Metasomatic Veins Research Articles

TiO 2 enrichment in ocean island basalts

An extensive dataset of ocean island basalt (OIB) compositions highlight that their titanium (Ti) contents are generally too high to be derived from melting of a peridotitic mantle. Even vanishingly small degree melts of a primitive mantle source have insufficient Ti to reproduce the compositions of many OIB. A Ti-enriched material is thus a necessary additive to many OIB sources. Of the enriched components commonly invoked to explain the range in OIB radiogenic isotopic signatures, only the addition of small amounts (∼ 1–10%) of recycled mafic crust is compatible with all geochemical constraints. Since such mafic crust starts melting deeper than its host peridotite, small degree melts formed beneath thick lithosphere will preferentially sample this enriched heterogeneity. This is compatible with the observation that the most Ti rich OIB also have the most enriched and variable signatures in terms of radiogenic isotope ratios. However, recycled mafic crust itself is insufficiently extreme isotopically to be able to account for the full range of isotopic ratios in OIB, therefore further, associated components (e.g. sediments, metasomatic veins) seem necessary in some cases.

Fluorine-beryllium deposits of the Vitim Highland, western Transbaikal region: Mineral types, localization conditions, magmatism, and age

The fluorine-beryllium deposits of the Vitim Highland are represented by two mineral types: feld-spar-fluorite-phenakite-bertrandite and thorite-fluorite-phenakite. Their localization is controlled by fault zones of various orders, folds, and stocks and dikes of the Early Mesozoic subalkali quartz syenite and syenite porphyry. The ore is represented by mineralized crush zones and metasomatic veins hosted in intercalated carbonate and aluminosilicate rocks. The deposits were formed under hydrothermal conditions at 360–90°C; their age is estimated at (243 ± 3)-(260 ± 2) Ma.

The metasomatic alternative for ocean island basalt chemical heterogeneity

Subduction of oceanic lithosphere is thought to be responsible for producing heterogeneity in Earth's mantle; the heterogeneity is most clearly preserved in the compositions of ocean island basalts (OIB). The variation of trace element and isotopic ratios in OIB is commonly explained by recycling of ancient oceanic crust associated with terrigenous or pelagic sediment. However a variety of chemical and physical arguments seem to indicate that subducted oceanic crust is not the source of OIB. In particular, experimental petrologic studies indicate that the most plausible source for OIB is the partial melting of peridotite in the presence of CO 2 or silica-deficient pyroxenites. Alternative hypotheses for the source of OIB are subducted oceanic basal lithosphere enriched by metasomatic liquids and delaminated metasomatised continental lithosphere. Lithospheric metasomatism is common to both hypotheses and could produce silica-deficient pyroxenite; however, the exact chemical and physical nature of the process and how it leads to chemical variations in recycled lherzolite that produce the various OIB isotopic end-member “signatures” is unclear. Chemical variations observed in the Cantal basalt (France) are interpreted as the result of a lithospheric metasomatic mechanism and provide important new constraints on the nature of this metasomatic process. The Cantal basalts, similar to OIB in composition, show unusual variations in Nb / Th, Nb / U, La / Nb and Ce / Pb ratios from the first (13–9 Ma) to the last (9–3 Ma) emitted basalt. The basalts are homogeneous with respect to their Sr, Nd, and Pb isotopic composition, ruling out variable sediment contamination of their mantle sources. We postulate that these trace element variations result from an evolution of metasomatic vein compositions present in a vein plus enclosing lithospheric mantle source. Cantal basalt compositions may be used to constrain the trace element variation generated by the evolution of a metasomatic agent within the lithosphere. Reconstruction of compositions in different parts of the lithosphere indicates significant heterogeneities in Th / Pb, U / Pb and Sm / Nd resulting from the metasomatic process. Subduction and isolation of this metasomatic lithosphere generates over 1–1.8 Ga, Nd and Pb isotopic variations similar to those observed in oceanic island basalts. Moreover, these isotopic variations are correlated with variations in trace element ratios commonly interpreted as the product of mixing between different end-member mantle components (HIMU and EM). The model calculations suggest that isotopic and trace element variations observed in OIB may more likely be the result of melting metasomatised subducted oceanic lithosphere rather than a mixture of chemically distinct mantle sources that formed and evolved independently from one another.

