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What remains of a 30 g sample, first recognized as a meteorite in 1989 during characterization of metalworking debris from Danebury, an Iron Age hillfort, in Hampshire, England, has been classified as an H5 ordinary chondrite. Its arrival on Earth has been dated as 2350 ± 120 yr BP, making it contemporary with the period of maximum human activity at the recovery site. Despite its considerable terrestrial residence age, the interior of the specimen is remarkably fresh with a weathering index of W1/2. There is, however, no evidence of human intervention in its preservation. Its near-pristine state is explained in terms of its serendipitous burial during the back-fill of a pit dug into chalk by prehistoric people for the storage of grain. This chance discovery has interesting ramifications for the survival of meteorites in areas having a high pH because of a natural lime content arising as a result of the local geology.

The spectacular arrival of a huge meteorite in central Siberia on February 15th 2013 was the largest event of its kind for more than a hundred years. Oxygen isotope analysis reveals the object involved was an ordinary chondrite of type LL. Petrological examination of the material analysed shows two main lithologies, metamorphic grade 5, were present both having veins of shock produced glass. All three types of material were investigated for carbon, nitrogen and noble gas content and isotopic compositions. The relatively low abundance of carbon and nitrogen indicate that Chelyabinsk is uncontaminated by comparison with other samples in meteorite collections so that indigenous components may be recognized. All the samples studied contained minimal amounts of cosmogenic and primordial noble gases, evidence that the pre-atmospheric size of the object was large, greater than two metres in diameter and that the explosion and break-up was accompanied by catastrophic degassing. The implications for other major meteorite falls and the Tunguska event are discussed.

We investigated exterior and interior subsamples from the Martian shergottite meteorites Allan Hills (ALH) A77005 and Roberts Massif (RBT) 04261 for secondary minerals, oxygen isotopes, Ar-Ar, and noble gas signatures. Electron microprobe investigations revealed that RBT 04261 does not contain any visible alteration even in its most exterior fractures, whereas fracture fillings in ALHA77005 penetrate into the meteorite up to 300 μm, beyond which the fractures are devoid of secondary minerals. Light noble gases seem to be almost unaffected by terrestrially induced alteration in both meteorites. Thus, a shock metamorphic overprint of 30–35 GPa can be deduced from the helium measurements in RBT 04261. Oxygen isotopes also seem unaffected by terrestrially weathering and variations can easily be reconciled with the differences in modal mineralogy of the exterior and interior subsamples. The measurements on irradiated samples (Ar-Ar) showed a clear Martian atmospheric contribution in ALHA77005, but this is less apparent in our sample of RBT 04261. Exterior and interior subsamples show slight differences in apparent ages, but the overall results are very similar between the two. In contrast, krypton and xenon are severely affected by terrestrial contamination, demonstrating the ubiquitous presence of elementally fractionated air in RBT 04261. Although seemingly contradictory, our results indicate that RBT 04261 was more affected by contamination than ALHA77005. We conclude that irrespective of on which planet the alteration occurred, exposure of Martian rocks to atmosphere (or brine) introduces noble gases with signatures elementally fractionated relative to the respective atmospheric composition into the rock, and relationships of that process with oxygen isotopes or mineralogical observations are not straightforward.

Kerlingarfjoll central volcano is Iceland’s second largest outcrop of Quaternary rhyolite and is part of the Icelandic Western Rift Zone. Geochemical and Ar/Ar age data show that at least 21 different rhyolite eruptions have taken place at Kerlingarfjoll over the last 350 ka. Ar/Ar dating was carried out on samples of obsidian which showed variable reproducibility, illustrating the difficulty in dating young Icelandic volcanics. Nevertheless, reasonable estimates of eruption age have been derived for a number of eruptive units that are consistent with observed stratigraphy, enabling an understanding of the temporal evolution of Kerlingarfjoll. Two rhyolite magma types are present. The first is an older, low-Nb rhyolite that was erupted episodically along a cryptic curved fracture system, to form a discontinuous ring of rhyolite mountains, between 350 and 250 ka. This discontinuous ring is similar to structures observed at other volcanoes in Iceland, suggesting that the development of a curved fracture that acts as a pathway for episodic silicic eruptions is a feature of central volcano development. The second magma is a younger, high-Nb rhyolite that was erupted episodically between 250 and 68 ka in the northern part of Kerlingarfjoll, forming two clusters, both of which have areas of intense hydrothermal activity. Repose periods for rhyolite volcanism are thought to be on the order of tens of thousands of years, and it is possible that Kerlingarfjoll will erupt rhyolite again in the future.

Garnet-bearing mantle xenoliths have been recovered from Quaternary alkali basalts, both within and peripheral to the Hangay dome of central Mongolia. Microfabric analysis and thermobaromery, combining empirical thermobarometers and the self-consistent dataset of THERMOCALC, indicate that garnet websterites from the Shavaryn-Tsaram volcanic centre at the dome core were formed in the spinel-lherzolite upper mantle at pressures of 17–18 kbars and temperatures of 1,070–1,090°C, whereas garnet lherzolites were derived from greater depths (18–20 kbars). Garnet lherzolites from the Baga Togo Uul vents near the dome edge were formed at 18–22 kbars under significantly cooler conditions (960–1,000°C). These xenoliths reveal reaction coronas of (1) orthopyroxene, clinopyroxene, plagioclase and spinel mantling garnets; (2) spongy rims of olivine replacing orthopyroxene and (3) low-Na, low-Al clinopyroxene replacing primary clinopyroxene. Trace-element abundances indicate that clinopyroxene from these coronas is in chemical equilibrium with the host magma. The thermobarometric and textural data suggest that lherzolite xenoliths from both sites were derived from depths of 60–70 km and entrained in magma at 1,200–1,300°C. The average rate of ascent, as determined by olivine zoning, lies in the range 0.2–0.3 m s−1. The contrast in thermal profiles of the upper mantle between the two sites is consistent with a mantle plume beneath the Hangay dome with elevated thermal conditions beneath the core of the dome being comparable to estimates of the Pleistocene geotherm beneath the Baikal rift.

