Ionic association in H2O–CO2 fluids at mid-crustal conditions

The equilibrium constant, K _a, of the association reaction to form ion pairs from charged solute species in supercritical solutions can be calculated from a model based on published equations. Log K _a at constant pressure is a linear function of the inverse in the dielectric constant of the fluid times temperature. The dielectric properties of H₂O and CO₂ at supercritical pressures and temperatures can also be evaluated using the Kirkwood equation. Using Looyenga mixing rules, the dielectric constant of H₂O–CO₂ mixtures can be obtained and the change in log K _a with addition of CO₂ in aqueous solutions evaluated. These changes in log K _a with addition of CO₂ are consistent with measured changes of log K _a with addition of Ar in supercritical H₂O–Ar solutions.
Log K _a of KCl and NaCl increase to an increasing extent as the mole fraction of CO₂ increases in H₂O–CO₂ solutions. For instance, at 2 kbar and constant temperature between 400 and 600° C, log K _a of KCl increases by about two orders of magnitude whilst that of NaCl increases by over four orders of magnitude as the CO₂ mole fraction increases from 0.0 to 0.35. Such changes in log K _a will have dramatic effects on the solubility of minerals in CO₂-rich environments.     

Primary carbonate/CO2 inclusions in sapphirine-bearing granulites from central Sri Lanka

High-density CO₂-rich fluid inclusions from a sapphirine-bearing granulite (Hakurutale, Sri Lanka) have been studied by microthermometry, Raman spectrometry and SEM analysis. Based on textural evidence, two groups of inclusions can be identified: primary, negative crystal shaped inclusions (group I) and pseudo-secondary inclusions, which experienced a local, limited post-trapping modification (group II). Both groups contain magnesite as a daughter mineral, occurring in a relatively constant fluid/solid inclusion volume ratio (vol_solid =0.15 total volume). CO₂ densities for group I and II differ only slightly. Both groups contain a fluid, which was initially trapped at peak metamorphic conditions as a homogeneous (CO₂+MgCO₃) mixture. Thermodynamic calculations suggest that such a fluid (CO₂+15 vol% MgCO₃) is stable under granulite facies conditions. After trapping, magnesite separated upon cooling, while the remaining CO₂ density suffered minor re-adjustments. A model isochore based on the integration of the magnesite molar volume in the CO₂ fluid passes about 1.5–2 kbar below peak metamorphic conditions. This remaining discrepancy can be explained by the possible role of a small quantity of additional water.     

Refinements in BaSO4 to CO2 Preparation and δ18 O Calibration of the Sulfate Reference Materials NBS-127, IAEA SO-5 and IAEA SO-6

Refinements have been made to achieve over 99% yield in the conversion of CO to CO₂ in order to improve the reproducibility and accuracy of δ¹⁸ O measurements in sulfates. BaSO₄ (10-15 mg) was mixed with an identical amount of spectrographic-grade graphite and loaded into a Pt boat. The mixture was gradually heated to 1100 °C to reduce sulfate to CO and CO₂; the former gas was simultaneously converted to CO₂ by a glow discharge between Pt electrodes immersed in a magnetic field (produced by a pair of external neodymium magnets). A small memory effect was noticed during the analysis (less than 0.3‰ per 10‰ difference in δ¹⁸ O between two subsequently analysed samples). The memory effect, however, was suppressed by repetitive preparation of the same specimen. CO₂ produced in this way from sulfate reference samples was analysed on a dual inlet and triple collector mass spectrometer along with CO₂ equilibrated with VSMOW, GISP and SLAP water reference samples. To avoid large departures of measured isotope ratios from ¹⁸O/¹⁶O of the working calibrator we used CO₂ gas prepared from ocean water sulfate for this purpose. The calibrated δ¹⁸ O values (in ‰) obtained in this way for NBS-127, IAEA SO-5 and IAEA SO-6 reference materials were 8.73 ± 0.05, 12.20 ± 0.07 and -10.43 ± 0.12, respectively.     