Zn and Cu isotopic variability in the Alexandrinka volcanic-hosted massive sulphide (VHMS) ore deposit, Urals, Russia

Copper and Zn isotope ratios of well-characterized samples from three ore facies in the Devonian Alexandrinka volcanic-hosted massive sulphide (VHMS) deposit, southern Urals, were measured using multi collector ICP-MS (MC-ICP-MS) and show variations linked to depositional environment and mineralogy. The samples analysed derived from: a) hydrothermal–metasomatic vein stockwork, b) a hydrothermal vent chimney, and c) reworked clastic sulphides. As the deposit has not been significantly deformed or metamorphosed after its formation, it represents a pristine example of ancient seafloor mineralization. Variations in δ 65Cu (where δ 65Cu = [( 65Cu / 63Cu) sample / ( 65Cu / 63Cu) standard − 1] * 1000) and δ 66Zn (where δ 66Zn = [( 66Zn / 64Zn) sample / ( 66Zn / 64Zn) standard − 1] * 1000) of 0.63 and 0.66‰, respectively, are significantly greater than analytical uncertainty for both isotope ratios (± 0.07‰, 2 σ). Very limited isotopic fractionation is observed in primary Cu minerals from the stockwork and chimney, whereas the Zn isotopic composition of the stockwork varies significantly with the mineralogy. Chalcopyrite-bearing samples from the stockwork have lighter δ 66Zn by ∼ 0.4‰ relative to sphalerite dominated samples, which may be due to equilibrium partitioning of isotopically light Zn into chalcopyrite during its precipitation. δ 66Zn also showed significant variation in the chimney, with an enrichment in heavy isotopes toward the chimney rim of ∼ 0.26‰, which may be caused by changing temperature (hence fractionation factor), or Raleigh distillation. Post-depositional seafloor oxidative dissolution and re-precipitation in the clastic sediments, possibly coupled with leaching, led to systematic negative shifts in Cu and Zn isotope compositions relative to the primary sulphides. Copper shows the most pronounced fractionation, consistent with the reduction of Cu(II) to Cu(I) during supergene mineralization. However, the restricted range in δ 65Cu is unlike modern sulphides at mid oceanic ridges where a large range of Cu isotope, of up to 3‰ has been observed [ Rouxel, O., Fouquet, Y., Ludden, J.N., 2004. Copper isotope systematics of the Lucky Strike, Rainbow, and Logatchev sea-floor hydrothermal fields on the Mid-Atlantic Ridge. Econ. Geol. 99, 585–600; Zhu, X.K., O'Nions, R.K., Guo, Y., Belshaw, N.S., Rickard, D., 2000. Determination of natural Cu-isotope variation by plasma source mass spectrometry: implications for use as geochemical tracers. Chem. Geol. 163, 139–149].

Tertiary Ultrapotassic Volcanism in Serbia: Constraints on Petrogenesis and Mantle Source Characteristics

The Serbian province of Tertiary ultrapotassic volcanism is related to a post-collisional tectonic regime that followed the closure of the Tethyan Vardar Ocean by Late Cretaceous subduction beneath the southern European continental margin. Rocks of this province form two ultrapotassic groups; one with affinities to lamproites, which is concentrated mostly in the central parts of the Vardar ophiolitic suture zone, and the other with affinities to kamafugites, which crops out in volcanoes restricted to the western part of Serbia. The lamproitic group is characterized by a wide range of Sr/Sri (0 70735–0 71299) and Nd/Ndi (0 51251–0 51216), whereas the kamafugitic group is isotopically more homogeneous with a limited range of Sr/Sri (0 70599–0 70674) and Nd/Ndi (0 51263–0 51256). The Pb isotope compositions of both groups are very similar (Pb/Pb 18 58–18 83, Pb/Pb 15 62–15 70 and Pb/Pb 38 74–38 99), falling within the pelagic sediment field and resembling Mesozoic flysch sediments from the Vardar suture zone. The Sr and Nd isotopic signatures of the primitive lamproitic rocks correlate with rare earth element fractionation and enrichment of most high field strength elements (HFSE), and can be explained by melting of a heterogeneous mantle source consisting of metasomatic veins with phlogopite, clinopyroxene and F-apatite that are out of isotopic equilibrium with the peridotite wall-rock. Decompression melting, with varying contributions from depleted peridotite and ultramafic veins to the final melt, accounts for consistent HFSE enrichment and isotopic variations in the lamproitic group. Conversely, the most primitive kamafugitic rocks show relatively uniform Sr and Nd isotopic compositions and trace element patterns, and small but regular variations of HFSE, indicating variable degrees of partial melting of a relatively homogeneously metasomatized mantle source. Geochemical modelling supports a role for phlogopite, apatite and Ti-oxide in the source of the kamafugitic rocks. The presence of two contrasting ultrapotassic suites in a restricted geographical area is attributable to the complex geodynamic situation involving recent collision of a number of microcontinents with contrasting histories and metasomatic imprints in their mantle lithosphere. The geochemistry of the Serbian ultrapotassic rocks suggests that the enrichment events that modified the source of both lamproitic and kamafugitic groups were related to Mesozoic subduction events. The postcollisional environment of the northern Balkan region with many extensional episodes is consistent at regional and local levels with the occurrence of ultrapotassic rocks, providing a straightforward relationship between geodynamics and volcanism.

