Alkali Magma Research Articles

Multiple magmatic processes revealed by distinct clinopyroxene populations in the magma plumbing system: A case study from a Miocene volcano in West Qinling, Central China

Abstract The application of whole-rock compositions to trace magma evolution or crystal-melt equilibrium may be called into question when foreign crystals are incorporated into host magmas. To address this challenge, establishing the origins (orthocrysts, antecrysts, xenocrysts, etc.) of minerals with variable textures in magmatic plumbing systems is necessary. In this paper, we describe complex oscillatory zoning patterns of clinopyroxenes (Cpx) from the Miocene Majuangou (MJG) alkali basalts from West Qinling, China. Our detailed petrographic, mineralogical, and geochemical analyses unravel the origins of various Cpx crystals with distinct textural patterns, thereby providing information about magma storage, recharging and/or mixing, and transportation, as well as the reactions between crystals and melts/fluids. Based on textural patterns, Cpx may be divided into four types: normal (Type-1 Cpx), simple oscillatory (Type-2 Cpx), complex oscillatory (Type-3 Cpx), and grains that lack zoning (Type-4 Cpx, suggested to be orthocrysts). Through the textural characterization of Cpx, the comparison between different types of Cpx, and the relationships between Cpx major compositions from different lithologies, we concluded that Type-1–3 Cpx cores are antecrysts or xenocrysts with diverse origins: primitive magma (Type-1 Cpx cores), magma mush (Type-2 Cpx cores), and crustal granulite (Type-3 Cpx cores). The zoning patterns and the compositions of these Cpx crystals indicate at least three batches of magmatic activity, i.e., the Batch-1 low-Mg# magma (Mg#: 47.4–53.3), the Batch-2 primitive magma (Mg#: 57.2–64.5), and the Batch-3 low-Mg# host alkali magma (Mg#: 47.2–54.6). Cpx-melt thermobarometry demonstrates that at least two crustal magma reservoirs existed in the magma plumbing system at depths of 30.1 and 40.9 km. The antecrystic/xenocrystic Cpx cores were captured by, continued to grow in, and subsequently reacted with ascending K-rich melt/fluid. The spongy textures in Cpx cores/mantles are attributed to this reaction, which may be expressed as: Melt 1 (primitive or evolved) + K-rich melts/fluids + Cpx (CaMgSi2O6) = K-feldspar (KAlSi3O8) + ilmenite (FeTiO3) + Melt 2 (derivative). The products of this reaction (K-feldspar and ilmenite) filled the sieves in the spongy zones of Type-1–3 Cpx. This detailed investigation of compositional and textural features of Cpx antecrysts/xenocrysts suggests that the interactions between various interconnected magma reservoirs are widespread beneath the magmatic plumbing system. Our study emphasizes the importance of the incorporation of foreign crystals and the Cpx-melt/fluid reaction in magmatic plumbing system, which can significantly modify the whole-rock compositions and lead to the formation of spongy textures without the need for fractures and cracks in minerals.

Multistage evolution of subcontinental lithospheric mantle of northwestern Deccan volcanic province, India: Constraints from the ultramafic xenoliths in alkali magma

Multistage evolution of subcontinental lithospheric mantle of northwestern Deccan volcanic province, India: Constraints from the ultramafic xenoliths in alkali magma

Structure and properties of alkali aluminosilicate glasses and melts: Insights from deep learning