The Greater Olkaria Volcanic Complex is a young (20 ka) multi-centred system in the central Kenya Rift Valley, mainly represented at outcrop by peralkaline rhyolites. The rhyolites show significant compositional variation; peralkalinity [mol. (Na2O + K2O)/Al2O3] varies from 1·01 to 1·55, Zr contents from 442 to 3640 ppm and Rb contents from 262 to 1056 ppm. More peralkaline rhyolites were generated along multiple, but generally closely similar, liquid lines of descent by 75% fractional crystallization of alkali feldspar–quartz-dominated assemblages from mildly peralkaline parental magmas, themselves probably derived by fractionation of trachytic magmas. Crystal fractionation took place at more than one upper crustal level. The rhyolite magmas were erupted from 13 centres, each having an eruptive history and geochemical evolution broadly similar to, but distinct from, those of the other centres. For most of the life span of the Olkaria system, almost the whole spectrum of peralkaline rhyolite compositions was erupted from vents in the complex at any one time. Apparent partition coefficients are presented for 34 trace elements in sanidine, fayalite, ferrohedenbergite, amphibole, biotite, ilmenite and chevkinite-(Ce). With the exception of certain values for Ba, Rb and Sr in sanidine, most coefficients vary systematically with whole-rock composition.

Through weathering processes, volcanic rocks contribute both to nutrient flux into the biosphere and atmospheric CO 2 drawdown. As rhyolitic rocks are of higher silica content and have lower concentrations of biologically-important elements than basalts they might be expected to be less easily weathered by a biota. Investigations on the microbial diversity and weathering of silica-rich rhyolitic glass (obsidian) from a lava flow in Iceland are reported. 16S rDNA analysis of rock whole genome DNA shows that the rock hosts remarkable eubacterial diversity. Irregular pitted weathering textures correspond to regions of eubacterial colonization as shown by FISH. Weathering processes proceed at alteration fronts, with a preference for potentially nutrient-rich regions containing plagioclase and pyroxene crystals, although these features are less well defined than those previously reported from basaltic glass, consistent with the lower rates of chemical weathering previously reported for rhyolites compared to ba...

Germanium abundances, determined by laser ablation-inductively coupled plasma-mass spectrometry, are presented for phenocrysts and glass matrices from a metaluminous trachyte and four peralkaline rhyolites from the Greater Olkaria Volcanic Complex, Kenya Rift Valley, Africa. Abundances (in ppm) are: sanidine 0.45–0.61; fayalite 4.8–11.7; hedenbergite 5.1–9.0; titanomagnetite 2.7; ilmenite 0.48; amphibole 8.3–8.9; biotite 7.0; chevkinite-(Ce) 309; trachyte glass 3.0; rhyolitic glasses 2.3–3.9. These values are generally greater than those recorded for silicic rocks in the literature, whilst the chevkinite-(Ce) value is the largest yet found in a magmatic mineral. Apparent partition coefficients range from 0.15–0.26 in sanidine to 124 in chevkinite-(Ce). Those for fayalite and hedenbergite increase with whole-rock peralkalinity and Fe content. The possibility of a role for accessory phases in influencing Ge distribution in rock-forming minerals is also raised.

St Kitts forms one of the northern group of volcanic islands in the Lesser Antilles arc. Eruptive products from the Mt Liamuiga centre are predominantly olivine + hypersthene-normative, low-K basalts through basaltic andesites to quartz-normative, low-K andesites. Higher-Al and lower-Al groups can be distinguished in the suite. Mineral assemblages include olivine, clinopyroxene, orthopyroxene, plagioclase and titanomagnetite with rarer amphibole, ilmenite and apatite. Eruptive temperatures of the andesites are estimated as 963–950°C at fO2 NNO + 1 (where NNO is the nickel–nickel oxide buffer). Field and mineral chemical data provide evidence for magma mixing. Glass (melt) inclusions in the phenocrysts range in composition from andesite to high-silica rhyolite. Compositional variations are broadly consistent with the evolution of more evolved magmas by crystal fractionation of basaltic parental magmas. The absence of any covariation between 87Sr/86Sr or 143Nd/144Nd and SiO2 rules out assimilation of older silicic crust. However, positive correlations between Ba/La, La/Sm and 208Pb/204Pb and between 208Pb/204Pb and SiO2 are consistent with assimilation of small amounts (

Hollandite and cryptomelane occur as intergrown botryoidal masses with goethite in a quartz vein hosted by the Leinster Granite at Cloghleagh Mine, near Blessington, Co. Wicklow. Coronadite and romanA¨chite accompany pyromorphite in an oxidised Pb–Zni?½quartz vein at Old Luganure Mine, Glendasan, also in the Leinster Granite. Quantitative analyses show the Cloghleagh material to consist of bands of varying K:Ba ratio ranging from cryptomelane (K > Ba) to hollandite (Ba > K), but neither approaches end member composition. The Luganure material is mostly coronadite of near Pb-end member composition, in some cases mixed, or intergrown with, romanA¨chite.

Liddicoatite, a rare species of tourmaline, has been found in the McCombe pegmatite, of complex type and spodumene subtype, an example of the LCT (lithium, cesium, tantalum) family. It is one of a group of beryl- and complex-type pegmatites exposed along the Uchi–English River subprovincial boundary-zone, Ontario, in the Superior Province of the Canadian Shield. The mineral assemblage containing zoned "fluor-elbaite"i?½liddicoatite crystals was the last product of primary, near-solidus crystallization of the pegmatite and is unique not only to this subprovincial boundary, but also throughout the vast number of rare-element pegmatites distributed across Ontario. The most calcic composition of liddicoatite analyzed here corresponds to (Ca0.5Na0.40.1) (Li1.5Al1.2Mn0.2Fe0.1) Al6(Si6O18) (BO3)3 (OH3.4F0.6). Other calcium-enriched species found in the McCombe pegmatite include microlite (Ca1.2Na0.60.2)(Ta1.6Nb0.3Sn0.1)O6(F0.7OH0.3), Ca-enriched almandine (Alm44.6Sps26.4Grs26.2), Ca-enriched spessartine (Sps69.3Alm19.8Grs10.9), and fluorapatite. Evidence from progressively evolved units of the pegmatite and mineral zonation shows that contamination of the pegmatite-forming melt occurred, and that fluids from the pegmatite migrated out into the mafic metavolcanic host-rocks, creating a Li-, Cs-, and B-enriched metasomatic aureole. The evolution of the pegmatite-forming melt resulted in increased activity of fluorine, and the essentially in situ fractionation of the contaminated melt led to the crystallization of the rare-element-enriched minerals. Patterns of zonation seen in back-scattered-electron images show that primary crystallization of columbite–tantalite, garnet and tourmaline species involved multiple episodes of growth and dissolution in a dynamic system. Liddicoatite was able to form, not because of any one of these processes, but because they all acted together to concentrate both Li and Ca, a sequence of events that is apparently very rare in nature.