Genesis of Kanggur Gold Deposit in Eastern Tianshan Orogenic Belt, NW China: Fluid Inclusion and Oxygen and Hydrogen Isotope Constraints

Abstract. Primary fluid inclusions in quartz and carbonates from the Kanggur gold deposit are dominated by aqueous inclusions, with subsidiary CO₂-H₂O inclusions that have a constant range in CO₂ content (10–20 vol %). Microthermometric results indicate that total homogenization temperatures have a wide but similar range for both aqueous inclusions (120 to 310C) and CO₂-H₂O inclusions (140 to 340C). Estimates of fluid salinity for CO₂-H₂O inclusions are quite restricted (5.9∼10.3 equiv. wt% NaCl), whereas aqueous inclusions show much wider salinity ranging from 2.2 to 15.6 equivalent wt %NaCl.
The 6D values of fluid inclusions in carbonates vary from -45 to -61 %, in well accord with the published δD values of fluid inclusions in quartz (-46 to -66 %). Most of the δ¹⁸O and δD values of the ore-forming fluids can be achieved by exchanged meteoric water after isotopic equilibration with wall rock by fluid/rock interaction at a low water/rock ratio. However, the exchanged meteoric water alone cannot explain the full range of δ¹⁸O and δD values, magmatic and/or meta-morphic water should also be involved. The wide salinity in aqueous inclusions may also result from mixing of meteoric water and magmatic and/or metamorphic water.     

Shallow-level metamorphic fluids in a high uplift rat metamorphic belt; Alpine Schist, New Zealand

Abstract Fluids, some of which are CO₂-rich (up to 40 mol.% CO₂) and some of which are highly saline (up to 18 wt% NaCl equivalent), are trapped as fluid inclusions in quartz-calcite (∼ metallic minerals) veins which cross-cut the pumpellyite-actinolite to amphibolite facies rocks of the Alpine Schist. Fluids were commonly trapped as immiscible liquid-vapour mixes in quartz and calcite showing open-space growth textures. Fluid entrapment occurred at fluid pressures near 500 bars (possibly as low as 150 bars) at temperatures ranging from 260 to 330° C. Saline fluids may have formed by partitioning of dissolved salts into an aqueous phase on segregation of immiscible fluids from a low-density CO₂-rich fluid. Calcite deposited by these fluids has δ¹³C ranging from – 8.4 to – 11.5 and δ¹⁸O from + 4 to + 13. Isotopic data, fluid compositions and mode of occurrence suggest that the fluids are derived from high-grade metamorphic rocks. Fluid interaction with wall-rock has caused biotite crystallization and/or recrystallization in some rocks and retrogression of biotite to chlorite in other rocks.
Fluid penetration through the rock is almost pervasive in many areas where permeability, probably related to Alpine Fault activity, has focussed fluids on a regional scale into fractured rocks. The fluid flow process is made possible by high uplift-rates (in excess of 10 mm/year) bringing hot rocks near to the surface.     

Impure marbles from the UHP Brossasco-Isasca Unit (Dora-Maira Massif, western Alps): evidence for Alpine equilibration in the diamond stability field and evaluation of the X(CO2) fluid evolution

Two impure ultrahigh-pressure (UHP) marbles, a calcite marble with the peak assemblage Grt + Phe + Cpx + Rt + (Arg) and a dolomite marble with the peak assemblage Crn + Chl + Rt + Dol (±Arg), from the same lens from the polymetamorphic complex of the Brossasco-Isasca Unit (BIU) (southern Dora-Maira Massif) have been petrologically investigated and modelled by calculating P – T phase-diagram projections for H₂O–CO₂ mixed-volatile systems. Thermobarometric data obtained from the calcite marble suggest Alpine peak conditions in the diamond stability field (4.0 GPa at 730 °C), and allow reconstruction of the earlier portion of the Alpine retrograde P – T path, which is characterized by a significant decompression coupled with a moderate and continuous cooling to 650 °C at 2.50 GPa. The modelled fluid compositions at peak conditions point to 0.025 ≤  X (CO₂) ≤ 0.10 and X (CO₂) ≤ 0.0012 in the calcite marble and dolomite marble, respectively, suggesting fluid heterogeneity at the local scale and an internally buffered fluid evolution of the studied impure marbles. The lack of micro-diamond in the BIU marbles is explained by the very-low X (CO₂) values, which favoured relatively high f O₂-conditions, preventing the formation of diamond at the UHP peak metamorphic conditions.     