Aluminosilicate glasses and melts are of paramount importance for geo- and materials sciences. They include most magmas, and are used to produce a wide variety of everyday materials, from windows to smartphone displays. Despite this importance, no general model exists with which to predict the atomic structure, thermodynamic and viscous properties of aluminosilicate melts. To address this, we introduce a deep learning framework, ‘i-Melt’, which combines a deep artificial neural network with thermodynamic equations. It is trained to predict 18 different latent and observed properties of melts and glasses in the K2O-Na2O-Al2O3-SiO2 system, including configurational entropy, viscosity, optical refractive index, density, and Raman signals. Viscosity can be predicted in the 100–1015 log10 Pa·s range using five different theoretical frameworks (Adam-Gibbs, Free Volume, MYEGA, VFT, Avramov-Milchev), with a precision equal to, or better than, 0.4 log10 Pa·s on unseen data. Density and optical refractive index (through the Sellmeier equation) can be predicted with errors equal or lower than 0.02 and 0.006, respectively. Raman spectra for K2O-Na2O-Al2O3-SiO2 glasses are also predicted, with a relatively high mean error of ∼25% due to the limited data set available for training. Latent variables can also be predicted with good precisions. For example, the glass transition temperature, Tg, can be predicted to within 19 K, while the melt configurational entropy at the glass transition, Sconf(Tg), can be predicted to within 0.8 J mol−1 K−1.Applied to rhyolite compositions, i-Melt shows that the rheological threshold separating explosive and effusive eruptions correlates with an increase in the fraction of non-bridging oxygens in rhyolite melts as their alkali/Al ratio becomes larger than 1. Exploring further the effect of the K/(K + Na) ratio on the properties of alkali aluminosilicate melts with compositions varying along a simplified alkali magmatic series trend, we observe that K-rich melts have systematically different structures and higher viscosities compared to Na-rich melts. Combined with the effects of the K/(K + Na) ratio on other parameters, such as the solubility, solution mechanisms and speciation of volatile elements, this could ultimately influence the eruptive dynamics of volcanic systems emitting Na-rich or K-rich alkali magmas.

Petrogenesis of Neogene polymagmatic suites at a monogenetic low-volume volcanic province, Bahariya depression, Western Desert, Egypt

Monogenetic volcanoes can yield eruptive suites displaying substantial complexity in compositional characteristics. The Bahariya monogenetic volcanoes (BMV) in the Western Desert, Egypt are a good example. They comprise complex stratigraphic deposits involving scoria cone, lava flows, and subvolcanic sills and dikes related to diverse eruptive styles. Whole-rock and mineral chemistry and 40Ar–39Ar geochronology are used here to document the petrogenesis, source characteristics, and evolution of the Bahariya volcanoes. The architecture of the BMV is the product of two alkali magma batches: pyroclastics and lava flows forming explosive scoria cone (batch 1) and subvolcanic sills (batch 2). The two batches show contrast in the concentrations of incompatible trace elements and REE as well as element ratios such as Nb/Yb, Gd/Yb, Nb/U, and Ce/Pb (36, 5.0, 44, 30 vs. 17, 4.0, 39, 24 for batch 1 and 2, respectively). New whole-rock 40Ar/39Ar dating displays consistent age of 23.71 ± 0.06 and 23.73 ± 0.01 Ma for magma emplacement of batch 2. Batches 1 and 2 share common LILE and LREE enrichments and HFSE depletions, analogous to a HIMU-like, mantle-derived OIB source described from other Paleogene–Oligocene intraplate magmatic provinces in North Africa and worldwide. Trace element modeling proposes a derivation of the Bahariya volcanoes from parental melts generated by 8–12% partial melting of garnet lherzolite and amphibole-bearing garnet lherzolite at 2.18 ± 0.33 and 1.77 ± 0.33 GPa for batch 1 and batch 2, respectively, across the lithosphere–asthenosphere boundary at c. 70–90 km depth (2.14–2.76 GPa). These sources had been earlier metasomatized by volatile-, LILE- and HFSE-rich fluid(s) originating from Neoproterozoic subduction or a Phanerozoic plume. Fractional crystallization involved olivine + clinopyroxene in both batches followed by Fe–Ti oxides + apatite in batch 2. Furthermore, crust contamination/assimilation was an irrelevant process at crustal level during magma ascend to the surface. Data results of the geo-barometric computations disclose two magma storage levels involving intermediate to lower crustal levels at c. 35 km (1.05 GPa) for batch 1 and mid-crustal level at c. 25 km depth (0.75 GPa) for batch 2. This study delivers proof that magmas emitted at Bahariya depression can undergo complex polymagmatic processes during their storage and passage in the crust, mainly due to the existence of a multilevel plumbing system. The origin of the BMV, as with other within-plate volcanoes in North Egypt, appears to be allied to extension-induced asthenosphere upwelling activated by limited exclusion of thickened lithospheric root under a passive rift tectonic regime coupled with the development of lithospheric thinning and continental breakup in North Africa.

