Fluid and melt inclusion evidence for the origin of idiomorphic quartz crystals in topaz-bearing granite from the Salmi batholith, Karelia, Russia

Idiomorphic quartz crystals in topaz-bearing granite from the Salmi batholith contain primary inclusions of silicate melt and abundant mostly secondary aqueous fluid inclusions. Microthermometric measurements on melt inclusions give estimates for the granite solidus and liquidus of 640–680°C and 770–830°C, respectively. Using published solubility models for H₂O in granitic melts and the obtained solidus/liquidus temperatures from melt inclusions, the initial water concentration of the magma is deduced to have been approximately 3 wt.% and the minimum pressure about 2 kbar. At this initial stage, volatile-undersaturation conditions of magma were assumed. These results indicate that the idiomorphic quartz crystals are magmatic in origin and thus real phenocrysts. During subsolidus cooling and fracturing of the granite, several generations of aqueous fluid inclusions were trapped into the quartz phenocrysts. The H₂O inclusions have salinities and densities of 1–41 wt.% NaCl eq. and 0.53–1.18 g/cm³, respectively.     

Compositional diversity among Late Devonian peraluminous granitoid intrusions in the Meguma Zone of Nova Scotia, Canada

Late Devonian (385−370 Ma) granitoid intrusions in the Meguma Zone of southwestern Nova Scotia represent two geographically separate magmatic suites that show subtly different lithological, geochemical and isotopic characteristics. “Central intrusions” crop out with satellite mafic-intermediate intrusions, range in composition from granodiorite to leucogranite, contain two micas, have exclusively peraluminous compositions (molar A/CNK 1.1-1.3), variably high values for FeO_T (0.4–6.0 wt.%), Ba (5–980 ppm), Y (6–50 ppm), Pb (2–50 ppm), Ga (11–53 ppm), ⁸⁷Sr/⁸⁶Sr_i (0.7081-0.7130), δ¹⁸O (9.8–13.0) and δ³⁴S (4.5–11.9), in conjunction with low values for εNd (−1 to −6.5). In contrast, “peripheral plutons” crop out with synplutonic mafic-intermediate intrusions, range in composition from tonalite to leucogranite, may contain minor hornblende, have dominantly peraluminous compositions (molar A/CNK 0.9-1.3), variably high concentrations of TiO₂ (0.1-1.1 wt.%), Al₂O₃ (12.0–19.7 wt.%), CaO (0.2–4.9 wt.%), Sr (7–720 ppm), Cr (3–111 ppm) and V (1–136 ppm), higher εNd values (−2.0 to 3.2), and lower values for ⁸⁷Sr/⁸⁶Sr_i (0.7040-0.7079), δ¹⁸⁸O (7.6–10.5) and δ³⁴S (0–4.6). Such regional diversity is explained by inferring that upper crustal contamination dominated the central granitoid compositions and mixing with mantle-derived mafic-intermediate magmas dominated peripheral granitoid compositions. However, additional contributions from heterogeneous lower crust cannot be excluded.     

Sr–barite droplets associated with sulfide blebs in clinopyroxene megacrysts from basaltic tuff (Szentbékkálla, western Hungary)

A unique occurrence of strontian barite droplets associated with oriented planes of primary sulfide blebs in several clinopyroxene megacrysts was found in the Plio-Pleistocene basaltic tuff of Szentbékkálla (Bakony–Balaton Highland, western Hungary). The barite droplets contain 3.22–14.7 wt.% SrO, variable amount of FeO (up to 3.25 wt.%), minor CaO, CuO and NiO. Textural arrangements of oriented sulfide planes show that they were trapped as immiscible sulfide melt during crystallization of the clinopyroxene. Temperature and pressure estimation (978–1094 °C and 1.0–1.1 GPa) of the host clinopyroxenes indicate that formation of clinopyroxene megacrysts and entrapment of sulfide blebs happened under upper mantle conditions.

A genetic relationship between the sulfate and sulfide blebs is doubtful. Textural features suggest that the barite droplets might have been trapped together with the sulfide blebs from immiscible sulfide melts. However, it cannot be excluded that the barite formed during metasomatic event following the sulfide formation.     