Re-Os Model Age Constraints on the Genesis of the Moss Molybdenite Pegmatite-Aplite Deposit, Southwestern Grenville Province, Quyon, Quebec, Canada

The Moss molybdenite deposit, which was mined during both world wars, is hosted in late Grenvillian granitic pegmatite dikes. These dikes are locally aplitic, and intrude the late tectonic Onslow syenite (probably correlative with the ~1090–1070 Ma Kensington suite), which is in turn intrusive into the 1170–1155 Ma Chevreuil intrusive suite of southwestern Quebec. A Re-Os model age of 1053 ± 4 Ma (8.3 ppm Re) was obtained for molybdenite from a pegmatitic veinlet within an aplitic phase of the dike. A much younger Re-Os date of 860.9 ± 9.2 Ma (6.9 ppm Re) obtained previously from a pyrrhotite-and pyrite-rich metasomatic vein might reflect Os diffusion during slow cooling of the Grenville province, because this age is consistent with K-Ar dates reported from the region. Alternatively, the similarity between these younger dates may be fortuitous, and may instead relate to sampling issues for molybdenite. The Mo-rich granitic dikes at the Moss deposit are similar to other Mo-rich A-type pegmatite-aplite granitic dike systems that formed immediately after the peak of the Ottawan orogeny (1070–1090 Ma), and possibly are more closely related to the Kensington suite than previously thought.

Rb/Sr record of fluid-rock interaction in eclogites: The Marun-Keu complex, Polar Urals, Russia

Rb/Sr internal mineral isochrons in the eclogite facies Marun-Keu metamorphic complex, Polar Urals, Russia, date periods of fluid-rock interaction and record the metamorphic reaction history. The Marun-Keu complex consists of Late Proterozoic to Early Ordovician, mostly igneous rocks that experienced a subduction-related, non-pervasive eclogite facies metamorphism, followed by a local decompression-related amphibolite facies overprint, during the Uralian orogeny. Field observations show that metamorphic reactions as well as ductile deformation are controlled by local availability of a free fluid phase. Isotopic data reveals that availability of fluids similarly exerts control on isotope distribution. From a relic gabbro which has never been infiltrated by free fluids, a premetamorphic Rb/Sr age of 467 ± 39 Ma was obtained. Rb/Sr isochron ages for 14 samples of eclogite and amphibolite facies assemblages, sampled from within or close to metamorphic fluid veins, range from 352 ± 5 Ma to 360 ± 3 Ma. A Sm/Nd isochron for a metagranite yields an age of 354 ± 4 Ma. Taken together, the ages for both prograde and retrograde metamorphic assemblages overlap within analytical uncertainty and yield an average value of 355.5 ± 1.4 Ma, indicating that the metamorphic evolution and incipient exhumation of the Marun-Keu complex proceeded rapidly. The results demonstrate that assemblages preserve their Rb/Sr isotopic signatures as long as they remain devoid of free fluids, and that only fluid-rock interaction may cause Sr isotope redistribution. In addition, the data suggest local fluid-rock equilibrium, low fluid-rock ratios with overall fluid deficiency, and limited fluid mobility at depth. However, some fluids must have been mobile on the km-scale since they can be traced into the suprasubduction zone mantle wedge. Metasomatic veins in the Rai-Iz ophiolite yield a Rb/Sr mineral isochron age of 373.1 ± 5.4 Ma. They are interpreted as evidence for suprasubduction zone metasomatism in an oceanic setting, prior to subduction of the East European margin and associated formation of eclogites in the Marun-Keu complex.We propose that Rb/Sr mineral-isochron ages provide hygrochronological rather than thermochronological constraints. They define the cooling history only in combination with zircon and apatite fission track data. The straightforward interpretation of Rb/Sr mineral ages as cooling ages is obsolete.

Evidence for high silicic melt circulation and metasomatic events in the mantle beneath alkaline provinces: the Na–Fe-augitic green-core pyroxenes in the Tertiary alkali basalts of the Cantal massif (French Massif Central)