On the origin of CO2-rich fluid inclusions in migmatites

Abstract Nearly pure CO₂ fluid inclusions are abundant in migmatites although H₂O-rich fluids are predicted from the phase equilibria. Processes which may play a role in this observation include (1) the effects of decompression on melt, (2) generation of a CO₂-bearing volatile phase by the reaction graphite + quartz + biotite + plagioclase = melt + orthopyroxene + CO₂-rich vapour, (3) selective leakage of H₂O from CO₂+ H₂O inclusions when the pressure in the inclusion exceeds the confining pressure during decompression, and (4) enrichment of grain-boundary vapour in CO₂ by subsolidus retrograde hydration reactions.     

Fluid inclusion evidence for basement decompression during Permo-Triassic extension, SE New England, USA

Abstract CO₂-bearing fluid inclusions in strongly lineated but weakly foliated late Precambrian gneisses within the Hope Valley Shear zone of Connecticut and Rhode Island are of mixed composition ( X _co2± 0.1; 7 wt% NaCl equivalent) and variable density (0.59–0.86 g/ml) and occur mainly as isolated inclusions. Also present are dilute (3 wt% NaCl equivalent) aqueous inclusions which occur on healed fractures related to greenschist facies retrograde metamorphism. Isochores for dense isolated CO₂-bearing inclusions indicate pressures of 7.5–9 kbar at 500–600° C, the estimated temperature conditions of peak metamorphism. Published ⁴⁰Ar/³⁹Ar hornblende plateau age spectra indicate cooling through about 500° C at 265 ± 5 Ma. Isochores for low-density CO₂-bearing inclusions and aqueous inclusions intersect at the conditions of retrograde metamorphism (325–400° C) and indicate pressures of 3–4 kbar. Published ⁴⁰Ar/³⁹Ar biotite plateau ages indicate cooling through about 300° C at 250 ± 5 Ma. These data define a P–T uplift curve for the region which is convex towards the temperature axis and indicate uplift rates between 0.4 and 3.3 mm/year in Permian time. Exhumation of basement gneisses was coeval with normal (west-down) motion along the regional basement–cover contact (Honey Hill–Lake Char–Willimantic fault system), and is interpreted as due to post-orogenic extensional collapse of the Alleghanian orogeny.     

Anthropogenic CO2 in Southern Ocean surface waters: evidence from stable organic carbon isotopes

We present an approach for tracing the fate of anthropogenic CO₂, compiling a large data set of stable organic carbon isotope ratios from surface sediments, plankton, and sinking matter in the Atlantic Ocean. The δ¹³C values of sinking matter are generally lower by 0.5–4.6‰ compared to the surface sediments. This difference increases with increasing latitude, which is explained by a stronger modern increase in surface water [CO₂ (aq)] in the Southern Ocean relative to the Tropical/Subtropical Ocean. Preindustrial dissolved CO₂ concentrations in Atlantic surface waters, estimated from the δ¹³C_org of surface sediments, are compared to recently measured surface water [CO₂ (aq)] values taken from literature. We obtain only a slight increase in [CO₂ (aq)] at lower latitudes but a significant change of about 7 ± 2 μ m in high latitudinal surface waters which we attribute to anthropogenic perturbation. Our results suggest that CO₂ released by human activities has been stored in Southern Ocean surface waters.     

Dehydration reaction and isotope front transport induced by CO2 infiltration at Nuliyam, South India

The field relations from a quarry at Nuliyam, South India, illustrate dehydration of an amphibolite facies gneiss to granulite facies charnockite by CO₂ influx, over a scale of 30 m. Both the calc-silicate source of the fluids and the full extent of their penetration into the gneiss are preserved in a continuous section. Fluid flow is by a hydraulic fracture mechanism, but is thought to be pervasive. The sharp reaction front predicted by the continuum mechanical theory for advective fluid transport is not observed. The front spreading is on too large a scale for either diffusive or dispersive control and is due to local kinetic disequilibrium between the fluid and rock, although the divariant nature of the reaction may also have a limited effect. The time-integrated fluid flux varies from the instantaneous porosity at the fluid front to 20 vol. % adjacent to the calc-silicate. Carbon isotope budgets suggest that decarbonation of the calc-silicate by a Rayleigh fractionation process provides a sufficient source for the CO₂ influxing into the gneiss. Graphite abundances vary from 0.01 to 0.1% (by weight), it is principally derived by precipitation from the fluid and may be modelled from phase equilibria. Carbon isotope fronts coincide with the reaction front on the scale of sampling, although isotopic disequilibrium between graphite and inclusion-CO₂ also implies local fluid-rock disequilibrium.     