Conditions of pocket formation in the Oktyabrskaya tourmaline-rich gem pegmatite (the Malkhan field, Central Transbaikalia, Russia)

Coexisting melt (MI), fluid-melt (FMI) and fluid (FI) inclusions in quartz from the Oktaybrskaya pegmatite, central Transbaikalia, have been studied and the thermodynamic modeling of PVTX-properties of aqueous orthoboric-acid fluids has been carried out to define the conditions of pocket formation. At room temperature, FMI in early pocket quartz and in quartz from the coarse-grained quartz–oligoclase host pegmatite contain crystalline aggregates and an orthoboric-acid fluid. The portion of FMI in inclusion assemblages decreases and the volume of fluid in inclusions increases from the early to the late growth zones in the pocket quartz. No FMI have been found in the late growth zones. Significant variations of solid/fluid ratios in the neighboring FMI result from heterogeneous entrapment of coexisting melts and fluids by a host mineral. Raman spectroscopy, SEM EDS and EMPA indicate that the crystalline aggregates in FMI are dominated by mica minerals of the boron-rich muscovite–nanpingite CsAl₂[AlSi₃O₁₀](OH,F)₂ series as well as lepidolite. Topaz, quartz, potassium feldspar and several unidentified minerals occur in much lower amounts. Fluid isolations in FMI and FI have similar total salinity (4–8 wt.% NaCl eq.) and H₃BO₃ contents (12–16 wt.%). The melt inclusions in host-pegmatite quartz homogenize at 570–600 °C. The silicate crystalline aggregates in large inclusions in pocket quartz completely melt at 615 °C. However, even after those inclusions were significantly overheated at 650±10 °C and 2.5 kbar during 24 h they remained non-homogeneous and displayed two types: (i) glass+unmelted crystals and (ii) fluid+glass. The FMI glasses contain 1.94–2.73 wt.% F, 2.51 wt.% B₂O₃, 3.64–5.20 wt.% Cs₂O, 0.54 wt.% Li₂O, 0.57 wt.% Ta₂O₅, 0.10 wt.% Nb₂O₅, 0.12 wt.% BeO. The H₂O content of the glass could exceed 12 wt.%. Such compositions suggest that the residual melts of the latest magmatic stage were strongly enriched in H₂O, B, F, Cs and contained elevated concentrations of Li, Be, Ta, and Nb. FMI microthermometry showed that those melts could have crystallized at 615–550 °C.

Crystallization of quartz–feldspar pegmatite matrix leads to the formation of H₂O-, B- and F-enriched residual melts and associated fluids (prototypes of pockets). Fluids of different compositions and residual melts of different liquidus–solidus P–T-conditions would form pockets with various internal fluid pressures. During crystallization, those melts release more aqueous fluids resulting in a further increase of the fluid pressure in pockets. A significant overpressure and a possible pressure gradient between the neighboring pockets would induce fracturing of pockets and “fluid explosions”. The fracturing commonly results in the crushing of pocket walls, formation of new fractures connecting adjacent pockets, heterogenization and mixing of pocket fluids. Such newly formed fluids would interact with a primary pegmatite matrix along the fractures and cause autometasomatic alteration, recrystallization, leaching and formation of “primary–secondary” pockets.     

High-pressure–low-temperature metamorphism of metasedimentary rocks, southern Menderes Massif, western Turkey

Kilometer-scale lenses of quartz-rich metasedimentary rocks crop out in a discontinuous belt along the southern margin of the Menderes Massif, Turkey, and preserve evidence for high-pressure–low-temperature (HP–LT) metamorphism related to subduction of a continental margin during Alpine orogeny. Kyanite schist, quartzite, and quartz veins contain kyanite + phengite + Mg-chlorite, and the veins also contain magnesiocarpholite. A deformed carbonate metaconglomerate juxtaposed with the quartzite-dominated unit does not contain HP index minerals, and likely represents the tectonized boundary of the siliceous rocks with adjacent marble. The HP–LT rocks (10–12 kbar, 470–570 °C) record different pressure conditions than the adjacent, apparently lower pressure Menderes metasedimentary sequence. Despite this difference there is disagreement as to whether these HP–LT rocks are part of the Menderes sequence or are related to the tectonically overlying Cycladic blueschist unit. If the former, the entire southern Menderes Massif experienced HP–LT metamorphism but the evidence has been obliterated from most rocks; if the latter, rocks recording different metamorphic-kinematic conditions experienced different tectonic histories and were tectonically juxtaposed during thrusting. Based on observations and data in this study, the second model better accounts for the differences in P–T-deformation histories of the southern Menderes Massif rocks, and suggests that the HP–LT rocks are not part of the Menderes cover sequence.     

The isotopic composition of zircon and garnet: A record of the metamorphic history of Naxos, Greece