¶Na–Fe augitic green-core pyroxenes (hereafter called GCPX) are common in the silica-undersaturated basaltic rocks of many magmatic alkaline provinces. In the Cantal massif, green-core pyroxenes occur in nearly all the Supracantalian basalts (9.5 to 2 Ma), in contrast to the Infracantalian basalts (13 to 9.5 Ma) where they are rarely observed. An electron microprobe study demonstrates that the GCPX crystallized from evolved melts at intermediate to high pressures. Both Supra- and Infracantalian basalts show major and trace-element compositions similar to those of Ocean Island Basalts (OIB). However, the Supracantalian basalts exhibit an additional enrichment in Nb and Ta which we ascribe to a metasomatic event in the magma source. This enrichment could be related to the remobilization of metasomatic Ta and Nb-rich oxides located on grain boundaries in mantle peridotites (Bodinier et al., 1996). The hypothesis of the percolation of Si and K-rich metasomatic melts through the mantle is invoked to explain the deposition of these Ta–Nb oxides. On the basis of mineralogical and geochemical arguments, we suggest a relationship between these melts, the timing of the percolation and the formation of GCPX observed in Cantal basalts. We propose that GCPX crystallized in a melt differentiated by “percolative fractional crystallization” (PFC –Harte et al., 1993), a process which would account for the similar major-element composition of the GCPX and pyroxenes observed in the phonolites of the Cantal massif. Amphibolitic siliceous metasomatic veins in peridotite xenoliths recently described by Wulff-Pedersen et al. (1999) contain augites similar in composition to the GCPX in Cantal basalts, in addition to Nb-rich oxides. This confirms the genetic relationship between metasomatic melts, GCPX and Nb-rich oxide parageneses. GCPX would therefore seem to be linked to a metasomatic event which took place both/either during and/or between the two main stages of basaltic outpouring, and is consequently contemporaneous with the volcanism. We suggest that this process may represent a common mechanism in many alkaline provinces, where asthenospheric upwelling generates silicic metasomatizing liquids by PFC. This metasomatic process is inferred to deposit accessory phases within the lithospheric mantle, a process followed by the subsequent inheritance of this metasomatic imprint by the younger basalts.

Melting experiments on SiO 2 -rich lamproites to 6.4 GPa and their bearing on the sources of lamproite magmas

Supra-solidus phase relations at temperatures and pressures ranging from 800 to 1700 °C and 2 to 6.4 GPa have been determined experimentally for three silica-rich lamproites: hyalo-leucite phlogopite lamproite (Oscar, West Kimberley); sanidine richterite lamproite (Cancarix, Murcia-Almeria); and phlogopite transitional madupitic lamproite (Middle Table Mountain, Wyoming). All samples have extended melting intervals (500–600 °C). Bulk composition has a significant control on the nature of the initial liquidus phases, with orthopyroxene occurring at low pressures ( 6 GPa) orthopyroxene is replaced by garnet plus clinopyroxene as near-liquidus phases in the Oscar lamproite and by orthopyroxene plus clinopyroxene in the Cancarix sample. Clinopyroxene is a near-liquidus phase at all pressures in the Middle Table Mountain lamproite. Near-solidus phase assemblages at high pressure (>5 GPa) are: clinopyroxene + phlogopite + coesite + rutile + garnet (Oscar); clinopyroxene + garnet + coesite + K–Ti-silicate (Cancarix); clinopyroxene + phlogopite + apatite + K–Ti-silicate (Middle Table Mountain). In all compositions olivine is never found as a liquidus phase at any of the temperatures or pressures studied here. The phase relationships are interpreted to suggest that silica-rich lamproites cannot be derived by the partial melting of lherzolitic sources. Their genesis is considered to involve high degrees of partial melting of ancient metasomatic veins within a harzburgitic-lherzolitic lithospheric substrate mantle. The veins are considered in their mineralogy to be similar to the experimentally-observed, high pressure, near-solidus phase assemblages. The composition of silica-rich primary lamproite magmas differs between cratons as a consequence of differing mineralogical modes of the source veins and different relative contributions from the veins and wall-rocks to the partial melts.

Geikielite exsolution in spinel

Euhedral crystals of purple spinel are associated with calcite and forsterite in the central zone of composite metasomatic veins of the Adamello contact aureole (Northern Italy). In thin section, spinel is generally colorless, but exhibits concentric zones of dark brown color. These zones are aligned parallel to prominent crystal faces and contain abundant anisotropic, lath-shaped inclusions with their long axis oriented in the six directions of spinel. This orientation results from the alignment of {001}Geik and the {111}Sp crystallographic planes. The small (up to approximately 20 × 1 μm) inclusions were identified as geikielite by means of element distribution maps, micro-Raman spectroscopy, and transmission electron microscopy. Electron microprobe data revealed that spinel consists mainly of MgAl2O4, with only minor amounts of other components. The overall chemical variation within individual crystals is characterized by a continuous increase in FeOtot and ZnO from center to rim (0.6–2.1 wt% and <0.1–1.0 wt%, respectively). Whereas V2O3 is relatively constant (0.15 wt%), the bulk TiO2 concentration (analysis spot: 20 μm diameter) increases in general from center (0.12 wt%) to rim (1.2 wt%). This variation, however, is marked by several discontinuities that correspond to the zones containing geikielite inclusions. Where geikielite occurs, the spinel matrix is depleted in Ti relative to the inclusion-free zones. Geikielite was analyzed by analytical electron microscopy and has the formula Mg0.83Fe0.15Mn0.02(Ti0.99Fe0.01)O3. The textural relationships between geikielite inclusions and spinel matrix, combined with the analytical data, suggest that geikielite laths represent exsolution features, thus, a miscibility gap exists between Mg-Al-spinel and geikielite. The purple color results from scattering of light by the geikielite inclusions. Spinel with similar features was also observed in metasomatic veins of the Bergell contact aureole (Italy) and in granulite-facies marbles of Southern India.