The Geochronology and Geochemistry of Zircon as Evidence for the Reconcentration of REE in the Triassic Period in the Chungju Area, South Korea

The Chungju rare-earth element (REE) deposit is located in the central part of the Okcheon Metamorphic Belt (OMB) in the Southern Korean Peninsula and research on REE mineralization in the Gyemyeongsan Formation has been continuous since the first report in 1989. The genesis of the REE mineralization that occurred in the Gyemyeongsan Formation has been reported by previous researchers; theories include the fractional crystallization of alkali magma, magmatic hydrothermal alteration, and recurrent mineralization during metamorphism. In the Gyemyeongsan Formation, we discovered an allanite-rich vein that displays the paragenetic relationship of quartz, allanite, and zircon, and we investigated the chemistry and chronology of zircon obtained from this vein. We analyzed the zircon’s chemistry with an electron probe X-ray micro analyzer (EPMA) and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). The grain size of the zircon is as large as 50 µm and has an inherited core (up to 15 µm) and micrometer-sized sector zoning (up to several micrometers in size). In a previous study, the zircon ages were not obtained because the grain size was too small to analyze. In this study, we analyzed the zircon with laser ablation multi-collector inductively coupled plasma mass spectrometry (LA-MC-ICP-MS) for dating purposes. The REE patterns and occurrence of zircon in the quartz–allanite vein match well with previous reported recrystallized zircon, while the behavior of the trace elements shows differences with magmatic and hydrothermal zircon. The 206Pb/238U ages obtained from the zircon in the quartz–allanite vein are from 240.1 ± 2.9 to 257.1 ± 3.5 Ma and this age is included in the tectonic evolution period of the study area. Therefore, we suggest that the quartz–allanite veins in the Gyemyeongsan Formation were formed during the late Permian to early Triassic metamorphic period and the zircon was recrystallized at that time. The Triassic age is the first reported age with zircon dating in the Gyemyeongsan Formation and will be an important data-point for the study of the tectonic evolution of the OMB.

Bands of Zircon, Allanite and Magnetite in Paleozoic Alkali Granite in the Chungju Unit, South Korea, and Origin of REE Mineralizations

High-grade Zr–Nb–Y–rare earth element (REE) mineralization occurs as zircon–allanite–magnetite bands in layered Paleozoic alkali rocks which intruded the Gyemyeongsan Formation of the Chungju unit, South Korea. The mineralization period and genesis have been controversial. We investigated the petrological and mineralogical properties of the newly discovered zircon–allanite–magnetite bands and the geochronological properties of zircon within the bands in the alkali granite. We analyzed the zircon with laser ablation multi-collector inductively coupled plasma mass spectrometry (LA-MC-ICP-MS). The repeated quartz–feldspar-rich layers in the alkali granite show grain-sized grading textures and equilibrium igneous textures. Magnetite and allanite grains in these layers varied in size and exhibited isolated, aggregated, and coalesced textures. In addition, the settling texture of zircon grains onto the other minerals was observed. These observations could reasonably be explained by the process of gravitational accumulation during the solidification of magma. The 206Pb/238U ages obtained from zircon from the zircon–allanite–magnetite-rich layer and the alkali aplite were 331.1 ± 1.5 Ma and 334.5 ± 8.9 Ma, respectively. Therefore, we suggest that the Zr–Y–Nb–REE mineralization developed in the alkali rocks and the Gyemyeongsan Formation in the Chungju unit were formed by fractional crystallization of alkali magma and hydrothermal fluids which evolved from alkali magma fractional crystallization, respectively. The correlation between alkaline granite and REE mineralization found in this study could be used as a tool for REE exploration in other regions where the permeable geological unit is intruded by the alkali granite.

Petit-spot as definitive evidence for partial melting in the asthenosphere caused by CO2

The deep carbon cycle plays an important role on the chemical differentiation and physical properties of the Earth’s mantle. Especially in the asthenosphere, seismic low-velocity and high electrical conductivity due to carbon dioxide (CO2)-induced partial melting are expected but not directly observed. Here we discuss the experimental results relevant to the genesis of primitive CO2-rich alkali magma forming petit-spot volcanoes at the deformation front of the outer rise of the northwestern Pacific plate. The results suggest that primitive melt last equilibrated with depleted peridotite at 1.8–2.1 GPa and 1,280–1,290 °C. Although the equilibration pressure corresponds to the pressure of the lower lithosphere, by considering an equilibration temperature higher than the solidus in the volatile–peridotite system along with the temperature of the lower lithosphere, we conclude that CO2-rich silicate melt is always produced in the asthenosphere. The melt subsequently ascends into and equilibrates with the lower lithosphere before eruption.

