Olivine Microphenocrysts Research Articles

Magnesian Andesites from Kibblewhite Volcano in the Kermadec Arc, New Zealand

Abstract Primary andesitic magmas could be an important component of arc magma genesis and might have played a key role in the advent of continents. Recent studies hypothesized that primary andesitic magmas occur in the oceanic arc, where the crust is thin. The Kermadec arc has the thinnest crust among all the studied oceanic arcs (<15 km in thickness); however, there are no studies that corroborate the formation of primary andesitic magmas in the arc. The aim of this study is to develop a better understanding of primary andesites in oceanic arcs through the petrology of the Kermadec arc. Here, we present the petrology of volcanic rocks dredged from the Kibblewhite Volcano in the Kermadec arc during the R/V SONNE SO-255 expedition in 2017. Magma types range from andesite to rhyolite at the Kibblewhite Volcano, but basalts dominate at the neighboring cones. This study focuses on magnesian andesites from the northeastern flank of this volcano. The magnesian andesites are nearly aphyric and plagioclase free but contain microphenocrysts of olivine (Fo84–86) and clinopyroxene (Mg# = 81–87). Using olivine addition models, the primary magmas were estimated to contain 55–56 wt % SiO2 and 10–12 wt % MgO, similar to the high-Mg andesites observed in other convergent plate margins, indicating the generation of primary andesitic magma beneath the Kibblewhite Volcano. The trace element and isotopic characteristics of the magnesian andesites are typical of volcanic rocks from the Kermadec arc. This indicates that the subduction of a young plate or melting of a pyroxenitic source is not necessary to produce magnesian andesites. Instead, we propose that the magnesian andesites were produced by the direct melting of the uppermost mantle of the Kermadec arc. The thin crust of the Kermadec arc should yield low-pressure conditions in the uppermost mantle, allowing the sub-arc mantle to generate primary andesitic melts. This study supports the hypothesis that primary andesitic magmas generate in the arc where the crust is thin and provides a new insight into the magma genesis of the Kermadec arc.

Mineralogy of K-Rich Rocks from the Jharia Basin in Jharkhand: Indications for Cretaceous Lamproite Magmatism in Eastern India

AbstractPotassic dikes of early Cretaceous age (ca. 115 Ma) are emplaced in the Gondwana sedimentary basins of the Damodar valley in eastern India. Two potassic dikes in the Jharia basin (Ena and Moonidih) are characterized by the presence of olivine macrocrysts and phenocrysts, together with microphenocrysts of olivine, phlogopite–biotite, ilmenite, and apatite. These are set in a groundmass of phlogopite, apatite, ilmenite, spinel, and K-feldspar. Microphenocrystal and groundmass Al-Na-poor diopside and Al-poor Ti-rich amphibole are present in the Ena dike, whereas rutile, pyrite, galena, Zr-Ti-rich garnet, and quartz are minor phases occurring in the Moonidih dike. Two populations of relatively fresh olivine (I and II) are found in the Ena dike, in contrast to the Moonidih dike, where olivine macrocrysts and microphenocrysts are completely replaced by secondary magnesite, serpentine, and talc. The occurrence of fresh olivine in the Ena dike is reported for the first time in the Jharia basin. High forsterite (Fo) and Ni together with low Ca and Mn contents of the olivine-I macrocrysts suggest a mantle-derived xenocrystic origin. In contrast, the olivine-II population has high Ca and Mn, but relatively low Fo and Ni, suggesting a magmatic origin. Rimward decreasing Al and increasing Fe contents in zoned phlogopite crystals, plus the presence of groundmass spinel with compositions along the titanomagnetite trend (T2), indicate the lamproitic affinity of the K-rich dike rocks. Using a mineralogical-genetic classification scheme, the Ena and Moonidih dikes are classified as olivine-phlogopite-diopside-apatite-amphibole-feldspar lamproite and olivine-phlogopite-apatite-feldspar lamproite, respectively. We use the compositional variations of phlogopite, amphibole, apatite, and spinel to understand better the magmatic evolution and crystallization history of these potassic dikes. The Cretaceous lamproite magmatic event in eastern India was sourced from ancient metasomatized lithospheric mantle, with low-degree partial melting either triggered in the periphery of the impinging Kerguelen mantle plume head or induced by distant subduction.

Magmatism of the Beka Volcanic Massifs (Cameroon Volcanic Line, West-Central Africa): New Petrographical and Mineralogical Data

The Beka volcanic massifs are located northeast of Ngaoundere region, within the Adamawa plateau. It consists mainly of basanites, trachytes and phonolites. The petrographic study shows that all the basanite lavas have porphyritic microlitic textures with a more pronounced magmatic fluidity than the felsic lavas displaying trachytic textures. The lavas are composed of phenocrysts, microlites and microphenocrysts of olivine, clinopyroxene, plagioclase and iron-titanium oxides for the basanites and of greenish clinopyroxene, alkali feldspar, and titanomagnetite for the felsic lavas. Chemical microprobe analysis indicates that the olivine crystals are magnesian (Fo73-78). Clinopyroxene crystals have a composition of diopside (Wo47-) in the basaltic lavas and diopside near the hedenbergite pole in the trachytes phonoliths and titanomagnetite (TiO2: 21.13% - 22.36% and FeO: 68% - 68%). Chemical analyses on whole rocks show that all the lavas belong to the same series and the felsic lavas come from the differentiation of basanite lavas by fractional crystallization of the minerals therein. The basanites originate from a low rate of partial melting of an OIB-type mantle. Contamination and mixing processes are suspected. Lavas of similar composition are found in other volcanic centres of the Adamawa plateau and the continental and oceanic sectors of the Cameroon Volcanic Line, in particular the Kapsiki plateau, Mounts Cameroon and Bamenda.