Growth of zircon with respect to that of garnet has been studied using a combination of petrography, U–Pb dating and oxygen isotope analysis. The aim is to document the mechanism and pressure–temperature conditions of zircon growth during metamorphism in order to better constrain the Tertiary metamorphic history of Naxos, Greece. Two metamorphisms are recognised: (1) an Eocene Franciscan metamorphism (M1) and (2) a widespread Miocene Barrovian metamorphism (M2) that increases from greenschist facies up to partial melting. An amphibolite sample contains zircon crystals characterised by a magmatic core and two metamorphic rims, denoted as A and B, dated at 200–270, 42–69, and 14–19 Ma, respectively. The first metamorphic rim A (δ¹⁸O = 7 ± 1‰) preserves the δ¹⁸O value of the magmatic core (6.2 ± 0.8‰), whereas rim B is characterised by higher δ¹⁸O values (7.8 ± 1.8‰). These observations indicate the formation of A rims by solid-state recrystallisation in a closed system with regard to oxygen and those of B in an open system. Compositional zoning in garnet is interpreted as the result of decompressional heating. Zircon B rims and garnet rims display similar δ¹⁸O values which indicates a contemporaneous growth of garnet and zircon rims during the Miocene Barrovian event (M2). Calcic gneiss and metapelite samples contain zircon crystals with single metamorphic overgrowths aged 41–57 Ma. δ¹⁸O values measured in zircon overgrowths (11.8 ± 1.4‰) from the calcic gneiss are similar to those measured in garnet rims (11.4 ± 1.1‰) from the same rock. This suggests that garnet rims and zircon overgrowths grew during the high pressure–low temperature event in equilibrium with prograde fluids. In the metapelite sample, δ¹⁸O values are similar in garnet cores (14.8 ± 0.2‰) and in zircon metamorphic overgrowths (14.2 ± 0.5‰). As zircon overgrowths have been dated at ca. 50 Ma by U–Pb, garnet cores and zircon overgrowths are interpreted to have grown during the high pressure event.

As demonstrated here for the island of Naxos, correlating the crystallisation of zircon with that of metamorphic index minerals such as garnet using stable isotope composition and U–Pb determination is a powerful tool for deciphering the mechanism of zircon growth and pin-pointing zircon crystallisation within the metamorphic history of a terrain. This approach is potentially hampered by an inability to verify the degree of textural equilibrium of zircon with other mineral phases, and the possible preservation (in metamorphic rims) of isotopic signatures from pre-existing zircon when they form by recrystallisation. Nevertheless, this study illustrates the application of this approach in providing key constraints on the timing and mechanism of growth of minerals important to understanding metamorphic petrogenesis.     

Kinematics of a deep-seated landslide derived from photogrammetric, GPS and geophysical data

The deep-seated landslide located at the intersection of the Gradenbach and Möll valleys near Döllach, Austria, has been the target of many previous investigations [Kronfellner-Kraus, G., 1980. Neue Untersuchungsergebnisse in Wildbächen — Der Talzuschub in Abhängigkeit von Niederschlägen. Int. Symp. Interpraevent Bad Ischl 1: 179–192; Weidner, S., Moser, M., Lang, E., 1998. Influence of hydrology on sagging of mountain slopes (“Talzuschübe”) — New results of time series analysis. 8th International IAEG Congress, Vancouver, Canada, Balkema, Rotterdam, 1259–1266; Weidner, S., 2000. Kinematik und Mechanismus tiefgreifender alpiner Hangdeformationen unter besonderer Berücksichtigung der hydrologischen Verhältnisse. Dissertation, Friedrich–Alexander-Universität Erlangen–Nürnberg, 246.]. In this paper, photogrammetric, GPS and geophysical data have been utilized to derive a constraint on the kinematics of the sagging process. The photogrammetric models have been based on aerial photographs from 1962 and 1996. Displacement vectors of about 50 individual characteristic points have been determined; these clearly show the area of the sagging slope. From 1999 to 2004, eleven GPS campaigns have been carried out, yielding very accurate displacement vectors at four monitoring points. Information about the internal structure of the slope was determined using seismic surveys.

The displacements observed by photogrammetry and GPS have shown a rather uniform movement of the whole sagging slope, with a slight longitudinal compression of 0.6%. In order to extend the kinematic consideration to 3D, a special form of the equation of continuity (conservation of mass) has been applied to the 1962–1996 photogrammetric data, together with the cross-sections derived from the seismic data. The average velocities through cross-sections have been calculated by integrals of the mass-balance above and below these cross-sections. Changes of the total rock mass, due to accumulation and especially to erosion, as well as an overall compaction of 1.5% between 1962 and 1996, have been considered. The calculated average velocities through a cross-section and the surface velocities independently determined by photogrammetry agree within an uncertainty of 15%. The results indicate a block movement with shear concentration at the basal sliding surface.     

Magnetite whiskers and platelets in the ALH84001 Martian meteorite: evidence of vapor phase growth

Nanometer-sized magnetite crystals associated with carbonates in fracture zones within Martian meteorite ALH84001 have been examined using analytical transmission electron microscopy. Some of the the crystals exhibit distinctive morphologies: filamentary rods and ribbons, and platelets. The rods and ribbons are elongated along the crystallographic [100] and [111] directions. Some of the rods contain microstructural defects indicating that they grew by spiral growth about screw dislocations. Platelets are flattened along the [100] and [110] directions. These unique morphologies and microstructures constrain the growth conditions of magnetite. The whiskers and platelets most likely formed in the temperature range 500–800°C by direct condensation from a vapor or precipitation from a supercritical fluid, and their properties are inconsistent with a biogenic origin.     

