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Experimental determination of over seventy sets of clinopyroxene/silicate liquid (glass) partitition coefficients (D) for four rare earth elements (REE — La, Sm, Ho, Lu) in a range of REE-enriched natural rock compositions (basalt, basaltic andesite, andesite and rhyodacite) demonstrate a convex upward pattern, favouring the heavy REE (Ho, Lu) and markedly discriminating against the light REE (La). These patterns are consistent with previously documented clinopyroxene D values reported from natural phenocryst/matrix pairs and from experimental work using either REE-enriched compositions and electron microprobe analytical techniques (as in the present study) or natural or synthetic undoped compositions and mass spectrometric, ion probe or X-ray autoradiographic analytical techniques. However, the large data base in the present study allows evaluation of the effect of compositional and physical parameters on REE partitioning relationships. Considering DHo, it is shown that (1) D increases 6-fold with increasing SiO2 content of the coexisting liquid from ∼ 50 to ∼ 70 wt% SiO2 (2) D increases 4-fold with decreasing temperature from 1,120°C to 900° C (3) D increases 2-fold with increasing pressure from ∼ 2.5 to 20 kb. (4) D increases ∼ 2-fold fO2 increases from approximately that of the MW buffer to the HM buffer (5) D remains unchanged within experimental error as the water content of the melt changes from ∼ 0.3 to ≳ 10% by weight H2O. The absolute REE content of the clinopyroxene shows no consistent trend with temperature, but decreases slightly with increasing pressure, paralleling an increase in the jadeite component of the pyroxene. Thus the increase in D with increasing pressure is attributed to changes in the silicate liquid structure, which discriminate against accommodation of REE with increasing pressure. The clinopyroxene REE content increases with increasing fO2, and in this case the increase in D with increasing fO2 may be attributed mainly to this change in the clinopyroxene composition. Application of the present results to geochemical modelling allows a more appropriate choice of D values, according to the liquid composition and physical conditions applicable in the modelled system. They may also be used to evaluate cognate or xenocrystic relationships between clinopyroxene megacrysts and their host matrix.

Amphibole/liquid partition coefficients for the REE(Damph/liqREE) obtained from natural rocks increase systematically with bulk rock compositional change from basalt to rhyolite and are higher for the middle to heavy REE. Five new experimentally determined sets of DREE (La, Sm, “Eu2+”, Ho, Lu)and 4 published sets are similar to these data and provide values for use in geochemical modelling of magmatic processes involving amphibole, over a range of temperature, pressure and oxygen fugacity. The partition coefficients increase significantly with decreasing temperature, and increase slightly with increasing oxygen fugacity. Pressure does not appear to have a major effect, although one data set suggests increased pressure lowers Damph/liqREE in a basaltic composition.

Experimental determination of partition coefficients (D) for Nb and Ta between Ti-rich minerals (sphene, rutile, ilmenite and Ti-magnetite) and coexisting silicate liquids of basaltic andesite, andesite and trachyte composition, indicate that these elements partition strongly into sphene and rutile, and moderately into ilmenite and Ti-magnetite. The D values for sphene increase with decreasing temperature and with increasing SiO2 content of the coexisting liquid, but appear unchanged by pressure between 7.5 and 16 kb, and oxygen fugacity between magnetite-wustite and haematite-magnetite buffers. Also, DNbDTa. varies from 0.3–0.4 for sphene, increasing to 0.6–0.7 for rutile, 0.7–0.8 for ilmenite and 0.8 for Ti-magnetite. Thus fractionation involving these minerals may cause an increase in the NbTa ratio of any derivative liquid; this will be most noticeable where sphene is involved.

Over 80 sets of partition coefficients (D) for 4 rare-earth elements (REE) (La, Sm, Ho, Lu) partitioning between sphene and coexisting silicate liquid at 7.5 to 30 kbar and 900° to 1120°C, demonstrate that D-values increase with: (a) increasing SiO2 content of the coexisting liquid; (b) increasing pressure; and (c) decreasing temperature. The overall D pattern is convex upwards, with sphene strongly favouring the middle REE (Sm, Ho), and favouring Lu slightly more than La. Using Sr2+ as an analog for Eu2+, it is shown that sphene does not accept Eu2+, in contrast to its ready accommodation of the trivalent REE. This REE substitution appears to be coupled: Ca 2+ +Ti 4+ ⇌∑REE 3+ +(Al,Fe) 3+ Sphenes with between 1 and 21 wt. % ∑RE2O3 have been grown experimentally, and it is shown that for this composition range D is independent of the REE content of the sphere. However, these D-values are less than most determined from phenocryst—groundmass pairs. Thus, since there is only a partial overlap of REE concentration between the natural and synthetic sphenes, caution is needed in the direct application of these results to natural rocks. Nevertheless, the data confirm the potential importance of sphene in controlling REE distribution and indicate the parameters affecting REE partitioning, which need to be considered when choosing D-values for sphene in geochemical modelling of magmatic processes.

