a close resemblance to the chemical stratigraphy observed within the Cape Smith belt, leading us to consider the Roberts Lake Syncline as the southeasternmost portion of the Ungava Orogen rather than the northernmost part of the Labrador Trough. These insights contribute to a revised LIP and plume centre map for this NE portion of the Superior craton and help to constrain the large-scale tectonic architecture of the margins of the Superior Province.

Implications for the origins of Eoarchean ultramafic rocks of the North Atlantic Craton: a study of the Tussaap Ultramafic complex, Itsaq Gneiss complex, southern West Greenland

Ultramafic rocks found within the ~ 3.81 Ga Itsaq Gneiss Complex (IGC) have some mantle-like geochemical characteristics that have led to them being used to directly constrain the nature of the Eoarchean mantle. The discrimination of mantle peridotites that are the residues of partial melting, from cumulate peridotites generated by crystal accumulation from mantle-derived magmas can be difficult in ancient, altered ultramafic rocks whose field relations have been obscured by multiple tectonic episodes. Hence it is important to scrutinize significant individual occurrences of Eoarchean ultramafic rocks in some detail prior to using them to constrain the nature of Earth’s early mantle. Here we present mineral chemistry, whole rock major-, trace-, and platinum-group-element abundances, and Re–Os isotope compositions of a previously unstudied large ultramafic enclave in the IGC—the Tussaap Ultramafic Complex (TUC)—with the aim of documenting its origin. High FeO contents of up to 15.5 wt% and correlations between MgO and Os provide strong evidence that the TUC evolved through fractional crystallization rather than partial melt extraction. In addition, co-variations of major elements in the TUC lithologies can be modeled via fractional crystallization of picritic basalts using MELTS. Later alteration and metasomatism of these ultramafic rocks has largely overprinted primary mineral chemistry and resulted in a redistribution of light rare earth elements, rendering these tools ineffective for ascertaining the origin of the TUC or quantifying some of the petrogenetic processes that formed the body. In addition, it is clear that many geochemical features used to identify residual mantle peridotites can also be produced by cumulate or alteration processes, such as some variations in olivine and chromite chemistry, whole rock Al/Si vs Mg/Si systematics, and trace and platinum group element patterns. Finally, combined discrimination diagrams for high field strength elements and moderately high 187Os/188Os ratios suggest the parental melt of the TUC partially assimilated basaltic crust prior to precipitating the TUC cumulates. As such, these rocks represent a variably obscured record of Eoarchean crystal fractionation from mantle-derived melts. Despite not being prima facie mantle rocks, it is possible that such early formed ultramafic cumulates in nascent continents found their way into the later-stabilized roots of Archean cratons, helping to explain the high compositional variability of cratonic peridotites.

Пикритовые базальты как нетрадиционные руды никеля

Пикритовые базальты как нетрадиционные руды никеля

Evolution of High-Magnesium Melts During Crystallization of Rocks in Ultramafic Massifs, Siberian Platform

The results of study of primary melt inclusions in accessory Cr–spinelides from dunite have revealed features of the chemical compositions of high-magnesium picrite–basalt melts that formed ultramafic massifs of the Siberian Platform. Computational modeling (with the PETROLOG and PLUTON software) using the inclusion compositions allowed determination of the crystallization temperatures of olivines from dunites of the Guli (1520–1420°С), Konder (1545–1430°С), Inagli (1530–1430°С), and Chad (1460–1420°C) massifs. The Cr–spinellids from the same complexes crystallized at Guli (1410–1370°С), Konder (1420–1380°С), Inagli (up to 1430°С), and Chad (1430–1330°С). Overall, computational modeling has revealed two types of trends of high-Mg melt evolution on the Siberian Platform. The first type is characteristic of the Guli Pluton, with the following rock sequence: dunite–pyroxenite–alkalic rocks. The second type occurs in the Konder massif, with a different sequence: dunite–kosvite–pyroxenite and olivine–diopside rocks.