The zeolite group minerals brewsterite–Ba and harmotome occur as minor latestage primary phases in the low-temperature Pb–Zn veins at Glendalough and Glendasan, Co. Wicklow. Brewsterite–Ba was identified on a single specimen from the Old Luganure Mine. Harmotome occurs at the Glendalough Mine, the Foxrock Mine and the Old Luganure Mine. Quantitative analyses of crystals of brewsterite–Ba reveal the most barium-rich composition yet reported: (Ba1.88, Sr0.02, K0.01, Na0.01) [Al4.03Si12.02O32]·nH2O. This is close to the hypothetical barium end member. The harmotome is also barium-rich. It contains lesser amounts of sodium and potassium, but no calcium. This is the first record of brewsterite–Ba from Ireland.

Rare-element pegmatites may host several economic commodities, such as tantalum (Ta-oxide minerals), tin (cassiterite), lithium (ceramic-grade spodumene and petalite), and cesium (pollucite). Key geological features that are common to pegmatites in the Superior province of Ontario and Manitoba, Canada, and in other large tantalum deposits worldwide, can be used in exploration. An exploration project for rare-element pegmatites should begin with an examination of a regional geology map. Rare-element pegmatites occur along large regional-scale faults in greenschist and amphibolite facies metamorphic terranes. They are typically hosted by mafic metavolcanic or metasedimentary rocks, and are located near peraluminous granite plutons (A/CNK > 1.0). Once a peraluminous granite pluton has been identified, then the next step is to determine if the pluton is barren or fertile. Fertile granites have elevated rare element contents, Mg/Li ratio < 10, and Nb/Ta ratio < 8. They commonly contain blocky K-feldspar and green muscovite. Key fractionation indicators can be plotted on a map of the fertile granite pluton to determine the fractionation direction: presence of tourmaline, beryl, and ferrocolumbite; Mn content in garnet; Rb content in bulk K-feldspar; and Mg/Li and Nb/Ta ratios in bulk granite samples. Pegmatite dikes with the most economic potential for Li-Cs-Ta deposits occur the greatest distance (up to 10 km) from the parent granite. Metasomatized host rocks are an indication of a nearby rare-element pegmatite. Metasomatic aureoles can be identified by their geochemistry: elevated Li, Rb, Cs, B, and F contents; and by their mineralogy: presence of tourmaline, (Rb, Cs)-enriched biotite, holmquistite, muscovite, and rarely garnet. Once a pegmatite dike has been located, the next step is to assess its potential to contain Ta mineralization. Pegmatites with the highest degree of fractionation (and thus the most economic potential for Li-Cs-Ta) contain blocky K-feldspar with >3,000 ppm Rb, K/Rb 100 ppm Cs; and coarse-grained green muscovite with >2,000 ppm Li, >10,000 ppm Rb, >500 ppm Cs, and >65 ppm Ta. Pegmatites with Ta mineralization usually contain Li-rich minerals (e.g., spodumene, petalite, lepidolite, elbaite, amblygonite, and lithiophilite) and may contain Cs-rich minerals (e.g., pollucite, Cs-rich beryl). The ore minerals of Ta are commonly manganotantalite, manganocolumbite, wodginite, and microlite; Ta-rich cassiterite is also commonly present. Tantalum mineralization tends to occur in albitic aplite, mica-rich (lepidolite, cleavelandite ± lepidolite), and spodumene/petalite pegmatite zones.

A combined metamorphic and isotopic study of lit-par-lit migmatites exposed in the hanging wall of the Main Central Thrust (MCT) from Sikkim has provided a unique insight into the pressure-temperature- time path of the High Himalayan Crystalline Series of the eastern Himalaya. The petrology and geochemistry of one such migmatite indicates that the leucosome comprises a crystallized peraluminous granite coexisting with sillimanite and alkali feldspar. Large garnet crystals (2-3 mm across) are strongly zoned and grew initially within the kyanite stability field. The melanosome is a biotite-garnet pelitic gneiss, with fibrolitic sillimanite resulting from polymorphic inversion of kyanite. By combining garnet zoning profiles with the NaCaMnKFMASHTO pseudosection appropriate to the bulk composition of a migmatite retrieved from c. 1 km above the thrust zone, it has been established that early garnet formed at pressures of 10-12 kbar, and that subsequent decompression caused the rock to enter the melt field at c. 8 kbar and c. 750 � C, generating peritectic sillimanite and alkali feldspar by the incongruent melting of muscovite. Continuing exhumation resulted in resorption of garnet. Sm-Nd growth ages of garnet cores and rim, indicate pre-decompression garnet growth at 23 ± 3 Ma and near-peak temperatures during melting at 16 ± 2 Ma. This provides a decompression rate of 2 ± 1 mm yr )1 that is consistent with exhumation rates inferred from mineral cooling ages from the eastern Himalaya. Simple 1D thermal modelling confirms that exhumation at this rate would result in a near-isothermal decompres- sion path, a result that is supported by the phase relations in both the melanosome and leucosome components of the migmatite. Results from this study suggest that anatexis of Miocene granite protoliths from the Himalaya was a consequence of rapid decompression, probably in response to movement on the MCT and on the South Tibetan detachment to the north.