Hydrous metasomatism of oceanic sub-arc mantle, Lihir, Papua New Guinea: Part 2. Trace element characteristics of slab-derived fluids

Spinel peridotite xenoliths recovered from the Tubaf and Edison volcanoes, south of Lihir Island in the Tabar–Lihir–Tanga–Feni island arc in Papua New Guinea, are predominantly fresh, refractory harzburgites. Many of the harzburgite xenoliths have cross-cutting vein networks and show evidence of modal metasomatism. These metasomatic veins contain a secondary mineral assemblage consisting of fibrous, radiating orthopyroxene and fine-grained Fe–Ni sulfide with minor olivine, clinopyroxene, phlogopite, amphibole and magnetite. Adjacent to the veins, primary clinopyroxene is cloudy while orthopyroxene exhibits replacement by secondary fibrous orthopyroxene, similar in habit to orthopyroxene occurring in the veins. The mineralogical and geochemical characteristics of the Tubaf mantle xenoliths are the product of two major processes: an early partial melting depletion event that was overprinted by oxidation and alkali enrichment related to percolation of slab-derived, hydrous melts. HREE and MREE concentrations in clinopyroxene from the least metasomatised harzburgites indicate that they are the residues from a 15% to 25% partial melting event, consistent with formation in a MOR setting. The secondary vein assemblages show strong enrichment in the LILE (primarily Sr, Ba, Rb, Th, U and Pb) and the REE (primarily La, Ce, Nd, Sm, Eu and Gd), while the HFSE (Nb, Ta, Zr, Hf, and Ti) are neither enriched nor depleted. The mineral precipitates in the vein assemblages have high LREE/HFSE and LILE/HFSE, and reflect the relative solubility of these elements in hydrous melts. These trace element characteristics are similar to those of the Tabar–Lihir–Tanga–Feni arc lavas, and display the commonly observed HFSE depletion of arc magmatism. These findings support the hypothesis that this so-called “arc signature” is primarily dependent on the relative solubility of elements in slab-derived, hydrous melts, and the enrichment of these soluble elements in metasomatised mantle regions that are prone to preferential partial melting.

The distribution and behaviour of rhenium and osmium amongst mantle minerals and the age of the lithospheric mantle beneath Tanzania

Rhenium–osmium (Re–Os) isotope and elemental abundances have been obtained for primary mantle minerals, metasomatic phases, and a range of mantle rock types from xenoliths in recent volcanics in northern Tanzania. Re and Os abundances for sulphide and coexisting silicates in garnet lherzolites from Lashaine confirm that sulphide dominates the Os budget, but also show that Re is almost exclusively sited in silicate phases. Silicate minerals from two different samples yield 187Re– 188Os ages of 15.4±6.1 and 31.4±6.3 Myr, respectively. Comparison with 232Th– 208Pb (267.1±4.4 Myr) 147Sm– 143Nd (164±18 Myr) and 87Rb– 87Sr (in equilibrium at the present-day) ages for the same silicate minerals suggests differential closure between these isotope systems, and a closure temperature of ≥670°C for the Re–Os system. Remarkably, sulphide inclusions were not affected by diffusional equilibration between the silicates, and preserve significantly older age information. Model calculations suggest that sulphide–silicate equilibration ceased some 200–300 Ma, and the Os isotope composition of the sulphide ( 187Os/ 188Os=0.10432±0.00013) suggests a minimum age of 3.4 Gyr. Most xenoliths possess Os isotope compositions that are less radiogenic than the present-day chondritic mantle indicating that they experienced Re-loss some time ago. Samples showing evidence for modal metasomatism have high Re concentrations and Re/Os ratios, but their relatively unradiogenic Os isotope compositions suggests that the metasomatism occurred recently, consistent with data for metasomatic vein minerals. In contrast, some dunites possess both high Re/Os ratios and radiogenic Os isotope compositions. These samples differ from those affected by modal metasomatism in having low Re and exceptionally low Os concentrations. These results provide quantitative constraints on the distribution of Re and Os amongst mantle minerals, highlight the potential of Re–Os isotope dating of sulphide inclusions for establishing the early history of mantle mineral assemblages, and demonstrate that mantle processes themselves (metasomatism and dunite formation) can significantly modify the Os isotope chemistry of mantle rocks.

