K-bearing Phases Research Articles

Contrasting riverine K and Li isotope signatures during silicate weathering in the southeastern Tibetan Plateau

Mountain uplift produces steep slopes and rapid erosion rates, and hence exposes more silicate rocks to weathering and accelerates the drawdown of atmospheric CO2, driving global cooling over the Cenozoic. Riverine potassium (K) and lithium (Li) isotopes are two promising proxies for silicate weathering because the isotopes fractionate during secondary mineral formation associated with weathering and these two elements are highly enriched in silicate rocks. This study explores similarity and difference between K and Li isotope behaviors during weathering across the Lancang River in the southeastern Tibetan Plateau, in order to evaluate the role of these two isotopes in tracing silicate weathering in tectonically active mountains. Dissolved K and Li in the Lancang River mainstream are mainly derived from the dissolution of silicates. River waters are enriched in the heavy isotope of K (−0.35‰ to +0.05‰) and Li (+7.1‰ to +20.4‰) relative to suspended sediments (−0.56‰ to −0.42‰ and −2.3‰ to 0‰, respectively), and the element partitioning between dissolved and suspended loads controls dissolved δ7Li and δ41K values. Interesting, dissolved δ7Li and δ41K show the opposite evolution trends toward downstream. As the weathering intensity increases downstream (higher Si/(Na + K)⁎ ratios), the dissolved δ7Li values increase from 7.4‰ to 13.6‰, whereas dissolved δ41K values decrease from −0.04‰ to −0.26‰. Compared to its headwater with limited weathering, more secondary minerals (e.g., clays) are formed in the lower reaches (e.g., floodplains) with low physical erosion rates and high annual precipitation, driving more 6Li incorporated into clays, thus high dissolved δ7Li values downstream. While dissolved δ41K values and the fraction of 39K incorporated into secondary minerals are low in floodplains, this is consistent with observations that illite is a main K-bearing phase among clay minerals, and its content in the clay fraction in river sediment decreases as weathering intensity increases. Our study further supported that the increase in Cenozoic seawater δ7Li is related to weathering in floodplains rather than weathering of mountains, whereas K isotope fractionation mainly occurs in mountain regions. If Cenozoic seawater δ41K rise, as with δ7Li change, weathering at higher elevation may exerts more effect on high riverine and seawater δ41K values. This study improves our knowledge of K-Li isotope behaviors during present-day silicate weathering, and the response of K-Li isotopes in sedimentary archives to silicate weathering over geological time.

The timescale of solid-state deformation in the Northern Adamello igneous intrusive suite

The Cenozoic Adamello batholith in the Southern Alps records solid-state deformation structures including, in order of decreasing relative age, cooling joints, shear zones and faults. In the present study we constrained age and duration of each phase with 40 Ar/ 39 Ar geochronology. The host granodiorite, cooling joints, mylonites, pseudotachylytes and cataclasites were characterized through microstructural and mineralogical analysis, micro-computerized tomography and electron probe microanalysis. The dated K-bearing phases are (1) magmatic biotite, (2) biotite and K-feldspar in joints and mylonites, (3) pseudotachylytes and (4) hydrothermal K-feldspar in cataclasites. The wall-rock biotite has an age of 33.2 ± 0.2 Ma, independent of grain size, overlapping with the age of the cooling joints. Bulk biotite-rich mylonites have ages between 32.4 ± 0.5 and 30.8 ± 0.08 Ma. The K-feldspar cementing cataclasite has an age of 25.5 ± 1.1 Ma. Pseudotachylyte ages cluster between 29.8 ± 0.3 and 31.7 ± 3.1 Ma, with one of 25.6 ± 0.3 Ma. The resolvable difference in age between magmatic biotite and mylonites indicates that biotite is not a thermochronometer, as its age is mostly controlled by deformation and fluid–rock interactions. 40 Ar/ 39 Ar dates mostly confirm the relative ages determined from field relationships, with mylonites active within a time window of 1.6 myr and subsequent seismic faulting occurring for almost 6 myr. Supplementary material: Full analytical data are available at https://doi.org/10.6084/m9.figshare.c.5838146 Thematic collection: This article is part of the Isotopic Dating of Deformation collection available at: https://www.lyellcollection.org/cc/isotopic-dating-of-deformation

Potassium isotopic fractionation in a humid and an arid soil–plant system in Hawai‘i