U-Pb isotopic analyses using TIMS and Laser Ablation-Multi Collector-ICP-MS were carried out on columbite-tantalite minerals from three suites of rare-element (Li, Cs, Ta) pegmatites in the Superior Province of Canada. Conventional TIMS analyses of these columbite-tantalite crystals produce scattered data and reverse discordance even after HF leaching of the grains prior to dissolution, possibly reflecting the incomplete removal of the open-system metamict segments during sample preparation. LA-MC-ICP-MS analyses of unleached, primary columbite free from inclusions and alteration give consistent and precise (

Monazite is a mineral of choice for dating metamorphism in amphibolite- and granulite-grade metapelites. However, there exist a number of difficulties that complicate the interpretation of monazite geochronological data and prevent its application to many geological problems. The two main obstacles addressed in this contribution are firstly, the minor but significant (e.g. 1–30 Ma) dispersal in duplicate isotope dilution thermal ionisation mass spectrometry (ID-TIMS) U–Pb age data commonly recorded from a single rock, and secondly, the difficulty of attaching monazite age data to pressure and temperature information. Through a multidisciplinary approach utilising TIMS and laser ablation multicollector inductively coupled plasma mass spectrometry (LA-MC-ICPMS) isotope data, quantitative and qualitative EMP chemical analyses of monazite, and textural studies, we assess the significance of Pb loss, older components, and continuous and episodic monazite growth in the generation of dispersed age data. Three samples from the Canadian Cordillera and one sample from the Himalaya of Pakistan are examined. Each sample exhibits an age dispersion of between 1 and 12 Ma for single crystal and multi-grain TIMS U–Pb monazite age determinations. Consideration of the closure temperature for Pb diffusion in monazite and the metamorphic temperatures experienced by these samples suggests diffusive Pb loss did not play a significant part in generating this age dispersal. The LA-MC-ICPMS study indicates that an older component (

Tourmaline in petalite-subtype granitic pegmatites from the Pakeagama Lake and Separation Lake areas of northwestern Ontario provides evidence for the compositional evolution of pegmatite-forming melts through magmatic crystallization and pegmatite melt – host-rock interactions. At the former locality, tourmaline from the pegmatite units is essentially magmatic, whereas at the latter locality, tourmaline in many of the Fe-suite pegmatites displays characteristics (such as elevated Mg, Ti and Ca contents) that indicate significant contamination through the incorporation and digestion of components of amphibolite (mafic metavolcanic units) and banded iron-formation (BIF) host-rocks. The presence of tourmaline in mafic host-rocks indicates that cation migration also involved the introduction of B, Li and Al from pegmatite-generated fluids to form metasomatic haloes. The Li + Mn versus Mg + Ti + Ca diagram is introduced to discriminate tourmaline with a dominantly magmatic signature from tourmaline with a signature influenced by pegmatite – host-rock interaction. The behavior of Ca in tourmaline is also shown to be potentially useful in recognizing late-stage Ca-enrichment in pegmatite-forming melts as a result of Ca–F complexing, but can give similar trends to tourmaline that has suffered Ca-metasomatism as a result of interaction with granitic host-rocks. The tourmaline data suggest that at Separation Rapids, pegmatites mainly crystallized in an open system (i.e., pegmatite – host-rock interaction was common), whereas at Pakeagama Lake, the pegmatite units were predominantly unaffected by interaction with host rocks (i.e., they crystallized in a closed system). This has implications for concentrating rare-elements such as Rb and Cs, which otherwise would be dispersed throughout pegmatite and host rocks (as at Separation Lake). The potential for a buried pollucite deposit associated with the SE pegmatite at Pakeagama Lake has increased as a result of these observations, particularly as Cs is elevated in associated K-feldspar and mica, and is also present in a late pollucite-bearing vein.

The Separation Lake area is host to the most important rare-element pegmatites in Ontario, Canada. They include the Big Whopper and Big Mack petalite pegmatite systems which potentially represent the world's second largest lithium deposit of this type. The pegmatites occur in two distinct clusters adjacent to the Separation Rapids pluton which is thought to be the source of the rare-elements. Beryl-type and complex-, petalite-subtype pegmatites are the most common and a few pegmatites have characteristics similar to the lepidolite-subtype. This study reveals that columbite-tantalite in the pegmatites has an extremely wide range of composition from primitive ferrocolumbite to evolved, almost end-member manganotantalite. Evidence is provided that melt evolution resulted in increased fluorine activity (as seen in microlite compositions) and that in situ fractionation of magma within individual pegmatites often led to the crystallization of rare-element-enriched, Li mica-fluorapatite-cleavelandite pods. Zonation patterns seen in backscattered electron images show primary compositions of columbite-tantalite were modified by secondary processes related to extreme fractionation and involving the late stage development of albitic units in individual pegmatites. This alteration led to recrystallization of columbite-tantalite and produced compositions with lower Ta contents, but with little change in Mn content.

Hydrothermal fluid evolution north of the St Austell granite, southwest England, has been studied through geochemical analysis of tourmaline from a fault breccia of

Kelut volcano, East Java, is an active volcanic complex hosting a summit crater lake that has been the source of some of Indonesia’s most destructive lahars. In November 2007, an effusive eruption lasting approximately 7 months led to the formation of a 260-m-high and 400-m-wide lava dome that displaced most of the crater lake. The 2007–2008 Kelut dome comprises crystal-rich basaltic andesite with a texturally complex crystal cargo of strongly zoned and in part resorbed plagioclase (An47–94), orthopyroxene (En64–72, Fs24–32, Wo2–4), clinopyroxene (En40–48, Fs14–19, Wo34–46), Ti-magnetite (Usp16–34) and trace amounts of apatite, as well as ubiquitous glomerocrysts of varying magmatic mineral assemblages. In addition, the notable occurrence of magmatic and crustal xenoliths (meta-basalts, amphibole-bearing cumulates, and skarn-type calc-silicates and meta-volcaniclastic rocks) is a distinct feature of the dome. New petrographical, whole rock major and trace element data, mineral chemistry as well as oxygen isotope data for both whole rocks and minerals indicate a complex regime of magma-mixing, decompression-driven resorption, degassing and crystallisation and crustal assimilation within the Kelut plumbing system prior to extrusion of the dome. Detailed investigation of plagioclase textures alongside crystal size distribution analyses provide evidence for magma mixing as a major pre-eruptive process that blends multiple crystal cargoes together. Distinct magma storage zones are postulated, with a deeper zone at lower crustal levels or near the crust-mantle boundary (>15 km depth), a second zone at mid-crustal levels (~10 km depth) and several magma storage zones distributed throughout the uppermost crust (