The effects of liquid immiscibility and thermal diffusion on oxygen isotopes in silicate liquids

Differences between the δ18O values of Si- and Fe-rich immiscible liquids in the system Fe2SiO4-KAlSi2O6-SiO2 (Fa-Lc-Q) in isothermal experiments at 0.1 MPa have been determined experimentally to be 0.6 permil. The observed partition of 18O into the Si-rich liquid is consistent with previous experience with the preferential partition of 18O into Si-rich minerals in isothermal equilibrium with minerals of less polymerized structure. Crystallochemical principles affect the distribution of oxygen isotopes in coexisting isothermal liquids in the same way as they apply to isothermally coexisting crystals. The effects of Soret (thermal) diffusion on the distribution of oxygen isotopes in silicate liquids above the solvus in the system Fa-Lc-Q under conditions of an imposed temperature gradient of ca. 250 °C over 4 mm and at 2 GPa have also been investigated experimentally. Both the magnitude and the direction of separation of oxygen isotopes as a result of Soret diffusion are unexpected. For each of the silicate liquids, the cold end of the charge is enriched in 18O by up to 4.7 permil, and the highest δ18O values are associated with the most silica-poor compositions. The distribution of oxygen isotopes appears to be similar in each liquid, regardless of their chemical compositions, which is in contrast to the behaviour of cations whose distributions are compositionally dependent and characterized by strong crystallochemical effects wherein network-forming species such as Si and Al separate to the hot end and Mg, Fe and Ca are segregated preferentially to the cold end. Structural units in the melts are evidently less selective between oxygen isotopes than between cations, because oxygen redistribution over all possible sites in these units proceeds according to mass. Self-diffusion coefficients of oxygen in basaltic liquids estimated from the Soret experiments are in accord with those from other isotope tracer experiments, and comparable to those of Si. The possible effects of Soret diffusion on the oxygen isotopic composition of metasomatic veins in the mantle are examined in light of these data, and indicate that decay of the thermal gradients in the veins exceeds that of the diffusion of oxygen needed to produce variations in the δ18O values of mantle minerals. Variations in oxygen isotope ratios in most natural systems as a result of Soret effects are unlikely.

Isotope and Trace Element Geochemistry of Cretaceous Damaraland Lamprophyres and Carbonatites, Northwestern Namibia: Evidence for Plume--Lithosphere Interactions

Major and trace element and radiogenic and stable isotope data are INTRODUCTION reported for Cretaceous lamprophyres and carbonatites associated The close association of alkaline silicate rocks (ijolites, with alkaline igneous complexes in Damaraland, northwestern nephelinites and phonolites) and carbonatites within cerNamibia. Lamprophyres, emplaced as late-stage dykes, stocks and tain intra-plate intrusive complexes is a well-recognized diatremes, include both alkaline and ultramafic varieties. Many of and geographically widespread phenomenon (Le Bas, the latter contain abundant carbonate-rich ocelli. The carbonatites 1987; Barker, 1989). However, the genetic relationship are primarily sovites and Fe-rich beforsites emplaced as plugs and between them and the processes involved in their gendykes. Both lamprophyres and carbonatites are characterized by eration are still not fully understood. Considerable debate strong, but variable, enrichment in incompatible trace elements (e.g. exists as to whether carbonatite magmas are derived via La 45–7232 ppm; Nb 89–1961 ppm; Sr 700–43 000 ppm), the direct partial melting of mantle peridotite (e.g. Gittins, and variable but coherent initial Sr/Sr (0·70351–0·70468), 1988; Green & Wallace, 1988; Wallace & Green, 1988; Nd/Nd (0·51244–0·51266) and Pb isotope ratios (e.g. Yaxley et al., 1991), or whether they represent mantlePb/Pb = 17·77–19·99). Individual lamprophyre types derived magmas which have undergone modification at show distinct, but restricted, ranges in isotopes and incompatible lower pressures by a process of silicate–carbonate liquid element ratios (Zr/Nb = 0·9–2·0; Y/Nb = 0·15–0·34; La/ immiscibility from, or extensive fractionation of, a carYbn= 20–58), indicating derivation from a trace element enriched, bonated silicate parental magma (e.g. Middlemost, 1974; heterogeneous mantle source. Lamprophyres and carbonatites found Koster van Groos, 1975; Le Bas, 1987, 1989; Kjarsgaard within the same complexes have similar isotope ratios, indicating & Hamilton, 1989; Lee & Wyllie, 1994). derivation from a common source. Isotope and trace element variA linear series of anorogenic igneous complexes extend ations, coupled with the temporal and spatial association with the through Damaraland in northwestern Namibia (Fig. 1) palaeoposition of the Tristan plume, are thought to indicate that and provide an example of the contiguous alkaline silicate the Damaraland lamprophyre and carbonatite magmatism formed and carbonatite lithologies. The composition of these as a consequence of melting of metasomatic vein material introduced complexes shows a general northeasterly spatial trend into an isotopically depleted subcontinental lithospheric mantle by from granitic associations close to the Atlantic coast, alkaline melts or fluids derived from the upwelling Tristan mantle through differentiated basic, to peralkaline and carplume at the time of continental break-up. bonatitic compositions located ~250–350 km inland (Martin et al., 1960). In addition to the silicate rock– carbonatite associations within three of the Damaraland complexes, carbonate-rich globular structures (ocelli) have also been described within the late-stage nephelinitic