Plants play a critical role in the cycling of potassium (K) and the fractionation of its isotopes. However, little is known about K stable isotopic compositions in natural soil–plant systems and possible fractionation during intra-plant transport and root-soil uptake of K. This study focuses on K isotopic fractionation within a humid and an arid soil–plant system sampled on the windward and leeward sides of Kohala Mountain, Hawai‘i. We determined the K isotopic compositions of < 2-mm bulk soil, soil saturation extraction, and selected plant tissues by multi-collector inductively coupled plasma mass spectrometry and X-ray absorption spectroscopy. We studied soils and individual tissue samples such as roots, stems, barks, shoots (a sum of stems and fresh leaves), leaves (fresh and dead), seeds, and flowers of trees and grasses. The results demonstrated that: (i) tissue δ41K values ranged from −1.06 ± 0.06 to 1.15 ± 0.09‰; (ii) within the same plant, stems (barks), dead leaves, and reproductive tissues (flowers and seeds) were isotopically lighter compared to fresh leaves, and to a lesser extent,roots; (iii) δ41K values of the humid soil (-0.54 ± 0.07 to −0.49 ± 0.06‰) were lower than those of the arid soil (-0.24 ± 0.07 to −0.14 ± 0.06‰); and (iv) soil bioavailable pool δ41K (saturation extracts) ranged from −0.63 ± 0.08 to 0.34 ± 0.08‰ and 0.48 ± 0.08 to 0.54 ± 0.10‰ in the humid and arid soils, respectively. From synchrotron-based analysis of K atoms, we identified two major K-bearing phases co-existing as ionic K+ and K-pectate association of different fractions. Based on K isotopic and synchrotron data, we conclude that two dominant processes are responsible for plant-mediated K isotopic fractionation, including (1) K redistribution during intra-plant circulation and (2) uptake at the root-soil interface. For intra-plant circulation of K, there is a high affinity of isotopically lighter K to organic complexes as K-pectate, and K-pectate is particularly enriched in roots and fresh leaves. For K uptake at root-soil interface, isotopically lighter K is preferentially taken by roots from soil bioavailable pools following a low-affinity (passive) transport path. Soil K budget in two sites reflects strong source mixing effects with limited plant imprints. This work provides exploratory data on the biogeochemical fractionation of K isotopes in the soil–plant system.

Structural and chemical resetting processes in white mica and their effect on K-Ar data during low temperature metamorphism

White mica has been widely used to date microstructures and tectonic events in faults, shear-zones and folds because of its suitability for radiogenic dating. However, complex (i) microstructural evolution, (ii) individual chemical evolution of the K-bearing phases, (iii) mixing of ‘detrital grains’ with newly formed and/or recrystallized or chemically reset grains as well as (iv) volume diffusion may result in apparent K-Ar ages. Here, specimens from a prograde sediment sequence of the exhumed fossil European Alpine accretionary wedge were used to investigate resetting processes of white mica by the type and intensity of deformation as well as peak metamorphic conditions. We combine the K-Ar system with mass and mineral quantities from grain size fractions to calculate the amount of recrystallized white mica in each grain size fraction along the metamorphic gradient. Increasing recrystallization with increasing metamorphic grade is related to thermally activated pressure solution and dissolution-precipitation creep, as seen by the formation of a spaced cleavage of recrystallized phyllosilicates documented through Synchrotron X-ray Fluorescence Microscopy and Scanning Electron Microscope imaging techniques. Increasing recrystallization by dissolution-precipitation processes induces chemical resetting of the isotopic system, resulting in a prograde decrease of apparent K-Ar ages. We demonstrate that Ar volume diffusion does not play a significant role for the low-temperature samples, promoting recrystallization as the important physico-chemical process for age resetting. However, white mica chemistry reveals that no simple relation between isotopic resetting and grain size exists along the prograde path. Reliable age information can therefore only be obtained in the case of (nearly) complete resetting, which accounts only for the smallest grain size fraction at the highest metamorphic temperature. These findings could shed new light on accurate dating of mica-rich fault rocks, where the time constraints depend not only on the temperature, but also on the amount and type of deformation.

Thermal and Mechanical Activation of Sericite for Recovery of Potash Values

Sericite sample having potash (K2O) content ~ 9% was evaluated as a source of potash. The mineralogical studies revealed the fine grain structure of sericite containing muscovite, orthoclase, and phlogopite as the major potash bearing phases. The feed sample directly leached in different lixiviants yielded low potash dissolution values. Planetary ball milling was carried out to liberate the potash values from silicate matrix. It was observed that the K-bearing phases were quite stable, with milling action of about 8 h yielded ~ 30% values. The heat treatment of sericite with NaOH resulted in ~ 90% K-dissolution at optimum conditions (60% flux, 600 °C, 30 min) with the formation of sodium silicate and kalsilite. Mechanical activation had a positive effect on heat treatment kinetics as temperature decreased from 600 to 250 °C with ~ 100% potash recovery. CaCl2 and Na2CO3 were also evaluated as flux and based on a comparison of process variables and yield, NaOH was found to be the best flux.

Microstructures, mineral chemistry and geochronology of white micas along a retrograde evolution: An example from the Aar massif (Central Alps, Switzerland)

This study investigates gneissic tectonites of different mechanical origins, and aims at unravelling the link between microstructures, white mica chemistry and K-Ar data. The highest temperature mylonitic deformation, which is investigated, is dominated by dislocation creep in monomineralic quartz, and viscous granular flow in polymineralic domains. These mylonites show homogenous mineral chemistry, resulting in robust and consistent K-Ar age data. The lowest temperature deformation investigated, is that of a retrograde brittle deformation, producing cohesionless fault gouges. Within the cohesionless fault gouges, frictional granular flow is considered as the main deformation mechanism. The K-bearing phases inside the fault gouges record no chemical resetting during this brittle overprint, but contain mechanically fragmented K-bearing micas and significant amounts of newly produced smectite. This yields K-Ar ages inside of a gouge that are in the range of adjacent mylonite ages, and may not represent the timing of gouge formation. In an effort to understand these uncertainties in K-Ar ages, detailed observations of the microstructural context of K-bearing white mica were made. Microstructural criteria in combination with quantitative element mapping allowed for the discrimination of five important mica-related processes: (1) fracturing/recrystallization of white mica; (2) (re)-precipitation of new fine-grained white mica inside the deforming rock matrix; (3) pseudomorphic replacement of precursor minerals by white mica and chlorite; (4) pseudomorphic replacement of pre-existing white mica without grain size reduction; and (5) remnant grains. The combination of these different processes led to a high variability in mineral chemistry and isotope data for tectonites, which relates directly to variations in the size of the equilibrium volumes. Despite the samples being highly deformed in the presence of fluids, inherited sheet silicates may survive. If inherited sheet silicates are present they contribute to an increase in both the chemical heterogeneity and the apparent K-Ar ages of a sample.