Pyroxenite in the mantle source of basanites at the Youkou maar, Adamawa Volcanic Massif (Cameroon Volcanic Line, West Africa)

The Adamawa Volcanic Massif of the Cameroon Volcanic Line (CVL) is dominated by silica-undersaturated basaltic lavas, which have been attributed to plume volcanism. However, the relative contributions of peridotite and pyroxenite sources have not been assessed. Here, we present mineral and whole-rock data for the lavas to consider this issue and to better understand their petrogenesis and source mineralogy. The lavas are porphyritic, with phenocrysts and micro-phenocrysts of olivine, and to a lesser extent, clinopyroxene, in a groundmass of plagioclase, oxides, and apatite. The lava compositions are all basanites. They are silica-undersaturated and are primitive, mantle-derived melts. The basanites display lower CaO and CaO/Al2O3 ratio but higher Fe/Mn values than expected for melts of peridotite, consistent with an appreciable contribution from melting of pyroxenite in the source.Olivine phenocrysts are characterized by low CaO and NiO contents but higher Fe/Mn ratios than those of olivine in melts of peridotitic sources, supporting the contribution of pyroxenite to the primary melts, and that the main mantle source lithology for the basanites was a mixture of spinel-facies peridotite and silica-deficient pyroxenite. The presence of both pyroxenite and peridotite xenoliths in this area attests to the contribution of these two mantle lithologies to the primary melts, and their relative contribution varies on a very local scale. Significant contamination by crustal materials during magma ascent is ruled out. The Youkou basanites provide a key for improving the general understanding of melt production in lithologically heterogeneous mantle beneath the CVL. Beneath the Youkou volcano, partial melting of pyroxenite by decompression formed silica-deficient melts at 60-80 km depth, which modified the peridotite mantle source by a low degree melt enrichment process indicated by the enrichment in incompatible elements. This modified mantle source then underwent low-degree melting to produce the basanitic melts, which are concentrated in a linear zone focused by the combination of the northern edge of the Congo craton and the Central African Shear Zone.

Geochemistry of continental alkali basalts in the Sabzevar region, northern Iran: implications for the role of pyroxenite in magma genesis

A geochemical study was undertaken on basaltic lava flows intercalated with Oligocene to Miocene strata in the Sabzevar region, northern Iran, to examine their petrogenesis in a regional tectonic framework. The lavas are either aphyric or have phenocrysts and micro-phenocrysts of olivine and, to a lesser extent, clinopyroxene. Geochemically, the lavas are silica-undersaturated alkali basalts characterized by relatively high Mg# (~ 57–66) and Na2O/K2O (~ 2.0–6.7). They have distinctive trace element patterns characterized by strong rare-earth element fractionation, negative Nb–Ta and Zr-Hf anomalies and a positive Sr anomaly. Significant contamination by crustal materials either in the magma source or during ascent is ruled out on the basis of trace element compositions and Sr–Nd isotopic compositions (87Sr/86Sr = 0.7037–0.7048 and 143Nd/144Nd = 0.5128–0.5130), both of which differ markedly from continental crustal rocks. Phenocryst assemblage, analysis of multiple saturation points in lherzolite systems, and covariations of La/Yb with MgO of the studied lavas are generally consistent with an origin involving high-pressure fractionation of peridotite-derived melts. Primary magma compositions calculated by reversed fractionation of clinopyroxene and olivine for the relatively primitive samples (> 9 wt.% MgO), however, do not plot on the lherzolite multiple saturation points. Also, high-pressure fractionation predicts increasing trends of silica undersaturation and alkalinitiy with differentiation, and such trends are not indicated by the geochemical data. We suggest that the mixed trends shown by the data might be related to melt generation from both peridotite and silica-deficient pyroxenite sources, superimposed by variable degrees of high-pressure fractionation. The role of pyroxenite in magma genesis is indicated not only by the positive Sr anomaly shown by the trace element patterns, but also first-row transition element systematics of the studied lavas. The silica-deficient pyroxenites contributing to melt generation might have been transformed from mafic–ultramafic cumulates in subducted, lower oceanic crust, or might have formed in the lower crust or mantle lithosphere under continents during earlier magmatic episodes.