The radioelement distribution and content of the Lundy granite, a coarse-grained megacrystic granite of Tertiary age, has been measured using a portable gamma-ray spectrometer in order to assess fractionation and alteration processes in the granite. Results indicate a systematic variation of K, Th and U (with a few notable exceptions) that follows a partially concentric distribution to lower concentrations inland. The plateau region of the island (particularly the southern half) is relatively depleted in all radioelements. Over the island, measurements of K vary from 1.3–4.9 wt %, Th varies from 5.0–20.3 ppm and U varies from 2.0–12.5 ppm. A petrographic, electron microprobe and autoradiography examination of the granite indicates that the radioelements mainly reside in discrete major and accessory minerals, of which K-feldspar (K), biotite (K), monazite (Th), xenotime (U), tungsteniferous columbite (U) and uraninite (U) are the most important. Uraninite is rare, being preserved only in fresh samples which come mainly from abandoned quarries. Mass balance modelling indicates that up to 76.6% of uranium could reside in uraninite and where this has been leached by secondary processes such as hydrothermal alteration or weathering then the present radioelement content no longer reflects the original rock composition. Fission track evidence is presented to show the pathways along which uranium has been mobilized from or within the granite. Secondary sites of radioelements include fractures cross-cutting all major minerals (but especially quartz), grain boundaries, altered cores of plagioclase feldspar and occasionally yellowy brown mixed chlorite/smectite replacement product after biotite. Biotite itself may exhibit secondary tracks along cleavage traces. Combined effects of crystal fractionation (primary variation) and secondary alteration best explain the distribution of radioelements, with K controlled by fractionation of the major phases K-feldspar and biotite, Th by fractionation of the accessory mineral monazite (±xenotime and uraninite) and U contents by uraninite and tungsteniferous columbite. Secondary processes have removed much of the uraninite leaving behind indeterminate Fe—U material along fractures and residual U (and Th) enrichment within altered major minerals. There is some evidence to suggest that late radioelement-bearing fluids precipitated monazite and uraniferous zircon along fractures during the waning stages of magmatic activity.

A Microsoft Excel™ spreadsheet program has been developed to calculate structural formulae of microprobe-derived amphibole analyses and determine classification parameters according to the International Mineralogical Association scheme for amphibole nomenclature. The program has been tested in the determination of formulae and classification of 138 amphibole analyses from the literature. Of these, the small percentage (14%) of discrepancies were determined mainly to be the result of (i) differences between measured (wet chemical) and estimated values for FeO and Fe2O3 and (ii) differences inherent in summing structural formula of microprobe data to 23 oxygens and wet chemical data to 24 oxygens. The program has been developed for the Apple Macintosh but a PC version also is available. Spreadsheet implementations of four published geobarometers based on the total aluminium content of certain calcic amphiboles also are presented. These geobarometers are applicable to metaluminous plutonic (and associated volcanic) rocks containing the assemblage amphibole-biotite-quartz-plagioclase-(orthoclase)-sphene-FeTi oxides and apply to a pressure range of 2.5–13 kbar.

Several compositionally varied suites of plutonic rocks (gabbro-quartz monzogabbro, granite, quartz monzodiorite) in the Bunger Hills were emplaced at depths of ∼ 20 km over a relatively short time interval (1170-1150 Ma) during the waning stages of granulite-facies metamorphism, the peak of which occurred ∼ 1190 Ma ago. Alkaline (syenite to granite) rocks were emplaced west of the Denman Glacier at David Island ∼ 515 Ma ago. Most of these rocks crystallised from magmas derived by melting of heterogeneous mantle comprising at least two components — Nb-poor, long-term enriched lithosphere, and a Nb-rich OIB (ocean island basalt) -type within-plate component. Metasomatic enrichment of the source of the plutonic rocks, as well as that of 500-Ma alkaline dykes, may have been contemporaneous with continental crust formation in the area during the late Archaean or Early Proterozoic. Much of the chemical variation of the different suites can be explained by fractional crystallisation, but other processes, particularly variations in the extent and/or P–T conditions of melting were also important. Crustal contamination does not appear to have been a major factor in the petrogenesis of the Bunger Hills magmas, but cannot be entirely discounted, particularly for some of the granitic rocks; the more evolved David Island syenitic intrusives may have been derived by melting of lower-crustal rocks.

The island of Lundy forms the southernmost igneous complex of the British Tertiary Volcanic Province (BTVP) and consists of granite ( 90%) emplaced into deformed Devonian sedimentary rocks (Pilton Shale) and associated with a swarm of dykes of dolerite/basalt, minor trachyte and rhyolite composition. The dolerites are of varied olivine basalt composition and are associated with peralkaline trachyte and subalkaline/peralkaline rhyolite with alkali feldspar and quartz ± alkali amphibole ± pyroxene mineralogy. The dyke swarm is therefore an anorogenic bimodal dolerite/basalt-trachyte/rhyolite BTVP association. Although the dyke association is bimodal in major element terms between dolerite/basalt and minor trachyte/rhyolite, the mineralogy and trace element geochemistry indicate that the dykes may be regarded as a cogenetic dolerite - peralkaline trachyte/rhyolite association with minor subalkaline rhyolites. Sr and Nd isotope data indicate derivation of these magmas from a similar BTVP mantle source (with or without minor contamination by Pilton Shale, or possibly Lundy granite). The petrogenesis of the Lundy dyke association is therefore interpreted in terms of extensive fractional crystallization of basaltic magma in a magma chamber of complex geometry below the (exposed) Lundy granite. Fractional crystallization of a representative dolerite magma (olivine ± clinopyroxene ± plagioclase) yields trachyte magma from which the crystallization of alkali feldspar (anorthoclase) ± plagioclase (oligoclase) + Fe-Ti oxide + apatite results in peralkaline rhyolite. Rarer subalkaline rhyolites result from fractionation from a similar dolerite source which did not achieve a peralkaline composition so allowing the crystallization and fractionation of zircon. The basalt-(minor trachyte)/rhyolite bimodality reflects rapid crystallization of basalt magma to trachyte (and rhyolite) over a relatively small temperature interval (mass fraction of melt, F = 0.15). The rapid high level emplacement of basalt, trachyte and rhyolite dyke magmas is likely to have been associated with the development of a substantial composite bimodal basalt-(minor trachytel)/rhyolite volcano above the BTVP Lundy granite in the Bristol Channel.