Isotope and Trace Element Geochemistry of Cretaceous Damaraland Lamprophyres and Carbonatites, Northwestern Namibia: Evidence for Plume-Lithosphere Interactions

Major and trace element and radiogenic and stable isotope data are reported for Cretaceous lamprophyres and carbonatites associated with alkaline igneous complexes in Damaraland, northwestern Namibia. Lamprophyres, emplaced as late-stage dykes, stocks and diatremes, include both alkaline and ultramafic varieties. Many of the latter contain abundant carbonate-rich ocelli. The carbonatites are primarily sövites and Fe-rich beforsites emplaced as plugs and dykes. Both lamprophyres and carbonatites are characterized by strong, but variable, enrichment in incompatible trace elements (e.g. La 45−7232 ppm; Nb 89–1961 ppm; Sr 700–43 000 ppm), and variable but coherent initial 87Sr/86Sr (0.70351–0.70468), 143Nd/144Nd (0.51244–0.51266) and Pb isotope ratios (e.g. 206Pb/204Pb = 17.77–19.99). Individual lamprophyre types show distinct, but restricted, ranges in isotopes and incompatible element ratios (Zr/Nb = 0.9–2.0; Y/Nb = 0.15–0.34; La/Ybn = 20–58), indicating derivation from a trace element enriched, heterogeneous mantle source. Lamprophyres and carbonatites found within the same complexes have similar isotope ratios, indicating derivation from a common source. Isotope and trace element variations, coupled with the temporal and spatial association with the palaeoposition of the Tristan plume, are thought to indicate that the Damaraland lamprophyre and carbonatite magmatism formed as a consequence of melting of metasomatic vein material introduced into an isotopically depleted subcontinental lithospheric mantle by alkaline melts or fluids derived from the upwelling Tristan mantle plume at the time of continental break-up.

Phase equilibrium and stable isotope constraints on the formation of metasomatic garnet-vesuvianite veins (SW Adamello, N Italy)

Metasomatic garnet-vesuvianite veins occur within the contact metamorphic marble sequence of the Lower Triassic Prezzo formation in a narrow, 1–5 m wide zone along an intrusive marble-granodiorite contact at the southwestern border of the Tertiary Adamello batholith. The metasomatic mineral assemblage is comprised of garnet, vesuvianite, clinopyroxene, wollastonite, and pyrrhotite, which were precipitated from the vein-forming fluid in a preexisting calcite matrix at conditions of about 2800 bars and 630° C. The veins are enriched in silicon, aluminum, iron, magnesium, titanium and depleted in calcium with respect to the unaltered contact metamorphic marble. Graphite, which is present in the unaltered Prezzo Marble is absent in the veins. Irregularly shaped mineralogically distinct zones with different degrees of silicification can be distinguished within the veins. The isotopic compositions of calcite (cc) in the unaltered marble are about δ18O (SMOW; Standard mean Ocean Water)=21.0‰ and δ13C(PDB; Peedee belemnite)=0.0‰. They are reset to significantly lower values within the veins, where δ18Occ is 15.0 to 16.0‰ and δ13Ccc is −4.5 to −3.5‰. The isotopic front coincides with an abrupt change in the microscopic texture of matrix carbonate which occurs at the sharp boundary between graphite-bearing and graphite-free material. Within the veins the oxygen isotope fractionation between calcite and garnet (gar) varies systematically with distance from highly silicified zones. The variations in Δ18Occ-gar are as large as 2‰, on a millimeter scale, indicating garnet-calcite isotopic disequilibrium. Vein formation was due to the infiltration of a water rich fluid of magmatic provenance into the carbonate country rock along fractures. Removal of graphite from the wall rock by dissolution through the metasomatic fluid induced recrystallization of matrix calcite. Permeability was enhanced during calcite recrystallization facilitating material transport into the wall rock and metasomatic alteration. Vein garnet was precipitated in isotopic equilibrium with the metasomatic fluid. The isotopic composition of preexisting calcite was initially out of equilibrium with the vein-forming fluid and it was shifted towards equilibrium by surface-reaction controlled calcite-fluid isotopic exchange during calcite recrystallization. Due to the short lifetime of the metasomatic system, calcite-fluid isotopic equilibrium was generally not attained. Within the veins, oxygen and carbon transport was fast relative to mineral-fluid exchange of their isotopes and the geometry of the isotopic pattern is largely controlled by the kinetics of mineral-fluid exchange.