Large-volume basaltic hyaloclastite eruption along a propagating land/lake lithosphere fracture at Lake Van (Eastern Anatolia): impact of volcanism on the evolution of Lake Van V

The Incekaya hyaloclastite cone (eastern Anatolia, Turkey), the focal point along a major eruptive fissure, was the main source of an unusually large explosive basaltic eruption. The ca. 80 ka-old eruption began onshore with scoria cones from a 5 km N-S fracture propagating toward Lake Van (surface area of 3755 km2). At the intersection with the fault-bounded lake basin, a ca. 400-m-high subaerial hyaloclastite edifice formed, which can be crudely subdivided into a main lower massive bulk of hydrothermally altered lithic-rich hyaloclastites (CL) topped unconformably by a > 30-m-thick, well-bedded fallout tephra (CU). The CU tephras are correlated with (1) widespread onshore hyaloclastite fallout deposits mostly west-southwest of the cone and (2) a ca. 2-m-thick, ca 80-ka-old bedded hyaloclastite (V-60), part of a 220 m ICDP (International Continental Scientific Drilling Program) core, drilled in Lake Van, 27 km N of Incekaya. The hyaloclastite unit was seismically identified as being the most widespread and well-defined reflector throughout much of western Lake Van. A minimum volume of > 9 km3 fallout hyaloclastite tephra is estimated when the area of the seismic reflector is extrapolated to the coast and 2 km inland. Seismic reflectors also suggest at least two (hyaloclastite?) intralake cones rising up to 388 m above the lake sediment surface 1.5 km NW off Incekaya cone and were possibly erupted along the same fracture. The total volume of hyaloclastites includes (a) subaerial Incekaya cone, (b) the inferred subaqueous continuation of the cone(s), (c) the bedded intralake and onshore deposits, and, tentatively, (d) a widespread (seismically defined) mass flow deposits directly beneath Incekaya reflector of roughly 20 km3 and may represent the deposits of explosively erupted basaltic magma. Sideromelane shards, the main clast type, are dominantly angular, and most show ≪ 50 vol.% vesicles. Less common tachylite clasts are poorly vesicular (< 50 vol.%). Structural transitions and interlayering between tachylite and sideromelane are ubiquitous. Fluidal and pumiceous lapilli are present in the basal massive facies. Bulk rock and glass compositions indicate constant composition of the slightly evolved Al-rich basalt magma. Olivine (Fo78–82) and plagioclase (An70–80) microphenocrysts, many skeletal with growth features, and microlites make up < 1 vol.% and suggest rapid magma ascent. The high explosive energy of the eruption is interpreted to be due to (1) high magma discharge rates and shearing in the eruptive jet and (2) magma-water interaction conditions. Approximate temporal coincidences with the Incekaya eruption include the following: (a) an abrupt cessation in the supply of evolved tephra from the adjacent Suphan Volcano to the lake sediments, which ended abruptly for ca. 60 ky, (b) an extreme fall in lake level by ca. 150 m, and (c) a drastic increase in pore water salinity (Na+ and Cl− (g/L)) and pH.

Oldest lamproites from Peninsular India track the onset of Paleoproterozoic plume-induced rifting and the birth of Large Igneous Province

Potassic and ultrapotassic magmatism from deep lithospheric sources in intra-cratonic settings can be the signal of subsequent voluminous mafic magmatism and the formation of Large Igneous Provinces (LIPs) triggered by mantle plumes. Here we report for the first time, precise zircon U–Pb age data from a suite of lamproites in the Bastar Craton of central India that mark the onset of Paleoproterozoic rifting and culminating in the formation of extensive mafic dyke swarms as the bar codes of one of the major LIP events during the Precambrian evolution of the Indian shield. The lamproites from the Nuapada field occur as dismembered dykes and are composed of phenocrysts and microphenocrysts of altered olivine together with microphenocrysts of phlogopite and magnetite within a groundmass of chlorite and calcite with accessory rutile, apatite and zircon. The rocks compositionally correspond to olivine phlogopite lamproite and phlogopite lamproite. Geochemical features of the lamproites correlate with their counterparts in Peninsular India and other similar suites elsewhere in the world related to rift settings, and also indicate OIB-like magma source. The associated syenite shows subduction-related features, possibly generated in a post-collisional setting. Magmatic zircon grains with high Th/U ratios in the syenite from the Nuapada lamproite form a coherent group with an upper intercept age of 2473±8Ma representing the timing of emplacement of the magma. Zircon grains in three lamproite samples yield four distinct age groups at ca. 2.4Ga, 2.2Ga, 2.0Ga and 0.8Ga. The 2.4Ga group corresponds to xenocrysts entrained from the syenite whereas the 2.2Ga group is considered to represent the timing of emplacement of the lamproites. The ca. 2.0Ga zircon grains correlate with the major thermal imprint associated with mafic magmatism and dyke emplacement in southern Bastar and the adjacent Dharwar Cratons. A few young zircon grains in the syenite and lamproites show a range of early to middle Neoproterozoic ages from 879 to 651Ma corresponding to younger thermal event(s) as also represented by granitic veins cutting across these rocks and extensive silicification. Zircon Lu–Hf isotope data suggest magma derivation from a refertilized Paleo-Mesoarchean sub-continental lithospheric mantle, or OIB-type sources. The differences in Hf-isotope composition among the zircon grains from different age groups indicate that the mantle sources of the lamproite are heterogeneous at the regional scale. A combination of the features from geochemical and zircon Hf isotope data is consistent with asthenosphere-lithosphere interaction during the lamproite magma evolution.The timing of lamproite emplacement in central India correlates with the global 2.2Ga record of LIPs. We link the origin of the related mantle plume to the recycling of subducted slabs associated with the prolonged subduction-accretion history prior to the Neoarchean cratonization, as well as the thermal blanket effect of the Earth's oldest supercontinent. Pulsating plumes and continued rifting generated voluminous dyke swarms across the Bastar and Dharwar Cratons, forming part of a major global rifting and LIP event.