Fluorescent in situ hybridization (FISH) and 16S rDNA analysis were used to characterize the endolithic colonization of silica-rich rhyolitic glass (obsidian) in a barren terrestrial volcanic environment in Iceland. The rocks were inhabited by a diverse eubacterial assemblage. In the interior of the rock, we identified cyanobacterial and algal 16S (plastid) sequences and visualized phototrophs by FISH, which demonstrates that molecular methods can be used to characterize phototrophs at the limits of photosynthetically active radiation (PAR). Temperatures on the surface of the dark rocks can exceed 40 degrees C but are below freezing for much of the winter. The rocks effectively shield the organisms within from ultraviolet radiation. Although PAR sufficient for photosynthesis cannot penetrate more than approximately 250 mum into the solid rock, the phototrophs inhabit cavities; and we hypothesize that by weathering the rock they may contribute to the formation of cavities in a feedback process, which allows them to acquire sufficient PAR at greater depths. These observations show how pioneer phototrophs can colonize the interior of volcanic glasses and rocks, despite the opaque nature of these materials. The data show that protected microhabitats in volcanic rocky environments would have been available for phototrophs on early Earth.

The Greater Olkaria Volcanic Complex is a young ( 20 ka) multi-centred lava and dome field dominated by the eruption of peralkaline rhyolites. Basaltic and trachytic magmas have been erupted peripherally to the complex and also form, with mugearites and benmoreites, an extensive suite of magmatic inclusions in the rhyolites. The eruptive rocks commonly represent mixed magmas and the magmatic inclusions are themselves two-, three- or four-component mixes. All rock types may carry xenocrysts of alkali feldspar, and less commonly plagioclase, derived from magma mixing and by remobilization of crystal mushes and/or plutonic rocks. Xenoliths in the range gabbro–syenite are common in the lavas and magmatic inclusions, the more salic varieties sometimes containing silicic glass representing partial melts and ranging in composition from anorthite ± corundum- to acmite-normative. The peralkaline varieties are broadly similar, in major element terms, to the eruptive peralkaline rhyolites. The basalt–trachyte suite formed by a combination of fractional crystallization, magma mixing and resorption of earlier-formed crystals. Matrix glass in metaluminous trachytes has a peralkaline rhyolitic composition, indicating that the eruptive rhyolites may have formed by fractional crystallization of trachyte. Anomalous trace element enrichments (e.g. 2000 ppm Y in a benmoreite) and negative Ce anomalies may have resulted from various Na- and K-enriched fluids evolving from melts of intermediate composition and either being lost from the system or enriched in other parts of the reservoirs. A small group of nepheline-normative, usually peralkaline, magmatic inclusions was formed by fluid transfer between peralkaline rhyolitic and benmoreitic magmas. The plumbing system of the complex consists of several independent reservoirs and conduits, repeatedly recharged by batches of mafic magma, with ubiquitous magma mixing.

The British Tertiary Volcanic Province (BTVP) comprises within-plate central igneous complexes associated with plateau lavas and regional dyke swarms. Lundy is the southernmost complex of the BTVP and comprises granite (small tilde90%) emplaced into deformed Devonian sedimentary rocks within the Hercynian Cornubian granite province of southwest England. The complex is intruded by a northwest-southeast trending dyke swarm. In common with other BTVP igneous complexes, Lundy is associated with positive gravity and magnetic anomalies which are interpreted in terms of the presena of an underlying basic intrusion at shallow depth, with a volume exceeding that of the overlying granite. The Lundy intrusion is a coarse-grained megacrystic granite containing up to 20% alkali feldspar megacrysts in a coarse-grained groundmass composed of alkali feldspar, quartz, lithium-bearing muscovite, and 'biotite' (lithian siderophyllite), with a range of aaxssory minerals. The main granite has a coarse-grained (locally miarolitic) pegmatitic facies and is intruded by thin sheets and veins of fine-grained aplite and microgranite. The mineralogy indicates crystallization of the Lundy granite from a highly fractionated H2O- and halogen-rich magma at a relatively shallow crustal level. The main Lundy granite is a peraluminous leucogranite with Na2O=3–4%, K2Osmall tilde5%, low TiO2, MeO, CaO, Zr, and Sr, and high Rb and Rb/Sr in comparison with many other peralurninous granites, including those from the Cornubian batholith and the BTVP. A new Rb-Sr whole-rock isochron for the granite yields an age of 58·7±1·6 Ma with an initial 87Sr/86Sr of 0·715±0·006. Nd values for the granite (i?½0·9 to '1·9) plot between contemporaneous mantle (positive Nd and Cornubian granites (Nd=ca.i?½11). The trace element data (Rb, Y, Nb) show affinities with syn-collision and within-plate granites. As the Sr isotope data indicate a major crustal component, and the Nd isotope data suggest both mantle and crustal components, we propose that the Lundy granite is derived from a parental magma comprising crustal components (derived from a similar source to that of the Cornubian granite batholith) and a mantle-derived component (derived from a differentiate of contemporaneous basaltic magma This magma experienced fractional crystallization of plagioclase, alkai feldspar, Fe-Mg minerals, and REE-bearing accessory minerals before emplacement, and the Lundy granite experienced further in situ fractional crystallization, associateded with crustal contamination by the Devonian shale after emplacement.

Perim Island is an eroded fragment of the southwest flank of a late Miocene (10.5 ± 1.0 Ma) volcano whose centre lay on the southwesternmost tip of Arabia. The volcano is the westernmost of the E–W line of six central vent volcanoes (the Aden Line) that extends 200 km along the south coast of Arabia from Perim to Aden. Major oxide and trace element abundances are given for 35 Perim specimens and these show that the volcano has within-plate trace element characteristics and consists of a petrographically and geochemically simple suite of alumina-poor olivine basalts, andesites, and transitional andesite–trachyandesites. Six specimens, however, are markedly enriched in Al2O3 and CaO, and contain abundant (20–30 mode %) highly calcic (An77–83) plagioclase phenocrysts. Geochemical modelling suggests that the main Perim volcanic sequence was produced by the fractional crystallization (o1 + cpx + Ti-mt + plag) of a silica saturated (SiO2 c. 45%) basic melt. The high A1, high Ca, magmas appear to be mixing products of plagioclase-enriched basic magmas with more evolved melts. Perim is the oldest volcano of the Aden line, which becomes increasingly younger and alkalic eastward. It is suggested that the volcanism is related to an eastwards-propagating rift produced before the most recent stage of sea-floor spreading in the Gulf of Aden (4.5 Ma–present).