Phase relations of phlogopite and pyroxene with magnesite from 4 to 8 GPa: KCMAS–H2O and KCMAS–H2O–CO2

To constrain the melting phase relationships of phlogopite and magnesite in the presence of clino- and orthopyroxene, we performed experiments in the K2O–CaO–MgO–Al2O3–SiO2–H2O (KCMAS–H2O) and K2O–CaO–MgO–Al2O3–SiO2–H2O–CO2 (KCMAS–H2O–CO2) systems at pressures of 4–8 GPa and temperatures from 1100 to 1600 °C. We bracketed the carbonate-free solidus between 1250 and 1300 °C at 4 and 5 GPa, and between 1300 and 1350 °C at 6, 7 and 8 GPa. The carbonate-bearing solidus was bracketed between 1150 and 1200 °C at 4, 5 and 6 GPa, and between 1100 and 1150 °C at 7 and 8 GPa. Below the solidus in both systems at 4–6 GPa, phlogopite is in equilibrium with enstatite, diopside, garnet (plus magnesite in the carbonate-bearing system) and a fluid. At 7 GPa, phlogopite coexists with KK-richterite, enstatite, diopside, garnet (plus magnesite in the carbonate-bearing system) and a fluid. KK-richterite is the only stable K-bearing phase at 8 GPa and coexists with enstatite, diopside, garnet (plus magnesite in the carbonate-bearing system) and a fluid. In KCMAS–H2O, phlogopite is present to ~100 °C above the solidus. Olivine forms at the solidus and coexists with enstatite, diopside, garnet and melt. At depth in a subcontinental lithospheric mantle keel, phlogopite would be stable with orthopyroxene, clinopyroxene and magnesite to ~5 GPa along a 40 mW/m2 geotherm. A hydrous, potassic and CO2-bearing melt that intrudes the subcontinental mantle can react with olivine, enstatite and garnet, crystallizing phlogopite, magnesite and potentially liberating a hydrous fluid.

Insights into the lithosphere to asthenosphere melting transition in northeast Africa: Evidence from the Tertiary volcanism in middle Egypt

Throughout the northeast African and Arabian plates, small-volume basaltic volcanism has erupted sporadically over the last ~35Myr. While magmatic activity associated with the Afar plume and subsequent rifting of the Red Sea, Gulf of Aden and East African rift provide a mechanism for melt generation in those areas, the origin of melt elsewhere in the region has remained enigmatic. We have identified two types of intraplate basalt eruptions from middle Egypt, which we suggest were generated by distinct melting processes: sub-alkaline, hypersthene-normative basalts and basaltic andesites (2.8–4.9wt% total alkalis); and nepheline-normative alkali basalts and trachybasaltic flows (4.9–5.5wt% total alkalis). Estimations of the primary melt compositions suggest about twice as much fractionation in the sub-alkaline basalts (F=32–58%, dominantly clinopyroxene and olivine removal) in comparison to the alkaline basalts (F=18–30%, dominated by clinopyroxene removal). Application of the geochemical model HAMMS1 generates results which suggest the alkaline basalts are likely derived from low degrees of partial melting (F<2%) of a pyroxenite-bearing mantle source at moderate potential temperatures (TP) ~1386–1449°C, and pressures >3.7GPa. These conditions require melting at or below the base of lithospheric mantle. In contrast, the sub-alkaline basalts exhibit negative K-anomalies for all fractionation-corrected melts, suggesting a source with a K-bearing phase (likely amphibole), located within lithospheric mantle, constrained by the stability of amphibole to be <3GPa and 1170°C. We suggest that the enrichment in the lithospheric mantle is best explained by the percolation of fluids/or hydrous melts either from the Late Neo-Proterozoic subduction zone during accretion the Arabian-Nubian Shield, or from an old mantle plume that impinged the base of the lithosphere during the Phanerozoic. Destabilization of these enriched metasomes was likely a function of diffuse stretching associated with the opening of the Red Sea at ca. 30Ma, thermal perturbation from a slightly warmer asthenosphere, and the passage of asthenospheric melts through the lithospheric mantle. The strong correlation between these small-volume lavas and regional structural lineaments (e.g., Baharyia NE-SW Fault), and smaller NW-SE fractures associated with the Red Sea system, suggests that the eruption of lavas in the region is controlled by melt focusing into the lithosphere and favorable stress conditions in the upper crust. Thus, while the potential for melt generation may be present beneath much of north-east Africa, the rarity at which volcanism is discovered at the surface reflects what may be an unusual convergence of circumstances.