Experiments on hydrous mafic, intermediate and felsic compositions at 7.5–30 kbar and 900–1100°C allow broad delineation of TiO 2 contents for a range of silicate liquids coexisting with a Ti-rich accessory phase. Different starting compositions, including unadulterated fused natural rocks, and rocks enriched with various Ti-rich phases, give mutually consistent results. The TiO 2 content of liquids saturated in a Ti-rich accessory phase is shown to decrease with decreasing temperature, increasing pressure, increasing SiO 2 , alkali and rare-earth element content (at % level) of the liquid. Water content has little observable effect, while increasing oxygen fugacity lowers the TiO 2 content of liquid coexisting with a Ti-rich phase. A separate Ti-rich accessory phase may be expected to crystallize from liquids at 1000°C in the deep crust (pressures corresponding to 7.5–12 kbar) if the TiO 2 content exceeds ∼ 3.0 wt.% for mafic, ∼ 1.8 wt.% for intermediate, and ∼ 1.2 wt.% for felsic compositions. At the same depth but at 950°C, these TiO 2 values change to ∼ 1.7, 1.2 and 0.9 wt.%, respectively. At upper-mantle depths (20–30 kbar) the corresponding TiO 2 saturation levels at 1000°C are ∼ 2.0, 1 and 0.7 wt.%, respectively. In natural igneous rock series the contrasting TiO 2 SiO 2 systematics in alkaline and tholeiitic series compared with calc-alkaline series are well established, as are the characteristic, though non-unique, low TiO 2 values for rock series in convergent plate regions. The present results allow experimental confirmation of observed and/or predicted crystallization of Ti-rich phases in these rock series. They also provide reasonable constraints on conditions under which TiO 2 contents of convergent plate magmas may be buffered by a residual Ti-rich phase in the source region, and indicate that TiO 2 contents of mafic parent magmas in these areas are unlikely to be controlled in such a manner. In contrast, it is almost inevitable that felsic magmas, generated by partial melting of the lower crust, are saturated with respect to a Ti-rich phase in their source regions.

Geochemical modelling of magma generation and crystal fractionation processes at pressures corresponding to those of the lower crust1 and upper mantle2 is critically dependent on the trace element partition coefficients used in the calculations. The effects of temperature and melt composition on partition coefficients are reasonably well established3,4, but the possible effect of pressure has not been closely evaluated, apart from some work on nickel partitioning5. Rare earth elements (REE) are important in geochemical modelling, and the need for data on the pressure effect on REE partitioning has been pointed out3. We have done a series of experiments in which the role of pressure was isolated from that of temperature and melt composition (or melt structure), and we are able to demonstrate a systematic increase in La, Sm, Ho and Lu partition coefficients for sphene/silicate liquid and clinopyroxene/silicate liquid as pressure increases from 7.5 to 30 kbar.

International audience; Chromitite pods in the Mayarí-Cristal ophiolitic massif (eastern Cuba) were formed in the Late Cretaceous when island arc tholeiites and MORB-like back-arc basin basalts reacted with residual mantle peridotites and generated chromite-rich bodies enclosed in dunite envelopes. Platinum-group minerals (PGM) in the podiform chromitites exhibit important Os-isotope heterogeneities at the kilometric, hand sample and thin section scales. 187Os/188Os calculated at the time of chromitite crystallization (~90 Ma) ranges between 0.1185 and 0.1295 (γOs = −7.1 to +1.6, relative to enstatite chondrite), and all but one PGM have subchondritic 187Os/188Os. Grains in a single hand sample have initial 187Os/188Os that spans from 0.1185 to 0.1274, and in one thin section it varies between 0.1185 and 0.1232 in two PGM included in chromite which are only several millimeters apart. As the Os budget of a single micrometric grain derives from a mantle region that was at least several m3 in size, the variable Os isotopic composition of PGM in the Mayarí-Cristal chromitites probably reflects the heterogeneity of their mantle sources on the 10-100 m scale. Our results show that this heterogeneity was not erased by pooling and mingling of individual melt batches during chromitite crystallization but was transferred to the ore deposits on mineral scale. The distribution of the Os model ages calculated for PGM shows four main peaks, at ~100, 500, 750 and 1,000 Ma. These variable Os model ages reflect the presence of different depleted domains in the oceanic (Pacific-related) upper mantle of the Greater Antilles paleo-subduction zone. The concordance between the age of crystallization of the Mayarí-Cristal chromitites and the most recent peak of the Os model age distribution in PGM supports that Os in several grains was derived from fertile domains of the upper mantle, whose bulk Os isotopic composition is best approximated by that of enstatite chondrites; on the other hand, most PGM are crystallized by melts that tapped highly refractory mantle sources.

