Basalts from the volcano-sedimentary Eriksfjord Formation (Gardar Province, South Greenland) were erupted at around 1.2 Ga into rift-related graben structures. The basalts have compositions transitional between tholeiite and alkaline basalt with MgO contents <7 wt.% and they display LREE-enrichment relative to a chondritic source. Most of the trace element and REE characteristics are similar to those of basalts derived from OIB-like mantle sources. Initial 87Sr/86Sr ratios of clinopyroxene separates range from 0.70278 to 0.70383 and initial ϵNd values vary from –3.2 to +2.1. The most unradiogenic samples overlap with the field defined by carbonatites of similar age and can be explained by mixing of isotopically depleted and enriched mantle components. Using AFC modelling equations, the Sr-Nd isotope data of the more radiogenic basalts can successfully be modelled by addition of <5% lower crustal granulite-facies gneisses as contaminants. δ18Ov-smow values of separated clinopyroxene range from +5.2 to +6.0% and fall within the range of typical mantle-derived rocks. However, up to 10% mixing with an average lower crustal component are permitted by the data.

Carbonatite magmas are considered to be ultimately derived from mantle sources, which may include lithospheric and asthenospheric reservoirs. Isotopic studies of carbonatite magmatism around the globe have typically suggested that more than one source needs to be invoked for generation of the parental melts to carbonatites, often involving the interaction of asthenosphere and lithosphere.

In the rift-related, Proterozoic Gardar Igneous Province of SW Greenland, carbonatite occurs as dykes within the Igaliko Nepheline Syenite Complex, as eruptive rocks and diatremes at Qassiarsuk, as a late plug associated with nepheline syenite at Grønnedal-Íka, and as small bodies associated with ultramafic lamprophyre dykes. The well-known cryolite deposit at Ivittuut was also rich in magmatic carbonate. The carbonatites are derived from the mantle with relatively little crustal contamination, and therefore should provide important information about the mantle sources of Gardar magmas. In particular, they are found intruded both into Archaean and Proterozoic crust, and hence provide a test for the involvement of lithospheric mantle.

A synthesis of new and previously published major and trace element, Sr, Nd, C and O isotope data for carbonatites and associated lamprophyres from the Gardar Province is presented. The majority of Gardar carbonatites and lamprophyres have consistent geochemical and isotopic signatures that are similar to those typically found in ocean island basalts. The geochemical characteristics of the two suites of magmas are similar enough to suggest that they were derived from the same mantle source. C and O isotope data are also consistent with a mantle derivation for the carbonatite magmas, and support the theory of a cogenetic origin for the carbonatites and the lamprophyres. The differences between the carbonatites and lamprophyres are considered to represent differing degrees of partial melting of a similar source.We suggest that the ultimate source of these magmas is the asthenospheric mantle, since there is no geochemical or isotopic evidence for their having been derived directly from ancient, enriched sub-continental lithospheric mantle. However, it is likely that the magmas actually formed through a two-stage process, with small-degree volatile-rich partial melts rising from the asthenospheric mantle and being ‘frozen in’ as metasomites, which were then rapidly remobilized during Gardar rifting.

The North Qôroq centre comprises a series of concentric nepheline syenite intrusions and forms part of the Igaliko Nepheline Syenite Complex, in the rift-related Gardar Province of South Greenland. The North Qôroq syenites range from mildly undersaturated augite syenite to strongly peralkaline agpaitic nepheline syenite. Extensive in situ fractional crystallization has been postulated for the chemical variation both within units and throughout the centre. Many of the rocks have been affected by metasomatic fluids associated with the emplacement of younger syenite units, and this complicates their interpretation. In this study, the trends and compositions exhibited by pyroxene and amphibole from North Qôroq are examined and related to either primary crystallization or metasomatic activity (e.g. controls of fO2, peralkalinity). Implications thus drawn are used to interpret the chemical processes inherent in the chemical and fluid evolution of alkaline magmas, and, in particular, the transition from miaskitic to agpaitic magmatism. In general, the major phases of the North Qôroq syenites records the increasing evolution of the units by crystal fractionation, towards peralkaline compositions. The composition of olivine, in the least evolved syenites, also points to a relatively high state of fractionation of the parent magma, whilst pyroxene and amphibole record an overall decrease in Mg/Mg+Fe), and a general increase in Fe3+ and alkali content, with increased fractionation. The increasing peralkalinity of the magma also governs the evolution of pyroxene and, to a lesser degree, amphibole towards higher Zr and Ti contents in the more Na-rich compositions. The trends for pyroxene from metasomatized syenite show similar patterns, but lower Fe2+ enrichment, suggesting the source of the metasomatic fluids is similar to the evolving syenites. The presence of amphiboles in metasomatic rocks, and high F contents attest to the F-rich nature of the metasomatic fluids, which is in agreement with results previously reported for metasomatic fluorapatite.