Mantle xenolith-xenocryst-bearing monogenetic alkali basaltic lava field from Kutch Basin, Gujarat, Western India: Estimation of magma ascent rate

Kutch rift basin of northwestern India is characterized by a topography that is controlled by a number of fault controlled uplifted blocks. Kutch Mainland Uplift, the largest uplifted block in the central part of the basin, contains alkali basalt plugs and tholeiitic basalt flows of the Deccan age. Alkali plugs often contain small, discoidal mantle xenoliths of spinel lherzolite and spinel wehrlite composition. Olivine occurs as xenocrysts (coarse, fractured, broken olivine grains with embayed margin; Fo>90), phenocrysts (euhedral, smaller, and less forsteritic ~Fo80), and as groundmass grains (small, anhedral, Fo75) in these alkali basalts. In a few cases, the alkali plugs are connected with feeder dykes. Based on the width of feeder dykes, on the sizes of the xenocrysts and xenoliths, thickness of alteration rim around olivine xenocryst, we estimate that the alkali magmas erupted at a minimum speed of 0.37km per hour. The speed was likely greater because of the fact that the xenoliths broke up into smaller fragments as their host magma ascended through the lithosphere.

Water in Hawaiian garnet pyroxenites: Implications for water heterogeneity in the mantle

Mapping the compositional variability of the Earth's mantle is fundamental for understanding mantle dynamics, crustal recycling and melt generation. The geochemistry of intraplate oceanic basalts in particular points to a heterogeneous convecting mantle, often explained by variable proportions of fertile (pyroxenite, eclogite) and depleted (peridotitic) domains. A key parameter necessary to constrain the Earth's deep processes is water because it influences melting and affects the physical properties of the Earth's mantle. However, there are only a few direct water determinations on samples of the oceanic mantle. Here we report water concentrations by FTIR on garnet pyroxenite xenoliths from Salt Lake Crater vent, Oahu, Hawaii, in order to constrain the role of lithological variability in the distribution of water in the upper oceanic mantle. The clinopyroxenes have 260 to 576ppm H2O and the orthopyroxenes about half these amounts. Curiously, garnets have water concentrations below detection limits (<0.5ppm H2O). Calculated melts in equilibrium with the pyroxenes have, on average, 2.3±0.4wt.% H2O and 243±83 H2O/Ce. These H2O concentrations are similar to estimates for the H2O concentration of primary magmas of the rejuvenated Hawaiian stage volcanism, and 3 to 5 times higher than primary magmas of the shield stage. This supports earlier conclusions where the pyroxenites are interpreted as high pressure cumulates from alkali magmas similar to the rejuvenated stage magmas within the Pacific lithosphere. The reconstructed bulk pyroxenites have 211 to 467ppm H2O, similar to estimates for the source of Hawaiian magmas and two to four times higher than estimates for the depleted MORB source (~100ppm H2O). Despite their high water concentrations, however, the bulk pyroxenites have much lower H2O/Ce ratios than the MORB source (35–115 vs. 150–210, respectively) or Hawaiian magma sources (>160). The discrepancy between the low H2O/Ce ratios in the bulk pyroxenites and high H2O/Ce in the equilibrium melts is consistent with experimental data that predicts 4 to 5 times higher partition coefficient for Ce than H in these pyroxenes. Therefore, the process of high-pressure crystallization in the oceanic lithosphere will create pyroxene-rich lithologies which are paradoxically, both “wet” (i.e., high H2O concentrations) and “dry” (low H2O/Ce ratios) compared to the source of their parental melts. Phlogopite is present as a trace phase in these rocks (<0.4% modal) with relatively minor contribution on the bulk water contents. The coupled high H2O, low H2O/Ce ratios of the pyroxenites are similar to the inferred source of several Enriched Mantle (EM)-type Ocean Island Basalts (OIB), like the Samoa, Pitcairn, Society, and Kergulen hot spots, as well as the EM-1 type Koolau endmember of the Hawaiian magmas. We suggest that recycling of pyroxenite-bearing oceanic lithosphere can explain the relatively high H2O and low H2O/Ce ratios of some EM-type OIB. Our data suggests a link between lithological variability and heterogeneous water distribution in the upper mantle.

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.