The Cerro Bitiche Andesitic Field: petrological diversity and implications for magmatic evolution of mafic volcanic centers from the northern Puna

The Cerro Bitiche Andesitic Field (CBAF) is one of the two largest mafic volcanic fields in northern Puna (22–24° S) and is spatially and temporally associated with ignimbrites erupted from some central Andean Altiplano-Puna Volcanic Complex calderas. The CBAF comprises seven scoria cones and widespread high-K calcalkaline lava flows that cover an area of 200 km2. Although all erupted rocks have a relatively narrow chemical range (56–62 % SiO2, 3–6 % MgO), there is a broad diversity of mineral compositions and textures. The least evolved lavas (∼58–61 % SiO2) are high-Mg andesites with scarce (<10 %) microphenocrysts of either olivine or orthopyroxene. The small compositional range and low phenocryst content indicate evolution controlled by low percentages (<10 %) of fractional crystallization of olivine and clinopyroxene of magmas similar to the least evolved rocks from the field, accompanied by assimilation during rapid ascent through the crust. Evolved andesites (∼62 wt% SiO2), on the other hand, are porphyritic rocks with plagioclase + orthopyroxene + biotite and ubiquitous phenocryst disequilibrium textures. These magmas were likely stored in crustal reservoirs, where they experienced convection caused by mafic magma underplating, magma mixing, and/or assimilation. Trace element and mineral compositions of CBAF lavas provide evidence for complex evolution of distinct magma batches.

CV and CM chondrite impact melts

Volatile-rich and typically oxidized carbonaceous chondrites, such as CV and CM chondrites, potentially respond to impacts differently than do other chondritic materials. Understanding impact melting of carbonaceous chondrites has been hampered by the dearth of recognized impact melt samples. In this study we identify five carbonaceous chondrite impact melt clasts in three host meteorites: a CV3red chondrite, a CV3oxA chondrite, and a regolithic howardite. The impact melt clasts in these meteorites respectively formed from CV3red chondrite, CV3oxA chondrite, and CM chondrite protoliths. We identified these impact melt clasts and interpreted their precursors based on their texture, mineral chemistry, silicate bulk elemental composition, and in the case of the CM chondrite impact melt clast, in situ measurement of oxygen three-isotope signatures in olivine. These impact melts typically contain euhedral–subhedral olivine microphenocrysts, sometimes with relict cores, in glassy groundmasses. Based on petrography and Raman spectroscopy, four of the impact melt clasts exhibit evidence for volatile loss: these melt clasts either contain vesicles or are depleted in H2O relative to their precursors. Volatile loss (i.e., H2O) may have reduced the redox state of the CM chondrite impact melt clast. The clasts that formed from the more oxidized precursors (CV3oxA and CM chondrites) exhibit phase and bulk silicate elemental compositions consistent with higher intrinsic oxygen fugacities relative to the clast that formed from a more reduced precursor (CV3red chondrite). The mineral chemistries and assemblages of the CV and CM chondrite impact melt clasts identified here provide a template for recognizing carbonaceous chondrite impact melts on the surfaces of asteroids.

Pliocene–Quaternary basalts from the Harrat Tufail, western Saudi Arabia: Recycling of ancient oceanic slabs and generation of alkaline intra-plate magma

Harrat Tufail represents a Caenozoic basalt suite at the western margin of the Arabian plate. This rift-related suite includes voluminous Quaternary non-vesicular basalt (with fragments of earlier Pliocene vesicular flow) that forms a cap sheet over Miocene rhyolite and minor vesicular basalt. The contact between rhyolite and the basaltic cap is erosional with remarkable denudations indicating long time gap between the felsic and mafic eruptions. The geochemical data prove alkaline, sodic and low-Ti nature of the olivine basalt cap sheet. The combined whole-rock and mineral spot analyses by the electron microprobe (EMPA) suggest magma generation from low degree of partial melting (∼5%) from spinel- and garnet-lherzolite mantle source. Derivation from a mantle source is supported by low Na content in clinopyroxene (ferroan diopside) whereas high Mg content in ilmenite is an evidence of fractional crystallization trajectory. Accordingly, the Pliocene basaltic cap of Harrat Tufail is a product of mantle melt that originates by recycling in the asthenosphere during subduction of ancient oceanic slab(s). The whole-rock chemistry suggests an ancient ocean island basaltic slab (OIB) whereas the EMPA of Al-rich spinel inclusions in olivine phenocrysts are in favour of a mid-ocean ridge basaltic source (MORB). Calculations of oxygen fugacity based on the composition of co-existing Fe–Ti oxide suggest fluctuation from highly to moderately oxidizing conditions with propagation of crystallization (log10 fO2 from −22.09 to −12.50). Clinopyroxene composition and pressure calculation indicates low-pressure (0.4–2 kbar). Cores of olivine phenocrysts formed at highest temperature (1086–1151 °C) whereas the rims and olivine micro-phenocrysts formed at 712–9–796 °C which is contemporaneous to formation of clinopyroxene at 611–782 °C. Fe–Ti oxides crystallized over a long range (652–992 °C) where it started to form at outer peripheries of olivine phenocrysts and as interstitial phase with clinopyroxene.