Based on petrography, mineral chemistry, and petrology, the physico-chemical evolution of an agpaitic dyke was found to be very similar to that of the neighbouring Ilímaussaq complex. Various mineral assemblages were used to reconstruct the crystallization conditions of the dyke rock for different stages during cooling. The early magmatic phenocryst assemblage is alkali feldspar + nepheline + augite + olivine + magnetite and indicates liquidus temperatures of ∼850ºC, silica activities of ∼0.5, and oxygen fugacities of FMQ –1.5 to –3. The groundmass assemblage albite + microcline + nepheline + sodalite + arfvedsonite + aegirine + aenigmatite + astrophyllite indicates lower temperatures of between 600 and 450ºC, at silica activities of 0.25, and oxygen fugacities around the FMQ buffer. Amphibole composition strongly responds to fluorite saturation and proves crystallization occurred in a system closed to oxygen. Late-stage hydrothermal conditions are indicated by the conversion of nepheline and sodalite to analcime and the growth of aegirine on arfvedsonite. These late-stage reactions are constrained to temperatures of <300°C, water activities of between 0.5 and unity, and oxygen fugacities above MH. The dyke has to be regarded as a small equivalent of the larger Ilímaussaq complex, in which identical differentiation processes proceeded at a scale very different in terms of magma volume and cooling times.

A carbonatite sample from Phalaborwa, South Africa, consists of apatite, magnetite and a calcitedolomite ‘perthite’ which is interpreted as being due to exsolution of dolomite from a high-Mg calcite precursor. Carbon and oxygen isotope data indicate that the carbonates are equilibrated. In situ ionmicroprobe analyses for Fe, Mn, Na, Si, Y, the REEs, Pb, Th and U give the following average concentrations (in ppm) in the sequence apatite, calcite, dolomite: Fe 98, 1680, 8190; Mn 61, 510, 615; Na 1171, 627, 125; Si 368; 1.6, 0.2; Sr 4447, 5418, 2393; Ba 37, 2189, 75; La 1245, 300, 67; Y 121, 50, 5.8; Pb 16, 5.4, 1.4; Th 20, 0.02, 0; U 2.4, 0, 0.01. The concentrations are reasonably uniform in both apatite and dolomite, but in calcite are more variable. Na, Si, Y, the REEs, Pb, Th and U partition into apatite relative to both carbonates (and, hence, the precursor carbonate); KD ap/cc for REE decreases from ∽4 for La to ∽2 for Tm. There is almost equal partitioning of Sr between apatite and calcite. During separation of dolomite from calcite, Sr and Ba partition strongly into calcite and all the other analysed elements, except Fe and Mn, also preferentially enter calcite. The REEs prefer calcite relative to dolomite, and the KD dol/cc is reasonably constant, only varying from 0.23 to 0.17. Sr, Ba and Pb in the carbonates, and their partitioning between the calcite and dolomite, differ from other carbonatite carbonates reported in the literature.

The ∽16 Ma Rangwa Caldera Complex, part of the large Kisingiri nephelinite-carbonatite volcano, Homa Bay District, western Kenya (0º34’S; 34º09’E) contains carbonatitic lapilli and ash tuffs, agglomerate and tuffisite, and a number of intrusive calcite carbonatites. A detailed petrographic and electron microprobe study has been performed on 20 fresh samples from the collection at The Natural History Museum, London.