Calculated mineral equilibria for eclogites in CaO–Na2O–FeO–MgO–Al2O3–SiO2–H2O: application to the Pouébo Terrane, Pam Peninsula, New Caledonia

The high- P , medium- T  Pouébo terrane of the Pam Peninsula, northern New Caledonia includes barroisite- and glaucophane-bearing eclogite and variably rehydrated equivalents. The metamorphic evolution of the Pouébo terrane is inferred from calculated P–T  and P–T  – X H2O pseudosections for bulk compositions appropriate to these rocks in the model system CaO–Na₂O–FeO–MgO–Al₂O₃–SiO₂–H₂O. The eclogites experienced a clockwise P–T  path that reached P ≈19  kbar and T  ≈600  °C. The eclogitic mineral assemblages are preserved because reaction consequent upon decompression consumed the rocks' fluid. Extensive reaction occurred only in rocks with fluid influx during decompression of the Pouébo terrane.     

Thermochemical sulphate reduction in the Upper Devonian Cairn Formation of the Fairholme carbonate complex (South-West Alberta, Canadian Rockies): evidence from fluid inclusions and isotopic data

The Fairholme carbonate complex is part of the extensively dolomitized Upper Devonian carbonate reefs in west-central Alberta. The studied formations contain moulds (up to 10 cm in diameter), which are filled partially with (saddle) dolomite, quartz and calcite cements. These cements precipitated from a mixture of brines that acquired high salinity by dissolution of halite and brines derived from evaporated sea water. The fluids were warm (homogenization temperature of primary fluid inclusions of 76 to 200 °C) and saline (20 to 25 wt% NaCl equivalent) and testify to thermochemical sulphate reduction processes. The latter is deduced from S in solid inclusions, CO₂ and H₂S in volatile-rich aqueous inclusions and depleted δ¹³C values down to −26‰ Vienna Pee Dee Belemnite. High ⁸⁷Sr/⁸⁶Sr values (0·7094 to 0·7110) of the cements also indicate interaction of the fluids with siliciclastic sequences. The thermochemical sulphate reduction-related cements probably formed during early Laramide burial. Another (younger) calcite phase, characterized by depleted δ¹⁸O values (−23·9‰ to −13·9‰ Vienna Pee Dee Belemnite), low Na (27 to 37 p.p.m.) and Sr (39 to 150 p.p.m.) concentrations and non-saline (∼0 wt% NaCl equivalent) fluid inclusions, is attributed to post-Laramide meteoric water.     

CO2 transfer between the upper mantle and the atmosphere: temporary storage in the lower continental crust

The CO₂ atmospheric content has shown large variations over geological times. High contents (up to one order of magnitude more than present-day values) ultimately correspond to discrete episodes of mantle degassing, either juvenile, or subduction-related (carbon recycling). A number of arguments (e.g. the continuous volume increase of carbonate-bearing sediments with time) suggest that, throughout the Earth's history, juvenile CO₂ has formed a major contribution to the global carbon budget of the Earth.
The absence of a direct relationship between major volcanic episodes and the average CO₂ atmospheric content suggests that volcanoes might not be the only way by which mantle CO₂ is transported to the surface. It is proposed that large quantities of juvenile CO₂ could temporarily be stored in the lower continental crust during major episodes of granulite formation. These are primarily caused by magmatic underplating and they result in a vertical accretion of the crust by accumulation of CO₂-bearing, mantle-derived magmas. Most of the CO₂ migrates through the crust during post-metamorphic evolution and isostatic restoration of the normal continental thickness. However, large quantities of CO₂ can still be present in some areas, notably as high-density fluids enclosed in minerals.     