Conditions of formation of Paleozoic picrite basalt complexes of the basement of the Western Siberian Plate

The carried out researches with the help of 40Ar/39Ar analysis have shown a coincidence of formation (about 490 million years) of Paleozoic picrite and basalt complexes of the basement of the Western Siberian Plate. The data on petrochemistry, geochemistry of rare and rare-earth elements, and also on compositions of clinopyroxenes, testifies to formation of picrites of borehole № 11 (Chkalovsky area) as a result of action of intra plate magmatic systems of oceanic islands - OIB. Calculations on the basis of clinopyroxene compositions speak about considerable parameters of crystallization of picrite porphyrite minerals: 1215-1275 °C, 4.5-8 kbar. As a whole, it is established that considered picrite basalt complexes were formed with the assistance of enriched plume magmatic systems in intra plate conditions of ancient ocean near to its active margin.

Setting of the Formation of Paleozoic Picrite Basalt Complexes in the West Siberian Plate Basement

40Ar/39Ar analysis showed a simultaneous (at about 490 Ma) formation of the Paleozoic picrite and basalt complexes of the West Siberian Plate basement. The petrochemistry, trace and REE geochemistry, and composition of clinopyroxene indicate the formation of the picrite of well no. 11 (Chkalov area) as a result of intraplate magmatism of the OIB type. Calculations based on the compositions of clinopyroxene allowed crystallization of minerals of porphyric picrite at 1215–1275°C and 4.5–8 kbar. In general, it has been found that the picrite basalt complexes considered were formed from enriched igneous plume systems under intraplate conditions near the active margin of the ancient ocean.

Multiple Magma Conduits Model of the Jinchuan Ni-Cu-(PGE) Deposit, Northwestern China: Constraints from the Geochemistry of Platinum-Group Elements

The giant Jinchuan nickel-copper-platinum-group element (PGE) deposit is hosted by two individual sub-vertical intrusions, referred to as the western and eastern intrusions (including segment II-W and segment II-E). Exactly how the Jinchuan deposit was formed by a system of sub-vertical magma conduits is still not well understood. This paper reports new major elements, trace elements and PGEs data from the Jinchuan deposit to study the formation mechanism of sulfide ores with different textures and their relationship with the magma conduit system. Our study shows that the PGE tenors of disseminated and net-textured sulfide in segment II-E are significantly lower than segment II-W and the western intrusion, but the Cu/Pd ratios are opposite. In addition, net-textured sulfides in segment II-W show a negative correlation between IPGE (Ir, Ru and Rh) and PPGE (Pt and Pd) in contrast to the positive correlation in segment II-E and the western intrusion. These features indicate the parental magma sources of the western intrusion, segment II-W and segment II-E were originally three different surges of PGE-depleted magma. Modeling of parental magma in the western intrusion, segment II-W and segment II-E suggests that they were formed by the same initial picritic basalt (100 ppm Cu, 1 ppb Ir and 10 ppb Pd) with different prior sulfide segregations (0.0075%, 0.0085% and 0.011%). The three parts of Jinchuan sulfides show that the Pt/Pd and (Pt + Pd)/(Ir + Ru + Rh) ratios decrease from section III-5 toward both sides in the western intrusion and decrease from section II-14 toward all sides, whereas no regular spatial variations occur in segment II-E, showing the different fractionation processes of sulfide melt. The massive sulfides in the western intrusion and segment II-E experienced a ~20% to 40% and ~40% to 60% fractionation of sulfide melt, respectively. We propose that the Jinchuan deposit was generated in a metallogenic system of multiple magma conduits, where sulfides entrained in parental magma experienced different amounts of prior removal.

The Hudesheng mafic–ultramafic intrusions in the Oulongbuluke Block, Qinghai Province, NW China: chronology, geochemistry, isotopic systematics and tectonic implications

AbstractThe Hudesheng mafic–ultramafic intrusions are located in the Oulongbuluke Block, north of the Qaidam Block in Qinghai Province, NW China. We carried out a detailed study of the intrusions, including field observations, petrology, zircon U–Pb geochronology, Lu–Hf isotopes, bulk-rock major- and trace-element geochemistry, and mineral compositions, to provide a better understanding of their properties and the regional tectonic evolution. Zircon U–Pb dating on gabbro and pyroxenite samples yielded ages of 465 and 455 Ma, respectively. Geochemical data, in conjunction with the field observations and petrological features, suggest that the complex is Alaskan-type and the magma was derived from a depleted mantle source that was hydrous picritic basalt in composition and influenced by crustal contamination and slab-derived fluid metasomatism. Based on all the chronological, petrological, mineralogical and geochemical and regional geological data, we conclude that the palaeo-ocean closed diachronously from west to east between the Qaidam and Oulongbuluke blocks, and that the ocean in the east of the North Qaidam region closed after ∼455 Ma.