Most of the juvenile lapilli and ash particles are either predominantly composed of devitrified silicate glass (now biotite/phlogopite but probably also originally potassic silicate) or calcite carbonatite, which suggests that two molten liquids were erupted simultaneously. Some 10 mm-diameter lapilli contain quench-textured calcite crystals set in devitrified glass. They are interpreted as having crystallized from a molten silicate-carbonate melt at, or very near, the surface.

The extrusive carbonate is mostly composed of calcite, consistent with intrusive calcite compositions at Rangwa. Other key minerals are magnetite, two types of mica (magnesian-biotite phenocrysts and phlogopite xenocrysts) and fluorapatite.

The pyroclastic rocks contain many calcite carbonatite clasts, and fragments of calcite, aegirine and diopside, fluorapatite, magnetite, plus some phlogopite, titanite, K-feldspar, fenite and glimmerite; ijolite lithics are rare. Thus, there is no evidence for a cognate nephelinitic (ijolitic) or melilitic magma nor evidence for a direct relationship with the nephelinites of the Kisingiri volcano.

Two hypotheses are discussed. A rising silicate and K-rich carbonatite liquid may have evolved towards a carbonate-rich K-silicate liquid after crystallization of calcite, phlogopite, apatite and magnetite. Preservation of the the potassic component may be rare, with a more usual scenario being that potassic component separates as fenitizing fluids. The alternative is that the silicate component is remobilized fenite, formed from country rock that was mobilized by supercritical K-rich, fenitizing fluids associated with the carbonatite. Both scenarios require generation of a K-rich carbonatite magma, probably from a carbonated phlogopite-rich metasomatized mantle.

Some samples of hollandite-type titanates from the Murun alkaline complex (Yakutia, Russia) contain appreciable amounts of Pb (up to 12.5 wt.% PbO). These titanates occur in a pegmatitic K-feldsparaegirine rock containing subordinate K-rich batisite, titanite, wadeite and other minerals. The Pb-bearing crystals coexist with hollandite-type phases devoid of detectable Pb and zoned from a Kdominant (priderite) core to a Ba-dominant (henrymeyerite) rim. Recalculation of the microprobe analyses on the stoichiometric basis indicates that most of the Fe occurs in this mineral in trivalent form, suggesting the existence of a solid solution between the Ba(Ti6Fe)O16, K2(Ti6Fe)O16 and Pb(Ti6Fe)O16 end-members. The maximum proportion of the latter end-member in the Murun titanates is ∽45 mol.%. The Ba-free compositions [Pb1.0–1.3(Ti,Fe)8O16] and intermediate members of the (Ba1–xPbx)(Ti6Fe)O16 series were synthesized at 1050 –1100ºC. The synthesis products comprise tetragonal hollandites of various stoichiometry intermixed with rutile, a pseudobrookite-type phase and (for the Ba-free compositions) minor macedonite. Electron microprobe analyses of the hollandites indicate that there is a continuous series of compositions between the two hexatitanate end-members, Ba(Ti6Fe)O16 and Pb(Ti6Fe)O16. The crystal structure of one intermediate member was refined by the Rietveld method in space group I4/m, and found to differ from the hollandite archetype (i.e. Pb-bearing Ba manganate) in that Pb is preferentially partitioned into the 2b tunnel site at (0,0,½), whereas Ba is partitioned into the larger 4e site at (0,0,∽0.8).

Conventional experiments designed to investigate the mechanical properties of polycrystalline geological materials are generally restricted to measurements of whole-rock properties. However, when comparing the measurements with theoretical models, it is frequently essential to understand how the deformation is accommodated at the grain-scale. This is particularly true for polymineralic rocks because in this case most theories express the whole-rock properties as some function of the properties of their constituent minerals, and hence the contribution which each phase makes to those properties must be measured if the theories are to be fully assessed. The penetrating nature of neutrons offers a method of addressing this problem. By performing deformation experiments in the neutron beam-line and collecting neutron diffraction patterns at different applied loads, the lattice parameters of all the mineral phases present may be determined as a function of load. The elastic strain experienced by each phase is then easily determined. Moreover, the strain in different lattice directions is also obtained. From this information a wide range of problems relevant for the characterization of the elastic and plastic deformation behaviour of polymineralic geological materials can be explored. An experimental technique for carrying out such experiments is described, and its validity is demonstrated by showing that the results obtained from deforming an elastically isotropic olivine + magnesiowüstite sample agree, to within very tight bounds, with the behaviour predicted by theory for elastically isotropic composites.