Exploring the Moon's surface for remnants of the lunar mantle 1. Dunite xenoliths in mare basalts. A crustal or mantle origin?

AbstractRemotely sensed observations from recent missions (e.g., GRAIL, Kaguya, Chandrayaan‐1) have been interpreted as indicating that the deep crust and upper mantle are close to or at the lunar surface in many large impact basins (e.g., Crisium, Apollo, Moscoviense). If this is correct, the capability of either impact or volcanic processes to transport mantle lithologies to the lunar surface should be enhanced in these regions. Somewhat problematic to these observations and interpretations is that examples of mantle lithologies in the lunar sample collection (Apollo Program, Luna Program, lunar meteorites) are at best ambiguous. Dunite xenoliths in high‐Ti mare basalt 74275 are one of these ambiguous examples. In this high‐Ti mare basalt, olivine occurs in three generations: olivine associated with dunite xenoliths, olivine megacrysts, and olivine microphenocrysts. The dunite xenoliths are anhedral in shape and are generally greater than 800 μm in diameter. The interior of the xenoliths are fairly homogeneous with regard to many divalent cations. For example, the Mg# (Mg/Mg + Fe × 100) ranges from 82 to 83 in their interiors and decreases from 82 to 68 over the 10–30 μm wide outer rim. Titanium and phosphorus X‐ray maps of the xenolith illustrate that these slow diffusing elements preserve primary cumulate zoning textures. These textures indicate that the xenoliths consist of many individual olivine grains approximately 150–200 μm in diameter with low Ti, Al, and P cores. These highly incompatible elements are enriched in the outer Fe‐rich rims of the xenoliths and slightly enriched in the rims of the individual olivine grains. Highly compatible elements in olivine such as Ni exhibit a decrease in the rim surrounding the xenolith, an increase in the incompatible element depleted cores of the individual olivine grains, and a slight decrease in the “interior rims” of the individual olivine grains. Inferred melt composition, liquid lines of descent, and zoning profiles enable the reconstruction of the petrogenesis of the dunite xenoliths. Preservation of primary magmatic zoning (Ti, P, Al) and lack of textures similar to high‐pressure mineral assemblages exhibited by the Mg‐suite (Shearer et al. 2015) indicate that these xenoliths do not represent deep crustal or shallow mantle lithologies. Further, they are chemically and mineralogically distinct from Mg‐suite dunites identified from the Apollo 17 site. More likely, they represent olivine cumulates that crystallized from a low‐Ti mare basalt at intermediate to shallow crustal levels. The parent basalt to the dunite xenolith lithology was more primitive than low‐Ti basalts thus far returned from the Moon. Furthermore, this parental magma and its more evolved daughter magmas are not represented in the basalt sample suite returned from the Taurus‐Littrow Valley by the Apollo 17 mission. The dunite xenolith records several episodes of crystallization and re‐equilibration. During the last episode of re‐equilibration, the dunite cumulate was sampled by the 74275 high‐Ti basalt and transported over a period of 30–70 days to the lunar surface.

Hydrothermal alteration of plagioclase microphenocrysts and glass in basalts from the East Pacific Rise near 13°N: An SEM-EDS study

The interactions of seafloor hydrothermal fluid with igneous rocks can result in leaching elements from the rocks, creating potential ore-forming fluids and influencing the chemical compositions of near-bottom seawater. The hydrothermal alteration of plagioclase microphenocrysts and basaltic glass in the pillow basalts from one dredge station (103 degrees 57.62'W, 12 degrees 50.55'N, water depth 2480 m) on the East Pacific Rise (EPR) near 13 degrees N were analyzed using a scanning electron microscope (SEM) and energy dispersive X-ray spectrometry (EDS). The results show that the edges of the plagioclase microphenocrysts and the basaltic glass fragments are altered but the pyroxene and olivine microphenocrysts in the interior of the pillow basalts appear to be unaffected by the hydrothermal fluids. In addition, our results show that the chemical alteration at the rims of the plagioclase microphenocrysts and the edges of basaltic glass fragments can be divided into separate types of alteration. The chemical difference in hydrothermal alteration of the plagioclase microphenocrysts and the basaltic glass indicate that different degrees of hydrothermal fluid-solid phase interaction have taken place at the surface of the pillow basalts. If the degree of hydrothermal fluid-solid phase interaction is relatively minor, Si, Al, Ca and Na diffuse from the inside of the solid phase out and as a result these elements have a tendency to accumulate in the edge of the plagioclase microphenocrysts or basaltic glass. If the degree of hydrothermal fluid-solid phase interaction is relatively strong, Si, Al, Ca and Na also diffuse from the inside of solid phase out but these elements will have a relatively low concentration in the edge of the plagioclase microphenocrysts or basaltic glass. Based on the chemical variation observed in the edges of plagioclase microphenocrysts and basaltic glass, we estimate that the content of Si, Al and Fe in the edges of plagioclase microphenocrysts can have a variation of 10.69%, 17.59% and 109%, respectively. Similarly, the Si, Al and Fe concentrations in the edges of basaltic glass can have a variation of 9.79%, 16.30% and 37.83%, respectively, during the interaction of hydrothermal fluids and seafloor pillow basalt.