Geochemical characteristics and new eruption ages of ruby-related basalts from southeast Kenya

Two ruby-related basaltic fields were recently discovered in the southeast region of Kenya, exposed in the Nguu and Ngulai Hills vicinities. These fields host abundant deep-seated xenoliths, including corundum-bearing granulites. The basalts are alkaline affinity having compositions from foidrite to basanite. The Ngulai basalts have a wider range of SiO2 (38.2 wt.%–44.8 wt.%) covering those of the Nguu basalts (38.7 wt.%–42.3 wt.%). This overlapping behavior also holds for other major oxides and trace elements, e.g., Al2O3, Na2O, K2O, Cr, Ni, Rb and Ga. The overall OIB-like incompatible patterns with strong K depletion and slight spike of Ti enrichment signatures imply low degrees of partial melting of the upper mantle region source induced under a mantle plume-related process. The K-depletion signature indicates a residual K-bearing phase still retained in the source domain. Chondrite-normalized REE patterns exhibiting strong LREE enrichment without Eu anomalies suggest that plagioclase fractionation is insignificant. New 40Ar/39Ar ages indicate eruption events occurred during the Pleistocene times, which are around 2 Ma for the Ngulai basalts and 0.9 to 1.6 Ma for the Nguu basalts. Clinopyroxene-basalt thermobarometric calculations yield the equilibrium P-T ranges of ∼8-29 kbar and 1 200–1 450 °C.

Microscopy studies in support of OSL dating of mortars of historical buildings

There are several problems regarding the dating of the works in the historical buildings (see a review in Sanjurjo-Sánchez, 2012). Mortars are a potential material for dating of the built environment works, being ubiquitous and not reusable. Optical stimulated luminescence (OSL) dating can be applied to mortars with and without lime and considering the age interval of mortars from thousands of years up to a few decades. OSL dating is based on measuring the accumulated luminescence signal of aliquots (of one or more mineral grains) of constituents from the mortars, resulting from the absorbed dose of surrounding radioactivity (at a constant dose rate), considering contributions both from environment and the radioactive phases of the mortars.In this abstract is considered the relevance of microscopy studies as support for the preparation of samples for dating and the precise age calculations.Microscopy studies can allow identifying samples that cannot be dated due to lack of enough grains of suitable phases as is illustrated in Figure 1a. It is also possible to assess the presence of datable phases. Usually quartz and potassium (K) feldspar are used for dating (phases that can be easily distinguished by scanning electron microscopy and generally by polarizing optical microscopy). For dating, the OSL signal of such datable phases of a mortar must have been bleached before the mortar manufacture and settling, typically by daylight exposure of such minerals. Quartz grains have an easy to bleach signal. K-feldspars grains have a significant internal dose making them less susceptible to environmental variations but they present signal anomalous fading and the luminescence signal is harder to bleach than in quartz grains. However, according to Li B. et al., 2007, the effect of anomalous fading in K-feldspar may be negligible for samples younger than 100 years and there also some proposals for overcoming signal fading and the combination of measurements on K-feldspars and quartz. On the preparation of the sample, microscopy studies can assess the risk of contamination by other phases and, in analogy with what is done in mining studies, define a liberation size that relates to the obtainment of monomineral aliquots. It can also contribute to the identification of the presence of different grainsize fractions of the phases to use in dating (see example in Figure 1b), information that can be used for comparative studies of dose in the different grainsize fractions, such as isochron measurement procedures. One of the main issues for dating is the assessment of the radioactive dose rate that affected the mortar constituents related to the distribution of the contents of U, Th, Rb and K that can occur disperse through the sample or concentrated on some grains. Most OSL studies on sediments relate K content to K-feldspar occurrences. However, in some mortars there could be an important presence of K-micas such as muscovite and the biotite series (see Figure 1b,c) that have a lower K content and a different morphology. The presence of these mica grains has also been considered as potentially disturbing of the signal measurements and procedures for its removal have been proposed by Kortekaas and Murray, 2005. The grainsize distributions of radioactive grains and dated grains can also affect the way the radioactive dose affect the measured dose given that the same amount of K could be distributed in grains with different volume and hence different relation area/mass. Additionally, K-bearing phases can have contents of other radioactive elements affecting the beta dose as is illustrated by Figure 1c showing the presence of halos resulting from radioactive decay of inclusions in biotite crystals. While in general it is admitted that quartz grains had a zero internal dose, the presence of inclusions such as rutile (that can be assessed by optical microscopy) can endanger this assumption. Microscopy studies can also be used to assess precipitation and dissolution process that occur after the preparation of the mortar and that can affect the beta dose for the grains (see Figure 1d). Finally, microscopy studies can be used after dose estimates and can help solve problems that have baffled researchers such as low content of feldspar separations. In the case of mortar samples, for which can be obtained representative sections, the previous considerations show that petrographic models can help in the definition of spatial models for dose rate modelling and the establishment of appropriated age models.Support from Portuguese-Spanish collaboration Project “Ação Integrada E-141/10” (Fundação das Universidades Portuguesas)/“Acción Integrada PT2009-0077” (Ministerio de Ciencia e Innovación) and the Fundação para a Ciência e Tecnologia (pluriannual funding program for research units, PEst-OE/CTE/UI0697/2011, Portuguese funds).