Damage due to the Mw 7.7 Kutch, India earthquake of 2001

This paper presents a study of the damage due to the M_w 7.6–7.7 intraplate Kutch earthquake of 26 January 2001. It was a powerful earthquake with a high stress drop of about 20 MPa. Aftershocks (up to M 4) have continued for 2.5 years. The distribution of early aftershocks indicates a rupture plane of 20–25 km radius at depths of 10–45 km along an E–W-trending and south-dipping hidden fault situated approximately 25 km north of the Kutch Mainland Fault. The moment tensor solution determined from regional broadband data indicates reverse motion along a south-dipping (by 47°) fault. The earthquake is the largest event in India in the last 50 years and the most destructive in the recorded history in terms of socioeconomic losses with 13,819 deaths (including 14 in Pakistan), collapse/severe damage of over a million houses and US$10 billion economic loss. Surface faulting was not observed. However, intense land deformations have been observed in a 40×20-km meizoseismal area. These include lateral spreading, ground uplifts (about a meter), ground slumping and deep cracks. Liquefaction with ejection of sand and copious water was widespread in the Banni grassland, Rann areas (salt plains), along rivers and also in the coastal areas up to 200 km distance from the epicenter in areas of intensity VII to X+. Stray incidences of liquefaction have occurred up to distances of at least 300 km. For the first time in India, multistory buildings have been destroyed/damaged by an earthquake. The maximum acceleration is inferred to be 700 cm/s² and intensities are 1–3 units higher in soil-covered areas than expected from the decay rate of acceleration for hard rock.     

Peralkaline syenite autoliths from Kilombe volcano, Kenya Rift Valley: Evidence for subvolcanic interaction with carbonatitic fluids

Mineral chemistry, textures and geochemistry of syenite autoliths from Kilombe volcano indicate that they crystallized in the upper parts of a magma chamber from peralkaline trachytic magmas that compositionally straddle the alkali feldspar join in the “residuum system” (ne = 0–1.03; qz = 0–0.77). Mineral reaction and/or overgrowth processes were responsible for the replacement of (i) Mg–hedenbergite by aegirine–augite, Ca–aegirine and/or aegirine, (ii) fayalite by amphiboles, and (iii) magnetite by aenigmatite. Ti–magnetite in silica-saturated syenites generally shows ilmenite exsolution, partly promoted by circulating fluids.

By contrast, the Fe–Ti oxides in the silica-undersaturated (sodalite-bearing) syenites show no signs of deuteric alteration. These syenites were ejected shortly after completion of crystallization. Ilmenite–magnetite equilibria indicate fO₂ between − 19.5 and − 23.1 log units (T 679–578 °C), slightly below the FMQ buffer. The subsequent crystallization of aenigmatite and Na-rich pyroxenes suggests an increase in the oxidation state of the late-magmatic liquids and implies the influence of post-magmatic fluids.

Irrespective of silica saturation, the syenites can be divided into (1) “normal” syenites, characterized by Ce/Ce ratios between 0.83 and 0.99 and (2) Ce-anomalous syenites, showing distinct negative Ce-anomalies (Ce/Ce 0.77–0.24). “Normal” silica-saturated syenites evolved towards pantelleritic trachyte. The Ce-anomalous syenites are relatively depleted in Zr, Hf, Th, Nb and Ta but, with the exception of Ce, are significantly enriched in REE.

The silica-saturated syenites contain REE–fluorcarbonates (synchysite-bastnaesite series) with negative Ce-anomalies (Ce/Ce 0.4–0.8, mean 0.6), corroded monazite group minerals with LREE-rich patches, and hydrated, Fe- and P-rich phyllosilicates. Each of these is inferred to be of non-magmatic origin. Fractures in feldspars and pyroxenes contain Pb-, REE- and Mn-rich cryptocrystalline or amorphous material. The monazite minerals are characterized by the most prominent negative Ce-anomalies (Ce/Ce_mean = 0.5), and in the most altered and Ca-rich areas (depleted in REE), Ce/Ce is less than 0.2.

It is inferred that carbonatitic fluids rich in F, Na and lanthanides but depleted in Ce by fractional crystallization of cerian pyrochlore, percolated into the subvolcanic system and interacted with the syenites at the thermal boundary layers of the magma chamber, during and shortly after their crystallization.

Chevkinite–(Ce), pyrochlore, monazite and synchysite-bastnaesite, occurring as accessory minerals, have been found for the first time at Kilombe together with eudialyte, nacareniobsite–(Ce) and thorite. These latter represent new mineral occurrences in Kenya.     