Primary melt from Sannome-gata volcano, NE Japan arc: constraints on generation conditions of rear-arc magmas

The conditions under which rear-arc magmas are generated were estimated using primary basalts from the Sannome-gata volcano, located in the rear of the NE Japan arc. Scoriae from the volcano occur with abundant crustal and mantle xenoliths, suggesting that the magma ascended rapidly from the upper mantle. The scoriae show significant variations in their whole-rock compositions (7.9–11.1 wt% MgO). High-MgO scoriae (MgO > ~9.5 wt%) have mostly homogeneous 87Sr/86Sr ratios (0.70318–0.70320), whereas low-MgO scoriae (MgO 0.70327); ratios tend to increase with decreasing MgO content. The high-MgO scoriae are aphyric, containing ~5 vol% olivine microphenocrysts with Mg# [100 × Mg/(Mg + Fe2+)] of up to 90. In contrast, the low-MgO scoriae have crustal xenocrysts of plagioclase, alkali feldspar, and quartz, and the mineralogic modes correlate negatively with whole-rock MgO content. On the basis of these observations, it is inferred that the high-MgO scoriae represent primary or near-primary melts, while the low-MgO scoriae underwent considerable interaction with the crust. Using thermodynamic analysis of the observed petrological features of the high-MgO scoriae, the eruption temperature of the magmas was constrained to 1,160–1,220 °C. Given that the source mantle was depleted MORB-source mantle, the primary magma was plausibly generated by ~7 % melting of a garnet-bearing spinel peridotite; taking this into consideration, and considering the constraints of multi-component thermodynamics, we estimated that the primary Sannome-gata magma was generated in the source mantle with 0.5–0.6 wt% H2O at 1,220–1,230 °C and at ~1.8 GPa, and that the H2O content of the primary magma was 6–7 wt%. The rear-arc Sannome-gata magma was generated by a lower degree of melting of the mantle at greater depths and lower temperatures than the frontal-arc magma from the Iwate volcano, which was also estimated to be generated by ~15 % melting of the source mantle with 0.6–0.7 wt% H2O at ~1,250 °C and at ~1.3 GPa.

Discovery of Lamproites near Vattikod Area, NW Margin of the Cuddapah Basin, Eastern Dharwar Craton, Southern India

Abstract A cluster of lamproite dykes are located 1 km west of Vattikod village at the NW margin of the Cuddapah basin, Eastern Dharwar craton, southern India, during the pursuit for locating primary diamond source rocks by adapting multifarious applications. These exotic rocks are emplaced along WNW-ESE to NW-SE trending fractures in the granitic rocks belonging to the Peninsular Gneissic Complex. Ten out of twelve lamproites occur near Vattikod village and one each is located in the vicinity of Marepalli and Gundrapalli villages respectively. These lamproites, though highly altered, contain microphenocrysts of altered olivine, clinopyroxene, phlogopite, leucite and sanidine and translucent to opaque, amoeboid shaped patches of glass set in a groundmass rich in carbonate, phlogopite, serpentine, and chlorite. This new cluster of lamproites constitutes a part of the recently discovered Ramadugu lamproite field. The Vattikod and Ramadugu lamproites, together with those from Krishna lamproite field and the Cuddapah basin, constitute, a wide spectrum of ultrapotassic magmatism emplaced in and around the Palaeo-Mesoproterozoic Cuddapah basin in southern India.

Magmatic controls on eruption dynamics of the 1950 yr B.P. eruption of San Antonio Volcano, Tacaná Volcanic Complex, Mexico–Guatemala

San Antonio Volcano, in the Tacaná Volcanic Complex, erupted ~1950yr. B.P., with a Pelean type eruption that produced andesitic pyroclastic surges and block-and-ash flows destroying part of the volcano summit and producing a horse-shoe shaped crater open to the SW. Between 1950 and 800yrB.P. the eruption continued with effusive andesites followed by a dacite lava flow and a summit dome, all from a single magma batch. All products consist of phenocrysts and microphenocrysts of zoned plagioclase, amphibole, pyroxene, magnetite±ilmenite, set in partially crystallized groundmass of glass and microlites of the same mineral phases, except for the lack of amphibole. Included in the andesitic blocks of the block-and-ash flow deposit are basaltic andesite enclaves with elongated and ellipsoidal forms and chilled margins. The enclaves have intersertal textures with brown glass between microphenocrysts of plagioclase, hornblende, pyroxene, and olivine, and minor proportions of phenocrysts of plagioclase, hornblende, and pyroxene. A compositional range obtained of blocks and enclaves resulted from mixing between andesite (866°C±22) and basaltic andesite (enclaves, 932°C±22), which may have triggered the explosive Pelean eruption. Vestiges of that mixing are preserved as complex compositional zones in plagioclase and clinopyroxene-rich reaction rims in amphibole in the andesite. Whole-rock chemistry, geothermometry, experimental petrology and modeling results suggest that after the mixing event the eruption tapped hybrid andesitic magma (≤900°C) and ended with effusive dacitic magma (~825°C), all of which were stored at ~200MPa water pressure. A complex open-system evolution that involved crustal end-members best explains the generation of effusive dacite from the hybrid andesite. Amphibole in the dacite is rimmed by reaction products of plagioclase, orthopyroxene, and Fe–Ti oxides produced by decompression during ascent. Amphibole in the andesite, however, lacks such rims. Because the andesite was at 866±22°C and the dacite was at ~825°C, the reaction rims indicate that the andesitic magma ascended at 0.023ms−1 during the explosive phase of the eruption, whereas the dacitic magma rose more slowly at ~0.002–0.004ms−1.