Prestahnúkur is a 570m high rhyolite glaciovolcanic edifice in Iceland’s Western Rift Zone with a volume of 0.6 km3. Uniform whole rock, mineral and glass compositions suggest that Prestahnúkur was constructed during the eruption of one magma batch. Ar-Ar dating gives an age of 89± 24 ka, which implies eruption during the transition (Oxygen Isotope substages 5d to 5a) between the Eemian interglacial and the Weichselian glacial period. Prestahnu´kur is unique among published accounts of rhyolite tuyas because a base of magmatically-fragmented tephra appears to be absent. Instead, basal exposures consist of glassy lava lobes and coarse hyaloclastite, above which are single and multiple lava sheets with matrix-supported basal breccias and hyaloclastite upper carapaces. Steepening ramp structures at sheet termini are interpreted as ice-contact features. Interactions between erupting magma and water/ice have affected all lithologies. A preliminary model for the construction of Prestahnúkur involves an effusive subglacial eruption between 2–19 years duration which never became emergent, into an ice sheet over 700m thick. If 700m of ice had built up during this interglacial–glacial transition, this would corroborate models arguing for the swift accumulation of land-based ice in rapid response to global cooling.

Between the Rodrigues Triple Junction and the Marie Celeste fracture zone, basalts from the Central Indian Ridge (CIR) exhibit an enrichment in incompatible elements that increases in intensity northward. In addition, H2O/TiO2, Al[8], and Dy/Yb ratios increase, while Na[8] remains unchanged and Fe[8] decreases. Evolution of the enriched magma appears to be affected by elevated water contents, which lower the mantle solidus, thereby increasing the initial depth of melting, as well as delaying plagioclase crystallization. However, the enrichment affecting the northern samples is not a just function of hydrous mantle melting and crystallization. Instead of trending toward a small melt fraction from the mantle, as predicted by hydrous melting models, the CIR samples lie on a mixing line between N-MORB and a source component that closely resembles present-day Reunion hot spot lavas. Thus it appears that while hydrous melting and crystallization affect the CIR, the enriched and wet mantle originates from the Reunion hot spot, where it migrates eastward toward the CIR, against the direction of motion of the lithosphere.

The variation of heat production with respect to magma evolution is examined in four UK granites in which fractional crystallization accounts for the range of rock compositions present. Accessory minerals are the dominant hosts of U and Th in these granites and their role in controlling variations of these heat producing elements is assessed. Melt compositional parameters determine which accessory minerals precipitate (solubility control) and influence the partitioning of radioelements between early-crystallizing (fractionating) minerals and residual liquid. As a result, parental magma compositions and the effect of critical properties, such as alkalinity, aluminosity and volatile contents (particularly F-complexing) on magma evolution are considered important in controlling the primary distribution of radioelements in granites.

Two examples of uplifted basement have been studied in the Lhasa Terrane of the Tibetan Plateau. The Nyainqentanglha orthogneisses are bounded by staurolite-garnet schists to the north which record prograde metamorphism at 5 .0 ± 1.3 kbar, 610 ± 70 °C. Garnet sillimanite xenoliths within the orthogneiss suggest that peak temperatures reached at least 700 ± 70 °C at 5.1 ± 2 .5 kbar. These P / T fields reflect high T /low P metamorphism during Eocene subduction, and indicate that the syntectonic Nyainqentanglha orthogneiss was emplaced at depths greater than 10 km. Sillimanite-bearing assemblages from the Amdo gneisses in the northern Lhasa Terrane provide evidence of crustal anatexis at temperatures > 680 °C. This event is poorly constrained in time but is probably Cambrian or earlier. Within the Kunlun Terrane, biotite and garnet isograds north of the Xidatan Fault indicate an increase in metamorphic grade from north to south, reaching peak metamorphism at 470 ± 30 °C, 4 .3 ± 1.5 kbar synchronous with the emplacement of the Triassic batholith. Regional metamorphism was followed by uplift of at least 2 km before emplacement of post-tectonic, early Jurassic granites.

The 1983 hawaiite of Mount Etna was sampled and analyzed for groundmass and mineral compositions, rare-earth-element concentrations and Sr-Nd isotope ratios. Microprobe data for coexisting mineral phases and glass show crystallization temperatures of around 1100° C from a rather differentiated hawaiite magma at rather highfO2 (10−8 at 1100° C), well above the QFM buffer. The hawaiites are characterized by a marked enrichment in the light REE similar to other alkaline magmas: the (Ce)N/(Yb)N is greater than 10, a feature these hawaiites have in common with alkaline magmas erupted earlier on Mount Etna. Since the Ce/Yb ratio cannot be affected by fractionation of clinopyroxene and plagioclase, it is taken as an accurate reflection of the LREE-enriched nature of the hawaiites. From this point of view, the Etnean hawaiites are identical to within-plate alkaline magmas erupted on the Hawaiian islands. This similarity extends to the Nd-Sr isotope features. Two hawaiites have87Sr/86Sr=0.70346 and 0.70352 and143Nd/144Nd=0.51286 and 0.51284. These data indicate a source similar to oceanic-island basalts, a source depleted in Rb/Sr and Nd/Sm for some period of time. The Sr isotope data are identical to that previously reported for Mount Etna. Extraction of hawaiites from depleted source regions requires either recent enrichment events, mixing of asthenospheric and lithospheric melts, or variable degrees of melting. At present, the data do not allow a clear decision. The peculiar tectonic setting of Mount Etna, between the relatively undeformed African foreland and the active Aeolian volcanic islands, may suggest contributions to the source region from subduction and within-plate processes. Etnean lavas have a geochemical imprint of subduction-related enrichment processes, and they also share petrological and chemical features identical to oceanic-island basalts whose source region has been affected by within-palte enrichment processes.

This paper presents a model for the partial melting of quartz diorite and greywacke in the upper crust based on the mineralogy and geochemistry of enclaves within the Loch Doon granitic intrusion of southern Scotland. The melting of quartz diorite was modelled using autoliths, which represent fragments of cogenetic igneous rocks that became incorporated in the fractionating magma. Compared to their quartz diorite parents, the autoliths are enriched to varying degrees in some elements (notably Rb, Nb, Ta, Sm, Y, Yb) and depleted in others (Sr, and Ba); Eu and P are also depleted in the more assimitated autoliths. The compositions of melts that could be derived from assimilation of the autoliths have also been calculated: their REE patterns reveal a light REE enrichment, low concentrations of heavy REEs (1–3 x chondrite) and a positive Eu anomaly. The calculated degrees of melting vary from 35% in the least assimilated to 84% in the most assimilated autolith (assuming a bulk distribution coefficient of 10 for the most compatible element). Results from modelling of xenolith compositions (derived from metasediments) are also reported, but because of uncertainties in the composition of the parental sediment, these data are subject to larger errors. They do, however, indicate that resultant partial melts are distinctly different from those derived by partial melting of autoliths. In particular, the REE pattern of a greywacke-derived melt shows a slight enrichment in light REEs, greater concentrations of heavy REEs (10 x chondrite) and a small negative Eu anomaly. The calculated degrees of melting of the xenoliths fall in the range of 66–88% (assuming a bulk distribution coefficient of 10 for the most compatible element). The results have direct implications for assimilation and melting of the upper crust. By taking into account how the nature of residual phases is likely to change with depth, it can be demonstrated that some Archaean tonalite gneisses could represent liquids derived by partial melting of igneous material.

