Yann Lahaye

THE INFLUENCE OF ALTERATION ON THE TRACE-ELEMENT AND ND ISOTOPIC COMPOSITIONS OF KOMATIITES

Abstract To investigate the effects of hydrothermal alteration and metamorphism on the chemical and isotopic compositions of komatiites, we studied samples from the Alexo and Texmont regions in the 2.7-Ga Abitibi belt of Canada and from the Weltevreden, Mendon and Komati Formations of the 3.2–3.5-Ga Barberton belt of South Africa. Particular emphasis was placed on multiple samples from individual layered spinifex-olivine cumulate flows, the argument being that if these flows showed variations in the ratios of elements incompatible with olivine, then these variations were most likely due to chemical mobility during alteration. Data for rare-earth (REE) and high-field-strength elements (HFSE) reveal anomalies (non-chondritic HFSE/REE ratios), particularly in the most altered samples. After taking into account the limits of analytical precision, these anomalies are attributed to element mobility, not to fractionation of high-pressure minerals, as suggested by other authors for Abitibi komatiites. In Barberton komatiites three separate processes produced HFSE anomalies: element mobility during alteration, crust assimilation in some samples, and majorite fractionation. The effects of the latter process were recognized from systematic relationships between HFSE/REE and Al/Ti that coincided with calculated majorite fractionation trends. Initial Nd isotopic compositions of the komatiites were determined by analysis of magmatic pyroxenes. The initial ϵNd of pyroxene from the 2.7-Ga Alexo komatiite is +3.8, slightly higher than in whole rocks ( +0.6 to +3.5, average ∼ +2.5). In the carbonatized Texmont samples, the range in initial ϵNd is far greater ( −6.2 to +8.8). The variable initial ϵNd-values in the rocks are attributed to isotopic exchange of Nd with surrounding rocks during early alteration, and fractionation of Sm/Nd during later events. Pyroxene from 3.3-Ga Barberton komatiitic basalt had initial ϵNd of + 2.3.

The lead and copper isotopic composition of copper ores from the Sierra Morena (Spain)

The paper presents lead and copper isotope analyses of 51 copper ore samples from the Sierra Morena, South of Spain. They are from ancient mines of the Iberian Peninsula collected by Claude Domergue during various field campaigns in the central Sierra Morena from 1965 to 1975. Most samples consist of copper oxide minerals such as malachite, azurite and chrysocolla and stem from the surficial sections of the ore deposits. The aim of the study was to supplement the existing reference data bank on lead isotopic compositions of ancient copper mines from the Iberian Peninsula. This is particularly important for the Sierra Morena for which data exist mostly for lead-zinc but not for copper ores. The lead isotope ratios range from 18.165 to 19.712 (206Pb/204Pb), 0.797 to 0.859 (207Pb/206Pb) and 1.955 to 2.108 (208Pb/206Pb). Two separate fields can be distinguished with a major field intermediate between the ore deposits from SW and SE Spain and a second at higher 208Pb/206Pb values. Copper isotopes were analysed additionally to provide further constraints for provenance studies. The copper isotope ratios δ65Cu of the copper oxide samples are mostly positive and higher on average than those of sulphide minerals. They are a potential tool to distinguish between either sulphide ore or oxide ore deposit derived artefacts.

1.8 Ga magmatism in southern Finland: strongly enriched mantle and juvenile crustal sources in a post-collisional setting

Whole-rock and isotope geochemistry of six ∼1.8 Ga post-kinematic intrusions, emplaced along the ∼1.9 Ga Southern Svecofennian Arc Complex (SSAC) and in the SW part of the Karelian Domain in Finland, was studied. The intrusive age [U–Pb secondary ion mass spectrometer (SIMS)] of one of these, the Petravaara Pluton, was determined as 1811 ± 6 Ma. Basic-intermediate rocks are alkali-rich (K2O + Na2O > 4 wt.%) and typically shoshonitic, strongly enriched in large ion lithophile elements and light rare earth elements, but relatively depleted in high field strength elements and heavy rare earth elements. The enrichment is much higher than can be accounted for by crustal contamination and requires previously melt-depleted mantle sources, subjected to variable metasomatism by carbonate-rich fluids and sediment-derived melts. These sources are inferred to consist of phlogopite ± amphibole-bearing peridotites from depths below the spinel–garnet transition, as shown by the high Ce/Yb ratios. 87Sr/86Sr(1.8 Ga) ratios in the range 0.7027–0.7031 and ‘mildly depleted’ ϵNd(1.8 Ga) values (+0.1 to +1.4), with T DM values <2.1 Ga, suggest that mantle enrichment was associated with the previous Svecofennian subduction–accretion process, when enriched sub-Svecofennian mantle sections developed, dominantly characterized by 147Sm/144Nd ratios of 0.14–0.17. The associated granitoids are diversified. One group is marginally peraluminous, transitional between I (volcanic-arc) and S (syn-collisional) types, and was derived from mixed igneous and sedimentary, but juvenile Svecofennian source rocks, as supported by near-chondritic ϵNd(1.8 Ga) and somewhat elevated 87Sr/86Sr(1.8 Ga). The other group is transitional between I and A (within-plate) types in character and had dominantly igneous protoliths. The whole-rock geochemistry and isotopes suggest that the compositional variation between ∼50 and 70 wt.% SiO2 may be explained by hybridization between strongly enriched mantle-derived magmas and anatectic granitic magmas from the juvenile Svecofennian crust. One intrusion in the east contains a significant portion of Archaean, mostly igneous protolithic material (ϵNd(1.8 Ga) = –2.8 and ϵHf(t) for zircons between +2.8 and −11.9, with an average of −4.9). The ∼1.8 Ga post-kinematic intrusions were emplaced within the SSAC subsequent to the continental collision with the Volgo-Sarmatia craton from the SE, during a shift from contraction to extension, that is, in a post-collisional setting.