Phase transitions in the BaAl2O4–SrAl2O4 solid solution have been analysed as a function of temperature and composition using infrared (IR) powder absorption spectroscopy. The improper ferroelectric phase transition P6322 → P63 (2A superstructure) in the BaAl2O4 end-member can be detected through a change in slope of the wavenumbers of hard modes at ∽450 K. A change in line widths at ∽520 K appears to correlate with the development of diffuse intensity in a*-b* planes of electron diffraction patterns reported elsewhere in the literature. The same shift in wavenumber of hard modes is not observed in spectra from samples with compositions corresponding to 60, 80 and 90% of BaAl2O4 component, but a change in line widths at ∽500 K has been tentatively explained in terms of a different phase transition, from a P6322 parent structure to a P63 (√3A superstructure) product. Strain analysis of published high-temperature lattice parameter data suggests that the hexagonal → monoclinic transition in Sr-rich members of the solid solution may consist of two discrete transitions, and a sequence P6322 → C2 → P21 is suggested. The second transition could be related to instabilities in the hexagonal solid solution. Autocorrelation analysis of the IR spectra reveals a large positive deviation from linear behaviour across the solid solution, which is interpreted in terms of microscopic strain effects. These microscopic strains are probably responsible for the different transformation behaviour shown by samples with different compositions across the solid solution.

We report in this paper the first occurrence of the rare assemblage corundum + quartz in the high-grade metamorphic zone in the core of the Namaqualand Metamorphic Complex (NMC) (South Africa). The magnetite-bearing rocks hosting the corundum-quartz assemblage are very rare, and occur discontinuous bands a few cm wide, associated with diorite, tonalite and anorthosite within quartzfeldspar ± sillimanite ± biotite ± spinel supracrustal rocks extensively intruded by a foliated granite gneiss. The magnetite-bearing rocks are characterized by the presence of abundant crystals of magnetite together with ilmenite, spinel and large (∽1 cm in diameter) euhedral crystals of corundum both displaying a sharp contact with magnetite and quartz. However, in some micro-domains, spinel and quartz, as well as corundum and quartz, are separated by a thin corona of sillimanite with or without magnetite. These textural features suggest that the following reactions have taken place: quartz + corundum → sillimanite, and quartz + spinel + O2 → sillimanite + magnetite. These reactions are interpreted as occurring at higher, as yet unknown, P-T conditions than previously estimated for the NMC. Whether corundum and quartz were once in equilibrium is worth investigating, as it could constitute a geothermobarometric assemblage for ultra-high T granulites.

Fahlores [~(Cu,Ag)10(Zn,Fe)2Sb4S13] from the Keno Hill mining district, central Yukon, Canada record virtually the entire petrogenetic history of a Cretaceous hydrothermal system extending over 40 km outward from the Mayo Lake granitic pluton. These fahlores are an essential constituent of polymetallic sulphide veins developed in a graphitic Mississippian quartzite, where they occur in association with sphalerite, pyrargyrite, galena and siderite. Fahlores exhibit pronounced east-west zoning in average Ag/(Ag+Cu) and Zn/(Zn+Fe) values, with these simultaneously increasing and decreasing from east to west over 20 km of hydrothermal activity. These zonations are coupled with average Ag/(Ag+Cu) and Zn/(Zn+Fe) values in fahlore roughly paralleling the 300°C isotherm for fahlores in equilibrium with pyrargyrite, miargyrite and sphalerite in the simple system Ag2S-Cu2S-ZnS-FeS-Sb2S3. Early high-Ag, high-Zn fahlores from the eastern and western mines have Ag/(Ag+Cu) and Zn/(Zn+Fe) values requiring temperatures ≥400°C, in agreement with temperatures established from the As-content of arsenopyrite coexisting with pyrite, pyrrhotite and sphalerite. Ag/(Ag+Cu) and Zn/(Zn + Fe) values in later, main-stage fahlores are consistent with the 250–310°C range of temperatures established for boiling of Keno Hill fluids. Finally, Ag- and Fe-rich fahlores were produced by retrograde Fe-Zn exchange with sphalerite or crystallized from late-stage epithermal fluids which produced polybasite, stephanite, acanthite and wire silver. One such fahlore exhibits unmixing into high-Ag and low-Ag varieties. This is the first reported miscibility gap for freibergite fahlores and confirms the earlier prediction of such gaps.