Mechanisms driving polymagmatic activity at a monogenetic volcano, Udo, Jeju Island, South Korea

High-resolution, stratigraphically ordered samples of the Udo tuff cone and lava shield offshore of Jeju Island, South Korea, show complex geochemical variation in the basaltic magmas that fed the eruption sequence. The eruption began explosively, producing phreatomagmatic deposits with relatively evolved alkali magma. The magma became more primitive over the course of the eruption, but the last magma to be explosively erupted had shifted back to a relatively evolved composition. A separate sub-alkali magma batch was subsequently effusively erupted to form a lava shield. Absence of weathering and only minor reworking between the tuff and overlying lava implies that there was no significant time break between the eruptions of the two magma batches. Modelling of the alkali magma suggests that it was generated from a parent melt in garnet peridotite at c. 3 to 3.5 GPa and underwent mainly clinopyroxene + olivine ± spinel fractionation at c. 1.5 to 2 GPa. The sub-alkali magma was, by contrast, generated from a chemically different peridotite with residual garnet at c. 2.5 GPa and evolved through olivine fractionation at a shallower level compared to its alkali contemporary. The continuous chemostratigraphic trend in the tuff cone, from relatively evolved to primitive and back to evolved, is interpreted to have resulted from a magma batch having risen through a single dyke and erupted the batch’s head, core and margins, respectively. The alkali magma acted as a path-opener for the sub-alkali magma. The occurrence of the two distinct batches suggests that different magmatic systems in the Jeju Island Volcanic Field have interacted throughout its history. The polymagmatic nature of this monogenetic eruption has important implications for hazard forecasting and for our understanding of basaltic field volcanism.

Origin of the adakite–high-Nb basalt association and its implications for postsubduction magmatism in Baja California, Mexico: Discussion

Constraining the origin of the adakite–high-Nb basalt (HNB) association in Baja California, Mexico, is critical to a better understanding of global arc magmatism. Currently the preferred explanation for the close spatial and temporal association of the two rock suites is through melting of the basaltic portion of the subducted Farallon-Cocos plate, thus providing support for the slab-melting origin of adakites elsewhere. Moreover, a tectono-magmatic model involving the production of both adakite and HNB from slab melts offers a comprehensive explanation for the origin of the atypical, arc-related, postsubduction magmatism in Baja California. This paper proposes alternative models for the origin of HNB and postsubduction magmatism in Baja California, wherein the unusual geologic setting of western Mexico and westward movement of North America permitted the influx of Pacific asthenosphere beneath the adjacent Gulf of California after the cessation of subduction. Unlike the previous tectono-magmatic model, the new models propose that the asthenosphere provided a direct source for postsubduction tholeiitic and rare alkali magmas that were erupted in Baja California as tholeiites and HNB, respectively. Fractional crystallization of some of the HNB magmas plus assimilation of tholeiitic materials produced Nb-enriched basalts (NEB). The influx of Pacific asthenosphere after the cessation of subduction also provided thermal energy to melt the mafic lower Baja California crust, producing adakite rocks, and the preexist-ing metasomatized mantle wedge, producing bajaites and calc-alkaline magmas.

Characteristics of alkali magma differentiation at the Cape Verde Islands

Parental magmas of the Cape Verde Islands are high-Mg foidites, picrobasalts, and basanites. The rocks can be classified into the following two differentiated series: (1) a high-alkali series, which includes picrites, high-Mg foidites, low-Mg-foidites, and phonolites; and (2) a series of moderate alkalinity, which includes picrobasalts, basanites, tephrites, tephrophonolites, phonotephrites, phonolites, and trachytes.

Origin of the adakite-high-Nb basalt association and its implications for postsubduction magmatism in Baja California, Mexico