Strontium isotope disequilibrium of plagioclase in the Upper Critical Zone of the Bushveld Complex: evidence for mixing of crystal slurries

We report in situ Sr isotope data for plagioclase of the Bushveld Complex. We found disequilibrium Sr isotopic compositions on several scales, (1) between cores and rims of plagioclase grains in the Merensky pyroxenite, the Bastard anorthosite, and the UG1 unit and its noritic footwall, (2) between cores of different plagioclase grains within thin sections of anorthosite and pyroxenite of the Merensky unit, the footwall anorthosite of the Merensky reef and the footwall norite of the UG1 chromitite. The data are consistent with a model of co-accumulation of cumulus plagioclase grains that had crystallized from different magmas, followed by late-stage overgrowth of the cumulus grains in a residual liquid derived from a different level of the compacting cumulate pile. We propose that the rocks formed through slumping of semi-consolidated crystal slurries at the top of the Critical Zone during subsidence of the center of the intrusion. Slumping led to sorting of crystals based on density differences, resulting in a layered interval of pyroxenites, norites and anorthosites.

The effect of sodium on the solubilities of metals in silicate melts

Abstract We have studied experimentally the effects of variable Na2O concentrations on the solubility of four metals (Fe, Co, Ni, and Pd) in silicate melts of anorthite-diopside eutectic composition. Experiments were done at fixed temperatures of 1300 °C for Pd and 1400 °C for Ni, Co, and Fe. In the Fe solubility experiments, Na losses were severe. Therefore, a new experimental device (crucible-supported loop technique or CSLT) was designed to prevent Na losses of experimental charges primarily for experiments at low oxygen fugacities. The CSLT was tested extensively, and it was demonstrated that high Na partial pressures could be kept in a semi-closed crucible for at least 20 h. In experiments on Co solubilities, the CSLT was tested with reversed experiments. The Fe and Co solubilities clearly decrease with increasing Na2O contents. The Ni solubility is independent of Na2O contents up to 4.5 wt%. At oxidizing conditions, a small decrease in Ni solubility with increasing Na content was found. Palladium shows a mixed behavior, decreasing solubility with increasing Na2O, up to about 4 wt%. At higher Na contents, Pd solubility is independent of Na content. The increase of FeO activity coeficients with increasing Na content found in this study may provide an explanation for decreasing FeO with decreasing degrees of partial melting in experimentally produced mantle melts (Hirschmann et al. 1998), as melts at low melting degrees are Na-rich.

The effect of TiO2 on Pd, Ni, and Fe solubilities in silicate melts

Abstract We have determined Pd, Ni, and Fe solubilities in silicate melts of anorthite-diopside eutectic composition with variable TiO2 concentrations, from TiO2-free melts to melts with up to 26 wt% TiO2. The experiments were conducted with metal loops at 1300 °C, one atm total pressure, and at a wide range of oxygen fugacities. The glasses were analyzed with the electron microprobe and by laser ablation inductively coupled plasma mass spectrometry (LA ICP-MS). The behavior of Ni was found to be nonlinear. At given T-fO₂ conditions, its solubility is relatively constant for melts with up to about 4 wt% (3 mol%) TiO2. At higher TiO2 concentrations, Ni solubility increases strongly with TiO2 contents. The results obtained for Pd at oxidizing conditions are similar to those obtained for Ni. The solubility of Fe increases uniformly with TiO2 contents within the whole TiO2 concentration range. In some experiments Pd-containing alloy micronuggets were encountered. In this case analyses were done in areas free of nuggets. Experiments in which the nugget density was too high were discarded. The regular behavior of Pd-even in nugget-containing silicates-indicates that Pd-oxide solubility and nugget formation are independent processes. The occurrence of Ti-rich phases (karrooite and armalcolite) on the liquidus of TiO2-saturated melts at reducing conditions, and the Pd partition coefficients between rutile and silicate melts, were determined and are briefly discussed.