Petrogenesis of Mafic to Felsic Lavas from the Oligocene Siebengebirge Volcanic Field (Germany): Implications for the Origin of Intracontinental Volcanism in Central Europe

Magmatism in the Cenozoic Central European Volcanic Province (CEVP) has been related to two geodynamic scenarios, either extensional tectonics in the north Alpine realm or upwelling of deep mantle material. The Oligocene ( 30^19 Ma) Siebengebirge Volcanic Field (SVF) is a major part of the German portion of the CEVP and consists of erosional remnants of mafic to felsic volcanic edifices. It covers an area of 35 km (NW^SE) by 25 km (SW^NE) with eruptive centres concentrated near the eastern shore of the Rhine river in the vicinity of the city of Bonn. Mafic rocks in the SVF comprise strongly SiO2-undersaturated basanites to alkaline basalts. Occurrences of alkaline basalts are confined to an inner NW^SE-striking zone, whereas the more SiO2-undersaturated basanites dominate the western and eastern periphery of the SVF. Radiogenic isotope compositions (Sr/Sr 0·70335^ 0·70371; eNd þ3·1 to þ4·5; eHf þ6·5 to þ8·0; Pb/Pb 19·46^19·69; Pb/Pb 15·63^15·66; Pb/Pb 39·34^ 39·62) indicate a common asthenospheric mantle end-member with HIMU-like characteristics for all mafic rocks, similar to the European Asthenospheric Reservoir (EAR). A lithospheric mantle source component with a residual K-bearing phase (phlogopite or amphibole) is inferred from negative K anomalies. Incompatible trace element modelling indicates that melting took place in the spinel^garnet transition zone with low degrees of melting at higher pressures generating the basanitic magmas (LaN/YbN1⁄4 20^25), whereas the alkaline basalts (LaN/YbN1⁄414^18) are the result of higher melting degrees at shallower average melting depths. Differentiation of basanitic primary melts generated tephritic to tephriphonolitic magmas that, for instance, erupted at the Lo« wenburgVolcanic Complex in the central SVF. Latites and trachytes, such as the prominent Drachenfels andWolkenburg protrusions, are more common in the central portion of the SVF.These compositions originate from parental alkaline basaltic melts. All differentiated samples show evidence for crustal contamination, possibly with lowerto mid-crustal material comprising mafic granulites as found in Eifel basalt xenoliths and metapelites. Based on the spatial and temporal distribution of the various volcanic rock types, a model for the temporal evolution of the SVF can be proposed. During the initial phase of volcanism, low-degree basanitic melts were generated as a result of decompression following tectonic rifting and formation of the Cologne Embayment, a northward extension of the Rhine Graben. In a second stage, alkali basalts were generated at shallower depths and higher degrees of melting as a result of continued lithospheric thinning and passive upwelling of asthenospheric mantle. These conclusions strengthen previous models suggesting

Potassium-bearing clinopyroxene: a review of experimental, crystal chemical and thermodynamic data with petrological applications

AbstractAvailable experimental data on chemical composition and crystal structure of K-bearing clinopyroxenes are compiled together with the results of atomistic simulations and thermodynamic calculations of mineral equilibria. It is shown that the limited solubility of K2O in clinopyroxene from crustal rocks cannot be ascribed to the strong non-ideality of mixing between diopside (CaMgSi2O6) and K-jadeite (KAlSi2O6) components. The more likely reason is the instability of the potassic endmember with respect to other K-bearing phases. As the volume effects of typical K-jadeite-forming reactions are negative, the incorporation of K in the clinopyroxene structure becomes less difficult at higher pressure. Atomistic simulations predict that the thermodynamic mixing properties of diopside-K-jadeite solid-solutions at high temperature approach those of a regular mixture with a relatively small positive excess enthalpy. The standard enthalpy of formation(ΔfH° =—2932.7 kJ/mol), the standard volume(V° =6.479 J mol–1bar–1) and the isothermal bulk modulus(K0=145 GPa) of K-jadeite were calculated from first principles, and the standard entropy(S°= 141.24 J mol–1K–1) and thermal-expansion coefficient (α = 3.3 x 1CP–5K–1) of the K-jadeite endmember were estimated using quasi-harmonic lattice-dynamic calculations based on a force-field model. The estimated thermodynamic data are used to compute compositions of K-bearing clinopyroxenes in diverse mineral assemblages within a wideP-Tinterval. The review substantiates the conclusion that clinopyroxene can serve as an effective container for K at upper-mantle conditions.

Geochemical characteristics of the high- and low-Ti basaltic rocks from the uplifted shoulder of the Ohře (Eger) Rift, Western Bohemia