Borehole samples of carbonatites and phlogopite-pyroxenites from the Angico dos Dias (AdD) intrusive alkaline complex, State of Bahia, Brazil, have been investigated in terms of mineralogy, geochemistry and C-O-Sr-Nd isotopes. The AdD complex, of Early Proterozoic age (2 Ga), intrudes the northern side of the São Francisco Craton. Mineralogy and petrography indicate that the studied rocks only partially preserved their magmatic textural features owing to their metamorphic re-equilibration (greenschist facies). The REE contents and LREE/HREE ratios of the AdD carbonatites are very high (mean 3979±718 ppm and La/Yb = 215±23, respectively), as for most Precambrian magmatic carbonatites. The AdD carbonatites are also enriched in 18O (δ18O = 11.9 to 15.8‰), possibly due to secondary processes (e.g. metamorphism, alteration) whereas carbon isotopes are in the range of ‘primary carbonatites’ (δ13C = –5.7 to –7.1‰). Most of the initial 87Sr/86Sr and 143Nd/144Nd values of the studied carbonatites were not appreciably modified by secondary processes. Their εtSr and εtNd values (20.0 to 25 and 0.7 to –4.5, respectively) indicate enriched mantle sources very different from the ‘depleted’ ones related to many Precambrian carbonatites from North America (0.6 –2.6 Ga) and Africa (0.5 –2.0 Ga). The Early Proterozoic Sr-Nd isotopic signatures of the AdD carbonatites are similar to those of the Early Cretaceous carbonatites from the Paraná basin. The latter carbonatites show a great isotopic variability ranging from Bulk Earth to the related potassic magmatism from Asunción-Sapucai graben in the Eastern Paraguay (K-ASU magmatism: εtSr = 35 to 50 and εtNd = –12 to –20). The very similar isotopic compositions of Precambrian and post-Palaeozoic carbonatites worldwide indicate that the subcontinental mantle variability lasted for long periods of time and indicate a large-scale mantle heterogeneity.

Au-Cu mineralization at Palai-Islica occurs as disseminations in massive silicified volcanic rocks and, more abundantly, in sulphide-bearing quartz veins. The major ore minerals in the deposit are pyrite ± chalcopyrite, sphalerite and galena and there is a great variety of accessory minerals, including Au-Ag alloys and native gold. Pyrite, the most abundant sulphide, is closely associated with gold. Seven different types of pyrite have been distinguished with a variable concentration of different trace elements. Among these, the only one free of trace elements (type IV) is related to Au-Ag alloys. Pyrite associated with these Au-Ag alloys has cubic and pentagonal dodecahedral habits, whereas pyrite with pentagonal dodecahedral habit only is from barren zones. In addition, there is no significant invisible gold in the pyrite, but there is a relatively large amount of Ag in collomorphic pyrite (up to 0.20 wt.%) or type III pyrite (up to 1.47 wt.%). Arsenic is the most abundant trace element in pyrite (up to 6.11 wt.%), present as a metastable solid solution or as a non-stoichiometric element. A variety of marcasite related to the gold levels also has a considerable amount of trace elements (As up to 1.15 wt.%, Sb up to 0.40 wt.%).