Intra-orogenic Svecofennian magmatism in SW Finland constrained by LA-MC-ICP-MS zircon dating and geochemistry

We have studied plutonic rocks from the Korpo and Rauma areas of south-western Finland which can be categorized as intra-orogenic, i.e. they were intruded during a proposed extensional period between the two main Svecofennian orogenic cycles: the Fennian and Svecobaltic orogenies. The diorite from Rauma yielded an age of 1865 ± 9 Ma and the diorite from Korpo an age of 1852 ± 4 Ma. The adjacent garnet-bearing Korpo granite was 1849 ± 8 Ma in age. Zircons from the granite also included inherited Archaean and older Palaeoproterozic zircons, as well as metamorphic c. 1820 Ma rims. The diorites are high-K to shoshonitic, mantle-derived magmas, rich in Fe, P, F and light rare earth elements. The Korpo granites show typical features of crustal-derived melts and form hybrids with the diorites in contact zones. Both the mantle-derived and crustal-derived intra-orogenic magmatism are considered to have had a causal effect on the subsequent late Svecofennian (Svecobaltic) thermal evolution in southern Finland which culminated in granulite facies metamorphism and large-scale crustal melting.

Mantle source of the 2.44–2.50-Ga mantle plume-related magmatism in the Fennoscandian Shield: evidence from Os, Nd, and Sr isotope compositions of the Monchepluton and Kemi intrusions

Significant PGE and Cr mineralization occurs in a number of 2.44–2.50-Ga mafic layered intrusions located across the Karelian and Kola cratons. The intrusions have been interpreted to be related to mantle plume activity. Most of the intrusions have negative εNd values of about −1 to −2 and slightly radiogenic initial Sr isotope compositions of about 0.702 to 0.703. One potential explanation is crustal contamination of a magma derived from a mantle plume, but another possibility is that the magma was derived from metasomatized sub-continental lithospheric mantle. Samples from the upper chromitite layers of the Kemi intrusion and most samples from the previously studied Koitelainen and Akanvaara intrusions have supra-chondritic γOs values indicating some crustal contamination, which may have contributed to the formation of chromitites in these intrusions. Chromite separates from the main ore zone of the Kemi and Monchepluton intrusions show nearly chondritic γOs, similar to the coeval Vetreny belt komatiites. We suggest that the Os isotope composition of the primitive magma was not significantly changed by crustal contamination due to a high Os content of the magma and a low Os content of the contaminant. Modeling suggests that the Os and Nd isotope compositions of the Monchepluton and Kemi intrusions cannot be explained by assuming a magma source in the sub-continental lithospheric mantle with sub-chondritic γOs. A better match for the isotope data would be a plume mantle source with chondritic Re/Os and Os isotope composition, followed by crustal contamination.

Metamorphic evolution of the Ilomantsi greenstone belt in the Archaean Karelia Province, eastern Finland

Abstract The Ilomantsi greenstone belt is a Neoarchaean, c. 2.75–2.70 Ga volcanic–sedimentary complex in which metamorphic grade increases from staurolite grade in the SW of the belt to sillimanite grade in the NE. In the staurolite zone, prograde garnet zoning indicates pressure and temperature increases from 480–500°C at 2–4 kbar to 560–570°C at 6–7 kbar. Within the sillimanite zone temperatures peaked at 660–670°C at pressures of around 6 kbar. The U–Pb age determinations on monazite from the sillimanite zone yielded both Archaean and Proterozoic ages. One sample contains an exclusively Archaean monazite population of 2620±24 Ma, while another sample has two generations of monazite, with ages of 2664±33 Ma and 1837±13 Ma. The monazite data confirm that the Ilomantsi greenstone belt was metamorphosed simultaneously with the surrounding Neoarchaean migmatite complexes. The apparent clockwise PT path and medium P/T-type metamorphism are consistent with collisional tectonic settings, but the two distinct metamorphic events recorded by monazite indicate that a second, Palaeoproterozoic thermal event caused recrystallization and new mineral growth, in line with previous evidence from other isotopic systems. Accordingly, great care is necessary in defining metamorphic evolutionary P–T–t paths in rocks with complex mineral assemblages, to ensure correct identification of truly coeval mineral assemblages.

Offset-type PGE mineralisation in the Sotkavaara Intrusion, northern Finland: an association with zones of low-cr clinopyroxenite

isation is correspondingly rich, or that placer gold is a consequence of authigenic processes. Straight twin boundaries (Figure 1) are a consequence of annealing above 250°C, and these are present in all hypogene grains studied (Hough et al. 2007). Placer grains exhibit the same feature, invalidating an authigenic origin. Furthermore deformation of the twin boundaries could find application in predicting distance travelled in the surficial environment. This study is the first to systematically characterise the internal crystallography of gold from a lode and placer source. Future research will integrate data from composition studies (electron microprobe, trace element mapping by LA-ICP-MS) with gold crystallography with a view to further illuminating ore forming processes at specific localities.