Magmatic and hydrothermal processes in the Bouvet Triple Junction Region (South Atlantic)

Results of electron microprobe and microthermometric studies of samples collected from the Bouvet Triple Junction Region (BTJR) during a joint Russian-Italian geological expedition on the R/V Academician Nikolaj Strakhov (1994) have revealed new data on the composition of basaltic magmas and oceanic hydrothermal fluids connected with magmatic processes. Detailed analysis of basaltic glasses shows that the modem Mid-Atlantic Ridge (MAR) rift valley is composed of normal mid-ocean ridge basalts with low concentrations of K₂ O and TiO_z (N-MORB), while its flanks are more enriched with these components approaching E-MORB. A marked influence of the Bouvet hot spot volcanism on magma generation on the South-West Indian Ridge (SWIR) near Bouvet Island is observed. Basaltic melts in this area belong to alkalic and transitional series and have maximum contents of K₂O, TiO₂, H₂O.
Microthermometric analyses of fluid inclusions in the samples from the BTJR have revealed major differences in the oceanic hydrothermal fluid systems on the MAR and near SWIR, which depends on the peculiarities of magma. In the area of the MAR (with dry melts) only H₂O solution inclusions in quartz were found; thus, seawater is probably the only primary source of hydrothermal fluids (NaCl + MgCl₂+ H₂O; T = 170–200°C). In the SWIR area (with the high content of water in melts) syngenetic liquid CO₂ and H₂O solution inclusions in quartz indicate the influence of the magmatic fluid component on the ore-forming water/carbon dioxide solutions (NaCl + CaC1₂+ H₂O + CO₂; T = 200–310 °C; P = 900–1700 bar).     

Grain boundary migration of water and carbon dioxide during uplift of garnet-zone Alpine Schist, New Zealand

Abstract Deformed quartz veins in garnet-zone schist adjacent to the active Alpine Fault, New Zealand, have fluid inclusions trapped along quartz grain boundaries. Textures suggest that the inclusions formed in their present shapes during annealing of the deformed veins. Many of the inclusions are empty, but some contain carbon dioxide with densities that range from 0.16 to 0.80 g cm⁻³. No water, nitrogen or methane was detected. The inclusions are considerably more CO₂-rich than either the primary metamorphic fluid (<5% CO₂) or fluids trapped in fracture-related situations in the same, or related, rocks (<50% CO₂). Enrichment of CO₂ is inferred to have resulted from selective migration (wicking) of saline water from the inclusions along water-wet grain boundaries after cooling-induced immiscibility of a water-CO₂ mixture. Inclusion volumes changed after loss of water. Non-wetting CO₂ remained trapped in the inclusions until further percolation progressively removed CO₂ in solution. This mechanism of fluid migration dominated in ductile quartz-rich rocks near, but below, the brittle-ductile transition. At deeper levels, hydraulic fracturing is also an important mechanism for fluid migration, whereas at shallower levels advection through open fractures dominates the fluid flow regime.     

The amphibolite-granulite facies transition in West Uusimaa, S.W. Finland. A fluid inclusion study

This work presents the results of a fluid inclusion study of an amphibolite-granulite facies transition in West Uusimaa, S.W. Finland. Early fluid-inclusions in the granulite facies area are characteristically carbonic (CO₂), in contrast to predominantly aqueous early inclusions in the amphibolite facies area. These early inclusions can be related to peak metamorphic conditions (750-820°C and 3-5 kbar for peak granulite facies metamorphism). Relatively young CO₂ inclusions with low densities (<0.8g/cm³) indicate that the first part of the cooling history of the rocks was characterized by a near isothermal uplift.
N₂-CH₄ inclusions, with compositions ranging between pure CH₄ and pure N₂ (Raman spectral analysis), were found in the whole area. They are probably syn- or even pre-early inclusions. Only nearly critical homogenizing inclusions have been found (low density). Pressure estimates, based on densities of early fluid inclusions, show that the rapid transition of amphibolite towards granulite facies metamorphism is virtually isobaric. Granulite facies metamorphism in West Uusimaa is a thermal event, probably induced by the influx of hot, CO₂-bearing fluids.     

P-T-X (CO2) conditions in mafic and calc-silicate hornfelses from Oberon, New South Wales, Australia

Abstract The Rockley Volcanics from near Oberon, New South Wales occur within the aureole of the Carboniferous Bathurst Batholith and have been contact metamorphosed at P ∼ 100 ± 50MPa (10.5kbar) and a maximum T ∼ 565°C in the presence of a C–O–H fluid. Prior to contact metamorphism the volcanics were regionally metamorphosed and altered with the extensive development of actinolite, chlorite, plagioclase, quartz and calcite. The contact metamorphosed equivalents of these rocks have been subdivided into: Ca-poor (cordierite + gedrite), Mg-rich (amphibole + olivine + spinel), mafic (amphibole + plagioclase) and Ca-rich (amphibole + garnet + diopside; diopside + plagioclase; garnet + diopside + wollastonite) rocks.
The chemistry of the minerals in the hornfelses was controlled by the bulk rock chemistry and fluid composition. Pargasites and hastingsites as well as an unusual phlogopite with blue green pleochroism, are found in Ca-rich hornfelses. A comparison of the assemblages with experimentally derived equilibria suggests that the fluid phase associated with the Ca-rich hornfelses was water-rich (X_co2= 0.1 to 0.3) while that associated with the Mg-rich hornfelses was enriched in CO₂ (X_co2 > 0.7). The different hornfels types have reacted to contact metamorphism independently in both their solid and fluid chemistries.     