Fluorine-rich prismatine, (□,Fe,Mg)(Mg,Al,Fe)5Al4(Si,B,Al)5O21(OH,F), with F/(OH+F) = 0.36–0.40 and hercynite are major constituents of a Fe-Al-B-rich lens in ultrahigh-temperature granulite-facies quartz-sillimanite-hypersthene-cordierite gneisses of the Eastern Ghats belt, Andhra Pradesh, India. Hemo-ilmenite, sapphirine, magnetite, biotite and sillimanite are subordinate. Lithium, Be and B are concentrated in prismatine (140 ppm Li, 170 ppm Be, and 2.8 –3.0 wt.% B2O3). Another Fe-rich lens is dominantly magnetite, which encloses fine-grained zincian ferrohögbomite-2N2S, (Fe2+,Mg,Zn,Al)6 (Al,Fe3+,Ti)16O30(OH)2, containing minor Ga2O3 (0.30 –0.92 wt.%). Fe-Al-B-rich lenses with prismatine (or kornerupine) constitute a distinctive type of B-enrichment in granulite-facies rocks and have been reported from four other localities worldwide. A scenario involving a tourmalineenriched lateritic precursor affected by dehydration melting is our preferred explanation for the origin of the Fe-Al-B-rich lenses at the five localities. Whole-rock analyses and field relationships at another of these localities, Bok se Puts, Namaqualand, South Africa, are consistent with this scenario. Under granulite-facies conditions, tourmaline would have broken down to give kornerupine-prismatine (±other borosilicates) plus a sodic melt containing H2O and B. Removal of this melt depleted the rock in Na and B, but the formation of ferromagnesian borosilicate phases in the restite prevented total loss of B.

Arhbarite was described in 1982 as a new copper arsenate, Cu2(OH)AsO4·6H2O, from the Arhbar mine, Morocco. Due to the very small grain size no crystallographic data other than the X-ray powder diffraction data could be given. Reinvestigation of the type material and of new finds from the El Guanaco mine, Chile, is based on Rietveld refinement of X-ray powder data, electron microprobe investigations, optical studies as well as infrared and Raman spectra. The main results are: (1) arhbarite has the ideal formula Cu2Mg(AsO4)(OH)3 with magnesium oxide as an additional constituent and (2) arhbarite is isotypic with gilmarite, Cu3(AsO4)(OH)3. Arhbarite is triclinic, space group P1 (no. 1), unit-cell parameters (type material; refined from powder data) a= 5.315(4), b= 5.978(6), c= 5.030(6)Å, α = 113.58(6), β = 97.14(7), γ = 89.30(8)°, V= 145.2(1)Å3, Z= 1. Microprobe analyses of the type material gave MgO 10.20, CuO 47.46, NiO 0.20, CoO 0.24, As2O5 33.85, SiO2 0.10, H2O 7.58 (calc.), total 99.63 wt.%. The empirical formula is Cu1.98(Mg0.88Cu0.09Ni0.01Co0.01)S0.99(AsO4)1.02(OH)2.92, based on 7 O atoms. The Chilean arhbarite shows a slightly higher Cu content than the type material; it has the formula Cu1.99(Mg0.73Cu0.25Zn0.01)∑0.99[(AsO4)1.01(SiO4)0.01]∑1.02(OH)2.92. The formal redefinition of arhbarite has been approved by the CNMMN of the IMA.

The crystal structure of hügelite, Pb2[(UO2)3O2(AsO4)2](H2O)5, monoclinic, space group P21/m, a= 31.066(3)Å, b= 17.303(2)Å, c= 7.043(1)Å, β = 96.492(2)°, V= 3761.6(1)Å3, Z= 8, Dcalc = 5.74 g/cm3, was solved by direct methods using data from a crystal twinned by pseudo-merohedry, and was refined by full-matrix least-squares techniques on the basis of F2 to agreement indices R1 of 3.3% calculated for 5519 unique observed reflections (|Fo| 5 ≥ 4σF), and wR2 of 6.7% for all data. Intensity data were collected at room temperature using Mo-Kα radiation and a CCD-based area detector. Hügelite is a member of the phosphuranylite group and is the arsenate counterpart of dumontite. The sheets of uranyl pentagonal and hexagonal bipyramids and arsenate tetrahedra in hügelite are oriented parallel to (100), and the interlayer contains four symmetrically independent Pb atoms, each of which is coordinated by two oxygen atoms from uranyl ions, two oxygen atoms from arsenate tetrahedra, and three symmetrically distinct H2O groups. The unit-cell volume is four times larger than that previously reported for hügelite, or expected by comparison to dumontite, Pb2[(UO2)3O2(PO4)2](H2O)5; the larger cell probably results from the accommodation of the larger As atoms (relative to P) in the structure, and consequent subtle variations in the coordination geometries of the U and Pb positions.