Constraining the origin of the adakite–high-Nb basalt (HNB) association in Baja California, Mexico, is critical to a better understanding of global arc magmatism. Currently the preferred explanation for the close spatial and temporal association of the two rock suites is through melting of the basaltic portion of the subducted Farallon-Cocos plate, thus providing support for the slab-melting origin of adakites elsewhere. Moreover, a tectono-magmatic model involving the production of both adakite and HNB from slab melts offers a comprehensive explanation for the origin of the atypical, arc-related, postsubduction magmatism in Baja California. This paper proposes alternative models for the origin of HNB and postsubduction magmatism in Baja California, wherein the unusual geologic setting of western Mexico and westward movement of North America permitted the influx of Pacific asthenosphere beneath the adjacent Gulf of California after the cessation of subduction. Unlike the previous tectono-magmatic model, the new models propose that the asthenosphere provided a direct source for postsubduction tholeiitic and rare alkali magmas that were erupted in Baja California as tholeiites and HNB, respectively. Fractional crystallization of some of the HNB magmas plus assimilation of tholeiitic materials produced Nb-enriched basalts (NEB). The influx of Pacific asthenosphere after the cessation of subduction also provided thermal energy to melt the mafic lower Baja California crust, producing adakite rocks, and the preexist-ing metasomatized mantle wedge, producing bajaites and calc-alkaline magmas.

The age and chronostratigraphical significance of North Atlantic Ash zone II

AbstractRhyolitic tephra with the geochemical characteristics of North Atlantic Ash Zone (NAAZ) II are described from the giant piston core MD95‐2006 from the northeast Atlantic Ocean. Shard size distribution indicates that the tephra represent a wind‐sorted, primary air‐fall deposit, but with a mode close to 200 μm they are too coarse to be air‐fall deposits at the core site, which lies ca. 1000 km from the possible tephra source. Randomly sampled shards reveal a single geochemical population belonging to the Icelandic transitional alkali magma series, suggesting that they are unlikely to represent ice rafted debris derived from Icelandic icebergs. The tephra probably represent air‐fall deposits, transported to the core site by sea ice within the northeast Atlantic gyre. The NAAZ II peak coincides with the rapid climate transition (cooling) at the end of interstadial 15, which can be assigned an age of 53 260±2660 yr BP from direct correlation with the Greenland ice‐core (GISP2) record. A comparison of the MD95‐2006 Neogloboquadrina pachyderma (sinistral) relative abundance and GISP2 δ18O records, relative to the NAAZ II isochron, suggests that this climatic event was synchronous across the North Atlantic. By direct correlation of interstadial maxima between the two records, the GISP2 time‐scale is transferred to MD95‐2006. Comparison of corrected and calibrated radiocarbon ages derived from monospecific foraminifers with the GISP2 ages at the same stratigraphical horizon suggest major age differences. These probably result from large variations in atmospheric 14C concentration and highlight the significant uncertainties associated with radiocarbon calibration during marine isotope stage 3. Copyright © 2004 John Wiley &amp; Sons, Ltd.

La diversité des basaltes de Patagonie à la latitude du point triple du Chili (46°–47° lat. S) : données complémentaires et implications sur les conditions de la subduction

Abstract The basalts exposed around the Lago General Carrera–Buenos Aires display a wide range of chemical compositions. These include: Cretaceous calc-alkaline magmas typical of the Andean arc, Eocene to Quaternary intraplate alkali magmas and Quaternary magmas, the geochemical signature of which shows similarities with those of Chile ridge MORB. Such diversity is related to the evolution of the subduction processes below Patagonia, starting with a ‘normal’ subduction regime, followed by collision–subduction of the active Farallon–Aluk and Chile ridges together with the development of ‘asthenospheric windows’ during the Eocene and since the Upper Miocene respectively.

Pétrologie de l'intrusion alcaline mésozoïque de la région d'Anemzi, Haut Atlas Central, Maroc

The Mesozoic Anemzi intrusion belongs to the peri-Atlantic Mesozoic alkali magmatism. It is composed of gabbros (with or without olivine), monzodiorites and quartz-bearing syenites. Dolerite dykes of alcaline affinity occur nearby. The alkaline rocks are intrusive into mid-Jurassic limestones. Liquidus phases crystallised following the characteristic order of alkali magmas, i.e. olivine + spinel, plagioclase, clinopyroxene and amphibole. Clinopyroxene and amphibole compositions are governed by Ti-tschermakite substitutions. Clinopyroxene and amphibole in syenites are enriched in aegirine and richterite components, respectively. The mineralogical and chemical evolution in the Anemzi intrusion is consistent with low-pressure fractional crystallisation from an alkali lava under low to moderate fO 2 ( MW + [1 − 2 log units]). However, the parental liquid shows some geochemical characteristics of transitional magma, such as a high SiO 2 content (48 wt%) and low La/Yb N ratio (5). Silica activity decreased from the parental magma (0.56) to 0.08 after fractionation of the gabbros and then increased to 1.0 at the end of the magmatic differentiation. Compared with rocks of similar ages, belonging to the peri-Atlantic Cretaceous alkaline magmatism, the Anemzi intrusion is distinguished by low REE contents ([ La/ Yb] N = ∼ 6) and transitional basalt(s) as parental magma. This magma was emplaced through reactivation of Palæozoic fault systems.