Transition zone origin of potassic basalts from Wudalianchi volcano, northeast China

The origin of historic potassic basalts (from the 1719 to 1721AD eruption) from the Wudalianchi volcanic field, northeast China, is investigated. The samples examined in this study are aphyric, and contain ~5vol.% olivine microphenocrysts. Geochemical analyses of the samples, along with published data for material from the 1719 to 1721AD eruption, show that the basalts exhibit linear compositional trends in whole-rock variation diagrams, and are characterized by K2O enrichment (>4wt.%) and an EM1-like isotopic signature. Using thermodynamic calculations and the observed petrological features of the basalts, the temperature of the magmas shortly before eruption is estimated to have been ~1250°C, and the water content of the magma at depth is estimated to have been >~1.1wt.%. Because the estimated temperatures are significantly higher than the projected maximum temperature of the lithospheric mantle beneath the Wudalianchi volcanic field, the magmas were likely derived from the asthenospheric mantle. We suggest that both the potassic- and EM1-like natures of the basalts originated from the mantle transition zone, metasomatized by K-rich sediment fluids ~1.5Ga ago through a stagnation of an ancient slab. The linear whole-rock compositional trends of the basalts primarily reflect the geochemical heterogeneity in the mantle transition zone, with variable but coupled contributions of ~1.5Ga sediments in the ancient stagnant slab, and recent sediments and peridotites in the stagnant Pacific slab. We infer that the Wudalianchi magmatism was caused by an upwelling of a hydrous mantle plume from the mantle transition zone, which was hydrated through the stagnation of the ancient subducted slab and the recent Pacific slab.

제주도 현무암 내 각섬석의 성인에 대한 연구(I): 비석거리 하와이아이트

제주도 사라봉 부근의 산지등대, 비석거리에서 나타나는 하와이아이트에는 티탄이 풍부한 각섬석(캘수타이트)과 사장석이 반정으로 나타난다. 또한 휘석, 감람석 미반정을 포함하며, 인회석과 소량의 K장석 래스도 나타난다. 캘수타이트는 자형 또는 반자형으로 불투명광물 반응연을 가진 반정으로 주로 산출되며, 해안가 시료의 캘수타이트는 산화철 형태로 치환되어 각섬석 가상을 관찰할 수 있다. 아주 드문 형태로 휘석 결정 내에 캘수타이트가 반응연 관계로 나타나는 형태를 볼 수 있으며, 이를 통해 유체에 의한 2차적인 수화반응이 있었음이 추정된다. 압력-<TEX>$Al^T$</TEX> 지질압력계 관계식에 적용하여 결정화작용 압력을 추정한 결과, 산지등대의 시료에서는 약 6.3 kb, 비석거리의 경우 약 4.9 kb의 값을 얻을 수 있었다. 이를 통해 산지등대의 각섬석은 비석거리에 형성된 각섬석보다 더 깊은 곳에서 유체의 유입이 있었으며, 자형 반정의 형태와 인회석 포유물의 존재로부터 호스트마그마로부터 결정화작용을 통해 성장되었을 것으로 추정된다. Hawaiite which distributed in Sanjideungdae of Sarabong cinder cone and Biseokgeori area in northern part of Jeju island, contains phenocrysts of titanium-rich hornblende (kaersutite) and plagioclase with microphenocrysts of olivine, pyroxene and very small amounts of K-feldspar lath and apatite. Kaersutite is mostly euhedral or subhedral phenocrysts having opaque reaction rim. And kaersutite in Sanjideungdae area completely replaced to opaque minerals showing pseudomorph. Also it may be seen partly replacement of pyroxene by kaersutire as reaction rim. It is considered that hydration reaction had occurred with fluids. The crystallization pressure of kaersutite using pressure-<TEX>$Al^T$</TEX> geobarometer is approximately 6.3 kb in Sanjideungdae area and 4.9 kb in Biseokgeori area, respectively. As a result, fluid injection to magma and crystallization of kaersutite of Sanjideungdae hawaiite is deeper than that of Biseokgeori hawaiite, and it was growed to phenocrysts through crystallization. It is estimated that kaersutite of Biseokgeori hawaiite originated from crystallization from the host magma, based on the euhedral nature of the phenocrysts and on the presence of apatite inclusions.