Pre-eruptive conditions revealed by mega- and pheno-cryst compositions from the Quaternary Erzincan Volcanics, Eastern Turkey: Insights into the magma processes

Quaternary Erzincan Volcanics (QEVs) from the Erzincan Basin consist of mega- and pheno-cryst-bearing high-K calc-alkaline dome lavas. Fourteen nearly phenocrystic domes, with a range of basaltic-andesite, andesite, dacite and rhyolite compositions, were emplaced in the North Anatolian Fault Zone. The emplacement ages yielded by the unspiked K–Ar technique range from 102 to 140 ka. The andesitic domes (each less than 3 km in diameter) contain amphibole megacrysts. Amphibole compositions show a linear variation from ferro-edenite, edenite to pargasite from rhyolite to andesite. Pargasitic amphibole megacrysts scattered into the groundmass are very similar in composition to the microlites. All plagioclases are 53 mol%. Oscillation types are An₃₂₋₅₀ whose variations range from 10 to 16 mol% An and have 10–150 μm in thickness. Pre-eruptive conditions, calculated from mega- and pheno-cryst composition, using pyroxene and two oxide thermometers and the Al-in-hornblende barometer, ranged from 918 to 837 °C and 6.6 to 4.3 kbar for andesitic magma, 824–755 °C and 4.6–4.2 kbar for dacitic magma to 803–692 °C and 4.3–3.9 kbar for rhyolitic magma, which correspond to a depth of >10 km for storage region of the crust. The fO₂ values vary from −14.25 to −15.35 log units which are plotted just below nickel–nickel oxide (NNO) buffers. The systematic decrease in thermobarometric results from andesite to rhyolite is consistent with a single magma reservoir moving upward through the crust followed by fractional crystallization. Textural and compositional relationships of mega- and pheno-crystic phases suggest that magma mixing, fluid input to the reservoir and fractional crystallization processes, with a small amount crustal contamination play key role in evolution of the QEVs.     

Zircon ages of high-grade gneisses in the Eastern Erzgebirge (Central European Variscides)—constraints on origin of the rocks and Precambrian to Ordovician magmatic events in the Variscan foldbelt

This study is an attempt to unravel the tectono-metamorphic history of high-grade metamorphic rocks in the Eastern Erzgebirge region. Metamorphism has strongly disturbed the primary petrological genetic characteristics of the rocks. We compare geological, geochemical, and petrological data, and zircon populations as well as isotope and geochronological data for the major gneiss units of the Eastern Erzgebirge; (1) coarse- to medium-grained “Inner Grey Gneiss”, (2) fine-grained “Outer Grey Gneiss”, and (3) “Red Gneiss”. The Inner and Outer Grey Gneiss units (MP–MT overprinted) have very similar geochemical and mineralogical compositions, but they contain different zircon populations. The Inner Grey Gneiss is found to be of primary igneous origin as documented by the presence of long-prismatic, oscillatory zoned zircons (540 Ma) and relics of granitic textures. Geochemical and isotope data classify the igneous precursor as a S-type granite. In contrast, Outer Grey Gneiss samples are free of long-prismatic zircons and contain zircons with signs of mechanical rounding through sedimentary transport. Geochemical data indicate greywackes as main previous precursor. The most euhedral zircons are zoned and document Neoproterozoic (ca. 575 Ma) source rocks eroded to form these greywackes. U–Pb-SHRIMP measurements revealed three further ancient sources, which zircons survived in both the Inner and Outer Grey Gneiss: Neoproterozoic (600–700 Ma), Paleoproterozoic (2100–2200 Ma), and Archaean (2700–2800 Ma). These results point to absence of Grenvillian type sources and derivation of the crust from the West African Craton. The granite magma of the Inner Grey Gneiss was probably derived through in situ melting of the Outer Grey Gneiss sedimentary protolith as indicated by geological relationships, similar geochemical composition, similar Nd model ages, and inherited zircon ages. Red Gneiss occurs as separate bodies within fine- and medium-grained grey gneisses of the gneiss–eclogite zone (HP–HT overprinted). In comparison to Grey Gneisses, the Red Gneiss clearly differs in geochemical composition by lower contents of refractory elements. Rocks contain long-prismatic zircons (480–500 Ma) with oscillatory zonation indicating an igneous precursor for Red Gneiss protoliths. Geochemical data display obvious characteristics of S-type granites derived through partial melting from deeper crustal source rocks. The obtained time marks of magmatic activity (ca. 575 Ma, ca. 540 Ma, ca. 500–480 Ma) of the Eastern Erzgebirge are compared with adjacent units of the Saxothuringian zone. In all these units, similar time marks and geochemical pattern of igneous rocks prove a similar tectono-metamorphic evolution during Neoproterozoic–Ordovician time.     