Isotopic Disequilibrium During Metasomatic Vein Formation

At the southwestern border of the Adamello batholith metasomatic garnet-vesuvianite veins occur within the contact metamorphic marble sequence of the Prezzo formation. The veins are c h a r a c t e r i z e d by the assemblage garne t ( C a 3 A l l o F 3+ 9 eo.4SisO12) + v e s u v i a n i t e (Ca19Mg1.2Fel.2Allo.4Tio.zSilsO69[(OH)r.sFl.5) + clinopyroxene (CaMgo.sFeo.2Si2Or) + wollastonite + calcite + pyrrhotite (FeSI.1). Within the veins irregularly shaped microscopic domains with different degrees of silicification can be distinguished. Graphite is present in the contact metamorphic Prezzo Marble at 0.5 wt% concentration, but is absent in the metasomatically altered rocks.

Solid carbon dioxide in a natural diamond

CARBON and hydrogen are only trace constituents of the Earth's mantle, yet carbon- and hydrogen-bearing fluids have an important effect on magma genesis, mantle rheology and mantle chemistry. Many mantle-derived rocks record interactions with such fluids, but direct samples of the fluids themselves are rare. Diamonds, owing to their robust nature, constitute effective sampling devices for such deep-seated fluids1; for example, carbonates and water have been found2,3 in fluid inclusions in fibrous diamonds4. In addition, CO2, H2O, CO, CH4, H2 and N2 have been detected in gases released from diamonds by crushing5 or heating6,7, but their primary nature could not be confirmed beyond doubt1. Here we report the discovery of solid CO2 in a natural diamond. Infrared spectroscopy indicates that the CO2 is presently at a pressure of 5 GPa (50 kbar), and must therefore have been trapped at even greater pressures in the hot mantle, corresponding to depths of about 220–270 km. At these pressures, free CO2 should react with olivine and pyroxene; thus, its survival indicates the presence at depth of an environment of different mineralogy such as a fully carbonated metasomatic vein, or a block of subducted sediments.

Norman L. Bowen Award to John Barry Dawson

Barry Dawson is just right for th e first Norman L. Bowen Award in Volcanology, Geochemistry, and Petrology. Like Norman L. Bowen, he has the inspiration to combine ideas from different disciplines to obtain a higher synthesis. Consider his brilliant talk of yesterday morning (Eos 69, p. 502, abstract V11C‐06, Veined and metasomatised upper mantle beneath the Eastern African Rift: Evidence from N. Tanzania xenoliths). Barry rapidly reviewed the tectonic setting of east Africa, showed some nice photographs of the volcanic cones of Pello and Oldoinyo Lengai, focused on the upper‐mantle xenoliths in scoria at Pello, moved in on the microscopy and mineral chemistry of the metasomatic veins in the peridotites, snowed how the lower density of a metasomatized peridotite would fit the middle of a geophysical profile from seismic surveys, and brought off a grand finale in which the Kenya dome was interpreted to result from inflation related to metasomatization of underlying peridotite. Undoubtedly, this will lead to many further studies of doming adjacent to rift systems around the world.

Metasomatic phase relations in dolomites of the Adamello Alps

Intrusion of Tertiary quartz diorite into Triassic dolomite produced metasomatic veins up to 5 cm in width at or near the intrusive contact. Progressing from dolomite to quartz diorite, five zones of mineral assemblages can be distinguished: (1) calcite + forsterite, (2) diopside, (3) diopside + tremolite + clinozoisite + chlorite in the contact zone, (4) clinozoisite + or - plagioclase, and (5) a silica-enriched zone. Thermodynamic data for minerals and aqueous species in the system CaO-MgO-SiO ₂ -Al ₂ O ₃ -H ₂ O-CO ₂ permit determination of the fluid composition in local equilibrium with the various phase assemblages observed in the metasomatic veins. Geologic observations, together with comparative analysis of thermodynamic phase relations and fluid compositions over a range of pressures and temperatures, suggest that the metasomatic assemblages formed over a period of thousands of years by reaction of the country rock with an H ₂ O-rich fluid.--Modified journal abstract.