Tephrostratigraphy in the Ulleung Basin, East Sea: Late Pleistocene to Holocene

Three sediment cores, obtained from the Ulleung Basin, reveal five tephra layers that are easily distinguished from the other core intervals by coarse grain size, density grading, and sediment color. The tephra layers are well correlated among the cores and show a systematic lateral decrease in thickness and grain size away from the source of the explosive volcanic eruption. Three marker tephra layers were derived from Ulleung Island and consist predominantly of white vesiculate pumices of phonolitic or trachytic alkali magma. The disorganized Ulleung-I tephra layer was due to the last explosive eruption in Ulleung Island. It consists of only white vesiculate pumices, disturbed by strong bioturbation. The Ulleung-II tephra (ca. 9300 yr B.P.), the most explosive one among the Ulleung Island-derived tephras, shows two units: a lower density-graded unit and an upper pumice-scattered unit. The Ulleung-II tephra layer shows little bioturbation and is characterized by pumice-scattered texture. An unnamed tephra layer shows a distinct density grading by dark fine-grained crystals and white vesiculate pumices. This marker tephra originated from a phonolitic alkali magma of unknown source. The Aira-Tanzawa ash (ca. 22000 yr B.P.), derived from the Aira Caldera in southern Japan, consists commonly of bubblewalled glass shards from rhyolitic subalkali magma. During the Holocene period, sediments were accumulated at a rate of 17.5–24.7 cm/kyr, based on the Ulleung-II tephra, which increased toward the center of the basin. During the last glacial maximum (LGM) period which is represented by the interval between the Ulleung-II tephra layer and the Aira-Tanzawa ash, a very thick turbidite mud was deposited in the unchannelized Ulleung Basin plain with higher accumulation rate (mean 26.1 cm/kyr) than that during the Holocene period. The very thick turbidite mud of LGM period occurs only in the unchannelized Ulleung Basin plain, which is not correlated with other layers in the channelized basin plain.

Beryllium—Geochemistry, Mineralogy and Beneficiation

This paper presents a review of the geology, geochemistry and mineralogy of beryllium, and methods of beneficiation of beryl including the safety measures to be adopted while crushing and grinding of beryllium minerals. A typical lithophile element, beryllium forms a characteristic four fold coordination with oxygen forming the [BeO4]6− complex. Geochemically it accumulates in the acid and alkalic magmas during magmatic differentiation process. During crystallization of these two magmas beryllium follows different course; in the case of acid magma it gets concentrated in the residual pegmatitic and hydrothermal stage, while in the case of alkali magma it enters into the lattice of several rock forming and accessory minerals by diadochic capture preventing its concentration in the residual stage. Beryllium enters in the structure of as many as 40 minerals as an essential constituent and another 50 minerals as occasional and minor diadochic constituent. Of the 40 beryllium minerals, 26 are silicates reflect...

Geochemistry and petrology of the late cenozoic alkali basalts of the western Carpathians (Czechoslovakia)

Miocene to Quaternary alkali volcanism in the Western Carpathians followed calcalkali volcanism in the same region. The alkali volcanics have restricted composition ranging from alkali basalts and basanites to rare trachyandesites. They contain ultramafic xenoliths, megacrysts and xenocrysts of olivine, pyroxene and amphibole. Pyroxene xenocryst cores differ in color compared to pyroxene phenocrysts. Alkali basaltic rocks are enriched in LILE, LREE (Lan/Ybn = 10–19) and HFS elements. Based on major and trace element distribution and Nd, Sr and Pb isotopic composition, we suggest that metasomatically-enriched manlte peridotite was the source material of the alkali magmas. The range in composition of rocks and their minerals was probably controlled by (1) inhomogenities in the mantle-source and/or different degrees of melting, and (2) metasomatism associated with infiltration of alkali basalt magma through peridotite during ascent through the subcontinental lithosphere. The origin of fluids metasomatically enriching the mande is unclear. Their derivation from the subducted slab cannot be excluded.