High-Ti melanephelinite (3.8–5.9 wt% TiO 2), medium-Ti (phono)tephrite (2.7–3.1 wt% TiO 2), and low-Ti olivine melanephelinite/basanite (1.9–2.3 wt.% TiO 2) are subordinate rock types in the central European Cenozoic Volcanic Province. A contrasting melanephelinite to (phono)tephrite series occurs in the Loučná–Oberwiesenthal Volcanic Centre (37–28 Ma) and also as satellite volcanic bodies (26–12 Ma) together with olivine melanephelinite/basanite (17–20 Ma) on the southwestern periphery of the Krušné hory mountains (Erzgebirge). The volcanic rocks intrude the Variscan basement of the uplifted shoulder of the Ohře/Eger Rift in the Krušné hory mountains of the Bohemian Massif. Low Mg# (44–59) and Cr, Ni contents and enrichment of LILE, Zr, Hf, Nb, Ta, U, Th and LREE in the high-Ti melanephelinites contrast with the composition of primitive low-Ti olivine melanephelinites/basanites displaying high Mg# (63–74) and high contents of compatible elements. The high-Ti melanephelinites reveal a wide range in initial 87Sr/ 86Sr of ca. 0.7034–0.7038 and ε Nd of 2.4–4.9. The low-Ti melanephelinites show an overlapping range of initial 87Sr/ 86Sr of ca. 0.7035–0.7036 and ε Nd of 4.3–5.5. The large variation in initial 87Sr/ 86Sr ratios at similar ε Nd values in those rock types is interpreted as evidence for melting of metasomatized lithospheric mantle sources comprising K-bearing phases with radiogenic Sr. Modification of the olivine-free alkali basaltic magmas by differentiation or crustal contamination could give rise to the medium-Ti (phono) tephrites. The initial isotope ratios of all samples are consistent with HIMU-mantle sources and contributions from lithospheric mantle. The olivine-free melanephelinitic rocks often contain alkali pyroxenite–ijolite xenoliths with initial 87Sr/ 86Sr ratios of ca. 0.7036 and ε Nd of 3.0. We interpret these xenoliths as samples of an intra-crustal alkali complex derived from similar mantle sources as those for the basaltic volcanic rocks.

Partial Dissolution of Glauconitic Samples: Implications for the Methodology of K-Ar and Rb-Sr Dating

AbstractThe K-Ar dating of glauconite has been used as an important stratigraphic tool for many decades. The application of this technique is limited to pure glauconites, free of detrital contamination by K-bearing phases, often not easy to detect. This study extends the application of isotope dating to the contaminated glauconites and offers a precise technique for detecting the detrital contamination of glauconites.The most common K-bearing detrital contaminants have smaller (K-feldspars, Al-rich dioctahedral micas) or greater (trioctahedral micas) dissolution rates than glauconite in extremely low pH solutions. The differences in the dissolution rates can be applied to evaluate the purity of the glauconite and its crystallization age.The interlaboratory GLO glauconite standard and grain-size fractions separated from glauconitic sandstones of the Paleogene (sample GL) and Jurassic (sample GW8) ages were treated with acid (3M HCl, at 99±2°C) for different reaction times (0.5–7 h) and measured for their apparent isotopic ages.Microporous amorphous silica with large specific surface area is the solid product of the reaction and its content increases with reaction time. The K-Ar dates (apparent ages) of the solid residues increase significantly with reaction time: from 44.6 to 107 Ma for the GL sample and from 125.7 to 394.7 Ma for GW8. The increase is negligible in the case of the GLO standard. The Rb-Sr data of the GL sample were modeled using initial 87Sr/Sr ratios of 0.707–0.709, which resulted in a 29.9–35.8 Ma date for the untreated portions of GL, and ∼42.6 Ma after 7 h of treatment.The increase of isotopic K-Ar date with increasing time of dissolution is interpreted to be a result of increasing concentration of detrital, acid-resistant, K-bearing minerals, observed also with the electron microscope and X-ray diffraction. Probabilistic modeling based on single (K-Ar) or double (K-Ar and Rb-Sr) isotopic systems evaluated the isotopic ages of the detrital and authigenic minerals, and their K2O and Rb concentrations. The crystallization ages computed using these two methods are: 24.0, 26.5, and 32.3 Ma for the GL material, and 117.3–121.8 Ma for the GW8 series.The proposed method based on partial dissolution is a potential tool for evaluating the reliability of glauconite dating.

A new model for the evolution of diamond-forming fluids: Evidence from microinclusion-bearing diamonds from Kankan, Guinea

We report major, trace and volatile element compositions for seven coated diamonds from Kankan, Guinea. Four diamonds trapped microinclusions with high-Mg carbonatitic high-density fluids (HDFs) and three carry silicic to low-Mg carbonatitic HDFs. Mineral inclusions and nitrogen aggregation in the cores indicate growth at ∼ 5 GPa, 1100–1200 °C. Significant differences in the nitrogen aggregation state (A + B, A and A + C centres in cores, inner coats and rims of three diamonds) indicate growth during multiple events, well separated in time. All diamonds show radial chemical evolution. The HDFs in high-Mg carbonatitic diamonds grow more carbonatitic towards the rim, the silicic one grows more silicic, and the zoned diamonds show contrasting trends in the different parts of the coats. The HDFs are highly enriched in Ba, Th, U and the light REE; enrichment in K, Rb and Cs is somewhat lower. Normalized Sr, Ti and Y concentrations are low relative to the middle and heavy REE. Trace elements in the low-Mg carbonatitic to silicic suite vary systematically with increasing silicic character of the HDFs. The high-Mg HDFs commonly deviate from such arrays. A new model is proposed for the evolution of diamond-forming fluids, suggesting that high-Mg carbonatitic HDFs are formed through interaction of saline HDF with peridotitic rock, while low-Mg silicic to carbonatitic HDFs are the result of penetration of K-rich hydrous fluid into eclogitic rock. Another possible source for the high potassium content of the HDFs is K-bearing phases in the mineral assemblage of the rock. Sinusoidal REE patterns that are common in garnet inclusions in monocrystalline diamonds may form when HDFs with steep REE patterns are introduced into a LREE-depleted harzburgite. Unless garnet and diamond were formed in two separate events, this similarity suggests growth of monocrystalline diamonds from fluids similar to the HDFs.