KIMBERLITE SILLS AND DYKES ASSOCIATED WITH THE WESSELTON KIMBERLITE PIPE, KIMBERLEY, SOUTH AFRICA

Field relations, petrography and bulk rock geochemistry constrain the emplacement mechanisms and petrogenesis of a kimberlite sill complex exposed in water tunnels at Wesselton Mine, Kimberley. The sill complex, together with associated dykes, is precursor to the main Wesselton pipe and contains: 1. aphanitic kimberlite with a carbonatitic groundmass of calcite, perovskite, spinel, serpentine and phlogopite, with variations from rocks rich in olivine microphenocrysts to rocks with little or no olivine; 2. macrocrystic kimberlite containing entrained harzburgite microxenoliths, olivine macrocrysts and strongly reacted orthopyroxene; and, 3. a ‘green’ aphanitic kimberlite. The sills were emplaced as multiple intrusions into the upper Dwyka shales immediately below a Karoo dolerite sill in a zone several metres thick. Emplacement of the sill complex was governed by the rigidity of the dolerite sill, which prevented the magma from breaching the surface. Feeder dykes to the sills were influenced by a regional crustal stress system with the principal vector of horizontal compression being between eastwest and northwest-southeast. Serpentinisation has pseudomorphed olivine, replaced calcite, apatite and phlogopite in the groundmass and thereby changed the bulk compositions. Geochemcal and mineral chemistry data on spinels and phlogopites indicate intrusion of several magma batches with different petrogenetic histories. The magmas are transitional to carbonatite with compositional variations linked to partial melting, assimilation of xenocrysts, entrainment of xenoliths, xenocrysts and macrocryst, fractional crystallization and flow differentiation during ascent and emplacement.

Mafic and silica-rich glasses in mantle xenoliths from Wau-en-Namus, Libya: Textural and geochemical evidence for peridotite–melt reactions

Anhydrous spinel peridotite xenoliths in Quaternary nepheline-basanite and melilite- or sodalite-bearing lavas of the Wau-en-Namus volcano in S Libya range from lherzolites to harzburgites recording melt extraction in a shallow setting (≤ 2 GPa). Primary clinopyroxenes have distinct trace element characteristics documenting LILE (large ion lithophile element) depletion or enrichment events predating the formation of glass pockets and veins in the xenoliths. These glasses are aluminous and alkali-rich, range in composition from ultrabasic to silicic (43–67 wt.% SiO 2) and may contain empty vugs and micro-phenocrysts of olivine, clinopyroxene, spinel, plagioclase, sodalite, apatite that are similar in composition to phenocrysts in the host lavas. Reactions of infiltrating melt and xenolith minerals are documented by diffuse Fe–Ca-rich rims of olivine in contact with glass, and by spongy-textured reaction domains caused by incongruent dissolution of primary pyroxenes and spinel. Some glasses have trace element characteristics similar to that of the host Ne-basanite, suggesting they were derived from the same source during entrainment and transport to the surface. Incompatible element enrichment and Sr–Nd isotopic compositions of the analyzed host lava are similar to HIMU (high μ; μ = 238Pb/ 204Pb)-type magmas, but the Pb isotopic composition is less radiogenic compared to other intra-plate Neogene magmatic rocks from N Africa.

Petrogenesis of lamprophyres from Chhota Udepur area, Narmada rift zone, and its relation to Deccan magmatism

Geological setting, petrology and geochemistry of a new find of lamprophyre dykes, intruding the Precambrian basement gneisses from the Chhota Udepur area, Narmada rift zone, Western India, are presented. Of the three lamprophyre dykes, two of them display E–W trend paralleling that of the Narmada rift, extend up to ∼2.5 km in length and constitute the longest yet recorded lamprophyre dykes from the Indian shield. The Chhota Udepur lamprophyres (CUL) show several overlapping mineralogical and geochemical characteristic features displayed by alkaline- and calc-alkaline lamprophyres and hence it is not straight forward to pigeon-hole them in existing lamprophyre classification schemes. They are porphyritic–panidiomorphic with euhedral to sub-hedral phenocrysts and microphenocrysts of olivine (Fo 78.33–78.42 Fa 21.24–21.37), diopsidic augite to titaniferous augite (Wo 54.83–59.03 En 36.75–41.36 Fs 0.1–5.66), biotite and amphibole (kaersutite). Feldspar (orthoclase perthite and plagioclase) is essentially confined to the groundmass along with accessory phases such as apatite and spinel (ti-magnetite). Rare carbonate ocelli are also present. In terms of major element geochemistry the CUL are nepeheline-normative and predominantly sodic (Na 2O > K 2O); yet their K 2O and SiO 2 contents are sufficiently high to signal traits of calc-alkaline lamprophyres. On the other hand, the CUL exclude negative spikes at Nb–Ta which are considered to be a characteristic of subduction-related (calc-alkaline) magmas, are highly evolved (Mg#: 27.82–55.45), enriched in incompatible trace elements (high La/Nb, Zr/Nb) and have fractionated REE distribution patterns (La/Yb N ratios: 35–44). Post-emplacement alteration and crustal contamination have had little influence on their geochemistry. Th/Yb, Ta/Yb, Nb/U and Ce/Pb ratios of the CUL imply an OIB type-enriched mantle source, similar to that displayed by other Deccan-related lamprophyres. A metasomatised (enriched) garnet lherzolite mantle source with very low degrees of partial melting that has subsequently undergone a large degrees of fractional crystallisation is capable of accounting observed REE geochemistry of the CUL. We infer the lamprophyres of Chhota Udepur area to be a part of the alkaline rock spectrum of the Deccan Large Igneous Province and their emplacement, together with that of other alkaline complexes in the Chhota Udepur area, is envisaged to be strongly controlled by pre-existing crustal weakness accentuated by the mantle plume–lithosphere interaction during the end-Cretaceous in the Narmada rift zone.