Abiogenic Fischer–Tropsch synthesis of hydrocarbons in alkaline igneous rocks; fluid inclusion, textural and isotopic evidence from the Lovozero complex, N.W. Russia

A detailed fluid inclusion study has been carried out on the hydrocarbon-bearing fluids found in the peralkaline complex, Lovozero. Petrographic, microthermometric, laser Raman and bulk gas data are presented and discussed in context with previously published data from Lovozero and similar hydrocarbon-bearing alkaline complexes in order to further understand the processes which have generated these hydrocarbons. CH₄-dominated inclusions have been identified in all Lovozero samples. They occur predominantly as secondary inclusions trapped along cleavage planes and healed fractures together with rare H₂O-dominant inclusions. They are consistently observed in close association with either arfvedsonite crystals, partially replaced by aegirine, aegirine crystals or areas of zeolitization. The majority of inclusions consist of a low-density fluid with CH₄ homogenisation temperatures between −25 and −120 °C. Those in near-surface hand specimens contain CH₄+H₂ (up to 40 mol%)±higher hydrocarbons. However, inclusions in borehole samples contain CH₄+higher hydrocarbons±H₂ indicating that, at depth, higher hydrocarbons are more likely to form. Estimated entrapment temperatures and pressures for these inclusions are 350 °C and 0.2–0.7 kbar. A population of high-density, liquid, CH₄-dominant inclusions have also been recorded, mainly in the borehole samples, homogenising between −78 and −99 °C. These consist of pure CH₄, trapped between 1.2 and 2.1 kbar and may represent an early CH₄-bearing fluid overprinted by the low-density population. The microthermometric and laser Raman data are in agreement with bulk gas data, which have recorded significant concentrations of H₂ and higher hydrocarbons up to C₆H₁₂ in these samples. These data, combined with published isotopic data for the gases CH₄, C₂H₆, H₂, He and Ar indicate that these hydrocarbons have an abiogenic, crustal origin and were generated during postmagmatic, low temperature, alteration reactions of the mineral assemblage. This would suggest that these data favour a model for formation of hydrocarbons through Fischer–Tropsch type reactions involving an early CO₂-rich fluid and H₂ derived from alteration reactions. This is in contrast to the late-magmatic model suggested for the formation of hydrocarbons in the similar peralkaline intrusion, Ilímaussaq, at temperatures between 400 and 500 °C.     

The origin of medium-K ankaramitic arc magmas from Lombok (Sunda arc, Indonesia): Mineral and melt inclusion evidence

High-calcium, nepheline-normative ankaramitic basalts (MgO > 10 wt.%, CaO/Al₂O₃ > 1) from Rinjani volcano, Lombok (Sunda arc, Indonesia) contain phenocrysts of clinopyroxene and olivine (Fo_85–92) with inclusions of spinel (Cr# 58–77) and crystallised melt. Olivine crystals have variable but on average low NiO (0.10–0.23 wt.%) and high CaO (0.22–0.35 wt.%) contents for their forsterite number. The CaO content of Fo_89–91 olivine is negatively correlated with the Al₂O₃ content of enclosed spinel (9–15 wt.%) and positively correlated with the CaO/Al₂O₃ ratios of melt inclusions (0.9–1.5). Major and trace element patterns of melt inclusions are similar to that of the host rock, indicating that the magma could have formed by accumulation of small batches of melt, with compositions similar to the melt inclusions. The liquidus temperature of the magma was  1275 °C, and its oxygen fugacity ≤ FMQ + 2.5. Correlations between K₂O, Zr, Th and LREE in the melt inclusions are interpreted to reflect variable degrees of melting of the source; correlations between Al₂O₃, Na₂O, Y and HREE are influenced by variations in the mineralogy of the source. The melts probably formed from a water-poor, clinopyroxene-rich mantle source.     

Effects of initial soil-water conditions on frost heaving characteristics

Frost heavings have been observed on the soils in waterproof basins (four basins denoted by A, B, C and D) at the Tomakomai field site, Hokkaido, Japan. Basins A, B and C were filled with the same silty soil, and Basin D with four layers, namely, pebbles (0–5 cm below the surface), a mixture of pebbles and silty soil (5–20 cm), silty soil (20–60 cm) and sand (60–160 cm). In the winter of 1976–1977 water levels in the basins were set at the levels of the ground surface in A, 40 cm below the surface in B, the bottom in C and 30 cm below the surface in D. Basin C had no free groundwater. The highest surface heave was 32 cm in A, 26 cm in B, 19 cm in C and 14 cm in D. A water supply from the free groundwater toward the freezing front extended over 220 cm through the silty soil, but was limited to 63 cm through the sand. The migration speeds of the soil water supplied from the unfrozen part were calculated for A, B and C from the measurements of their volumetric water-content profiles.     