Alkali in phlogopite and amphibole and their effects on phase relations in metasomatized peridotites: a high-pressure study

Subsolidus phase relations for a K-doped lherzolite are investigated in the model system K2O–Na2O–CaO–FeO–MgO–Al2O3–SiO2–H2O at 1.5–6.0 GPa and 680–1,000°C. Phlogopite is ubiquitous and coexists with Ca-amphibole up to 3.2 GPa and 900°C. High-pressure phlogopites show a peculiar mineral chemistry dependent on pressure: e.g., at 5.5 GPa and 680°C, excess of Si (up to 3.4 apfu) coupled with deficiency in Al (as low as 0.58 apfu) and K + Na (as low as 0.97 apfu), suggest a significant amount of a talc/10 A phase component ([v]XIISi1K−1Al −1 IV , where [v]XII is interlayer vacancy). Mixed layering or solid solution relations between high-pressure phlogopites and the 10 A phase, Mg3Si4O10(OH)2 nH2O, are envisaged. Phlogopite modal abundance, derived by weighted least squares, is maximum at high-pressure and relative low-temperature conditions and therefore along the slab–mantle interface (10.3 ± 0.7 wt.%, at 4.8 GPa, 680°C). In phlogopite-bearing systems, Ca-amphibole breaks down between 2.5 and 3.0 GPa, and 1,000°C, through the water conservative reaction 5(pa + 0.2 KNa−1) + 17en + 15phl = (10di + 4jd) + 5py + 12fo + 20(phl + 0.2 talc), governed by bulk composition and pressure-dependent variations of K/OH in K-bearing phases and as a result, it does not necessarily imply a release of fluid.

Trace elements and Sr–Nd–Pb isotopes of K-rich, shoshonitic, and calc-alkaline magmatism of the Western Mediterranean Region: Genesis of ultrapotassic to calc-alkaline magmatic associations in a post-collisional geodynamic setting

High-MgO ultrapotassic rocks are found in four different areas of the Western Mediterranean basin associated in space and time with shoshonitic and calc-alkaline rocks. They represent different magmatic events at the active continental plate margin from Oligocene to Pleistocene. These rocks are found within the Western Alps (Northern Italy), in Corsica (France), in Murcia-Almeria (South-Eastern Spain), and in Southern Tuscany (Central Italy). Ultrapotassic terms are mostly lamprophyres, but olivine latitic lavas with a clear lamproitic affinity are also found. Lamproite-like rocks range from slightly silica under-saturated to silica over-saturated, and they are characterised by low Al 2O 3, CaO, and Na 2O contents. They are plagioclase-free rocks, but K-feldspar is abundant beside other K-bearing phases. Shoshonitic and calc-alkaline rocks are invariably space associated to lamproites, and they either precede or follow them. High-Mg ultrapotassic rocks are characterised by strong enrichment of incompatible elements, which prevent further enrichment due to shallow level crustal contamination. K 2O and incompatible element contents decrease passing from high-Mg ultrapotassic to high-Mg shoshonitic and calc-alkaline rocks suggesting that K and incompatible trace elements enrichments are a primary characteristic. Ultrapotassic to calc-alkaline rocks from Western Mediterranean regions, in spite of their different age of emplacement, are characterised by similar incompatible trace elements distribution. Depletion of High Field Strength elements with respect to Large Ion Lithophile elements is observed. Positive spikes at Th, U, and Pb, with negative spikes at Ba, Nb, Ta, Sr, P, and Ti, are common characteristics of ultrapotassic (lamproitic) to high-K calc-alkaline rocks. Ultrapotassic rocks are extremely enriched in radiogenic Sr and unradiogenic Nd with respect to the associated shoshonitic and calc-alkaline rocks. Different isotopic values are distinctive of the different magmatic provinces irrespective of magmatic affinities. 87Sr/ 86Sr i ranges between 0.71645 and 0.71759 for Western Alps lamproites, between 0.71226 and 0.71230 for Corsica lamproite, between 0.71642 and 0.72259 for Murcia-Almeria lamproites, and between 0.71578 and 0.71672 for Tuscany lamproites. Radiogenic Sr decreases along with K 2O through shoshonitic to calc-alkaline rocks. Conversely 143Nd/ 144Nd i values increase with decreasing K 2O, with the highest value of 0.51243 found for the one samples from Murcia-Almeria. Contrasting trends are observed among initial values of lead isotopes, but all falling well within the field of upper crustal rocks. Different trends of 207Pb/ 204Pb i and 208Pb/ 204Pb i vs. 206Pb/ 204Pb i for samples from the different provinces are observed. Several evidences indicate that most of the magmas of the different provinces have been generated in a depleted upper mantle (i.e., lithospheric) modified by metasomatism, but an asthenospheric component is also recognised in Corsica. At least two different subduction-related metasomatic agents re-fertilised the depleted original upper mantle source. Carbonate-free siliciclastic sediments and carbonate-rich sediments have been recycled within the upper mantle through subduction and partial melting. Assuming that metasomatic component is concentrated in a vein network, in Tuscany and Corsica, time relationships indicate that low degree of partial melting of the pure vein produced lamproitic-like magmas, whereas an increase in the partial melting involve the surrounding upper mantle, then diluting the alkaline component and produced the entire spectra of magma observed. In South-Eastern Spain calc-alkaline magmatism preceded lamproitic ones, and might be generated by partial melting of mantle wedge metasomatised by fluids from oceanic slab prior to collision. Lamproitic magmas followed after melt-dominated metasomatic agents invaded the lithospheric upper mantle domain. Migration of the magmatism with time is the result of eastward migration of subduction with subsequent opening of Balearic, Ligure-Provençal, and Tyrrhenian basins.