Zircon frequently occurs as a minor mineral in kimberlites, and is recognised as a member of a suite of mantle-derived megacryst minerals. Cathodoluminescence (CL) microscopy and laser ablation ICPMS analysis were used to study the internal structure and chemical composition of zircon crystals from southern African kimberlites. Zoning revealed by CL ranges from fine oscillatory to broad homogeneous cores and overgrowths. The ICPMS data show that kimberlite zircons have distinctive trace element contents, with well defined ranges for REE, Y, U, Th, P and some other trace elements. Both low REE contents (ΣREE < 50 ppm), and distinctive chondrite-normalised REE patterns with low and flat HREE are characteristic of kimberlite zircons. Samples or zones with yellow CL have higher Th, U, Y, and REE than those with blue-violet CL. Variations in the concentrations of a range of trace elements lead to different amounts of lattice defects, creating the possibility for different levels of direct excitation of luminescence centres, and therefore different CL colours. The distinctive CL and compositional features described here can rapidly identify kimberlite zircons in prospecting samples taken during exploration for kimberlite bodies.

The Slave Craton in Canada, is a small Archaean fragment, bounded on the east by the Thelon magmatic arc (2.0-1.9 Ga) on the western edge of the Archaean Rae Province and on the west by the Great Bear magmatic arc of the Wopmay Orogen (1.88-1.84 Ga). The northern and northeastern part is overlapped by Late Proterozoic and Phanerozoic supracrustal rocks. On the south, the Slave Craton is separated by the Great Slave Lake Shear Zone from the Early Proterozoic Chinchaga and Buffalo Head terranes. We use robust geochemical methods based on mantle-derived xenoliths, heavy mineral concentrates from over 25 kimberlites, and representative diamond populations and their inclusions to construct sections that delineate the composition, structure and thermal state of the lithospheric mantle across the Slave Craton. This analysis reveals a distinct two-layered lithosphere beneath the craton: a shallow ultradepleted, olivine-rich layer and a deeper less depleted layer, interpreted as an Archaean plume head. We have mapped variations in the gravity/topography relationships across the Slave Province in terms of the effective elastic thickness (Te). The results show that the northern part of the craton is characterised by a relatively weak lithosphere (Te 56 km). A N-S zone of low Te along the middle of the craton may map the deep extension of the suture between the ancient continental block making up the western part of the craton, and the younger accreted terranes that make up the eastern part. The zone of low Te gradient coincides with an area of strongly conductive upper mantle, and with the Nd/Pb isotope lines which define a major crustal boundary at depth, and is a major locus of kimberlite intrusion.

youngest and least well-understood part of the Baltic Shield. These data allow us to identify the Lu–Hf signatures of different crustal domains in the region, to

Quenched carbonate-silicate inclusions in lherzolitic clinopyroxene macrocrysts, derived from 200 km beneath the Slave craton in northern Canada, are interpreted as natural samples of mantle carbonatites. Oxygen, carbon, and strontium isotope data provide evidence for the involvement of subducted crustal material in the origin of these carbonatites, supporting suggestions that carbon recycling by subduction is an important prerequisite for carbonatite magmatism. The compositional range of the inclusions suggests that the parent melt was decreasing in silica content as it was trapped in the host crystal, a trend that is predicted experimentally. Isotopic disequilibrium between the carbonatitic inclusions and the host clinopyroxene indicates that they were trapped shortly before kimberlite eruption, suggesting a temporal link between the entrapment of the carbonatite in the host and the Paleocene eruption of the kimberlite.