Layered mantle at the Karelian Craton margin: P–T of mantle xenocrysts and xenoliths from the Kaavi–Kuopio kimberlites, Finland

Peridotitic clinopyroxene (cpx) and pyrope garnet xenocrysts from four kimberlite pipes in the Kaavi–Kuopio area of Eastern Finland have been studied using major and trace element geochemistry to obtain information on the vertical compositional variability of the underlying mantle. The xenocryst data, when combined with the petrological constraints provided by peridotite xenoliths, yield a relatively complete section through the lithospheric mantle. Single-grain cpx thermobarometry fits with a 36-mW/m² geotherm calculated using heat flow constraints and xenolith modes and geophysical properties. Ni thermometry on pyrope xenocrysts gives 700–1350 °C and, based on the cpx xenocryst/xenolith geotherm, indicates a wide sampling interval, ca. 80–230 km. Plotting pyrope major and trace element compositions as a function of temperature shows there are three distinct layers in the local lithospheric mantle:

(1) A low-temperature (<850 °C) harzburgite layer distinguished by Ca-rich but Ti-, Y- and Zr-depleted pyropes. The xenoliths originating from this layer are all fine-grained garnet-spinel harzburgites with secondary cpx.

(2) A variably depleted lherzolitic, harzburgitic and wehrlitic horizon from 950 to 1150 °C or 130 to 180 km.

(3) A deep layer from 180 to 240 km composed largely of fertile material.

The peridotitic diamond window at Kaavi–Kuopio stretches from the top of the diamond stability field at 140 km to the base of the harzburgite-bearing mantle at about 180 km, implying a roughly 40-km-wide prospective zone.     

Main features of crustal structure in western and southern Europe based on data of explosion seismology

During the past two decades deep seismic sounding measurements have been carried out in western and southern Europe, mainly using the refraction method. These investigations were performed partly on a national basis but as well within international cooperative programs under the sponsorship of the European Seismological Commission.

In France, a systematic study has been executed to determine the main feature of deep structures under the Central Massif and the Paris Basin. In the Forez and Margeride regions, the sub-crustal velocity is lower (7.2 km/sec) than the normal value (8.0 km/sec) observed in the adjacent areas.

The central and southern part of Western Germany is covered by an extensive network of refraction profiles. The crustal thickness varies, similarly to France, from 25 to 35 km. A great amount of deep reflection data was obtained by commercial and special reflection work. The crust beneath the Rhinegraben area shows the typical “rift system” structure with a low subcrustal velocity (7.4–7.7 km/sec).

Very intensive refraction work has been carried out in the Alpine area. The maximum crustal thickness found near the axis of the negative gravity anomaly is about 55–60 km. Furthermore, a clear lowvelocity layer at a depth between 10 and 30 km has been detected. A key position with regard to the geotectonic structure of the Alps is held by the zone of Ivrea characterized by a pronounced gravity high. From the refraction work it may be concluded that there material of the lower crust and the upper mantle (7.2–7.5 km/sec) is overlying a layer of extremely low velocity (5.0 km/sec) which is interpreted as sialic crust.

Three years ago, a systematic study of crustal structure of the Italian peninsula has been started. Reversed profiles were observed on Sicily, in Calabria, and in Puglia. On Sicily, the structure is very complicated; the crust of the western part looks like a transition between a continental and oceanic structure whereas the eastern side shows a continental-type crust. In Calabria and Puglia, the crustal thickness has been determined to be about 25–35 km.     

Torngat ultramafic lamprophyres and their relation to the North Atlantic Alkaline Province

Geological mapping and diamond exploration in northern Quebec and Labrador has revealed an undeformed ultramafic dyke swarm in the northern Torngat Mountains. The dyke rocks are dominated by an olivine-phlogopite mineralogy and contain varying amounts of primary carbonate. Their mineralogy, mineral compositional trends and the presence of typomorphic minerals (e.g. kimzeyitic garnet), indicate that these dykes comprise an ultramafic lamprophyre suite grading into carbonatite. Recognized rock varieties are aillikite, mela-aillikite and subordinate carbonatite. Carbonatite and aillikite have in common high carbonate content and a lack of clinopyroxene. In contrast, mela-aillikites are richer in mafic silicate minerals, in particular clinopyroxene and amphibole, and contain only small amounts of primary carbonate. The modal mineralogy and textures of the dyke varieties are gradational, indicating that they represent end-members in a compositional continuum.