CRYSTALLOGRAPHICALLY ORIENTED POLYPHASE RODS IN RED WILLEMITE FROM FRANKLIN, NEW JERSEY

The color of red willemite (Zn 2 SiO 4 ) from Franklin, New Jersey, has been attributed in the past to inclusions of franklinite. We have examined these inclusions, which are rod-shaped and 5–20 μm in diameter, and found them to contain multiple phases. Polarized-light microscopy shows the rods to be oriented uniformly parallel to the c axis of willemite, and to have hexagonal or pseudohexagonal cross-sections. Phases identified by standardless EDS analysis and electron back-scatter diffraction include franklinite, friedelite, zincian kutnohorite, serpentine and rhodonite, as well as a sparse, unidentified K-bearing phase. Possible explanations for the origin of the rods include exsolution, primary fluid inclusions, and epitactic growth contemporaneous with willemite crystallization. Although rod-like exsolution is known in some geological settings, and an original exsolved phase might have been altered to the present assemblage, some of the elements in that assemblage ( e.g., H, C, Cl, K, and Ca) are incompatible with the willemite structure at any temperature. An origin as fluid inclusions is supported by the shapes of the rods ( i.e. , negative crystals with a hexagonal cross-sections), the presence of phases containing OH, Cl, Ca, K, and CO 3 , and void space visible around solid phases. The tapered shapes of the rods seems inconsistent with epitaxy. We suggest that willemite-generating metasomatic fluids were preserved as inclusions rich in elements incompatible with willemite, and that subsequent conditions led to their crystallization, possibly by reaction with the host. Reaction ceased with complete consumption of the fluid, resulting in relict fluid inclusions that impart color on the macro scale.

Thermal models and clay diagenesis in the Tertiary-Cretaceous sediments of the Alava block (Basque-Cantabrian basin, Spain)

AbstractDiagenesis in the Cretaceous and Tertiary sediments of the Alava Block (Basque- Cantabrian basin) has been studied using the clay mineralogy (X-ray diffraction) of cuttings from three representative wells of a N—S cross-section. More than 5500 m of various lithologies (marls, mudstones and sandstones) have been drilled in the northern part of the domain, and 2100 m in the southern zone. The illitization of smectite and the disappearance of kaolinite, due to diagenesis, are the most characteristic features in the northern well. Evolution of smectite to illite has been differentiated into four zones, from top to bottom of the series, each showing specific I-S interstratified clay assemblages. The disappearance of smectite and the distribution of kaolinite in the other two wells are explained based on source-area considerations. Burial and thermal history have been reconstructed, revealing a northward increase in thermal flow until the Oligocene (Alpine orogeny paroxysm). In the northern well, the thermal model suggests temperatures of 160 and 270°C for the disappearance of smectite (R0) and illite-smectite (I-S) mixed-layer R1 clay minerals, respectively. The disappearance of kaolinite is related to a temperature of 230°C, a temperature never attained in the other two wells. Retardation of these processes, in relation to temperature values in the literature, is a consequence of the poor reactivity of marly lithologies, due to the low availability of cations. In this regard, the scarcity of reactants (K-bearing phases) and the absence of pathways (low permeability) for their access and circulation imply that illitization could have taken place in a closed system, by diffusion, on a very small scale, i.e. that of the original smectite grains.

EVOLUÇÃO GEOQUÍMICA E PETROGÊNESE DE CHARNOCKITOS ÍGNEOS NO CINTURÃO MÓVEL COSTEIRO DO BRASIL

Rochas máficas e félsicas com hiperstênio são importantes componentes da zona de transição entre o cráton do São Francisco e o Cinturão Móvel Costeiro Neoproterozóico no extremo sul do Estado da Bahia. Os charnockitóides possuem teores 54 a 70% de SiO2, correspondendo de termos intermediários a ácidos. A geoquímica revela um caráter metaluminoso e uma assinatura cálcio-alcalina/álcali-cálcica, compreendendo rochas ricas em Ti, P, Zr, Ba e ETR, ligeiramente enriquecidas em Fe e K e pobres em Mg, Al e V, sendo variavelmente empobrecidas em Ca, se comparadas com rochas granitóides de teor de sílica similar. Possuem teores altos de ETR, com padrões moderadamente a fortemente fracionados para os termos mais básicos e ácidos. A interpretação petrogenética baseia-se na(s): (i) natureza dos contatos, os quais mostram que as rochas félsicas intrudiram nas rochas máficas; (ii) texturas magmáticas das rochas charnockíticas félsicas em contraste com texturas inequivocamente metamórficas dos membros finais máficos; (iii) mineralogia predominantemente anidra; e (iv) trends homogêneos em muitos diagramas de variação, além do caráter geoquímico não-depletado das rochas félsicas; sugerindo que magmas tipo C são produzidos pela fusão de fontes máficas a intermediárias de alto-K, nas quais a hornblenda foi previamente desidratada. O baixo conteúdo de CaO e alta razão Ca/Sr dos magmas tipo A é uma evidência que suporta plagioclásio residual, mas não hornblenda, nas rochas fontes da região.