Mantle source heterogeneity in continental mafic Large Igneous Provinces: insights from the Panjal, Rajmahal and Deccan basalts, India

Abstract In this paper we illustrate the geochemical diversity of the basalts of Panjal ( c. 289 Ma), Rajmahal ( c. 117 Ma) and Deccan ( c. 65 Ma) continental large igneous provinces (continental LIPs) from India as controlled by multiple mantle sources. The Panjal primitive basalts are formed by 5–20%, Rajmahal basalts by 10–20% and Deccan picritic basalts by 1–20% melting of the mantle. The depth of melting beneath Deccan is greater than beneath Rajmahal, which greater than beneath Panjal. We have categorized the Panjal, Rajmahal and Deccan basalts into Type I and Type II. The Type I basalts with lower La/Nb, Th/Nb and Th/Yb ratios and higher ε N d i values ( ε N d i = −3 to +8) are derived from the sublithospheric sources; some of the Type I basalts represent lithosphere-contaminated melts. The Type II basalts with higher La/Nb, Th/Nb and Th/Yb ratios and lower ε N d i values ( ε N d i = −20 to −3) are produced from the subcontinental lithosphere. We estimate that lithosphere and sublithosphere (plume and/or E-MORB patches within asthenosphere) have contributed, respectively, approximately 67 and 33% to the Panjal, 52 and 48% to Rajmahal, and 28 and 72% to Deccan LIPs. Fertility of the Indian subcontinental lithosphere may have increased with decreasing age related to the break-up of Gondwana.

Geology, mineralogy and geochemistry of the Kekem dyke swarm (Western Cameroon): Insights into Paleozoic–Mesozoic magmatism and geodynamic implications

The broadly N70°–90°E-trending dykes swarm at Kekem cut across the Paleoproterozoic-to-Achean terranes of West Cameroon remobilized during the Pan-African orogeny. They are picrite basalts and basalts with tholeiitic/transitional affinity, as shown by mineralogical and geochemical data, with variable major and trace element contents, MgO ranges from 7.3 to 12.4wt.%, Cr from 190 to 411ppm, Ni from 15 to 234ppm. All the dykes are light REE enriched with LaN/YbN values of 5.3–8.1, suggesting a co-magmatic origin. They originated from a 2.8% partial melting of a spinel-mantle source with no or little crustal input. The geochemical features of Kekem dykes are similar to those of Paleozoic and Mesozoic dykes recorded in North and Central Africa, suggesting multiple reactivations of pre-existing fractures that resulted in the fragmentation of western Gondwana and the opening of Central and South Atlantic Oceans.

The Influence of Mineral Matrices on the Thermal Behavior of Glycine.

On the Hadean-Early Archean Earth, the first islands must have provided hot and dry environments for abiotically formed organic molecules. The heat sources, mainly volcanism and meteorite impacts, were also available on Mars during the Noachian period. In recent work simulating this scenario, we have shown that neat glycine forms a black, sparingly water-soluble polymer ("thermomelanoid") when dry-heated at 200°C under pure nitrogen. The present study explores whether relevant minerals and mineral mixtures can change this thermal behavior. Most experiments were conducted at 200 or 250°C for 2 or 7days. The mineral matrices used were phyllosilicates (Ca-montmorillonites SAz-1 and STx-1, Na-montmorillonite SAz-1-Na, nontronite NAu-1, kaolinite KGa-1), salts (NaCl, NaCl-KCl, CaCl2, artificial sea salt, gypsum, magnesite), picritic basalt, and three Martian regolith simulants (P-MRS, S-MRS, JSC Mars-1A). The main analytical method employed was high-performance liquid chromatography (HPLC). Glycine intercalated in SAz-1 and SAz-1-Na was well protected against thermomelanoid formation and sublimation at 200°C: after 2days, 95 and 79%, respectively, had either survived unaltered or been transformed into the cyclic dipeptide (DKP) and linear peptides up to Gly6. The glycine survival rate followed the order SAz-1 > SAz-1-Na > STx-1 ≈ NAu-1 > KGa-1. Very good protection was also provided by artificial sea salt (84% unaltered glycine after 200°C for 7days). P-MRS promoted the condensation up to Gly6, consistent with its high phyllosilicate content. The remaining matrices were less effective in preserving glycine as such or as peptides.