The Torngat ultramafic lamprophyres are characterized by high but variable MgO (10–25 wt.%), CaO (5–20 wt.%), TiO₂ (3–10 wt.%) and K₂O (1–4 wt.%), but low SiO₂ (22–37 wt.%) and Al₂O₃ (2–6 wt.%). Higher SiO₂, Al₂O₃, Na₂O and lower CO₂ content distinguish the mela-aillikites from the aillikites. Whereas the bulk rock major and trace element concentrations of the aillikites and mela-aillikites overlap, there is no fractional crystallization relation between them. The major and trace element characteristics imply related parental magmas, with minor olivine and Cr-spinel fractionation accounting for intra-group variation.

The Torngat ultramafic lamprophyres have a Neoproterozoic age and are spatially and compositionally closely related with the Neoproterozoic ultramafic lamprophyres from central West Greenland. Ultramafic potassic-to-carbonatitic magmatism occurred in both eastern Laurentia and western Baltica during the Late Neoproterozoic. It can be inferred from the emplacement ages of the alkaline complexes and timing of Late Proterozoic processes in the North Atlantic region that this volatile-rich, deep-seated igneous activity was a distal effect of the breakup of Rodinia. This occurred during and/or after the rift-to-drift transition that led to the opening of the Iapetus Ocean.     

High-frequency sequences and their stacking patterns: sequence-stratigraphic evidence of high-frequency eustatic cycles

A hierarchy of interpreted eustatic cyclicity in siliciclastic sedimentary rocks has a pattern of superposed cycles with frequencies in the ranges of 9–10 m.y., 1–2 m.y., 0.1–0.2 m.y., and 0.01–0.02 m.y. (second- through fifth-order cyclicity, respectively). Stratigraphic units displaying this cyclicity include composite sequences, sequences, and parasequences. On the Exxon global cycle chart, fundamental third-order cycles (1–2 m.y. average duration) stack into related groups (second-order cycles: 9–10 m.y. duration). A much larger pattern (about 200 m.y.) is interpreted as tectonically controlled eustasy probably related to sea-floor spreading rates.

One and probably two higher orders of cyclicity (fourth-order: 0.1–0.2 m.y.; and fifth-order: 0.01–0.02 m.y.) are now observed in work with well logs, cores, and outcrops in areas of very rapid deposition. These frequencies are in the range of Milankovitch cycles, and may represent part of the Milankovitch hierarchy which has been widely interpreted for cyclical units in carbonate rocks.

High-frequency (fourth-order) sequences, which form at a 0.1–0.2 m.y. cyclicity, have all the stratal attributes of conventional sequences, including constituent parasequences and systems tracts, and play a dominant role controling reservoir, source, and sealing rock distribution. A consistent hierarchy of stratigraphy is observed. Parasequences (probable fifth-order cyclicity) stack into sets to form systems tracts in fourth-order sequences. Groups (sets) of fourth-order sequences are deposited between major third-order boundaries within third-order composite sequences. Sequences in these sets stack in prograding and backstepping patterns to form third-order lowstand, transgressive, and highstand sequence sets.

Third-order sequence boundaries are marked by greater basinward shifts in facies, by larger more widespread incised valleys, and by more extensive onlap than are fourth-order sequence boundaries. Third-order condensed sections commonly are widespread, faunally rich, and widely correlated biozone and mapping markers. Fourth-order sequence analysis helps to understand reservoir, source, and seal distribution at the play and prospect scale. An example from the Gulf of Mexico is discussed.     

Petrogenesis of the Early Jurassic Nandaling flood basalts in the Yanshan belt, North China Craton: A correlation between magmatic underplating and lithospheric thinning

Integrated geochemical and Sr–Nd–Pb isotopic studies of the Early Jurassic Nandaling flood basalts (NFB) in the Yanshan belt, northern margin of the North China Craton (NCC), are presented in this paper. These sub-alkaline basalts evolved from a more magnesium-rich parental magma through fractional crystallization of olivine and clinopyroxene. The primitive magma of the NFB originated from 2–5% partial melting of spinel to garnet transitional peridotite at about 70–80 km depth in the Mesozoic lithosphere mantle. The NFB contain a distinctive lithospheric component, characterized by Nb (Ta), Th, U and Ti depletions, LREE enrichments, moderate Sr, and low Nd and Pb initial isotopic ratios, as a result of an interaction between lower crust (15–25%) and primitive magma evoked by magmatic underplating at crust–mantle boundary. The Early Jurassic NFB extruded in an intraplate extensional setting related to post-orogenic collapse in the northern margin of the NCC, indicating an event of lithospheric modification earlier than that in the southern margin (Early Cretaceous). The temporal similarity of the Jurassic–Cretaceous mantle-derived mafic rocks to lower crust replacement, and the decoupling of surface shortening with lithospheric thinning during the Late Jurassic–Early Cretaceous, suggest the important role of magmatic underplating and subsequent crust–mantle interaction accompanied by asthenosphere upwelling on the evolution of the Mesozoic lithosphere of the NCC. The correlation between lithospheric thinning and magmatic underplating may be an important process in continental rifting.