A Fluid Inclusion Study on Columnar Adularia from the Hishikari Low-sulfidation Epithermal Gold Deposit, Japan

Abstract: In the Hishikari low-sulfidation epithermal gold deposit, Japan, columnar adularia crystals commonly precipitated directly on to the fracture surface of wall rock, and then electrum precipitated on the columnar adularia with fine-grained adularia and quartz. To reveal the characteristics of mineralizing fluids and the elevation of paleo-water tables at the earliest stage of mineralization in the Honko-Sanjin zone of the Hishikari deposit, the fluid inclusions in the columnar adularia in gold-bearing quartz–adularia veins were studied.
Coexistence of vapor-rich and liquid-rich two–phase primary fluid inclusions indicates the deposition of columnar adularia from boiling fluids. The precipitation temperatures range from 175 to 215C, and generally increase with depth. The temperatures of final melting point of ice range from –1.2 to –0.1C with an average of –0.5C, corresponding to salinity ranging from 0.2 to 2.1 wt% NaCl equivalent with an average of 0.9 wt% NaCl (eq.). Concentrations of non-condensable gases such as CO₂ were under the detection limit of a laser Raman microprobe spectrometer. From the precipitation temperature of columnar adularia in the Hosen–2 vein and the boiling point – depth curve for a 0.9 wt% NaCl (eq.) fluid, paleo-water table was estimated to be at an elevation of about +170 m. The elevation of the paleo-water tables for other veins was estimated to range from +140 to +215 m.     

An Improved Model for Calculating CO_2 Solubility in Aqueous NaCl Solutions and the Application to CO_2-H_2O-NaCl Fluid Inclusions

<正>To determine compositions,homogenization pressures and isopleths of CO2-H2O-NaCl fluid inclusions,an improved activity-fugacity model is developed to calculate CO2solubility in aqueous NaCl solutions.The model can predict the solubility of CO2in aqueous NaCl solutions from 273 K to 723 K,from1 bar to 1500 bar and from 0 to 4.5 mol kg-1of NaCl,within or close to experimental uncertainties.The average deviation between the solubility predicted by     

Determination of Rare Earth Elements and Y in Ultramafic Rocks by ICP-MS After Preconcentration Using Fe(OH)3 and Mg(OH)2 Coprecipitation

A simple and reliable method to separate rare earth elements (REE) from Mg, Fe, K, Na, Ca and Ba in ultramafic rocks has been developed, thereby concentrating their abundances. The sample (0.3 g) was digested with HF and HNO₃ in a PTFE bomb, placed in a stainless steel container and, after drying, the insoluble residue was dissolved in 6 ml of 10% v/v HNO₃. Following the addition of 50% triethanolamine and 30% m/v NaOH solution, the REE were precipitated along with Mg(OH)₂, such that the majority of Fe, K and Na in the solution could be separated by centrifuging. The precipitate was dissolved in 1 ml HNO₃ and a buffer solution of NH₄Cl/NH₄OH at pH = 9.0 was added to precipitate the REE along with any remaining Fe as Fe(OH)₃, and so achieve separation from Mg, Ca and Ba, which remained in the solution. In this way, REE could be separated from major elements and were concentrated by a factor of about 60. The recovery of REE was more than 95% using this method. Four ultramafic rock reference materials, PCC-1 (USGS), JP-1 (GSJ), DZE-1, DZE-2 (IGGE) and one new proficiency testing sample GeoPT12 (GAS Serpentinite) were analysed by ICP-MS using indium as an internal standard. The quantitation limits were about 0.02–0.2 ng g⁻¹. Smooth chondrite-normalised REE patterns were obtained with a precision for REE determination of about 2–9%.