Granites may be subdivided according to their intrusive settings into four main groups—ocean ridge granites (ORG), volcanic arc granites (VAG), within plate granites (WPG) and collision granites (COLG)—and the granites within each group may be further subdivided according to their precise settings and petrological characteristics. Using a data bank containing over 600 high quality trace element analyses of granites from known settings, it can be demonstrated using ORG-normalized geochemical patterns and element-SiO2 plots that most of these granite groups exhibit distinctive trace element characteristics. Discrimination of ORG, VAG, WPG and syn-COLG is most effective in Rb-Y-Nb and Rb-Yb-Ta space, particularly on projections of Y-Nb, Yb-Ta, Rb-(Y + Nb) and Rb—(Yb + Ta). Discrimination boundaries, though drawn empirically, can be shown by geochemical modelling to have a theoretical basis in the different petrogenetic histories of the various granite groups. Post-collision granites present the main problem of tectonic classification, since their characteristics depend on the thickness and composition of the lithosphere involved in the collision event and on the precise timing and location of magmatism. Provided they are coupled with a consideration of geological constraints, however, studies of trace element compositions in granites can clearly help in theelucidation of post-Archaean tectonic settings.

Electron microprobe data are presented from zoned niobian wolframite crystals in a quartz-wolframite-(cassiterite) vein and wall-rock from a zinnwaldite-bearing granite pluton that forms part of the late-Caledonian Glen Gairn granitic complex in the Eastern Highlands of Scotland. Compositions range from Nb- and Ta-rich wolframite, (Fe,Mn)(W,Nb,Ta)O 4 , forming strongly zoned, difficult-to-polish regions of crystals, to almost pure hubnerite MnWO 4 , forming relatively unzoned, easily polished regions. Nb and Ta substitution is very restricted in Mn-rich (>80 mol% hubnerite) wolframites but increases regularly in more Fe-rich compositions through a coupled substitution mechanism between Mn 2+ W 6+ and Fe 3+ (Nb,Ta) 5+ . Most of the wolframite crystals contain two forms of zonation: a physical zonation, observed optically and by electron imaging, and a compositional zonation revealed by X-ray traverses. Compositional zoning is progressive, involving gradual changes in mineral chemistry from core to rim, on which is superimposed a form of oscillatory zoning involving repeated resetting of compositions. Physical zones are layers of uniform width separated by distinct boundaries; they are arranged parallel to crystal faces and reflect intermittent crystal growth. These zones vary in width from less than 5 m in the niobium-rich wolframite, to over 200 m in regions of niobium-poor hubneritic wolframite. Hydrothermal alteration, particularly of the intensely zoned niobium-rich regions, has resulted in assemblages of altered Nb- and Ta-rich wolframite (tungstite?), secondary hubneritic wolframite and scheelite. Nb-rich inclusions 5-10 m across are found sporadically in the larger wolframite crystals. Progressive variation in wolframite chemistry is readily explained in terms of gradually changing physiochemical conditions, such as pH, f O 2 , temperature, pressure and fluid composition, during crystallization. The oscillatory zonation reflects cyclicity of those conditions. Plausible models which could explain such effects include: 1. (a) intermittent mixing of chemically distinct magmatic and meteoric fluids 2. (b) repeated hydraulic fracturing and injection of fluids leading to fluctuating pressure and temperature of the hydrothermal fluid. Volatile species (F, Cl, CO 2 ) may have been effective as complexing agents thus affecting the supply of Nb from the fluid. Deposition of Nb- and Ta-rich compositions were probably favoured by high f O 2 , low pH conditions.

The late Caledonian Loch Doon granitic intrusion ranges in composition from hypersthene diorite at the margin, through quartz diorite, granodiorite and granite to cordierite microgranite at its core. Petrogenetic modelling of trace element variations and least squares analysis of major elements indicate that two distinct magmas are involved, each magma controlled by crystallization of plag-opx-cpx-bio. Late stage rocks related to the second magma include the cordierite microgranites and aplites, which are interpreted as the final residue which crystallized rapidly after a build up and loss of volatiles. Analyses of whole rocks and minerals for REE's and other elements of moderate-high ionic potential indicate that these elements are strongly controlled by minor phase crystallization; apatite, zircon, sphene and allanite are dominant at intermediate compositions but other accessory minerals such as monazite and xenotime may also become important at acid compositions. It is probable that within each magma the mechanisms of crystal settling and filter pressing operated, the former being initially dominant, and the latter becoming more important with increasing degree of fractional crystallization.

The Loch Doon igneous complex lies to the N of the Solway–Shannon Iapetus Ocean suture line through the British Isles, but in a region lacking the Proterozoic continental basement which is known to exist further N. It comprises a genetically related magma suite ranging from hypersthene diorite margins through quartz monzonite to granite at the centre. Petrochemical data, studies of xenolith and autolith densities and field radiometric mapping indicate that the suite lacks sharp internal boundaries but is gradational and based on a monzodiorite parental magma which evolved through a two-stage crystal fractionation process. The early crystallization of dioritic margins was dominated by pyroxene-plagioclase precipitation and these minerals were joined later by biotite as acidic residual liquids developed. Biotite formation and fractionation at the expense of pyroxenes is related to contamination of the magma by water from the roof zone. Partial assimilation of roof material, particularly in the form of xenoliths, and mixing between cumulates and residual liquids are also recognized but are of relatively minor importance. These petrochemical, geophysical and isotopic studies of the Southern Uplands granites indicate the generation of parental magmas by the partial fusion of metabasaltic lower crust or crustal underplate as a late event in the evolution of the Iapetus Ocean.